Biomedical subjects

Peter M Groffman

Publications and source records attributed to Peter M Groffman.

10 recordsLinked to original sources

PubMed2006

Chloride effects on nitrogen dynamics in forested and suburban stream debris dams.

Organic debris dams (accumulations of organic material) can function as "hotspots" of nitrogen (N) processing in streams. Suburban streams are often characterized by high flows that prevent the accumulation of organic debris and by elevated concentrations of solutes, especially nitrate (NO(3)(-)) and chloride (Cl(-)). In this study we (1) studied the effects of urbanization on the extent and characteristics of debris dams in large and small streams and (2) evaluated the effects of NO(3)(-) and Cl(-) on rates of N cycle processes in these debris dams. In some suburban streams debris dams were small and rare, but in others factors that reduce the effects of high stream flows fostered the maintenance of debris dams. Ambient denitrification enzyme activity (DEA) in these suburban and forested streams was positively correlated with stream NO(3)(-) concentrations. In laboratory microcosms, DEA in debris dam material from a forested reference stream was increased by NO(3)(-) additions. Chloride additions constrained the response of DEA to NO(3)(-) additions in material from the forested stream, but had no effect on DEA in material from streams with a history of high Cl(-) levels. Chloride additions changed the sign of net N mineralization from negative (consumption of inorganic N) to positive in debris dam material from the forested reference stream, but had no effect on net mineralization in material from streams with a history of exposure to Cl(-). Understanding the factors regulating the maintenance and N cycling activity of organic debris, and incorporating them into urban stream management plans could have important effects on N dynamics in suburban watersheds.

Chlorides↗

PubMed2006

A distinct urban biogeochemistry?

Most of the global human population lives in urban areas where biogeochemical cycles are controlled by complex interactions between society and the environment. Urban ecology is an emerging discipline that seeks to understand these interactions, and one of the grand challenges for urban ecologists is to develop models that encompass the myriad influences of people on biogeochemistry. We suggest here that existing models, developed primarily in unmanaged and agricultural ecosystems, work poorly in urban ecosystems because they do not include human biogeochemical controls such as impervious surface proliferation, engineered aqueous flow paths, landscaping choices, and human demographic trends. Incorporating these human controls into biogeochemical models will advance urban ecology and will require enhanced collaborations with engineers and social scientists.

Biochemical Phenomena↗

PubMed2006

Patterns of litter disappearance in a northern hardwood forest invaded by exotic earthworms.

A field study was conducted to evaluate the effects of exotic earthworm invasions on the rates of leaf litter disappearance in a northern hardwood forest in southcentral New York, USA. Specifically, we assessed whether differences in litter quality and the species composition of exotic earthworm communities affected leaf litter disappearance rates. Two forest sites with contrasting communities of exotic earthworms were selected, and disappearance rates of sugar maple and red oak litter were estimated in litter boxes in adjacent earthworm-free, transition, and earthworm-invaded plots within each site. After 540 days in the field, 1.7-3 times more litter remained in the reference plots than in the earthworm-invaded plots. In the earthworm-invaded plots, rates of disappearance of sugar maple litter were higher than for oak litter during the first year, but by the end of the experiment, the amount of sugar maple and oak litter remaining in the earthworm-invaded plots was identical within each site. The composition of the earthworm communities significantly affected the patterns of litter disappearance. In the site dominated by the anecic earthworm Lumbricus terrestris and the endogeic Aporrectodea tuberculata, the percentage of litter remaining after 540 days (approximately 17%) was significantly less than at the site dominated by L. rubellus and Octolasion tyrtaeum (approximately 27%). This difference may be attributed to the differences in feeding behavior of the two litter-feeding species: L. terrestris buries entire leaves in vertical burrows, whereas L. rubellus usually feeds on litter at the soil surface, leaving behind leaf petioles and veins. Our results showed that earthworms not only accelerate litter disappearance rates, but also may reduce the differences in decomposition rates that result from different litter qualities at later stages of decay. Similarly, our results indicate that earthworm effects on decomposition vary with earthworm community composition. Furthermore, because earthworm invasion can involve a predictable shift in community structure along invasion fronts or through time, the community dynamics of invasion are important in predicting the spatial and temporal effects of earthworm invasion on litter decomposition, especially at later stages of decay.

Animals↗

PubMed2006

Methods for measuring denitrification: diverse approaches to a difficult problem.

