Soil Bulk Research Articles

Stability and heavy metals accumulation of soil aggregates under different land uses in the southwest coastal Bangladesh

Agricultural soil contamination is increasing day-by-day and becoming a major problem over the globe. Trace elements accumulation in the bulk soil is frequently documented, however, there is no precise mechanism of their distribution in the different soil aggregates level. We collected twelve composite soil samples from banana fields, fallow land, rice cultivated with pond water (rice field-I), and rice cultivated with rain water (rice field-II). We separated soil samples into four different size of aggregates (4-2, 2–0.25, 0.25–0.053, <0.053-mm) and then, aggregate stability (MWD), soil organic carbon (SOC), and heavy metals content (Pb, Cd, Cr, As, Fe, Mn, Zn, Ni, Co, Cu) in the soil samples were measured with different techniques. Results showed that MWD was higher in the rice-based land use, which was significantly contributed by SOC (p < 0.001). The concentration of Pb, As, Cd, Fe, and Mn were increased, while Cu and Zn concentration were reduced with increasing aggregate size (p < 0.05). In contrast, aggregate size did not influence on Ni and Co accumulation (p > 0.05). Moreover, macroaggregate acted as an accumulator for Fe, Mn, and As, while all the aggregate fractions acted as accumulators for Cu and Zn. Our study indicated that MWD, SOC, aggregate size and composition, and metal species were the controlling factors of trace elements accumulation and distribution in the various sizes of soil aggregates.

Root-associated bacterial communities of vegetable Brassica parachinensis enrich pollutant-degrading taxa and functions for enhancing phthalate dissipation

Soil pollution by organic contaminants like phthalates (PAEs) significantly impacts root-associated microbial community and soil health. However, the rhizosphere effects of different vegetable cultivars on root-associated bacterial communities and their effects on PAE degradation in rhizosphere are not well uncovered. Here, two cultivars (i.e., Huaguan and Lvbao) of vegetable Brassica parachinensis significantly enhanced the dissipation of di-(2-ethylhexyl) phthalate (DEHP) in rhizosphere (higher by 23.9 % - 86.8 % compared with bulk soil). PERMANOVA tests demonstrated that the bacterial community structure was significantly impacted by niche variation and DEHP pollution, but not by vegetable genotypes, with significant gradient divergences along bulk soil to (far-) rhizosphere to rhizoplane. Niche variation and DEHP pollution also remarkably influenced the ecological networks of bacterial communities in soil-root continuum with high proportions of positive microbial interaction. Furthermore, rhizosphere and DEHP pollution significantly enriched PAE-degrading functions and bacteria (e.g., Aeromicrobium and TM7a, as keystone taxa), which is beneficial for enhancing DEHP degradation. Meanwhile, metagenomic binning identified various bacterial families such as Chitinophagaceae and Nocardioidaceae capable of degrading PAEs in rhizosphere, which may play important roles in PAE biodegradation through cooperation and co-metabolism. This study advances our understandings on the promotion of vegetables for PAE removal in rhizosphere and its related mechanisms.

The occurrence of banana Fusarium wilt aggravates antibiotic resistance genes dissemination in soil

