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Understanding the transport of dissolved organic carbon (DOC) and nitrogen (N) as water flows through headwater basins is important for predicting downstream water quality. With increased recognition of climatic impact on nutrient transport, more studies are needed in headwater basins experiencing a Mediterranean-type climate, such as those of the Sierra Nevada, California. We analyzed water samples collected over 5 years from eight low-order and mixed-conifer watersheds to elucidate the temporal variation of water chemistry and evaluate their responses to prolonged drought and low-intensity forest thinning. We observed higher stream DOC concentrations in October compared to other months within water years prior to drought and thinning, suggesting the importance of antecedent moisture conditions on seasonal C export. In unthinned watersheds, stream DOC concentrations were lower (62%) and DOC aromaticity was higher (68 and 92%, depending on the index used) during drought compared to non-drought years. In thinned watersheds during drought years, stream water had higher DOC concentrations (66–94% in three consecutive years following thinning) and dissolved inorganic N (24%, in the third year following thinning) compared to unthinned watersheds during drought. Additionally, lower stream DOC concentrations were found in watersheds with higher elevations and lower drainage densities in the year with near-average precipitation; however, these correlations were not significant in years with greater or extremely low precipitation. Taken together, our results suggest that stream concentrations of DOC and dissolved N in Mediterranean headwater basins are extremely variable over time due to the high temporal climatic variabilities and periodic management practices. [ABSTRACT FROM AUTHOR]

Dissolved organic matter (DOM) plays several important roles in forest ecosystem development, undergoing chemical, physical and/or biological reactions that affect ecosystem nutrient retention. Very few studies have focused on gross rates of DOM production, and we know of no study that has directly measured DOM production from root litter. Our objectives were to quantify major sources of total potentially water-soluble organic matter (DOMtps) production, with an emphasis on production from root litter, to quantify and compare total potentially soluble organic C, N, and P (DOCtps, DONtps, and DOPtps) production, and to quantify changes in their production during forest primary succession and ecosystem development at the Mt. Shasta Mudflows ecosystem chronosequence. To do so, we exhaustively extracted freshly senesced root and leaf and other aboveground litter for DOCtps, DONtps, and DOPtps by vegetation category, and we calculated DOMtps production (g m−2 y−1) at the ecosystem level using data for annual production of fine root and aboveground litter. DOM production from throughfall was calculated by measuring throughfall volume and concentration over 2 years. Results showed that DOMtps production from root litter was a very important source of DOMtps in the Mount Shasta mudflow ecosystems, in some cases comparable to production from leaf litter for DONtps and larger than production from leaf litter for DOPtps. Total DOCtps and DONtps production from all sources increased early in succession from the 77- to the 255-year-old ecosystem. However, total DOPtps production across the ecosystem chronosequence showed a unique pattern. Generally, the relative importance of root litter for total fine detrital DOCtps and DONtps production increased significantly during ecosystem development. Furthermore, DOCtps and DONtps production were predominantly driven by changes in biomass production during ecosystem development, whereas changes in litter solubility due to changes in species composition had a smaller effect. We suggest that DOMtps production from root litter may be an important source of organic matter for the accumulation of SOM during forest ecosystem development. [ABSTRACT FROM AUTHOR]