Interactive Effects of Soil Ph and No3- Availability Managements on the Magnitude and Stoichiometry of Denitrification Products from Subtropical Acidic Agricultural Soil

Abstract

Soil pH and nitrate (NO 3 - ) concentration have been suggested as the main regulators of nitrous oxide (N 2 O) and dinitrogen (N 2 ) emissions from denitrification. However, rather little is known about the mechanisms how soil pH and NO 3 - concentration individually or in combination affect N 2 O and N 2 production by denitrification or the N 2 O/(N 2 O+N 2 ) product ratio (N 2 Oi). Here, we used the 15 N gas flux method under N 2 -depleted artificial atmosphere to directly quantify the dynamics of N 2 O and N 2 emissions by denitrification under anoxic conditions at varying pH levels (pH 4.5, 6.1, and 7.2; the higher pH levels were adjusted by lime additions to the soil) and two soil NO 3 - concentrations (30 and 80 mg N kg -1 ) over a 10 days incubation period. N 2 Oi was negatively and non-linearly correlated with soil pH ( p <0.01) as liming of soils significantly enhanced cumulative N 2 emissions over the incubation period. On average, liming decreased cumulative N 2 O emissions by about a factor of 2 in treatments with low NO 3 - concentrations ( p <0.01), while cumulative N 2 O emissions remained constant in treatments with high NO 3 - concentrations. The positive effects of liming on N 2 O reduction could be partly overshadowed by high NO 3 - concentration. N 2 Oi values were observed sharply decreased when the NO 3 - concentration reached a threshold value and liming non-linearly increased the threshold value. Hence, there were significant interaction effects of soil pH and soil NO 3 - concentration levels on N 2 Oi values. The present study shows that denitrification models need to consider both, soil pH as well as NO 3 - concentrations while predicting rates of denitrification and its product ratios. Moreover, our study suggests that liming of acidic agricultural soils is only an effective N 2 O mitigation strategy if over-supply of soils with N fertilizers is avoided.

Keywords: Denitrification, Dinitrogen (N2), Nitrous oxide (N2O), Denitrification product ratio, Soil pH, Nitrate (NO3-)