Colloidal nitrogen is an important and highly-mobile form of nitrogen discharging into the Great Barrier Reef lagoon.

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.