Elevated nutrient inputs to marshes differentially impact carbon and nitrogen cycling in two northern Gulf of Mexico saltmarsh plants

Peatlands have accumulated vast quantities of organic carbon over thousands of years but it is unclear how these sensitive ecosystems will respond to future climate change. If emissions of methane from peatlands increase, then they may contribute increasingly towards climatic warming due to the higher greenhouse warming potential of this gas. We investigated the radiocarbon concentration of methane emissions from a temperate bog over 1.5 years, which we supported with measurements of the surface flux of methane and carbon dioxide. The radiocarbon content of methane emissions varied greatly, from modern (i.e. fixed from the atmosphere within recent decades) to ~ 1400 years BP. Flux rates of methane were spatially and temporally highly variable. A vegetation clipping experiment showed that plants had a great influence on the carbon isotope composition and flux of methane emitted from the peat surface, consistent with earlier studies showing the key role of plants in peatland methane emissions. When plants were absent, emission rates were 70–94% lower and the radiocarbon age of methane emissions was much younger and less variable. Our radiocarbon measurements show that at this peatland, plant-associated methane emissions contain carbon originally fixed from the atmosphere up to hundreds of years earlier, consistent with a contribution from plant mediated transport of methane sourced from sub-surface layers.

     

Chronic Nitrogen Enrichment at the Watershed Scale Does Not Enhance Microbial Phosphorus Limitation

Increased N inputs through chronic atmospheric deposition has enriched temperate forest ecosystems, altering critical ecosystem functions such as decomposition and potentially resulting in a shift to P limitation. We used a combination of microbial biomass stoichiometry and enzymatic activity analyses to evaluate the potential for microbial nutrient limitation over the course of a growing season in response to multi-decadal, whole-watershed N enrichments and a one time, plot-scale P addition that occurred in the 22nd year of whole-watershed treatments. The one-time P addition increased microbial biomass threefold and reduced N-acetyl-glucosaminidase (NAG) and acid phosphatase (AP) activity 1 week after application, but there was no interaction with long-term experimental N enrichment to indicate a shift to P limitation. However, both N and P treatments increased C limitation independently of each other over the duration of the study based on measured increases in β-1,4-glucosidase (BG) activity relative to NAG and AP. Microbial biomass stoichiometry and enzyme activity indicated that BBWM is P limited regardless of N status. Our findings highlight the complex interactions between C, N, and P use and limitation in a forested ecosystem subjected to long-term N enrichment.     

Nitrate reduction capacity is limited by belowground plant recovery in a 32‐year‐old created salt marsh

Human activities have decreased global salt marsh surface area with a subsequent loss in the ecosystem functions they provide. The creation of marshes in terrestrial systems has been used to mitigate this loss in marsh cover. Although these constructed marshes may rapidly recover ecosystem structure, biogeochemical processes may be slow to recover. We compared denitrification and dissimilatory nitrate reduction to ammonium (DNRA) rates between a 32‐year‐old excavation‐created salt marsh (CON‐2) and a nearby natural reference salt marsh (NAT) to assess the recovery of ecosystem function. These process rates were measured at 5 cm increments to a depth of 25 cm to assess how plant rooting depth and organic matter accumulation impact N‐cycling. We found that, for both marshes, denitrification and DNRA declined with depth with the highest rates occurring in the top 10 cm. In both systems, N‐retention by DNRA accounted for upwards of 75% of nitrate reduction, but denitrification and DNRA rates were nearly 2× and 3× higher in NAT than CON‐2, respectively. Organic matter was 6× lower in CON‐2, likely due to limited plant belowground biomass production. However, there was no response to glucose additions, suggesting that the microbial functional community, not substrate limitation, limited nitrate reduction recovery. Response ratios showed that denitrification in CON‐2 recovered in surficial sediments where belowground biomass was highest, even though biomass recovery was minimal. This indicates that although recovery of ecosystem function was constrained, it occurred on a faster trajectory than that of ecosystem structure.