Biogenic NO emissions from savanna soils as a function of fire regime, soil type, soil nitrogen, and water status

A study of NOx emissions from soils representative of nutrient-poor and nutrient-rich savannas and their response to burning and soil water content was carried out in the southern Kruger National Park, South Africa. The study spanned the end of the dry season and the beginning of the wet season (September-December 1992). Nitrogen mineralization rates were measured using an in situ technique simultaneously with measurements of NOx emissions. NOx emissions were almost entirely as NO. The relationship between NO emission rate and soil moisture was parabolic regardless of soil type and management practice, with the lowest NO emission rates being measured at low (<0.087) and high (>0.542) water-filled pore space values. The initial increase in NO emission rates with increasing soil moisture are paralleled by increases in the nitrate concentration in the soil. The highest NO emission rates (20 ng N-NO m(-2) s(-1)-excluding the brief initial peak) were measured on plots from which fire had been excluded for 35 years. The next highest rates (8 ng N-NO m(-2) s(-1)) were measured on the more fertile soils. Infertile soils, burned every second year, had rates of 3.5 ng N-NO m(-2) s(-1). The NO emission rates show a positive correlation with soil total N content and N nitrification rate. The effect of excluding fire from a savanna is to increase the soil nitrogen content through increased litter inputs, which in turn increases nitrification rates and soil NO emissions.