Abstract
Soil-borne colloids have been linked to long-distance transport of radionuclides, metal(loid)s and nutrients. Colloid-associated nitrogen (N) will have different mechanisms of biogeochemical cycling and potential for water-borne transport over longer distances compared to dissolved N. The role that colloids play in the supply and mobility of N within catchments discharging into the Great Barrier Reef (GBR) lagoon is unexplored. Here, we examine water-dispersible clay (WDC) from soil samples collected from gullies and agricultural drains within three different land uses (sugarcane, non-agricultural land and grazing) within the Townsville area. The proportion of soil N associated with WDC was inversely correlated with total soil N, with up to 45% of the total soil N being colloid-associated in low N gully soils. Within the <0.45 µm fraction of the WDC, only 17–25% of the N was truly dissolved (<3 kDa) at the gully sites compared to 58% in the sugarcane sites. Our results demonstrate the importance of colloidal N and the inaccuracy of assuming N < 0.45 µm is dissolved in the sampled areas, as well as providing an alternate explanation for the large amounts of what has previously been defined as dissolved inorganic N in runoff from non-fertilized grazing land. In particular, they describe why non-fertilized land uses can contribute significant N < 0.45 µm, and why catchment models of nutrient export based on soil N concentrations can over-estimate loads of particulate nitrogen derived from monitoring data (N > 0.45 µm). The findings suggest that managing soil erosion may also contribute to managing N < 0.45 µm.
Highlights
As reported in earlier work examining colloid-facilitated transport of contaminants[25,26,27], the concentration of N in the water-dispersible clay (WDC) was strongly correlated with % C (Pr > F = < 0.001), WDC concentration (Pr > F = < 0.01), soil clay % plus silt % (Pr > F = < 0.01), and soil N content (Pr > F = < 0.001) across all sampling sites, with % C having the strongest correlation to WDC N (Figs 3 and 4)
Within the body of literature on N transport that does exist, studies have reported that colloidal N is an important component of N flux[27]
One study examining N transport in the Southeastern United States reported that colloids were involved in the transport of N and that colloid-bound N could desorb from particulates upon entering an estuary[17]
Summary
Since European settlement of Australia in the late 18th century[1], sediment and nutrient loads to the Great Barrier Reef (GBR) marine zone have increased dramatically as a result of land-use changes such as urbanization, mining and agricultural intensification[1,2,3]. During this time period, total suspended solids (TSS), total nitrogen (N) and total phosphorus (P) delivery to the GBR lagoon has increased 5.5, 5.7, and 8.9 fold, respectively[3]. As a result of this long-distance mobility, colloids have the potential to be a vector by which nutrients are transported longer distances than would otherwise be possible[17], entering the GBR lagoon and possibly contributing to degradation of water quality and ecosystem health[5,6,18]
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