High Nitrogen Availability but Limited Potential Carbon Storage in Anaerobic Digestates from Cover Crops

Abstract

Cover crops are increasingly used for biogas production, a renewable energy source, without competing for food production. The behavior of the resulting digestates after soil application is poorly understood, which prevents their efficient recycling in agriculture and the environmental assessment of their application. The objective of this study was to quantify the nitrogen availability and potential carbon storage of cover crop-issued digestates after soil application. A total of 10 raw digestates, 2 liquid phases, and 3 solid phases after phase separation were sampled. Main cover crops used in the sampled biogas plants were winter barley, rye, and maize. Classical physicochemical analyses and laboratory incubations to study their C and N mineralization were conducted. Despite a moderate C mineralization of raw and liquid digestates after 91 days, their initial limited carbon content induced, in the end, a low contribution to soil organic carbon (13 and 11 kg remaining C Mg −1 FM, respectively), similar to a pig slurry and much lower than a bovine manure. With a higher initial carbon content and lower C mineralization, the contribution of solid digestates to carbon storage could be higher if applied at a sufficient rate. Organic N mineralization of raw and liquid digestates was moderate, but their N availability was high (3 and 4 kg available N Mg −1 FM, respectively), thanks to their mineral nitrogen contents, similar again to a pig slurry. In contrast, that of solid digestate was almost null with a very low mineral N content and no organic N mineralization. Finally, all the digestates also brought significant amounts of P and K.

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

“…We tested alternative values in uncertainty analysis (see Section 2.5). We also approximated the performance of Sys‐Metha for the prediction of %C dig by simulating biogas plants with a majority of cover crops in their feedstock reported in Levavasseur et al (2022). The comparison between predicted and measured digestate C content is reported in Figure S2.…”

“…I ROC has been defined as a predictor of C remaining from exogenous organic matter after long‐term incubation with soil under controlled conditions. We, thus, used the mean I ROC value (0.5) of the raw digestates of cover crops reported in Levavasseur et al (2022). The uncertainty concerning this value was also tested (Section 2.5).…”

“…We considered biogas plants with a hydraulic retention time greater than 100 days (Levavasseur et al, 2022), leading to an AD yield of 105% in Sys-Metha. %C loss storage was equal to 8% for raw digestate.…”

“…The exact digestate C input resulting from the AD of the energy cover crop was computed with Sys-Metha (Section 2.3.2). Typically, an energy cover crop yield of 7 t DM ha −1 (with 20% DM content) corresponded approximately to a digestate application of 35 m 3 ha −1 once in the rotation, which was a classical application rate (Levavasseur et al, 2022). Alternative lower and higher application rates of digestate were considered in the uncertainty analysis (Section 2.5) to explore the effect of a discrepancy between fields producing energy cover crops and fields where digestate was applied.…”

“…Alternative lower and higher application rates of digestate were considered in the uncertainty analysis (Section 2.5) to explore the effect of a discrepancy between fields producing energy cover crops and fields where digestate was applied. Wet mesophilic digestion with an absence of phase separation and an uncovered storage of raw digestate were considered in the AD scenario, as typical AD of energy cover crops operated (Levavasseur et al, 2022). Except for the insertion of energy cover crops and the application of digestate, no modification of the cropping system was considered in the AD scenario.…”