Total Soil Organic Matter Content Research Articles

Efficient removal of lead from polluted paddy soil by one-pot synthesized Nano-Fe3O4 incorporated stable lignin hydrogel

Lead (Pb) contamination in agricultural soils poses a significant threat to both ecosystems and human health. While nano-Fe3O4 exhibits promising potential for Pb remediation, its practical application in the soil is hindered by its’ biotoxicity, easy aggregation, and the risk of secondary pollution. Thus, this study presents a novel approach wherein Fe3O4 was incorporated into hydrogel via a one-pot synthetic strategy (Fe3O4@LH). This incorporation enhanced the mechanical properties and environmental stability of the hydrogel composites. Based on the mechanical properties, environmental stability, and single-point adsorption results for Pb, we selected Fe3O4@LH-4 for further research. The removal mechanism and the feasibility of employing Fe3O4@LH-4 for Pb removal from paddy soil were investigated through batch adsorption experiments and soil culture studies. Results showed that the adsorption process was primarily governed by swelling adsorption, electrostatic adsorption, ion exchange, precipitation, nanometer effect, and complexation mechanisms. The application of Fe3O4@LH-4 significantly led to the reduction of 16.7 %–25.4 % in soil Pb content, with removal rates escalating alongside increased dosage and application periods of Fe3O4@LH-4. Fitting results of the prediction model indicated that the Pb content in mildly Pb-contaminated soil (186.55 mg/kg) would decrease to be below the risk control standard for soil contamination of agricultural land in China (140 mg/kg) after 112 days of continuous application. Concurrently, cadmium and arsenic contents in the soil decreased by 5.2 %–10.8 % and 7.1 %–16.7 %, respectively. Moreover, the application of Fe3O4@LH-4 positively influenced soil nutrient levels, with total nitrogen and soil organic matter content significant increments of 13.6 %–41.0 % and 4.6 %–16.1 %, respectively. Furthermore, Fe3O4@LH-4 recovery exceeded 88.3 % after a 90-day application period. These findings underscore the potential of Fe3O4 incorporated hydrogel as a promising agent for the sustained removal of heavy metal Pb from paddy soil.

Is it necessary to increase the maize planting density in China?

Increasing the maize planting density is considered a potential approach for increasing the grain yield in China, but there is no consensus regarding its yield-increasing effect and the influence of specific factors. Thus, we established a database (2721 pairs of data from 187 publications) to quantify the effects of increasing the maize planting density on the phenotypic traits and yield, to determine an appropriate maize planting density for each planting area, and to quantify the effects of environmental factors and agricultural imputs on the outcomes of increasing the planting density. We found that increasing the planting density increased the individual competition among maize plants, with negative effects on their growth, but the grain yield increased by 11.18–13.43 % due to the increased population biomass. Using the database, we found large differences in the optimal density and peak grain yield among maize planting areas, and the factors that caused these differences were analyzed based on subgroup analysis. Field management practices significantly influenced the outcomes of increasing the planting density. In particular, higher agricultural inputs (irrigation amount, and nitrogen and phosphorus application rates) enhanced the positive effect of increasing plant density on the optimal plant density and peak grain yield. In addition, the effects of increasing the planting density were significantly influenced by environmental (climate and soil) factors. When the mean annual temperature was 7–14°C and the mean annual precipitation was 400–800 mm, the yield and maximum peak yield were highest in relatively fertile (high total nitrogen, available nitrogen, and soil organic matter contents) and neutral (pH 6–8) soils. Our results highlight the need to increase the current maize planting density and provide a scientific basis for determining reasonable planting densities for different maize growing areas in China.

Uptake and Transport of Selenium in a Soil–Tea Plant–Tea Infusion System: A Study of Typical Tea Plantations in a Selenium-Rich Area of China

Selenium is an important indicator for the evaluation of tea quality. However, the relationship between selenium uptake by tea plants (Camellia sinensis) and soil properties, as well as selenium transport and distribution in a tea plantation soil–tea plant–tea infusion production system, remain unclear. In this study, 12 tea plantations situated in a typical selenium-rich area of China were selected, and the characteristics and crucial factors influencing the uptake and transport of selenium were analyzed using a plantation soil–tea plant–tea infusion production system. The soil total selenium content ranged from 1.12 to 6.67 mg kg−1, with an average of 2.57 mg kg−1. The average available selenium content was 53.56 µg kg−1, and the activation rate of soil selenium was 2.27%. Soil-available selenium was significantly positively correlated with total selenium, available potassium, and soil organic matter contents, and was significantly negatively correlated with soil pH. The selenium content in old leaves ranged from 0.29 to 2.73 mg kg−1, which met the standard for selenium-rich tea, whereas only 33% of young leaves met this standard. The selenium enrichment factor was highest in the fibrous root and lowest in the young leaves. The average selenium transport factors from fibrous roots to main roots, from main roots to main stems, from main stems to lateral stems, from lateral stems to young leaves, and from lateral stems to old leaves were 0.53, 0.92, 0.67, 0.97, and 2.30, respectively. The selenium concentration of tea infusion ranged from 1.88 to 12.49 μg L−1, and the average selenium dissolution rate was 22.62% after one brewing. This study identified critical factors that influence soil-selenium availability. The selenium content in tea plant organs is indicated to be strongly associated with the selenium content in the main roots.

