The nutrients in rainwater that reach the soil by throughfall or stemflow may represent significant nutrient fluxes in agroforestry systems with low external input. However, there is no information about these processes in agroecosystems of semi-arid northeastern Brazil. This study was conducted in an alley cropping system with Gliricidia and maize in Esperança, PB, Brazil. The objectives of the study were to quantify: (a) the proportion of rainfall that reaches the soil as throughfall or stemflow and the amount intercepted by the tree canopy; and (b) the input of N, P and K to the soil as throughfall, stemflow and direct rainfall water. Four plots were established in the experimental area and throughfall water collectors were placed at a distance of 0.50 m from the tree trunks. Stemflow water was collected in each plot using collars around the tree trunks that conducted the water to collectors while four collectors were installed in adjacent areas without trees to collect rainfall water. Of the total rainfall in 2004, 67 % reached the soil as throughfall, 0.74 % as stemflow and 32 % was intercepted by tree canopies. N and P concentrations were similar in throughfall and stemflow water samples, but these were around 300 % greater than those in rainfall water. The K concentration in stemflow water was 100 and 600 % greater than in samples from throughfall and rainfall, respectively. On average, N, P, and K inputs to the soil were 5, 1 and 24 kg ha-1 in rainfall water; 9, 2 and 62 kg ha-1 in throughfall water; and 0.12, 0.02 and 1 kg ha-1 in stemflow water, respectively. The results demonstrate the importance of agroforestry for the sustainability of agricultural systems with low external input in semi-arid NE Brazil.
semi-arid region; atmospheric deposition; nutrient cycling
