Decomposition rates of organic amendments and soil organic matter fractions as indicators of soil quality: An on-farm study of organic and conventional dairy farms in Ohio.

摘要

Soil organic matter (SOM) decomposition as mediated by microorganisms and invertebrates is a major pathway for energy flow and nutrient cycles in soils. In agriculture these mechanisms play a central role maintaining soil fertility, especially under organic and low-chemical management conditions. In this study, decomposition rates and subsequent nitrogen release from crop residues and animal manure were investigated in an organic and a conventional dairy farm in Wayne county, Ohio. Both farms practiced corn (Zea mays )-small grains (Avena and Triticale)-hay ( Medicago and Dactylis) crop rotation with some variation, and the conventional farm applied synthetic fertilizers, herbicides, and insecticides. Two experiments documenting decomposition and nitrogen release from crop residues and cattle manure in (1) summer and (2) winter-summer, were conducted. During two summers (1992--1993), mesh bags containing orchard grass ( Dactylis glomerata), alfalfa (Medicago sativa), cow manure, and mixtures of grass-alfalfa, grass-alfalfa-cow manure were buried in corn soil. Also, decomposition of corn residue, four common weeds ( Setaria faveri, Polygonum pensylvanicum, Amaranthus hybridus, Chenopodium album), and a mixture of these four species were studied. Soil organic matter fractions (i.e., particulate organic matter (POM)) were quantified before and after corn residue addition to soils. Earthworm (Lumbricidae) population density also was assessed. During summer months, mesh bags with alfalfa, and grass-alfalfa had the most rapid decomposition rates whereas cow manure was slowest. Residue mass loss in both farms was greater in 1992 than in 1993, and also it was greater in the organic than in the conventional farm during 1992, where more precipitation was recorded. There were positive correlations between decay rates and initial N concentration in litter, and in 1992 the correlation was higher for the organic farm. In the winter-summer decomposition experiment there were significant differences in decomposition in the summer, corn residues decomposed most rapidly and Chenopodium the slowest. Also, residues placed in the organic farm decomposed significantly faster than in the conventional farm in the summer. Total SOM, 2-mm POM, residue biomass loss, and earthworm numbers were significantly higher in the organic than in the conventional farm. Residues that had the highest levels of nitrogen concentration, decomposed the most rapidly.