13C and 15N stabilization dynamics in soil organic matter fractions during needle and fine root decomposition

Little is known about how the chemical composition of plant litter affects the amount and nature of C and N stabilized as soil organic matter (SOM). We examined the fate of dual labeled 13C and 15N Ponderosa pine fine roots (<2 mm) and needles decomposing for 2 yr in situ in a temperate conifer forest soil in the Sierra Nevada, CA, USA. We compared the distribution and stabilization of litter derived C and N in four SOM pools using a density fractionation procedure followed by an alkaline extraction of the dense fraction into fulvic, humic, and humin fractions. The C turnover times (estimated with natural abundance 14C) of these SOM fractions were distinct and ranged from 5 yr (light fraction) to 260 yr (insoluble humin). Input of C as roots resulted in 28% more total C retained in soil when compared to inputs as needles. Twice as much root 13C was present in the particulate soil (>2 mm) than for needles, while bulk soil (<2 mm) 13C and 15N recoveries were similar between litters. SOM fractions provided greater sensitivity than bulk soil and showed significant differences between litters in both the amount and chemical composition of 13C and 15N compounds recovered within SOM fractions. More needle 13C was retained in humic and humin fractions than was 13C from roots. The chemical composition of stabilized organic molecules differed fundamentally between needle and root sources within the dense fraction SOM pools, especially during the first year. Root inputs were stabilized predominately as N-rich biomolecules in the humic and humin fractions, while needles contributed C-rich biomolecules to these dense fraction SOM pools. The large pulse of C-rich compounds from the more labile needles recovered in the humic and humin fractions did not persist after 1.5 yr, suggesting that low C:N ratio compounds derived from decomposing litters may stabilize more strongly and persist within the dense fraction SOM pools. The fundamental differences in C and N pathways during decomposition and stabilization of below ground (root) and above ground (needle) litters suggest that shifts in plant C allocation may influence the long term stability of plant-derived C in soil.