Carbon burial by shallow lakes on the Yangtze floodplain and its relevance to regional carbon sequestration

Floodplain lakes may play an important role in the cycling of organic matter at the landscape scale. For those lakes on the middle and lower reaches of the Yangtze (MLY) floodplain which are subjected to intense anthropogenic disturbance, carbon burial rates should, theoretically, be substantial due to the high nutrient input, increased primary production and high sediment accumulation rates. There are more than 600 lakes >1 similar to km2 on the Yangtze floodplain including 18 lakes >100 similar to km2 and most are shallow and eutrophic. 210Pb-dated cores were combined with total organic carbon (TOC) analyses to determine annual C accumulation rates (C AR; g similar to C similar to m-2 similar to yr-1) and the total C stock (since similar to 1850). The sediment TOC content is relatively low with an average <2% in most lakes. C AR ranged from similar to 5 to 373 similar to g similar to C similar to m-2 similar to yr-1, resulting in C standing stocks of 0.6015.3 similar to kg similar to C similar to m-2 (mean: similar to 5 similar to kg similar to C similar to m-2) since similar to 1850. A multicore study of Chaohu lake (770 similar to km2) indicated that spatial variability of C burial was not a significant problem for regional upscaling. The possible effect of changes in lake size and catchment land use on C burial was examined at Taibai lake and indicated that lake shrinkage and declining arable agriculture had limited effects on C AR. The organic C standing stock in individual lakes is, however, significantly dependent on lake size, allowing a simple linear scaling for all the MLY lakes. Total regional C sequestration was similar to 80 Tg C since similar to 1850, equivalent to similar to 11% of C sequestration by soils, but in similar to 3% of the land area. Shallow lakes from MLY are a substantial regional C sink, although strong mineralization occurs due to their shallow nature and their role as C sinks is threatened due to lake drainage.