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.