Biological nitrogen fixation in Malaysian oil palm: how legume cover crops reduce synthetic fertiliser dependence

Nitrogen is the nutrient that oil palm demands in the largest quantity after potassium, and nitrogen fertilisation represents one of the largest single input costs in Malaysian plantation management. Nitrogen leaves the system in harvested bunches, and more is lost through leaching, volatilisation from applied urea, and denitrification in waterlogged soils. Replacing this nitrogen with synthetic urea, which is prone to volatilisation losses and carries a significant carbon footprint, is both expensive and environmentally costly.

Leguminous cover crops offer a biological alternative to part of this nitrogen demand. Through their symbiosis with soil bacteria of the Bradyrhizobium genus, legumes convert atmospheric nitrogen gas (N2) into plant-available ammonium: a process called biological nitrogen fixation (BNF). The nitrogen fixed by a well-managed leguminous cover crop system in Malaysian oil palm represents a substantial, renewable nitrogen supply that reduces the amount of synthetic urea needed and improves the overall sustainability profile of the plantation.

The biology of nitrogen fixation in tropical legumes

BNF in leguminous cover crops occurs within root nodules: small, pinkish-red structures visible on the roots of actively fixing plants. Inside each nodule, Bradyrhizobium bacteria (called bacteroids in their nodule-resident state) maintain conditions where the nitrogenase enzyme can convert N2 gas into NH4+, which the host plant can absorb directly. The nitrogenase reaction requires significant energy (about 16 ATP molecules per N2 molecule fixed), provided by the host plant through photosynthate supply to the nodules. This energy cost means that actively fixing plants invest a significant fraction of their photosynthetic output in the symbiosis, which is why nitrogen fixation rates are closely tied to above-ground biomass production.

The ureide method, which measures ureide compounds (allantoin and allantoate) in stem extracts or xylem sap, is used to quantify BNF in tropical legumes without the expensive acetylene reduction assay. Using this technique under oil palm in Papua New Guinea, Pipai et al. (2023) estimated 48 to 102 kg N per hectare per year for the legume cover as a whole in young plantations, a figure for the mixed cover rather than for any single species. Species-level figures measure different things: for Pueraria javanica, up to 250 kg N per hectare per year accumulated in above-ground biomass in one northeast Thailand trial (Clermont-Dauphin et al., 2016); for Mucuna bracteata, 67 to 84% of plant nitrogen from fixation in a 15N study under three-year-old oil palm (Cheah, Zaharah and Aminuddin, 2010), with no routine annual rate asserted.

Why BNF often underperforms on Malaysian acid soils

The nitrogen fixation potential described above is achievable, but often not achieved in practice on Malaysian acid Ultisols. Three factors consistently limit BNF in the field relative to theoretical potential: soil pH effects on Bradyrhizobium populations, inadequate inoculation, and soil nitrate inhibition.

Bradyrhizobium strains that nodulate Malaysian cover crop legumes grow and nodulate best in moderately acid to neutral soil. In the strongly acid soils common under Malaysian oil palm, native Bradyrhizobium populations can be low, and nodulation without a supplemental inoculant can be poor. Plants growing without functional nodules fix no nitrogen, regardless of how much N2 is in the soil atmosphere. Leaf symptoms of nitrogen deficiency in un-nodulated cover crops are indistinguishable from those of simply nitrogen-deficient non-legumes: the biological nitrogen source has not been activated.

Inoculation with a viable, species-appropriate Bradyrhizobium inoculant at planting helps overcome this barrier on acid soils, especially on land where the legume has not been grown before and native rhizobia are scarce. Check nodulation a few weeks after emergence by digging up a few plants: healthy, actively fixing nodules are pink to red inside.

High soil nitrate concentration, from recent urea applications or from intensive mineralisation of organic matter, suppresses BNF by reducing the plant's energetic investment in nodule maintenance. This is why nitrogen fixation estimates from plantations receiving heavy synthetic nitrogen inputs are typically lower than from lower-input systems. Reducing synthetic nitrogen applications (as BNF begins to supply more of the plant's N requirement) actually increases BNF output in a positive feedback loop, but this requires confidence in the cover crop system's fixation capacity before reducing synthetic inputs.

Quantifying the fertiliser substitution value

For scale, every 100 kg of nitrogen that reaches the crop is equivalent to about 217 kg of urea (urea is 46% N); multiply by your own delivered urea price to see the input cost this represents, before accounting for the volatilisation losses that further reduce urea efficiency. What your estate saves depends on the nitrogen your own stand actually supplies.

Any reduction in synthetic nitrogen applications once the cover is established and well nodulated should be set from leaf and soil analysis with your agronomist, not from a fixed percentage. The nitrogen fixed by cover crops is released gradually from their residues after biomass turnover, and its release does not match the timing of palm demand, so synthetic nitrogen remains important for timing-sensitive nutrition (particularly during bunch development). SoilBoost EA, a soil conditioner rather than a nitrogen source, can be used alongside cover crops to support soil structure and root-zone conditions.

Using Seed Activator at planting of Mucuna bracteata, Pueraria javanica, Calopogonium mucunoides, or Centrosema pubescens, alongside an appropriate Bradyrhizobium inoculant, helps lay the biological foundation for good BNF from the start of cover crop establishment. Investing in good establishment and nodulation is one of the more cost-effective agronomic decisions in Malaysian oil palm: it adds biologically fixed nitrogen to the system without the volatilisation losses, input costs and carbon footprint of synthetic urea.