Soluble organic nitrogen in plant and soil processes in the Upper Coastal Plain
Jin, Virginia Lee
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Primary productivity in many terrestrial ecosystems is limited by nitrogen (N) availability, so plant uptake of soluble organic N (SON, free amino-N) compounds is potentially important in plant N nutrition and ecosystem N cycling in Upper Coastal Plain ecosystems. The potential effects of plant organic N uptake on ecosystem processes were initially explored in floodplain ecosystems using a nutrient model that incorporated a plant-available organic N pool. The model predicted that short-term organic N uptake by plants was most affected by competition with soil microorganisms for available N, while longer-term organic N uptake was more strongly affected by soil physicochemical properties. In a two-year field survey of four major ecosystem types in the Upper Coastal Plain (floodplain, upland hardwood, oldfield, sandhills), seasonal concentrations of soluble organic and inorganic N were highly variable in both forest floor and mineral soils of all ecosystem types and were strongly related to litterfall and rainfall events. Concentrations of soluble N were greatest in forest floors and decreased with soil depth, and the release of N from forest floors strongly influenced N concentrations in the mineral soils of all ecosystems. On a landscape scale, soluble organic and inorganic N concentrations were correlated to ecosystem characteristics reflecting broad environmental gradients. In a field study using stable isotope tracer techniques, roots of both Acer rubrum L. and Pinus palustris Miller demonstrated the ability to compete well against soil N immobilization factors for available ammonium-N in floodplain and sandhills soils, respectively, while they did not use N or C applied as the amino acid glycine. The relationships between soil N availability and plant N uptake were highly variable and differed over time, reflecting the complexity of plant N use patterns. While organic N uptake as glycine was not unequivocally traced in either tree species examined here, soil soluble organic N clearly plays a pivotal role in soil N dynamics and plant N nutrition in these two contrasting temperate ecosystems.