The spread of antibiotic resistance genes (ARGs) and subsequent soil-borne disease outbreaks are major threats to soil health and sustainable crop production. However, the relationship between occurrences of soil-borne diseases and the transmission of soil ARGs remains unclear. Here, soil ARGs, mobile genetic elements and microbial communities from co-located disease suppressive and conducive banana orchards were deciphered using metagenomics and metatranscriptomics approaches. In total, 23 ARG types, with 399 subtypes, were detected using a metagenomics approach, whereas 23 ARG types, with 452 subtypes, were discovered using a metatranscriptomics method. Furthermore, the metagenomics analysis revealed that the ARG total abundance levels were greater in rhizospheres (0.45 ARGs/16S rRNA on average) compared with bulk (0.32 ARGs/16S rRNA on average) soils. Interestingly, metatranscriptomics revealed that the total ARG abundances were greater in disease-conducive (8.85 ARGs/16S rRNA on average) soils than disease suppressive (1.45 ARGs/16S rRNA on average) soils. Mobile genetic elements showed the same trends as ARGs. Network and binning analyses indicated that Mycobacterium, Streptomyces, and Blastomonas are the main potential hosts of ARGs. Furthermore, Bacillus was significantly and negatively correlated with Fusarium (P < 0.05, r = −0.84) and hosts of ARGs (i.e., Mycobacterium, Streptomyces, and Blastomonas). By comparing metagenomic and metatranscriptomic analyses，this study demonstrated that metatranscriptomics may be more sensitive in indicating ARGs activities in soil. Our findings enable the more accurate assessment of the transmission risk of ARGs. The data provide a new perspective for recognizing soil health, in which soil-borne disease outbreaks appear to be associated with ARG spread, whereas beneficial microbe enrichment may mitigate wilt disease and ARG transmission.

Rhizobacterial Community Structure Differs Between Landrace and Cultivar of Rice Under Drought Conditions.

The rhizosphere plays a critical role in crop growth and fitness, particularly under moisture-stress conditions. Modern breeding has diminished the ability of crops to recruit beneficial microbiomes, making them more vulnerable to drought, unlike wild types and landraces that adapt better. This study aims to elucidate how rice landrace and cultivar influence the rhizosphere bacterial communities and biochemical attributes under normal and moisture stress conditions. Rhizospheres of rice landrace, Norungan, and high-yielding cultivar, Co51, were assessed using soil biochemical and 16S rRNA gene sequencing approaches. Drought negatively impacted soil carbon pools, enzymes, and respiration in the rhizospheres of both genotypes. However, Norungan's rhizosphere showed less harm than Co51's. During drought, reductions in soil organic carbon (3.94%), microbial biomass carbon (14.26%), labile carbon (1.94%), dehydrogenase (10.1%), urease (21.27%), phosphatase (9.61%), and respiration rate (15.02%) were more pronounced in Co51 than in Norungan. Alpha diversity of rhizosphere bacterial communities was significantly lower than bulk soil, with drought further reducing diversity in both genotypes. Drought decreased the abundance of Firmicutes and Bacteroidetes in Norungan's rhizosphere while it increased the abundance of Acidobacteria, Actinobacteria, Chloroflexi, and Proteobacteria. Conversely, in Co51's rhizosphere, drought enhanced the abundance of Firmicutes and Bacteroidetes while reducing the abundance of Acidobacteria and Proteobacteria. These results suggest that under moisture-stress conditions, the landrace Norungan recruits less-diversified, specific groups of microorganisms to augment the rhizosphere's functioning. This study underscores the need to develop strategies for cultivars and hybrids with enhanced rhizosphere resilience during drought conditions.

Tillage influence on agrophysical soil properties and crop productivity in the irrigated conditions of the steppe zone of Ukraine

The main goal of the study was to establish the influence of various tillage systems on the agrophysical properties of dark-chestnut soil under short-grain crop rotation in the irrigated conditions of southern Ukraine. The research was carried out during 2021-2022 in a stationary experiment on a four-field crop rotation: grain maize – winter rapeseed – winter wheat – soybeans. The experimental field was in the semi-arid steppe climate zone at the Institute of Climate-Smart Agriculture of the National Academy of Agrarian Sciences of Ukraine. Three tillage systems were studied for their influence on soil bulk density, porosity, and water permeability, namely: mouldboard-differentiated ploughing tillage; differentiated chisel tillage; and differentiated ploughless tillage with soil slitting. Soil bulk density was determined using the core method. Soil porosity was calculated as the ratio of total bulk density to solid fraction bulk density. Water infiltration rates, established through the water absorption test method, were used to measure soil water permeability. The results of the study were statistically analysed using the common ANOVA procedure with Fisher’s least significant difference test at P&lt;0.05. In addition to the agrophysical parameters of the soil, the energy output of the crop rotation was assessed. It was established that mouldboard tillage did not provide significant benefits in terms of bulk density and soil porosity. However, ploughing showed the best results for soil water permeability across all crops in the rotation. The highest energy output of crop rotation (119.1 GJ/ha) was recorded for the ploughless-differentiated tillage system with soil slitting, whereas the mouldboard ploughing and chisel tillage systems produced somewhat lower energy yields of 112.0 and 108.6 GJ/ha, respectively. Therefore, ploughless-differentiated tillage with soil slitting is the most effective option for short-grain crop rotations in irrigated conditions of southern Ukraine, in terms of creating optimal soil agrophysical properties and achieving the highest crop productivity

