Environmental controls on density‐based soil organic carbon fractionations in global terrestrial ecosystems

Abstract

Categorization of soil organic carbon (SOC) into different functional subpools according to their recalcitrance and protective mechanisms helps better understand ecosystems organic carbon (OC) dynamics, and various attempts have been made to explore the suitable experimental fractionation method for such purpose. However, most previous studies neglected the influences of environmental factors on the effectiveness of varying fractionation methods. Density fractionation has shown great promise in elucidating SOC immobilization mechanisms. Here, we compared three varying types of density fractionation methods (density, density + dispersion, and density + other procedures) for categorizing the SOC into three functional pools, that is, active OC (OCactive), moderately stable OC (OCm‐stable), and stable OC, (OCstable) using global data compiled for 95 sites in 31 published studies, and examined the influences of climate (mean annual temperature [MAT] and annual precipitation), vegetation type, and soil properties (soil depth, clay content, and soil type) on SOC fractions determined by the three density fractionation methods. The percentage of OCm‐stable fraction was found to be highest using the density method and lowest using the density + dispersion method, due to differential density ranges between the two methods. At a global scale, the contents of total SOC and its OC fractions decreased with temperature. Precipitation had no apparent influences on the subdivided SOC fractions using either the density + dispersion method or the method of density + other procedures, whereas soil type constrained the effect of precipitation on SOC fractions using the density method. The percentage of OCm‐stable determined by the density + dispersion method was more responsive to MAT and vegetation type than that by the other two methods. The percentage of OCstable determined by the method of density + other procedures was significantly and positively related to the clay content as the OCstable based on this method included small particles. For all the three methods of fractionation, soil type had a greater influence than the clay content on the SOC fractions, especially the OCm‐stable and the OCstable. For soil type characterized by rich metal oxides, both the density method and the method of density + other procedures could be used for SOC fractionation. For soil type rich in nutrients, the density + dispersion method would have higher sensitivity for distinguishing the OCm‐stable.

Global pattern of organic carbon pools in forest soils

“…The similar MAOC pool sizes among contrasting climatic zones can be explained by the overriding controls by factors such as soil physico‐chemical properties (i.e., the contents of Fe/Al oxides; Figure 7; Figure S11d,e), because stronger ligand exchange bonding also occurs despite the increasing decomposition and desorption of mineral‐OC bonds under warmer temperature (Conant et al., 2011; Pignatello, 1999). Consequently, mineral protection in SOC persistence overrides the controls by restricted substrate accessibility and microbial activity (Gentsch et al., 2018; Lavallee et al., 2020; Qin et al., 2021; Sun et al., 2023). The MAOC pool increased under wetter climate in forests of the temperate and sub/tropical zones (Figure S11b), likely explainable by the accumulation of soil microbial necromass under more humid conditions.…”

“…Inconsistent with our first hypothesis and some prior studies (Lavallee et al., 2020; Lugato et al., 2021), here we found that the MAOC pool decreased with increasing MAT globally, especially in forests of the sub/tropical zones (Figure 6c). This response is likely caused by faster turnover of organic materials under warmer conditions that overrides the protection of SOC via the mechanism of reactive metal ions such as Fe and Al (Doetterl et al., 2015; Sun et al., 2023).…”