Where Is Needle- and Root-Derived Soil Organic Matter After 10 Years of Decomposition in a Temperate Forest?

All soil organic matter (SOM) is ultimately derived from plant litter. The fate of plant litter in ecosystems determines soil carbon (C) storage and nutrient availability with far-reaching implications for ecosystems and global change. However, little is known about the process by which litter becomes SOM (as opposed to the well-studied controls on rates of C and nitrogen (N) loss from litter). We are investigating whether litter type affects where in soils litter-derived C and N eventually reside. Specifically, we are investigating whether litter type affects which minerals the C and N are associated with and how much C is in the microbial pool after a decade. We incubated 15N and 13C-labeled Pinus ponderosa needle and fine root litter in the Blodgett Experimental Forest in the Sierra Nevada foothills for 10 years. A two-way factorial design was used with needle and root litter placed into O and A soil horizons. In 2001, litter was inserted into the given horizon within soil mesocosms (10.2 cm diameter x 24 cm long PVC) that had two 5 x 5 cm mesh windows to allow contact with the surrounding soil. After 0.5, 1, 1.5, 4.5, and 10 years, the soil mesocosms were collected from the field. Isotopes were used to measure the percent recovery of the litter C and N in the bulk soil of the O and A horizons. To investigate mineral associations of the added litter C and N after 10 years, we sequentially fractionated the soils by density. The fractions were a free light fraction (<1.75 g cm-3), a fraction dominated by phyllosilicate secondary minerals (1.75-2.5 g cm-3), a quartz and feldspar-dominated fraction (2.5-2.78 g cm-3), and a fraction dominated by biotite with kaolinite and iron oxide coatings (>2.78 g cm-3). To quantify the amount of litter-derived C actively cycling in the microbial pool after 10 years and use of the C by different microbial groups, we measured the 13C in phospholipid fatty acids (PLFAs). After 10 years, more root litter C (about 40%) was retained in the soil than needle litter C (about 25%). Less than 0.15% of the remaining litter C (0.06% of originally applied) was found actively cycling in microbial PLFA's. Needle and root C did not differ in the amount remaining still in the active microbial pool. Preliminary data indicate that unlike after one year, there were no microbial groups with strong preferences for the added root or needle C relative to other microbial groups. The amount of root and needle C and N associated with the different mineral groups will also be presented.