Coupled carbon and silicon cycling and characteristics in a subtropical watershed, China

Abstract Silicon (Si) and carbon (C) can couple at different timescales. Phytolith carbon fixation and silicate weathering are two key processes of this type of coupling. Silicate rock weathering sequesters CO2 and thus generates a net C sink, which is one way that C is removed from ecosystems. This study selected the Xiangxi River watershed as the research object, which is characteristic of typical watersheds found in subtropical regions, and continuously monitored for Si and C transfers in this basin. We were able to ascertain the Si and C coupled cycling relationship in this basin by analyzing the influence of environmental changes on these two elements. The main findings were as follows: (1) The atmospheric chemical CO2 sequestration rate in this subtropical basin was 14.2 kg hm−2 yr−1, wherein carbonates sequestered atmospheric CO2 at a rate of 5.4 kg hm−2 yr−1 and silicate rock sequestered atmospheric CO2 at a rate of 8.8 kg hm−2 yr−1. (2) Both rainfall and vegetation type had significant effects on Si and C migration. Different characteristics of canopy interception of each tree species had obvious dissociation effects on dissolved organic carbon (DOC), while the absorption or dissociation effect on dissolved silicate (DSi) differed among the various tree species. Basin soil was enriched with a large amount of organic C during the dry season, which was subsequently exported during the rainy season. (3) The Si input into the watershed was less than the Si output (the net accumulation of Si was −27 kg hm−2 yr−1), and the C input was greater than the C output (the net accumulation of C was + 2785 kg hm−2 yr−1). This study determined that the Xiangxi River watershed was an Si “source” and a C “sink”. The results of this study can provide a reference for further study of the coupling relationship between Si and C, and provide support for the study of ecosystem material balance.