Periodic Reporting for period 1 - MarshFlux (The effect of future global climate and land-use change on greenhouse gas fluxes and microbial processes in salt marshes)
Reporting period: 2020-01-04 to 2022-01-03
Coastal wetlands are globally important ecosystems providing valuable ecosystem services, such as carbon sequestration over long timescales, affecting global carbon cycling and climate modulation. The amount of carbon sequestered, and therefore the net long-term global cooling potential of coastal marshes, however, is affected by complex biogeochemical reactions in marsh soils, which may produce and/or consume all three of the major greenhouse gases (GHGs) (carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O)). The magnitude and direction of these fluxes, and whether marsh soils act as a source or sink of GHGs, is affected by a variety of environmental factors which are predicted to vary with projected global change. MarshFlux, therefore, aims to address fundamental gaps in understanding of how the global cooling potential of coastal marshes will be affected by responses of biogeochemical reaction rates and GHG fluxes to global change. This research is critical for effective management of coastal wetlands to maintain their Blue Carbon value under future global change. This is particularly crucial as more countries look to natural climate solutions to offset their greenhouse gas emissions and there are increasing calls for blue carbon to be included in national greenhouse gas inventories.
The primary objective of MarshFlux is to determine the response of biogeochemical reaction rates and greenhouse gas (GHG) fluxes in salt marshes to a variety of environmental factors, which are expected to vary due to future global climate and land-use change. There are three research objectives o achieve this overall aim; 1) to determine the effect, both individually and combined, of changes in nutrient concentrations, salinity and temperature on GHG fluxes in soils from salt marshes along a climatic-gradient, 2) to determine the relative importance and strengths of the various pathways of N2O and CH4 production and consumption in marsh soils, under varying nutrient concentrations, salinity and temperature and 3) to develop indices of the magnitude and direction of GHG fluxes under projected future climate and land-use scenarios to allow more accurate prediction of future GHG emissions from salt marshes. These will be determined by measuring GHG fluxes from the entire marsh system, incorporating soil, water and vegetation, representative of in-situ conditions, under varying environmental stressors (nutrient concentrations, salinity, temperature) and vegetation-types.
The primary objective of MarshFlux is to determine the response of biogeochemical reaction rates and greenhouse gas (GHG) fluxes in salt marshes to a variety of environmental factors, which are expected to vary due to future global climate and land-use change. There are three research objectives o achieve this overall aim; 1) to determine the effect, both individually and combined, of changes in nutrient concentrations, salinity and temperature on GHG fluxes in soils from salt marshes along a climatic-gradient, 2) to determine the relative importance and strengths of the various pathways of N2O and CH4 production and consumption in marsh soils, under varying nutrient concentrations, salinity and temperature and 3) to develop indices of the magnitude and direction of GHG fluxes under projected future climate and land-use scenarios to allow more accurate prediction of future GHG emissions from salt marshes. These will be determined by measuring GHG fluxes from the entire marsh system, incorporating soil, water and vegetation, representative of in-situ conditions, under varying environmental stressors (nutrient concentrations, salinity, temperature) and vegetation-types.
Work so far has centred around two experiments, one in-situ and one laboratory incubation experiment, investigating the effect of global environmental change on salt marsh greenhouse gas fluxes and rates of nitrogen biogeochemistry to understand the climate cooling potential and nutrient filtering potential of salt marshes now and in the future. The in-situ experiment produced surprising results of high CH4 emissions, higher than most published values, from one elevation zone in the salt marsh. Salt marshes are typically expected to have low CH4 emissions and this zone was expected to have one of the lowest at this site due to higher salinity than any other zone. This may have large implications for climate feedbacks with sea level rise, where shifting elevation zones may reduce the climate cooling effect of the salt marsh. Additionally, this also suggests that salinity alone is not necessarily a good indicator of potential CH4 emissions, as we typically assume salt marshes to have low CH4 emissions because of their high salinity this could mean that the climate cooling effect of salt marshes is lower than previously thought.
The incubation experiments were conducted with soils from two vegetation types (Spartina alterniflora and Spartina patens) from two climatic regions (Quebec and Louisiana) resulting in four marsh sites at ambient and elevated treatments. The elevated treatments had increase temperature and nutrient loading to represent future global change. N2O fluxes were small and negative for all marsh sites but became large and positive under elevated treatments. CH4 fluxes showed more complicated responses between marsh sites and were higher from the Louisiana sites, where salinity was much lower. Denitrification-derived N2O fluxes under current conditions were all negligible but these emissions increased under elevated treatments with Quebec soils switching from more complete denitrification (N2 emission) under current conditions to more incomplete denitrification (N2O emission) under future conditions. Nitrification-derived N2O also increased from current to future treatments from the Spartina alterniflora sites. These results suggest that under future global change of increased temperature and nutrient loading, salt marshes will likely switch from minor sinks to sources of N2O, due to increased rates of nitrification and more incomplete denitrification. This has large implications for the future global cooling potential of salt marshes, especially as N2O is a greenhouse gas 298 times more potent than CO2, with increased N2O emissions offsetting some of the climate cooling feedback from carbon sequestration in salt marsh soils.
The incubation experiments were conducted with soils from two vegetation types (Spartina alterniflora and Spartina patens) from two climatic regions (Quebec and Louisiana) resulting in four marsh sites at ambient and elevated treatments. The elevated treatments had increase temperature and nutrient loading to represent future global change. N2O fluxes were small and negative for all marsh sites but became large and positive under elevated treatments. CH4 fluxes showed more complicated responses between marsh sites and were higher from the Louisiana sites, where salinity was much lower. Denitrification-derived N2O fluxes under current conditions were all negligible but these emissions increased under elevated treatments with Quebec soils switching from more complete denitrification (N2 emission) under current conditions to more incomplete denitrification (N2O emission) under future conditions. Nitrification-derived N2O also increased from current to future treatments from the Spartina alterniflora sites. These results suggest that under future global change of increased temperature and nutrient loading, salt marshes will likely switch from minor sinks to sources of N2O, due to increased rates of nitrification and more incomplete denitrification. This has large implications for the future global cooling potential of salt marshes, especially as N2O is a greenhouse gas 298 times more potent than CO2, with increased N2O emissions offsetting some of the climate cooling feedback from carbon sequestration in salt marsh soils.
Two manuscripts from the project are currently under review, with one more in preparation. Main results so far have been presented and discussed at multiple international conferences, local science/stakeholder meetings, departmental seminars and public outreach events. I expect that two or more manuscripts will be submitted this year for publication and results will be presented at 3 or more conferences. Further public engage work is also planned for example at university outreach events and school talks.