IMMOBILIZATION OF A N-15-LABELED NITRATE ADDITION BY DECOMPOSING FOREST LITTER

Effects of chronic HNO3 and H2SO4 additions on decomposition of senesc ed birch leaf, beech leaf, spruce needle, and wood chip litters were e xamined. Litters were incubated for up to 4 years in fiberglass mesh ( 1 mm) bags on experimental plots in a mixed-species forest near the Be ar Brooks Watershed Manipulation (BBWM) site in eastern Maine, United States. Plot treatments included HNO3 additions at 28 and 56 kg N . ha (-1) . year(-1), H2SO4 additions at 128 kg S . ha(-1) . year(-1), and a combined HNO3 and H2SO4 treatment at 28 kg N and 64 kg S . ha(-1) . year(-1). The N-15 content of all NO3 added was artificially increased to 344 parts per thousand delta(15)N. Litter bags were collect ed eac h fall and analyzed for organic matter loss, nitrogen concentration, a nd N-15 abundance throughout the 4-year experiment. Extractive (non-po lar-soluble+water-soluble), cellulose (acid-soluble), and lignin (acid -insoluble) fractions were analyzed for the first 2 years. In wood chi ps, nitrogen additions increased mass loss and N concentration, but no t the mass of N after 4 years. Neither N nor S additions had large eff ects on mass loss, N concentration, or N content of leaf litters. All litters immobilized and mineralized N simultaneously, but we were able to place a lower bound on gross N immobilization by mass balancing N- 15 additions. Birch and spruce litters showed net mineralization, whil e beech leaf and wood chip litters showed net immobilization. Net immo bilizing Litters were those with the highest initial cellulose concent ration (wood chips=80%, beech leaves=54%), and we attribute the higher capacity for immobilization to more readily available carbon. Lignin mass increased initially in all Litter types except spruce needles. Al so, extractives in net immobilizing litters either increased initially (wood chips) or decreased at a slower rate than bulk Litter (beech le aves). We calculate the potential of decomposing Litter to immobilize exogenous nitrate in this system to be 1-1.5 kg N . ha(-1) . year(-1), which is about half of the usual NO3 deposition at this site, but onl y a small fraction of the experimental addition.