Soil Organic Matter Stabilization Research Articles

Enhancing Carbon Sequestration in Mediterranean Agroforestry Systems: A Review

The agroforestry systems with a high potential for C sequestration are those degraded by poor management strategies. Studies on changes in soil C status in these ecosystems mostly take into account labile C pools. Labile and stable soil organic matter (SOM) fractions are affected by soil management and land-use changes. Stable C pools are essential to understanding effects of land-use on soil C storage in the long term. The SOM stability is partly enhanced by the interaction of SOM with minerals and its inclusion into soil aggregates. Recalcitrant substances (e.g., lignin and chitin) also contribute to the passive SOM fraction. Macroaggregates mostly reflect the influence of plant roots and coarse intra-aggregate particulate SOM (POM), whereas microaggregates reflect the influence of fine interaggregate POM, clay concentration and humified SOM fraction. Often, POM is more sensitive to soil management changes than total SOM. Glomalin is a recalcitrant protein consisting of chitin produced by mycorrhizal fungi to protect hyphae. Glomalin has implications on C sequestration in agroforestry soils, but further research is needed before any prediction can be made. One challenge is reducing the CO2 emission from roots, and increasing the recalcitrant root C.

Preliminary Research on Agricultural Cultivation Decreasing Amino Sugar Accumulation in Calcareous Soils in Subtropical Karst Region of China

Soil microbial residues play an important role in the formation and stabilization of soil organic matter and can be quantitatively characterized by amino sugars. However, the response of soil microbial residues to agricultural cultivation in karst areas remains unclear. In this study, we collected soil samples from natural reserved land as well as five plantation forests dominated by Citrus trees cultivated for 0, 1, 5, 15, 30 years to examine the effects of agricultural cultivation on the content of microbial residues (amino sugar analysis). Results showed that: (1) Soil Amino Sugars (ASs) contents were significantly reduced after agricultural cultivation along with the sharp decrease in soil organic carbon (SOC). After 30 years of cultivation, the contents of total ASs, glucosamine (GluN), galactosamine (GalN), and muramic acid (MurA) in cultivated soils decreased by 58.22%, 55.30%, 27.11%, respectively, compared with 0 yr.; (2) Microbial residual carbon contribution to SOC increased from 34.11% to 81.33% after 30 years of cultivation, including fungal residual carbon (FRC) (25.79% to 48.6%) and bacterial residual carbon (BRC) (8.32% to 32.72%); (3) Soil GluN/MurA values tended to decrease with increasing cultivation years. The results highlight the significant effect of cultivation years on amino sugar accumulation. It indicates that the years of reclamation in karst areas have different impacts on the organic fractions derived from various microbial communities in the soil organic matter pool, and the microbial residues indicated by amino sugar are of great significance for the interception of soil organic matter.

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

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.

The Molecular Composition of Humic Acids in Permafrost Peats in the European Arctic as Paleorecord of the Environmental Conditions of the Holocene

The purpose of our research is focused on examination of the transformation regularities of molecular composition of humic acids (HAs) in the hummocky frozen peatlands of the European Arctic as a marker of climatic changes in the Holocene, and assessment of the stabilization of soil organic matter under the conditions of modern climatic warming. Histosols located in the two subzones of the European Arctic served as the research subjects. This territory is actively used for reindeer breeding, which is a vital agricultural branch in the Far North of the Russian Federation. The data obtained reveal the main trends in the formation of HAs from Arctic peatlands under different environmental conditions. Modern peat sediments (top layers) in the middle and late Holocene period formed out of bryophyte residues and contained HAs with long-chain carbohydrate and paraffin structures in their composition. These structures enlarged the dynamic radii of HA molecules, and, thus, caused high average molecular weight values. The more favorable climatic conditions of the early Holocene (the Atlantic optimum) defined the botanical composition of peat, which was dominated by tree and sedge communities with high contents of lignin components and, as a consequence, a larger share of aromatic fragments, characterized by thermo-biodynamic resistance in HAs of horizons in the lower and central profile parts. The molecules of HAs are an archive of paleoclimatic records. The Subboreal and Subatlantic climatic conditions determined the specifics of vegetation precursors and, as a result, the molecular structure of HAs in seasonally thawed layers, with a predominance of long-chain aliphatic fragments. The conversion of HAs from Histosols led to an increase in the proportion of carbon in branched and short-chain paraffinic structures with their subsequent cyclization and aromatization. The results of this process are most clearly manifested in layers formed during the Holocene I and II climatic optima. Higher biologically active temperatures of the seasonally thawed layer of soils at bare spots (without vegetation) determined the accumulation of thermodynamically more stable HA molecules with a high content of aromatic fragments. This contributed to both the stabilization of the SOM and the conservation of peatlands in general.

