Transfer of litter-derived N to soil mineral-organic associations: Evidence from decadal 15N tracer experiments

Mineral-organic associations act as mediators of litter-derived N flow to the mineral soil, but the time scales and pathways involved are not well known. To close that gap, we took advantage of decade old N-15 litter labeling experiments conducted in two European forests. We fractionated surface soils by density with limited disaggregating treatment and investigated organic matter (OM) characteristics using delta C-13, delta N-15 and the C/N ratio. Mineral properties were studied by X-ray diffraction and selective dissolution of pedogenic oxides. Three types of associations were isolated: plant debris with few trapped minerals (<1.65 g/cm(3)), aggregates dominated by phyllosilicates (1.65-2.4 g/cm(3)), and single mineral grains and pedogenic oxides with little OM (>2.4 g/cm(3)). A small proportion of N-15 tracer was rapidly attached to single mineral grains, while most of it moved from plant debris to aggregates of low density and progressively to aggregates of higher density that contain a more microbially processed OM. After a decade, 60% of the N-15 tracer found in the investigated horizon was retained in aggregates, while plant debris still contained 40% of the tracer. We present a conceptual model of OM and N flow through soil mineral-organic associations, which accounts for changes in density, dynamics and chemistry of the isolated structures. It suggests that microbial reworking of OM entrapped within aggregates (1.65-2.4 g/cm(3)) causes the gradient of aggregate packing and, further on, controls the flow of litter-derived N through aggregates. For associations with denser material (>2.4 g/cm(3)), mineralogy determines the density of the association, the type of patchy OM attached to mineral surfaces and controls the extent of litter-derived N incorporation. (C) 2011 Elsevier Ltd. All rights reserved.