Mineralization of carbon and nitrogen from plant debris, as affected by debris size and depth of burial

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The relationship between particle size and mineralization has been well established for organic carbon (OC). It has repeatedly been reported that mineralization decreases with decreasing particle size, and that the OC of fine particles (fine silt, clay) has a much slower turnover than that of coarse organic fragments. For nitrogen (N), however, the available data are contradictory. Here, we study the behaviour of OC and N in a range of size fractions over two years of field decomposition. To this end, Medicago sativa ground plants, and Quercus ilex ground litter were mixed with an OC-poor red earth, and were buried at 5 and 40 cm depth, in nylon mesh bags. Selected samplings were taken to study the changes in the distribution of OC and N among the size fractions, obtained by ultrasonic dispersion: coarse sand (2000–200 μm), fine sand (200–50 μm), coarse silt (50–20 μm), fine silt (20–2 μm), and clay (<2 μm). In addition, we studied the clay-occluded OC and N fraction, which is the remains of the oxidation of clay with 30% H 2O 2. The losses of OC (as % of the total OC in the size fraction) consistently decreased with decreasing fraction size, and loss of this element was minimal for clays. In Quercus mixtures, a net input of OC to clays was observed, indicating OC redistribution during decomposition. The losses of N did not show such a clear pattern; in Medicago mixtures, no relationship with size was observed, whereas in Quercus mixtures, they fell slightly with decreasing size. Clay acted as a net sink for N in Quercus mixtures, especially at 40 cm depth. Overall, the clay-occluded OC and N was the most stable fraction. In most cases, losses were significantly greater at 40 cm depth; when a net input of either OC or N was observed for a given fraction, the input was also greater at this depth. Most detectable changes were observed in the light subfraction ( d<2.0) of each size fraction, while in the dense subfraction ( d≥2.0) changes were much smaller. The OC/N ratios of the coarse fractions (coarse sand, fine sand) decreased dramatically, whereas those of finer fractions were largely unaltered. There was a clear trend towards the uniformity of the OC/N ratios of the fractions, and we suggest that this trend might, to some extent, control the N dynamics. Hence, while OC dynamics is driven mainly by physical availability, which depends on fraction size, that of N is also controlled by the quality of the organic debris (OC/N ratio, among other indicators), and the interaction of both controlling factors may result in a size–mineralization relationship that is not as clear as that observed for OC.