SEMIANNUAL LOSSES OF NITROGEN AS NO AND N2O FROM UNBURNED AND BURNED CHAPARRAL

Nitric oxide (NO), nitrous oxide (N2O), and carbon dioxide (CO2) measurements were made at unburned and burned sites within a semiarid chaparral ecosystem located in southern California. Some sites were burned during the dry season (July); others at the start of the wet season (December). This report primarily describes measurements of gas fluxes and soil parameters at unburned sites and sites burned in December. Burning greatly increased the concentrations of ammonium (NH4+), nitrate (NO3-) and potentially mineralizable nitrogen in soil. Nitrate concentrations at sites burned in December were much higher than those at sites burned in July, indicating active nitrification and decreased water stress. Irrigation usually produced immediate but short-lived increases in NO flux; however, these increases were more moderate than those observed in July. Diurnal studies performed at irrigated and nonirrigated burned sites both in July and December indicated a high correlation of NO flux with temperature (r(2) > 0.81) at most sites, provided that the soil moisture was greater than 30%. Results of diurnal studies were used to normalize daily fluxes and to calculate semiannual losses of nitrogen from both burned and unburned sites. Over a six-month period, 0.3 g N m(-2) was lost as NO from burned chaparral, representing 75% of the exchangeable nitrogen lost over the same period and 3.9% of the potentially mineralizable nitrogen present in the soil. From unburned soil, 0.1 g N m(-2) was lost as NO, representing 33% of the exchangeable nitrogen and 3.3% of the potentially mineralizable nitrogen in soil. These studies indicate that over a six-month period following a burn, much of the NH4+ produced by the burning of aboveground vegetation is lost to the atmosphere as NO. These emissions may be expected to persist for periods in excess of six months because of the increased mineralizable nitrogen available in soil as a result of the burn. Losses of nitrogen both during and following a burn may impact soil fertility, contribute to both air and water pollution, and yield significant inputs to the global atmospheric budgets of both NO and N2O. (C) 1989 by the American Geophysica Union.