Browsing by Subject "Soil science"

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    Spatial and temporal pattern of nitrous oxide flux in an upland-bog watershed.
    (2011-08) Spence, Joan M.
    Problem Statement This study assessed the influence of landscape type and topographic position on N2O emissions and the factors that control it. Objective The objective was to explain how landscape, topography, vegetation and season influence the controls on N2O flux across an upland-bog watershed. Method Nitrous oxide flux was sampled on each of five upland hillslope positions and the hummock and hollow microtopographic positions in the lagg, lagg with alder (identified as alder) and bog landscape types during 2007 and 2008. Gravimetric water content and depth to water table were measured for each N2O flux sample at the time of sampling.Additional samples of water and soil were gathered by landscape type and upland hillslope position in June and September 2008 to assess soil and water properties. Additional soil samples by landscape type and upland hillslope position were tested for denitrification potential. The Generalized Estimating Equation was used to statistically analyze N2O flux, soil moisture content, depth to water table, and air and soil temperature. Univariate Analysis of Variance (ANOVA) and t-tests were used to statistically analyze soil for total carbon, total nitrogen, ammonia, nitrate, bulk density and pH. Univariate ANOVA and t-tests were used to analyze water for total carbon, total nitrogen, ammonia, nitrate and pH. Results For 2007 the alder landscape type had higher N2O flux than the bog landscape type; 7.52 ug N m-2 h-1 and 3.13 ug N m-2 h-1 respectively. During 2007, N2O flux in the alder hollow, 11.65 ug N m-2 h-1, was greater than the for lagg hummock, alder hummock and bog hummock and hollow. The lagg hollow; 5.27 ug N m-2 h-1 was greater than the bog hummock and hollow. Nitrous oxide flux for 2007 was higher than for 2008. Depth to water table and DOC were highest for 2007. DOC was greatest and pH was lowest for the bog. The average C:N for the upland was 20 with a positive relationship with denitrification potential. The average C:N for the peatland was 29 with a negative relationship with denitrification potential. There was a positive correlation between upland N2O flux and soil temperature for 2007 (r2=0.51) and 2008 (r2=0.22). There was a negative relationship between 2007 peatland N2O flux and DOC (r2=0.32) and a positive relationship between 2007 peatland N2O flux and dissolved NO3 - (r2=0.40). Denitrification potential for the upland, alder and lagg landscape types was limited by NO3 -. The alder landscape type appeared to have a more active microbial population than the lagg landscape type because only the alder landscape type had greater denitrification potential for the glucose+nitrate solution than the nitrate solution. Unlike previous studies, the denitrification potential for the bog landscape type was not greater with NO3 - solution. There was no difference in denitrification potential for June and September. Conclusions The N2O flux “hotspot” for the bog watershed was the alder hollow microtopographic position, which emitted the highest N2O flux during 2007. Denitrification potential was greater for the alder and glucose+nitrate solution than the nitrate solution; however soil carbon did not limit denitrification potential. Denitrification potential was limited by NO3 - for the upland. It was hypothesized that the toeslope would have the highest N2O flux for the upland. While the toeslope had higher soil total nitrogen and carbon than the other positions, N2O flux was not significantly greater. The factors controlling N2O flux in the peatland were not clear, N2O flux was highest for the alder hollow in 2007 but not 2008. Soil collected from the alder did not have significantly higher denitrification potential than the lagg. Denitrification potential and N2O flux for the bog were not always lower than the other landscape types pointing to the similar results from other studies indicating perhaps a different community of microbial nitrogen processors not necessarily smaller populations. Implications According to the IPCC 2007 assessment on global climate change; there is ample evidence that climate is warming over most of the earth and there is likely to be greater precipitation, higher incidence of storm events and earlier spring snowmelt. Increased temperatures and organic matter mineralization have the potential to increase N2O flux because those conditions support nitrification and denitrification. However, increased plant growth would increase competition among vegetation and microorganisms for NO3 - and NH4 + which would decrease substrates of N2O emissions. This means that N2O flux increases would likely be limited to the vegetative dormancy, the period of advantage for microbial processing. The conditions that cause earlier spring snow melt may not be the same conditions that could end vegetative dormancy, so this could result in a longer spring period when microbes have the advantage for NO3 - and NH4 + uptake and subsequent N2O emissions. Spring snowmelt is the time when water moves through upland soil bringing with it labile carbon, NO3 - and NH4 + to the toeslope and lagg. The water probably has sufficient carbon for active microbial communities and could result in early season N2O flux peaks. The hollow microtopographic positions in the lagg and alder landscapes were two places with the highest N2O flux. Increased temperatures may bring about increased mineralization and increase nitrogen processing rates. Increased precipitation may mean decreased depth to water table. These changes may increase N2O emissions in the hummocks while saturated conditions in the hollows may jointly limit nitrification which limits denitrification.