Biodegradability of soluble organic matter in maize-cropped soils

Abstract

Soluble organic matter and its biodegradability are important in relation to soil nutrient fluxes, carbon (C) sequestration, and water quality. The objective of this study was to evaluate the quantity and biodegradability of soluble organic matter in soil under monoculture maize or maize−soybean rotation, and different amendments: manure, inorganic fertilizer, or no amendment. We characterized organic matter extracted in cold and hot (80 °C) water by using a bioassay involving incubation of the extracts (after inoculation with soil microflora) at 35 °C for 42 days. Soluble organic C and organic and mineral nitrogen (N) were monitored during the incubation. Extracts of whole soil (0–15 cm depth) and three water-stable aggregate size classes (>1000, 1000–250, and 250–50 μm) were subjected to the bioassay in order to evaluate the physical disposition and kinetics of soluble organic matter. Hot water-soluble C accounted for about 70% of the total (cold+hot) water-soluble organic matter. Organic N was the major form of N in the extracts; it comprised 61–83% of the total N extracted with cold water and 87–97% of the total N extracted with hot water. The quantity of soluble organic matter that was biodegradable was related to the extraction procedure and the management history of the soil. The proportion of soluble C and organic N that was biodegradable was greatest in hot water extracts and greater in manured than nonmanured soils. During the bioassay, proportionately greater amounts of organic N were metabolized relative to organic C, so that the C:N ratios of the extracts widened as the incubation progressed, indicating that the soluble organic matter was rich in labile organic N. The rate at which soluble organic matter decomposed (i.e., turnover) was not related to the type of input. Decomposition data fitted a double exponential decay model, suggesting that the soluble organic matter comprised two fractions: a rapidly decomposable fraction (containing 29–36% of the total soluble C) with a turnover time of <1 day, and a slowly decomposable fraction with a turnover time of about 80 days. The concentration of soluble organic matter in soil aggregates was related to aggregate size; the concentration was greatest in the largest aggregates and it decreased with decreasing aggregate size. Soluble organic C extracted from aggregates also comprised two kinetically discrete pools. The turnover time (i.e., mean residence time) of each pool was not affected by amendment, crop rotation, or aggregate size. However, the slowly decomposable organic C pool was larger in microaggregates (<250 μm) than in macroaggregates, whereas the rapidly decomposable pool was larger in macroaggregates. We conclude that water-soluble organic matter in maize-cropped soils comprises two kinetically defined fractions and is readily decomposable due to its high content of hydrophilic, labile N-rich compounds.