Cover crops and Nitrogen transformation

Here, Carla Süß published the data from her incubation experiment with ¹⁵N-labelled cover crops. In her experiment, Carla investigated how the incorporation of different cover crops affects the C and N dynamics in a loamy soil. The factors of cover crop species (winter rye and oil radish), their compartments (root and shoot biomass) and the pretreatment of the cover crops (fresh and dried) were analyzed. For this purpose, soil columns with mixed, ¹⁵N-labeled cover crop biomass were incubated for 70 days at 8°C and 40% water-filled pore space. Continuous gas measurements (CO₂, N₂O) were carried out, and at several time points during the incubation, mineral nitrogen content, microbial biomass (C and N), water-soluble carbon, and the ¹⁵N signature of the various N fractions were determined.Overall, a significantly higher initial soil respiration was observed when dried cover crop biomass was incorporated compared to fresh biomass, indicating the increased microbial availability of cover crop C and N due to drying. Furthermore, the results showed that the fate of N and C during the decomposition of cover crop shoot and root biomass differed significantly, and that this was mainly influenced by the different compositions of shoot and roots (e.g., N content, C:N ratio and soluble cell components). During the decomposition of root biomass, a higher proportion of cover crop-derived N was found in the microbial biomass at the end of the incubation, indicating at least a temporary microbial immobilization of cover crop N. In line with this, the soil NO₃-N content was close to the control value. The emitted N₂O contained both soil N and cover crop N in all variants, with a higher proportion for shoot biomass (55-57%) than for the root variants where 75% of the N₂O-N originated from the soil N pool after application of fresh oil radish roots. The results suggest that cover crop roots and shoots may have different effects on soil N dynamics in the subsequent main crop. These differences may be relevant for optimizing fertilization, especially if aboveground cover crop biomass is harvested and the N dynamics are strongly influenced by decomposing cover crop roots.

https://doi.org/10.1002/jpln.202200437