Microbial biomass and activity in geomorphic features in forested and urban restored and degraded streams

Abstract

Geomorphic spatial heterogeneity affects sediment denitrification, an anaerobic microbial process that results in the loss of nitrogen (N), and other anaerobic microbial processes such as methanogenesis in urban streams. We measured sediment denitrification potential (DEA), net nitrification, methanogenesis, and a suite of ancillary microbial variables in geomorphic stream features (organic debris dams, pools, riffles, and sloughs) in forested, urban degraded and urban restored streams in the Baltimore, MD, USA metropolitan area, between June 2005 and November 2006. DEA was higher in organic debris dams (2783 ± 1405 ng N g −1 h −1, mean ± standard error) than in pools (505 ± 144), riffles (360 ± 78), and sloughs (270 ± 106), and was higher in forest (1439 ± 613) than in urban degraded (442 ± 98) and restored sites (391 ± 116), but the differences were not statistically significant. DEA was positively related to microbial biomass N (MBN) ( p < 0.0001) and percent sediment organic matter (SOM) ( p = 0.006). DEA and MBN were significantly higher in June 2005 and August 2006 than in November 2006, a temporal pattern that may have been driven by changes in microbial biomass. Methanogenesis was active in all stream geomorphic features across all study sites. Reach scale estimates of nitrification ranged from 157 to 344 mg N m −2 d −1 and were similar to reach scale DEA rates (97–230 mg N m −2 d −1), with no significant differences between restored and unrestored reaches. These results suggest that in-stream geomorphic features in urban restored and degraded sites have the potential to function as N sinks by maintaining anaerobic conditions and microbial biomass and activity that stimulate denitrification.