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  Soil Organic Carbon Under Conservation Agriculture In Mediterranean And Humid Subtropical Climates: Global Meta-Analysis

  Conservation agriculture (CA) is an agronomic system based on minimum soil disturbance (no-tillage, NT), permanent soil cover and species diversification. The effects of NT on soil organic carbon (SOC) changes have been widely studied, showing somewhat inconsistent conclusions, especially in relation to the Mediterranean and humid subtropical climates. These areas are highly vulnerable and predicted climate change is expected to accentuate desertification and, for these reasons, there is a need for clear agricultural guidelines to preserve or increment SOC. We quantitively summarized the results of 47 studies all around the world in these climates investigating the sources of variation in SOC responses to CA, such as soil characteristics, agricultural management, climate and geography. Within the climatic area considered, the overall effect of CA on SOC accumulation in the plough layer (0-0.3 m) was 12% greater in comparison to conventional agriculture. On average this result corresponds to a carbon increase of 0.48 Mg C ha-1 year-1. However, the effect was variable depending on the SOC content under conventional agriculture: it was 20% in soils which had ≤ 40 Mg C ha-1, while it was only 7% in soils that had > 40 Mg C ha-1. We proved that 10 years of CA impact the most on soil with SOC ≤ 40 Mg C ha-1. For soils with less than 40 Mg C ha-1, increasing the proportion of crops with bigger residue biomasses in a CA rotation was a solution to increase SOC. The effect of CA on SOC depended on clay content only with more than 40 Mg C ha-1 and become null with a SOC/clay index of 3.2. Annual rainfall (ranged between 331-1850 mm yr-1) and geography had specific effects on SOC depending on its content under conventional agriculture. In conclusion, SOC increments due to CA application can be achieved especially in agricultural soils with less than 40 Mg C ha-1 and located in the middle latitudes or in the dry conditions of Mediterranean and humid subtropical climates. This article is protected by copyright. All rights reserved.
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  Soil organic carbon under conservation agriculture in Mediterranean and humid subtropical climates: Global meta-analysis

  Conservation agriculture (CA) is an agronomic system based on minimum soil disturbance (no-tillage, NT), permanent soil cover, and species diversification. The effects of NT on soil organic carbon (SOC) changes have been widely studied, showing somewhat inconsistent conclusions, especially in relation to the Mediterranean and humid subtropical climates. These areas are highly vulnerable and predicted climate change is expected to accentuate desertification and, for these reasons, there is a need for clear agricultural guidelines to preserve or increment SOC. We quantitively summarized the results of 47 studies all around the world in these climates investigating the sources of variation in SOC responses to CA, such as soil characteristics, agricultural management, climate, and geography. Within the climatic area considered, the overall effect of CA on SOC accumulation in the plough layer (0–0.3 m) was 12% greater in comparison to conventional agriculture. On average, this result corresponds to a carbon increase of 0.48 Mg C ha−1 year−1. However, the effect was variable depending on the SOC content under conventional agriculture: it was 20% in soils which had ≤ 40 Mg C ha−1, while it was only 7% in soils that had > 40 Mg C ha−1. We proved that 10 years of CA impact the most on soil with SOC ≤ 40 Mg C ha−1. For soils with less than 40 Mg C ha−1, increasing the proportion of crops with bigger residue biomasses in a CA rotation was a solution to increase SOC. The effect of CA on SOC depended on clay content only in soils with more than 40 Mg C ha−1 and become null with a SOC/clay index of 3.2. Annual rainfall (that ranged between 331–1850 mm y−1) and geography had specific effects on SOC depending on its content under conventional agriculture. In conclusion, SOC increments due to CA application can be achieved especially in agricultural soils with less than 40 Mg C ha−1 and located in the middle latitudes or in the dry conditions of Mediterranean and humid subtropical climates. Highlights The results of 47 studies were quantitively summarized by using a meta-analysis SOC accumulation due to CA was 12% greater compared to conventional agriculture SOC increment due to CA can reach 20% in soils having less than 40 Mg C ha−1 The impacts of pedo-climatic factors and agronomic management practices were studied
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  Conservation agriculture practices drive maize yield by regulating soil nutrient availability, arbuscular mycorrhizas, and plant nutrient uptake

