Increasing organic stocks in agricultural soils: Knowledge gaps and potential innovations

Abstract

Recent initiatives, such as the United Nations declaring 2015 as the International Year of Soils and the French « 4 per 1000 » initiative call attention on soils and on the importance of maintaining and increasing soil organic matter stocks for soil fertility and food security, and for climate change adaptation and mitigation. We stress that soil organic carbon storage (i.e. an increase of soil organic carbon stocks) should be clearly differentiated from soil organic carbon sequestration, as the latter assumes a net removal of atmospheric CO 2 . Implementing management options that allow increasing soil organic carbon stocks at the local scale raises several questions, which are discussed in this article: how can we increase SOC stocks, at which rate and for how long; where do we prioritize SOC storage; how do we estimate the potential gain in C and which agricultural practices should we implement? We show that knowledge and tools are available to answer many of these questions, while further research remains necessary for others. A range of agricultural practices would require a re-assessment of their potential to store C and a better understanding of the underlying processes, such as no tillage and conservation agriculture, irrigation, practices increasing below ground inputs, organic amendments, and N fertilization. The vision emerging from the literature, showing the prominent role of soil microorganisms in the stabilization of soil organic matter, draw the attention to more exploratory potential levers, through changes in microbial physiology or soil biodiversity induced by agricultural practices, that require in-depth research.

Direct evidence using a controlled greenhouse study for threshold effects of soil organic matter on crop growth

“…, Chenu et al. ). These initiatives along with other organizations such as the USDA and Cornell Soil Heath Framework generally support the notion that “more is better” when it comes to managing for SOM (Oldfield et al.…”

“…A key question for policy makers that focus on increasing SOM in agricultural soils is how much and at what rate can SOC stocks be increased (Chenu et al. ). Our results demonstrate that building up SOM in soils, where biophysically feasible, from low existing concentrations will likely have beneficial impacts on productivity and may permit reductions in inorganic N fertilizer use and/or irrigation water.…”

Characterising the biophysical, economic and social impacts of soil carbon sequestration as a greenhouse gas removal technology

“…These measures transfer existing organic carbon to the soil pool. This in itself is soil C storage (Chenu et al, ), but where this transfer to the soil C pool (vs. other uses) increases long‐term C removal from the atmosphere, it represents net sequestration. Organic amendments may also improve crop primary productivity via increased nutrient availability and labile C fractions; this represents a secondary pathway by which this measure can influence net atmospheric C removal.…”

“…Optimal pH improves soil nutrient availability, increasing primary productivity and OM input to soil (Ahmad, Singh, Dijkstra, & Dalal, 2013;Holland, White, Glendining, Goulding, & McGrath, 2019). However, liming also increases C and N mineralisation (Chenu et al, 2019;Paradelo, Virto, & Chenu, 2015), accelerating losses as well as increasing inputs and making net SCS response context-specific.…”

Residue retention promotes soil carbon accumulation in minimum tillage systems: Implications for conservation tillage

“…The C loss is mainly caused by the post-harvest removal/loss of 64 crop residues and C distribution/decomposition processes from agricultural fields, such as 65 hay harvest or tillage (Lal, 2004a). Since SOC sequestration is crucial for soil fertility and 66 climate change mitigation, alternative production practices have been adopted globally to 67 enhance soil C storage (Chenu et al, 2019;Lal, 2004b;Sauvadet et al, 2018). 68 Conservation tillage practices, a term originally introduced by Wall (2006), 69 encompass three managerial components, i.e., minimum tillage (including no-tillage, NT, 70 and reduced tillage, RT) with or without crop residue retention.…”