Influence of natural and novel organic carbon sources on denitrification in forest, degraded urban, and restored streams

Abstract

Organic carbon is important in regulating ecosystem function, and its source and abundance may be altered by urbanization. We investigated shifts in organic carbon quantity and quality associated with urbanization and ecosystem restoration, and its potential effects on denitrification at the riparian–stream interface. Field measurements of streamwater chemistry, organic carbon characterization, and laboratory‐based denitrification experiments were completed at two forested, two restored, and two unrestored urban streams at the Baltimore Long‐Term Ecological Research site, Maryland, USA. Dissolved organic carbon (DOC) and nitrate loads increased with runoff according to a power‐law function that varied across sites. Stable isotopes and molar C:N ratios suggested that stream particulate organic matter (POM) was a mixture of periphyton, leaves, and grass that varied across site types. Stable‐isotope signatures and lipid biomarker analyses of sediments showed that terrestrial organic carbon sources in streams varied as a result of riparian vegetation. Laboratory experiments indicated that organic carbon amendments significantly increased rates of denitrification (35.1 ± 9.4 ng N·[g dry sediment]−1·h−1; mean ± SE) more than nitrate amendments (10.4 ± 4.0 ng N·[g dry sediment]−1·h−1) across streamflow conditions and sites. Denitrification experiments with naturally occurring carbon sources showed that denitrification was significantly higher with grass clippings from home lawns (1244 ± 331 ng N·g dry sediment−1·h−1), and overall unrestored urban sites showed significantly higher denitrification rates than restored and forest sites. We found that urbanization influences organic carbon sources and quality in streams, which can have substantial downstream impacts on ecosystem services such as denitrification.

Nutrient Retention in Restored Streams and Rivers: A Global Review and Synthesis

“…Plant dynamics included measurements of biomass, plant uptake, and nutrient utilization efficiency [64,65]. Stable isotope ratios were used to determine composition and microbial utilization of particulate organic material [65][66][67].…”

“…Coarse woody debris (CWD) treatments increased ammonium uptake velocity (V f ) by 23%-154% and uptake rate (U) by 61%-235% when compared to the control reaches [59]. As wood is characterized by higher C:N ratios than biofilms or microorganisms [100], its decomposition requires additional N sources, thereby increasing the nutrient demand of microbial decomposers [67]. Carbon limitation of N uptake typically occurs below DOC levels of 2 mg/L [101].…”

Urban Evolution: The Role of Water

“…More recently, stream restoration has been considered as an urban adaptation to influence ecosystem functions like the cycling of nutrients and organic matter instead of only focusing on structural attributes like habitat and biotic species composition [103,104]. There has been an increase in the number of studies that monitor restoration projects [20,50,[105][106][107][108].…”

Decomposition of tree leaf litter on pavement: implications for urban water quality

“…Although particulate matter derived from leaf inputs to streets may be removed by sweeping or trapped in retention basins, DOC losses from leaf litter leaching would be rapidly transported into storm drains, and, as for streams (Meyer et al 1998;McArthur and Richardson 2002;Slack and Feltz 1968), provide a pulsed input of labile organic matter for microbial metabolism. Leaf litter leaching of DOC also could help explain relatively higher concentrations of DOC observed in urban watersheds compared to nearby forested ones (Newcomer et al 2012;Petrone 2010) and in stormwater runoff, particularly during fall (unpublished data).…”