The effect of rainfall on NO and N2O emissions from Ugandan agroforest soils

Agroforestry systems, often incorporating N-fixing trees, are widely used in tropical countries. However, the effect of such systems on the emissions of atmospheric pollutants NO and N2O is largely unknown. Here we compare the emissions from two agroforest systems, one where the tree component is the nitrogen fixing tree species (Calliandra calothyrsus) and the other where the tree component does not fix N (Grevillea robusta). The experiment simulated the first rainfall of a wet season, which has been shown to be a major source of NO and N2O emissions in tropical soils. Light (10 mm rain) and heavy rainfall (25 mm rain) was added to small repacked soil columns. Light rainfall resulted in a brief flush of NO up to 96 mug NO-N m(-2) h(-1) within 1 day of wetting. Nitrous oxide emissions were negligible, In contrast heavy rainfall produced a flush in N2O emissions up to 2000 mug N2O-N m(-2) h(-1), but no NO. Responses were at least 4 times larger for soils from the N fixing trees compared to the non-N fixing trees.