Soil nitrogen dynamics along a gradient of long-term soil development in a Hawaiian wet montane rainforest.

摘要

The rates of net N mineralization and nitrification of soils from seven sites in a Hawaiian wet montane forest were determined. The sites differ in age, ranging from 400 to 4 100 000 years, but are comparable in other variables (all at 1200 m altitude with 4000 mm or more mean annual rainfall), and the chronosequence simulated a development of soils from basaltic lava. Soils were incubated for 20 days at 17.5degC, which is nearly equivalent to a mean field air temperature of the sites, and at an elevated temperature of 25.5degC under three treatments: field-wet without amendments; air dried to a permanent wilting point; and given phosphate (NaH2PO4) at the rate of 50 mug P per g dry soil. Both mineralization and nitrification rates varied significantlyamong the sites at the field temperature. Fractions of the mineralized organic matter (indexed by the N produced per g organic C) increased sharply from the youngest to the 5000-year site before declining abruptly to a near constant value from the 9000to the 1 400 000-year sites. Total organic C in the top soils (<15 cm deep) increased almost linearly with age across the sites. Consequently, net NH4- and NO3-N produced on an area basis (g m-2 20-d-1) increased sharply from 0.2 in the youngest site to1.2 in the 5000-year site, then both became depressed once but steadily increased again. The fraction of organic matter mineralized, and the net N turnover rates were outstandingly high in the oldest site where a large amount of organic matter was observed; the topsoil organic matter which was used in this analysis appeared to be highly labile, whereas the subsurface organic matter could be relatively recalcitrant. As suggested by earlier workers, the initial increase in N turnover seemed to correspondto the increasing quantity of N in the soils through atmospheric deposition and biological fixation. The later decline in fraction of organic matter mineralized seemed to relate to increasing soil C/N ratios, increasingly recalcitrant organic matter, andpoorer soil drainage with age. The elevated temperature treatment produced significantly higher amounts of N mineralization, except for the youngest site where N was most limiting, and for two sites where soil waterlogging might be severe. P fertilizerapplication invariably resulted in slower N turnovers, suggesting that soil microbes responded to added P causing N immobilization. The youngest site did not significantly respond to added P. The magnitude of immobilization was higher in older than in younger soils, suggesting that P more strongly limits microbial populations in the older soils.