Microbial biomass and soil organic matter dynamics in oil palm (Elaeis Guineensis Jacq.) plantations, west Malaysia

Abstract We used the chloroform fumigation-extraction method to investigate relationships between microbial biomass C and total organic C, and between microbial biomass ninhydrin-N and total organic N, in areas within oil palm plantations receiving different amounts of organic matter over 5, 10 and 20 y. The avenues and weeded circles beneath the palms, which comprised 65% and 15% and of the area respectively, had mean organic concentrations of 0.82% at 5 y, 1.76% at 10 y and 2.21% at 20 y. These areas received negligible inputs of litter from above-ground during the life of the plantation and the increments in soil organic matter must have been largely derived from root material. In contrast, windrows of pruned fronds, occupying 20% of the area, received annual inputs of up to 10 t ha −1 of organic matter (equivalent to 4.8 t C ha −1  y −1 ) but the carbon content of the underlying soils only increased from 0.82% at 5 y, to 2.47% at 10 y and 3.09% at 20 y. Values for biomass C varied from 109 to 390  μ g C g −1 with highest values under the palm fronds. Biomass ninhydrin-N (N nin ), varying between 2.2 and 14.3  μ g N g −1 , was significantly correlated with biomass C (C mic ), soil organic C (C org ) and organic N, and was shown to be a reliable alternative to biomass C determinations for estimating microbial biomass in these soils. The C-to-N ratio of microbial biomass (C mic :N nin ) varied between 26 and 50 with a mean of 31; no trends were evident in relation to age of site or location within sites. At 5 y % C mic in C org was larger (2.40%) under the frond piles than in the avenues (1.70%) and the weeded circles (1.36%). In the 10 and 20 y-old plantations, however, % C mic in C org declined under the frond piles relative to other treatments. It was concluded that microbial biomass C generally increased as a function of increasing total organic C related to the age of the plantation but was not consistently higher under frond piles which received much higher inputs of organic matter than the avenues and circles. This suggested that most of the frond material decomposed on the soil surface and did not significantly affect soil microbial biomass.