Mechanisms of N-doped microporous biochar decreased Cd transition in rhizosphere soils and its impact on soil bacterial community composition

Soil cadmium (Cd) contamination has garnered considerable attention. This study employed batch sorption experiments and rhizobox experiments to examine the impact of nitrogen-doped microporous biochar (NBB) on the temporal and spatial distribution of Cd in the rhizosphere of rice plants, with the aim of elucidating the underlying mechanisms. The results indicated that the adsorption of Cd(II) onto NBB was predominantly governed by chemical reactions. When applied to soil, the NBB significantly hindered the migration of Cd from the bulk soil to the rhizosphere. Additionally, the application of NBB decreased the redox potential (Eh) in the rhizosphere soil and increased the relative abundance of Anaeromyxobacteraceae, Geobacteraceae, Desulfurisporaceae, and Syntrophomonadaceae, which could facilitate the reduction of soil Cd availability. Furthermore, the NBB2 treatment encouraged the formation of iron plaque on the root surface, thereby limiting the uptake of Cd from the soil into the root system. Moreover, the N-doped microporous biochar treatment resulted in lower Cd levels in the stele of root, an effect that was associated with increased sulfur (S) content in the stele and epidermis, suggesting a potential role for S in Cd sequestration. Ultimately, the application of N-doped microporous biochar resulted in diminished Cd accumulation in the rice tissues.

Plant growth promoting endophyte modulates soil ecological characteristics during the enhancement process of cadmium phytoremediation

Endophyte assisted phytoremediation of cadmium (Cd) contaminated soil represents a promising strategy. However, the precise soil ecological regulatory mechanisms by which endophyte enhance the Cd phytoextraction remain unclear. Here, we employed the plant growth promoting endophyte (PGPE) Pseudomonas sp. E3, which has been validated to effectively enhance Cd extraction in Solanum nigrum L., to investigate its regulatory mechanism on soil ecology. The results demonstrated that while PGPE inoculation resulted in minimal alterations to the physicochemical properties of the bulk soil, it led to a notable increase in acid phosphatase activity by 17.86% and urease activity by 24.85% in the rhizosphere soil. This, in turn, significantly raised the available nitrogen and phosphorus contents by 16.93% and 21.27%, respectively, in the rhizosphere soil. Additionally, PGPE inoculation effectively replenished the bioavailable fractions of Fe and Cd, which had been depleted due to root uptake. Importantly, the inoculation specifically augmented the abundance of biomarkers p_Patescibacteria, f_Saccharimonadales, and g_Saccharimonadales in the rhizosphere soil. These biomarkers exhibited a significant positive correlation with the available nutrient and metal element contents. Moreover, the co-occurrence network analysis demonstrated that the inoculation resulted in a simplified bacterial community network, which may have facilitated community synergism by displacing bacteria with a negative association. This regulation appears to occur independently of PGPE colonization. Overall, our findings suggested that PGPE also exerts a regulatory influence on soil ecological features, significantly aiding hyperaccumulators in nutrient acquisition and heavy metal accumulation.

