Land use effect and hydrological control on nitrate yield in subtropical mountainous watersheds

Nitrate export in small subtropical watersheds is rarely observed and the estimation of individual land use nitrate yield from a mixed combination within catchments has scarcely been studied. In this study the nitrate concentrations at 16 nested catchments in the Chi-Chia-Wan watershed in Central Taiwan were measured during 2007-2008. A 3-layer TOPMODEL was applied to estimate daily discharge for ungauged sub-catchments. The observed nitrate concentrations and the simulated discharges were used for nitrate flux estimations through four flux methods. Meanwhile, a new deconvolution computation was developed to resolve the nitrate yield of each land use from within the mixed combinations. The results showed that the observed mean NO ₃-N concentration in relatively pristine catchments was approximately 0.145 ± 0.103 mg l ^-1, which is comparable with other forestry catchments around the world. However, the higher rainfall/runoff, substantial N deposition, and other nitrogen sources resulted in significantly higher annual export of approximately 238-1018 kg-N km ^-2 yr ^-1. Our deconvolution computation showed that the background yield of natural forestry was ∼351 ±62 kg-N km ^-2 yr ^-1. On the other hand, the extremely high nitrate yield of active farmland was ∼308, 170 ± 19 241 kg-N km ^-2 yr ^-1 due to over-fertilization. The deconvolution computation technique is capable of tracing the mixed signals at the outlet back to the nitrate productions from varied land use patterns. It advances the application of river monitoring network. The typical values of nitrate yields can serve as a guideline for land management. Comparing the nitrogen input and output, we found some nitrogen missing in the cycling which may indicate certain removal processes and we therefore suggest further study to be carried out to fully understand nitrogen cycling in subtropics.