Biological nitrogen fixation by cover legumes under oil palm plantations in Papua New Guinea.

Abstract

Sustainable management of soil nutrients, and more generally soil health, is a priority concern for the Papua New Guinea oil palm industry, as it is for most other agricultural systems in the world. Like other crops, oil palms need essential elements such as N, P, K and Mg in large amounts annually in order to maintain high fresh fruit bunches (FFB) production. Nutrients are supplied in the form of mineral fertilizers annually to meet nutritional requirements. Legume cover plants used under the oil palm for weed suppression and erosion control also contribute N to the oil palm system through biological N fixation, although amounts of N fixed have not been quantified for these legumes in PNG oil palm plantations. In this study, the xylem ureide technique was calibrated in a glasshouse experiment using ¹⁵N isotope dilution for the legume cover species Calopogonium mucunoides, Pueraria phaseoloides and Mucuna pruriens, before being applied in PNG oil palm plantations to assess N₂ fixation by these cover legume species and Calopogonium caeruleum. Legume standing shoot biomass under 2 to 25 year old plantations was 144 to 443 g/m² and litter was 100 to 804 g/m², equating to an estimated mean 400 kg/ha shoot biomass per plantation. Legume shoot N was 3.5 to 12g/m² while the litter N was 1.8 to 22 g/m² with a mean plantation shoot N estimate of 10 kg/ha. Dependence on N₂ fixation was highly variable, ranging from 18 (P. phaseoloides) to 75% (C. mucunoides), and did not show any relationship with age of plantation but was significantly lower where soil nitrate-N was high. Amounts of N fixed were 1.5 to 4.4 g/m² for standing shoot and 0.9 to 6.0 g/m² for litter equating to plantation estimates from 0.3 (C. mucunoides) to 34 (P. phaseoloides) kg N fixed/ha. These were conservative estimates since the study did not account for N in roots and furthermore only measured standing biomass rather than annual production. Estimates were based on measures of actual percent legume cover (0.6 to 44%) - hence indicated potential for increasing inputs of fixed N by managing for greater cover. Further research is recommended to quantify legume biomass production over time, including litter and root accumulation and turnover. Nevertheless, except for M. pruriens which did not transport a large proportion of fixed N as ureides, this study successfully calibrated the ureide technique to quantify input of biologically fixed N from cover legumes in the PNG oil palm system. With this knowledge, more informed decisions can be made regarding the effective management of N inputs from fertilisers and legumes in order to achieve sustainable oil palm cultivation.