Effects of Nitrogen (N) Addition On Soil Organic Carbon (SOC) and Root Decomposition in Switchgrass (Panicum virgatum L.).

Soil organic carbon (SOC) sequestration can help reducing carbon dioxide (CO₂) in the atmosphere as well as improving soil quality. However, whether N contributes to SOC sequestration is questionable because the effect of N addition on decomposition is inconsistent and often contradictory. This study was conducted to investigate the effect of external N and carbon (C) addition on decomposition of SOC. Soil has been incubated with 9 treatments (3 levels of N (N0=0, NL=0.021, NH=0.083 mg N/g soil) * 3 levels of C (C0=0, CL=2.5, CH=5.0 mg root/g soil). The flux of CO₂, microbial biomass carbon (MBC), inorganic N concentration, and soil pH were measured during 6 months of incubation. The CO₂ flux between 5 and 50 days decreased under a high level of N, but this N effect was not significant after 50 days. After 120 days, while “CH+NL” and “CH+N0” treatments produced the highest amount of CO₂-C (3.5 mg CO₂-C/g soil) in total, the production of CO₂ in C0 treatments was the lowest (1.7 mg CO₂-C/g soil) regardless of N treatments. When assuming that SOC decomposition in control was same as the other C treatments, the decomposition of root C from CH was 1.62 mg CO₂-C/g soil, which was twice than that from CL (0.89 mg CO₂-C/g soil). MBC was measured by chloroform fumigation-extraction method. The MBC did not significantly vary among N treatments and was not related to CO₂ fluxes either. The MBC was the highest in the CH treatment in 0, 45, and 100 days of incubation (C0, CL, CH = 0.35, 0.35, 0.41 at day 0; 0.28, 0.30, 0.33 at day 45; 0.22, 0.22, 0.25 mg C/g soil at day 100, respectively). This research showed that N addition may increase SOC sequestration by reducing SOC decomposition, but the mechanisms need to be studied further.