Quantification of biological N2 fixation of hedgerow trees in Northern Lampung

The contribution of the leguminous hedgerow trees Flemingia congesta and mixed Gliricidia sepium - Peltophorum dasyrrachis established on an Ultisol (Grossarenic Kandiudult) in Northern Lampung, Sumatra (Indonesia) to the N economy of the hedgerow intercropping system was assessed. N2 fixation estimates of hedgerow trees over a two year period after establishment were obtained by the 15N dilution method using Peltophorum dasyrrachis as the non-fixing reference plant. Average yearly tree pruning dry matter and total N yields of the two hedgerow systems were not significantly different although seasonal differences occurred. Gliricidia obtained a larger (average 51%) proportion of its N from N2 fixation than Flemingia (average 25%) the effect being consistent over the two years. Seasonal variation in the % N derived from N2 fixation was associated with changes in soil mineral N availability. The amount of N2 fixed was not significantly different between the two N fixing trees (26 vs. 35 kg N ha-1 yr-1 respectively). This was due to the higher total N yield of Flemingia compared to Gliricidia compensating for the smaller proportion of N derived from N2 fixation. The amount of tree soil N uptake was similar for the two hedgerow systems suggesting that the higher proportion of N derived from N2 fixation in Gliricidia was probably due to competition for soil mineral N by the associated non-fixing Peltophorum. Rapid temporal and spatial decline in plant available 15N occurred suggesting that matching of fixing and non-fixing reference plants is important. Compared with estimates of N2 fixation obtained by the natural abundance method the 15N dilution method overestimated N2 fixation by the tree legumes (an average of up to 18%), however the difference between the two fixing legume trees was maintained. N balance estimates suggested that N2 fixation inputs of alley trees were sufficient to sustain moderate crop yields but for higher food crop yields, N2 fixing leguminous crops would have to be included to balance N off-take