Microbial Processes and Plant Nutrient Availability in Arctic soils

Introduction

Several characteristics of arctic soils influence microbial activity, nutrient mineralization, and nutrient availability to plants and will certainly figure prominently in changes in these processes in a warmer arctic climate. Arctic soils are generally overlain by a dense mat of organic matter and vegetation, wet for at least part of the year and permanently frozen at some depth. These factors combine to lower summer soil temperatures, impede the progression and decrease the depth of seasonal thawing, and maintain relatively high soil moisture content. Cold, wet soil environments and short summers slow organic matter decomposition and nutrient mineralization and severely restrict nutrient availability to plants.

The accumulation of organic matter in arctic soils is determined largely by the combined effects of temperature and moisture on decomposition and primary production. Because of climatic variation among arctic regions, the amounts of organic matter and nutrients in tundra soils vary across broad geographic scales. Organic matter often accumulates at depth in permanently frozen peats in relatively wet arctic regions such as the coastal plain no northern Alaska¡K

¡K Organic carbon¡K increases with moisture, from low amounts in well-drained beach-ridge ecosystems with cushion plant-lichen communities¡K such an overall patter ¡V of organic carbon increasing with moisture from well- to poorly ¡Vdrained ecosystems ¡V also occurs in Alaska¡¦s coastal and foothill tundra regions

Well-drained soils are less common in patterned ground regions with little relief, such as the Alaskan coastal plain, where more than 85% of soils are moist to poorly drained. Moist soils with dense organic mats(5-40cm thick), intermediate thaw depths, and divers plant communities dominated by tussock-forming sedges occupy gently sloping land in much of the Low Arctic¡K

Organic matter and moisture content are important determinants of soil temperature, thaw depth, cation exchange capacity, aeration, redox potential, and other properties affecting biological processes in soils. Decomposition rates and soil moisture balances will likely be affected by the warmer temperatures predicted for the Arctic The resulting changes in soil organic matter, moisture and microbial processes in ecosystems will alter the amounts, seasonality, and forms of mineral nutrients available to plants. A warmer climate will likely have different overall effects on soil properties and on nutrient cycling in dry, moist, and wet arctic ecosystems.

Microbial and soil Processes

Nutrient cycling and fertilization studies in arctic ecosystems show that plant growth is strongly limited by nutrient availability. Primary production is often nitrogen-limited, but phosphorus(especially in organic soils) or nitrogen and phosphorus together can also limit production.

Arctic ecosystems are generally conservative of nutrients accumulating large amounts in soil organic matter pools with very long turnover times. Because of these characteristically slow turnover rates and, in some ecosystems, the gradual burial of organic matter in permafrost, nutrients become available to plants at very low rates. Long turnover time result from slow decomposition, which can become a bottleneck in nutrient cycling rates. Differences among ecosystem types in soil microclimate and decomposition may explain the inverse relationships between soil nutrient stocks and nutrient cycling rates or primary productivity as reported, for example, on Alaska¡¦s northern coast. Slow decomposition leads to greater accumulation of organic matter in soil and can lower nutrient mineralization rates, thereby decreasing primary productivity

A. Decomposition

Although the decomposition of organic matter in arctic ecosystems is generally much slower than at lower latitudes, it is controlled by the same factors. Decomposition rates increase globally with actual evapotranspiration(AET) from warm temperate to arctic regions and decrease with the percentage of lignin in litter. Field studies comparing decomposition of common material in tundra with forest ecosystems are consistent with this observation. Models based on laboratory and field studies at a number of arctic International Biological Programme(IBP) sites have successfully used temperature, moisture content, and substrate quality to predict the decomposition of litter and soil organic matter. Temperature is the most important predictor of decomposition rates in arctic ecosystems. Comparisons of arctic IZBP studies suggest that, overall, decomposition rates increase by about 20% per year for every 10000 degree-days above 0 C. Microbial respiration in tundra litter and soil organic matter is measurable at ¡V7 C and increases with temperature between about 5 and 20 to 30 C. There are, however, important functional differences in decomposition processes operating at low versus high ends of the range of typical summer field temperatures in arctic soils. Enzymatic degradation of cellulosics and simple organic compounds both increase with temperature above about 10C. Below 10C, however, degradation of soluble organic compounds continues to vary with temperature, whereas cellulose decomposition is restricted, both by low numbers of active cellulolytic microbes and by greater thermodynamic constraints on cellulase activities as compared with enzymes that degrade organic acids. This probably contributes to the sharp increase in temprature sensitivity of microbial reactions in general in soils above 10C. ((temperature can be immediately changed by machine activity in drilling))

