Phosphorus Binding to Nanoparticles and Colloids in Forest Stream Waters

Elemental contents in catchment headwaters are indicative of the load of nutrients and minerals cycled or released from ecosystems, yet little is known about natural colloids (1–1000 nm) and especially natural nanoparticles (NNP, 1–100 nm) as nutrient carriers in forested headwater streams. We hypothesize that the majority of P is bound to NNP in forest streams but that their size and composition varies for different forested headwater systems. Four forested sites in Germany and one in Norway, which differ in total P content, were sampled for stream water and analyzed for colloids. The samples were fractionated using field flow fractionation coupled to inductively coupled plasma–mass spectrometry and an organic C detector. The results showed that NNP and colloids from all sites could be separated into three distinct fractions (approximately 1–20 nm, >20–60 nm, and >60 nm). The elemental concentrations of P, organic C, Al, Si, Fe, and Mn in the fractions differed among the five sites. However, cluster analysis showed that each fraction had unique elemental signatures with different preferential P binding partners. Phosphorus was preferentially associated with Fe in the smallest size fraction, with an increasing contribution of organic-C-associated P as the fraction size increased. The largest fraction was dominated by clay minerals. Also, the data indicated that the relative contribution of the NNP and colloidal fractions for ecosystem nutrient supply rises as total P concentrations decline. The study highlighted the still underestimated importance of NNP for matter transport in forest streams and thus P cycling.