Similar specific mineralization rates of organic carbon and nitrogen in incubated soils under contrasted arable cropping systems

Abstract

No tillage is often thought to mitigate greenhouse gas emissions from agricultural land by increasing soil carbon storage, because of a reduced mineralization of soil organic carbon (SOC) and nitrogen (SON). Regrettably, most available references on this topic come from laboratory incubations of disrupted soil from superficial soil layer. Here, we compare SOC and SON mineralization rates in the long-term experiment of La Cage (France) under conventional (CON), low input (LI), conservation agriculture (CA) or organic (ORG) management. Disturbed soil samples from the 0−27 cm soil layer of all treatments were laboratory incubated for four months, while undisturbed CON and CA soil cores were incubated to account for tillage effects. Physical disturbance decreased SOC and SON mineralization. Model fitting showed that the size of the C labile pool and the C and N mineralization rates of the slow pool were 1.5–2.3-fold greater in undisturbed soil cores than in disturbed ones, which may be due to a higher abundance of labile SOC (e.g. plant residues) in undisturbed soil cores. All cropping systems exhibited similar specific rate of mineralization, expressed per unit of SOC, SON or microbial biomass C, both for disturbed and undisturbed soils. Similar mineralization in CA and CON undisturbed soil cores may result from the balance between higher amount of labile OM and less favourable soil structure for decomposition in CA. Similar mineralization rates in disturbed soil cores suggest that OM decomposability and environmental conditions for decomposers were similar between cropping systems. Overall, these results confirmed the hypothesis previously made in silico to explain the differences in SOC storage in this experiment (Autret et al., 2016). Our results together with the increased SOC stocks observed in CA and ORG treatments suggest that increased biomass returns to soil or changes in microbial physiology may be the main drivers of SOC storage.