Microbial Biomass and Soil Structure Associated with Corn, Grasses, and Legumes

Maintaining long-term soil productivity requires development of cropping systems that provide maintenance or improvement in soil structure and an understanding of associated rhizosphere microbial populations. The objectives of this study were to determine the effects of several crops on soil biomass C and biomass N contents, their within-season variability, and the relationships between changes in soil biomass C, biomass N, and soil structure on a Brookston clay loam soil (fine-loamy, mixed, mesic Typic Argiaquoll). Soil microbial biomass C, biomass N, and soil-structure parameters (wet aggregate stability [WAS], organic carbon [OC], dry aggregate mean weight diameter [MWD], bulk density, and total and air-filled porosity) were measured during the third year of corn (Zea mays L.), soybean (Glycine max [L.], Merr.) alfalfa (Medicago sativa L.), red clover (Trifolium pratense L.), reed canarygrass (Phalaris arundinacea L.), orchardgrass (Dactylis glomerata L.), and no-crop (bare, covered, and shaded) plots at monthly intervals (June, July, August, and September). Reed canarygrass resulted in greater biomass-C contents than both the corn and soybean at all four sampling dates. Soil biomass C under alfalfa was significantly greater than under corn and soybean for both the first and last sampling dates. Forage species did not affect the soil biomass-N content. No consistent effects of the no-crop treatment on biomass C or biomass N were observed between sampling dates. Biomass C was significantly correlated with WAS, OC, and MWD for the cropping treatments. Wet aggregate stability was negatively correlated with soil water content for both the no-crop and cropping treatments, indicating that improvements in structure were not solely the result of the cropping treatments and associated rhizosphere populations. Reed canarygrass resulted in greater soil biomass C/N than the alfalfa, corn, and orchard grass plots, suggesting that fungal activity, and therefore soil structure, may be preferentially enhanced in the presence of reed canarygrass. This study demonstrated the influence of forage species and seasonal variability on concurrent changes in microbial biomass and soil structural properties.