Electrical conductivity profiling for rapid contamination assessment in unsaturated zones: A case study of an MSW landfill

This study investigates the potential of using bulk soil electrical conductivity (ECbulk) to predict pore water conductivity (ECpw) for assessing the contamination in the unsaturated zone of an old municipal solid waste (MSW) landfill. ECbulk, ECpw, and water content were evaluated with depth at an old MSW landfill in Bhalswa, Delhi, using the Hydraulic Profiling Tool (HPT) and a dual tube soil sampling system. This data was also supplemented by a cone penetration test (CPTu) for high-resolution soil type identification. The correlation of ECbulk with ECpw was primarily influenced by volumetric water content and mineral conductivity with the latter being negligible at this site due to the high conductivity of the leachate. A reasonable linear correlation between normalized EC (ECbulk/ECpw) was observed with volumetric water content, except at low water contents. ECbulk and ECpw profiles with depth indicated attenuation of contaminants in clay layers, while sand layers exhibited constant profile with depth. ECpw was contributed by macro ions generally found in the leachate, including Na+, Mg2+, K+, Ca2+, NH4+, Cl−, SO42−, and HCO3−, as demonstrated by a strong correlation with their cumulative ionic strength. The results indicate that ECbulk profile can be used as a rapid semi-quantitative method for assessing contaminant migration in the unsaturated soil zone, supporting the remediation or control strategies at old landfills.

Regulation of photosynthetic carbon fate by plant diversity and nutrients in abandoned farmland on the Loess Plateau

Understanding the distribution and transfer of photosynthetic carbon (PC) in ecosystems is important to determine the fate of soil carbon (C). However, the allocation on plants of newly fixed PC during secondary succession is unknown. Using in-situ 13CO2 pulse labeling we report characteristics of PC turnover and driving factors that drive it in plants and soils at different successional stages on the Loess Plateau, China. Within 30 d of labeling, 13C abundance significantly decreased in plant leaves and stems, and significantly increased in roots, especially on farmland abandoned for 30a (from 114.67 mg·m−2 to 225.08 mg·m−2). The abundance of 13C in soils significantly increases over time, with 13C abundances in bulk soil 8a after abandonment being 8.17 % higher than those after 30a and 9.34 % after 15a. The contribution of newly fixed PC to soil organic carbon (SOC) was also significantly higher in soil 8a after abandonment than after 15a and 30a. Plant diversity and nutrients are the main factors to drive PC fixation and transfer. Because farmland abandoned for 8a has low plant diversity, high species evenness, and high plant nutrient contents, its plant-soil system fixes more PC. Plant diversity is a key predictor of PC fixation, and plant nutrients have an important influence on the transfer of PC. These results provide improve our understanding of vegetation restoration and terrestrial carbon sequestration in abandoned farmlands.

Gradient variations in rhizospheric soil exchangeable cations across a forest-steppe transect

The forest-steppe ecotone (FSE) is vulnerable to climate change and valuable to predict ecosystem succession. Although species diversity covaries with climatic factors and changes gradually across FSE transects, aligning with “Whittaker’s hierarchy theory”, whether soil nutrients follow a similar distribution pattern remains unclear. We studied the ecological gradient changes in effective cation-exchange capacity (ECEC, sum of exchangeable Ca2+, Mg2+, K+ and Na+) in the near-neutral-pH rhizosphere and bulk soils along the north–south zonal transect of the Hulunbuir FSE, China. The 230-km transect includes hierarchical transitions from two closed forests through two FSE to four meadow steppes. For both rhizosphere and bulk soils, soil ECEC were the highest in meadow steppes followed by closed forests and FSE area. ECEC were inversely correlated with soil water content but positively with soil pH. Therefore, the distribution of ECEC along the forest-steppe transect did not conform to a hierarchical distribution and decoupled from vegetation transitions, possibly due to the fact that the distribution of soil pH, plant uptake and leaching break the uniformity of ECEC variation along the gradient. In closed forests and FSE ecosystems, the ECEC and exchangeable Ca2+ in rhizosphere soils were higher than those in bulk soil, indicating that rhizosphere processes may alleviate cation limitation in these ecosystems. The study substantially improves our understanding of soil nutrient mobilization and soil fertility across forest-steppe ecotones which helps projecting ecosystem succession. It further highlights the critical role of rhizosphere processes in modulating hierarchical vegetation transition along environmental gradients and provides avenues for future research opportunities.