The influence of soil development on the depth distribution and structure of soil microbial communities

Although it has been shown that the interaction of climate and time shape the dynamics of soil organic matter (SOM) storage and preservation in soil, the role of soil microbial communities in this dynamic remains unclear. Microbial communities are present throughout soil profiles and likely play critical roles in SOM and nutrient cycling, however the influence of other factors such as soil development (i.e., age) and the composition SOM on microbial community variation with depth has yet to be quantified. Improving our understanding of the relationship between soil development, soil depth, and microbial communities may provide insight to the critical role they play in cycling and preservation of SOM, as well as more mechanistic predictions of the response of soil communities to change, such as landscape-scale changes in available moisture or temperature regimes. Here we compare soils spanning a soil age by climate gradient (i.e., climo-chronosequence) to better understand the mechanisms which influence soil microbial community structure and the molecular composition of SOM. While we observed little depth-dependence in metrics of microbial community structure (i.e., composition, diversity, dissimilarity) across the range of soil development under a wetter climate, we found significant depth-dependent changes in community metrics under a drier climate, which became more pronounced as soils became older. This shift in bacterial and archaeal community structure and diversity is most apparent below a clay-rich argillic horizon formed in the older, drier soils. The molecular composition of SOM as measured by high resolution mass spectrometry (i.e., FTICR MS) also exhibited similar shifts in composition with soil depth and age. Our results highlight how soil moisture shapes the interaction of soil development, SOM, and microbial community composition. Differences in the moisture regime between our two study sites drive differences in biogeochemical depth gradients and subsequent variation in soil microbes and SOM. This suggests that knowledge of not just the pedogenic trajectory of soil development, but also the spatial position relative to distinct pedogenic features, are important for explaining variations in the depth-dependencies of microbial communities and associated SOM.

Controls on labile and stabilized soil organic matter during long-term ecosystem development

A better understanding of factors controlling the distribution of soil organic matter (SOM) in differently stabilized pools in soils during ecosystem development is essential to more efficiently manage soil as a C sink and aid global warming mitigation. We, thus, investigated C and N stocks in bulk soil and differently stabilized soil fractions (free particulate organic matter (fPOM), micro- and macroaggregates, and mineral-associated organic matter (MAOM)), in seven chronosequences comprising primary and secondary successions at various locations in Europe and tested the influence of multiple environmental parameters on these C and N stocks.Best-subset multiple regression indicated that plant litter production and succession age were of minor importance. Above a certain threshold of organic matter inputs to the soil, changes in the quantity of such inputs may have little direct effect on the partitioning of organic matter to various soil fractions. Instead, biophysicochemical soil properties (such as pH and PLFA and clay contents) were the best predictors for both C and N stocks. We also found a shift in the relevance of biotic and abiotic factors from fPOM and the various aggregate fractions to MAOM, the latter being exclusively associated with pH and the clay content. This indicates that sites dominated by MAOM likely more slowly react to management interventions, such as increasing or diversifying organic matter inputs, than sites dominated by fractions more responsive to biotic factors, such as fPOM. Finally, as compared to primary successions, secondary successions were associated with higher C and N stocks in aggregate fractions, pointing towards better conditions for soil structure formation in the latter. Our results highlight the need to elucidate the form in which C and N exist in the soil for selection of proper management strategies and that soil biophysicochemical parameters may have a disparately larger impact on the partitioning of C and N to variously stabilized soil fractions than the rate of organic matter inputs.

Thermal-gradient analysis of soil organic matter using an elemental analyser – A tool for qualitative characterization?

