Soil microbial properties and nitrogen cycling in forage production systems receiving swine lagoon effluent.

摘要

Land application of swine lagoon effluent (SLE) is widespread in the southeastern USA. As a surrogate of synthetic N fertilizer, SLE is applied to forage crops primarily based on requirement of plant yield and of dry matter quality such as protein and fiber contents. Although soil microorganisms play a central role in soil N turnover, retention and therefore the environmental fate of soil and fertilizer N, the response of soil microbial communities to SLE application has received little attention. In this study, soil microbial properties in forage production systems receiving swine lagoon effluent were examined at both organismal and process levels. The specific objectives of this research were to (1) determine microbial community structure and catabolic function; (2) assess soil enzyme activities involved in C, N and P cycling; (3) quantify soil N mineralization, immobilization and nitrification; and (4) link the changes in microbial community with soil properties as a consequence of contrasting N fertilization (SLE versus a synthetic N fertilizer) or of contrasting forage management practices (hay production versus cattle grazing). A holistic delineation may improve our understanding of microbial ecology in managed forage production systems and accordingly facilitate best management practices, in particular N fertilization.; Soil microbial biomass, activity and rates of N transformation were heterogeneous in a grazed pasture, due mainly to non-uniform distribution of animal excreta. While soil microbial biomass, respiration activity and net N mineralization were positively correlated with the concentration of animal excreta and associated changes in soil C and N (P 0.05, Pearson correlation coefficient r ≈ 0.70), gross N mineralization and nitrification potential were not. This difference indicates that soil microbial community and its mediated processes may not respond linearly to N and C availability. Soil microbial community structure and functions were therefore examined in a hay production system supplied either SLE or ammonium nitrate (AN) at application rates of 0, 200, 400 or 600 kg plant available N ha-1 yr -1. Microbial biomass, respiration activity and net N mineralization peaked at the application rate of 200 or 400 kg available N ha-1 yr-1. Activities of several hydrolytic enzymes involved in C and N cycling were also highest at the application rate of 200 or 400 kg available N ha-1 yr-1. However, oxidative soil enzymes including phenol oxidase and peroxidase activity were positively correlated with the application rates of SLE (P 0.05), but not AN, suggesting that SLE and AN differed in their influence on soil microbial community or its mediated processes. Indeed, fingerprinting of phospholipid fatty acids and community-level physiology showed that microbial community composition differed between soils fertilized with SLE versus AN. The differences in microbial community and its mediated processes were highly correlated with soil pH. This research implies that as a consequence of changes in soil properties associated with a high application rate of animal waste, soil microbial community population size and community structure would adjust accordingly. However, this adjustment may not benefit soil fertility in the long run.