ORGANIC-MATTER CHARACTERISTICS AND WATER-STABLE AGGREGATION OF A SANDY LOAM SOIL AFTER 9 YEARS OF WOOD-RESIDUE APPLICATIONS

The long-term effects of wood-residue applications on soil properties are not well documented. This study was undertaken to characterize the organic matter and aggregation of a sandy loam after 9 yr of biennial application of wood residues (tree clippings) at rates of 25, 50 and 100 Mg ha-1 with and without nitrogen fertilization. Carbon (C) and ni trogen (N) contents of the whole soil were determined as well as the C content of the density fractions and of the fractions soluble and ins oluble to Na4P2O7. In comparison with the control, the whole-soil C co ntent was 16-24% higher following application of wood residues alone a nd 16-37% higher for application of wood residues supplemented with ni trogen. The treatments had no effect on soil water-stable macroaggrega tion ( > 250 mum). Wood-residue applications had no effect on the humi c material (soluble in Na4P2O7) but favored the humin-C content (the f ractions insoluble in Na4P2O7) by 25-60% relative to the control. The light-fraction organic matter was on average 68% larger, and the heavy fraction 17% larger, in the treated soils than in the control. On ave rage, 80% of the differences in total organic C induced by residue app lication could be attributed to differences in the humin and heavy fra ctions. The long-term effect of wood-residue applications to the soil was, therefore, reflected in an accumulation of the more stable organi c matter present as heavy and humin fractions. In addition, the differ ences in the light fractions suggested a short-term effect of wood-res idue applications.