G05_P02

Divergence of compound class persistence in a California grassland soil

Grant K¹,  Repasch M^1,2, Finstad K¹, Broek T^1,3, McFarlane K¹

¹Lawrence Livermore National Laboratory, Livermore, United States, ²Institute of Arctic and Alpine Research, University of Colorado, Boulder, Boulder, United States, ³National Ocean Sciences Accelerator Mass Spectrometry (NOSAMS) Facility, Woods Hole Oceanographic Institution, Woods Hole, United States

Soils store more carbon than the atmosphere and vegetation combined. Soil organic carbon (SOC) is composed of a complex mixture of plant and microbial derived organic compounds with distinct cycling timescales. The residence time of individual SOC components depends on a combination of factors, including compound reactivity, mineral association, and climate conditions, making it difficult to accurately quantify. However, radiocarbon analysis of specific compound classes can disentangle the mixture of SOC ages within a single sample. We modified methods to measure the Δ¹⁴C of distinct compound classes (lipids, amino acids, and carbohydrates) from bulk and physically fractionated grassland soils. Additionally, we measured the Δ¹⁴C of the water-extractable fraction (WEOC) and the residual acid-insoluble fraction. Samples were collected from a grassland meadow in Hopland, CA which receives 940 mm yr-1 of rainfall and is dominated by Avena barbata. We sampled a 1m soil pit at depth intervals (0-10, 10-20, 20-50, 50-100 m) to study changes in SOC persistence with depth. We used solid state ¹³C-NMR to measure the relative abundance of the target compound classes in soil. The Δ14C of bulk soil decreased from +28±6‰ at 0-10cm to -495±23‰ at the 50-100cm depth interval. The clay fraction (<63μm) had higher Δ¹⁴C values than both the bulk (<2mm) and sand (<2mm to >63μm) fractions. WEOC Δ¹⁴C values ranged from modern to about -45.6‰. Δ¹⁴C values of total extracted lipids ranged from 36±4‰ at the surface to -215±3‰ at depth. Quantifying the age distribution of distinct compound classes gives insight into SOC persistence.