Much research focuses on increasing carbon storage in mineral-associated organic matter (MAOM), in which carbon may persist for centuries to millennia. However, MAOM-targeted management is ...insufficient because the formation pathways of persistent soil organic matter are diverse and vary with environmental conditions. Effective management must also consider particulate organic matter (POM). In many soils, there is potential for enlarging POM pools, POM can persist over long time scales, and POM can be a direct precursor of MAOM. We present a framework for context-dependent management strategies that recognizes soils as complex systems in which environmental conditions constrain POM and MAOM formation.
Abstract
The largest terrestrial organic carbon pool, carbon in soils, is regulated by an intricate connection between plant carbon inputs, microbial activity, and the soil matrix. This is manifested ...by how microorganisms, the key players in transforming plant-derived carbon into soil organic carbon, are controlled by the physical arrangement of organic and inorganic soil particles. Here we conduct an incubation of isotopically labelled litter to study effects of soil structure on the fate of litter-derived organic matter. While microbial activity and fungal growth is enhanced in the coarser-textured soil, we show that occlusion of organic matter into aggregates and formation of organo-mineral associations occur concurrently on fresh litter surfaces regardless of soil structure. These two mechanisms—the two most prominent processes contributing to the persistence of organic matter—occur directly at plant–soil interfaces, where surfaces of litter constitute a nucleus in the build-up of soil carbon persistence. We extend the notion of plant litter, i.e., particulate organic matter, from solely an easily available and labile carbon substrate, to a functional component at which persistence of soil carbon is directly determined.
Rising atmospheric CO2 concentrations have increased interest in the potential for forest ecosystems and soils to act as carbon (C) sinks. While soil organic C contents often vary with tree species ...identity, little is known about if, and how, tree species influence the stability of C in soil. Using a 40 year old common garden experiment with replicated plots of eleven temperate tree species, we investigated relationships between soil organic matter (SOM) stability in mineral soils and 17 ecological factors (including tree tissue chemistry, magnitude of organic matter inputs to the soil and their turnover, microbial community descriptors, and soil physicochemical properties). We measured five SOM stability indices, including heterotrophic respiration, C in aggregate occluded particulate organic matter (POM) and mineral associated SOM, and bulk SOM δ15N and ∆14C. The stability of SOM varied substantially among tree species, and this variability was independent of the amount of organic C in soils. Thus, when considering forest soils as C sinks, the stability of C stocks must be considered in addition to their size. Further, our results suggest tree species regulate soil C stability via the composition of their tissues, especially roots. Stability of SOM appeared to be greater (as indicated by higher δ15N and reduced respiration) beneath species with higher concentrations of nitrogen and lower amounts of acid insoluble compounds in their roots, while SOM stability appeared to be lower (as indicated by higher respiration and lower proportions of C in aggregate occluded POM) beneath species with higher tissue calcium contents. The proportion of C in mineral associated SOM and bulk soil ∆14C, though, were negligibly dependent on tree species traits, likely reflecting an insensitivity of some SOM pools to decadal scale shifts in ecological factors. Strategies aiming to increase soil C stocks may thus focus on particulate C pools, which can more easily be manipulated and are most sensitive to climate change.
We investigated effects of tree species planted in replicated plots of a common garden on five indices of soil organic matter (SOM) stability (heterotrophic soil respiration, bulk soil δ15N and ∆14C, and C in particulate and mineral associated organic matter). Our results suggest tree species regulate SOM stability via the chemical composition of their tissues, especially roots. Some of our stability indices (C in mineral associated SOM and bulk soil ∆14C), though, were negligibly dependent on tree species traits, likely reflecting an insensitivity of some SOM pools to decadal scale shifts in ecological factors. Strategies aiming to increase soil C stocks may thus focus on particulate C pools, which can more easily be manipulated and are most sensitive to climate change.
