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.