Potential nitrogen immobilization in grassland soils across a soil organicmatter gradient

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 st udies have explicitly tested the relationship between soil carbon content a nd nitrogen retention. We initiated a laboratory study to directly assess t he influence of soil organic matter content on potential nitrogen immobiliz ation and turnover for soils collected from across a temperature gradient i n the Great Plains region of the U.S. We measured soil organic carbon, tota l nitrogen and carbon-nitrogen ratios and estimated carbon mineralization a nd net nitrogen mineralization over 5- and 30-day laboratory incubations. W e used the N-15 pool dilution assay to estimate;gross nitrogen immobilizati on and nitrogen turnover for 5 day laboratory incubations. Soil organic car bon concentration and soil carbon-nitrogen ratios were negatively correlate d with mean annual temperature in a linear regression model that accounted for 46-56% of the variability, respectively. Regional patterns in soil orga nic carbon content and small scale variability in substrate availability im posed by discontinuous plant cover together strongly influenced potential n itrogen immobilization. Potential carbon mineralization and nitrogen immobi lization increased with increasing soil organic matter content. Soil organi c carbon content accounted for 58% of the variation in potential rates of N immobilization. A strong correlation between nitrogen immobilization and c arbon mineralization further suggests that rapid stabilization of nitrogen is facilitated by an active microbial community and the availability of a r eadily mineralizable organic substrate. (C) 2000 Elsevier Science Ltd. All rights reserved.