Conservation agriculture through the lens of soil, climate, and management factors

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Conservation agriculture through the lens of soil, climate, and

management factors

Andrés E. Rucabado Gordo 12480665

22/12/2020

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Agriculture has been linked to a decrease in natural areas, terrestrial biodiversity, ecosystem services, and an increase in greenhouse gas emissions, which can further fuel climate change. Conventional agricultural practices have been regarded as responsible for these trends, as they have been shown to lead to a decrease in soil organic carbon (SOC), soil structural degradation, erosion, crusting, and soil compaction. While the use of machinery to prepare the soil for cultivation, i.e. tillage, contributes to the increase of greenhouse gas emissions. A common strategy to reduce and limit the impact of agriculture on climate change is to transition from conventional tillage practices to conservation tillage. The idea behind this is that practices like no-tillage (NT) would enhance soil carbon (C) sequestration and offset some of the CO2 emissions that come from the agroindustry. Moreover, the inclusion of other conservation agriculture (CA) practices, like cover crops, crop rotations, and residue management, in tandem with NT, can provide other benefits to agroecosystems besides an increase in soil C. However, previous study results are inconsistent as to whether these practices lead to a substantial increase in carbon sequestration, with some authors claiming an overestimation of its benefits. To breach this knowledge gap, in this review I evaluate the effect of NT practices on SOC based on management, climatic, and soil factors. The management practices studied here are crop rotations, cover crops, and residue management, and I also compare these practices between tropical and temperate regions. The results indicate that management practices that are aimed at increasing soil C inputs, such as cover crops and residue management, yielded the highest SOC content with 76 Mg C ha-1 and 56.70 Mg C ha-1 respectively. While the tropical region resulted in the highest SOC content, with an average of 64.20 Mg C ha-1 and 48.78 Mg C ha-1 for the temperate region. Subtropical soils were included in the tropical region category and represented the bulk of the studies in the region, while tropical soils were limited to four studies. It may be that the variability between soils in these two regions, and the overrepresentation of subtropical soils, led to this result. Finally, sandy soils yielded the highest SOC content of all the soil textures. This was likely due to high C inputs from cover crops and residue management, as sandy soils benefit more from an increase in C inputs than from a change in tillage management. Moreover, most of the sandy soils were in the tropics, and thus benefited from the high SOC contents found in this region. In conclusion, the adoption of CA practices can lead to an increase in SOC, but this will not be the case for all pedo-climatic conditions, therefore these conditions should be taken into consideration when applying such practices.

Keywords:

Soil organic carbon, tillage, no tillage, conservation agriculture, carbon

sequestration

1. Introduction

Land use change and land use management practices are one of the biggest drivers for global change, contributing to a significant reduction of natural areas, terrestrial biodiversity, ecosystem services, and an increase of greenhouse gas emissions (GHG) (Vliet, 2019). Particularly, agricultural intensification and cropland expansion has played an important role in these processes (Struck et al., 2020; Vliet et al., 2019). In addition, conventional agricultural practices such as tillage operations, have led to a decrease in soil organic carbon (SOC), soil structural degradation, erosion, crusting, and soil compaction (Modak et al., 2020). Tillage systems have been widely used for their short-term benefits such as crop yield security, seedbed preparation, water and soil management, and weed control. The long-term effects on soil health and the environment, however, may not be as beneficial (Higashi et al., 2014). In addition to tillage systems, other conventional agricultural practices can also prove detrimental to soil health. For example, stubble burning, residue removal, fallow periods, and other practices that leave the soil bare will increase soil erosion, decrease soil organic matter, and biomass, and will cause soil nutrient deficiencies (Derpsch et al., 2014;. Du, Ren & Hu, 2010).

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2 To reduce the negative impact of agricultural practices on ecosystems there is a need to develop sustainable land management practices. In the last few decades, there has been a move towards conservation agricultural practices, which include a variety of conservation tillage treatments characterized by their soil disturbance intensity, like reduced tillage (RT) and no-tillage (NT). In addition to conservation tillage, conservation agricultural practices include a variety of sustainable management systems that work to promote biodiversity and enhance biological processes. This can result in improved water and nutrient use efficiency, resilient crops, and improved and sustained crop production (González-Sánchez et al., 2019). This is achieved through a reduction in soil disturbance, and an increase in soil cover and crop rotations and diversity. Moreover, the holistic application of these practices aims to improve soil health and reduce the GHG emissions of conventional agriculture, while also mitigating the increase of anthropogenic GHG emissions from other sectors through changes in soil carbon dynamics leading to an increase in soil carbon (C) storage (González-Sánchez et al., 2019; Struck et al., 2020).

Multiple authors have found that agricultural soils managed by NT and conservation agricultural (CA) systems tend to increase soil C stocks (Bayer, Martin-Neto, Mielniczuk, Pavinato & Dieckow, 2006; Chivenge, Murwira, Giller, Mapfumo & Six, 2007; Derpsch et al., 2014; González-Sánchez et al., 2019; Struck et al., 2020). This presents an opportunity to use these soils as a sink for atmospheric C and thus help mitigate global warming (Bayer et al., 2006; González-Sánchez et al., 2019).

Soil C sequestration refers to the process in which vegetation removes CO2 from the atmosphere and

stores it in the soil biomass in the form of SOC, mainly through the process of photosynthesis (González-Sánchez et al., 2019). Conservation agriculture (CA) takes advantage of these processes to serve as a climate change mitigation tool. On one hand, by lessening soil disturbance via conservation tillage practices, the soil’s ability to protect and store SOC is enhanced, as described in section 2.2; and on the other hand, there is a decrease in CO2 emissions resulting from a reduction in the use of

tillage machinery and other field operations. While CA practices that control C inputs, such as cover crops and residue management, can lead to a positive net ecosystem carbon balance (Cates & Jackson, 2019).

