Plot-scale spatial variability of methane, respiration, and net nitrogen mineralization in muck-soil wetlands across a land use gradient

Abstract

Spatial patterns of soil microbial activity are central to understanding greenhouse gas dynamics in wetlands. Because agriculture reduces spatial heterogeneity in soil properties, we examined the hypothesis that soil microbial activity in natural wetlands would show a high degree of spatial autocorrelation when distances between samples were small; however, in wetlands that were disturbed by agriculture these spatial patterns would occur across larger sampling distances or disappear due to homogenization. We examined the hypothesis via methane (CH4) dynamics, respiration (as carbon dioxide (CO2) production), and net nitrogen (N) mineralization in muck soils from a natural wetland dominated by sedges (Carex lacustris) either with or without grey alder (Alnus rugosa). The disturbed wetland had corn production or restoration with either reed canary grass (Phalaris arundinacea) or purple loosestrife (Lythrum salicaria) growing for about 10years. Rates of anaerobic CH4 production ranged from 0.1 to 12,370nmolkg−1s−1 with the largest rates in the natural wetland with alder; the Reed Canary Grass site exhibited extremely slow rates of CH4 production. Oxic CH4 consumption occurred only in soils from the natural wetland, with no activity in soils from the Corn site and the two Restored sites. Rates of CO2 production ranged from −76 to 388nmolkg−1s−1 under anoxic conditions and from 0.1 to 402nmolkg−1s−1 under oxic conditions, with the highest rates in natural wetland surface soils. Despite broad spatial variation in rates across all sites, spatial patterns of CH4, respiration, and net N mineralization displayed limited spatial dependence, with autocorrelation at distances <8m only in the Purple Loosestrife site. The results suggest that large sample sizes are needed to characterize the high variability present in microbial activity and greenhouse gases in muck soils, but explanation(s) for spatial variability remain elusive.