The spatial distributions of selected soil properties in two adjacent sites in southwest Michigan were examined to evaluate the potential effects of chronic disturbance on resource heterogeneity. One site was a cultivated field that had been cleared, plowed, and cropped annually for decades prior to sampling while the other, uncultivated field was cleared of original forest in 1960 after which it was mown annually but never plowed or cropped. We took replicate samples from a 330-point unaligned grid across the sites for soil pH, gravimetric moisture, inorganic phosphorus, total carbon, and net nitrification and nitrogen mineralization potentials. Soils in the cultivated site contained less than half as much carbon as in the uncultivated site, but had higher levels of inorganic phosphorus and moisture, and higher soil pH. Potential net nitrogen mineralization and nitrification rates did not differ between sites. Geostatistical analysis showed that almost all properties examined were strongly autocorelated within each site; structural variance as a proportion of sample variance ranged from 30-95% for all properties, and for any given property differed little between sites. The distance over which this dependence was expressed, however, was for all properties but pH substantially less in the uncultivated site (7-26 m) as compared to the tilled site (48-108m), especially for total C and net nitrification and N mineralization. These results suggest that the spatial pattern and scale of soil variability can differ markedly among edaphically identical sites and that these differences can be related to disturbance history.

The degree to which soil resource availability is linked to patterns of microbial activity and plant productivity within ecosystems has important consequences for our understanding of how ecosystems are structured and for the management of systems for agricultural production. We studied this linkage in a 48-ha site in southwest Michigan, USA, that had been cultivated and planted to row crops for decades. Prior to seeding the site to genetically identical soybean plants (Glycine max) in early spring, we removed soil samples from 600 locations; plant biomass was harvested from these same locations later in the season. Soil samples were analyzed for physical properties (texture, bulk density), chemical properties (moisture, pH, total C, total N, inorganic N), and biological attributes (microbial biomass, microbial population size, respiration potential, and nitrification and N-mineralization potentials). Plant analyses included biomass and C and N contents. Soil resource variability across this long-cultivated site was remarkably high, as was variability in microbial activity and primary productivity. In almost all cases variability exhibited a strong spatially explicit structure: for most properties and processes 50% of sample variance was spatially dependent at a scale of 5-60 m. Exceptions included microtopog-raphy, soil pH, and inorganic P, which were spatially dependent across the entire 1-1200 m range of separation distances examined in this study, and the culturable-bacteria population , which was not spatially autocorrelated at any scale examined. Both topographic relief and soil pH exhibited strongly nested structures, with autocorrelation occurring within two (topography) or more (pH) distinct ranges. Multiple regression analysis showed surprisingly little correlation between biological processes (soybean productivity, soil N turnover , soil respiration), and static soil properties. The best predictor of soybean biomass at late reproductive stages (r 2 0.42) was a combination of nitrate N, bulk density, inorganic P, N-mineralization rates, and pH. Overall, results suggest a remarkable degree of spatial variability for a pedogenically homogeneous site that has been plowed and cropped mostly as a single field for 100 yr. Such variability is likely to be generic to most ecosystems and should be carefully evaluated when making inferences about ecological relationships in these systems and when considering alternative sampling and management strategies.

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Research on nitrate-nitrogen (NO 3 -N) leaching in turfgrass indicates that, in most cases, leaching poses little risk to the environment. Most of the research was conducted on sites that were recently established, and the potential for greater NO 3 -N leaching from mature turf sites is unknown. The fate of nitrogen (N) was examined for a 10-yr-old Kentucky bluegrass (Poa pratensis L.) turf using intact monolith lysimeters and microplots. From October 2000 through July 2002, half of the lysimeters and microplots were treated annually with urea at a high rate of 245 kg N ha 21 (49 kg N ha 21 application 21 ). The remaining lysimeters and microplots were treated annually with urea at a low rate of 98 kg N ha 21 (24.5 kg N ha 21 application 21 ). The Oct. 2000 urea application was made with 15 N double-labeled urea to facilitate fertilizer identification among clippings, verdure, thatch, soil, roots, and leachate. The average total recovery of applied labeled fertilizer nitrogen (LFN) for the low and high N rates was 78 and 74%, respectively. NO 3 -N concentrations in leachate for the low N rate were typically below 5 mg L 21 . For the high N rate, NO 3 -N concentrations in leachate were often greater than 20 mg L 21 . Over approximately 2 yr, 1 and 11% of LFN was recovered in leachate for the low and high N rates, respectively. This research indicates that single dose, high rate, water soluble N applications (49 kg N ha 21 application 21 ) to mature turfgrass stands should be avoided to minimize the potential for NO 3 -N leaching.

Abstract. The degree to which soil resource availability is linked to patterns of microbial activity and plant productivity within ecosystems has important consequences for our understanding of how ecosystems are structured and for the management of systems for agricultural production. We studied this linkage in a 48-ha site in southwest Michigan, USA, that had been cultivated and planted to row crops for decades. Prior to seeding the site to genetically identical soybean plants (Glycine max) in early spring, we removed soil samples from ഠ600 locations; plant biomass was harvested from these same locations later in the season. Soil samples were analyzed for physical properties (texture, bulk density), chemical properties (moisture, pH, total C, total N, inorganic N), and biological attributes (microbial biomass, microbial population size, respiration potential, and nitrification and N-mineralization potentials). Plant analyses included biomass and C and N contents. Soil resource variability across this long-cultivated site was remarkably high, as was variability in microbial activity and primary productivity. In almost all cases variability exhibited a strong spatially explicit structure: for most properties and processes Ͼ50% of sample variance was spatially dependent at a scale of 5-60 m. Exceptions included microtopography, soil pH, and inorganic P, which were spatially dependent across the entire 1-1200 m range of separation distances examined in this study, and the culturable-bacteria population, which was not spatially autocorrelated at any scale examined. Both topographic relief and soil pH exhibited strongly nested structures, with autocorrelation occurring within two (topography) or more (pH) distinct ranges. Multiple regression analysis showed surprisingly little correlation between biological processes (soybean productivity, soil N turnover, soil respiration), and static soil properties. The best predictor of soybean biomass at late reproductive stages (r 2 ϭ 0.42) was a combination of nitrate N, bulk density, inorganic P, N-mineralization rates, and pH. Overall, results suggest a remarkable degree of spatial variability for a pedogenically homogeneous site that has been plowed and cropped mostly as a single field for Ͼ100 yr. Such variability is likely to be generic to most ecosystems and should be carefully evaluated when making inferences about ecological relationships in these systems and when considering alternative sampling and management strategies.