However, it is still uncertain if conservation agricultural practices, especially conservation tillage treatments, have a significant impact on SOC sequestration, and if it’s at all beneficial for farmers’ livelihoods given the reduced crop yields found by some researchers, and additional unwanted and unexpected side effects on the environment and society (Giller, Witter, Corbeels & Tittonell, 2009). Furthermore, there has been a scarcity of standardized tillage research that includes a systems approach, resulting in a spectrum of studies with different conclusions and views on the role of conservation tillage on SOC sequestration (Derpsch et al., 2014; Haddaway et al., 2017; Giller et al., 2009). Most of the published research claims that CA is the solution to current agricultural and climate change problems. However, Giller et al. (2009) opposes this view and provides a more pragmatic perpective to this topic, they argue that CA is not appropriate for all farmers and farming systems, and that it may not always satisfy the conditions and necessities of these farming systems. On SOC sequestration, they conclude that there is not enough evidence to prove an increase in SOM and soil fertility under conservation tillage. This is echoed in part by a few researchers, like Derpsch et al. (2014), who attribute the shortcomings of CA research on a lack of standardization. Ultimately, this makes it harder to compare conservation tillage systems across different conservation management scenarios and different climatic and soil conditions. And so, the need arises to systematically study how management and pedo-climatic factors interact with conservation tillage systems and how, in turn, this influences the natural biological process of agricultural ecosystems.

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3 Moreover, it is likely that not all agricultural lands across the globe are suitable for C sequestration by means of NT and conservation agricultural practices. Soil dynamics are, after all, complex systems: the potential C storage capacity of soils depends on the primary productivity, which in turn is limited by climate, soil, and management system employed (Du, Ren & Hu, 2010; Luo, Wang & Jianxin Sun, 2010; Virto, Barré, Burlot & Chenu, 2012). This highlights the importance to understand the processes that result from the interaction of these factors. To that end, this literature review compares SOC content between different conservation agricultural practices, in addition to NT, between different soil types, and between two main climate groups, tropical and temperate.

2. Source of information and data analysis approach

Searches for the relevant literature were done so through Google Scholar and the academic database of the University of Amsterdam. A minimum of 15 references was required by the examination board of the University of Amsterdam. The keywords used to search in the databases were: ‘tillage’, ‘no-tillage’, ‘conservation agriculture’, ‘soil organic carbon’, ‘carbon sequestration’, ‘conservation tillage’. A selection criterion was set up to identify the literature that was most relevant to this literature review.

For the study locations, those studies performed in wet tropical and temperate locations were selected for the purpose of comparing SOC content under NT regimes in two different climatic regions. Additionally, relevant site information like soil type and texture had to be included in the research for selection. Studies that failed to report climate and soil characteristics were excluded.

For management practices and sampling, papers had to include NT practices in their research. Other conservation management practices also had to be considered in the study design in order to be selected. Such practices include crop rotation, residue management, cover crops, and crop diversity. Field studies with a minimum duration of 10 years were selected based on multiple findings stating that SOC changes under NT regimes were detectable after the 10 year mark.

Meta-data was extracted from the studies and organized in tables. These tables contain information on study location, climate zone, soil type and texture, duration of the study, soil sampling depth, management regime (crop rotation, residue management, etc.), soil C content, and citation. This information was then organized according to similar soil, climate, and management parameters to easily analyse the data. Some studies reported SOC content as g kg-1_{. In order to have a homogenous}

dataset, values were converted into Mg ha-1_{when needed.}

3. Soil, climate, and management practices: At the centre of soil organic carbon

dynamics in agroecosystems

The following sub-sections discuss the current knowledge and understanding of how soil, climate and management practices influence soil carbon dynamics, based on contemporary research on these processes.

2.1 Soil

The amount of C sequestered and stored by agroecosystems will significantly depend on soil type and its interaction with a given management system and the micro-environment (Higashi et al., 2014). Different soils have their own C saturation level; however, agricultural soils are not considered saturated owing to the C decomposition conditions present as the result of soil disturbance (Stewart, Paustian, Conant, Plante & Six, 2007). Instead, under dominating C decomposition conditions, these soils present an effective C stabilization capacity with increasing C inputs and thus will have a maximum C sequestration potential (Stewart et al., 2007). The stabilization capacity of SOC for

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4 different soils is due to their ability to provide physical and structural protection to organic matter, limiting decomposition and increasing its stability (Giller et al., 2009; Bayer et al., 2006).

According to Lehmann & Kleber (2015) SOM is in a continuous state of decomposition enabled by soil microorganisms. The decomposition of organic matter fragments causes large molecules to decrease in size; along the way these fragments may become recalcitrant to decomposition by adsorption to soil aggregates and mineral surfaces (Lehman & Kleber, 2015). According to these authors, SOM molecules are capable of recalcitrance by adsorption at any stage in the decomposition continuum, from large to small molecules. However, large and complex molecules will have slower adsorption rates and fast desorption rates, while smaller molecules will experience the opposite. Therefore, greater pool sizes of recalcitrant material will be found further along the decomposition continuum towards small size molecules. Furthermore, the different organic compounds entering the soil as C inputs will differ from each other in composition, and thus will have different turnover and temperature responses (Lehmann & Kleber, 2015). Yet, the authors emphasize that the variation of these responses will be more influenced by the environmental and biotic conditions of the soil ecosystem than by temperature.

