Key predictors of soil organic matter vulnerability to mineralization differ with depth at a continental scale

Abstract

Soil organic matter (SOM) is the largest terrestrial pool of organic carbon, and potential carbon-climate feedbacks involving SOM decomposition could exacerbate anthropogenic climate change. However, our understanding of the controls on SOM mineralization is still incomplete, and as such, our ability to predict carbon-climate feedbacks is limited. To improve our understanding of controls on SOM decomposition, A and upper B horizon soil samples from 26 National Ecological Observatory Network (NEON) sites spanning the conterminous U.S. were incubated for 52 weeks under conditions representing site-specific mean summer temperature and sample-specific field capacity (−33 kPa) water potential. Cumulative carbon dioxide respired was periodically measured and normalized by soil organic C content to calculate cumulative specific respiration (CSR), a metric of SOM vulnerability to mineralization. The Boruta algorithm, a feature selection algorithm, was used to select important predictors of CSR from 159 variables. A diverse suite of predictors was selected (12 for A horizons, 7 for B horizons) with predictors falling into three categories corresponding to SOM chemistry, reactive Fe and Al phases, and site moisture availability. The relationship between SOM chemistry predictors and CSR was complex, while sites that had greater concentrations of reactive Fe and Al phases or were wetter had lower CSR. Only three predictors were selected for both horizon types, suggesting dominant controls on SOM decomposition differ by horizon. Our findings contribute to the emerging consensus that a broad array of controls regulates SOM decomposition at large scales and highlight the need to consider changing controls with depth.

Climate Effects on Subsoil Carbon Loss Mediated by Soil Chemistry

“…Total organic C of density fractions was determined using an IsoPrime 100 EA-IRMS instrument (IsoPrime Ltd, Cheadle, UK). Water-extractable OC (WEOC) (also described in ref 56) was determined by extraction at a 1:30 soil water For the 20 sites falling on the gradient from higher Ca and Mg to higher Fe and Al, the first principal component axis of the two mineralogical axes was computed using the "stats" package ("prcomp" function, centered and scaled) in R (v. 3.5.3) 57 in RStudio (v. 1.1.423), 58 using natural log-transformed AO-extractable Fe + Al (mol g dry soil −1 ) and natural log-transformed AAextractable Ca + Mg (mol g dry soil −1 ). The first principal component axis is from here described as the "soil chemistry axis."…”

“…Density distribution of SOC into the free light fraction (FLF) and DF was determined following refs − and is also described in ref . Sodium polytungstate (SPT) solution was adjusted to a density of 1.65 g cm –3 and sonication applied at 750 J g soil –1 (20 g of soil in 50 mL SPT, sonicated for 5 min at a rate of 50 J s –1 , corresponding to 1500 J total, or 30 J mL –1 ).…”

“…Total organic C of density fractions was determined using an IsoPrime 100 EA-IRMS instrument (IsoPrime Ltd, Cheadle, UK). Water-extractable OC (WEOC) (also described in ref ) was determined by extraction at a 1:30 soil water ratio, with 2 h shaking at 20 °C followed by centrifugation (8 min at 3824 rcf) and filtration with combusted glass fibers (Whatman GF/F, 0.7 μm pore size). Total organic C concentrations in extracts were determined using a Shimadzu TOC-V (Shimadzu Scientific Instruments, Columbia, MD, USA) analyzer with non-dispersive infrared (NDIR) detection.…”

“…Incubations were conducted over the period 2016–2020. Reference includes the incubation methodology for 365 d incubations of A and upper B-horizons conducted at MST and field capacity (−33 kPa). In this study, we used bulk (<2 mm) upper B horizon soils (drawn from a five-core composite sample, n = 1 for each temperature-by-moisture-by-site treatment combination) incubated for 91 d at temperature levels corresponding to the site MST and at MST +5 °C (Table S5) and variable moisture levels (−400, −150, and −33 kPa).…”

“…Incubation headspace gas sampling is described in ref . Briefly, the headspace gas carbon dioxide (CO 2 ) concentration was measured at time intervals determined by the previous cycle respiration rate, which was estimated by measuring headspace CO 2 on a LI-COR LI-6250 unit (LI-COR Biosciences, Lincoln, NE, USA) at the time of sampling (Figure S4).…”