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 mi neral soil under field conditions in an old-growth Douglas-fir/western heml ock/western red cedar stand in the central Oregon Cascades. Additionally, l aboratory assays and incubations were used to elucidate the controls on net N transformations in these materials. Net N mineralization under field con ditions 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 minera lization per mass of total N were similar or higher in well-decayed boles c ompared to mineral soil. These results are surprising, given that the C:N r atio 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 f orm or that C compounds in well-decayed bole material are less readily meta bolized by microbial heterotrophs (i.e., C availability is lower) than in m ineral soil, resulting in a higher ratio of net-to-gross N mineralization b ecause of reduced N demand by a C-limited microflora. A lower microbial res piration rate and smaller microbial biomass C relative to the total C pool in well-decayed boles than in mineral soil support the latter hypothesis. F urthermore, bole material exhibited a higher specific respiration rate (res pired C per unit microbial biomass C), suggesting either a lower C-use effi ciency of bole microflora or a lower C availability in boles compared to mi neral soil. Both well-decayed bole and mineral soil materials showed low an nual rates of net nitrification under field conditions. Using an estimate o f the mass of class 4 and 5 boles in similar forest stands in the Pacific N orthwest, 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 interna l N fluxes within this forest, as well as lower than the rate of N input fr om the atmosphere. Total plant uptake in a nearby old-growth Douglas-fir fo rest 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 th is study indicate that the C:N ratio is a poor predictor of net N release f rom contrasting forest detrital pools.