Fire effects on ecosystem nitrogen cycling in a Californian bishop pine forest.

Fire can cause severe nitrogen (N) losses from grassland, chaparral, and temperate and boreal forest ecosystems. Paradoxically, soil ammonium levels are markedly increased by fire, resulting in high rates of primary production in re-establishing plant communities. In a manipulative experiment, we examined the influence of wild-fire ash residues on soil, microbial and plant N pools in a recently burned Californian bishop pine (Pinus muricata D. Don) forest. Ash stimulated post-fire primary production and ecosystem N retention through direct N inputs from ash to soils, as well as indirect ash effects on soil N availability to plants. These results suggest that redistribution of surface ash after fire by wind or water may cause substantial heterogeneity in soil N availability to plants, and could be an important mechanism contributing to vegetation patchiness in fire-prone ecosystems. In addition, we investigated the impact of fire on ecosystem N cycling by comparing 15N natural abundance values from recently burned and nearby unburned P. muricata forest communities. At the burned site, 15N natural abundance in recolonising species was similar to that in bulk soil organic matter. By contrast, there was a marked 15N depletion in the same species relative to the total soil N pool at the unburned site. These results suggest that plant uptake of nitrate (which tends to be strongly depleted in 15N because of fractionation during nitrification) is low in recently burned forest communities but could be an important component of eco- system N cycling in mature conifer stands.

Soil microbiota in two annual grasslands: responses to elevated atmospheric CO2.

We measured soil bacteria, fungi, protozoa, nematodes, and biological activity in serpentine and sandstone annual grasslands after 4 years of exposure to elevated atmospheric CO2. Measurements were made during the early part of the season, when plants were in vegetative growth, and later in the season, when plants were approaching their maximum biomass. In general, under ambient CO2, bacterial biomass, total protozoan numbers, and numbers of bactivorous nematodes were similar in the two grasslands. Active and total fungal biomasses were higher on the more productive sandstone grassland compared to the serpentine. However, serpentine soils contained nearly twice the number of fungivorous nematodes compared to the sandstone, perhaps explaining the lower standing crop of fungal biomass in the serpentine and suggesting higher rates of energy flow through the fungal-based soil food web. Furthermore, root biomass in the surface soils of these grasslands is comparable, but the serpentine contains 6 times more phytophagous nematodes compared to the sandstone, indicating greater below-ground grazing pressure on plants in stressful serpentine soils. Elevated CO2 increased the biomass of active fungi and the numbers of flagellates in both grasslands during the early part of the season and increased the number of phytophagous nematodes in the serpentine. Elevated CO2 had no effect on the total numbers of bactivorous or fungivorous nematodes, but decreased the diversity of the nematode assemblage in the serpentine at both sampling dates. Excepting this reduction in nematode diversity, the effects of elevated CO2 disappeared later in the season as plants approached their maximum biomass. Elevated CO2 had no effect on total and active bacterial biomass, total fungal biomass, or the total numbers of amoebae and ciliates in either grassland during either sampling period. However, soil metabolic activity was higher in the sandstone grassland in the early season under elevated CO2, and elevated CO2 altered the patterns of use of individual carbon substrates in both grasslands at this time. Rates of substrate use were also significantly higher in the sandstone, indicating increased bacterial metabolic activity. These changes in soil microbiota are likely due to an increase in the flux of carbon from roots to soil in elevated CO2, as has been previously reported for these grasslands. Results presented here suggest that some of the carbon distributed below ground in response to elevated CO2 affects the soil microbial food web, but that these effects may be more pronounced during the early part of the growing season.

Initial effects of experimental warming on above- and belowground components of net ecosystem CO2 exchange in arctic tundra.

The Arctic contains extensive soil carbon reserves that could provide a substantial positive feedback to atmospheric CO2 concentrations and global warming. Evaluation of this hypothesis requires a mechanistic understanding of the in situ responses of individual components of tundra net ecosystem CO2 exchange (NEE) to warming. In this study, we measured NEE, total ecosystem respiration and respiration from below ground in experimentally warmed plots within Alaskan acidic tussock tundra. Soil warming of 2-4°C during a single growing season caused strong increases in total ecosystem respiration and belowground respiration from moss-dominated inter-tussock areas, and similar trends from sedge-dominated tussocks. Consequently, the overall effect of the manipulation was to substantially enhance net ecosystem carbon loss during mid-summer. Components of vascular plant biomass were closely correlated with total ecosystem respiration and belowground respiration in control plots of both microsites, but not in warmed plots. By contrast, in the warmed inter-tussock areas, belowground respiration was most closely correlated with organic-layer depth. Warming in tussock areas was associated with increased leaf nutrient pools, indicating enhanced rates of soil nutrient mineralisation. Together, these results suggest that warming enhanced net ecosystem CO2 efflux primarily by stimulating decomposition of soil organic matter, rather than by increasing plant-associated respiration. Our short-term experiment provides field evidence to support previous growth chamber and modelling studies indicating that arctic soil C reserves are relatively sensitive to warming and could supply an initial positive feedback to rising atmospheric CO2 concentrations/changing climate.

Stimulation of grassland nitrogen cycling under carbon dioxide enrichment.

Nitrogen (N) limits plant growth in many terrestrial ecosystems, potentially constraining terrestrial ecosystem response to elevated CO2. In this study, elevated CO2 stimulated gross N mineralization and plant N uptake in two annual grasslands. In contrast to other studies that have invoked increased C input to soil as the mechanism altering soil N cycling in response to elevated CO2, increased soil moisture, due to decreased plant transpiration in elevated CO2, best explains the changes we observed. This study suggests that atmospheric CO2 concentration may influence ecosystem biogeochemistry through plant control of soil moisture.