Nitrogen transformations in fallen tree boles and mineral soil of an old-growth forest

I measured net N transformation rates of well-decayed boles and adjacent mineral soil under field conditions in an old-growth Douglas-fir/western hemlock/western red cedar stand in the central Oregon Cascades. Additionally, laboratory assays and incubations were used to elucidate the controls on net N transformations in these materials. Net N mineralization under field conditions was similar for well-decayed boles and mineral soils (0.83 and 0.61 g N.m(-2) material.yr(-1), respectively). Laboratory rates of net N mineralization per mass of total N were similar or higher in well-decayed boles compared to mineral soil. These results are surprising, given that the C:N ratio of well-decayed boles was much greater than that of mineral soil (117 vs. 26, respectively), and given the vastly different physical structure of these materials. Higher field and laboratory rates of net N mineralization relative to total N in boles compared to mineral soil suggest either that organically bound N contained in boles is in a more readily mineralizable form or that C compounds in well-decayed bole material are less readily metabolized by microbial heterotrophs (i.e., C availability is lower) than in mineral soil, resulting in a higher ratio of net-to-gross N mineralization because of reduced N demand by a C-limited microflora. A lower microbial respiration rate and smaller microbial biomass C relative to the total C pool in well-decayed boles than in mineral soil support the latter hypothesis. Furthermore, bole material exhibited a higher specific respiration rate (respired C per unit microbial biomass C), suggesting either a lower C-use efficiency of bole microflora or a lower C availability in boles compared to mineral soil. Both well-decayed bole and mineral soil materials showed low annual rates of net nitrification under field conditions. Using an estimate of the mass of class 4 and 5 boles in similar forest stands in the Pacific Northwest, I estimate that well-decayed boles contribute about 0.16-0.25 g N.m(-2).yr(-1) of plant-available N. This N flux is lower than other internal N fluxes within this forest, as well as lower than the rate of N input from the atmosphere. Total plant uptake in a nearby old-growth Douglas-fir forest has been estimated at about 4.0 g N.m(-2).yr(-1), suggesting that well-decayed boles may contribute about 4-6% of plant N uptake. Results from this study indicate that the C:N ratio is a poor predictor of net N release from contrasting forest detrital pools.