Faba bean in crop rotation shapes bacterial and fungal communities and nutrient contents under conventional tillage of triticale

Legumes in cropping sequence can strongly moderate soil biodiversity and its biochemical status, which could influence soil fertility and productivity. However, their impact on soil microbes and their relationships with soil chemical variables during the subsequent crop growth is not understood enough. In this study, we analyzed the changes in bacterial and fungal communities structure through 16S rRNA and ITS amplicon sequencing, respectively, across crop rotation with faba bean (faba bean-wheat-triticale, F) and without faba bean (wheat-wheat-triticale, W). Rhizosphere and bulk soil samples were taken during the triticale growth stages (stem elongation and maturity). Soil enzymatic activity and chemical properties were also determined. Study factors (crop rotation, soil compartment, growth stage) affected N, C, and P transformations, as indicated by the activity of soil enzymes, and F was more beneficial than W for protease, urease, and acid phosphomonoesterase activities, as opposed to cellulase activity. Changes in the chemical properties led to the shift in soil microbial communities with different bacterial and fungal communities' responses. F treatment enhanced the abundance of the bacterial genera representatives such as Streptomyces and Candidatus Udaeobacter while suppressing the abundance of Jatrophihabitans and Terrabacter. Crop rotation significantly influenced fungal genera and a greater abundance of Helgardia, Pseudogymnoascus, Monocillium, Fusarium, Chaetomium, and Vishniacozyma under F than W was noted and the adverse situation was noted for Peziza and Rhizopus. Rotation type significantly affected the alfa-diversity of the fungal, but not bacterial community. Beta-diversity analyses (nMDS, cluster) indicated that the main factor grouping samples were soil compartment and growth stage for the bacterial and fungal microbiome, respectively. The PERMANOVA results revealed significant effects of all factors on bacterial and fungal microbiomes. Different soil chemical variables governed bacterial and fungal communities. Corg, pH, P, Mg, and Corg, pH were the most important soil factors regulating bacterial and fungal community structure by crop rotation, respectively. Bacterial and fungal communities were more related to the content of Ntot in rotation with than without faba bean. The findings of this study contribute to a deeper insight into the relation between the faba bean in cropping sequence and soil microbiome, and modulation crucial soil conditions for the productivity of the successive crop.

No-tillage facilitates soil organic carbon sequestration by enhancing arbuscular mycorrhizal fungi-related soil proteins accumulation and aggregation

No-tillage is known to optimize soil structure and enhance soil organic carbon (SOC) stocks in cropland. However, the exact mechanisms driving the accumulation of SOC are still unclear, especially concerning the regulation of arbuscular mycorrhizal fungi (AMF) communities and diversity in SOC sequestration. Here, this study aims to elucidate the intricate relationship between AMF community, glomalin-related soil proteins (GRSP), and SOC within bulk soil and aggregates across four tillage treatments (i.e. FA, fallow; RT, rotary tillage; DT, deep tillage; NT, no-tillage) based on a 7-year tillage experiment. Results showed that the contents of SOC and GRSP were significantly higher by 1.14–1.46 mg/g and 0.43–0.72 mg/g in the bulk soil under NT relative to RT and DT, respectively. The contribution of GRSP-C to SOC under NT was also higher than RT and DT, especially in > 53 μm particle size. Additionally, NT increased AMF diversity and the abundance of glomerales and diversisporales, all showing a strong positive correlation with GRSP (p < 0.05), indicating their potential regulatory role in GRSP production. The positive correlations between GRSP and the mass percentage of the > 53 μm particle size fraction (R2 = 0.74; p < 0.01) and MWD (R2 = 0.63; p < 0.01) suggested that no-tillage may drive large aggregates (>53 μm) formation and enhance aggregate stability through GRSP levels. Overall, increased AMF diversity and keystone taxa abundance at the order level via no-tillage promoted SOC accumulation through the production of GRSP and the protection of large aggregates. This study highlights that no-tillage is an effective and sustainable soil management strategy for enhancing soil quality in agricultural ecosystems.

