Initial Decomposition and Humification Dynamics of Ponderosa Pine Fine Roots and Needles

To understand the influence of litter chemistry and microclimate on the long-term stabilization of plant inputs, it is essential to better understand biological and chemical regulation of the conversion of litter to stable soil organic matter (SOM). We present first-year results from a 3-year field study examining the fate of 13C- and 15N- labeled Pinus ponderosa in an 80-year-old conifer forest in the Sierra Nevada, CA on an Ultic Haploxeralf. Our objectives are to assess the effects of litter type (fine roots vs. needles) and substrate placement depth (O vs. A horizon) on rates of C and N mineralization, immobilization into microbial biomass and specific microbial groups, and stabilization into SOM fractions. Data will be presented on recovery of 13C and 15N in soil microbial, mineral and SOM fractions after 152 d and C respiration over the initial 300 d. In situ litter decomposition, as estimated by 13C respiration, of needles exceeded that of roots by 270% at 61 d, by 140% at 152 d and was similar for the two substrates at 221 d. Comparing the effect of soil depth, pine needles had greater 13C respiration in the O horizon than in the A horizon through 152 d, while the rate of fine root decay was not significantly different between soil depths through 221 d.