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

Abstract

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 increases significantly (P < .01) in relation to apparent N uptake by vegetation (N " u) as calculated using field measurers of net N mineralization (N m ) 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 N u (° = —0.71, P < .05; ° —0.63, ° < .10, respectively). However, only °50% of N m 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 (N f r ) was a constant fraction of N uptake. Therefore, N f r increased in absolute terms with both N m and apparent N uptake. Fine—root N turnover rates (or N f r /fine—root N content) also increased as N m and N u increased. Provided that fine—root biomass and N turnover rates were similar within sites, allocation of production to belowground biomass also increased relative to increases in soil N availability. Furthermore, the proportion of total net production allocated to belowground biomass did not decrease with increased N availability.