Variability of heterotrophic metabolism in small stream corridors of an early successional watershed

Abstract

[1] Metabolic activity in stream corridors is regulated by a complex combination of factors that are difficult to disentangle in mature ecosystems. Chicken Creek in Germany, an experimentally created watershed in an early successional stage, offers the opportunity to assess the spatiotemporal variation in metabolic activity in a simplified system. We measured microbial respiration in soils and sediments along the hydrologic flow path from upland terrestrial to ephemeral to perennial sites of three stream corridors. Dry soils and sediments were rewetted before respiration measurements to mimic periods of activity during and after rainfall. Respiration rates and organic matter contents of soil and sediment were generally low. The presence of algae and accretion of vascular plant fragments in the perennial stream reaches increased respiration rates, pointing to the importance of particulate organic matter. Contrary to expectation, respiration rates of rewetted soil and sediment from dry stream channels were similar to rates measured with sediments collected in the perennial channel sections. This suggests that permanent water availability was not a main factor determining metabolic potential in the early successional Chicken Creek watershed. Carbon turnover in perennial channels was fourfold to eightfold higher than in ephemeral channels and terrestrial sites, as water was permanently available. However, this magnitude was insufficient for perennial channels to compensate for the large surface area of terrestrial soils: extrapolated to a year and the whole watershed, stream channels contributed only 5% to total carbon turnover, 95% being due to soils during and after rainfall events.

Disconnect of microbial structure and function: enzyme activities and bacterial communities in nascent stream corridors

“…This resemblance for all of the 10 enzymes studied is unexpected because sites, ranging from dry soil crusts to permanently submerged stream sediments, differed widely in terms of water availability, a key factor determining microbial metabolism. The unresponsiveness of potential enzyme activities to differences in site characteristics is in accordance with observations on general microbial metabolism measured as respiratory activity at various sites in a barren desert of Antarctica (McMurdo Valley) (Zeglin et al, 2009) and in the early successional Chicken Creek catchment (Gerull et al, 2011). This suggests, contrary to our hypotheses, that neither permanence of water availability (perennial versus ephemeral sites) nor direction of vertical water …”

“…However, in line with the lack of spatial variation in potential enzyme activities, organic matter content varied little in the Chicken Creek catchment, and although it was slightly higher at the terrestrial sites (Gerull et al, 2011), this difference was small and apparently insufficient to alter potential enzyme activities.…”

“…One such factor is light. Although neither particularly strong (r ¼ 0.24-0.33) nor necessarily indicative of cause-and-effect relationships, highly significant correlations between chlorophyll-a content in soil and sediments (data from Gerull et al, 2011) and potential activities of phosphatase and all carbon-acquiring hydrolytic enzymes might suggest that algae were instrumental in increasing potential enzyme activities in summer. This could be due directly to algal enzyme production, especially in the case of phosphatase.…”

“…It was deliberately created to study ecosystem succession. Average annual precipitation and air temperature are 559 mm and 9. were taken at each site (Gerull et al, 2011) and immediately preserved. Approximately 3 g of soil or sediment was preserved in 10 ml of 2% formalin containing 0.1% pyrophosphate and stored at 4 1C for determination of bacterial abundance.…”

“…As a result, effects of community change on ecosystem functions can be easily masked. However, this masking effect could be alleviated by taking advantage of the physically and biologically simple structure of nascent ecosystems formed by volcanism (Vitousek, 2004), glacier retreat (Milner et al, 2007;Brankatschk et al, 2011) or humans (Gerwin et al, 2009;Gerull et al, 2011), where species numbers are reduced but artifacts caused by greatly simplified laboratory systems are avoided.…”