Controls on Natural Nitrogen‐15 and Carbon‐13 Abundances in Forest Soil Organic Matter

We used δ 15 N and δ 13 C 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 δ 15 N and δ 13 C values while decomposition increased δ 15 N and δ 13 C values. Leaf and root litter inputs averaged −3.8 and −1.6‰ δ 15 N and −27.3 and −28.2‰ δ 13 C, respectively. Field experiments showed that low surface soil δ 15 N and δ 13 C values resulted when litter inputs were high. Laboratory experiments showed that overall isotopic fractionation during decomposition left residual soil N and C enriched in 15 N and 13 C, and could explain the high δ 15 N and δ 13 C values observed in deeper forest soils (+5.9‰ δ 15 N and −23.6‰ δ 13 C for 10 to 20 cm soils). Our results suggest two pools of naturally labeled N in forest soils: surface N with low δ 15 N values and subsoil N with high δ 15 N 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 13 C depleted litter components, such as lignin, during long‐term decomposition in forest soils.