Soil Resources, Microbial Activity, and Primary Production Across an Agricultural Ecosystem

Abstract

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.

Spatial Variation in Soil Properties among North American Ecosystems and Guidelines for Sampling Designs

“…Similarly, Robertson et al [11] were surprised by the large variability in a wide range of soil properties from a soybean field that visibly appeared homogeneous, supporting the notion that agricultural ecosystems are typically more variable than many researchers appreciate.…”

Spatial Heterogeneity of Stream Water Nutrient Concentrations over Successional Time

“…Several different models can be used in semi-variogram analysis (Webster 1985); we chose to use spherical models because they rigorously define a sill and range (Fig. 2), which provide a quantitative measure of patch size, and because they are commonly used for soil nutrient semi-variograms (Webster 1985, Schlesinger et al 1996, Robertson et al 1997…”

“…Geostatistics have been used to describe variation in nutrient concentrations in agricultural fields (Robertson et al 1993(Robertson et al , 1997, desert shrubland and grassland (Schlesinger et al 1996), forests (Palmer 1990, Lechowicz and Bell 1991), and lakes (Harris 1986), but not in running waters. Streams are extremely heterogeneous systems, varying both temporally and spatially in many properties, including flow rate, temperature, light, substrate, and resource availability.…”

Effects of Added Organic Matter and Water on Soil Carbon Sequestration in an Arid Region

“…Microbial activity requires a certain soil water content range, and is limited by soil continually drying out [20], [21]. Contact between the microbes and the available substrate and the physiological performance of microbes are limited at low soil water contents [22]. Therefore, droughts could reduce the decrease in soil C by inhibiting microbial decomposition.…”