Nutrient dynamics in the arctic tundra of Alaska

Global warming is proceeding the fastest at extreme latitudes. If this speeds the decomposition of thick organic tundra soils, they may release more CO₂ and accelerate warming. Alternatively, N release from soil organic matter (SOM) may also increase, enhancing plant growth, which is N limited in the tundra, and offsetting soil C losses. The direction the arctic C balance will shift is unclear and depends on poorly understood interactions between C and N cycling. Our goal was to elucidate the controls on changes in the C balance of the arctic with warming. We performed a laboratory incubation of tundra soils to study the controls on tundra SOM mineralization; we measured amino acids, NH₄+, NO₃− , DON and DOC, and PO₄= over the arctic growing season to characterize the seasonal dynamics of available nutrients; we measured soil proteins, and proteolysis rates to determine whether amino acid production is controlled by enzyme activity or substrate availability; we incubated cores of plants and soil with labeled NH₄+ , amino acids, or other organic N compounds to determine how they partition between plants and soil; and we incubated roots and soils with these N compounds at a range of concentrations to determine if their partitioning is controlled by their uptake kinetics. The model of nutrient availability that emerges is that tundra plants can take up mineralized N, amino acids, and other organic N forms released from the breakdown of SOM by microbial extra-cellular enzymes. The main controls on N partitioning between plants and soil appear to be plant community composition and root access to soil N. Plant nutrient uptake and root C inputs to the soil causes peaks in nutrient demand at the height of the arctic growing season, stimulating increases in soil enzyme activity, which in turn results in pulses of soluble proteins and PO₄=. Thus the timing and magnitude of plant root growth and nutrient uptake appear to be the major factors determining the seasonal dynamics of available nutrients in tundra soils. As a result, changes in plant community composition have the potential to substantially alter nutrient cycling in tundra soils.