Stoichiometry constraints challenge the potential of agroecological practices for the soil C storage. A review

To date, the stoichiometry concept has been poorly used to explore C, N, and P cycles in agroecosystems. As agriculture is now under pressure to reduce the use of synthetic inputs, ecological processes and alternative agricultural practices will become the main regulators of the relationships between C, N, and P and thereby of nutrient availability and C storage in soils. In this paper, we review the ecological theories underpinning the concepts of homeostasis and stoichiometric flexibility, their application to agroecosystems, and how stoichiometry could constraint agroecological practices related to nutrient availability and soil C storage. Our main findings are (1) optimal C:N:P ratios exist at the species level, reflecting a range of ecological strategies and representing a keystone constraint for the coupling of C, N, and P cycles, resulting in canonical ratios at the community level. Stoichiometric flexibility nevertheless exists from the organism level-autotrophs having higher flexibility than heterotrophs-to the community level, depending on assembly rules. (2) Agroecosystems are stoichiometrically constrained especially in the soil compartment, due to the low stoichiometric flexibility of microorganisms at the community level. (3) Agricultural practices such as fertilization decrease N:P ratios in soil surface when total P is considered, while C:N ratios remained constant. (4) Stoichiometry homeostasis constraints for soil C storage require the availability of N and P. They can be supported by agroecological practices that promote nutrient recycling (organic fertilization, permanent soil cover, N fixation). The 45-Tg N and 4.8-Tg P needed to increase the C stock of cropped soils by "4 per mille per year" can be also provided by suppressing nutrient losses. We conclude that two soil compartments should be more investigated to evaluate their potential to bypass stoichiometric constraints and foster C storage while reducing chemical inputs: deep soil horizons and particulate organic matter fraction.