Effects of complex carbon sources on vertical distribution and exchange flux of dissolved nutrients at sediment-water interface

To investigate the purification mechanism of composite carbon source, taking largemouth bass (Micropterus salmoides) breeding system as the experimental object, we added the treatment group with composite carbon source of corn husk and composite carbon source of rice straw, but did not add the control group with composite carbon source, and then collected the in situ water samples from the vertical profiles of each group of the sediment-water interface by using with the Peeper (Pore water equilibriums) technique, so as to analyze the vertical distribution of nutrient salts in each group, estimate the exchange fluxes at the sediment-water interface, and investigate the effects of the composite carbon sources on the nitrogen transport and transformation at the sediment-water interface. The results show that: 1) Each group of nutrient salts at the sediment-water interface had a strong vertical distribution pattern, except for ammonium nitrogen (NH₄ ⁺-N) and total nitrogen (TN), while the other nutrient salts showed a decreasing trend with depth. 2) NO₃ ⁻-N was the main endogenously released nutrient salts in the sediments of the largemouth bass aquaculture system. Compared with the control group, the two groups of carbon sources both promoted the release of NH₄ ⁺-N in the sediments. The corn husk carbon source slowed down the release of nitrate nitrogen (NO₃ ⁻-N), nitrite nitrogen (NO₂ ⁻-N) and phosphate (PO₄ ³⁻-P), and the effect of the rice straw carbon source was insignificant. 3) The addition of the composite carbon source promoted the nutrient salts removal from the water body. Compared with the rice straw carbon source, the corn husk carbon source had higher TN, PO₄ ³⁻-P and chemical oxygen demand (COD) removal rates of 73%, 53% and 48%, respectively. In conclusion, adding composite carbon sources affects the vertical distribution characteristics of nutrient salts and diffusion fluxes at the sediment-water interface; the corn husk carbon source slowed down the release of nutrient salts from the sediment and had a better effect on water purification.