Showing papers on "Soil organic matter published in 2022"

Life and death in the soil microbiome: how ecological processes influence biogeochemistry

Abstract: Soil microorganisms shape global element cycles in life and death. Living soil microorganisms are a major engine of terrestrial biogeochemistry, driving the turnover of soil organic matter — Earth’s largest terrestrial carbon pool and the primary source of plant nutrients. Their metabolic functions are influenced by ecological interactions with other soil microbial populations, soil fauna and plants, and the surrounding soil environment. Remnants of dead microbial cells serve as fuel for these biogeochemical engines because their chemical constituents persist as soil organic matter. This non-living microbial biomass accretes over time in soil, forming one of the largest pools of organic matter on the planet. In this Review, we discuss how the biogeochemical cycling of organic matter depends on both living and dead soil microorganisms, their functional traits, and their interactions with the soil matrix and other organisms. With recent omics advances, many of the traits that frame microbial population dynamics and their ecophysiological adaptations can be deciphered directly from assembled genomes or patterns of gene or protein expression. Thus, it is now possible to leverage a trait-based understanding of microbial life and death within improved biogeochemical models and to better predict ecosystem functioning under new climate regimes.

Soil organic matter formation, persistence, and functioning: A synthesis of current understanding to inform its conservation and regeneration

Abstract: Soil organic matter (SOM) provides vital services to humanity. Its preservation and further accrual are key to sustain food production and avoid an irreversible climate crisis. Here we present the processes and drivers of SOM formation and persistence within a coherent state-of-the-art framework. We posit that SOM forms via two distinct pathways depending on whether inputs are water soluble and/or easily solubilized entering the soil as dissolved organic matter (DOM), or they are structural. These distinct inputs form mineral-associated organic matter (MAOM), and particulate organic matter (POM), respectively. Both these SOM fractions have plant and microbial components but in different proportions, with MAOM being more highly microbial. SOM persistence will depend on microbial activity inhibition, the degree of its limitation and carbon use efficiency, and microbial access constraints, primarily due to association to minerals and occlusions in fine aggregates. Climate is the overarching control of SOM persistence, also by affecting ecosystem traits, when persistence is driven by microbial activity inhibition or limitation, largely responsible for POM storage. Soil geochemical traits are the overarching control of SOM persistence driven by microbial access constraints, particularly in the subsoil, specifically controlling MAOM storage. SOM affects soil properties (aggregation, porosity, and cation exchange capacity) which in turn determine the soil's capacity for functioning and ability to provide desired outcomes including erosion and flood prevention, plant productivity, and climate mitigation. The specific properties of SOM which influence its contributions to these functions are discussed, with implications for SOM conservation and regeneration to promote desired outcomes.

Hyphae move matter and microbes to mineral microsites: Integrating the hyphosphere into conceptual models of soil organic matter stabilization

Abstract: Associations between soil minerals and microbially derived organic matter (often referred to as mineral‐associated organic matter or MAOM) form a large pool of slowly cycling carbon (C). The rhizosphere, soil immediately adjacent to roots, is thought to control the spatial extent of MAOM formation because it is the dominant entry point of new C inputs to soil. However, emphasis on the rhizosphere implicitly assumes that microbial redistribution of C into bulk (non‐rhizosphere) soils is minimal. We question this assumption, arguing that because of extensive fungal exploration and rapid hyphal turnover, fungal redistribution of soil C from the rhizosphere to bulk soil minerals is common, and encourages MAOM formation. First, we summarize published estimates of fungal hyphal length density and turnover rates and demonstrate that fungal C inputs are high throughout the rhizosphere–bulk soil continuum. Second, because colonization of hyphal surfaces is a common dispersal mechanism for soil bacteria, we argue that hyphal exploration allows for the non‐random colonization of mineral surfaces by hyphae‐associated taxa. Third, these bacterial communities and their fungal hosts determine the chemical form of organic matter deposited on colonized mineral surfaces. Collectively, our analysis demonstrates that omission of the hyphosphere from conceptual models of soil C flow overlooks key mechanisms for MAOM formation in bulk soils. Moving forward, there is a clear need for spatially explicit, quantitative research characterizing the environmental drivers of hyphal exploration and hyphosphere community composition across systems, as these are important controls over the rate and organic chemistry of C deposited on minerals.

