Shrub encroachment increases soil organic and inorganic carbon in coastal wetlands

PurposeTemperate herbaceous wetlands have been widely experiencing shrub encroachment due to global climate change and human activities. We attempted to investigate the effect size and pathway of shrub-induced C sequestration in coastal wetlands.MethodsIn the shrub-encroached wetlands of the Yellow River Delta, China, two transects were selected randomly, including two landscapes (flat wetland, FW, and tidal ditch wetland, TDW). Based on the distances to shrub Tamarix chinensis, four soil sampling sites (base, mid, edge, and between) for each shrub were determined. We analyzed spatial patterns of soil organic and inorganic carbon (SOC and SIC) and total nitrogen contents (TN) and stable isotope ratios (delta C-13 and delta N-15), evaluated the direct and indirect effects of shrub on SOC and SIC by stepwise regression and structural equation modeling (SEM), and then determined the coupled relationships between soil C and N.Results and discussionShrub encroachment increased significantly both SOC (by 67.18%) and SIC (by 9.93%) contents, with higher SOC increase in FW whereas higher SIC increase in TDW. TN had the strongest positive effect on soil carbon (0.797 for SOC and 0.949 for SIC), and that C/N ratio had the second strongest positive influence on SOC (0.716) but negative influence on SIC (-0.567). This suggested a superimposed effect from both TN and C/N ratio on SOC and an offset effect on SIC. Additionally, TN had a greater effect on SOC in FW while clay and silt had a higher effect on SIC in TDW. Both delta C-13 and delta N-15 values decreased simultaneously (-0.69 parts per thousand and -0.16 parts per thousand) in TDW while delta C-13 decreased greatly (-2.36 parts per thousand) but delta N-15 increased greatly (0.39 parts per thousand) in FW. It means that a coupled relationship existed between soil C and N cycling in the former but a decoupled relationship in the latter. Finally, an interaction mechanism was proposed, where the coupled and decoupled relationships between soil C and N was one likely factor which drove higher SIC accumulation in better wetland but higher SOC sequestration in poor one.ConclusionsThe presence of shrubs increased SOC and SIC in coastal wetlands with significant landscape effects, resulted in tidal ditch wetland (better wetland) being potential sinks of inorganic carbon, and in flat wetland (nutrient-poor wetland) being significant sinks of organic carbon. The indirect effect of shrub on soil carbon was dominant, which was driven mainly by increasing TN and altering coupled relationships between soil C and N