Linking organic carbon inputs to microbial nitrogen mineralization-immobilization turnover and nitrous oxide dynamics using 15N tracer techniques

Soil carbon (C) and nitrogen (N) cycles are closely coupled, with the nature of organic C input to soils, from crop residues to recalcitrant biochar, strongly influencing microbial N mineralization-immobilization turnover (MIT) and associated N₂O/N₂ emissions. However, the extent to which different organic C inputs regulate MIT and thereby control soil N retention and greenhouse gas emissions in agricultural systems remains poorly understood. An incubation experiment using field soils from a long-term fertilization trial (NPK application since 1954, with or without biochar since 2022) on a loess-derived Cambisol soil from Northern Austria (Grabenegg) was carried out to evaluate the short-term effects of maize-derived crop residues and biochar amendment on MIT. ¹⁵N-labeled fertilizers (¹⁵NH₄NO₃, NH₄¹⁵NO₃; 150 kg N/ha) were applied to quantify gross N mineralization and immobilization, gross nitrification and NO₃⁻ immobilization, fertilizer N retention, and N₂O and N₂ emissions. Microbial biomass N (MBN), mineral N pools (¹⁵NH₄⁺, ¹⁵NO₃⁻), and gaseous N fluxes (¹⁵N₂O and ¹⁵N₂) were measured using established ¹⁵N isotope tracing and mass spectrometric techniques, allowing to track crop residue and biochar amendment effects on the partitioning of N transformation pathways and N₂O reduction to N₂.

Preliminary results revealed amendment-specific effects. After one week of incubation, laboratory amendment with crop residues increased NH₄⁺ availability by 26% (2.50 ± 0.47 mg N kg⁻¹) in soil with long-term biochar, but slightly decreased it by 8% (3.14 ± 0.69 mg N kg⁻¹) in soil without long-term biochar treatment, relative to unamended controls (2.00 ± 0.79 mg N kg⁻¹; 3.42 ± 0.82 mg N kg⁻¹). In contrast, lab amended biochar strongly decreased NH₄⁺ availability (~99%) in both field soils (0.03 ± 0.01 mg N kg⁻¹; 0.04 ± 0.04 mg N kg⁻¹), indicating a consistent response across soils regardless of field biochar application. Gross N mineralization, derived using ¹⁵N isotopic techniques with ¹⁵NH₄NO₃, was strongly stimulated by crop residues during first week, increasing rates by 172% and 290% relative to controls in soils with and without long-term biochar treatment, respectively, whereas lab amended biochar caused moderate increases of 59% and 39%. Compared to biochar, crop residues enhanced gross N mineralization 1.7-fold and 2.8-fold in soils with and without long-term biochar treatment, highlighting the stronger stimulation of N mineralization by labile C inputs. These findings show highly amendment-specific responses of MIT, differentially affecting soil N retention, and the mitigation of agricultural greenhouse gas emissions.

Keywords: nitrogen cycling, mineralization-immobilization turnover, organic amendments, biochar, crop residue, N₂O emissions, N₂ emissions, ¹⁵N tracer, climate-smart agriculture