Redox-induced transformation of potentially toxic elements with organic carbon in soil

Abstract: Abstract Soil organic carbon (SOC) is a crucial component that significantly affects the soil fertility, soil remediation, and carbon sequestration. Here, we review the redox-induced transformation of potentially toxic elements (PTEs) through the abiotic impact of SOC. The complex composition of SOC includes humus, pyrogenic carbon (e.g., biochar), dissolved organic matter, and anthropogenic carbon (e.g., compost), with varying concentrations and properties. The primary redox moieties on organic carbon are surface functionalities (e.g., phenol, quinone, and N/S-containing functional groups), environmentally persistent free radicals, and graphitic structures, and their contents are highly variable. Owing to these rich redox moieties, organic carbon can directly affect the reduction and oxidation of PTEs in the soil, such as Cr(VI) reduction and As(III) oxidation. In addition, the interactions between organic carbon and soil redox moieties (i.e., O 2 , Fe, and Mn minerals) cause the transformation of PTEs. The formation of reactive oxygen species, Fe(II), and Mn(III)/Mn(II) is the main contributor to the redox-induced transformation of PTEs, including Cr(VI) reduction and As(III)/Cr(III)/Tl(I) oxidation. We articulated both the positive and negative effects of organic carbon on the redox-induced transformation of PTEs, which could guide soil remediation efforts. Further scientific studies are necessary to better understand the potential transformations of PTEs by SOC, considering the complicated soil moieties, variable organic carbon composition, and both biotic and abiotic transformations of PTEs in the environment. Graphical Abstract

Long-term combined effects of tillage and rice cultivation with phosphogypsum or farmyard manure on the concentration of salts, minerals, and heavy metals of saline-sodic paddy fields in Northeast China

Abstract: Soil sodicity is a major ecological problem in the western Songnen Plain of Northeast China and rice cultivation is the main approach used to mitigate saline-sodic soils. However, rice cultivation alone may not be the most effective practice. This study aimed to investigate the combined effects of annual tillage and rice cultivation with either phosphogypsum or farmyard manure on soil salinity, mineral status, and concentration of heavy metals in saline-sodic paddy fields. Treatments were: 1) untreated (no amendments), untilled, and uncultivated (no rice) saline-sodic native grasslands (UG); 2) untreated, tilled, rice-cultivated paddy fields (PFU); 3) tilled, rice-cultivated, amended paddy fields with phosphogypsum (PFPG); and 4) tilled, rice-cultivated, amended paddy fields with farmyard manure (PFFM). The effectiveness of these treatments on soil improvement was evaluated after a 10-year field experiment. Compared to the UG control, the 0–20 cm topsoil layer of PFU, PFPG, and PFFM had respective decreases in Na+ concentrations of 42.9%, 61.5%, and 60.9%; in CO32- + HCO3- concentrations of 18.9%, 63.2%, and 57.9%; in Cl- concentration of 64.6%, 75.7%, and 79.9%; in pH units of 0.57, 1.05, and 1.30; in soil electrical conductivity (EC1:5) of 18.3%, 49.1%, and 48.3%; and in exchange sodium percentage (ESP) of 47.2%, 66.9%, and 72.5%. Also, the 0–20 cm topsoil layer of PFPG and PFFM had its concentrations of soil organic matter (SOM), available nitrogen (AN), and available phosphorus (AP) significantly (P

Clarifying the evidence for microbial‐ and plant‐derived soil organic matter, and the path toward a more quantitative understanding

Abstract: Predicting and mitigating changes in soil carbon (C) stocks under global change requires a coherent understanding of the factors regulating soil organic matter (SOM) formation and persistence, including knowledge of the direct sources of SOM (plants vs. microbes). In recent years, conceptual models of SOM formation have emphasized the primacy of microbial‐derived organic matter inputs, proposing that microbial physiological traits (e.g., growth efficiency) are dominant controls on SOM quantity. However, recent quantitative studies have challenged this view, suggesting that plants make larger direct contributions to SOM than is currently recognized by this paradigm. In this review, we attempt to reconcile these perspectives by highlighting that variation across estimates of plant‐ versus microbial‐derived SOM may arise in part from methodological limitations. We show that all major methods used to estimate plant versus microbial contributions to SOM have substantial shortcomings, highlighting the uncertainty in our current quantitative estimates. We demonstrate that there is significant overlap in the chemical signatures of compounds produced by microbes, plant roots, and through the extracellular decomposition of plant litter, which introduces uncertainty into the use of common biomarkers for parsing plant‐ and microbial‐derived SOM, especially in the mineral‐associated organic matter (MAOM) fraction. Although the studies that we review have contributed to a deeper understanding of microbial contributions to SOM, limitations with current methods constrain quantitative estimates. In light of recent advances, we suggest that now is a critical time to re‐evaluate long‐standing methods, clearly define their limitations, and develop a strategic plan for improving the quantification of plant‐ and microbial‐derived SOM. From our synthesis, we outline key questions and challenges for future research on the mechanisms of SOM formation and stabilization from plant and microbial pathways.

