5.4 Anthropogenic nitrogen discharge on DIN transport in global rivers
Excess nutrients from fertilizer applications, pollution discharge, and regulated outflow through rivers from lands to oceans, can seriously impact coastal ecosystems. A reasonable representation of these processes in LSMs and RTMs is very important for understanding human–environment interactions. The implementation of the nitrogen transport module in CAS-FGOALS (Fig. 1) was like the coupling process shown in Fig. 1 in Liu et al. (2019). Note that currently the N transport to the ocean is not added and merits future work. Figure 13 shows the forcing of point-source N and its transport in global rivers simulated by CAS-FGOALS-g3. It was seen that several large point sources of N existed around the globe, with three centers including the middle of the USA (Mississippi River), western Europe, and the north of China (Yellow River and Yangtze River). The annual forcing rate of the source N ranges between 1000–8000 mg∙N∙m-2∙yr-1. Corresponding to this forcing, we saw that almost all the large rivers in the world have been affected by widespread human activities (Fig. 7(b)). The rivers in western Europe and eastern China were the most polluted, where the annual DIN increased by 25–50 Gg∙N∙yr-1. The nitrogen discharge by both runoff N and point source could directly and markedly augment the amount of DIN in most rivers across the world, and was hence an important factor related to riverine environmental problems. Undoubtedly, the Mississippi River Basin, Yellow River Basin, Yangtze River Basin, and western Europe were the most effected regions.
The process of N transport within a river is another important process in terms of N transport. The river temperature, which affected the reactions of the N transported in the river, is shown in Fig. 14(a). The river temperatures overall followed a change with latitude, where the temperature decreased as the geographical position extended northward. Another significant feature was the change with altitude, such as the river temperatures in mountain regions like the Tibetan Plateau or the Andes Mountains being lower than those in the surrounding regions. As a result of the river temperature and transport reactions therein, DIN retention in rivers was also shown in Fig. 14(b), where most of the retention was in western Europe, North China, middle USA, and the south-eastern parts of Australia and South America. In addition to N retention, N transport was also impacted by surface water regulation (Fig. 14(c)), which tended to withdraw DIN at large dams. The withdrawn DIN, corresponding to large water-regulation activities such as in northern India, North China, and parts of the USA, are shown in Fig. 14(d). In general, our results suggested that incorporating schemes related to riverine nitrogen transport and human activities into the model could be an effective way to monitor the global river water quality and evaluate the performance of the global land surface modeling. In future work, the developed model in this work will be coupled with atmospheric and ocean models to simulate and project the global nitrogen cycle.