The C stabilization capacity of soils will also depend on the management system employed. NT systems, for example, will increase the C saturation limit of soils due to better soil aggregation and a reduced soil disturbance (Giller et al., 2009). Nevertheless, this may not be the case for all soil types. Tillage treatments, or conservation tillage treatments for that matter, will not have a significant effect on coarse textured soils (i.e. sandy soils) compared to finer textured soils (i.e. clayey soils)(Chivenge, Murwira, Giller, Mapfumo & Six, 2007). This is because coarse soils lack physical and structural protection of SOM, and thus may experience faster decomposition rates (Giller et al., 2009). As a result, soil C sequestration on these soils will significantly depend on C inputs from vegetation, cover crops, and crop residues rather than on a management shift to NT (Chivenge et al., 2007; Giller et al., 2009)

In comparison, in finer textured soils SOC accumulation will be strongly influenced by conservation tillage systems since these soils are significantly affected by aggregate turnover, which is faster in tilled soils due to soil disturbance (Chivenge et al., 2007). For example, on soils with high clay content, stabilization of SOC is possible thanks to the association of organic matter with the soil’s clay particles in aggregates (Chivenge et al., 2007). This can easily be seen in the study by Bayer et al. (2006) where they reported higher annual C sequestration rates on clayey soils (0.60 Mg C ha1_{) compared to that of}

the sandy clay loam site (0.30 Mg C ha-1_{) after a shift from CT to NT and without any residues or cover}

crops.

2.2 Climate

The distribution of the carbon stock within an ecosystem can vary with latitude. In temperate climates, 72% of the organic carbon can be found in the soil, and 28% in the vegetation. On the other hand, in tropical ecosystems 38% of organic carbon is found in the soil, while the majority (62%) can be found in the plant biomass (Blais, Lorrain, Plourde & Varfalvy, 2005). With estimates of the SOC content of tropical and temperate soils varying between 85 Mg C ha-1_{and 139 Mg C ha}-1_{(Blais et al., 2005).}

However, in agricultural systems, the soil C content will not only be dependent on climate, but also on the type of tillage and management system employed (Higashi et al., 2014).

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5 Figure I. Ratios of SOC under conservation tillage systems compared to conventional tillage in four different climatic subregions. Adapted from "Agricultural management impacts on soil organic carbon storage under moist and dry climatic conditions of temperate and tropical regions” by S. Ogle, F. Breidt & K. Paustian, 2005, Biogeochemistry, 72, 96. Copyright 2005 by “Springer”.

A meta-analysis by Ogle, Breidt & Paustian (2005) found that soils with conservation tillage treatments on tropical dry and wet climates experienced on average a 7% increase in SOC across the soil profile (30cm) when compared to temperate dry and wet climates. Between the two conservation practices the impact on SOC was variable; NT systems showed higher SOC concentrations than reduced tillage (RT) systems when changed from CT (Figure I). On RT soils, SOC was 3% higher than conventionally tilled soils under a temperate dry climate, 9% under temperate moist, 10% under tropical dry, and 16% under tropical moist. Conversely, for NT, SOC was 10% higher than conventionally tilled soils under temperate dry climates, 16% under temperate moist, 17% under tropical dry, and 23% under tropical moist. Additionally, the authors also studied the effects of long-term (<20 years) cultivation on SOC. For this, the reference SOC stock from native soils were compared with the SOC stocks of cultivated soil (Figure II). They found significant SOC losses after long-term cultivation, declining to 58% of the amount found in native soils under a tropical moist climate. This was followed by tropical dry climates at 69%, temperate moist at 71%, and temperate dry at 82%.

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6 Figure II. Ratios of SOC on a long term cultivated soil compared to the native soil in four different climatic subregions. Adapted from "Agricultural management impacts on soil organic carbon storage under moist and dry climatic conditions of temperate and tropical regions” by S. Ogle, F. Breidt & K. Paustian, 2005, Biogeochemistry, 72, 94. Copyright 2005 by “Springer”.

Contrary to these results, other authors have found that SOC sequestration on agricultural systems will perform better in temperate climates. Under conservation tillage treatments, there is a greater C sequestration potential in regions with a temperate climate, given a reduction in SOC mineralization and decomposition (Dimassi et al., 2014). While under the same management systems, soils that experience an increase in water content following wet years, or as a result of irrigation, will have an increase in soil organic matter decomposition, specifically in the surface layer when crop residues are used (Dimassi et al., 2014).

In temperate climates with high precipitation rates, biomass productivity is increased and thus resulting in higher C content. However, there is a threshold in the water balance after which SOC will decrease due to SOM decomposition or a decline in productivity, this will also be dependent on temperature. Luo et al. (2010) found that regions in Australia with 500-600 mm of annual rainfall under conservation tillage experienced a 38.6% increase in soil C content at the surface layer; whilst regions with less than 300-400 mm and more than 600 mm of precipitation had a significantly lower soil C content. The latter regions had a slight increase (25.3%) of soil C content under stubble retention management. The same study also investigated how temperature influences C sequestration and decomposition; they found that soil C content had the highest increase and retention in low temperature areas (13.5-15C). In these regions, the lower temperatures allow for a slower decomposition rate of SOC than in areas with higher temperatures.

2.3 Conservation management practices

The goal of conservation management is to minimize the negative effects conventional agriculture has on the environment, and more recently to serve as a climate change mitigation tool by using soils as an atmospheric carbon sink (Derpsch et al., 2005; González-Sánchez et al., 2019). Conservation

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7 management practices, however, not only refers to NT or conservation tillage systems, but to a range of agricultural practices which include crop rotation, cover crop, crop diversity, and residue management (Derpsch et al., 2014). As mentioned before, management systems, and their interaction with pedo-climatic factors, will influence soil organic carbon dynamics and storage.

2.3.1 Conservation tillage

Tillage and other conventional management practices significantly influence the distribution and concentration of SOC on soils. This occurs via various chemical and physical mechanisms through the soil profile. For instance, tillage is characterized by a high disturbance intensity of soil aggregates, this results in an increase of the surface area on soil particles, leaving organic matter unprotected to microbial attacks which decompose the originally protected SOC (Luo et al., 2010). Additionally, tillage also mixes different soil particles and incorporates fresh organic matter from the soil surface into the deeper layers, this provides energy for microbial growth and thus enhancing SOC decomposition (Luo et al., 2010).

