Deforestation for oil palm alters the fundamental balance of the soil N cycle

Expansion of commercial agriculture in equatorial regions has significant implications for regional nitrogen (N) budgets. Here we investigate changes in N availability and turnover in Southeast Asia following the replacement of tropical forest with oil palm plantations along a chronosequence of oil palm maturity (3-months to 15-year-old stands) and secondary to primary forest succession in Sabah, Malaysian Borneo. Ten sites were sampled during March and April 2012 and rates of gross ammonium (NH4+) and nitrate (NO3-) production (mineralisation and nitrification) and consumption (n = 8), potential denitrification and “anaerobic ammonium oxidation” (“anammox”) (n = 12) were determined using 15N isotope additions to soil cores and slurries respectively. Gross mineralisation rates (0.05 – 3.08 g N m-2 d-1) remained unchanged in oil palm relative to forests. However, a significant reduction in gross nitrification (0.04 – 2.31 g N m-2 d-1) and an increase in NH4+ immobilisation disrupt the pathway to nitrogen gas (N2) production substantially reducing (by > 90%) rates of denitrification and “anammox” in recently planted oil palm relative to primary forest. Potential nitrous oxide (N2O) emissions were greater than potential N2 production and remained unchanged across the chronosequence indicating a potentially increased ratio of N2O:N2 emission when soils were first disturbed. These results are an important precursor to studies that could yield improved estimates of regional N turnover and loss in Southeast Asia which will have global implications for N biogeochemical cycling.