title: Modeling the terrestrial N processes in a small mountain catchment through INCA-N: A case study in Taiwan.
abstract: Riverine dissolved inorganic nitrogen (DIN) is an important indicator of trophic status of aquatic ecosystems. High riverine DIN export in Taiwan, ~3800kg-Nkm-2yr-1, which is ~18 times higher than the global average, urges the need of thorough understanding of N cycling processes. We applied INCA-N (Integrated Nitrogen Catchment Model) to simulate riverine DIN export and infer terrestrial N processes using weekly rainwater and streamwater samples collected at the Fushan Experimental Forest (FEF) of northern Taiwan. Results showed that the modeled discharge and nitrate export are in good agreement with observations, suggesting the validity of our application. Based on our modeling, the three main N removal processes, in the order of descending importance, were plant uptake, riverine N transport and denitrification at FEF. The high plant uptake rate, 4920kg-Nkm-2yr-1, should have led to accumulation of large biomass but biomass at FEF was relatively small compared to other tropical forests, likely due to periodic typhoon disruptions. The low nitrate concentration but high DIN export highlights the importance of hydrological control over DIN export, particularly during typhoons. The denitrification rate, 750kg-Nkm-2yr-1, at FEF was also low compared to other tropical forest ecosystems, likely resulting from quick water drainage through the coarse-loamy top soils. The high DIN export to atmospheric deposition ratio, 0.45, suggests that FEF may be in advanced stages of N excess. This simulation provides useful insights for establishing monitoring programs and improves our understanding N cycling in subtropical watersheds.
authors.affiliation: Department of Geography, National Taiwan University, Taipei, Taiwan.
authors.forename: Meng-Chang
authors.initials: MC
authors.lastname: Lu
authors.affiliation: Department of Geography, National Taiwan University, Taipei, Taiwan.
authors.forename: Chung-Te
authors.initials: CT
authors.lastname: Chang
authors.affiliation: Department of Life Science, National Taiwan Normal University, Taipei, Taiwan.
authors.forename: Teng-Chiu
authors.initials: TC
authors.lastname: Lin
authors.affiliation: School of Forestry and Resource Conservation, National Taiwan University, Taipei, Taiwan.
authors.forename: Lih-Jih
authors.initials: LJ
authors.lastname: Wang
authors.affiliation: Division of Silviculture, Taiwan Forestry Research Institute (TFRI), Taipei, Taiwan.
authors.forename: Chiao-Ping
authors.initials: CP
authors.lastname: Wang
authors.affiliation: Department of Geography, National Taiwan University, Taipei, Taiwan.
authors.forename: Ting-Chang
authors.initials: TC
authors.lastname: Hsu
authors.affiliation: Department of Geography, National Taiwan University, Taipei, Taiwan. Electronic address: riverhuang@ntu.edu.tw.
authors.forename: Jr-Chuan
authors.initials: JC
authors.lastname: Huang
keywords: Denitrification; Dissolved inorganic nitrogen (DIN); Fushan Experimental Forest; Plant uptake; Subtropical catchment