On the other hand, conservation tillage practices, defined as “any tillage and planting system that covers 30 percent or more of the soil surface with crop residue, after planting, to reduce soil erosion by water.” (CTIC, 2002), reduces the intensity of soil disturbance, preventing soil degradation, and SOC decomposition (Luo et al., 2010). Moreover, conservation tillage can also work towards conserving and increasing organic matter concentrations, especially when applied in combination with cover crops (Higashi et al., 2014) and other conservation management practices (Derpsch et al., 2014). As a result of its seeming ability to conserve SOM, conservation tillage, especially NT, has been proposed by many as a climate mitigation tool.

On natural ecosystems, a significant amount of soil C to a depth of 1 meter is concentrated in the surface 0.20 meter of the soil profile (Luo et al., 2010). However, on agricultural soils under CT systems, soil C is evenly distributed through the soil profile to the depth of cultivation (Powlson et al., 2012). Like natural ecosystems, conservation tillage systems, especially NT, concentrate the majority of SOC in the soil surface layers, since root distribution is concentrated here and reflecting the use of crop residues when applied (Powlson et al., 2012). Because SOC is concentrated in the surface layers of the soil profile, microbial activity will be more active here, and thus the SOC located in the sub-surface layers will experience lower decomposition rates (Higashi et al., 2014).

Some researchers warn that the effect of conservation tillage practices on SOC sequestration may not be as significant as most of the studies report (Chivenge et al., 2007; Giller et al., 2009). Giller et al. (2009) for example, advises caution in interpreting results that show an increase in SOC under NT since, as mentioned above, there is a stratification of soil C due to a lack of mixing. Therefore, SOC has been accumulated in the soil surface rather that throughout the soil profile, leading researchers to overestimate the beneficial effects of conservation tillage on SOM. While Chivenge et al. (2007) acknowledge that a reduction of soil disturbance can lead to an increase of SOC, he introduces a caveat. The authors argue that conservation tillage can lead to an enhancement in SOC accumulation on clay soils, however, this effect is not as significant on sandy soils. Instead, the increment of C inputs on these soils should be given priority over a reduction of tillage intensity.

In short, conservation tillage attempts to emulate soil conditions under natural ecosystems, where soil disturbance is limited to soil biota and burrowing fauna. In doing so, the soil’s ability to protect SOC is enhanced, and SOC stocks are increased.

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2.3.2 Cover crops

Among other things, management systems aim to control the amount of C inputs that go into the soil, these C inputs will significantly influence changes in SOC content (Dimassi et al., 2014). One management practice that determines C inputs and is usually coupled to CA, is cover crops. Cover crops are crop systems aimed at providing permanent soil cover between the growing seasons of normal crop production for the purpose of soil and seed protection and soil improvement (Baumhardt & Blanco-Canqui, 2014). Cover crops are a significant source of C inputs to agricultural soils; they have been shown to be the main element that corresponds to an increase of SOC when changing from CT to NT (Virto et al., 2012). The general idea is that, particularly in sandy soils, cover crops will enhance SOM build up within the soil, and thus increasing SOC content (Chivenge et al., 2007). Additionally, cover crops work to protect the soil from erosion, which would otherwise remove organic matter from the soil, and to maintain moisture and regulate temperature (Derpsch et al., 2014).

A study by Higashi et al. (2014) on tillage systems and cover crops on Japanese soils showed that SOC content will be higher on NT soils with cover crops while CT will lead to a lower SOC content. However, the authors found that the biomass incorporated into the soil by cover crops will be significantly influenced by the cover crop species and N input levels.

Many farmers that employ the use of cover crops often turn to legumes, clovers, and vetch as these species have been shown to increase soil N as well as soil C. To maximize the efficiency of these species, some researchers are studying how a biculture of legumes and nonlegumes can further enhance soil productivity by increasing their biomass input and N mineralisation (Sainju et al., 2007). For example, Sainju et al. (2007) hypothesized that a mixture of legumes and nonlegumes would be followed by a greater increase of soil C than a cover crop monoculture. At one of the experimental locations, a dryland cotton crop in central Georgia, USA, the researchers did not find any significant changes in SOC. They conclude that the dry conditions and the limited soil water content in the field slowed organic C mineralisation and turnover rate to soil C. Additionally, the experiment only lasted two years, not enough to detect any significant changes in SOC, especially under dryland cotton. However, in their second experimental field, irrigated cotton, the researchers found significant differences between cover crops treatment on SOC. They found greater C inputs when a combination of rye and blend (a mixture of different legume species) was used in comparison with the use of blend and crimson clover by themselves. The authors conclude the use of cover crops is more efficient under irrigated cotton, though they mention the need to study these dynamics under a longer period of time. The use of cover crops results in multiple benefits for soil health and SOC stocks. They have the ability of protecting the soil from erosion, regulate soil temperature, and maintain soil moisture; this, coupled with their capacity to control C inputs, can lead to an enhancement of SOM build-up. However, some crop species are better adept at functioning as cover crops than others, as is the case with leguminous crops.

