This study examines how the availability of soil nitrogen (N) regulates changes in plant productivity and species composition during old field succession on N-poor soils at CCNHA. First, a simulation model of soil N and carbon (C) cycling (NCSWAP) was used to examine changes in the mechanisms regulating N availability and primary production along a chronosequence that included 16-, 25-, and 50-year old fields. The model closely simulated observed successional changes in net N mineralization, aboveground net primary production, microbial biomass, and humads (the non-living fraction of soil organic matter actively involved in soil C and N turnover). Second, the hypothesis that spatial and temporal differences in N uptake facilitate species' coexistence in a N-limited, 50-year old field plant community was investigated. Differences among the six most abundant species were assessed by measuring uptake of 15N injected at two soil depths at each of three times during the growing season. For a detailed description of this experiment, see McKane's PhD thesis entitled "Regulation of Carbon and Nitrogen Cycling and Plant Community Structure During old field Succession" [1101].