We measured δ13C and Δ14C for both DOC and POC for four selected months (January, April, July and October) to determine and compare the sources of DOC and POC and their seasonal changes of organic inputs as carried by the two rivers (Table 2). Values of δ13C ranged from −25.6‰ to −32.1‰ and −28.8‰ to −32.1‰ for DOC and −23.4‰ to −25.6‰ and −23.1‰ to −24.7‰ for POC, in the Yellow River and Changjiang, respectively. DOC and POC transported by the two rivers had similar δ13C mean values but DOC in both rivers was more depleted in δ13C than POC. In their studies, Wu et al.  and Yu et al.  reported similar δ13C values (−24.3‰ to −26.8‰) for POC collected in the mainstreams of the lower to upper reaches of Changjiang. These δ13C values reflect the typical stable carbon isotope source signatures of terrestrial organic matter, similar to the values reported for the other large river systems such as the Amazon [Hedges et al., 2000; Raymond and Bauer, 2001a, 2001b; Townsend-Small et al., 2007], Mississippi [Wang et al., 2004], and Yukon River and several other large arctic rivers [Guo and Macdonald, 2006; Neff et al., 2006; Raymond et al., 2007]. In comparison, the values of Δ14C measured for both DOC and POC showed distinct differences between the two rivers. As plotted in Figure 6, the 14C ages of POC transported by the Yellow River are extremely old, ranging from 4,110 to 8,040 years, while the DOC are relatively younger with 14C ages ranging from 400 to 1,070 years. In Contrast, the 14C ages of DOC in Changjiang ranged from 305 to 1570 years, comparable to the values of the Yellow River DOC. However, the 14C ages of POC in Changjiang (815–1060 years) are much younger than the POC ages of the Yellow River. These distinct 14C age differences of DOC and POC found in the two rivers clearly indicate that the input sources of POC to the two rivers are different. The very old POC carried by the Yellow River suggests that these POC were not from the recent-fixed freshwater plankton and terrestrial plant materials. Rather, these POC were primarily derived from the highly decomposed soil, clay minerals and quaternary loess in the middle reach of the river [Zhang et al., 1995], and perhaps weathering of old rocks and ancient kerogen. The unmatched 14C ages of POC and DOC in the Yellow River also support our discussion earlier that those old POC were extremely refractory, bond tightly with the clay particles, and could not be easily released into the DOC pool from either chemical dissolution/desorption or biological processes during the river transport. In comparison, the relatively young 14C age POC in Changjiang and DOC in both rivers suggest that these carbon pools contained a mixture of both old and a large fraction of recent-fixed modern terrestrial organic materials. This great difference of14C ages between DOC and POC has also been reported for other river systems. For example, Raymond and Bauer [2001a] measured Δ14C of DOC and POC in the Amazon and Hudson Rivers and a few small rivers (York, Parker) in the Northeast of the United States. They found that the 14C ages of DOC were all younger than POC in these rivers. DOC had a more modern 14C age than POC (1,258 years BP) in the Amazon, and POC in the Hudson River was 3,000 years older than the riverine DOC (1,380 year BP). Young terrigenous DOC was also measured in several Arctic rivers such as the Yenisey, Mackenzie and Yukon rivers [Amon and Meon, 2004; Benner et al., 2004; Guo and Macdonald, 2006; Neff et al., 2006; Raymond et al., 2007]. These studies suggest that the predominant sources of organic matter that contributed to the young ages of riverine DOC were from the leaching and decomposition of recent-fixed carbon in plant litter and upper soil horizons. The14C age differences found between POC and DOC also support our discussion above suggesting that the two OC pools represent two different organic sources and they are isotopically disjointed. POC and DOC pools are regulated by chemical and biological processes at different rates and timescales during their transport in the rivers. In Figure 7, we also plotted the correlation of δ13C and 14C ages for DOC and POC. It is quite clear that POC transported by the Yellow River and Changjiang can be separated into two groups with similar δ13C values but different 14C ages. DOC in the two rivers (with one exception) joined in one group with relatively young 14C age and more depleted 13C values, consistent with our discussion above on the source variations of organic matter transported by the two rivers.
 The sources of POC and DOC transported by the Yellow River and Chanjiang also showed strong seasonal variations (Figure 6). The 14C ages of POC in the Yellow River decreased from winter (January) to the spring (April) and summer (July) and then increased again in the late fall. This observed decrease in the POC ages was likely due to the contribution of some recent-fixed organic carbon from freshwater plankton and terrestrial plants during the spring and summer months when the primary production is high. In late fall when the contribution of the modern14C decreased, the POC age then increased. In response, we see the same trend of seasonal changes for the 14C ages of DOC in the Yellow River, indicating that some young organic carbon was added into the DOC pool as well in the spring and summer months. In comparison, the seasonal variation was more pronounced for DOC than POC in the Changjiang River in 2009. The 14C ages of DOC decreased rapidly from winter to the spring and summer months and then increased again in the late fall. The POC ages, however, remained relatively constant with the season, suggesting that the sources of POC to Changjiang were primarily from the terrestrial plants and perhaps mixed with some old organic carbon from the upper soil deposits. In their series studies, Bianchi et al.  and Duan and Bianchi  investigated the seasonal variability in sources of DOC and POC transported in the low Mississippi and Pearl Rivers. They found that in situ phytoplankton production played an important role for the observed seasonal variations of DOC and POC. Based on the lignin and 13C NMR analyses, they indicated that autochthonous production in the rivers could be more significant than previously thought which could affect on the age and lability of riverine organic matter entering the ocean. The more rapid seasonal changes of the 14C ages of DOC than POC found in the Yellow and Changjiang rivers in our study could also suggest that the microbial turnover rate of DOC was much faster than POC and the terrestrial DOC had much shorter resident time than POC during the transport processes in the rivers. Since the 14C ages of DOC in the Yellow River showed less seasonal variation than DOC in Changjiang, it is also possible that relatively low light intensity or high shading effect in the turbid Yellow River water could limit the phytoplankton production in some degrees [Duan and Bianchi, 2006], resulting in less young aged DOC production during the spring and summer months.
 Since the recent-fixed plant organic matter has a young radiocarbon age (assume Δ14C = +50‰) and is labile, we can estimate how much recent-fixed labile organic carbon was added to the POC and DOC pools in spring and summer months assuming that the winter (January) POC and DOC were relatively refractory (as measured aged Δ14C values). Our calculations indicate that at least 7% and 34% recent-fixed C and 31% and 42% recent-fixed C had been added to the POC and DOC pools in April and July in the Yellow River; 31% and 60% recent-fixed C were contributed to the DOC pool in April and July in Changjiang. Although the transformation and fate of these labile and refractory terrestrial organic carbon transported by the Yellow River and Changjiang have not been well studied, our study suggests that the seasonal changes in organic sources and the contributions of labile and refractory POC and DOC by the two rivers could have an important influence not only on the biogeochemical and ecosystem processes in the estuaries and coastal waters, but also on the carbon cycle and budget in the ECS as well. The differences in organic source inputs and seasonal variations of POC and DOC transported by the Yellow River and Changjiang provide a perfect comparison to study and address this important yet still unanswered question.