Denitrification, the reduction of the nitrogen (N) oxides, nitrate (NO3-) and nitrite (NO2-), to the gases nitric oxide (NO), nitrous oxide (N2O), and dinitrogen (N2), is important to primary production, water quality, and the chemistry and physics of the atmosphere at ecosystem, landscape, regional, and global scales. Unfortunately, this process is very difficult to measure, and existing methods are problematic for different reasons in different places at different times. In this paper, we review the major approaches that have been taken to measure denitrification in terrestrial and aquatic environments and discuss the strengths, weaknesses, and future prospects for the different methods. Methodological approaches covered include (1) acetylene-based methods, (2) 15N tracers, (3) direct N2 quantification, (4) N2:Ar ratio quantification, (5) mass balance approaches, (6) stoichiometric approaches, (7) methods based on stable isotopes, (8) in situ gradients with atmospheric environmental tracers, and (9) molecular approaches. Our review makes it clear that the prospects for improved quantification of denitrification vary greatly in different environments and at different scales. While current methodology allows for the production of accurate estimates of denitrification at scales relevant to water and air quality and ecosystem fertility questions in some systems (e.g., aquatic sediments, well-defined aquifers), methodology for other systems, especially upland terrestrial areas, still needs development. Comparison of mass balance and stoichiometric approaches that constrain estimates of denitrification at large scales with point measurements (made using multiple methods), in multiple systems, is likely to propel more improvement in denitrification methods over the next few years.

Acetylene↗

PubMed2005

Increased salinization of fresh water in the northeastern United States.

Chloride concentrations are increasing at a rate that threatens the availability of fresh water in the northeastern United States. Increases in roadways and deicer use are now salinizing fresh waters, degrading habitat for aquatic organisms, and impacting large supplies of drinking water for humans throughout the region. We observed chloride concentrations of up to 25% of the concentration of seawater in streams of Maryland, New York, and New Hampshire during winters, and chloride concentrations remaining up to 100 times greater than unimpacted forest streams during summers. Mean annual chloride concentration increased as a function of impervious surface and exceeded tolerance for freshwater life in suburban and urban watersheds. Our analysis shows that if salinity were to continue to increase at its present rate due to changes in impervious surface coverage and current management practices, many surface waters in the northeastern United States would not be potable for human consumption and would become toxic to freshwater life within the next century.

Chlorides↗

PubMed2003

Role of soil freezing events in interannual patterns of stream chemistry at the Hubbard Brook Experimental Forest, New Hampshire.

Soil freezing is a disturbance of the below ground environment, potentially resulting in increased losses of NO3- and surface water acidification. Here, we report the effects of soil freezing on interannual variation in stream chemistry at the Hubbard Brook Experimental Forest, New Hampshire. Data from 1970 to 1997 of soil frost depth, snow cover, precipitation, air temperature, and stream discharge and chemistry were used in a stepwise linear regression model to select the variables that best predicted deviations of annual stream concentrations from 4-year running averages. Variables quantifying soil freezing severity were selected as significant predictors of short-term fluctuations in stream K+, NO3-, Ca2+, and Mg2+ concentrations from 1970 to 1989, explaining 59 and 47% of the short-term variability in K+ and NO3-, respectively. Fine-root mortality and disturbance of root-soil-microbe interactions, with subsequent effects on decomposition and nutrient uptake, likely contributed to the mobilization of K+ and NO3- to streamwater following severe soil freezing events. The relationship between soil freezing and stream chemistry, however, weakened during the period 1990-1997. Because soil freezing has had inconsistent effects on stream chemistry during the period 1970-1997, it is unclear whether future changes in the frequency, duration, and depth of soil freezing events as the result of changes in the snow cover regime under a warmer climate will have significant impacts on the losses of NO3- and nutrient-base cations from temperate northern ecosystems.

Climate↗

PubMed2002

Soil nitrogen cycle processes in urban riparian zones.

Riparian zones have been found to function as "sinks" for nitrate (NO3-), the most common groundwater pollutant in the U. S., in many areas. The vast majority of riparian research, however, has focused on agricultural watersheds. There has been little analysis of riparian zones in urban watersheds, despite the fact that urban areas are important sources of NO3- to nitrogen (N)-sensitive coastal waters in many locations. In this study, we measured stream incision, water table depths, and pools, production (mineralization, nitrification), and consumption (denitrification) of NO3- in urban soils. Samples were taken from soil profiles (0-100 cm) of three forested urban and suburban zones and one forested reference riparian zone in the Baltimore, Maryland metropolitan area. Our objectives were to determine (1) if stream incision associated with urbanization results in lower riparian water tables, and (2) if pools, production, and consumption of NO3- vary systematically with stream incision and riparian water table levels. Two of the three urban and suburban streams were more incised and all three had lower water tables in their riparian zones than the forested reference stream. Urban and suburban riparian zones had higher NO3- pools and nitrification rates than the forested reference riparian zone, which was likely due to more aerobic soil profiles, lower levels of available soil carbon, and greater N enrichment in the urban and suburban sites. At all sites, denitrification potential decreased markedly with depth in the soil profile. Lower water tables in the urban and suburban riparian zones thus inhibit interaction of groundwater-borne NO3- with near surface soils that have the highest denitrification potential. These results suggest that urban hydrologic factors can increase the production and reduce the consumption of NO3- in riparian zones, reducing their ability to function as sinks for NO3- in the landscape.

