Retention of soluble organic nutrients by a forested ecosystem

We document an example of a forested watershed at the Coweeta Hydrologic Laboratory with an extraordinary tendency to retain dissolved organic matter (DOM) generated in large quantities within the ecosystem. Our objectives were to determine fluxes of dissolved organic C, N, and P (DOC, DON, DOP, respectively), in water draining through each stratum of the ecosystem and synthesize information on the physicochemical, biological and hydrologic factors leading to retention of dissolved organic nutrients in this ecosystem. The ecosystem retained 99.3, 97.3, and 99.0% of water soluble organic C, N and P, respectively, produced in litterfall, throughfall, and root exudates. Exports in streamwater were 4.1 kg ha yr of DOC, 0.191 kg ha yr of DON, and 0.011 kg ha yr of DOP. Fluxes of DON were greater than those of inorganic N in all strata. Most DOC, DON, and DOP was removed from solution in the A and B horizons, with DOC being rapidly adsorbed to Fe and Al oxyhydroxides, most likely by ligand exchange. DON and DOC were released gradually from the forest floor over the year. Water soluble organic C produced in litterfall and throughfall had a disjoint distribution of half-decay times with very labile and very refractory fractions so that most labile DOC was decomposed before being leached into the mineral soil and refractory fractions dominated the DOC transported through the ecosystem. We hypothesize that this watershed retained soluble organic nutrients to an extraordinary degree because the soils have very high contents of Fe and Al oxyhydroxides with high adsorption capacities and because the predominant hydrologic pathway is downwards as unsaturated flow through a strongly adsorbing A and B horizon. The well recognized retention mechanisms for inorganic nutrients combine with adsorption of DOM and hydrologic pathway to efficiently prevent leaching of both soluble inorganic and organic nutrients in this watershed. Abbreviations: DOC – dissolved organic carbon DOM – dissolved organic matter DON – dissolved organic nitrogen DOP – dissolved organic phosphorous