BIOMASS AND CO₂FLUX IN WET SEDGE TUNDRAS: RESPONSES TO NUTRIENTS, TEMPERATURE, AND LIGHT

Abstract

The aim of this research was to analyze the effects of increased N or P availability, increased air temperature, and decreased light intensity on wet sedge tundra in northern Alaska. Nutrient availability was increased for 6–9 growing seasons, using N and P fertilizers in factorial experiments at three separate field sites. Air temperature was increased for six growing seasons, using plastic greenhouses at two sites, both with and without N + P fertilizer. Light intensity (photosynthetically active photon flux) was reduced by 50% for six growing seasons at the same two sites, using optically neutral shade cloth. Responses of wet sedge tundra to these treatments were documented as changes in vegetation biomass, N mass, and P mass, changes in whole‐system CO2 fluxes, and changes in species composition and leaf‐level photosynthesis. Biomass, N mass, and P mass accumulation were all strongly P limited, and biomass and N mass accumulation also responded significantly to N addition with a small N × P interaction. Greenhouse warming alone had no significant effect on biomass, N mass, or P mass, although there was a consistent trend toward increased mass in the greenhouse treatments. There was a significant negative interaction between the greenhouse treatment and the N + P fertilizer treatment, i.e., the effect of the two treatments combined was to reduce biomass and N mass significantly below that of the fertilizer treatment only. Six years of shading had no significant effect on biomass, N mass, or P mass. Ecosystem CO2 fluxes included net ecosystem production (NEP; net CO2 flux), ecosystem respiration (RE, including both plant and soil respiration), and gross ecosystem production (GEP; gross ecosystem photosynthesis). All three fluxes responded to the fertilizer treatments in a pattern similar to the responses of biomass, N mass, and P mass, i.e., with a strong P response and a small, but significant, N response and N × P interaction. The greenhouse treatment also increased all three fluxes, but the greenhouse plus N + P treatment caused a significant decrease in NEP because RE increased more than GEP in this treatment. The shade treatment increased both GEP and RE, but had no effect on NEP. Most of the changes in CO2 fluxes per unit area of ground were due to changes in plant biomass, although there were additional, smaller treatment effects on CO2 fluxes per unit biomass, per unit N mass, and per unit P mass. The vegetation was composed mainly of rhizomatous sedges and rushes, but changes in species composition may have contributed to the changes in vegetation nutrient content and ecosystem‐level CO2 fluxes. Carex cordorrhiza, the species with the highest nutrient concentrations in its tissues in control plots, was also the species with the greatest increase in abundance in the fertilized plots. In comparison with Eriophorum angustifolium, another species that was abundant in control plots, C. cordorrhiza had higher photosynthetic rates per unit leaf mass. Leaf photosynthesis and respiration of C. cordorrhiza also i...

Cloud cover and delayed herbivory relative to timing of spring onset interact to dampen climate change impacts on net ecosystem exchange in a coastal Alaskan wetland

“…Diffuse light on cloudy days can increase light use efficiency and partly offset reductions in PAR (Mercado et al 2009), although the diffuse light effect is most critical in forests where direct-beam irradiance results in considerable shadows (Roderick et al 2001). Other long-term experiments in Arctic locations simulating increased cloud cover through shading indicate that photosynthesis can be limited by light, but responses to shade were species specific (Chapin and Shaver 1985), and not always observed in NEE (Shaver et al 1998).…”

Microbiogeochemical Traits to Identify Nitrogen Hotspots in Permafrost Regions

“…Recent studies on Yedoma permafrost estimated total N stocks in permafrost to be as high as 97 Pg N at depths to more than 3 m [42]. However, most of the nitrogen is firmly bound in soil organic matter (SOM) by tight N mineralizationimmobilization-turnover (MIT) or in the perennially frozen ground, therefore tundra ecosystems are generally described as severely N-limited [30,[43][44][45][46][47][48][49][50][51][52][53][54][55][56]. Nitrogen limitation is enhanced by low internal N inputs due to low or even negative net N mineralization rates, including ammonification and nitrification under cold and wet conditions (summarized in [53,[57][58][59]) and low external N inputs due to biological N fixation (20-2000 mg m −2 y −1 , [53,[59][60][61]) combined with low atmospheric N deposition (<200-300 mg m −2 y −1 , [62,63]), and no external N fertilization.…”