Initial radiocarbon (14C) results of compound class persistence across a climate gradient in California grassland soils

Abstract

Soils mediate the rapid cycling of carbon through the critical zone. 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 series of grassland meadows across California ranging in climatic conditions including temperature and precipitation. Sites include grassland meadows in Angelo Coast Range Reserve, Hopland Extension Reserve and Sedgwick reserve, and which receives 2160, 940, and 380 mm yr-1 of rainfall and is dominated by Avena spp. We sampled 1m soil pits by ~10cm intervals 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 about +50‰ to about +10‰ in the O-horizons to a range of about -150‰ to about -650‰ in the deepest horizons. At the Hopland site, 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 a direct measurement of the persistence between these phases.