Microbial immobilization and the retention of anthropogenic nitrate in a northern hardwood forest

To determine the importance of microorganisms in regulating the retention of anthropogenic NO3-, we followed the belowground fate and flow of (NO3-)-N-15 in a mature northern hardwood forest. dominated by Acer saccharum Marsh. Total recovery of added N-15 (29.5 mg N-15/m(2) as NaNO3) in inorganic N, microbial immobilization in forest floor and soil microbial biomass, soil organic matter, and root biomass pools (0-10 cm depth) was 93% two hours following application of the (NO3-)-N-15 but rapidly dropped to similar to 29% within one month, presumably due to movement of the isotope into other plant tissues or deeper into soil. Microbial immobilization was initially (i.e., at 2 h) the largest sink for (NO3-)-N-15 (21% in forest floor; 16% in soil microbial biomass). After one month, total N-15 recovery varied little (24-18%) throughout the remainder of the growing season, suggesting that the major N transfers among pools occurred relatively rapidly. At the end of the four-month experiment, the main fates of the N-15 label were in soil organic matter (7%), root biomass (6%), and N immobilized in forest floor and soil microbial biomass (6%). Temporal changes in the N-15 enrichment (atom % excess N-15) Of plant and soil pools during the first month of the experiment indicated the dynamic nature of NO3 cycling in this Forest. The N-15 enrichment of soil microbial biomass and the forest floor significantly increased two hours after isotope additions, suggesting rapid microbial immobilization of NO3-. In contrast, the N-15 enrichment of soil organic matter did not peak until day 1, presumably because much of the added N-15 cycled through microorganisms before becoming stabilized in soil organic matter, or it directly entered soil organic matter via physical processes. Furthermore, the N-15 enrichment of root biomass (<0.5-mm diameter and 0.5-2.0 mm diameter) was greatest between day 7 and day 28, following significant increases in the N-15 enrichment of soil organic matter (day 1) and, more importantly, NH4+ (day 2). From these data we conclude that microorganisms are immediate. short-term sinks for anthropogenic NO3-. Although the long-term fate of NO3- additions to this forest is likely in soil organic matter and plants, the cycling of N through microorganisms appears to be the major short-term factor influencing patterns of NO3- retention in this ecosystem.