Global distribution, formation and fate of mineral‐associated soil organic matter under a changing climate: A trait‐based perspective

Abstract

Soil organic matter (SOM) is the largest actively cycling reservoir of terrestrial carbon (C), and the majority of SOM in Earth's mineral soils (~65%) is mineral‐associated organic matter (MAOM). Thus, the formation and fate of MAOM can exert substantial influence on the global C cycle. To predict future changes to Earth's climate, it is critical to mechanistically understand the processes by which MAOM is formed and decomposed, and to accurately represent this process‐based understanding in biogeochemical and Earth system models. In this review, we use a trait‐based framework to synthesize the interacting roles of plants, soil micro‐organisms, and the mineral matrix in regulating MAOM formation and decomposition. Our proposed framework differentiates between plant and microbial traits that influence total OM inputs to the soil (‘feedstock traits’) versus traits that influence the proportion of OM inputs that are ultimately incorporated into MAOM (‘MAOM formation traits’). We discuss how these feedstock and MAOM formation traits may be altered by warming, altered precipitation and elevated carbon dioxide. At a planetary scale, these feedstock and MAOM formation traits help shape the distribution of MAOM across Earth's biomes, and modulate biome‐specific responses of MAOM to climate change. We leverage a global synthesis of MAOM measurements to provide estimates of the total amount of MAOM‐C globally (~840–1540 Pg C; 34%–51% of total terrestrial organic C), and its distribution across Earth's biomes. We show that MAOM‐C concentration is highest in temperate forests and grasslands, and lowest in shrublands and savannas. Grasslands and croplands have the highest proportion of soil organic carbon (SOC) in the MAOM fraction (i.e. the MAOM‐C:SOC ratio), while boreal forests and tundra have the lowest MAOM‐C:SOC ratio. Drawing on our trait framework, we then review experimental data and posit the effects of climate change on MAOM pools in different biomes. We conclude by discussing how MAOM is integrated into soil C models, and how feedstock and MAOM formation traits may be included in these models. We also summarize the projected fate of MAOM under climate change scenarios (Representative Concentration Pathways 4.5 and 8.5) and discuss key model uncertainties. Read the free Plain Language Summary for this article on the Journal blog.

Emerging relationships among soil microbes, carbon dynamics and climate change

“…(2022) also elaborate the concepts and mechanisms of abiotically mediated priming effect, exploring the linkages with the mineral‐associated organic matter and microbial biomass concepts as defined in Buckeridge et al. (2022) and Sokol et al (2022), and discuss how to introduce the priming effect into models to advance the understanding of soil C responses to climate change.…”

“…However, we still ignore the spatial distribution of MAOM across different biomes, and most importantly, whether MAOM is sensitive to climate change in time‐scales relevant to the natural climate solutions needed in the 21st century. Sokol et al (2022) identify meaningful plant and microbial traits that regulate MAOM formation and decomposition, as a framework to address climate change effects. Using a literature survey, they build a global database describing MAOM concentration (as a measure of stocks) and proportion (as a measure of the functionality of the SOM pool) across terrestrial biomes.…”

“…their temperature and moisture sensitivity at different time‐scales), a suite of ecosystem and Earth system models explicitly incorporates the interactions between plants, microbes and minerals. In their review, Sokol et al (2022) assess the uncertainties related to MAOM predictions under emission scenarios, and pave the road for the inclusion of physico‐chemical mechanisms when investigating the land C‐climate feedback and the potential of soils as natural climate solutions.…”

“…From here, the authors discuss potential applications of the Y‐A‐S framework that are highly relevant for this Special Feature. For instance, how drought‐induced shifts and trade‐offs between microbial traits can impact necromass production and determine the stabilization of soil C in the mineral‐associated pool (see Buckeridge et al., 2022 and Sokol et al, 2022 in this Special Feature), or how microbial adaptation to drying–rewetting cycles can result in nonlinear relationships between soil moisture and soil respiration (see Evans et al., 2022 in this Special Feature). As such, the review of Malik et al (2022) integrates some of the most important aspects investigated in this Special Feature, from the abiotic control on microbial processes to variation of microbial‐moisture relationships across spatiotemporal scales.…”

“…Nottingham et al (2022) focus on humid tropical forest soils across multiple time-scales because they store a vast amount of C globally. Buckeridge et al (2022) and Sokol et al (2022) deal with the new paradigm of plant inputs and microbial necromass interactions with mineral surfaces as mechanisms driving soil C accrual and persistence. Then, two reviews propose new frameworks to address the impacts of changing precipitation patterns on soil microbes and C using moisture response curves (Evans et al, 2022) and microbial traits (Malik & Bouskill, 2022).…”