Analysing the thermal stability of soil organic matter (SOM) by heating soil material up to 900 °C and recording the time/temperature-dependent evolution of CO2, using an elemental analyser, may be an easily available tool to obtain information on the composition and stabilization of SOM. We studied four sample sets, including size fractions of mineral horizons, particulate organic matter (POM), soil aggregates, and thermally treated materials. All analyses were conducted with a thermal gradient of 70 K min−1, with i) stages of constant temperature at 400, 600 and 900 °C (‘DIN 19359′), ii) stages at 450, 600 and 900 °C (modified DIN method), or iii) continual heating. With both stage methods, the large majority of organic carbon (OC) was present in the least thermally stable fraction, thermally decomposing at T ≤400 or 450 °C, respectively. However, neither did these methods result in the same relative proportions of fractions of similar thermal stability, nor did they result in time/temperature-dependent CO2-evolution patterns characteristically reflecting the different sample sets. Thus, results of the stage-heating methods were operationally defined, as different SOM fractions in terms of composition or stabilization mechanism were combusted, but showing similar patterns. Nonetheless, most POM samples revealed less thermal stability than the other fractions with any method. Evaluation of CO2 development during continual heating provided more options, including T of maximum CO2 evolution, T50, that is, T at which 50% of CO2 has evolved, and peak deconvolution using Gauss functions. The mineral samples showed at least two maxima and variable T50 values up to 550 °C. Most POM fractions had lower T50, but not in general, possibly caused by the presence of black carbon or strongly complexing cations. The aggregate samples were mostly in between the mineral and POM samples. Irrespective of the method applied, direct unambiguous identification of samples in terms of composition or stabilization mechanisms was not possible. Nonetheless, we consider an elemental analyser a valuable tool to evaluate the thermal stability of large samples sets at relatively low cost and time effort. Particularly peak deconvolution from continual heating offers the option of quantifying OC fractions, differing in thermal stability.

Bacterial community dynamics explain carbon mineralization and assimilation in soils of different land-use history.

Soil dwelling microorganisms are key players in the terrestrial carbon cycle, driving both the degradation and stabilization of soil organic matter. Bacterial community structure and function vary with respect to land use; yet the ecological drivers of this variation remain poorly described and difficult to predict. We conducted a multi-substrate DNA-stable isotope probing experiment across cropland, old-field, and forest habitats to link carbon mineralization dynamics with the dynamics of bacterial growth and carbon assimilation. We tracked the movement of 13 C derived from five distinct carbon sources as it was assimilated into bacterial DNA over time. We show that carbon mineralization, community composition, and carbon assimilation dynamics all differed with respect to land use. We also show that microbial community dynamics affect carbon assimilation dynamics and are associated with soil DNA content. Soil DNA yield is easy to measure and may be useful in predicting microbial community dynamics linked to soil carbon cycling. Soil dwelling microorganisms are key players in the terrestrial carbon cycle, driving both the degradation and stabilization of soil organic matter. Microbial communities vary with respect to land use, but we still have an incomplete understanding of how variation in community structure links to variation in community function. DNA stable isotope probing (DNA-SIP) is a high-resolution method that can identify specific microbial taxa that assimilate carbon in situ. We conducted a large-scale multi-substrate DNA-SIP experiment to explore differences in bacterial activity across land-use regimes. We show that microbial community dynamics vary with land use, that these dynamics are linked to soil carbon cycling, and that they are associated with easily measured soil properties.

Carbon-enriched organic amendments differently affect the soil chemical, biological properties and plant biomass in a cultivation time-dependent manner

