Microbial biomass and activities in soil aggregates affected by winter cover crops

Winter cover crops may increase soil organic matter (SOM) and improve soil structure in intensively managed summer vegetable cropping systems. Our study examined the influence of three cover crop treatments (fallow, cereal, and legume within a summer crop rotation of sweet corn and broccoli) on microbiological properties associated with five soil aggregate-size classes (< 0.25, 0.25 to 0.5, 0.5 to 1.0, 1.0 to 2.0, and 2.0 to 5.0 mm) of a Willamette silt loam (Pachic Ultic Argixerolls). The distributions of total organic C (TOC), total Kjeldahl N (TKN), soil microbial biomass C (SMBC), readily mineralizable C and N, and two enzyme activities (beta-glucosidase and fluorescein diacetate [FDA] hydrolysis) were compared among aggregate-size classes of surface (0-20 cm) soils at seedbed preparation (June 1996) and after harvest of either broccoli (September 1995) or sweet corn (September 1996). Although cover cropping did not significantly influence aggregate-size distribution, TOC, nor TKN on any sampling date, it did significantly (P < 0.05) influence SMBC in September 1995, mineralizable C and N and FDA hydrolysis and beta-glucosidase activities in June 1996, and levels of mineralizable C and N and beta-glucosidase activity in September 1996. Significant (P < 0.01) interactions occurred among the cover crop treatments and aggregate-size classes for SMBC in September 1995, and among mineralizable C and N and FDA hydrolysis in June and September 1996. Mineralizable C and N and FDA hydrolysis differed (P < 0.01) among aggregate-size classes between June and September 1996; these changes were significantly (P < 0.01) affected by cover crop treatment. These studies identified focal points for further research on the effects of aggregate-size classes and cover cropping on the activities and dynamics of their respective microbial communities.