Abstract
We investigated the decomposability of soil organic matter (SOM) along a chronosequence of rainforest sites in Hawaii that form a natural fertility gradient and at two long-term fertilization experiments. To estimate turnover times and pool sizes of organic matter, we used two independent methods: (1) long-term incubations and (2) a three-box soil model constrained by radiocarbon measurements. Turnover times of slow-pool SOM (the intermediate pool between active and passive pools) calculated from incubations ranged from 6 to 20 y in the O horizon and were roughly half as fast in the A horizon. The radiocarbon-based model yielded a similar pattern but slower turnover times. The calculation of the 14C turnover times is sensitive to the lag time between photosynthesis and incorporation of organic C into SOM in a given horizon. By either method, turnover times at the different sites varied two- or threefold in soils with the same climate and vegetation community. Turnover times were fastest at the sites of highest soil fertility and were correlated with litter decay rates and primary productivity. However, experimental fertilization at the two least-fertile sites had only a small and inconsistent effect on turnover, with N slowing turnover and P slightly speeding it at one site. These results support studies of litter decomposition in suggesting that while plant productivity can respond rapidly to nutrient additions, decomposition may respond much more slowly to added nutrients.
Highlights
Substantial uncertainties remain in estimates of soil organic matter (SOM) turnover times in different ecosystems, including how turnover is affected by the nutrient status of ecosystems and by soil mineral composition
We expected that the influence of site fertility or added nutrients, if any, would be more important in the O horizon, whereas the A horizon might reflect the influence of minerals on carbon decomposition and stabilization
The radiocarbon content of SOM in each horizon was determined for a single composite of the 10 soil pits sampled at each site, using a split of the ground soil used for total carbon and nitrogen analysis
Summary
Anderson 1988; Beyer 1991; Nadelhoffer and others 1991; Raich and Schlesinger 1992; Howard and Howard 1993; Trumbore 1993; Biederbeck and others 1994; Hassink 1994; Townsend and others 1995; Trumbore and others 1996). There are three methods for estimating the turnover time of intermediate-cycling SOM: (1) by long-term (>6 months) soil incubations (Townsend and others 1997); (2) by modeling the 14C content of the soil, which reflects the incorporation and loss of the atmospheric ‘‘bomb’’ spike (14C produced by aboveground nuclear weapons testing, mostly between 1959 and 1963) by organic matter (Trumbore 2000); and (3) after a change between C3 and C4 vegetation, by using the 13C content of SOM to calculate turnover time under certain equilibrium assumptions (Balesdent and others 1988; Veldkamp 1994). We expected that the influence of site fertility or added nutrients, if any, would be more important in the O horizon, whereas the A horizon might reflect the influence of minerals on carbon decomposition and stabilization
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