BackgroundThe farmyard manure application maintains quality of arable soils, provides nutrients, mitigates climate change by soil carbon sequestration. Biochar and other complex carbon rich amendments may stabilize organic matter derived by composting and decelerate organic carbon mineralization. However, how the combined utilization of biochar, humic substances and manure effects on soil chemical and biological properties have been least explored, especially their effect on soil basal and substrate induced respirations are needed to be further explored. Therefore, the potential of biochar and Humac (a commercial humic substances product) in combination with manure to improve the soil properties and plant growth was investigated in this experiment using barley under a short-term (12 weeks) and maize under long-term (following 12 weeks, a total of 24 weeks) cultivation.ResultsIn the early phase of cultivation (12 weeks) Humac- or biochar-enriched manures (M + H, M + B, respectively) enhanced the contents of nutrient elements (carbon + 5.6% and + 7%, nitrogen + 6.7% and − 5%, sulphur − 7.9% and + 18.4%), the activity of enzymes including (β-glucosidase + 32% and + 9.6%, phosphatase + 11% and 6.3%), and dry aboveground biomass (+ 21% and + 32%), compared to the control and manure-treated soil. However, these impacts of M + H and M + B manures were reduced under longer period, i.e., at the experiment end (24 weeks). After 24 weeks of cultivation, a decrease in absolute values of all determined enzyme activities indicated putative reduction of mineralization rate due to presumed higher recalcitrance of manure-derived organic matter, with Humac, biochar amendments. Increased stability of soil organic matter reduced microbial activity due to lower availability of nutrients. Possibly, the shortened period of manure maturation could help preserve a higher amount of less degraded organic matter in the enriched manures to counteract these observed features.ConclusionsWe summarized that the biochar and humic substances combined with manure have the potential to improve the soil characteristics, plant biomass and soil health indicators but the improvements faded away in a cultivation time-dependent manner. Further studies are required to explore the structure and functioning of microbial activities under long-term experimental conditions.Graphical abstract

Stabilisation of soil organic matter: interactions between clay and microbes

Soil organic matter (SOM) plays a central role in the global carbon balance and in mitigating climate change. It will therefore be important to understand mechanisms of SOM decomposition and stabilisation. SOM stabilisation is controlled by biotic factors, such as the efficiency by which microbes use and produce organic compounds varying in chemistry, but also by abiotic factors, such as adsorption of plant- and microbially-derived organic matter onto soil minerals. Indeed, the physicochemical adsorption of organic matter onto soil minerals, forming mineral associated organic matter (MAOM), is one of the significant processes for SOM stabilisation. We integrate existing frameworks of SOM stabilisation and illustrate how microbial control over SOM stabilisation interacts with soil minerals. In our new integrated framework, we emphasise the interplay between substrate characteristics and the abundance of active clay surfaces on microbial processes such as carbon use efficiency and recycling. We postulate that microbial use and recycling of plant- and microbially-derived substrates decline with increased abundance of active clay surfaces, and that the shape of these relationships depend on the affinity of each substrate to adsorb, thereby affecting the efficiency by which organic matter remains in the soil and is stabilised into MAOM. Our framework provides avenues for novel research and ideas to incorporate interactions between clay surfaces and microbes on SOM stabilisation in biogeochemical models.Graphical abstract

Decomposition of soil organic matter by ectomycorrhizal fungi: Mechanisms and consequences for organic nitrogen uptake and soil carbon stabilization

A major fraction of nitrogen (N) in boreal forest soils is found in organic forms associated with soil organic matter (SOM) and mineral particles. The capacity of ectomycorrhizal (ECM) fungal symbionts to access this N is debated, considering that these fungi have lost many of the genes for decomposing organic matter that were present in their saprotrophic ancestors. To gain a molecular-level understanding of the N-mining processes in ECM fungi, we developed an experimental approach where the processes of decomposition were studied in parallel with the changes in the structure and properties of the organic matter. We showed that ECM fungi have significant capacities to assimilate organic N associated with SOM and mineral surfaces. The decomposition mechanisms differ between species, reflecting the lignocellulose decomposition mechanisms found in their saprotrophic ancestors. During N-mining, the ECM fungi processed the SOM to a material with increased adsorptive properties to iron oxide mineral particles. Two pathways contributed to these changes: Extracellular modifications of the SOM and secretion of mineral surface reactive metabolites. Some of these metabolites have iron(III)-reducing activities and can participate in extracellular Fenton reactions and redox reactions at iron oxide mineral surfaces. We conclude that the traditional framework for understanding organic N acquisition by ECM fungi from recalcitrant SOM must be extended to a framework that includes how those decomposition activities affect the stabilization and reactivity of mineral-associated SOM. The activity through these complex networks of reactions is decisive for the overall effect of ECM fungal decomposition on nutrients and C-cycling in forest ecosystems.

