Potential nitrogen immobilization in grassland soils across a soil organic matter gradient

Abstract

Nitrogen additions to grasslands have increased historically and are likely to continue increasing given the current and projected land use patterns, urbanization and fossil fuel use. Nitrogen retention in both grassland and forest soils is often limited by organic substrate availability, but few studies have explicitly tested the relationship between soil carbon content and nitrogen retention. We initiated a laboratory study to directly assess the influence of soil organic matter content on potential nitrogen immobilization and turnover for soils collected from across a temperature gradient in the Great Plains region of the U.S. We measured soil organic carbon, total nitrogen and carbon–nitrogen ratios and estimated carbon mineralization and net nitrogen mineralization over 5- and 30-day laboratory incubations. We used the 15 N pool dilution assay to estimate gross nitrogen immobilization and nitrogen turnover for 5 day laboratory incubations. Soil organic carbon concentration and soil carbon–nitrogen ratios were negatively correlated with mean annual temperature in a linear regression model that accounted for 46–56% of the variability, respectively. Regional patterns in soil organic carbon content and small scale variability in substrate availability imposed by discontinuous plant cover together strongly influenced potential nitrogen immobilization. Potential carbon mineralization and nitrogen immobilization increased with increasing soil organic matter content. Soil organic carbon content accounted for 58% of the variation in potential rates of N immobilization. A strong correlation between nitrogen immobilization and carbon mineralization further suggests that rapid stabilization of nitrogen is facilitated by an active microbial community and the availability of a readily mineralizable organic substrate.