Nitrous oxide, nitric oxide, and methane fluxes from soils following clearing and burning of tropical secondary forest

Abstract

Conversion of humid tropical forest to agriculture significantly alters trace gas emissions from soils. We report nitrous oxide (N2O), nitric oxide (NO), and methane (CH4) fluxes from secondary forest soils prior to and during deforestation, and throughout the first agricultural cropping. Annual average nitrogen oxide emissions from forest soils were 1.5 ng N cm−2 h−1 for N2O and 0.9 ng N cm−2 h−1 for NO. Forest clearing increased the level of extractable nitrate in soils and average nitrogen oxides fluxes (2.7 ng N cm−2 h−1 for N2O, and 8.1 ng N cm−2 h−1 for NO). Immediately after biomass burning, short‐term peaks of N2O and NO (123 ng N cm−2 h−1 for N2O, and 41 ng N cm−2 h−1 for NO) were superimposed on generally increased fluxes. Peak emissions declined within 3 days after burning. Postburn fluxes stayed higher than measured on adjacent forest sites for 3–4 months (averages for postburn fluxes were 17.5 ng N cm−2 h−1 for N2O, and 19.2 ng N cm−2 h−1 for NO). Increased N2O and NO emissions after clearing and until cropping were probably due to a combination of increased rates of nitrogen cycling and higher gaseous diffusion in drying soils. Compared to emissions from young pastures in the region, fluxes of nitrogen oxides from unfertilized agricultural areas were low (3.9 ng N cm−2 h−1 for N2O and 3.4 ng N cm−2 h−1 for NO), probably due to nitrogen uptake by fast growing corn plants and losses by leaching with draining soil water in the wet season. Variation in CH4 fluxes was high for all land use periods. Forest soils consumed an average of 1.0 mg CH4 m−2 d−1, which slightly increased in drier soils after clearing (1.2 mg CH4 m−2 d−1). Postburn CH4 consumption by soils was slightly reduced (0.8 mg CH4 m−2 d−1) compared to forest soils. Unfertilized agricultural soils consumed less CH4 than forest soils.