Perspectives from the Fritz‐Scheffer Awardee 2021: Soil organic matter storage and functions determined by patchy and piled‐up arrangements at the microscale

AbstractMounting evidence of recent spectromicroscopic insights have revealed that the distribution of mineral‐associated organic matter (OM) at the microscale and nanoscale is organized heterogeneously in patchy and piled‐up arrangements of varying thickness. Spectromicroscopic approaches could show local deterministic features of distinct OM and mineral composition that influence the heterogeneous lateral OM distribution. OM–OM interactions shape vertical and three‐dimensional OM structures with potentially multilayered composition. Conceptualizing mineral‐associated OM as patchy‐distributed and piled‐up has critical implications for our understanding of soil ecosystem functions as it defines their functional properties in compartmentalized regions at the microscale and nanoscale. The concentrated storage of OM associated to only a minor part of mineral surfaces implies that carbon sequestration may be decoupled from a direct limitation by the amount of fine mineral particles while sustaining mineral surface functionality in other parts. At the microscale and nanoscale, differences in altered surface properties, compartmentalized microhabitats, and biotic architectures shape a conceptual understanding where OM storage and other soil functions are driven by spatially resolved interactions. This novel conceptual framework warrants experimental approaches to incorporate the patchy and piled‐up arrangement of OM and upscale potential effects of its heterogeneous arrangement to systematically understand the effectiveness of soil functions.

Climate warming weakens the negative effect of nitrogen addition on the microbial contribution to soil carbon pool in an alpine meadow

Microbial-derived soil organic carbon has attracted increased attention as an important source of soil organic matter that may affect soil carbon persistence. Ecosystems at high elevation and latitude contain massive amounts of soil organic carbon that is potentially vulnerable to rapid climate change. However, little is known about the mechanisms that govern the production and accumulation of carbon derived from microbes in the context of global changes for these regions. Here, we investigate soil biomarkers from membrane lipids and microbial residues in an experiment with six-year warming and nitrogen (N) addition treatments in an alpine meadow of the Tibetan Plateau. The results showed low accumulation of amino sugars in soil (167.51 to 297.36 μg·g−1) for this alpine meadow. After the six-year manipulative experiment, warming promoted accumulation of microbial-derived carbon in soil with both increased bacterial (increased by 53.56%) and fungal carbon (increased by 25.60%). Nitrogen addition inhibited microbial-derived carbon accumulation (decreased by 25.37%) and significantly decreased the fungal carbon content relative to bacterial carbon content. Climate warming tended to weaken this N-driven negative effect. As microbial residues relate closely with soil organic matter stability, these results highlight the potential of global change factors (warming and N addition) to alter the structure and persistence of soil organic matter, with important implications in refining carbon cycle-climate models.

Responses of Q10 to anthropogenic soil redistribution: Synthetic effects of SOM composition and r/K-shifts of microbial community

The temperature sensitivity of soil respiration (Q10) is a pivotal parameter for forecasting the fate of soil carbon (C) under global warming. Although a plethora of studies have elucidated the changes in Q10 in farmland or natural ecosystems, the response of Q10 to soil redistribution remains unclear. A simulated anthropogenic soil redistribution experiment by examining subsoil exposure (E20) and topsoil addition (A20) was performed to explore the influences of soil organic matter (SOM) composition and r/K-shifts of microbial community on Q10. Compared with the undisturbed soil, the average annual Q10 in the E20 plot was significantly increased, whereas the mean annual Q10 in the A20 plot decreased. The ratios of CC to COO and CC to CO, which indicate the stability of SOM, were significantly increased by 14.8%− 44.2% and 33.9%− 45.4% in the E20 plot but decreased by 18.6%− 28.1% and 3.8%− 11.2% in the A20 plot, respectively. The ratios of oligotrophs to copiotrophs in both the bacterial and fungal communities were significantly increased in the E20 plot (K-shift) but decreased in the A20 plot (r-shift). The β-D-xylosidase and cellobiohydrolase activities in the E20 plot were significantly decreased but prominently increased in the A20 plot, whereas the activity of polyphenol oxidase showed the opposite trend. The high aromaticity of recalcitrant SOM in the E20 plot determined the prevalence of K-shift in the bacterial and fungal communities, which might be attributed to the production of enzymes that decompose recalcitrant SOM, which then further enhanced Q10 and vice versa. Overall, our findings indicated that the feedback of ecological strategy of microbial community on the changes in SOM functional groups should be considered when evaluating the response of regional C cycling to anthropogenic soil redistribution.

