Decomposing litter as a sink for 15N-enriched additions to an oak forest and a red pine plantation

The importance of surface litter as a sink for atmospheric N deposition on forests was investigated using 15N tracers to quantify the effects of N input rate and litter age on retention of N inputs by forest litter. The study was conducted in plots receiving ambient (0.8 g N m^-2 per year) and chronically elevated N inputs (ambient+5 g NH4NO3-N m^-2 per year) in both a hardwood forest and a red pine plantation. Plots (30 m×30 m) were part of the Chronic Nitrogen Amendment Study at the Harvard Forest, Massachusetts, USA. To allow estimates of N sink strength in litter cohorts of two different ages, litterbags containing oak leaves, maple leaves, pine needles, or maple wood chips were placed in the plots in years 1 and 3 of fertilizer additions. Uptake of N inputs into decomposing litter was estimated using 15N tracers applied as 15NH4 or 15NO3 during years 4 and 5 of fertilizer additions. Thus 15N additions occurred to litters in their fourth and fifth years of decay (set out in year 1 of N additions) and to litters in their second and third years of decay (set out in year 3 of N additions). The effect of N input level on mass loss and N dynamics varied among litter types and appeared to increase with litter age. Fertilization decreased mass loss and loss of initial N content in maple and oak leaf litters, but it enhanced mass loss in wood and had little effect on pine needles. Decomposing litter cohorts in both forests retained significant proportions of N inputs at both ambient (20% in hardwood, 9% in pine) and elevated levels (7% in hardwood, 6% in pine) over the 2 years of tracer additions. Tracer assimilation into the 2-year-old litter cohort decreased under elevated N inputs, NO3- assimilation decreased relative to NH4+, and assimilation of both forms was lower in 4-yearold material on a per unit litter mass basis. Ecosystem-scale retention of N inputs by the 2+4-year-old litter cohorts in both forests increased with N input level, from 0.32 and 0.15 g N m^-2 over 2 years (ambient hardwood and pine plots) to 0.86 and 0.70 g N m^-2 over 2 years (fertilized hardwood and pine plots). Although N sink strength of litter pools increased more strongly in the pine forest in response to elevated N inputs, the hardwood forest retained more N under both input levels, suggesting that it may be a larger sink for atmospheric N deposition than the pine forest. Recently formed litter pools assimilate more NO3- than NH4+ under ambient N deposition, but may lose capacity to assimilate NO3- relative to NH4+ under potential future increases in N deposition.