Carbon availability mediates the effect of nitrogen on CO2 release from soils

Nitrogen availability affects decomposition of soil organic carbon (SOC), and hence impacts soil quality. It remains unclear whether N availability determines the direction of SOC transformation and how this may interact with C availability in soil. Nitrogen-deficient topsoil from two depths (0–5 cm, Soil5; 5–10 cm, Soil10) were supplied with eight rates of Ca(NO3)2 (0–160 mg N kg−1 soil), and incubated for four weeks. Two experiments, with and without weekly addition of glucose at 0.5 mg C g−1, were conducted. Without glucose, N addition at 40–80 mg kg−1 to Soil5 stimulated CO2 release, but further supply up to 120 mg kg−1 showed no effect on CO2 release in Week 1. Compared with N addition alone, glucose addition lowered N requirement (20 vs. 40 mg kg−1) for the peak CO2 release in Soil5, indicating that labile-C supply decreased N demand for microbial activity. With glucose addition, CO2 release rates in Soil5 remained constant at N supply ranging from 20 to 160 mg kg−1. In contrast, CO2 release rates in Soil10 was affected by N supply only when glucose was supplied, suggesting the important role of C availability in regulating N-induced CO2 efflux in soil. Irrespective of N supply, specific CO2 release per unit SOC following glucose addition was higher in Soil10 than Soil5, albeit no differences between two soils without glucose addition. These results suggest that enhanced N availability up to 20–40 mg kg−1 favors soil CO2 release when C availability is not limiting.