Quantitative and qualitative changes of humus in whole soils and their particle size fractions as influenced by different levels of compost application

Effect of long-term application (ca. 30 years) of compost at different levels on humus composition of whole soils and their particle size fractions in a field subjected mainly to double cropping (barley and paddy rice) was investigated. Soil samples were collected from three plots of different types of management: (a) F plot, only chemical fertilizers containing N, P and K were applied; (b) F+LC plot, both chemical fertilizers and a low level of compost were applied; (c) F+HC plot, both chemical fertilizers and a high level of compost were applied (the amount of compost applied in the F+HC plot was three times larger than that applied in the F+LC plot). Each soil sample was divided into coarse sand(CSA), medium sand-(MSA) and fine sand-(FSA) sized aggregate, silt-sized aggregate (SIA) and clay-sized aggregate (CLA) fractions by wet-sieving and sedimentation. In addition, the CSA and MSA fractions were subdivided into “mineral particles” (MP) and “decayed plants” (DP) by a density fractionation. Humus composition was influenced depending upon the level of compost applied. The application induced an increase in the amounts of total humus (TH), humic acid (HA) and fulvic acid (FA) in the whole soil and many size fractions, particularly, SIA fraction. The increase was remarkable in the F+HC plot. In the CSA and MSA fractions, the amounts of TH, HA and FA were much larger in the CSAand MSA-DP fractions than in the CSAand MSA-MP fractions. The amounts of TH, HA and FA in the SIA fraction were larger than those in the CLA fraction for the F+HC and F+LC plots, and the reverse was true for the F plot. On the other hand, the degrees of humification of humic acids in whole soils and many size fractions, particularly SIA fraction, decreased by compost application. The decrease was markedly in the F+HC plot. These findings suggest that the SIA fraction play an important role in the quantitative and qualitative changes of humus, including HA and FA, as influenced by a long-term compost application.