Deposition of C and N in the soil from legume-based cover crops – quantification and mineralization.

Legume-based cover crops (CC) provide a wide range of ecosystem services, including nitrate leaching reduction, N fertilizer and soil C sequestration effects. However, the scarcity of information on belowground C and N increases the uncertainty on the magnitude of these effects. With the present study, we quantified soil C and N deposition from legume-based CC (two pure legumes and two mixtures) grown under field conditions, in Denmark. To do so, we labeled CC plants with 13C (CO2) and 15N (leaf-labeling) throughout their growing period, by following a frequent multiple-pulse labeling scheme. Then, we incubated labeled CC roots and soil to investigate C and N mineralization. Cover crops with red clover produced about 2.7 Mg ha-1 aboveground biomass, and deposited in the soil approximately 160 kg C ha-1 and 40 kg N ha-1, with approximately equal allocation of C and N to above- and beloground fractions. After four months of incubation at 10 °C, mineralization of root C was 11-18 %, while N mineralization was 39-56 %. Mineral N derived from soil N deposition was on average 7 kg N ha-1, after four months of incubation, accounting for approximately 10 % of the mineral N estimated based on red clover shoot and root biomass. Overall, C and N inputs from legume-based CC are greatly underestimated if soil deposition is not taken into account. In addition, net mineral N derived from soil deposition represents an important contribution to the N fertilizer effect of legume-based CC