Prescribed defoliation strategies influence soil carbon and nitrous oxide potential in pastures

AbstractCool‐season perennial grasslands are commonly used for livestock pasture. However, understanding of the belowground effects of forage defoliation (selective pressure applied by grazing) management is nascent. We investigated how defoliation management affects aboveground (forage productivity) and belowground processes (soil C/N), microbial nutrient cycling, and potential nitrous oxide [N2O] emissions). We hypothesized that rotational (moderate frequency) defoliation at low severity will increase soil organic matter (SOM) storage and microbial capacity to cycle C, N, and P, but that these changes would also lead to greater soil N2O emission potential. Data were collected from organically (OF) and conventionally managed (CF) farms with differing botanical composition in West Virginia, where three frequencies and two severities of forage defoliation were imposed. At OF, more severe defoliation decreased labile soil C content (–26%), while moderate defoliation resulted in a 39% reduction in microbial enzymatic capacity to cycle C, but not at CF. The N2O emission potential (ratio of nirK/nosZ functional gene abundance), was differentially affected by defoliation treatments at both sites. At OF, defoliation severity altered N2O emission potential, whereby low severity resulted in an 82% decrease in potential N2O emissions compared with high severity treatments. At CF, continuously defoliated plots exhibited a 71% lower N2O emission potential relative to less frequent defoliation. Belowground microbial parameters were more responsive to defoliation at OF, suggesting past management may influence how grazing management asserts belowground changes. Results suggest that defoliation management could enhance soil C storage, but potentially at the risk of increased N2O emissions.

Phage‐bacterium interactions and nutrient availability can shape C and N retention in microbial biomass

AbstractPhage‐bacterium interactions influence soil microbial life and ecological functions, including microbial evolution, community patterns, and nutrient cycling. However, understanding of phage‐mediated soil bacterial lysis dynamics, and impacts on soil carbon (C) and nitrogen (N) cycling, remains elusive. Here, we used a short‐term laboratory incubation microcosm model system (which did not contain actual soil) consisting of a soil bacterium Bacillus cereus LB2 and an exclusive B. cereus phage SWEP1 to illustrate how a single soil phage manipulates bacterial lysis and the associated necromass formation under different nutrient conditions, and the associated changes on C and N dynamics. Results showed that the phage‐induced bacterial lysis significantly increased the total amounts of necromass C and N, which was correlated to nutrient conditions. Bacteria populations were effectively eliminated by phages under eutrophic conditions, whereas the lysis rate of SWEP1 slowed down under oligotrophic conditions. The presence of phage clearly stimulated necromass accumulation, albeit with a reduced proportion of dissolved NH4+‐N content from the supernatant. Therefore, phages may enhance microbial necromass formation, and actively contribute to soil organic matter (SOM) stabilisation and C and N cycling in soil ecosystem.Highlights Phage‐mediated bacterial lysis increased C and N contents in necromass Eutrophic conditions facilitate phage‐mediated bacteria elimination The lysis rate of phage slows down under oligotrophic conditions The contribution of bacterial necromass to SOM may be underestimated

Soil nitrogen status can be improved through no-tillage adoption particularly in the surface soil layer: A global meta-analysis

