Nitrification activities and N mineralization in paddy soils are insensitive to oxygen concentration

Abstract

ABSTRACTOxygen concentration is considered to be the most important factor influencing nitrification and mineralization rates in agricultural soils. However, the sensitivities of nitrification and N mineralization in paddy soils to oxygen concentrations are not well known. We examined nitrification activities and N mineralization rates of six paddy soils with pH ranging from 5.23 to 7.83 and incubated at 25°C and 60% water-holding capacity in laboratory after ammonium was added at concentrations of 10, 30 and 50 mg N kg−1 of soil and the headspace gases were replaced with stock gases whose oxygen concentrations were 20%, 10% and 2%, respectively. The tested paddy soils had a very wide range of nitrification activities so that the nitrate ratio in inorganic N varied from > 95% after 1 day incubation to < 25% after 7 days of incubation. The nitrate ratio correlated with the soil pH. Nitrate content and its ratio, and mineralization rate were not suppressed when the oxygen concentration in the headspace decreased from 20% to 2%. Our results suggested that nitrifiers and microbial communities involved in N mineralization might have adapted to the environment with low oxygen concentration in paddy soils in which oxygen concentration is deficient during the flooding period. However, oxygen concentration would be even lower than 2% in paddy soils under the flooding conditions. Thus, the minimum oxygen concentration at which nitrification activities in paddy soils are suppressed needs to be explored further.