Coupling of nutrient cycling and carbon dynamics in the Arctic, integration of soil microbial and plant processes

Most studies of nutrient cycling in arctic ecosystems have either addressed questions of plant nutrient acquisition or of decomposition and mineraliza tion processes, while few studies have integrated processes in both the soi l and plant compartments. Hen, we synthesize information on nutrient cyclin g within, and between, the soil/microbial and the plant compartments of the ecosystems and integrate the cycling of nutrients with the turnover of org anic matter and the carbon balance in tundra ecosystems. Based on this comp ilation and integration, we discuss implications for ecosystem function in response to predicted climatic changes. Many arctic ecosystems have high amounts of nutrients in the microbial biom ass compared to the pools in the plant biomass both due to large nutrient-c ontaining organic deposits in the soil and low plant biomass. The microbial pools of N and P, which are the most commonly limiting nutrients for plant production, may approach (N) or even exceed (P) the plant pools. Net nutri ent mineralization is low, the residence time of nutrients in the soil is l ong and the nutrients an strongly immobilized in the soil microorganisms. T his contributes to pronounced nutrient limitation for plant productivity, i mplies that the microbial sink strength for nutrients is strong and that th e microbes may compete with plants for nutrients, but also that they are a potential source of plant nutrients during periods of declining microbial p opulations. The extent of this competition is poorly explored and it is unc ertain whether plants mainly take up nutrients continuously during the summ er when the microbial activity and, presumably, also the microbial sink str ength is high, or whether the main nutrient uptake occurs during pulses of nutrient release when the microbial sink strength declines. Improved knowledge of mechanisms for plant-microbial interactions in these nutrient-limited systems is important, because it will form a basis also fo r our understanding of the C exchange between the ecosystems and the atmosp here under the predicted, future climatic change. High microbial nutrient i mmobilization, i.e, low release of plant-available nutrients, paired with h igh microbial decomposition of soil organic matter will lead to a loss of C from the soil to the atmosphere, which may not be compensated fully by inc reased plant C fixation. Hence, the system will be a net source of atmosphe ric C. Conversely, if plants are able to sequester extra nutrients efficien tly, their productivity will increase and the systems may accumulate more C and turn into a C sink, particularly if nutrients are allocated to woody t issues of low nutrient concentrations. (C) 1999 Elsevier Science B.V.