4:00 PM - 4:15 PM
[MIS14-09] Effect of long-term nitrogen fertilization on stream water nitrate concentration in a forested watershed
Keywords:nitrate, natural forest, cation, paired catchment method
Biogeochemical cycling in forest is influenced by interaction between each component, such as vegetation, soil, and microbe, resulting in forming stream water quality. We can identify average biogeochemical cycling in forests by investigating stream water quality. In addition, input of atmospheric substances changes the balance of biogeochemical cycling within the ecosystems, affecting stream water quality. For example, increase of anthropogenic atmospheric nitrogen deposition accelerates nitrate leaching from forest to stream. However, in situ empirical study for the effect of nitrogen addition on stream nitrogen leaching in forested watershed is not enough. In addition, long-term nitrogen fertilization experiment and observation which simulate changing atmospheric nitrogen deposition enables us to understand the ecosystem response and underlying mechanism. This study aimed to clarify the effect of long-term application of nitrogen fertilization in forested watershed on stream water quality with special focus on nitrate concentration.
Study site was located in a natural forest in Nakagawa Experimental Forest of Hokkaido University in northern Hokkaido, Japan. The forest type was conifer-broadleaved mixed forest and understory dwarf bamboo (Sasa senanensis, Sasa kurilensis) grows. We established two watersheds (fertilized [1.4 ha] and control [1.1 ha]) using paired catchment method. In fertilized watershed, we applied granular NH4NO3 (50kgN ha–1 yr–1) once a year from 2002. We collected stream water biweekly during April or May to October or November at the outlet of each watershed. We monitored stream discharge at the gauging weir downstream confluence of the two watersheds. We transported the collected water to the laboratory using cool-box, measured pH and EC, and filtrated. We analyzed the concentrations of anion (nitrate, Chloride, Sulfate) and cation (K+, Na+, Mg2+, Ca2+) in the filtrated water. Here we report the change during 2003 and 2016.
Long-term trend of nitrate concentration showed increasing trend (P<0.001) in fertilized watershed whereas no significant trend in control watershed. In 2010, eight years following the onset of nitrogen fertilization, nitrate concentration increased dramatically and maintained the high range thereafter. We did not observe the substantial increase during first several years, indicating that the forest ecosystem had the capacity to retain the added nitrogen. However, continuous nitrogen load would have accumulated nitrogen in the ecosystem, leading to excess soil nitrogen availability over the ecosystem demand which accelerated nitrate leaching. In fertilized watershed, there was an inter-annual variation in the range of nitrate concentration before substantial increase in 2010: the range was high in 2007 and low in 2008 and 2009 again. This pattern of inter-annual variation was also seen in control watershed, suggesting that inter-annual variation in climate or biological factors would also have affected the nitrate concentration. In the relationship between stream discharge and nitrate flow, the slope was distinct between fertilized (steep) and control (gentle) watersheds, suggesting that runoff of nitrate accelerated with increasing discharge. In the comparison of cation concentrations among watersheds, Na+ and Mg2+ was higher in fertilized watershed. In contrast, Ca2+ was lower in fertilized watershed. No clear trend in K+ concentration. The relationship of nitrogen fertilization and cation concentration was different depending on the ion species. We demonstrated that continuous nitrogen load causes accelerated nitrogen leaching from the ecosystem though the forest has high nitrogen retention capacity in northern Japan.
Study site was located in a natural forest in Nakagawa Experimental Forest of Hokkaido University in northern Hokkaido, Japan. The forest type was conifer-broadleaved mixed forest and understory dwarf bamboo (Sasa senanensis, Sasa kurilensis) grows. We established two watersheds (fertilized [1.4 ha] and control [1.1 ha]) using paired catchment method. In fertilized watershed, we applied granular NH4NO3 (50kgN ha–1 yr–1) once a year from 2002. We collected stream water biweekly during April or May to October or November at the outlet of each watershed. We monitored stream discharge at the gauging weir downstream confluence of the two watersheds. We transported the collected water to the laboratory using cool-box, measured pH and EC, and filtrated. We analyzed the concentrations of anion (nitrate, Chloride, Sulfate) and cation (K+, Na+, Mg2+, Ca2+) in the filtrated water. Here we report the change during 2003 and 2016.
Long-term trend of nitrate concentration showed increasing trend (P<0.001) in fertilized watershed whereas no significant trend in control watershed. In 2010, eight years following the onset of nitrogen fertilization, nitrate concentration increased dramatically and maintained the high range thereafter. We did not observe the substantial increase during first several years, indicating that the forest ecosystem had the capacity to retain the added nitrogen. However, continuous nitrogen load would have accumulated nitrogen in the ecosystem, leading to excess soil nitrogen availability over the ecosystem demand which accelerated nitrate leaching. In fertilized watershed, there was an inter-annual variation in the range of nitrate concentration before substantial increase in 2010: the range was high in 2007 and low in 2008 and 2009 again. This pattern of inter-annual variation was also seen in control watershed, suggesting that inter-annual variation in climate or biological factors would also have affected the nitrate concentration. In the relationship between stream discharge and nitrate flow, the slope was distinct between fertilized (steep) and control (gentle) watersheds, suggesting that runoff of nitrate accelerated with increasing discharge. In the comparison of cation concentrations among watersheds, Na+ and Mg2+ was higher in fertilized watershed. In contrast, Ca2+ was lower in fertilized watershed. No clear trend in K+ concentration. The relationship of nitrogen fertilization and cation concentration was different depending on the ion species. We demonstrated that continuous nitrogen load causes accelerated nitrogen leaching from the ecosystem though the forest has high nitrogen retention capacity in northern Japan.