Immobilization of a ¹⁵N-labeled nitrate addition by decomposing forest litter

Effects of chronic HNO₃ and H₂SO₄ additions on decomposition of senesced birch leaf, beech leaf, spruce needle, and wood chip litters were examined. Litters were incubated for up to 4 years in fiberglass mesh (1 mm) bags on experimental plots in a mixed-species forest near the Bear 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 ¹⁵N content of all NO₃ added was artificially increased to 344% δ¹⁵N. Litter bags were collected each fall and analyzed for organic matter loss, nitrogen concentration, and ¹⁵N abundance throughout the 4-year experiment. Extractive (non-polar-soluble+water-soluble), cellulose (acid-soluble), and lignin (acid-insoluble) fractions were analyzed for the first 2 years. In wood chips, nitrogen additions increased mass loss and N concentration, but not the mass of N after 4 years. Neither N nor S additions had large effects 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 ¹⁵N additions. Birch and spruce litters showed net mineralization, while beech leaf and wood chip litters showed net immobilization. Net immobilizing litters were those with the highest initial cellulose concentration (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. Also, extractives in net immobilizing litters either increased initially (wood chips) or decreased at a slower rate than bulk litter (beech leaves). 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 only a small fraction of the experimental addition.