Changes in soil and plant carbon pools after 9 years of experimental summer warming and increased snow depth

Climate change can have positive and negative effects on the carbon pools and budgets in soil and plant fractions, but net effects are unclear and expected to vary widely within the arctic. We report responses after nine years (2012−2021) of increased snow depth (snow fences) and summer warming (open top chambers) and the combination on soil and plant carbon pools within a tundra ecosystem in West Greenland. Data included characteristics of depth-specific soil samples, including the rhizosphere soil, as well as vegetation responses of NDVI-derived traits, plant species cover and aboveground biomass, litter and roots. Furthermore, natural vegetation growth through the study period was quantified based on time-integrated NDVI Landsat 8 satellite imagery.Our results showed that summer warming resulted in a significant and positive vegetation response driven by the deciduous low shrub Betula nana (no other vascular plant species), while snow addition alone resulted in a significant negative response for Betula. A significant positive effect of summer warming was also observed for moss biomass, possibly driven increasing shade by Betula. The aboveground effects cascaded to belowground traits. The rhizosphere soil characteristics differed from those of the bulk soil regardless of treatment. Only the rhizosphere fraction showed responses to treatment, as soil organic C stock increased in near-surface and top 20 cm with summer warming. We observed no belowground effects from snow addition. The study highlights the plant species response to treatment followed by impacts on belowground C pools, mainly driven by dead fine roots via Betula nana. We conclude that the summer warming treatment and snow addition treatment separately showed opposing effects on ecosystem C pools, with lack of interactive effects between main factors in the combination treatment. Furthermore, changes in soil C are more clearly observed in the rhizosphere soil fraction, which should receive more attention in the future.

Microaggregates regulate the soil organic carbon sequestration and carbon flow of windproof sand fixation forests in desert ecosystems

Afforestation improves the soil organic carbon (SOC) sequestration by affecting aggregates formation. However, the impact of the establishment and development of windproof sand fixation forests on soil carbon (C) flow and sequestration in desert regions are largely unclear. The space-for-time method was used to elucidate changes in bulk soil, aggregate-associated organic carbon (OC) content, stock, and C flow with afforestation years and the main influencing factors. We sampled from 0–20 cm, 20–60 cm and 60–100 cm in natural desert (CK) and windproof sand fixation forests after 3, 7, and 10 years of afforestation. The direction of C flow within the aggregates was quantified using the δ13C natural abundance method. Results showed that compared to the CK, afforestation increased in the bulk soil OC stock in the 0 − 100 cm by 2.97, 5.34, and 1.67 Mg·ha−1 at 3, 7, and 10 years of afforestation, respectively. The bulk soil OC content decreased gradually, but the stock increased with soil depth. Furthermore, C sequestration in this region mainly relied on the microaggregates OC. Microaggregates acted as both a “source” and a “sink” in desert ecosystems. During the 7 years of afforestation, the δ13C in the aggregates decreased with increasing soil depth, indicating older C in the subsoil. The variations in bulk soil OC stock were mainly regulated by ammonium nitrogen, nitrate nitrogen, and total nitrogen. Therefore, the results suggest that 7 years of afforestation may be the optimal choice among the three afforestation years compared for C sequestration.

Long-term legacy of phytoremediation on plant succession and soil microbial communities in petroleum-contaminated sub-Arctic soils