Cities↗

PubMedDate not supplied

In situ push-pull method to determine ground water denitrification in riparian zones.

To quantify ground water denitrification in discrete locations of riparian aquifers, we modified and evaluated an in situ method based on conservative tracers and 15N-enriched nitrate. Ground water was "pushed" (i.e., injected) into a mini-piezometer and then "pulled" (i.e., extracted) from the same mini-piezometer after an incubation period. This push-pull method was applied in replicate mini-piezometers at two Rhode Island riparian sites, one fresh water and one brackish water. Conservative tracer pretests were conducted to determine incubation periods, ranging from 5 to 120 h, to optimize recovery of introduced plumes. For nitrate push-pull tests, we used two conservative tracers, sulfur hexafluoride and bromide, to provide insight into plume recovery. The two conservative tracers behaved similarly. The dosing solutions were amended with 15N-enriched nitrate that enabled us to quantify the mass of denitrification gases generated during the incubation period. The in situ push-pull method detected substantial denitrification rates at a site where we had previously observed high denitrification rates. At our brackish site, we found high rates of ground water denitrification in marsh locations and minimal denitrification in soils fringing the marsh. The push-pull method can provide useful insights into spatial and temporal patterns of denitrification in riparian zones. The method is robust and results are not seriously affected by dilution or degassing from ground water to soil air. In conjunction with measurements of ground water flow-paths, this method holds promise for evaluating the influence of site and management factors on the ground water nitrate removal capacity of riparian zones.

Ecosystem↗

PubMedDate not supplied

Denitrification potential in urban riparian zones.

Denitrification, the anaerobic microbial conversion of nitrate (NO3-) to nitrogen (N) gases, is an important process contributing to the ability of riparian zones to function as "sinks" for NO3- in watersheds. There has been little analysis of riparian zones in urban watersheds despite concerns about high NO3- concentrations in many urban streams. Vegetation and soils in urban ecosystems are often highly disturbed, and few studies have examined microbial processes like denitrification in these ecosystems. In this study, we measured denitrification potential and a suite of related microbial parameters (microbial biomass carbon [C] and N content, potential net N mineralization and nitrification, soil inorganic N pools) in four rural and four urban riparian zones in the Baltimore, MD metropolitan area. Two of the riparian zones were forested and two had herbaceous vegetation in each land use context. There were few differences between urban and rural and herbaceous and forest riparian zones, but variability was much higher in urban than rural sites. There were strong positive relationships between soil moisture and organic matter content and denitrification potential. Given the importance of surface runoff in urban watersheds, the high denitrification potential of the surface soils that we observed suggests that if surface runoff can be channeled through areas with high denitrification potential (e.g., stormwater detention basins with wetland vegetation), these areas could function as important NO3- sinks in urban watersheds.

Bacteria, Anaerobic↗

PubMedDate not supplied

Denitrification enzyme activity of fringe salt marshes in New England (USA).

Coastal salt marshes are a buffer between the uplands and adjacent coastal waters in New England (USA). With increasing N loads from developed watersheds, salt marshes could play an important role in the water quality maintenance of coastal waters. In this study we examined seasonal relationships between denitrification enzyme activity (DEA) in salt marshes of Narragansett Bay, Rhode Island, and watershed N loadings, land use, and terrestrial hydric soils. In a manipulative experiment, the effect of nutrient enrichment on DEA was examined in a saltmeadow cordgrass [Spartina patens (Aiton) Muhl.] marsh. In the high marsh, DEA significantly (p < 0.05) increased with watershed N loadings and decreased with the percent of hydric soils in a 200-m terrestrial buffer. In the low marsh, we found no significant relationships between DEA and watershed N loadings, residential land development, or terrestrial hydric soils. In the manipulation experiment, we measured increased DEA in N-amended treatments, but no effect in the P-amended treatments. The positive relationships between N loading and high marsh DEA support the hypothesis that salt marshes may be important buffers between the terrestrial landscape and estuaries, preventing the movement of land-derived N into coastal waters. The negative relationships between marsh DEA and the percent of hydric soils in the adjacent watershed illustrate the importance of natural buffers within the terrestrial landscape. Denitrification enzyme activity appears to be a useful index for comparing relative N exposure and the potential denitrification activity of coastal salt marshes.

Bacteria↗