CONVERTING TROPICAL RAIN-FOREST TO FOREST PLANTATION IN SABAH, MALAYSIA .1. DYNAMICS AND NET LOSSES OF NUTRIENTS IN CONTROL CATCHMENT STREAMS

Streamwater chemistry was monitored for five years in six streams in a paired catchment experiment in Mendolong, Sabah, Malaysia, including controls in rain forest and secondary vegetation after the 'Borneo fir e' of 1982-3 and comparing the effects of different ways of establishi ng forest plantations with Acacia mangium. Three catchments were cover ed with selectively logged lowland hill dipterocarp forest (W4-W6) and three (W1-W3) with secondary vegetation after forest fires. The contr ol catchments, W3 and W6 reported in this paper, had no treatments app lied. Reference monitoring at all streams was for 25 months and the to tal period of study reported here is 64 months. The soils in the catch ments were mainly Orthic Acrisol in W3 and Gleyic Podsol in W6 and a m ix of both soil types in the other catchments. Element baseflow concen trations were generally low and not significantly different from storm flow concentrations for all streams during the reference period. Conce ntrations were also generally consistently low for the two control str eams during the whole period of measurement. Chemical inputs as wet de position were low as a result of a high input from local convection. T he rain forest on the Podsol had a tight nutrient circulation indicate d by small net losses of macronutrients. The Podsol was found to have poorer conditions for soil mineralization and more surficial runoff, r esulting in higher loads of S, C and N in the organic phases, with hig her organic C/N ratio, in the discharge. Nitrogen was found to accumul ate in both catchments. An almost double accumulation of N in W3 was a ttributed to a larger biomass accumulation continuing after the forest fire 3-8 years earlier. On the other hand, the Acrisol in W3 had much larger net losses of S, Si, K, Ca, Mg and Na. Most of differences cou ld be attributed to differences in weathering between the soils and lo cal mineralogical differences.