Seasonal and diurnal variations in DIC, NO3− and TOC concentrations in spring-pond ecosystems under different land-uses at the Shawan Karst Test Site, SW China: Carbon limitation of aquatic photosynthesis

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Human activities have altered terrestrial carbon (C) and nitrogen (N) dynamics via changes to land cover and use such as deforestation, agriculture, application of fertilizers, etc. and have influenced the patterns of organic C input and eutrophication in downstream freshwater ecosystems. Biogeochemical cycling of C and N and the related organic carbon (OC) production may display correlated diurnal and seasonal variations due to photosynthesis and respiration in these ecosystems, whose underlying mechanisms still need to be resolved. In this study, we document the diurnal and seasonal variations measured in DIC (dissolved inorganic carbon), NO3−, TOC (total organic carbon) and other related hydrochemical parameters (pH and DO-dissolved oxygen) in five artificial spring-pond ecosystems with differing land-uses in tanks draining into springs and corresponding ponds, constructed at the Shawan Karst Test Site, SW China. It was found that diurnal changes in DIC, NO3− and TOC in all ponds were dominated by aquatic ecosystem metabolism (i.e., photosynthesis and respiration), as evidenced by the pertinent variations in DO and pH. Daily DIC and NO3− uptake and OC production were higher in October (growing period) and lower in January (dormant period), indicating seasonal differences in assimilation that were determined by both changes in weather (temperature and light) and nutrient inputs. Under conditions of bare rock or bare soil, there was very low DIC and NO3− additions to the spring-pond ecosystems, resulting in lower OC productivity in the ponds. Cropped land yielded higher DIC and NO3− to the pond, due to growth of corn and use of fertilizers that enhanced OC production. Highest productivity and densest vegetation cover on tanks with grassland or shrubs (with higher N retention in soils) resulted in higher DIC but limited NO3− addition to the ecosystems downstream. The highest DIC concentration (in the grassland) resulted in maximum OC production in the pond. These results indicate that OC production in the ponds with elevated pH was limited by DIC fertilization. In general, the supply of DIC is not considered to limit aquatic primary productivity because its concentration exceeds that of other plant macronutrients such as NO3 and PO43− by two or three orders of magnitude. Therefore, the carbon limitation detected here may indicate that photoautotrophs in karst dominated aquatic terrains (dominated by Charophyta and Spirogyra) cannot use the total DIC for photosynthesis but only the dissolved CO2, which comprises <1% of total DIC at pH > 8.2 that is characteristic in these environments. This may have implications for control of eutrophication in such alkaline aquatic ecosystems, i.e., rates of eutrophication in freshwater ecosystems may be regulated not only by N and/or P but also by C. It is also projected that there will be an increase in OC sequestration with the current land-use and global climate change-driven increases in DIC, due to carbon limitation of aquatic primary production.