Distribution of soil organic carbon and nitrogen fractions, enzyme activities and microbial diversity in temperate alley cropping systems

Environmental concerns over the effects of global warming and the long-term sustainability of conventional agricultural production has stimulated the search for alternative agroecosystems, such as alley cropping, which may lead to greater carbon sequestration and improved soil fertility. Differences in litter composition between crop and tree litter, and microclimate modifications in alley cropping systems may introduce spatial variation in soil organic carbon (SOC) and nitrogen (N) fractions and soil microbial properties. The objectives of this study were: (1) to assess the effects of litter quality, soil properties and microclimate differences on soil C and N mineralization in established alley cropping systems; (2) to determine the spatial variability of soil C and N fractions in these systems; and (3) to evaluate the effects of these systems on spatial differences in soil microbial activity and functional diversity. To address the first objective, microcosm and litterbag experiments were conducted in 2001 and 2002, respectively. Bulk soils were collected from the 0--20 cm depth at three sites: a 21-yr old pecan (Carya illinoinensis )/bluegrass (Poa trivials) intercrop (Pecan site) in north-central Missouri, a 12-yr old silver maple (Acer saccharinum )/soybean (Glycine max) - maize (Zea mays ) rotation (Maple site) in northeastern Missouri and a warm season grass restored prairie (MDC site) in southwestern Missouri. Seven litters with varying composition were collected including pecan, silver maple, chestnut and walnut leaf litter (tree litter) and maize, soybean and bluegrass hay residues (crop litter). Aerobic microcosm incubations were maintained at 25°C and a soil water potential of -47 kPa. Crop litter had a narrow C: N ratio (<25), low soluble polyphenols (<4%) and lignin (<15%) contents compared to tree litter. Carbon mineralization was highly correlated to litter quality indices, except soluble polyphenols, while the relationship between N dynamics and litter quality indices was less clear. To achieve the second objective, soils were collected to a depth of 30 cm from the Pecan and Maple sites, at the tree row and at the middle of the alley at each site, in fall 2001 and summer 2002. Soil total organic C and N and C and N fractions did not differ consistently with distance from the tree row and between years. Field moist soil sampled at the same time as those used for assessment of soil C and N fractions was used to evaluate spatial patterns in soil enzymes and microbial functional diversity using Biolog at the Pecan and Maple sites. Higher enzyme activities and functionally different soil microbial populations were present under tree canopies compared to cropped alleys. Microclimate (i.e. soil temperature and water content) effects in alley cropping systems varied between sites, possibly because of the differences in tree inter- and intra row spacing and orientation.