Terrain and land management practice influences on water quality in oil palm compared with forested catchments in Sabah, Malaysian Borneo

Oil palm plantations have become a major land-use in parts of the wet tropics over the past 40 years. There has been much concern about its hydrological, erosional, water quality and nitrous emissions (and hence climate change) consequences.  These impacts, however, may vary considerably with terrain  (hilly terrain with bench-terracing being very different from low- and    moderate-slope terrain without bench-tarracing), details of land management practices, and  over the c. 25 years life-cycle of oil palm plantations – but remain largely unassessed.  With regard to water quality, this paper presents evidence from catchment studies in hilly terrain since 2012 in the headwaters of the Brantian and Kalabakan river basins in Sabah (Malaysian Borneo), with a  particular focus on results from a bench-terraced, mature oil palm catchment (3.27 km²).  Studies there formed part of Stability of Altered Forest Ecosystems (SAFE) Project, within which comparisons were drawn between the oil palm (OP) catchment, a near-primary (VJR Virgin Jungle Reserve) catchment, and six multiple-logged catchments. All catchments were instrumented from 2011 with sensors recording 15-minute data on conductivity, turbidity, water temperature and water depth (and hence discharge). This was supplemented by (a) a programme of monthly spot sampling for water chemistry, (b) opportunistic storm event sampling of water chemistry since 2014, (c) a regional survey of baseflow water chemistry of oil palm catchments in 2014; and (d) exploratory application in June 2025 of a multi-isotope approach (using δ²H–H₂O, δ¹⁸O–H₂O, δ¹⁵N–NO₃⁻, and δ¹⁸O–NO₃⁻) in exploring water chemistry (particularly nitrate values) of the oil palm and VJR catchments. The regional survey highlights significant but relatively modest increases in nitrate, sulphate and chloride levels at baseflow that also vary in magnitude between catchments.   For the OP catchment, (1) nitrate levels at baseflow differ little from values in forested catchments, but levels of chloride and sulphate are much elevated; (2) both the conductivity records and storm-event sampling, however, indicate the importance of flushing of fertilizer-derived nitrates and sulphates during some (but not all) storm events.  Reasons for the regional survey and OP catchment results are explored, particularly the influence of (1) the enhanced stormflow and reduced baseflow of hilly oil palm terrain with its bench-terracing and high track density and (2) the varying degree of efficiency of fertilizer uptake linked to different application techniques and frequency strategies. Results of the multi-isotope (nitrogen, oxygen and hydrogen) isotope approach of 2025 highlight differences in isotope values (a) downstream within the OP catchment, (b) with changes in discharge before and after a rainstorm, (c) between the OP and VJR catchments and (d) between the OP catchment and published results from other studies in low slope terrain in Peninsular Malaysia.  Links with nitrate source processes are explored.  Possible ways in which impacts on pollution might be reduced are presented and discussed, including how to avoid possible conflicts with strategies to reduce erosion, storm runoff (and downstream flooding) and nitrous atmospheric emissions from oil palm terrain.