Fluxes of nitric oxide from soils following the clearing and burning of a secondary tropical rain forest

At sites in the Atlantic Lowlands of Costa Rica, clearing and burning of a secondary tropical rain forest caused a significant increase in soil nitric oxide (NO) emissions. Soil-atmosphere NO fluxes averaged 0.5 ng N cm−2 hr−1 prior to clearing and increased to 4.1 ng N cm−2 hr−1 following clearing and to greater than 12.0 ng N cm−2 hr−1 following burning. Soil NO emissions were elevated for a period of 3–4 months following clearing, and fluxes peaked for 1–3 days following burning. We conducted a series of experiments with intact soil cores to determine the probable mechanism responsible for elevated NO emissions from soils. In one set of experiments we added substrates for microbial nitrification (ammonium), denitrification (nitrate), and chemical denitrification (nitrite) to autoclaved (killed) and nonautoclaved (live) soil cores. Water-only additions were used as controls. Compared to water or nitrate additions, ammonium caused a significant increase in NO emissions from live cores. Water, ammonium, and nitrate additions had no effect on emissions from autoclaved cores. Nitrite solution additions resulted in highly significant increases in NO emissions from both autoclaved and nonautoclaved soil cores. In a second set of experiments we treated intact soil cores with acetylene (1 kPa C2H2) to selectively inhibit nitrification and oxygen to inhibit denitrification. The oxygen treatment had no effect on NO production while acetylene significantly reduced NO emissions. The results from the substrate addition and inhibition experiments demonstrate that microbial denitrification is not a major pathway for NO production in these soils. In contrast, microbial nitrification appears to be a critical process responsible for NO emissions throughout the clearing and burning period. Field experiments with acetylene as an inhibitor show that immediately following burning, chemical denitriflcation of nitrite deposited in ash supports a large peak in NO fluxes.