Spatiotemporal evolution of carbon sequestration of limestone weathering in China

Abstract

Carbonate carbon sequestration (CS) can aid in solving the problem of terrestrial residual carbon sinks and imbalances in the global carbon budget. Thus, complete understanding of the magnitude, spatiotemporal distribution, and evolution of this sequestration is highly desirable. On the basis of random forest regression and maximal potential dissolution model for carbonate, we estimated the CS of typical carbonate weathering in China from 2000 to 2014, that is, the sequestration of limestone weathering, using long-term ecologic, meteorological, hydrological raster data, and monitored data from 44 watersheds in China and surrounding regions. We extended our analyses by systematically exploring the spatiotemporal pattern and evolution trend of the flux and total sequestration. High levels of ionic activity coefficients of Ca2+ and HCO 3 − in limestone regions were observed to be mainly distributed in Northern and Northwestern China with a clear gradient from northwest to southeast. With a contrary spatial pattern, the annual average CS flux (CSF) of limestone weathering in China was estimated to be 4.28 t C km−2 yr−1, with high values mainly in the karst zones in Southeastern China. The mean CSF in different latitudes showed that Southern China (south of 28.14°N) was the region with the largest interannual fluctuation of flux and CSF increases as latitude decreases. The mean CSF in subtropical and tropical (TR) regions was the maximum of all major climate types, and for the frigid (F), mid-temperate (MTE), warm temperate (WTE), and temperate (TE) major climates; the CSF in the desert (D) subdivided climate was the minimum of these climates. By contrast, the values in grassland (G) and broad-leaved forest subdivided climate were the maximum. The pixel-based trend analysis indicated that the CSF of limestone weathering in China was slightly increasing in the period 2000–2014 with a rate of 0.036 t C km−2 yr−1. Furthermore, the annual total CS was estimated to be 7.07 Tg carbon per year (Tg C yr−1) with high levels in 2002, 2008, and 2010, and the minimum appeared in 2011 with a slightly increasing trend of the total CS being observed with a rate of 0.06 Tg C yr−1. Tibet Autonomous Region was the administrative division with the largest total CS of limestone weathering (1.20 Tg C yr−1) in China, and karst zones in Southeastern China had the largest total CS (4.95 Tg C yr−1) which accounts for 70.01% of that in the three divided karst regions. On the basis of the diversity of rock chemical weathering carbon cycle mechanisms of different carbonate rock types, we estimated that the total CS of carbonate weathering in China may reach 11.37 Tg C yr−1 (the sink was approximately 5.02 t C km−2 yr−1), which amounts to 16.20% of the total biomass CS in China, furthermore, the CSF of carbonate weathering in China can reach 6.54 t C km−2 yr−1 if excluding the interference of the negative runoff. This finding indicates that CS of carbonate weathering is an indispensable part of China’s terrestrial carbon sink system. The research pattern of this study was important for further improving the accuracy of the estimation for the global carbonate weathering carbon sink.