Fertilization and Soil Microbial Community: A Review

Abstract: The present paper reviews the most recent advances regarding the effects of chemical and organic fertilizers on soil microbial communities. Based on the results from the articles considered, some details are presented on how the use of various types of fertilizers affects the composition and activity of soil microbial communities. Soil microbes have different responses to fertilization based on differences in the total carbon (C), nitrogen (N) and phosphorus (P) contents in the soil, along with soil moisture and the presence of plant species. These articles show that the use of chemical fertilizers changes the abundance of microbial populations and stimulates their growth thanks to the nutrient supply added. Overall, however, the data revealed that chemical fertilizers have no significant influence on the richness and diversity of the bacteria and fungi. Instead, the abundance of individual bacterial or fungal species was sensitive to fertilization and was mainly attributed to the changes in the soil chemical properties induced by chemical or organic fertilization. Among the negative effects of chemical fertilization, the decrease in enzymatic activity has been highlighted by several papers, especially in soils that have received the largest amounts of fertilizers together with losses in organic matter.

Microscale carbon distribution around pores and particulate organic matter varies with soil moisture regime

Abstract: Abstract 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.

The role of plant input physical-chemical properties, and microbial and soil chemical diversity on the formation of particulate and mineral-associated organic matter

Abstract: Soil organic matter (SOM) is a fundamental resource to humanity for the many ecosystem services it provides. Increasing its stocks can significantly contribute to climate change mitigation and the sustainability of agricultural production. Elucidating the mechanisms and drivers of the formation of the main components of SOM, particulate (POM) and mineral associated (MAOM) organic matter, from the decomposition of plant inputs is therefore critical to inform management and policy designed to promote SOM regeneration. We designed a two-tiered laboratory incubation experiment using 13 C and 15 N labeled plant material to investigate the effects of the physical nature (i.e., structural versus soluble) of plant inputs as well as their chemical composition on (1) the pathways of SOM formation, (2) the soil microbial community and chemical diversity, and (3) their interaction on the stabilization efficiency of litter-derived C in POM and MAOM, in a topsoil and a subsoil. We found that: i) the physical nature of the plant input (structural vs soluble) drove both the pathways and efficiencies of SOM formation; ii) POM formation from the decomposition of structural residues increased in efficiency the more decomposed were the residues, and linearly with soil microbial and chemical diversity, the latter only for subsoil; ii) more input-derived C and N were retained in subsoil because of both higher stabilization in MAOM and POM, and slower residue decay. Our results also confirm the importance of direct sorption of soluble inputs to silt- and clay-sized minerals for the formation of MAOM in bulk soils. Taken together these finding suggest that the highest potential for SOM accrual is in subsoils characterized by higher C saturation deficit, from the separate addition of decomposed residues and soluble plant inputs. • Plant soluble vs structural inputs drive pathways and efficiencies of SOM formation. • MAOM forms most efficiently from soluble plant inputs, likely by direct sorption. • POM forms most efficiently from highly decomposed plant residues. • Soil microbial and chemical diversity promote POM but not MAOM formation. • POM and MAOM form most efficiently in subsoil due to lower decay and C saturation.

