Graduate Thesis Or Dissertation
 

Belowground carbon storage and soil organic matter quality following fertilizer and herbicide applications in ponderosa pine plantations along a site-quality gradient in northern California

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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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