2.3.3 Residue management

As with cover crops, residue management is linked to an improvement of SOC sequestration rates (Derpsch et al., 2014), especially in the tropics (Chivenge et al., 2007), and is considered a pillar of CA (Giller et al., 2009). Residue management is the use of crop residues, which are the materials left in a field after a crop has been harvested so as to reduce wind and water erosion (Sandretto, 1997), among other potential benefits which will be discussed in this section. The exclusion of crop residues from fields has been shown to accelerate SOC losses, in particular when done so under conventional tillage regimes (Chivenge et al., 2007; Dimassi et al., 2014). Crop residue retention aids in reducing and avoiding soil disturbance and soil erosion, it has been shown to increase biological activity and SOC

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9 sequestration and helps to conserve soil moisture, improve water usage, and moderate soil temperature (Derpsch et al., 20014). Additionally, it can increase soil aggregate stability and soil porosity (Giller et al., 2009), which in turn protects SOM from decomposition and helps stabilize SOC. On a long-term study on the effects of reduced tillage and residue management on soil C stabilisation, crop fields with mulch ripping treatments showed a higher soil C content than fields under clean ripping (no residues left on the surface) (Chivenge et al., 2007). This study was conducted on sandy and clayey soils. On both soil types, mulch ripping had higher soil C contents, however, there was a significant difference between mulch ripping and clean ripping on sandy soils. Although, in general, C inputs are critical for productivity, and thus larger inputs of organic matter, like crop residues, mulch, and manure, will correspond to increases in SOC (Chivenge et al., 2007).

While organic matter inputs aid in an increase of SOC content in the overall soil profile, there will be a stratification in the distribution of soil C that will depend on the tillage treatment used and the depth of the tillage operation. For NT treatments, the use of residues will lead to an increase of SOC in the surface layer since there is no incorporation of residues into the deeper layers of the soil. On the other hand, subsurface layers will have higher SOC content under CT and RT treatments as a result of the decomposition of buried residues (Du et al., 2010).

Although the implementation of residues has its benefits and can lead to an overall improvement in soil health, the use of this CA practice does not come without its risk and limitations. For instance, in many parts of the world, especially in places where crop-livestock activities are integrated, farmers will use crop residues for livestock feed rather than using it for mulch (Tittonell & Giller, 2013). When this is coupled with an intensification of livestock productions, resulting from an increase in worldwide demands, many farmers will rather opt for using their crop residues as a feed resource, thus providing competition to the use of residues for mulch (Giller et al., 2009). Another drawback of residue retention is the stimulation of pests, like rodents and termites, who can consume the residue cover in a matter of weeks (Giller et al., 2009). Residues can also attract grubs and cutworms which can, in the case of cereal crops, cut roots, and interfere in cereal growth. Finally, all of this can lead to an increase in labour, which is meant to decrease under CA, and can also result in lower crop yields, affecting the livelihoods of farmers, especially those whose economic stability heavily depends on agriculture. To sum up, like cover crops, residue management can be beneficial for soil health and for SOM build-up. By reducing soil disturbance and erosion, maintaining soil moisture and regulating soil temperature, there is an increase in biological activity as well as soil aggregate stability. All this works to protect and enhance SOC. Despite all its benefits, there may be circumstances where residue management may not be as viable. In some locations where mix crop-livestock systems are employed, residues are often use as livestock feed. Furthermore, residues can also stimulate pest infestations, outweighing the beneficial effects of this CA practice.

2.3.4 Crop rotation & crop diversity

Another way to reduce SOC losses and improve C sequestration rates is converting from monocultures to fallow-free rotation cropping systems and increasing crop diversity (West & Post, 2002). This can result in a significant increase in SOC (Luo et al., 2010). Page et al. (2013) demonstrated the importance of eliminating fallow periods when converting to NT. They found that the high rates of organic matter decomposition and minimal inputs of SOC in crop-fallow rotation systems can result in a decrease or stagnation of SOC content, independent of the management employed, be that NT or CT. On the other hand, Dimassi et al. (2014) study of different crop management scenarios and carbon dynamics showed that crop rotations with minimal C inputs can also lead to a decline of SOC, they attributed this to low amounts of crop residues.

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10 Hernanz, Sánchez-Girón & Navarrete (2009) conducted a 20 year study of SOC sequestration in a cereal/leguminous crop rotation under different tillage regimes. Besides an increase in SOC content resulting from a conversion of CT to NT, the authors also found that the change from a cereal-fallow rotation to a winter wheat-vetch rotation was followed by a significant increase of SOC content similar to that under the CT to NT shift. However, the wheat-vetch rotation did not lead to a significant increase of SOC under RT and was comparable to SOC stocks under CT. In addition, the incorporation of legumes into the rotation also resulted in higher organic matter inputs and higher soil N concentrations. Consequently, the authors looked into the temporal evolution of SOC, they noted a pattern found by many other authors, the first 10 years of the experiment showed a significant increase of SOC, while after this period SOC reached a steady state. For NT this happened at the 11 year mark, while under CT and RT a steady state was reached after 12 years.

Other studies on crop rotations and SOC sequestration have reported insignificant changes in SOC stocks when adopting a crop rotation management. For example, West & Post (2002) found that on average, enhancing crop rotation complexity did not result in a significant increase of SOC compared to a change to NT. Moreover, the change from continuous corn to corn-soybean rotation yielded no increases in SOC. They explain that this may be the result of a decrease in C inputs from corn residues since a corn-soybean rotation produces fewer residues than a continuous corn crop. When assessing the changes of SOC while applying crop rotations to a NT field, the authors found no significant increase in SOC. This is explained by the idea that SOC under NT will be closer to equilibrium than when under CT, and thus an enhancement of rotation complexity will have a less significant effect on SOC sequestration (West & Post, 2002). On the other, wheat rotations appeared to be more successful than the corn-soybean rotations. They found that changing from a wheat-fallow rotation to a continuous crop with rotations of wheat with other crops results in significantly higher SOC content. This highlights the importance of implementing a range of conservation agricultural practices when adopting NT or other conservation tillage systems, both in practice and research. Several studies on NT systems have omitted the element of management from the methodology and focused mainly on tillage itself, resulting in unrepresentative data (Derpsch et al., 2014). However, there are some studies that have taken a systems approach to tillage research and they have shown that NT will have a greater carbon sequestration potential when combined with other conservation agricultural practices (Derpsch et al., 2014; Higashi et al., 2004; Luo et al., 2010; West & Post, 2002).

