Broader Implications for land and water management in karst
area
Nitrogen fertilizer is a predominant contributor of nitrogen pollution
in many karst catchments worldwide (Panno et al, 2001; Minet et al.,
2017; Eller and Katz, 2017). Especially, in southwest China, one of the
largest and continuous karst areas in the world, the agricultural
activities are the main cause of aquifer nitrate pollution (Yang et al.,
2013). In this area, farmers usually apply fertilizer in May and
November. According to the transit time pdfs shown in Fig.9, in the dry
season, most rainfall inputs will be stored in aquifer for many months
before they exit the catchment. That means a significant portion of
nitrogen from the fertilizer used in dry period (November) will be
released slowly for a long time, leading the chronic aquifer nitrate
pollution. Denitrification from water via emission of gases is the main
mechanism of nitrogen removal, and reaction rates in karst areas are
very fast, and can reach 15 mg N·m2/d (Heffernan et
al., 2012). According to the results by Yue et al., (2015),
denitrification removes about 46.7% of the nitrate in this area, and
the ratio is similar to that in other karst area, such as 36% in
Kentucky, USA (Husic et al., 2019). Although long residence times may
increase the potential for denitrification (Albertin et al., 2012; Han
et al., 2015), there is also substantial residual nitrogen remaining in
the karstic aquifer (Yue et al., 2019). Therefore, improving the
efficiency of fertilization is an
urgent need to reduce the nitrate contamination.
The largest agricultural fertilization in the region where the Chenqi
catchment is based typically occurs in May, corresponding to the end of
the dry season and beginning of the wet season. The more rapid flows
with short transit times of rainfall inputs, occur via large fractures
(i.e. sinkholes) and from steeper hillslopes unit, and contributed most
flow at the outlet in wet season (Zhang et al., 2017, 2019). Hence,
during heavy rainfall events, nitrogen from fertilizer will rapidly
infiltrate to the aquifer and flow out the catchment through surface
stream or underground channel, leading to the nitrogen loss. Especially,
the sinkholes, mainly distributed in valleys covered by thick soil with
high N inputs due to local fertilizer applications, increase the
concentrated, rapid loss of nitrogen in karst critical zone during wet
period. According to the results from Yue et al., (2019), in the wet
season, up to 94% of annual nitrate export from the headwater
catchments occurred within two months. Therefore, fertilization timing
to earlier in the growing season and restricted application around
sinkholes may help to loss of N during large storm events.