Quantification of biological N 2 fixation of hedgerow trees in Northern Lampung

Abstract

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 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 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 15N 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 15N 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.