Abstract
Abstract. Oceania rivers are hotspots of DIN (dissolved inorganic nitrogen) and DIP (dissolved inorganic phosphorus) transport due to humid/warm climate, typhoon-induced episodic rainfall and high tectonic activity that create an environment favorable for high/rapid runoff and soil erosion. In spite of its uniqueness, effects of hydrologic controls and land use on the transport behaviors of DIN and DIP are rarely documented. A 2 yr monitoring study for DIN and DIP from three headwater catchments with different cultivation gradient (0 To 8.9%) was implemented during a ~ 3 day interval with an additional monitoring campaign at a 3 h interval during typhoon periods. Results showed the DIN yields in the pristine, moderately cultivated (2.7%), and intensively cultivated (8.9%) watersheds were 8.3, 26, and 37 kg N ha−1 yr−1, respectively. For the DIP yields, they were 0.36, 0.35, and 0.56 kg P ha−1 yr−1, respectively. Higher year-round DIN concentrations and five times larger in DIN yields in intensively cultivated watersheds indicate DIN is more sensitive to land use changes. The high background DIN yield from the relatively pristine watershed was likely due to high atmospheric nitrogen deposition and large subterranean N pool. The correlations between runoff and concentration reveals that typhoon floods purge out more DIN from the subterranean reservoir, i.e., soil, by contrast, runoff washes off surface soil resulting in higher suspended sediment with higher DIP. Collectively, typhoon runoff contributes 20–70% and 47–80%, respectively, to the annual DIN and DIP exports. The DIN yield to DIP yield ratio varied from 97 to 410, which is higher than the global mean of ~ 18. Such a high ratio indicates a P-limiting condition in stream and the downstream aquatic environment. Based on our field observation, we constructed a conceptual model illustrating different remobilization mechanisms for DIN and DIP from headwaters in a mountainous river, which is analogous to typical Oceania rivers and the headwater of large rivers in similar climate zones. Our study advanced our understanding about the role of cyclones, which exert hydrological control, and land use on nutrient export in the Oceania region, benefiting watershed management under the context of climate change.
Highlights
The global biogeochemical cycles of nitrogen (N) and phosphorus (P) have been significantly altered due to the increasing demand of food anSd oenliedrgyEcaorntshumption caused by increasing population and human activities (Galloway and Cowling, 2002; Seitzinger et al, 2010)
DIN concentration (DINC) : DIP concentration (DIPC) at the lowest given Q (Fig. 7b–d), i.e., groundwater, reflects the cultivation gradient as well, implying the long-term agricultural activities may have influenced the groundwater. Such high DINC : DIPC indicates the aquatic ecosystem is mainly limited by dissolved inorganic phosphorus (DIP) (Tseng et al, 2010)
The rainfall–runoff process plays a critical role in releasing dissolved inorganic nitrogen (DIN) and DIP via different mechanisms from subsurface and surface reservoirs, respectively, into streams, modulating the ratio of DINC to DIPC that may influence the production and phytoplankton/microbial community structures in the downstream aquatic system
Summary
The global biogeochemical cycles of nitrogen (N) and phosphorus (P) have been significantly altered due to the increasing demand of food anSd oenliedrgyEcaorntshumption caused by increasing population and human activities (Galloway and Cowling, 2002; Seitzinger et al, 2010). The phosphorus entering into the environment has been doubled due to mining and the use of rock phosphate as fertilizer, detergent additives, animal feed supplements, and other technical uses (Bennett et al, 2001; USGS, 2008). The increasing discharge of dissolved inorganic nitrogen (DIN) and phosphorus (DIP) from rivers may subsequently induce eutrophication in freshwater. The imbalanced export of DIN and DIP from land/soil that cause a change in the riverine DIN : DIP ratio may alter phytoplankton community structures, deteriorating the ecosystem (Justic et al, 1995; Howarth et al, 1996; Rabalais et al, 1996; Elser et al, 2009)
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