4. Meta-analysis on the interaction of climate, soil, and management practices

and their effects on SOC

This section discusses how climate, soil, and management practices work in tandem to influence SOC content. These results are based on 10 research papers (Table I) that studied one or more of the previously mentioned parameters. As a caveat to these results, it is important to mention that the majority of these studies did not report their findings based on a soil mass equivalent. This can make it difficult to realistically compare SOC content between the different locations presented here.

Table I. SOC content of temperate and tropical soils under different management scenarios. NT: no-tillage; CC: cover crops;

RM: residue management; CR: crop rotation.

Location Soil Type Climate Study

duration CMP Sampling depth (cm) C content (Mg C ha-1) Reference

Luziania, Brazil Oxisol Aw (Koppen) Tropical,

wet summers 15 N/A 0-20 41.00

Bayer et al. (2006)

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Location Soil Type Climate Study

duration CMP Sampling depth (cm) C content (Mg C ha-1) Reference

Costa Rica, Brazil Oxisol Aw (Koppen) Tropical,

wet summers 20 N/A 0-20 57.30

Bayer et al. (2006) Peñaflor CC, Aragon, Spain Hypercalcic Calcisol

BSk (Koppen) Cold semi

arid 19 CC, RM 0-40 60.23 Blanco-Moure et al. (2013) Peñaflor CF, Aragon, Spain Hypercalcic Calcisol

arid 20 RM 0-40 59.17 Blanco-Moure et al. (2013) Lanaja, Aragon, Spain Hypercalcic Calcisol

arid 10 CR, RM 0-40 62.75 Blanco-Moure et al. (2013) Torres de Alcanadre, Aragon, Spain Calcaric Cambisol

Cfa (Koppen) Humid

subtropical 9 CC, RM 0-40 58.78 Blanco-Moure et al. (2013) Undués de Lerda, Aragon, Spain Haplic Calcisol

Cfa (Koppen) Humid

subtropical 13 RM 0-40 88.54 Blanco-Moure et al. (2013) Artieda, Aragon, Spain Hypercalcic Calcisol Cfb (Koppen) Oceanic climate 19 CR 0-40 65.18 Blanco-Moure et al. (2013)

Ontario, Canada Typic Hapludalf

Dfb (Koppen) Warm summer humid continental

25 CR 0-60 52.35 Deen & Kataki (2003) Boigneville, France CM1 Haplic Luvisol Cfb (Koppen) Oceanic climate 41 RM 0-56.4 37.44 Dimassi et al. (2014) Boigneville, France CM2 Haplic Luvisol Cfb (Koppen) Oceanic climate 41 CR 0-56.1 40.8 Dimassi et al. (2014) Boigneville, France CM3 Haplic Luvisol Cfb (Koppen) Oceanic climate 41 CR 0-56.2 36.27 Dimassi et al. (2014) Boigneville, France CM4 Haplic Luvisol Cfb (Koppen) Oceanic climate 41 CR 0-55.4 36.10 Dimassi et al. (2014) Boigneville, France CM5 Haplic Luvisol Cfb (Koppen) Oceanic climate 41 CR 0-56.4 38.82 Dimassi et al. (2014) Boigneville, France CM6 Haplic Luvisol Cfb (Koppen) Oceanic climate 41 RM 0-56.7 37.75 Dimassi et al. (2014) Hebei province, China Vertisol

arid 11 RM 0-30 58.49 He et al. (2014)

Kanto province,

Japan Andosol

Cfa (Koppen) Humid

subtropical 9 CC, RM 0-30 75.43

Higashi et al. (2014) Kanto province,

Japan Andosol

Cfa (Koppen) Humid

Higashi et al. (2014) Kanto province,

Japan Andosol

Cfa (Koppen) Humid

Higashi et al. (2014)

Londriana, Brazil Oxisol Cfa (Koppen) Humid

subtropical 21 CR 0-40 62.90

Machado & Silva (2000) Passo Fundo,

Brazil Oxisol

Cfa (Koppen) Humid

subtropical 11 CR 0-40 86.10

Machado & Silva (2000) Dixon Springs,

Illinois, USA

Typic Fragiudalf

Cfa (Koppen) Humid

subtropical 10 CC 0-75 61.10 Olson et al. (2014) Hermitage, Queensland, Australia Black Vertosol

Cfa (Koppen) Humid

subtropical 40 N/A 0-30 56.00 Page et al. (2013)

Bengaluru, Karnataka, India Oxic Haplustalfs Aw (Koppen) Tropical, wet summers 10 CR 0-21 14.29 Prasad et al. (2016)

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12

4.1 Climate

Soils under temperate conditions are believed to have greater C sequestration potential under NT regimes, thanks to lower rates of SOC mineralization and decomposition (Dimassi et al., 2014) (see section 2.1). However, as several studies have shown (Bayer et al., 2006; Ogle, Breidt & Paustian, 2005; Six et al., 2005) when NT is applied with other CA practices in the tropics, there is an increase in SOC similar, or greater, to that in temperate soils. In addition, as with temperate soils, soils managed by NT systems under tropical climates have been found to show conditions resembling those found in adjacent natural soils (Bayer et al., 2006; Machado & Silva, 2001).

Table I shows a summary of the effects of NT systems, as well as a range of different management scenarios, on SOC content under tropical and temperate conditions. The tropical category also includes studies performed in subtropical climates as most of the soils in these regions share more characteristics with tropical soils than with temperate ones (Six et al., 2002). Additionally, an overwhelming majority of NT studies performed on tropical locations were done so in Brazil and the south of China, making it harder to find comparable studies across the tropical region.

Table II. SOC content on tropical and temperate soils under NT systems.

