The impacts of nutrient loading on greenhouse gas exchange in floodplain fens.

Abstract

This research quantified the impact of nutrient loading on GHG exchange within two English floodplain fens. GHG exchange was quantified in two sites of differing nutrient status under conservation management between June 2012 and September 2013 using closed static chambers. Annual reconstructed CH4 emission (diffuse and plant-mediated) was similar between sites (18 ± 2.6 and 15 ± 1.7 g CH4 m-2 yr-1 for nutrient-poor and nutrient-rich sites, respectively), whilst ecosystem respiration (2725 ± 10 and 3479 ± 154 g CO2 m-2 yr-1, respectively) and gross primary productivity (2814 ± 103 and 5039 ± 564 g CO2 m-2 yr-1, respectively) were significantly greater in the nutrient enriched site, translating into a significant difference in NEE (-90 ± 139 and -1560 ± 418 g CO2 m-2 yr-1, respectively). This difference was caused by a greater aboveground biomass at the nutrient enriched site sequestering more CO2. PO43- and NO3- were shown to be significant controlling factors on CH4 emission and CO2 exchange. Ebullition was shown to be an important transport mechanism for CH4 within floodplain fens; fluxes were within a similar order of magnitude to diffusive and plant-mediated fluxes, which has not previously been demonstrated. Additionally, shorter sampling periods (< 48 hours) result in more reliable ebullition estimates than > 48 hours due to less oxidation occurring within the funnel traps and should be used in highly productive environment such as floodplain fens. An ex situ short-term (< 15 days) laboratory fertilisation study under anaerobic conditions showed significant increase in potential methanogenesis with PO43- additions but suppression with NO3- additions. Anaerobic CH4 oxidation was observed in nutrient-poor peat after 144 hours, with the greatest oxidation rates in PO43- fertilised samples. Potential N2O production via denitrification only occurred in samples fertilised with NO3- and no difference was observed in fermentation between treatments.