Fine Roots, Net Primary Production, and Soil Nitrogen Availability: A New Hypothesis

The relationships between above- and belowground net primary production and soil nitrogen availability were studied at nine temperate forest sites. Annual allocations of nitrogen (N) and net primary production to leaf litter, perennial tissues (wood + bark), and aboveground biomass all increased significantly (P < .01) in relation to apparent N uptake by vegetation (NJ) as calculated using field measures of net N mineralization (Nj) and other major N fluxes to and from available N pools. Mean annual N content and biomass of fine roots (:<3.0 mm diameter) were both negatively correlated with NU (r= -0.71, P < .05; r -0.63, P < .10, respectively). However, only -50% of Nm at each site could be accounted for by allocation to aboveground litter and perennial tissues. Assuming that mineralized N not accounted for by allocation to these components was taken up by vegetation and allocated to fine roots, annual N allocation to fine roots (Nfr) was a constant fraction of N uptake. Therefore, Nfr increased in absolute terms with both Nm and apparent N uptake. Fine-root N turnover rates (or Nfl/fine-root N content) also increased as Nm and NU increased. Provided that fine-root biomass and N turnover rates were similar within individual sites, allocation of production to belowground biomass also increased relative to increases in soil N availability.Furthermore, the proportion of total net primary production allocated to belowground biomass did not decrease with increased N avail- ability.

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