Influence of Soil Organic Matter Concentrations on Carbon and Nitrogen Activity

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Abstract
The relationship of soil organic matter (OM) concentration to microbial biomass concentration and mineralizable OM is central to understanding the establishment and functioning of soil nutrient cycles. These parameters are presumably related to aboveground plant biomass and plant N concentration, but the mechanisms and controls of these interactions are not well understood. To further evaluate these relationships, a field study was established in a series of soil materials whose organic C concentrations ranged from 1 to 21 g kg−1. Surface soil (0‐15 cm) and vegetation samples were collected from plots of each treatment during the 1983 growing season. Microbial biomass was measured by chloroform fumigation and incubation; mineralizable C and N were measured in 20‐d laboratory incubations; plant growth was measured by weighing material clipped from 0.18‐m2 frames; and plant N concentration was measured by Kjeldahl digestion and colorimetric analysis. Microbial biomass increased linearly with soil OM concentration. Mineralizable N and plant production also increased with soil OM, but were greater with 7 than with 15 g kg−1 of organic C. Even though aboveground plant biomass was greater with either 7 or 15 than with 21 g kg−1 organic C, plant N concentrations were highest with 21 g kg−1. Soil OM concentrations were more closely related to microbial biomass than to mineralizable C and N, or to plant biomass and plant N concentrations. Nitrogen mineralization in the laboratory corresponded to plant N concentration in the field. Soil OM concentrations controlled microbial biomass C and N concentrations. However, additional factors also influenced the activity of the microbes and the resultant OM mineralization and plant N concentrations.