Nitrogen transformation as affected by decomposition of ¹⁵N‐labeled cover crop shoots and roots

Abstract Background Incorporation of cover crop (cc) shoot and root biomass can have different effects on nitrogen (N) dynamics and the transformation of soil‐derived N and cc N. Aims The objective was to determine the effects of different ccs, cc compartments (roots and shoots), and pretreatment of cc biomass (fresh vs. dried) on mineralization processes and on the transformation of soil and cc N following incorporation into a silty loam soil. Methods Soil columns with incorporated 15 N‐labeled root and shoot biomass of two cc species (winter rye and oil radish) and different pretreatments (dried and fresh) were incubated for 70 days at a constant temperature and soil moisture (8°C, 40% water‐filled pore space). Carbon and N transformation dynamics were determined repeatedly, distinguishing between N originating from cc biomass and from soil. Results Net CO 2 emission was related to the amount of soluble cell components added with ccs. Net N mineralization was negatively related to the C:N ratio of cc biomass. The incorporation of dried cc biomass caused higher initial soil respiration and N immobilization than fresh biomass. All treatments with cc incorporation showed increased N 2 O emission. Emitted N 2 O‐N consisted mainly of cc N (55%–57%) in treatments with fresh shoot biomass, whereas soil N was the main source of N 2 O (75%) in the treatment with fresh oil radish roots. Recovery of cc 15 N was affected by crop compartment and pretreatment. At the end of the incubation, it was 17.5%–42.3% in soil NO 3 − , 0.1%–8.1% in microbial biomass N, and less than 0.23% of cc N was found in cumulative N 2 O emission. Conclusion The incorporation of cc roots and shoots had different effects on N mobilization and immobilization processes and on the partitioning of cc N. These processes can be influenced significantly by pretreatment of the added plant biomass (dried vs. fresh).