  Conservation agriculture (CA) can sustainably increase crop productivity through improved soil chemical, physical, and biological properties, among others. However, the implementation of all its three main components (i.e., no-tillage, organic soil cover/mulch, and crop diversification) in southern Africa is often challenging, resulting in variable yield responses. Disentangling the contributions of CA practices is necessary to understand the drivers of maize grain yield within the region. Here we analysed two 6-year long component omission experiments, one at a sandy soil location and the other at a clay soil location. In these two experiments, soil chemical parameters, total plant nutrient uptake, rate of crop residue decomposition, and arbuscular mycorrhizal fungi (AMF) colonization of maize roots were assessed. Soil chemical properties only differed across systems at the sandy soil location with the mulched systems under no-tillage (NT) resulting in increased soil organic carbon levels, total nitrogen, and soil available phosphorus as compared to conventional tillage with no mulch or rotation (CT). Conventional tillage-based systems resulted in fastest decomposition of maize residues, while systems with NT and rotation resulted in highest AM fungal root colonization rate of maize at the clay soil location. Total plant N uptake was almost 2-fold higher in tilled and no-tilled systems with both mulch (M) and rotations (R) (i.e., NT+M+R and CT+M+R) as compared to CT. Structural equation modeling was used to disentangle the links between cropping systems, soil chemical and biological properties, plant nutrient uptake, and maize grain yield. Cropping systems had direct and indirect influences on yield at both locations. At both locations, cropping systems influenced yield via plant N uptake, with the NT+M+R and CT+M+R systems having more beneficial effects compared to other systems, as shown by their higher path coefficients. In conclusion, we recommend a more holistic approach to cropping system assessment that includes a higher number of abiotic and biotic determinants. This would allow for a more rigorous evaluation of the drivers of yield and increase our understanding of the effects and performance of practices under the prevailing agro-ecological conditions.
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  Climate drives global soil carbon sequestration and crop yield changes under conservation agriculture

  Conservation agriculture has been shown to have multiple benefits for soils, crop yield and the environment, and consequently, no-till, the central practice of conservation agriculture, has rapidly expanded. However, studies show that the potential for carbon (C) sequestration in no-till farming sometimes is not realized, let alone the ability to maintain or improve crop yield. Here we present a global analysis of no-till-induced changes of soil C and crop yield based on 260 and 1,970 paired studies; respectively. We show that, relative to local conventional tillage, arid regions can benefit the most from conservation agriculture by achieving a win-win outcome of enhanced C sequestration and increased crop yield. However, more humid regions are more likely to increase SOC only, while some colder regions have yield losses and soil C loss as likely as soil C gains. In addition to site-specific characteristics and management, a careful assessment of the regional climate is needed to determine the potential benefits of adopting conservation agriculture.
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  Carbon sequestration potential through conservation agriculture in Africa has been largely overestimated: Comment on: “Meta-analysis on carbon sequestration through conservation agriculture in Africa”

  Soil organic carbon (SOC) sequestration depends on several factors,including land use, pedo-climatic conditions, topographic position andthe initial SOC stock (Post and Kwon, 2000; Minasny et al., 2017). Atthe plot scale, a positive SOC balance is created by increasing the inputof organic matter to the soil to exceed the carbon (C) losses by miner-alization, leaching and erosion or by decreasing the rate of SOC de-composition. In Africa, agricultural soils are generally known to havepotential as a C sink due to previous SOC depletion (Vågen et al., 2005;Swanepoel et al., 2016). Two widely promoted crop managementpractices to store C in agricultural soils are conservation agriculture(CA) and agroforestry. Both practices can increase SOC through in-creased C inputs from higher biomass productivity and reduced C losses(through soil cover and reduced soil tillage), leading to a net transfer ofC from the atmosphere to the soil, thus contributing to the mitigation ofclimate change (Smith et al., 2005;Powlson et al., 2011; Griscom et al.,2017).