Region Locations Lowest (Mg C ha-1₎ _{Highest (Mg C ha}-1₎

Average SOC content (Mg C ha-1₎

Tropical 12/24 14.29 88.54 64.20

Temperate 12/24 36.10 65.18 48.78

As can be seen in Table II, SOC content in tropical soils was 1.3 times greater than that in temperate soils, throughout the soil profile under a NT system along with other conservation agricultural practices such as residue management, crop rotations and cover crops. There is a broad range of SOC content between all locations, with the lowest being 14.29 Mg C ha-1_{on a tropical soil in the east of}

India (Prasad et al., 2016) and the highest 88.54 Mg C ha-1_{in a subtropical soil in Spain (Blanco-Moure}

et al., 2013). In fact, the three sites under true tropical conditions have the lowest SOC content in the tropical region category, with an average of 37.53 Mg C ha-1_{, while the average SOC content for the}

subtropical soils is 74.60 Mg C ha-1_.

Even though tropical and subtropical soils share similar conditions, it may be that the variability between soils in these two regions is significant enough to produce contrasting results. However, Six et al. (2002) shows that while on average soil C on tropical soils has higher turnover rates, the ranges of estimated soil C turnovers between tropical and temperate are quite similar and there is a significant overlap between the C turnover rates of these two regions. Another possible explanation for the higher SOC contents on subtropical soils is that the experimental sites in the tropics only had NT as a CA practice, save for the site in India which had also crop rotations, and thus lacked practices intended to control C inputs, like cover crops and residue management. And it is in these tropical regions where these practices play a vital role, in conjunction with NT, in enhancing SOC sequestration rates (Chivenge et al., 2007; Derpsch et al., 2014). On the other hand, the subtropical sites included cover crops, residue management, and crop rotations in their conservation management when compared to the tropical sites.

It also does not help that literature in this subject is not distributed equally across different climatic and geographic regions, resulting in an underrepresentation of strictly tropical soils in this review,

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13 which may cause SOC content values to be skewed towards subtropical soils. For a better representation of this region, there is a need to conduct more research on tropical soils from different geographic locations and not only from a selected few.

4.2 Conservation management practices

This section focuses on three of the most widely used conservation management practices, residue management, cover crops, and crop rotations, and how their interaction with NT systems influences SOC content (Table III).

The conservation management practice with the highest value of SOC, as can be seen in Table III, was cover crop with an average of 76.38 Mg C ha-1_{. However, it should be noted that the majority of these}

locations had cover crop residues as mulch. Therefore, not only are there C inputs from cover crops, but also from their residues, which may very well increase the amount of SOC content than if only cover crops were applied. All the locations used to calculate this average are in the tropical region and have a humid subtropical climate.

Residue management, without the presence of cover crops, has the second highest value of SOC content with 56.70 Mg C ha-1_{. The residues covered >30% of the soil, per CA principles. Five out of the}

seven locations used to calculate this average are in the temperate region, with oceanic and cold semi-arid climates.

These results are consistent with the findings of multiple authors who conclude that a change to CA management practices, especially those responsible for soil C inputs like cover crops and residue management, will result in increases of SOC content and C sequestration rates (Chivenge et al., 2007; Derpsch et al., 2014; Six et al., 2002).

Table III. SOC content under three different conservation management practices.

Conservation management

practice Locations Lowest (Mg C ha-1₎ _{Highest (Mg C ha}-1₎

Cover crop 6/24 61.10 86.12 76.38

Residue management 7/24 37.44 88.54 56.70

Crop rotation 8/24 14.29 86.10 45.95

Crop rotations had the lowest value of SOC content with 45.95 Mg C ha-1_{for the eight locations}

surveyed. The crops used for the rotations where corn-soybean for Deen & Kataki (2003), a combination of maize-winter wheat rotations in the first years and winter wheat-barley-sugar beet-pea in the later stages of the experiment for Dimassi et al. (2014), soybean-wheat for Machado & Silva (2000), and finger millet-pigeon pea/horsegram for Prasad et al. (2016). Results from each location varied significantly from each other, this can be explained by the adoption of different plant species for their rotation in each site.

Four of the surveyed locations included soybeans for the full duration of the study (Deen & Kataki, 2003; Machado & Silva, 2000; Prasad et al., 2016), while the remaining locations only used legumes in the final years of the experiment. Out of these four locations, three had the highest SOC content in the crop rotation category; 52.35 Mg C ha-1 _{(Deen & Kataki, 2003), 62.90 Mg C ha}-1,_{and 86.1 Mg C ha} -1_{(Machado & Silva, 2000). In contrast, the experimental sites that only included leguminous crops in}

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14 their rotation on the final years of the study (Dimassi et al., 2014) had an average SOC content of 40.00 Mg C ha-1_{. These findings are consistent with those of (Hernanz et al., 2009; Sainju et al., 2007) who}

reported higher SOC contents in management regimes that included leguminous crops in their rotations. Moreover, these results also confirm West & Post (2002) conclusions that wheat rotations are more effective than a corn-soybean rotation in increasing SOC content, especially if legume crops are included in the rotation. This can be seen in the Machado & Silva (2002) experiment which had a soybean-wheat rotation and yielded the highest SOC, compared to the experiment by Deen & Kataki (2003) whose rotation consisted of corn-soybean.

4.3 Soil

Section 2.2 introduces the argument that any given soil will interact differently with climatic conditions and with management practices; and that the soil’s composition will determine how this interaction will influence soil carbon dynamics. To test the validity of these arguments, twenty-four experimental sites (Table I) were grouped according to their soil texture (Table IV) and their SOC content was averaged for each soil texture category.

Table IV. Average SOC content as influenced by soil texture under a NT system.

