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 mineralization processes, while few studies have integrated processes in both the soil and plant compartments. Here, we synthesize information on nutrient cycling within, and between, the soil/microbial and the plant compartments of the ecosystems and integrate the cycling of nutrients with the turnover of organic matter and the carbon balance in tundra ecosystems. Based on this compilation 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 biomass compared to the pools in the plant biomass both due to large nutrient-containing 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 nutrient mineralization is low, the residence time of nutrients in the soil is long and the nutrients are strongly immobilized in the soil microorganisms. This contributes to pronounced nutrient limitation for plant productivity, implies that the microbial sink strength for nutrients is strong and that the microbes may compete with plants for nutrients, but also that they are a potential source of plant nutrients during periods of declining microbial populations. The extent of this competition is poorly explored and it is uncertain whether plants mainly take up nutrients continuously during the summer when the microbial activity and, presumably, also the microbial sink strength 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 for our understanding of the C exchange between the ecosystems and the atmosphere under the predicted, future climatic change. High microbial nutrient immobilization, i.e. low release of plant-available nutrients, paired with high 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 increased plant C fixation. Hence, the system will be a net source of atmospheric C. Conversely, if plants are able to sequester extra nutrients efficiently, their productivity will increase and the systems may accumulate more C and turn into a C sink, particularly if nutrients are allocated to woody tissues of low nutrient concentrations.