Seasonal protein dynamics in Alaskan arctic tundra soils

In the arctic tundra of Alaska, plant growth is limited by N supply, especially in tussock tundra. Because proteins are the predominant form of soil organic N, proteolysis is considered to be the rate-limiting step in both the release of amino acids and in N mineralization. To help understand the controls on N availability in tundra soils, and to determine whether proteolysis is controlled by enzyme activity or by substrate availability, we measured soil protein concentrations, and proteolysis rates with and without added protein, every 1-2 weeks through the summer of 2000 and twice in the summer of 2001. Protease activity with added protein ('potential') was higher than without added protein ('actual'). However, differences between the two tended to be driven by relatively brief peaks in potential protease activity. In fact, actual and potential rates were usually similar, suggesting that much of the time proteolysis was not limited by substrate availability, but rather by enzyme activity. Our data suggest that protease activity was actually only substrate limited at the times when it was highest. Potential rates peaked at the same times that soluble proteins were also high. These increases in protease activity and soluble protein concentrations occurred when soil amino acid and NH4+ concentrations were at their lowest, drawn down by the seasonal peaks in root growth. The fact that the peaks in protease activity coincided with the peak in soil amino acid and NH4+ demand strongly suggests that proteolysis was stimulated by high soil amino acid demand, and resulted in increases in soluble protein concentrations caused by the solubilization of larger proteins.