Monday, November 13, 2006 - 2:15 PM

Surface N2O Fluxes and Microbial Process Rates of Planted and Mulched Soils.

Mary-Ann Bruns, Pennsylvania State University, Dept. of Crop & Soil Sci., 116 ASI Bldg., ., University Park, PA 16802, Loren B. Byrne, The College of Wooster, Dept. of Biology, 931 College Mall, Wooster, OH 44691, and Matthew T. McCoy, Dept. of Crop & Soil Sciences, The Pennsylvania State Univ, 116 Ag. Sci. & Ind., University Park, PA 16802.

Soil biogeochemical responses to common management treatments in urbanized areas have received little attention but can be readily demonstrated when practices like mowing and mulching alter the balance between plant and microbial competition for water and substrates. Inorganic N pools and net N process rates of unfertilized soils during the 2004 and 2005 growing seasons were measured in a randomized complete block experiment at Penn State’s Agronomy Research Farm in Central Pennsylvania, on shallow, well-drained lithic Hapludalfs (silt loams). Treatments of shredded bark mulch, limestone gravel mulch, and mowed lawn, with reference unmowed vegetation, were established in 2003 in an area that had been an annually mown old field for at least 25 years. In 2004 inorganic N pools in all treatments were high (>20 µg N g-1 soil) but were highest in bark mulch soils (consistently >40 µg N g-1 soil). For both types of mulched soils that year, potential denitrification rates measured in the laboratory after C2H2 flush were higher than for planted soils (respective ranges of 90-240 and -10-120 µg N2O-N m-2h-1). However, only gravel mulch soils exhibited higher surface N2O fluxes in the field (up to 1.3 µg N2O-N m-2h-1), while all other fluxes were low (<0.05 µg N2O-N m-2h-1). In 2005 both types of mulched soils had higher water-filled pore space than planted soils, but only gravel-mulched soils contained higher NO3--N pools than other treatments. Despite this, potential denitrification rates and field N2O fluxes from gravel-mulched plots were higher on only one out of five sampling dates in 2005. Factors that appeared to contribute to increased N2O fluxes from gravel-mulched soils were higher soil temperature, higher pH, and reduced activity of heterotrophic microbes. Microbial process-controlled soils beneath anthropogenic covers are widespread in urbanized areas, and they may be significant unrecognized sources of N2O emissions.