Modelling carbon responses of tundra ecosystems to historical and projected climate: a comparison of a plot- and a global-scale ecosystem model to identify process-based uncertainties.

We are developing a process-based modelling approach to investigate how carbon (C) storage of tundra across the entire Arctic will respond to projected climate change. To implement the approach, the processes that are least understood, and thus have the most uncertainty, need to be identified and studied. In this paper, we identified a key uncertainty by comparing the responses of C storage in tussock tundra at one site between the simulations of two models - one a global-scale ecosystem model (Terrestrial Ecosystem Model, TEM) and one a plot-scale ecosystem model (General Ecosystem Model, GEM). The simulations spanned the historical period (1921-94) and the projected period (1995-2100). In the historical period, the model simulations of net primary production (NPP) differed in their sensitivity to variability in climate. However, the long-term changes in C storage were similar in both simulations, because the dynamics of heterotrophic respiration (RH ) were similar in both models. In contrast, the responses of C storage in the two model simulations diverged during the projected period. In the GEM simulation for this period, increases in RH tracked increases in NPP, whereas in the TEM simulation increases in RH lagged increases in NPP. We were able to make the long-term C dynamics of the two simulations agree by parameterizing TEM to the fast soil C pools of GEM. We concluded that the differences between the long-term C dynamics of the two simulations lay in modelling the role of the recalcitrant soil C. These differences, which reflect an incomplete understanding of soil processes, lead to quite different projections of the response of pan-Arctic C storage to global change. For example, the reference parameterization of TEM resulted in an estimate of cumulative C storage of 2032 g C m-2 for moist tundra north of 50°N, which was substantially higher than the 463 g C m-2 estimated for a parameterization of fast soil C dynamics. This uncertainty in the depiction of the role of recalcitrant soil C in long-term ecosystem C dynamics resulted from our incomplete understanding of controls over C and N transformations in Arctic soils. Mechanistic studies of these issues are needed to improve our ability to model the response of Arctic ecosystems to global change.

Transformation of a tundra river from heterotrophy to autotrophy by addition of phosphorus.

Continuous enrichment of an arctic river with only 10 parts per billion phosphate-phosphorus caused an immediate growth of attached algae for more than 10 kilometers downstream, showing that phosphorus alone limited photosynthesis. As a result of the increased photosynthesis, there was an increase in bacterial activity in films on rocks on the bottom of the stream. The major source of energy became the photosynthetic carbon fixed in the stream rather than the organic material entering from the surrounding tundra, and the overall metabolism of the stream shifted from heterotrophy to autotrophy. An increase in the size and developmental stage of some of the dominant aquatic insects illustrates the food limitation in this nutrient-poor habitat.

Global deforestation: contribution to atmospheric carbon dioxide.

A study of effects of terrestrial biota on the amount of carbon dioxide in the atmosphere suggests that the global net release of carbon due to forest clearing between 1860 and 1980 was between 135 x 10(15) and 228 x 10(15) grams. Between 1.8 x 10(15) and 4.7 x 10(15) grams of carbon were released in 1980, of which nearly 80 percent was due to deforestation, principally in the tropics. The annual release of carbon from the biota and soils exceeded the release from fossil fuels until about 1960. Because the biotic release has been and remains much larger than is commonly assumed, the airborne fraction, usually considered to be about 50 percent of the release from fossil fuels, was probably between 22 and 43 percent of the total carbon released in 1980. The increase in carbon dioxide in the atmosphere is thought by some to be increasing the storage of carbon in the earth's remaining forests sufficiently to offset the release from deforestation. The interpretation of the evidence presented here suggests no such effect; deforestation appears to be the dominant biotic effect on atmospheric carbon dioxide. If deforestation increases in proportion to population, the biotic release of carbon will reach 9 x 10(15) grams per year before forests are exhausted early in the next century. The possibilities for limiting the accumulation of carbon dioxide in the atmosphere through reduction in use of fossil fuels and through management of forests may be greater than is commonly assumed.

Effect of litter nitrogen on decomposition and microbial biomass inSpartina alterniflora.

The effect on decomposition of 4 different levels of nitrogen in aerial tissue ofSpartina alterniflora, collected at the end of its growing season litter, was studied in laboratory percolators for 56 days at 20‡C. The CO2 evolution and the release of organic nitrogen and organic carbon were monitored. From these data, the ash-free dry weight (AFDW), nitrogen (N) content, and carbon∶nitrogen (C/N) ratio were calculated at various times during decomposition. Fungal biomass, bacterial biomass, and the relative autoradiographic activity of bacteria were measured at the end of the study. Decomposition was significantly affected by the nitrogen content of the litter. A 55% increase in plant N increased overall weight loss and k by 50% and 40%, respectively. Furthermore, k (calculated from time course weight loss data) responded linearly to the 4 different levels of nitrogen inSpartina tissue. Fungi appear to dominate the microbial community. At the end of the experiment, fungal biomass was between 2.23 and 3.08% of the AFDW, and was calculated to contain 12 to 22% of the nitrogen in the litter. Bacterial biomass was 1/10 of the fungal biomass, and 12-17% of the bacteria were active. The total microbial biomass was not affected by increased plant nitrogen. In the course of decomposition, the organic nitrogen and carbon were highest in the effluent water in all treatments during the first 8 days. The respiration rate (CO2 evolution) first increased to a maximum at day 18 and then decreased to a constant rate (1-2 mg C/day/g detritus). Respiration was highest in the high N litter. The C/N ratio in all treatments increased from the start to day 8, then decreased to day 20. In low N litter, C/N then increased again as a result of increased total organic nitrogen (TON) loss relative to carbon mineralization. In the high N, this was reversed.

Use of nuclepore filters for counting bacteria by fluorescence microscopy.

Polycarbonate Nuclepore filters are better than cellulose filters for the direct counting of bacteria because they have uniform pore size and a flat surface that retains all of the bacteria on top of the filter. Although cellulose filters also retain all of the bacteria, many are trapped inside the filter where they cannot be counted. Before use, the Nuclepore filters must be dyed with irgalan black to eliminate autofluorescence. Direct counts of bacteria in lake and ocean waters are twice as high with Nuclepore filters as with cellulose filters.

Amino acid flux in an estuary.

Dissolved organic matter in York River estuary included 38 micrograms of free amino acids per liter. The highest concentrations were of glycine, serine, and ornithine. Of the 14 amino acids studied for uptake by planktonic bacteria, glycine, methionine, and serine had the greatest flux rates. The total amino acid flux represented from 1 to 10 percent of the daily photosynthetic carbon fixation.