The amounts, sources and relative ages of inorganic and organic carbon pools were assessed in eight headwater streams draining watersheds dominated by either forest, pasture, cropland or urban development in the lower Chesapeake Bay region (Virginia, USA). Streams were sampled at baseflow conditions six different times over 1 year. The sources and ages of the carbon pools were characterized by isotopic (delta C-13 and a dagger C-14) analyses and excitation emission matrix fluorescence with parallel factor analysis (EEM-PARAFAC). The findings from this study showed that human land use may alter aquatic carbon cycling in three primary ways. First, human land use affects the sources and ages of DIC by controlling different rates of weathering and erosion. Relative to dissolved inorganic carbon (DIC) in forested streams which originated primarily from respiration of young, C-14-enriched organic matter (OM; delta C-13 = -22.2 +/- A 3 aEuro degrees; a dagger C-14 = 69 +/- A 14 aEuro degrees), DIC in urbanized streams was influenced more by sedimentary carbonate weathering (delta C-13 = -12.4 +/- A 1 aEuro degrees; a dagger C-14 = -270 +/- A 37 aEuro degrees) and one of pasture streams showed a greater influence from young soil carbonates (delta C-13 = -5.7 +/- A 2.5 aEuro degrees; a dagger C-14 = 69 aEuro degrees). Second, human land use alters the proportions of terrestrial versus autochthonous/microbial sources of stream water OM. Fluorescence properties of dissolved OM (DOM) and the C:N of particulate OM (POM) suggested that streams draining human-altered watersheds contained greater relative contributions of DOM and POM from autochthonous/microbial sources than forested streams. Third, human land uses can mobilize geologically aged inorganic carbon and enable its participation in contemporary carbon cycling. Aged DOM (a dagger C-14 = -248 to -202 aEuro degrees, equivalent(14)C ages of 1,811-2,284 years BP) and POM (a dagger C-14 = -90 to -88 aEuro degrees, C-14 ages of 669-887 years BP) were observed exclusively in urbanized streams, presumably a result of autotrophic fixation of aged DIC (-297 to -244 aEuro degrees, C-14 age = 2,251-2,833 years BP) from sedimentary shell dissolution and perhaps also watershed export of fossil fuel carbon. This study demonstrates that human land use may have significant impacts on the amounts, sources, ages and cycling of carbon in headwater streams and their associated watersheds.
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机译：在下切萨皮克湾地区（美国弗吉尼亚州）的森林，牧场，农田或城市发展为主导的八个流域排水流域中，评估了无机和有机碳库的数量，来源和相对年龄。在一年中，在基本流条件下对流进行了六次不同的采样。通过同位素（δC-13和匕首C-14）分析和激发发射矩阵荧光与平行因子分析（EEM-PARAFAC）表征碳库的来源和年龄。这项研究的结果表明，人类土地利用可能以三种主要方式改变水生碳循环。首先，人类土地利用通过控制不同的风化和侵蚀速率来影响DIC的来源和年龄。相对于森林流中的溶解性无机碳（DIC），其主要来自呼吸富含C-14的年轻有机物（OM;δC-13 = -22.2 +/- A 3 aEuro度;匕首C-14 = 69 +/- A 14 aEuro度），城市化河流中的DIC受碳酸盐沉积风化的影响更大（δC-13 = -12.4 +/- A 1 aEuro度;匕首C-14 = -270 +/- A 37 aEuro度）和一种牧场流对年轻土壤碳酸盐的影响更大（δC-13 = -5.7 +/- A 2.5 aEuro度;匕首C-14 = 69 aEuro度）。其次，人类土地利用改变了溪流水OM的陆地与本地/微生物源的比例。溶解性OM（DOM）和颗粒性OM（POM）的C：N的荧光特性表明，排水的人为流域比天然林/微生物源的DOM和POM相对贡献更大。第三，人类土地利用可以动员地质老化的无机碳，并使其参与当代碳循环。老年DOM（匕首C-14 = -248至-202 aEuro度，等效（14）C年龄为1,811-2,284年BP）和POM（匕首C-14 = -90至-88 aEuro度，C-14仅在城市化河流中观察到669-887岁BP年龄，这可能是由于沉积壳溶解以及可能是沉积壳溶解导致的DIC（-297至-244 aEuro度，C-14年龄= 2,251-2,833年BP）年龄的自养性固着也成为化石燃料碳出口的分水岭。这项研究表明，人类土地利用可能会对源头水流及其相关流域中碳的数量，来源，年龄和循环产生重大影响。
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