Fraksionasi bahan organik tanah dan kecepatan mineralisasinya pada ultisol Lampung Utara

The main objective of this study is to test to what extent the amount and quality density fractions respond to differences in input of residues through two-step experiment fractionation of soil organic matter (quantity aspect) and incubation of these fractions (quality aspect).

The soil organic matter ( >150 pm ) of various soils under species plots of forest; sugarcane; and Alley Cropping systems i.e : Peltophorum dasyrrachis, Gliricidia sepium; Leucaena leucocephala, Calliandra calothyrsus, Flemingia congesta, and control (without alley) which had received different inputs of organic residues for 10 years was fractionated into light, intermediate, and heavy fraction ( 2mm ). All of these fractions are mixed with quartz sand. The mixture were incubated in the leaching tubes to determine mineralization rates.

The dry Weight, the C, N, P content, and C : N ratio of soil organic matter fraction were much affected by residue treatments (species plots). For all species plots it is shown that the dry weight of litter > light > intermediate > heavy fraction. Whereas...all fractions of plot species showed that forest > Calliandra > Peltophorum > Flemingia > Gliricidia > Leucaena > Sugarcane > Control. Mineralization rates of soil organic matter fractions for all species plots were, in increasing order as follows : light > litter > intermediate > heavy. Whereas based on all fraction it is shown that : Leacaena > Gliricidia > Peltophorum > Calliandra > Flemingia >1 Control > Forest > Sugarcane. For all species plots, the fractions with high C : N ratio is mineralized faster than those with low C : N ratio. However, the fractions of species plots with high C N ratio is mineralized more slowly than those with low C : N ratio.

It was apperent from this study that not all alley plots were mineralized quickly, despite the high nitrogen concentration in the fractions. Nitrogen and P released by fractions of alley plots with high C : N ratio would be slower than those with low C : N ratio. It appeared that mineralization rates of various fractions in this incubation- method (quartz sand as media) was faster than those in incubation method with soil as media due to the absensce of complex of organo-mineral in the sand media, so that the rate of mineralization was dominantly controlled by its chemical composition besides the higher biological activity in the sand media.