Vegetation and soils were sampled in 22 old fields ranging in age from 1 to 56 yr since abandonment. Soil nitrogen concentration increased significantly with field age. Vegetation cover, total aboveground plant biomass, and litter cover increased significantly with soil nitrogen. Light penetration to the soil surface was negatively correlated with total plant biomass. Field age and soil nitrogen concentration were used as independent variables in simple regression and partial correlation analyses to determine the relative importance of such time—dependent processes as dispersals vs. the availability of a limiting resource (nitrogen) as predictors of patterns in species richness or the abundance of various plant groups. Species richness per field and within—field heterogeneity in species composition increased with field age. Local species richness decreased with increasing soil nitrogen. Cover of annuals and introduced species decreased with field age and nitrogen; however, annuals contributed an important part of total vegetative cover even n 25—yr—old fields. Cover of perennials and woody species increased with soil nitrogen and field age. Although the fields were bordered by woods, woody species contributed @<15% cover even in the oldest fields. For several plant groups the relationship between cover and soil nitrogen within individual fields was the opposite of that among all fields. These patterns suggest that while soil nitrogen is an important determinant of local species composition and abundance, dispersal and colonization, which are dependent on field age, determine which species are present in a field.

Measurements of carbon, nitrogen, and phosphorus content were carried out in the soils of a hillslope of shortgrass steppe. Plant biomass, soil morphology, and soil physical properties were also measured. Soil morphology indicated that the site had undergone several cycles of rapid erosion and deposition. Total mass of C, N, and P increased downslope, following a trend in soil depth, but the summit A horizon had higher C, N, and organic P concentrations than the backslope, reflecting a higher clay content. Laboratory and field incubations showed that N availability increased downslope, while relative N mineralization (N mineralized: total N) decreased. Organic matter content and mineralization rate were closely coupled to physical properties of the soil, which reflect the geomorphic history of the site.

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Nitrogen availability and its response to fertilizer amendments was measured by in situ incubation in four old fields ranging in age from 16 to >100 years at Cedar Creek Natural History Area. Net nitrogen mineralization in control plots increased with field age, from 4.4 g/m in the youngest field to 6.5 g/m in the oldest field. The proportion of total N mineralized decreased with field age, from 6.2% of total N mineralized in the youngest field to 4.8% mineralized in the oldest field, suggesting a decrease in organic matter quality with time. Unlike many forests in the region, nitrogen mineralization was correlated with total soil nitrogen content. Greater than 90% of the mineralized N was nitrified in most months. Analyses of variance indicate significant effects of field age and month of year on N mineralization and nitrification, but not effect of fertilizer treatment except in the oldest field. Fertilizer additions did not significantly increase standing pools of mineral N in the youngest or oldest fields but did in the 26 and 50 year old fields. However, changes in mineral N pools did not account for the amount added in fertilizer. Strong plant and microbial sinks for fertilizer and possibly leaching losses may be the reasons why fertilizer additions did not stimulate N mineralizations during the first two years in most fields.

We studied the decay and nitrogen dynamics of little bluestem (Schizachyrium scoparium) litter in fertilized and unfertilized Minnesota old fields, using the litterbag technique. Annual decay rates and nitrogen leaching losses during the first month of decay were highly correlated with N content of litter and not with fertilizer additions. After the first month of decay, nitrogen was immobilized for at least 18 months. In contrast to decay rates and early N leaching losses, these immobilization rates were correlated with the amount of ammonium nitrate added in fertilizer rather than with initial %N. Therefore, litter quality and exogenous nitrogen supply appear to have different and independent effects on decay rates and N dynamics of little bluestem litter.