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

Abstract

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.

Keywords

• Microbial response
• Mineralization
• Nitrogen supply
• Soil organic carbon
• Soil organic matter