Abstract. Phytoremediation can be a cost-effective method of restoring contaminated soils using plants and associated microorganisms. Most studies follow the impacts of phytoremediation solely across the treatment period and have not explored long-term ecological effects. In 1995, a phytoremediation study was initiated near Fairbanks, Alaska, to determine how the introduction of annual grasses and/or fertilizer would influence degradation of petroleum hydrocarbons (PHCs). After 1 year, grass and/or fertilizer-treated soils showed greater decreases in PHC concentrations compared to untreated plots. The site was then left for 15 years with no active site management. In 2011, we re-examined the site to explore the legacy of phytoremediation on contaminant disappearance, as well as on plant and soil microbial ecology. We found that the recruited vegetation and the current bulk soil microbial community structure and functioning were all heavily influenced by initial phytoremediation treatment. The number of diesel-degrading microorganisms (DDMs) was positively correlated with the percentage cover of vegetation at the site, which was influenced by initial treatment. Even 15 years later, the initial use of fertilizer had significant effects on microbial biomass, community structure, and activity. We conclude that phytoremediation treatment has long-term, legacy effects on the plant community, which, in turn, impact microbial community structure and functioning. It is therefore important to consider phytoremediation strategies that not only influence site remediation rates in the short-term but also prime the site for the restoration of vegetation over the long-term.

Assessing the formation and stability of paddy soil aggregate driven by organic carbon and Fe/Al oxides in rice straw cyclic utilization strategies: Insight from a six-year field trial

Soil organic carbon (SOC) and iron/aluminum (Fe/Al) oxides are key cementing agents in driving soil aggregate formation, yet their direct effects and interactions on aggregate under long-term rice straw cyclic utilization (LSCU) in cold regions are still unclear. We compared chemical fertilizer (CF) with LSCU strategy: rice-straw (RS), biochar (RB), and biochar-based fertilizer (BF). We showed that the increase of macroaggregate (2–0.25 mm) is associated with SOC, dissolved organic carbon (DOC), humin carbon (HUC), amorphous and organic complexed Fe/Al oxides (Feo, Fep, Alo, Alp), and in each size of the aggregate, there exists an interaction between SOC (fractions) and Fe/Al oxides. Furthermore, aggregate stability was determined by Feo, Fep, and Alo. LSCU enhances macroaggregate and aggregate stability by increasing SOC and Fe/Al oxides in the bulk soil and aggregates, but there are differences among LSCU. In all treatments, RS had more DOC, fulvic acid carbon (FAC), humic acid carbon (HAC) and Fep; while RB had more SOC, HUC, free Fe/Al oxides (Fed, Ald), Feo, Alp; and BF had more Alo in bulk soil. Over the years, RS increased the DOC, FAC and HAC, whereas RB enhanced the stable SOC fractions (HUC) and promoted high reactive Fe/Al oxides formation (Feo, Fep, Alo), and BF increased DOC, Feo, Fep and Alo. Moreover, RB increases the direct pathway of SOC and Fe/Al oxides to aggregate, promoting aggregate formation. Our study provides new perspective on the mechanisms and promising practice for improving rice straw utilization efficiently, paddy soil fertility and productivity sustainably in cold regions.

Benefit and risk: Keystone biomes in maize rhizosphere associated with crop yield under different fertilizations

The highly diverse beneficial and deleterious microbes in the rhizosphere influence plant growth and crop production in agricultural ecosystems. However, our understanding of the relationships between beneficial and risky phylotypes in the rhizosphere, and plant growth, in addition to their potentially varied response to different fertilization treatments in black soils, remains poor. Here, we investigated variations in multitrophic (e.g., bacteria, fungi, invertebrates, and protists) community composition and diversity under different five-year fertilization treatments, and identified the potential beneficial and potential risky keystone multitrophic biota in maize rhizosphere and bulk soils based on ecological networks and soil food web structure. Bacteria and protist community compositions exhibited greater variations under different fertilization regimes when compared with those of fungi and invertebrates. In addition, two keystone phylotype groups associated with crop yield and soil function were detected, and the potential beneficial keystone phylotypes exhibited higher relative abundances in C-fixation, N-fixation, and P-mineralization genes; conversely, the potential risky keystone phylotypes were associated with denitrification and nitrate reduction genes, and included more potential fungal pathogens. Furthermore, structural equation modeling results indicated that organic amendments, particularly cow manure, could improve crop yield by enriching beneficial keystone phylotypes and suppressing risky keystone phylotypes. Altogether, the results of the present study highlight the critical roles of rhizosphere biota in food production, and identify keystone phylotypes that are beneficial or harmful to crop production and soil function, which could facilitate the conservation of beneficial biota and the early detection of potential pathogens, as well as the exploitation of beneficial biota to increase crop yield.