It has been shown that reactive soil minerals, specifically iron(III) (oxyhydr)oxides, can trap organic carbon in soils overlying intact permafrost, and may limit carbon mobilization and degradation ...as it is observed in other environments. However, the use of iron(III)-bearing minerals as terminal electron acceptors in permafrost environments, and thus their stability and capacity to prevent carbon mobilization during permafrost thaw, is poorly understood. We have followed the dynamic interactions between iron and carbon using a space-for-time approach across a thaw gradient in Abisko (Sweden), where wetlands are expanding rapidly due to permafrost thaw. We show through bulk (selective extractions, EXAFS) and nanoscale analysis (correlative SEM and nanoSIMS) that organic carbon is bound to reactive Fe primarily in the transition between organic and mineral horizons in palsa underlain by intact permafrost (41.8 ± 10.8 mg carbon per g soil, 9.9 to 14.8% of total soil organic carbon). During permafrost thaw, water-logging and O
limitation lead to reducing conditions and an increase in abundance of Fe(III)-reducing bacteria which favor mineral dissolution and drive mobilization of both iron and carbon along the thaw gradient. By providing a terminal electron acceptor, this rusty carbon sink is effectively destroyed along the thaw gradient and cannot prevent carbon release with thaw.
Understanding the cycling of C and N in soils is important for maintaining soil fertility while also decreasing greenhouse gas emissions, but much remains unknown about how organic matter (OM) is ...stabilized in soils. We used nano‐scale secondary ion mass spectrometry (NanoSIMS) to investigate the changes in C and N in a Vertisol and an Alfisol incubated for 365 days with 13C and 15N pulse labeled lucerne (Medicago sativa L.) to discriminate new inputs of OM from the existing soil OM. We found that almost all OM within the free stable microaggregates of the soil was associated with mineral particles, emphasizing the importance of organo‐mineral interactions for the stabilization of C. Of particular importance, it was also found that 15N‐rich microbial products originating from decomposition often sorbed directly to mineral surfaces not previously associated with OM. Thus, we have shown that N‐rich microbial products preferentially attach to distinct areas of mineral surfaces compared to C‐dominated moieties, demonstrating the ability of soils to store additional OM in newly formed organo‐mineral associations on previously OM‐free mineral surfaces. Furthermore, differences in 15N enrichment were observed between the Vertisol and Alfisol presumably due to differences in mineralogy (smectite‐dominated compared to kaolinite‐dominated), demonstrating the importance of mineralogy in regulating the sorption of microbial products. Overall, our findings have important implications for the fundamental understanding of OM cycling in soils, including the immobilization and storage of N‐rich compounds derived from microbial decomposition and subsequent N mineralization to sustain plant growth.
Understanding how organic carbon (C) and nitrogen (N) are cycled in soils is important for decreasing emissions of greenhouse gases and for maintaining soil fertility. We used nano‐scale secondary ion mass spectrometry (NanoSIMS) to examine changes in C and N in two soils. We have found that N‐rich compounds derived from microbial degradation often sorb directly to mineral surfaces not previously associated with organic matter (OM). This is important as it shows the ability of soils to store additional OM in new associations, with this information also assisting in the development of models for OM and nutrient cycling.
The microenvironmental conditions in soil exert a major control on many ecosystem functions of soil. Their investigation in intact soil samples is impaired by methodological challenges in the joint ...investigation of structural heterogeneity that defines pathways for matter fluxes and biogeochemical heterogeneity that governs reaction patterns and microhabitats. Here we demonstrate how these challenges can be overcome with a novel protocol for correlative imaging based on image registration to combine three-dimensional microstructure analysis of X-ray tomography data with biogeochemical microscopic data of various modalities and scales (light microscopy, fluorescence microscopy, electron microscopy, secondary ion mass spectrometry). Correlative imaging of a microcosm study shows that the majority (75%) of bacteria are located in mesopores (<10 μm). Furthermore, they have a preference to forage near macropore surfaces and near fresh particulate organic matter. Ignoring the structural complexity coming from the third dimension is justified for metrics based on size and distances but leads to a substantial bias for metrics based on continuity. This versatile combination of imaging modalities with freely available software and protocols may open up completely new avenues for the investigation of many important biogeochemical and physical processes in structured soils.