Dynamics and composition of soil organic carbon in response to 15 years of straw return in a Mollisol

Abstract: Crop straw return is being widely applied as a sustainable soil management practice to improve soil organic carbon (SOC) storage in intensive agricultural ecosystems. However, the dynamics and chemical composition of SOC under long-term straw return are not fully understood. Based on a 15-year field experiment with soybean (Glycine max (L.) Merrill.)–maize (Zea mays L.) cropping system, we studied SOC temporal dynamics under no fertilization (NF), mineral fertilizers (NPK) and mineral fertilizers with straw return (NPKS). Meanwhile, we determined five labile carbon (C) pools, i.e., microbial biomass C (MBC), water-soluble organic C (WSOC), light fraction C (LFC), readily oxidizable organic C (ROC), and particulate organic C (POC), as well as δ13C values and organic functional groups of soil organic matter (SOM). After 15-year application of continuous straw return with mineral fertilizer application, SOC content increased by 14.2%. The δ13C values of SOC increased with time under all treatments, and its positive relationship with cumulative maize C input indicated a larger C contribution to SOC from maize than soybean residues. The NPKS treatment significantly increased the contents of MBC, WSOC, LFC, ROC, and POC and their proportion in SOC, compared with the NF treatment. The aliphatic and aromatic relative peak areas were significantly affected by straw return, with an increasing relative peak area at 2930 cm−1 and decreasing at 1620 cm−1, respectively. Our results demonstrate that straw return could continuously increase SOC in an inter-annual rotation of soybean and maize cropping system in Mollisol. Meanwhile, long-term fertilization can alter SOM chemical composition, with lower humification degree under the combined application of straw and mineral NPK fertilizers.

Resistant soil carbon is more vulnerable to priming effect than active soil carbon

Abstract: The newly added exogenous organic matter may change the decomposition rate of native soil organic matter (SOM) by the priming effect (PE) and further impact the terrestrial carbon (C) balance. Earth system models have not yet considered the distinct responses of soil C pools with different stability and turnover rates to PE. We addressed this knowledge gap by incubating three soils (from the same site but at different stages of SOM decomposition) developed under C3 vegetation with or without the addition of maize straw (C4) and its pyrolysis product-biochar (C4) in a 180 d-incubation at 22 and 32 °C. The three soils were sampled from 15-year old-field, 15-year bare-fallow, and 23-year bare-fallow plus additional laboratory 815-d incubation, representing active (old-field soil) and relatively resistant (two bare-fallow soils, with more stable fraction and less labile fraction) soil C pools, respectively. Soil-derived CO2 was distinguished by a two-component mixing model using a natural 13C isotopic tracing method (C3 vs. C4). The PE was by 30%–239% higher in two bare-fallow soils than in old-field soil regardless of types of new C input (straw and biochar) or temperatures (22 and 32 °C), and the straw-induced PE was by 90%–297% higher than the biochar-induced PE regardless of soils or temperatures. Moreover, the straw-induced PE was not altered with warming in the three soils, but the biochar-induced PE was significantly decreased by warming, especially in old-field soil. These results suggested that the PE varies with soil C pools with different stability. The relatively resistant soil C pool is more vulnerable to PE than the active soil C pool, but the resistant soil C pool can sequestrate more exogenous C and result in higher soil C storage than the active soil C pool, at least in the short term. In addition, more biochar-C can be retained in these soils than straw-C, but this difference would decrease with warming. Overall, this study suggests that future experimental and modeling studies should pay attention to the distinct vulnerability of soil C pools with different stability to PE and warming for accurately predicting soil C dynamics.

Trends in Microbial Community Composition and Function by Soil Depth

Abstract: Microbial communities play important roles in soil health, contributing to processes such as the turnover of organic matter and nutrient cycling. As soil edaphic properties such as chemical composition and physical structure change from surface layers to deeper ones, the soil microbiome similarly exhibits substantial variability with depth, with respect to both community composition and functional profiles. However, soil microbiome studies often neglect deeper soils, instead focusing on the top layer of soil. Here, we provide a synthesis on how the soil and its resident microbiome change with depth. We touch upon soil physicochemical properties, microbial diversity, composition, and functional profiles, with a special emphasis on carbon cycling. In doing so, we seek to highlight the importance of incorporating analyses of deeper soils in soil studies.

