Aim Agroforestry systems incorporating N-fixing trees have been shown to be socially beneficial and are thought to be environmentally friendly, both enriching and stabilizing soil. However, the effect of such systems on the emissions of the important greenhouse gas nitrous oxide (N2O) and the tropospheric ozone precursor nitric oxide (NO) is largely unknown.
Location Soil was collected from the research plots of Institut Sénégalais de Recherches Agricoles at Bandia and Bambey, Senegal, West Africa, and from neighbouring farmers’ fields. Trace gas flux measurements and chemical analysis of the soil were carried out at the Centre for Ecology and Hydrology (CEH), Edinburgh, UK.
Methods Nitric oxide (NO) and nitrous oxide (N2O) emissions were measured following simulated rainfall events (10 and 20 mm equivalents) from repacked soil cores collected under two tree species (Acacia raddiana) and Eucalyptus camaldulensis) in each of two provenance trails. In addition, soil samples were collected in local fields growing peanut (Arachis hypogaea) and Sorghum (Sorghum vulgare), close to the species trials in Bambey. NO was measured using a flow through system and was analysed by chemiluminescence. Nitrous oxide was measured from the repacked soil core headspace and was analysed by electron capture gas chromatography. Soil mineral N was extracted with KCl and analysed by colorimetric methods on separate soil columns.
Results Light rainfall, which increased the gravimetric soil moisture content to 20%, stimulated an increase in NO emission but there was no detectable N2O emission. A heavy rainfall event, which increased the gravimetric soil moisture to 30%, stimulated N2O emission with a subsequent peak in NO emissions when the soils became drier. Soil collected under the N-fixing tree species emitted significantly more N2O than soil collected under the N-fixing crop species (P < 0.01). NO and N2O emissions significantly correlated with soil available N (NH4 and NO3) (P < 0.05).
Main conclusions Rainfall intensity, supply of mineral N from organic matter and N fixation were the prime drivers of NO and N2O emissions from seasonally dry tropical soils. The improved soil fertility underneath the trees provided a larger pool of mineral N and yielded larger rates of NO and N2O emissions.