Seasonal Shifts in the Soil Microbial Community and Nitrogen Cycling in Wetlands under Three Types of Vegetation in the New Jersey Pinelands.
Shen Yu and Joan Ehrenfeld. Rutgers, 14 College Farm Rd, New Brunswick, NJ 08901
Vegetation distribution in the Pinelands region of southern New Jersey varies with groundwater table gradient, controlled by the surficial Kirkwood-Cohansey aquifer. Drought caused by groundwater over-withdrawal is thought to affect ecological functions, including nitrogen cycling, in the pitch pine-dominated forest in the Pinelands. We hypothesize that droughts, including seasonal droughts, shift soil microbial community composition and accelerate organic nitrogen mineralization. To test the hypotheses, monthly soil sampling and in situ nitrogen mineralization was carried out for one year, from April 2005 to April 2006, in two replicate sites, each consisting of three continuous topographic-gradient wetlands covered with pitch pine (Pinus rigida), pitch pine-hardwood, and Atlantic white cedar (Chamaecyparis thyoides), respectively. The seasonal drought was detected in September 2005 when moistures of the sandy soils in pine and pine-hardwood wetlands were less than 10% (v/v). Soil microbial community composition determined by phospholipid fatty acid analysis (PLFA), clearly differs between peat soil in the cedar-swamps and sandy soils in the pine and pine-hardwood wetlands. Net soil nitrogen mineralization rates were consistently higher in the cedar-swamps than in the pine and pine-hardwood wetlands, but there were no differences in net nitrification rates among the three wetlands. No significant seasonal changes in net nitrogen mineralization and nitrification have been observed. Low net mineralization rates were detected in sandy soils (<100 mg N m-3 day-1) and peat soils (<500 mg N m-3 day-1), indicating these forest soils have a very slow organic nitrogen turnover. Since very low concentrations of inorganic nitrogen were detected in incubated soils (<5 g N m-3), microbial nitrogen immobilization may take place in these nitrogen-limited forest soils when plant roots are excluded. The microbial nitrogen immobilization is a possible reason diminishing the drought effect on nitrogen mineralization processes.