Regional mapping of soil organic matter content using multitemporal synthetic Landsat 8 images in Google Earth Engine

Abstract: Accurate assessment of the spatial distribution of soil organic matter (SOM) is of great significance for regional sustainable development, especially in fertile black soil areas. The present study proposed a regional-scale high spatial resolution (30 m) SOM mapping method based on multitemporal synthetic images. The study area is located on the Songnen Plain of Northeast China. First, all available Landsat 8 surface reflectance (SR) data during the bare soil period (April and May) from 2014 to 2019 in the study area were screened in the Google Earth Engine (GEE), and the cloud mask was constructed. The median, average, maximum, and minimum values of the image set were synthesized according to single-year multimonth, multiyear single-month and multiyear multimonth time ranges, and the spectral index of the synthesized image was constructed. Second, the bands and spectral indices of different synthetic images were used as input to establish a random forest (RF) model of SOM prediction, and the accuracies of different spatial prediction models of SOM were compared to evaluate the optimal regional remote sensing prediction model of SOM. The following results were show. 1) The use of the spectral index combined with the image band as input had a greater improvement in the accuracy of SOM prediction than the use of only the image band. 2) Compared to the average, maximum and minimum synthesized images, the median synthesized image had higher accuracy in SOM prediction. 3) More years of synthesized images provided more robust SOM prediction results. 4) May was the best time window for SOM mapping on the Songnen Plain. This study presents a large-scale and high spatial resolution SOM mapping method that is suitable for black soil areas in Northeast China and extends the application of GEE in digital soil mapping.

Soil organic carbon variation determined by biogeographic patterns of microbial carbon and nutrient limitations across a 3, 000-km humidity gradient in China

Abstract: Soil extracellular enzyme stoichiometry has been used to characterize the acquisition strategies of soil microorganisms in obtaining carbon (C), nitrogen (N) and phosphorus (P). However, the variations in soil microbial resource limitation to changing precipitation regimes and edaphic factors remain poorly understood, particularly for a highly divergent soil organic C gradient along the transect from dry to wet areas. This study investigated soil microbial C and nutrient (P/N) limitations along a 3,000-km humidity gradient. The results revealed a downward unimodal-shaped relationship between the humidity index (HI) and soil microbial P/N limitation with a threshold of HI = 0.68 (corresponding mean annual precipitation ranged from 469 mm to 551 mm). Changes in microbial C limitation, and total- and available soil P contents along the humidity gradient further revealed the presence of this threshold. Soil microbial C limitation remained constant at a humidity level below HI = 0.68, and it increased above this threshold. Microbial P limitation decreased and N limitation increased as humidity increased to HI = 0.68. Above HI = 0.68, the microbial P limitation gradually elevated with an increase in humidity. We also found that humidity and soil nutrients are critical factors explaining the variations in microbial resource limitation, and soil nutrients control microbial resource limitation on either side of the HI = 0.68 threshold. These findings suggest that the acquisition of N and P by soil microorganisms stimulates the decomposition of soil organic matter, and future predictions of ecosystem C budgets should thus consider enzymatic processes.

Soil organic carbon variation determined by biogeographic patterns of microbial carbon and nutrient limitations across a 3, 000-km humidity gradient in China

Abstract: Soil extracellular enzyme stoichiometry has been used to characterize the acquisition strategies of soil microorganisms in obtaining carbon (C), nitrogen (N) and phosphorus (P). However, the variations in soil microbial resource limitation to changing precipitation regimes and edaphic factors remain poorly understood, particularly for a highly divergent soil organic C gradient along the transect from dry to wet areas. This study investigated soil microbial C and nutrient (P/N) limitations along a 3,000-km humidity gradient. The results revealed a downward unimodal-shaped relationship between the humidity index (HI) and soil microbial P/N limitation with a threshold of HI = 0.68 (corresponding mean annual precipitation ranged from 469 mm to 551 mm). Changes in microbial C limitation, and total- and available soil P contents along the humidity gradient further revealed the presence of this threshold. Soil microbial C limitation remained constant at a humidity level below HI = 0.68, and it increased above this threshold. Microbial P limitation decreased and N limitation increased as humidity increased to HI = 0.68. Above HI = 0.68, the microbial P limitation gradually elevated with an increase in humidity. We also found that humidity and soil nutrients are critical factors explaining the variations in microbial resource limitation, and soil nutrients control microbial resource limitation on either side of the HI = 0.68 threshold. These findings suggest that the acquisition of N and P by soil microorganisms stimulates the decomposition of soil organic matter, and future predictions of ecosystem C budgets should thus consider enzymatic processes.