United Nations ‘Sustainable Nitrogen Management’ (UNEP/EA.4/L.16) resolution calls for adopting best practices for recovery and recycling of N. Bulk of the soil N stock is in soil organic matter (SOM) form. The stabilization of SOM following conversion to no-till practice will, therefore, regulate N flux and storage in soils. To realize the no-tillage (NT) effect on soil N, a global meta-analysis was conducted with 2,268 pairs of data points from 327 peer-reviewed articles. Increases of 21 and 16% in total N concentration and N stock, respectively, were noted under NT compared to conventional tillage (CT) in the surface soil layer (0–10 cm). However, no such changes were accounted for down the profile. Continental and temperate climates recorded significant gains in total N in NT while the effect was negligible in tropical and dry climates. Soil N increased in NT irrespective of soil texture; coarser the texture, marginally higher was the gain. An incremental rise in soil N was evident with an increase in the duration of NT adoption. The cereal-cereal and cereal-legume rotations had marginal advantages with NT over other cropping systems. The surface 0–10 cm layer recorded the largest N stock under NT, which remained similar down the depth. A decrease in soil pH was noted with conversion to NT practice, but soil electrical conductivity was comparable with CT. No-tillage favoured the soil N and as a consequence, fertilizer-N use under no-till condition, must take into account of the potential N release from SOM for achieving higher N-use efficiency.

Soil Organic Matter Storage in Irrigated Tsitsikamma Dairy Farms with Minimum Tilled Pasture Mixtures: Case Studies

In recent years, pasture production changed from conventional tilled single pastures to minimum tilled mixed pastures in the Tsitsikamma region, South Africa. However, storage of soil organic matter (SOM) under minimum tilled mixed pastures is not yet quantified. This study evaluated SOM indices in the upper 60 cm soil of six-year-old mixed pastures in the Upper (UT) and Lower (LT) Tsitsikamma regions. Soil samples were collected at 0–15, 15–30, 30–45, and 45–60 cm soil layers of five farms (F1, F2, F3, F4, and F5) treated with different rates of fertilizer (NPK) alone and in combination with dairy effluent (DE) and/or poultry manure (PM). Results of this study indicated that there were no significant differences in bulk density, total nitrogen (N), and rate of potentially mineralizable N (PMN) between farms in the UT region. In the LT region, NPK, DE, and PM combinations improved soil C accumulation relative to the soil application of NPK. Higher C/N ratios in the LT region suggested adequate C for microbial energy and maintenance. Integrating manure into minimum tilled pasture mixtures as a replacement for synthetic fertilizers seems to be a feasible option to promote SOM storage, but remains only feasible by applying site-specific management strategies.

Microplastic additions alter soil organic matter stability and bacterial community under varying temperature in two contrasting soils

Microplastics can accumulate in soils and strongly affect the biogeochemical cycle. Biodegradable plastic films show potential as sustainable alternatives that could reduce microplastic soil contamination and accumulation. However, the effects of traditional and biodegradable microplastics on soil organic matter (SOM) stability are not sufficiently understood, particularly under varying temperatures. The objective of this study was to examine the effects of polyethylene (PE) and biodegradable polylactic acid (PLA) microplastics on SOM stability and bacterial community in two contrasting soils (Black soil vs. Loess soil) under varying temperature conditions (15 °C vs. 25 °C). Results showed that microplastics addition significantly enhanced cumulative CO2 emissions and DOC contents, particularly 1 % PLA treatment accelerated CO2 emissions by 19 % – 74 %, DOC content by 3 % – 23 % at 25 °C. A higher temperature sensitivity (Q10) at the PLA treatment indicated that PLA is more susceptible to elevated temperature compared to PE. The presence of both PE and PLA microplastics significantly changed the DOC spectral characteristics, i.e., high temperature increased the value of the specific UV absorbance (SUVA) in soil without microplastics, while decreased it in soil with microplastics. In comparison to soil without microplastics, soil exposed to 1 % microplastics had lower MBC concentrations and greater metabolic quotient. 16S rRNA gene sequencing showed that the presence of PLA microplastic significantly alters soil bacterial community. PE and CK had similar Bray-Curtis distances between two temperatures, while PLA increased the dissimilarity between CK compared to PE. Compared to the two soils, loess soil is more sensitive to microplastics addition. Microplastics have a non-ignorable effect on soil organic matter stability, the interaction between microplastics and soil environment should be considered.

Soil priorities in Russia

• The research priorities for soil science in Russia include the study of soil microbiome and metagenome and other topics: • The impact of global and regional climate changes soils and mechanisms of soil organic matter stabilization. • Technogenic and agrogenic soil pollution, the construction of improved and artificial soils. • Digital agriculture and soil information systems, remote and proximate sensing methods. • The economics of land degradation.