Abstract:In order to understand the biogeochemical process of controlling daily cycling of DIC and NO3-, and their daily variation in a karst stream. This paper installed two monitoring sites including Guancun subterranean river outlet (CK) and surface creek mouth (LY), which are located at Guancun Village and Leiya Village, Daliang Township, Rong’an County of Guangxi Zhuang Autonomous Region, China, to Carried out a diurnal monitoring and sampling work by high time-resolution for two days for physical and chemical parameters、N and C isotopes (δ13CDIC、δ15N-NO3- and δ18O-NO3-) in water. Research results revealed physical and chemical parameters in CK site were stable basically, while Ca2+、DIC and PCO2 in LY site showed apparent diurnal variations i.e. increased during the daytime and decreased at night, and showed a significant negative correlation with DO and pH. Compared to CK site, Photosynthesis led to the decline of DIC and the increased of δ13CDIC during the day, while respiration caused the increased of DIC and the decline of δ13CDIC in LY site which was even lower than δ13CDIC in CK site. NH4+ of stream decreased basically and NO3- increased at night and morning during monitoring, while δ15N-NO3- showed a downward trend and relatively close to the initial value of δ15N-NO3-, and NO3- appeared decline trend at afternoon. Results indicate that photosynthesis and respiration of aquatic plants control daily cycling of DIC in stream, and it was calculated through the mass balance equation knowing that the amount of inorganic carbon converted to organic carbon by photosynthesis was 0.94kgC•d-1, which could form a relative long-term stable carbon sink. Stream water was influenced mainly by nitrification at night and morning, which the amount of growth was 2.08 kgN•d-1, however stream water was influenced by assimilation at afternoon, which the amount of losing was 0.42 kgN•d-1. The output amount of stream’s NO3--N was 1.66 kgN•d-1, showing that it was conducive to the growth of aquatic photosynthetic organisms, and could promote the occurrence of N assimilation in karst streams with carbon-rich and calcium-rich. Thereby, this processes also reduced the output amount of inorganic N, indicating that water quality may be changed by the biogeochemical processes of nitrogen at diurnal scale.