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Belowground carbon (C) storage and quality of soil organic matter (SOM) in
forest soils have implications for sustainable forest management and C sequestration,
but how these pools change in response to management is poorly understood. I
investigated whether fertilization and competing vegetation control, applied alone or
in combination early in stand development, affected forest-floor, fine-root, and
mineral-soil C and nitrogen (N) pools to 1-m depth at three ponderosa pine (Pinus
ponderosa) plantations across a site-quality gradient in northern California. Secondly,
I assessed how these treatments affected surface SOM quality at these sites via 1)
density fractionation, 2) dissolved organic C (DOC) and dissolved N release during
225-day laboratory incubation, and 3) CO2 evolution and DOC mobilization during
16-day laboratory incubation.
Twenty years after plantation establishment, mean belowground C pools, were 83,
177, and 206 Mg C ha-1 for the low-, intermediate-, and high-quality sites,
respectively. Belowground N pools for the three sites were 5.6, 7.4, and 6.8 Mg N
ha-1, respectively. Responses of aboveground biomass to treatment were marked, but
changes in belowground C and N pools to fertilization and competing vegetation
control were limited. Fertilization increased total C and N pools at the low- and
intermediate-quality sites and increased the proportion of total belowground C and N
in the forest floor at all three sites. Competing vegetation control increased the forestfloor
C pool at the lowest quality site, but had no effect on total pools. Fertilizer
increased whole-soil and light-density-fraction N and decreased C:N ratios at two
sites, suggesting increased SOM quality. Fertilization decreased C mineralization at
the most productive site, had the opposite effect at the intermediate site, and had no
effect at the poor site. Competing vegetation control affected light-fraction C and N
concentrations and C:N ratios inconsistently among sites and decreased N
mineralization at the most productive site, suggesting decreased SOM quality.
Although forest floors were the most sensitive of the belowground C pools to these
silvicultural treatments, results suggest that the major mechanism for increased C
sequestration through management of these ponderosa pine forests will be through
increased tree growth, rather than belowground C storage. |
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