B. Nitrogen Mineralization
In the Arctic, as elsewhere, most of the inorganic nitrogen available for plant uptake is supplied by the mineralization of organic matter. Precipitation inputs of nitrogen to tundra ecosystems are generally <= 0.03 g m-2 yr-1 -- small relative to annual nitrogen uptake into vegetation (Barsdate and Alexander, 1975). Nitrogen fixation by rhizobial or actinorhizal symbionts in certain nodulated vascular plants (eg. Lupines, alders) or by symbiotic or freeliving blue-green algae can supply large proportions of the nitrogen taken up by vegetation in some arctic ecosystems

When soil microbes are active because of favorable physical donations and sources of readily oxidizable organic carbon, ammonium is released from organic matter to soil solution by enzymatically mediated processes (Burns, 1978). Active microbes are generally better competitors for mineral N than plant roots because of their greater surface area and closer proximity to microsites with actively decomposing organic matter. When microbes are subjected to substrate (carbon) limitations, low temperatures, low availabilities of electron acceptors(O2, sulfate, etc.) or other stresses, they often release NH4+ to soil solution. Mineralized NH4+ has several potential fates, including adsorption to negatively charged particles , chemical immobilization in clay lattices or humus, reimmobilization into microbial biomass, uptake by plants, and oxidation to nitrate(NO3-) by nitrifying bacteria(Paul and Clark, 1989). Evidence that nitrification can occur in arctic soils is accumulating. The occurrence of nitrification has important implications for plants in arctic ecosystems because species differ in nitrate uptake potential and assimilations efficiencies(Smirnoff and Stewart, 1985). NO3- is also more easily leached downslope than NH4+ can be reduced and exported from soils as N2O or N2.

The difference between the gross nitrogen flux from soil organic matter mineral forms and immobilization by microbes is net nitrogen mineralization. Net mineralization is generally positive during a growing season or across a year, but rates can be very low or negative during the arctic midsummer. Variations in N mineralization rates across growing seasons are determined largely by asynchronies in competitive success for mineral N between heterotrophic microbes and plant roots. (Gorham et al., 1979) Thus, the amount and quality of organic carbon in soils, soil microclimate, and microbial activity interact to determine the amounts, forms and seasonality of nitrogen availability to plants in arctic ecosystems.

Because ratios of primary production to N mineralization are higher in arctic ecosystems than in many others, processes such as the direct uptake of amino acids from soils, N fixation, and N retranslocation within plants are likely to be important in meeting nitrogen requirements for plant growth in arctic ecosystems.

Although net N mineralization is generally much lower in arctic soils than in soils of warmer regions, mineralization rates can vary among different arctic ecosystem types. These differences are largely due to variations in the quality of soil organic matter and microclimate among ecosystems. Nitrification can occur in arctic soils even though cold, wet conditions are foten not considered favorable for nitrifiers. Nitrate has been repoted in tissues of arctic plants growing on stream anks and well-drianed microsites and in arctic soil solution and surface waters. (Haag, 1974; Ulrich and Gersper, 1978) Nitrification accounted for about half the annual net nitrogen mineralization in the moist tundra but, in contrast to ammonificaiton, occurred mostly in midsummer. Because nitrification is more temperature- sensitive than mineralization(Paul and Clark, 1989) it may be largely restricted to the warmest part ofht the season I nthose arctic soils where it does occur.

Source: CHAPIN, F.S., JEFFERIES, R.L., REYNOLDS, J.F., SHAVER, G.R. Arctic Ecosystem in a Changing Climate: An Ecophysiological Perspective, 1991

Last Updated: 11/3/2003