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 to the N economy of the hedgerow intercropping system was assessed. N-2 fixation estimates of hedgerow trees over a two year period after establishment were obtained by the N-15 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 N-2 fixation than Flemingia (average 25%) the effect being consistent over the two years. Seasonal variation in the % N derived from N-2 fixation was associated with changes in soil mineral N availability. The amount of N-2 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 N-2 fixation. The amount of tree soil N uptake was similar for the two hedgerow systems suggesting that the higher proportion of N derived from N-2 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 N-15 occurred suggesting that matching of fixing and non-fixing reference plants is important. Compared with estimates of N-2 fixation obtained by the natural abundance method the N-15 dilution method overestimated N-2 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 N-2 fixation inputs of alley trees were sufficient to sustain moderate crop yields but for higher food crop yields, N-2 fixing leguminous crops would have to be included to balance N off-take.