Energy cover crops for biogas production increase soil organic carbon stocks: A modeling approach

Abstract

Energy cover crops for biogas production through anaerobic digestion (AD) are inserted between two primary crops. They replace either bare soil or nonharvested cover crops, and their management is usually intensified to produce more biomass. They allow the production of renewable energy as well as digestate, used as an organic fertilizer, without directly competing with food production. Because of the increased biomass production and export and of the return of a digested biomass to the soil, the impact of energy cover crops on soil organic carbon (SOC) is questioned. The objective of this paper was to study the difference in SOC stocks induced by the introduction of energy cover crops for AD coupled with the application of the resulting amount of digestate. We used the AD model Sys-Metha combined with the soil C model AMG to simulate SOC stocks for 13 case studies in France, with scenarios comparing different intercrop management practices, with or without cover crops, harvested or not. Our results indicated that the higher biomass production of energy cover crops (from 6.7 to 11.1 t DM ha −1 ) in comparison with nonharvested cover crops (2 t DM ha −1 ) or bare soil led to higher humified C input (belowground input and digestate), despite the high C fraction exported in AD. This resulted in an increase in SOC stocks in comparison with nonharvested cover crops or bare soil (from 0.01 to 0.12 t C ha −1 year −1 over 30 years). The uncertainties in the model parameters did not modify these results. However, in the case of equal biomass production between energy cover crops and nonharvested cover crops, SOC stocks would be lower with energy cover crops.

Impacts on water quality of producing biogas on pig farms as a function of the associated agricultural practices

“…Our results suggest a loss of soil organic matter in the soil under anaerobic digestion scenarios, contrary to several studies (Launay et al 2022, Levavasseur et al 2023. This divergence could be attributed to several factors.…”

“…Initially developed using livestock and food processing waste, AD development is actually largely based on ECC and in a lesser extent on fodder such as maize and grass (Launay et al 2022). The introduction of ECC in AD systems is widely used by several countries (Szerencsits et al 2016, Strauß et al 2019, Riau et al 2021, Levavasseur et al 2023, mainly to ensure a continuous supply of substrate for biogas units…”