Hydrological effects and nutrient losses of forest plantation establishment on tropical rainforest land in Sabah, Malaysia

Abstract

New measurements of field saturated hydraulic conductivity, soil moisture, rain intensity, surface runoff and erosion are presented with earlier published data on soil physical properties, stream water quality and runoff and dissolved nutrient losses from a paired catchment experiment in Sabah, Malaysia. The catchment experiment compared crawler tractor extraction and burning (normal practice, catchment W5) versus manual extraction and no burning (W4). Another catchment where secondary vegetation after forest fire was cleared and burned before planting was also studied (W1 + 2). Two separate control catchments were also included; one was for rainforest (W6) and the other comprised secondary vegetation (W3). Although hydraulic conductivity was reduced in clay soils at 20 and 40 cm depths under tractor tracks, the decrease was less drastic than for steady-state infiltrability on both clay and sand topsoils, since control topsoils (0–20 cm) were well aggregated in contrast to more compact subsoils with higher clay content. Topsoils were close to saturation at all times, also after treatments, but control forest surface runoff in slopes (excluding valley bottom) comprised only 2.9% of rainfall in a wet year. Except for just after burning, clear-felling did not increase slope surface runoff on undisturbed clay topsoils, and surface erosion was not increased, except for the erosion of ash after burning. On tractor tracks, extensive surface runoff caused surface and gully erosion. Elevated concentrations of suspended sediment in stormflow in W5 were back to pretreatment levels within 2 years. Under manual extraction suspended particulate loss was approximately half, most of which emanated from sediments activated by increasing stream runoff. Dissolved nutrient losses from catchment W5 in kilograms per hectare were 39.9 (N), 1.3 (P), 189 (K), 27 (Ca) and 16 (Mg) in the 33 months during and after treatments; these amounts were equivalent to between 10 and 185% of the removal in the harvest. For the ‘minimum disturbance’ of W4, the increased nutrient loss was halved, except for Ca. Burning of residues in W1 + 2 and W5 increased dissolved losses dramatically over a short period. The bulk of soil water was concluded to move quickly as troughflow in topsoil. Hence, ‘bypassing’ of retention possibilities in the largest pores with small specific surfaces led to large dissolved nutrient stream losses in this humid tropical environment, even without soil disturbance and extensive surface runoff.