Soil carbon sequestration arises from the interplay of carbon input and stabilization, which vary in space and time. Assessing the resulting microscale carbon distribution in an intact pore space, ...however, has so far eluded methodological accessibility. Here, we explore the role of soil moisture regimes in shaping microscale carbon gradients by a novel mapping protocol for particulate organic matter and carbon in the soil matrix based on a combination of Osmium staining, X-ray computed tomography, and machine learning. With three different soil types we show that the moisture regime governs C losses from particulate organic matter and the microscale carbon redistribution and stabilization patterns in the soil matrix. Carbon depletion around pores (aperture > 10 µm) occurs in a much larger soil volume (19-74%) than carbon enrichment around particulate organic matter (1%). Thus, interacting microscale processes shaped by the moisture regime are a decisive factor for overall soil carbon persistence.
Physical separation of soil into different soil organic matter (SOM) fractions is widely used to identify organic carbon pools that are differently stabilized and have distinct chemical composition. ...However, the mechanisms underlying these differences in stability and chemical composition are only partly understood. To provide new insights into the stabilization of different chemical compound classes in physically-separated SOM fractions, we assessed shifts in the biomolecular composition of bulk soils and individual particle size fractions that were incubated in the laboratory for 345 days. After the incubation, also the incubated bulk soil was fractionated. The chemical composition of organic matter in bulk soils and fractions was characterized by ¹³C-CPMAS nuclear magnetic resonance spectroscopy and sequential chemical extraction followed by GC/MS measurements. Plant-derived lipids and lignin were abundant in particulate organic matter (POM) fractions of sand-, silt-, and clay-size and the mineral-bound, clay-sized organic matter. These results indicate that recent conceptualizations of SOM stabilization probably understate the contribution of plant-derived organic matter to stable SOM pools. Although our data indicate that inherent recalcitrance could be important in soils with limited aggregation, organomineral interactions and aggregation were responsible for long-term SOM stabilization. In particular, we observed consistently higher concentrations of plant-derived lipids in POM fractions that were incubated individually, where aggregates were disrupted, as compared to those incubated as bulk soil, where aggregates stayed intact. This finding emphasizes the importance of aggregation for the stabilization of less ‘recalcitrant’ biomolecules in the POM fractions. Because also the abundance of lipids and lignin in clay-sized, mineral-associated SOM was substantially influenced by aggregation, the bioavailability of mineral-associated SOM likely increases after the destruction of intact soil structures.
Increased human‐derived nitrogen (N) loading in terrestrial ecosystems has caused widespread ecosystem‐level phosphorus (P) limitation. In response, plants and soil micro‐organisms adopt a series of ...P‐acquisition strategies to offset N loading‐induced P limitation. Many of these strategies impose costs on carbon (C) allocation by plants and soil micro‐organisms; however, it remains unclear how P‐acquisition strategies affect soil C cycling. Herein, we review the literature on the effects of N loading on P limitation and outline a conceptual overview of how plant and microbial P‐acquisition strategies may affect soil organic carbon (SOC) stabilization and decomposition in terrestrial ecosystems.
Excessive input of N significantly enhances plant biomass production, soil acidification, and produces plant litterfall with high N/P ratios, which can aggravate ecosystem‐level P limitation.
Long‐term N loading can cause plants and soil micro‐organisms to alter their functional traits to increase P acquisition. Plants can release carboxylate exudates and phosphatases, modify root morphological traits, facilitate the formation of symbiotic associations with mycorrhizal fungi and stimulate the abundance of P‐mineralizing and P‐solubilizing micro‐organisms. Releasing carboxylate exudates and phosphatases could accelerate SOC decomposition, whereas changing symbiotic associations and root morphological traits (e.g. an increase in fine root length) may contribute to higher SOC stabilization. Increased relative abundances of P‐mineralizing and P‐solubilizing bacteria can accelerate P mining and SOC decay, which may decrease microbial C use efficiency and subsequently lower SOC sequestration.
The trade‐offs between different plant P‐acquisition strategies under N loading should be among future research priorities due to their cascading impacts on soil C storage. Quantifying ecosystem thresholds for P adaption to increased N loading is important because P‐acquisition strategies are effective when N loading is below the N threshold. Moreover, understanding the response of P‐acquisition strategies at different levels of native soil N availability could provide insight to divergent P‐acquisition strategies across sites and ecosystems. Altogether, P‐acquisition strategies should be explicitly considered in Earth System Models to generate more realistic predictions of the effects of N loading on soil C cycling.
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