Controls on Natural Nitrogen-15 and Carbon-13 Abundances in Forest Soil Organic Matter
We used δ15N and δ13C measurements to study formation and decay of soil organic matter in surface soils of two oak (Quercus spp.) forests in Wisconsin. There were two controls of soil isotopic compositions: new litter inputs and overall isotopic fractionation during decomposition. Litter inputs lowered soil δ15N and δ13C values while decomposition increased δ15N and δ13C values. Leaf and root litter inputs averaged −3.8 and −1.6‰ δ15N and −27.3 and −28.2‰ δ13C, respectively. Field experiments showed that low surface soil δ15N and δ13C values resulted when litter inputs were high. Laboratory experiments showed that overall isotopic fractionation during decomposition left residual soil N and C enriched in 15N and 13C, and could explain the high δ15N and δ13C values observed in deeper forest soils (+5.9‰ δ15N and −23.6‰ δ13C for 10 to 20 cm soils). Our results suggest two pools of naturally labeled N in forest soils: surface N with low δ15N values and subsoil N with high δ15N values. Natural abundance measurements of soil N may be useful for following the importance of these two N pools in forest N cycles. We found no evidence for selective preservation of 13C depleted litter components, such as lignin, during long-term decomposition in forest soils.
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