Litter-Derived N in An Old-Growth Forest with Long-Term Organic Matter Exclusions: Does Changing the Litter Type Matter?.

In old-growth forests of Oregon, N availability is often considered limiting, and organic N and NH₄⁺ are dependent on soil microorganisms and organic matter inputs  to build and replenish these pools. Changes in the quantity and quality of organic matter can alter the availability of C and N in a system, thus potentially shifting the dominant processes occurring in the ecosystem N cycle. To examine these shifts in N cycling processes, particularly rate of depolymerization, we added leached, ¹⁵N-labeled litter to old-growth soils (CO), considered to be N limited, and to soils that have experienced 12 years of litter and root exclusions (NI), which lysimeter data suggest are N replete. Two litter types, red alder (RA) and Douglas fir (DF), and no litter addition controls (NL), were incubated in the laboratory, and harvested at four times (3, 10, 56, and 150 days). During the first 10 days of the experiment, the only treatment showing net N mineralization was the NI-NL treatment; all other treatments showed net N immobilization. By 56 days and at 150 days, all treatments clearly showed net N mineralization and nitrification, with CO soils always lower in N mineralization rates than NI soils. In both CO and NI, the addition of litter decreased N mineralization rates; however, there was no difference among litter qualities. Respiration also differed among the CO and NI, with NI treatments being lower. The addition of litter increased respiration but an effect of litter quality was only observed in NI soils, with the addition of DF having decreased respiration relative to RA. Throughout the incubation, we observed that litter type was inconsequential to net N mineralization in both the CO and NI soil, but was a factor when it came to C utilization.