Co-application of glucose and phosphorus with recalcitrant high-carbon soil amendments improves N retention in a reclaimed soil: a long-term incubation study.

Incorporation of soil amendments with high organic carbon content (HCA) can reduce losses of mineral nitrogen (N) from agricultural soils. The magnitude of N immobilization and remobilization is strongly controlled by the availability of carbon (C) and phosphorus (P). However, the exact mechanisms and interactions between C, N, and P availability are poorly understood. An eight-month incubation experiment was conducted on recultivated mine soil with low organic C, mineral N and P background concentrations to investigate the effects of HCA in combination with ¹³C-labelled glucose and mineral P fertilization on greenhouse gas emissions, soil nutrient status (dissolved organic C (DOC), nitrate (NO₃^-), extractable P), and microbial biomass growth. The experiment had a factorial design of one N level × two P levels × six C treatments (control, wheat straw, poplar sawdust, glucose, and combinations of wheat straw or sawdust with glucose). The HCA increased the cumulative CO₂ and CH₄ emissions but decreased N₂O emission, except for wheat straw. Addition of ¹³C-labelled glucose decreased the cumulative CH₄ emission by 59 and 85 % in the sawdust and sawdust + P treatment, respectively. Glucose application reduced the NO₃^- content in the HCA-amended soil by 26-64 %, while P fertilizer further decreased the NO₃^- content in the wheat straw and sawdust treatments by 20 and 24 %, respectively. Both HCA and glucose treatments promoted microbial biomass growth and reduced the soil mineral N content. The δ¹³C of microbial biomass (δ¹³C_MB) showed an increasing trend during the whole experiment, although ¹³C-labelled glucose was added only once at the beginning of the experiment. Addition of HCA decreased δ¹³C_MB, while P addition had the opposite effect. In conclusion, adding a readily available C source to HCA may increase the efficacy of retaining N in post-harvest soils, particularly of more recalcitrant types of HCA like sawdust.