Divergent controls on carbon concentration and persistence between forests and grasslands of the conterminous US

L. M. Matosziuk,J. A. Hatten,B. D. Strahm,A. R. Possinger,C. W. Swanston,M. Bowman,K. A. Heckman,L. E. Nave,M. SanClements,A. Gallo,C. Rasmussen,T. L. Weiglein

doi:10.1007/s10533-020-00725-z

Abstract

Variation in soil organic C (%OC) concentration has been associated with the concentration of reactive Fe- and Al-oxyhydroxide phases and exchangeable Ca, with the relative importance of these two stabilizing components shifting as soil pH moves from acid to alkaline. However, it is currently unknown if this pattern is similar or different with regard to measures of soil C persistence. We sampled soils from 3 horizons (uppermost A, uppermost B, C or lowest B horizons) across a pH gradient of 11 grass-dominated and 13 deciduous/mixed forest-dominated NEON sites to examine similarities and differences in the drivers of C concentration and persistence. Variation in C concentrations in all soils could be linked to abundances of Fe, Al and Ca, but were not significantly linked to variation in soil C persistence. Though pH was related to variation in Δ14OC, higher persistence was associated with more alkaline pH values. In forested soils, depth explained 75% of the variation in Δ14OC (p < 0.0001), with no significant additional correlations with extractable metal phases. In grasslands, soil organic C persistence was not associated with exchangeable Ca concentrations, but instead was explained by depth and inorganic C concentrations (R2 = 0.76, p < 0.0001), implying stabilization of organic C through association with carbonate precipitation. In grasslands, measures of substrate quality suggested greater persistence is also associated with a more advanced degree of decomposition. Results suggest that explanatory variables associated with C concentrations differ from those associated with persistence, and that reactive Fe- and Al-oxyhydroxide phases may not be present in high enough concentrations in most soils to offer any significant protective capacity. These results have significant implications for our understanding of how to model the soil C cycle and may suggest previously unrecognized stabilization mechanisms associated with carbonates and forms of extractable Si.

Highlights

Our understanding of the relative importance of drivers of soil organic C (SOC) persistence and turnover has been evolving over the past decades
On average (Table 1), %OC values did not differ between grassland and forest soils (p = 0.1228, Fig. 2), grasslands were significantly depleted in D14OC in comparison to forests (122.7 % ± 54.5 %, p = 0.0275, Fig. 3) and had lower C: N ratios (3.91 ± 1.38, p = 0.0060)
Across the suite of temperate forest and grassland soils from National Ecological Observatory Network (NEON) we found substantial differences in the drivers of C concentration and persistence, both across and between biomes

Summary

Introduction

Our understanding of the relative importance of drivers of soil organic C (SOC) persistence and turnover has been evolving over the past decades. Fe- and Aloxyhydroxide phases have been highlighted as possessing a unique role in the stabilization of SOC due to their ubiquitous nature, high specific surface area, and abundance of reactive hydroxyl groups. This is true of studies of both natural soils (Torn et al 1997; Rasmussen et al 2005; Kramer and Chadwick 2018, and others), and artificial soils used in laboratory incubation and batch flow-through experiments (see references in Kleber et al 2015). Exchangeable Ca has been associated with increasing SOC concentrations in more arid systems (Rasmussen et al 2018; Rowley et al 2018).

Methods

Results

Discussion

Conclusion