Application of 13C-labeled litter and root materials for in situ decomposition studies using phospholipid fatty acids

Microorganisms play a central role in litter decomposition and partitioning C between CO2 evolution and sequestration of C into semi-permanent pools in soils. At the ecosystem level, forest stand age influences rates of litter accumulation and quality, and micro-climatology which could affect the microbial community structure and C sequestration processes. Although numerous laboratory experiments have studied the decomposition of model 13C-labeled compounds, few studies have verified these findings under field conditions. The objective of this study was to track decomposition of 13C-labeled Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) materials into the soil microbial community using 13C-phospholipids fatty acid (PLFA) analysis in three different aged forest stands. A field experiment was conducted that had three forest stand age treatments: old-growth (>500 yrs); 8-year-old clear-cut (CC8); and 25-year-old clear-cut (CC25) (landscape reps of n = 2). Each stand age had in situ microcosms that were amended with either 13C-labeled surface litter or root material. Microcosms were destructively sampled seven times over a 22-month period and the soil was analyzed for the relative amounts of 13C incorporated (13C%INCORP) into PLFAs and the proportional distribution of 13C incorporated into PLFAs. The 13C%INCORP was affected by stand age and 13C source with greater 13C%INCORP in samples from CC8 than OG or CC25. Also, the level of 13C%INCORP was greater for labeled litter than root material in five out of the seven sample dates. In general, 18:1ω9 and 18:2ω6,9 (common fungal biomarkers) had the greatest amount of 13C incorporation throughout the study period in both clear-cut and old-growth sites, especially in plots with 13C-labeled litter. Our data showed a low fungal 13C-PLFA: bacterial 13C-PLFA ratio (0.45) 1 month after incubation was initiated compared to 5, 7 and 9 months after incubation (two of these dates were >1.0). This suggests that initially bacteria played a greater role in the decomposition of the added needles with fungi playing a more important role in subsequent sample dates. Our results illustrate that the use of 13C-labeled materials in field studies coupled with13C-PLFA profiling is a powerful tool for determining microbial dynamics during decomposition – enabling statistically significant detection of land management treatment effects on C acquisition by microbial functional groups.