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

The contribution of the leguminous hedgerow trees Flemingia congesta and mi xed Gliricidia sepium - Peltophorum dasyrrachis established on an Ultisol ( Grossarenic Kandiudult) in Northern Lampung, Sumatra to the N economy of th e hedgerow intercropping system was assessed. N-2 fixation estimates of hed gerow trees over a two year period after establishment were obtained by the N-15 dilution method using Peltophorum dasyrrachis as the non-fixing refer ence plant. Average yearly tree pruning dry matter and total N yields of the two hedger ow systems were not significantly different although seasonal differences o ccurred. Gliricidia obtained a larger (average 51%) proportion of its N fro m N-2 fixation than Flemingia (average 25%) the effect being consistent ove r the two years. Seasonal variation in the % N derived from N-2 fixation wa s 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 v s. 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 prop ortion 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 competiti on for soil mineral N by the associated non-fixing Peltophorum. Rapid tempo ral and spatial decline in plant available N-15 occurred suggesting that ma tching of fixing and non-fixing reference plants is important. Compared wit h 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 avera ge of up to 18%), however the difference between the two fixing legume tree s 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.