Soil texture Locations Lowest (Mg C ha-1₎ _{Highest (Mg C ha}-1₎

Sandy clay loam 2/24 14.29 41.00 27.65

Clayey 4/24 56.00 86.10 65.58

Sandy loam 9/24 51.38 88.54 70.14

Silty loam 3/24 52.35 61.10 57.31

Silty clay 6/24 36.10 40.80 37.86

The soils with the highest SOC content were those with a sandy loam texture, with an average of 70.14 Mg C ha-1_{. Soils in this category are both from the tropical and temperate regions; those that were}

located in the tropical region yielded the highest SOC content. In terms of management practices, all the experimental sites had at least one form of management practice that controlled C inputs. In addition, the clay percentage in these soils gradually increased with depth. Clayey soils had the second highest SOC content, with an average of 65.58 Mg C ha-1_{. These soils were located in the tropical}

region; none of them had a management practice aimed at increasing C inputs.

The soils dominated by higher concentrations of silt showed some of the lowest SOC contents, with an average of 57.31 and 37.86 Mg C ha-1_{for silty loam and silty clay respectively. The management}

scenario of these soils consisted mostly of crop rotations, only three of them incorporated practices aimed at increasing C inputs. Additionally, all but one of the sites were located in the temperate region. The lowest SOC content was found under the sandy clay loam textured soils, with an average of 27.65 Mg C ha-1_{. However, there were only two experimental sites within this category, one of them}

being the site from India with 14.29 Mg C ha-1_{, which is the outlier value in all categories.}

Of all the soil texture categories, the one that was expected to yield the highest SOC content average was clayey soils. This was expected because finer textured soils are more effective in stabilising SOC since organic matter particles can associate with clay particles in soil aggregates and thus become resistant to decomposition (Bayer et al., 2006; Chivenge et al., 2007). Additionally, clayey soils benefit

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15 from a change in tillage intensity, from CT to NT, as these soils are significantly affected by aggregate turnover under CT systems (Chivenge et al., 2007). However, this was not the case, as the soils with the highest SOC content average were those with a sandy loam textured.

The high amount of SOC content in sandy loam soils can be explained by the influence of climatic conditions and management factors. For instance, more than half of these soils were located in the tropics, the region that had the highest SOC content average, thus they benefited from the high productivity of their location. Additionally, the soils’ clay percentage gradually increased with depth, meaning that organic matter could associate with clay particles and resist decomposition. Though having a NT system means that organic matter concentrates on the surface layers of the soil profile and is not incorporated into the subsoil, where clay particles are in higher concentration, therefore any potential recalcitrance of organic matter by assimilation of clay particles is fairly limited. That said, the factor that most likely had a significant influence on SOC content was that of the management practices that control soil C inputs. As mentioned before, an increase in SOC content on coarse textured soils, like the one being discussed here, will significantly depend on soil C inputs (Chivenge et al., 2007; Giller et al., 2009). The reason being that these soils lack physical and structural protection of organic matter and will exhibit faster decomposition rates (Giller et al., 2009), and thus will significantly depend on C inputs that come from CA practices.

5. Conclusion

The results of this study showed that NT and other CA practices will influence SOC differently given varying climatic and soil conditions. As far as climate is concerned, the tropical regions showed the highest SOC content. However, strictly tropical soils were underrepresented in this study, reflecting a shortage of tillage research in this area, which in most cases is conducted in the same tropical regions. Additionally, for conservation management practices, higher SOC contents were found under cover crops, residue management and crop rotations, respectively. Indicating that practices aimed at controlling C inputs, cover crops and residue management, will lead to higher SOC contents. Lastly, sandy soils produced the highest SOC content of all soil texture categories, followed by clayey soils, which were expected to have the highest SOC content. Climatic and management factors can explain this trend, as there was not an equal representation of sandy soils in both climatic regions.

While NT and other CA practices can lead to an increase in SOC content and in turn serve as a climate change mitigation tool, there also needs to be a reduction in GHG emissions, not just in the agricultural sector but in all parts of society. Therefore climate change and global warming mitigation should not be the sole focus of CA research, bigger benefits may come from these practices like the improvement of soil health, erosion prevention, water management, weed control, higher yields, and as a way to reduce GHG emissions from heavy machinery in the agriculture sector. In the meantime, under a business as usual climate scenario, soil organic carbon sequestration may just be a positive side effect of conservation agricultural practices but should not be considered its champion.

What’s more, research about CA practices is plagued by inconsistencies in their methodology. Definitions of conservation tillage, for instance, varies greatly between research papers; sampling depth is often limited to the surface layers of the soil profile, and study durations most of the times don’t reach the 10 year mark where changes in SOC content start becoming significant after a change to NT systems. Additionally, it is often the case that researchers don’t incorporate other conservation management practices into their methodology, and even more common is the failure to study how different climates and soils interact with these practices to influence soil C dynamics, resulting in very narrowed and limited views of the process inherent in CA. In addition to this, CA research often leaves

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16 out the impact that these practices have on N2O and CH4 emissions, which may offset any gains in SOC

content (VandenBygaart, 2016).

In conclusion, the body of research that is available today is not enough to fully understand the processes and interactions that occur at the nexus of climate, soil, and management practices, and how these processes might help us mitigate our current climate crisis. To counter these shortcomings and improve the quality of conservation tillage research, it is necessary to develop a standard methodology with clear definitions of the varying tillage regimes, and the implementation of other CA practices. Soil sampling should also be defined to an agreed upon depth that clearly represents soil C dynamics of the whole soil profile and not just the surface layers. Furthermore, there is a need to conduct more research in tropical soils without limiting it to countries that are currently overrepresented in this area, such as Brazil and the tropical regions of China. Finally, an emphasis should be placed on a systems approach that looks into the interactions of climate, soil, and management factors.

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Conservation agriculture through the lens of soil, climate, and management factors