Response of denitrification in paddy soils with different nitrification rates to soil moisture and glucose addition.

Denitrification is one of the most important N loss pathways in paddy soil. The nitrification rate is a key natural feature for controlling denitrification N loss in paddy soil. However, the relationship between nitrification and denitrification under different conditions in paddy soil remains unknown. By using 15N tracing, we investigated the response of denitrification loss to soil moisture and glucose addition in six paddy soils, whose net nitrification rates ranged from 0.36 mg N kg-1 day-1 to 5.72 mg N kg-1 day-1. The soils were amended with or without glucose to simulate root exudates at rates of 100 mg kg-1 of soil and incubated under either 60% water holding capacity (WHC) or flooded (2 cm depth) at 25 °C for 15 days. Denitrification loss was calculated by the unrecovered 15NH4+. The results showed that the soil nitrification rate significantly affected the N recovery form and denitrification loss of the applied 15N. NH4+ was the main recovered N form of the applied 15N in soil with a low nitrification rates. Denitrification losses were higher in the high nitrification rate soil than soil with low nitrification rate in all treatments. The correlation between denitrification and nitrification rates was well fit by Michaelis-Menten kinetics during the incubation, irrespective of soil moisture and glucose addition, and the R2 ranged from 0.801 to 0.977 (P < 0.05). Glucose addition did not stimulate denitrification under either 60% WHC or flooded conditions. The results showed that nitrification rate, rather than labile organic supply, controlled denitrification in paddy soil.