Inverse isolation of dissolved inorganic nitrogen yield for individual land-uses from mosaic land-use patterns within a watershed

Abstract

Abstract. This study combines the observed riverine DIN (dissolved inorganic nitrogen) export and the controlling factors (land-use, population and discharge) to inversely estimate the effective DIN yield factors for individual land-use and DIN per capita loading. A total of 16 sub-catchments, with different land-use compositions on the Danshui River of Taiwan, were used in this study. Observed riverine DIN concentrations and yields varied from 20–450 μM and 400–10 000 kg N km−2 yr−1 corresponding to the increase of urbanization gradient (e.g. building and population). Meanwhile, the transport behaviors changed from hydrological enhancement to dilution with increasing urbanization as well. Our method shows that the DIN yield factors, independent of discharge, are 12.7, 63.9, and 1381.0 μM, for forest, agriculture, and building, respectively, which equals to 444.5, 2236.5, 48 335 kg N km−2 yr−1 at the given annual runoff of 2500 mm. The agriculture DIN yield only accounts for 10% of fertilizer application indicating the complicated N cascade and possible over fertilization. The DIN per capita loading (~0.49 kg N capita−1 yr−1) which is lower than the documented human N emission (1.6–5.5 kg N capita−1 yr−1) can be regarded as an effective export coefficient after treatment or retention. A conducted scenario experiment supports the observations demonstrating the capability for assessment. We therefore, can extrapolate all possible combinations of land-use, discharge, and population density for evaluation. This can provide a strong basis for watershed management and supplementary estimation for regional to global study.

Effects of mountain tea plantations on nutrient cycling at upstream watersheds

“…However, the effects of such uncertainties and omissions on estimating N retention ratio were discussed. We did not calculate the retention ratio for P because the majority of P in watersheds was in particulate forms (Smith et al, 1991) that were not analyzed in our study.…”

“…In most agricultural watersheds, the majority (> 90 %) of P leaves the watersheds in particulate form (Smith et al, 1991), and the loss in dissolved form (i.e., PO 3− 4 ) through runoff is relatively minor (Brady and Weil, 1999). Thus, while the dissolved form of P could respond to land use changes, a complete P budget at watershed scale still requires reliable estimates on the particulate P.…”

“…It has been reported that over-fertilization is common in Japan, Korea, and Taiwan, and despite an estimated 23-63 % over-fertilization the use of fertilizers is still increasing in the region (Ahmed, 1996). In the Danshui River of northeastern Taiwan, the output of dissolved inorganic N ranged from 3 kg ha −1 yr −1 in relatively pristine headwaters covered mostly by natural forests to 100 kg ha −1 yr −1 in a pop-ulated estuary (Lee et al, 2014;Shih et al, 2015). In humid southeastern China, N output from a watershed with 17.5 % of agricultural lands, steep slopes (the watershed has a mean slope of 21 % and the site is located in the hilly upstream region), and very heavy application of N fertilizers (300-1000 kg ha −1 yr −1 ) reached 73 kg ha −1 yr −1 , approximately the same magnitude as those reported here.…”

Effects of mountain agriculture on nutrient cycling at upstream watersheds

“…It has been reported that over-fertilization is common in Japan, Korea, and Taiwan, and despite an estimated 23-63 % over-fertilization the use of fertilizers is still increasing in the region (Ahmed, 1996). In the Danshui River of northeastern Taiwan, the output of dissolved inorganic N ranged from 3 kg ha −1 yr −1 in relatively pristine headwaters covered mostly by natural forests to 100 kg ha −1 yr −1 in a pop-T.-C. Lin et al: Effects of mountain tea plantations on nutrient cycling at upstream watersheds ulated estuary (Lee et al, 2014;Shih et al, 2015). In humid southeastern China, N output from a watershed with 17.5 % of agricultural lands, steep slopes (the watershed has a mean slope of 21 % and the site is located in the hilly upstream region), and very heavy application of N fertilizers (300-1000 kg ha −1 yr −1 ) reached 73 kg ha −1 yr −1 , approximately the same magnitude as those reported here.…”

“…In contrast to N, most of the P fertilizer was retained within the watershed or transported in particulate form so that dissolved P only accounts for a small proportion of the input. In most agricultural watersheds, the majority (> 90 %) of P leaves the watersheds in particulate form (Smith et al, 1991), and the loss in dissolved form (i.e., PO 3− 4 ) through runoff is relatively minor (Brady and Weil, 1999). Thus, while the dissolved form of P could respond to land use changes, a complete P budget at watershed scale still requires reliable estimates on the particulate P.…”