Denitrification potential in riparian zones and streams

Denitrification potential in riparian zones and streams Denitrification potential and a series of ancillary variables (inorganic nitrogen concentrations, moisture content, organic matter content, microbial biomass carbon and nitrogen content, potential net nitrogen mineralization and nitrification, microbial respiration, root biomass) has been measured in riparian zone soils and stream geomorphic features by a series of undergraduate and graduate student researchers as part of the Baltimore Ecosystem Study since the early 2000s. These studies often center on the series of sites where there has been long-term monitoring (since 2000) of riparian water tables and groundwater chemistry along four first or second order steams in and around the Gwynns Falls watershed in Baltimore City and County, MD (BES_0455/32; http://beslter.org/metacat_harvest_attribute_level_eml/html_metadata/bes_455.asp). One site is in the completely forested Pond Branch catchment that serves as a "reference" study area for the Baltimore LTER (BES). Two sites (Glyndon, Gwynbrook) are in suburban areas of the watershed; one just upstream from the Glyndon BES long-term stream monitoring site in the headwaters of the Gwynns Falls, and one along a tributary that enters the Gwynns Falls just above the Gwynnbrook BES long-term stream monitoring site farther downstream. The final, urban site (Cahill) is along a tributary to the Gwynns Falls in Leakin Park in the urban core of the watershed. Other sites were used in different studies as described in the publications associated with each study. The different studies also varied in just which ancillary variables were measured. Samples were stored at 4o C between sampling and analysis (from less than 1 week to up to three weeks). Samples were manually homogenized: all large rocks, roots, and other non-decomposed organic material were removed, and samples were thoroughly mixed. All samples were held at field moisture before analysis. Soil moisture content was determined by drying at 60 oC for 24 hours. Organic content was determined by "loss on ignition;" 450 oC for four hours. Roots were removed by hand from soil samples (a 15-minute time limit for root picking was established), rinsed twice (once by dipping into deionized water and once by shaking in deionized water), dried at 60_C for 24 hours, and weighed. Denitrification enzyme activity was measured using a short-term anaerobic assay. Sieved soils were amended with NO3- (100 mg N kg-1), dextrose or glucose (40 mg kg-1), chloramphenicol (10 mg kg-1) and acetylene (10 kPa) and were incubated under anaerobic conditions for 90 minutes. Gas samples were taken at 30 and 90 minutes, stored in evacuated glass tubes and analyzed for N2O by electron capture gas chromatography. Microbial biomass C and N content was measured using the chloroform fumigation-incubation method. Soils were fumigated to kill and lyse microbial cells in the sample. The fumigated sample was inoculated with fresh soil and sealed in a jar, and microorganisms from the fresh soil grew vigorously using the killed cells as substrate. The flushes of carbon dioxide (CO2) and 2 M KCl extractable inorganic N (NH4+ and NO3-) released by the actively growing cells during a 10-day incubation at field moisture content were assumed to be directly proportional to the amount of C and N in the microbial biomass of the original sample. A proportionality constant (0.41) was used to calculate biomass C from the CO2 flush in the fumigated samples. Biomass N is the total inorganic N flush in the fumigated samples. Inorganic N and CO2 production were also measured in "control" samples. Control samples were prepared in the same fashion as those listed above, but were not fumigated. These incubations provided estimates of microbial respiration and potential net N mineralization and nitrification. Microbial respiration was quantified from the amount of CO2 evolved over the 10-day incubation. Potential net N mineralization and nitrification were quantified from the accumulation of NH4+ plus NO3- and NO3- alone during the 10-day incubation. We measured 2 M KCl extractable inorganic N in the fresh soil samples to determine the initial soil NO3- and NH4+ concentrations. Carbon dioxide was measured by thermal conductivity gas chromatography. Inorganic N was measured colorometerically using an autoanalyzer. Column A Sampling date B Site name and description C Lead student researcher D Publication E Habiat F Land use context G Latitude (approximate) H Longitude (approximate) I Site code J Sampling depth K Replicate plot L Denitrification enzyme activity (ng/g/hr) M KCl extractable nitrate (mg N/kg) N KCL extractable ammonium (mg N/kg) O KCL extractable inorganic N (nitrate plus ammonium ) (mg N/kg) P Microbial respiration (mg C/kg/d) Q Potential net N mineralization (mg N/kg/d) R Potential net nitrification (mg N/kg/d) S Soil organic matter (g/g) T Moisture content (g/g) U Microbial biomass C (mg C/kg) V Microbial biomass N (mg N/kg) W Root biomass (g/g) These data have been published in the following papers: Bettez, N. D. and P. M. Groffman. 2012. Denitrification potential in stormwater control structures and natural riparian zones in an urban landscape. Environmental Science and Technology 46:10909 - 10917. Gift, D. M., P. M. Groffman, S. S. Kaushal, and P. M. Mayer. 2010. Denitrification potential, root biomass, and organic matter in degraded and restored urban riparian zones. Restoration Ecology 18:113-120. Groffman, P. M., N. J. Boulware, W. C. Zipperer, R. V. Pouyat, L. E. Band, and M. F. Colosimo. 2002. Soil nitrogen cycle processes in urban riparian zones. Environmental Science and Technology 36:4547-4552. Groffman, P. M. and M. K. Crawford. 2003. Denitrification potential in urban riparian zones. Journal of Environmental Quality 32:1144-1149. Groffman, P. M., A. M. Dorsey, and P. M. Mayer. 2005. N processing within geomorphic structures in urban streams. Pages 613-625. Hale, R. L. and P. M. Groffman. 2006. Chloride effects on nitrogen dynamics in forested and suburban stream debris dams. Journal of Environmental Quality 35:2425-2432. Harrison, M. D., P. M. Groffman, P. M. Mayer, and S. S. Kaushal. 2012. Microbial biomass and activity in geomorphic features in forested and urban restored and degraded streams. Ecological Engineering 38:1-10. Waters, E. R., J. L. Morse, N. D. Bettez, and P. M. Groffman. 2014. Differential carbon and nitrogen controls of denitrification in riparian zones and streams along an urban to exurban gradient. Journal of Environmental Quality 43:955-963.