Subsoil carbon input by cover crops depends on management history

Cover crops represents an agricultural management option, which has the potential to increase soil carbon (C) stocks and contribute to atmospheric CO2 reduction. However, there is a scarcity of studies quantifying the C inputs from cover crops, particularly into subsoil layers. Furthermore, estimates of plant C inputs to soils often lack accounting of root fragments (here defined as roots in the size class of 0.25–4 mm) and net phyllo- and rhizodeposition. Based on a field experiment with multiple 13C-CO2 pulse labeling, we investigated the short-term C inputs from cover crop mixtures to 1-m depth in two long-term cropping systems with organic and conventional management. We used a novel 13C-based approach to quantify the amount of C in root fragments, as young fragile cover crop roots are challenging to sample in soils. The results showed that the total belowground C input to 1-m depth (570–1000 kg C ha−1) was similar across long-term management (organic and conventional, with or without a history of cover crops), but with a higher belowground C input for mixtures established without winter vetch (Vicia villosa Roth). The C input to the subsoil (> 25 cm) by cover crops accounted for 11–42% of the total plant belowground C input, which was significantly and negatively correlated to the soil fertility measured as the initial mineral nitrogen (N) content in the topsoil. The results suggest that root exploitation, and thus plant C inputs into the subsoil, was enhanced in relatively N poor cropping systems. Root fragments and net phyllo- and rhizodeposition accounted for 39–59% and 5–37%, respectively, of the total cover crop belowground C input, i.e., both representing significant pools of plant C input. We conclude that future studies on the effects of cover crops on soil C storage need to account for subsoil C pools including root fragments and rhizodeposition to achieve a comprehensive estimation of plant C inputs.