Long-term field pH manipulation influence on microbial activity, water repellency and physical properties of soil

Abstract Studies across multiple soils find increasing pH decreases water repellency. In this study, water repellency and a range of other soil physical properties of bulk soils, aggregates and intact specimens were measured on a long-term pH field experiment on a single sandy loam soil under a ley-arable crop rotation, with soil pH adjustments occurring annually by adding FeSO4 or CaCO3, to lower or raise the pH, respectively. Crop impacts were investigated by comparing 3rd year grass-white clover to spring oats, at the beginning (May) and end (September) of the growing season to allow soil structure comparisons. As in previous research, increased CO2 microbial respiration (p&lt;0.05) was found with increasing pH along the gradient, but in this study, we found only the aggregate and soil bulk density affected by soil pH. Soil-water contact angles differed between crops (p&lt;0.05), as well as the repellency index of soil aggregates, however, there was no soil pH effect. Overall, differences in data were found to be a result of the various crops in the rotation rather than by soil pH, indicating only minor impacts on soil physical characteristics after &gt; 55 years of chemical additions to amend soil pH.

Insights into effects of conventional and biodegradable microplastics on organic carbon decomposition in different soil aggregates

The impacts of microplastics on soil ecological functions such as carbon recycling and soil structure maintenance have been extensively focused. However, the mechanisms underlying the impacts of microplastics on soil carbon transformation and soil microbial community at soil aggregate scale have not been clarified yet. In this work, the effects and action mechanisms of traditional microplastic polypropylene (PP) and degradable microplastic polylactic acid (PLA) on carbon transformation in three sizes of soil aggregates were investigated. The results showed that both PP and PLA promoted CO2 emission, and the effect depended on the type and content of microplastics, and the size of soil aggregates. Changes in soil carbon stocks were mainly driven by changes in organic carbon associated with macroaggregates. For macroaggregates, PP microplastics decreased soil organic carbon (SOC) as well as dissolved organic carbon (DOC). These changes were reversed in microaggregates and silt and clay. Interestingly, PLA increased the SOC, DOC and CO2 emissions in bulk soil and all three aggregates with a dose-effect response. These changes were associated with soil microbes, functional genes and enzymes associated with the degradation of labile and recalcitrant carbon fractions. Furthermore, PP and PLA reduced bacterial community diversities and shifted bacterial community structures in both the three aggregates and in bulk soil. Alterations of functional genes induced by microplastics were the key driving factors of their impacts on carbon transformation in soil aggregates. This research opened up a new insight into the mechanisms underlying the impacts of microplastics on soil carbon transformation, and helped us make rational assessments of the risks and the disturbances of microplastics on soil carbon cycling.

Abscisic acid increase correlates with the soil water threshold of transpiration decline during drought.

By regulating carbon uptake and water loss by plants, stomata are not only responsible for productivity but also survival during drought. The timing of the onset of stomatal closure is crucial for preventing excessive water loss during drought, but is poorly explained by plant hydraulics alone and what triggers stomatal closure remains disputed. We investigated whether the hormone abscisic acid (ABA) was this trigger in a highly embolism-resistant tree species Umbellularia californica. We tracked leaf ABA levels, determined the leaf water potential and gravimetric soil water content (gSWC) thresholds for stomatal closure and transpiration decline during a progressive drought. We found that U. californica plants have a peaking-type ABA dynamic, where ABA levels rise early in drought and then decline under prolonged drought conditions. The early increase in ABA levels correlated with the closing of stomata and reduced transpiration. Furthermore, we found that transpiration declined before any large decreases in predawn plant water status and could best be explained by transient drops in midday water potentials triggering increased ABA levels. Our results indicate that ABA-mediated stomatal regulation may be an integral mechanism for reducing transpiration during drought before major drops in bulk soil and plant water status.