The 4‰ initiative launched by the French government at COP21 in Paris in December 2015 aspires to increase global soil organic carbon (SOC) stocks at a rate of 0.4% per year. We conducted a systematic literature review on SOC storage under agroforestry and conservation agriculture systems in sub-Saharan Africa, where we reported 66 and 33 cases for both systems respectively. The results showed that SOC storage rates were significantly higher than 4‰ yr−1 in fallows and in multistrata agroforestry systems (P = 0.0001 and 0.0178, respectively), but not in alley cropping and parklands systems. For conservation agriculture, SOC storage rates were only significantly higher than 4‰ yr−1 (P = 0.0438) when all three principles were applied, i.e. no- or minimum tillage combined with crop residue retention and intercropping or rotation. The data showed very large variability in SOC storage rates as the result of various factors, including previous land-use history, experimental set up and approach used to determine SOC storage (diachronic versus synchronic approach), soil type, depth of soil sampling, type of crops and management, and duration of the experiment. SOC storage rates significantly decreased with time in the agroforestry systems (P = 0.0328). However, we were unable to find significant relationships with initial SOC stocks or tree density. Given the limited published data and the high variability in results, no significant relationships between SOC storage rates and site variables were found for conservation agriculture. We argue that there is a potential for SOC storage in agricultural soils of sub-Saharan Africa, as illustrated by SOC gaps observed on smallholder farms. Low SOC levels are, however, to a great extent the result of limited resources of most smallholder farmers. Practices such as agroforestry and conservation agriculture can restore SOC in these soils, but the 4‰ initiative has to be implemented on the grounds of the positive impact on crop productivity rather than on climate change mitigation. The efficiency in doing so will depend on the specific situations and will need economic support to smallholder farmers, including the promotion of good markets for sale of extra produce and for input supply, effective private support and policy, such as credit schemes and subsidies for inputs, and efficient extension services which incentivize farmers to invest in new technologies.

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The 4 per 1000 goal and soil carbon storage under agroforestry and conservation agriculture systems in sub-Saharan Africa

Most studies on plant nutrition tend to focus on the topsoil (plough layer) and frequently neglect subsoil processes. However, cereal roots can potentially acquire nutrients including organic and inorganic nitrogen (N) from deep in the soil profile. Greater knowledge on the interaction of plants and microbes in subsoil environments is required to evaluate whether deep rooting traits in cereals will achieve greater nutrient use efficiency and greater soil carbon (C) storage in cropping systems. This study aimed to evaluate the relationship between root distribution, organic and inorganic N availability and potential N supply at the critical growth period during the wheat cropping cycle in a sand textured Eutric Cambisol. Our results provide evidence of significant microbial capacity in the subsoil. The rate of plant residue turnover and the mineralization of organic C and N substrates (glucose, amino acids, peptides, protein) declined slightly with increasing soil depth; however, these rates were not correlated with basal soil respiration, microbial biomass or community structure. This suggests that the microbial population in subsoil is more C limited but that its activity can be readily stimulated upon C substrate addition. A significant potential for organic and inorganic N turnover was also demonstrated at depth with a similar abundance of ammonifiers and ammonia oxidizing bacteria (AOB) and archaea (AOA) throughout the soil profile. Again, N mineralization in subsoils appears to be substrate limited. Root density declined rapidly down the soil profile with few roots present past 50 cm; suggesting that this is the major factor limiting C recharge of soil organic matter and microbial activity in subsoils. Greater root proliferation at depth could allow greater capture of water and the recapture of N lost by leaching; however, our results suggest that plant-microbial competition for C and N is as intense in the subsoil as in the topsoil. We conclude that while deeper rooting may improve nutrient and water use efficiency it may not lead to much greater C sequestration in subsoils, at least in the short term.

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Microbial competition for nitrogen and carbon is as intense in the subsoil as in the topsoil

Digging Deeper for Agricultural Resources, the Value of Deep Rooting

Soil aeration processes and status are reviewed with regard to different soil, climatic, land-use, and crop types and with regard to diffusive and advective flow mechanisms. Factors affecting aeration status and its quantifiers are discussed and active soil aeration (“oxygation”) practices are presented. Movement of O2 from the soil surface into the soil profile and its transport into soil aggregates and toward plant roots is mainly diffusive. In most circumstances, root respiration is constrained by vertical O2 diffusion from the atmosphere to the root zone and by the diffusive resistance of the mucilage layer. Several O2–diffusive flow models are proposed and discussed with regard to the different geometries, relevant length scales, boundary conditions, and sinks. Soil aeration by advective O2 flow, driven by barometric pressure changes, may also be significant in dry, coarse-textured soils with no underlying impermeable layers. Respiration in the soil consists mainly of plant root and microbial O2 uptake, which are roughly of the same magnitude and strongly correlated through symbiotic and competitive relationships. The bulk, areal soil respiration rate varies from several to tens of grams of O2 per square meter per day depending on soil cover: fallow, pasture, forests, unirrigated and irrigated, cropped fields, and orchards (in general increasing order). Soil respiration rate is also affected by climatic conditions, where a temperature difference of 10°C increases O2 consumption (and CO2 production) twoto threefold. The ratio between emitted CO2 and inspired O2 (respiratory quotient) is not unity (on a molar basis) but rather depends on the types of respiring populations and environmental conditions. Reduced O2 and elevated CO2 concentrations negatively affect plant growth and productivity. These conditions are correlated mostly with wet and warm soils, such as intensively irrigated fields with fine-textured soils (high water retention) during the summer. Oxygen-availability quantifiers such as O2 concentration, air content, and O2 diffusion rate are superior to other quantifiers such as soil properties (e.g., soil texture, porosity) and redox potential. In the last few decades, several active aeration methods have been proposed and evaluated, such as adding air or O2 bubbles or H2O2 to the irrigation water and air injection into the soil. Although these methods have given mainly positive results, none is widely used in agricultural practice, due mainly to a lack of profitability potential, field-scale proof demonstration, and a coherent protocol for field application.

Jean Pierre Montoroi

Papers

Understanding deep roots and their functions in ecosystems: an advocacy for more unconventional research