The nitrate dynamics affected by land use and saltwater intrusion in the coastal hydrologic interaction systems

Nitrate dynamics in the coastal hydrological system is concerned with the impact of nitrate levels on ecological stability and plays an important role in the global nitrogen cycle. However, the complex nitrate source and transport processes in the coastal area and the contribution of nitrogen cycle transformation processes make it a challenge to reveal the nitrate dynamics in the coastal hydrological system. In this study, we investigated the effects of land use types and hydrological interactions on nitrate dynamics in the coastal region using nitrogen composition combined with multi-isotope and Bayesian mixing modeling, focusing on the differences in paleo-saltwater intrusion area (PSIA) and modern seawater intrusion area (MSIA). The results showed that for nitrate sources in PSIA, soil N contributed the highest proportion (45.8% - 77.7%), while the highest proportion is manure and sewage (36.0% - 45.0%) in MSIA. Land use type is the key factor controlling the nitrate dynamics by changing the source and hydrological interaction process. The main source in saline lands is soil N, while manure and sewage are the main source in urban lands. In agricultural lands, nitrate sources are mainly a mixture of fertilizer N, soil N, manure and sewage. Compared with modern seawater intrusion, factors such as deeper burial and strong closure of palaeo-saltwater have led to a slowdown in nitrogen source input, making soil N the main source in PSIA. Nitrification can be promoted within a certain range of TDS in MSIA and can be inhibited when TDS exceeds this range in PSIA. Denitrification was inhibited with elevated TDS in both areas. In addition, nitrification is suppressed in agricultural lands under the influence of palaeo-saltwater intrusion, resulting in lower levels of nitrate pollution than that in MSIA. Nitrate levels here are less related to external inputs and more related to internal nitrate accumulation.