Moderate organic fertilizer substitution for partial chemical fertilizer improved soil microbial carbon source utilization and bacterial community composition in rain-fed wheat fields: current year.

Organic fertilizers can partially replace chemical fertilizers to improve agricultural production and reduce negative environmental impacts. To study the effect of organic fertilizer on soil microbial carbon source utilization and bacterial community composition in the field of rain-fed wheat, we conducted a field experiment from 2016 to 2017 in a completely randomized block design with four treatments: the control with 100% NPK compound fertilizer (N: P2O5: K2O = 20:10:10) of 750 kg/ha (CK), a combination of 60% NPK compound fertilizer with organic fertilizer of 150 kg/ha (FO1), 300 kg/ha (FO2), and 450 kg/ha (FO3), respectively. We investigated the yield, soil property, the utilization of 31 carbon sources by soil microbes, soil bacterial community composition, and function prediction at the maturation stage. The results showed that (1) compared with CK, organic fertilizer substitution treatments improved ear number per hectare (13%-26%), grain numbers per spike (8%-14%), 1000-grain weight (7%-9%), and yield (3%-7%). Organic fertilizer substitution treatments increased the total nitrogen, available nitrogen, available phosphorus, and soil organic matter contents by 26%, 102%, 12%, and 26%, respectively, compared with CK treatments. Organic fertilizer substitution treatments significantly advanced the partial productivity of fertilizers. (2) Carbohydrates and amino acids were found to be the most sensitive carbon sources for soil microorganisms in different treatments. Particularly for FO3 treatment, the utilization of β-Methyl D-Glucoside, L-Asparagine acid, and glycogen by soil microorganisms was higher than other treatments and positively correlated with soil nutrients and wheat yield. (3) Compared with CK, organic fertilizer substitution treatments increased the relative abundance of Proteobacteria, Acidobacteria, and Gemmatimonadetes and decreased the relative abundance of Actinobacteria and Firmicutes. Interestingly, FO3 treatment improved the relative abundance of Nitrosovibrio, Kaistobacter, Balneimonas, Skermanella, Pseudomonas, and Burkholderia belonging to Proteobacteria and significantly boosted the relative abundance of function gene K02433 [the aspartyl-tRNA (Asn)/glutamyl-tRNA (Gln)]. Based on the abovementioned findings, we suggest FO3 as the most appropriate organic substitution method in rain-fed wheat fields.

Aging affects isomer-specific occurrence of dechlorane plus in soil profiles: A case study in a geographically isolated landfill from the Tibetan Plateau

Two major structural isomers in commercial dechlorane plus (DP) mixtures, anti-DP and syn-DP, generally displayed varied desorption and partitioning efficiencies in soils, which may be linked to their different aging rates. However, the molecular parameters that govern the degree of aging and its associated effects on the occurrence of DP isomers have not been comprehensively investigated. In this study, the relative abundance of rapid desorption concentration (Rrapid) was measured for anti-DP, syn-DP, anti-Cl11-DP, anti-Cl10-DP, Dechlorane-604 (Dec-604), and Dechlorane-602 (Dec-602) at a geographically isolated landfill area in the Tibetan Plateau. The Rrapid values were used as an indicator of aging degree, exhibiting a close correlation with the three-dimension conformation of the molecules for the dechlorane series compounds. This observation suggested that planar molecules may have a greater tendency to accumulate in the condensed phase of organic matter and undergo more rapid aging. The fractional abundances and dechlorinated products of anti-DP were found to be predominantly controlled by the aging degree of DP isomers. The multiple nonlinear regression model indicated that differences in aging between anti-CP and syn-DP were primarily driven by the total desorption concentration and soil organic matter content. Aging plays a significant role in both the transport processes and metabolism of DP isomers and should be taken into account to refine the assessment of their environmental behaviors.

Spatial Changes in Glomalin-Related Soil Protein and Their Correlation with Soil Properties in the Black Soil Region of Northeast China

Glomalin-related soil protein (GRSP), soil nutrients, and soil enzyme activities are closely related to soil fertility and land productivity, which play an important role in indicating soil quality. Little is known about the spatial variation in GRSP and its relationship with edaphic factors. Here, the spatial distribution of GRSP, soil chemical properties, and the soil enzyme activities of 0–20 cm depth farmland soil in the black soil region of northeast China were investigated, and the relationships among edaphic factors were analyzed collected from 41 sampling sites. The results indicate that GRSP, soil organic matter, total nitrogen, and acid phosphatase activities showed significant patterns of spatial variation, generally decreasing from north to south along a latitudinal gradient. Principal component analysis revealed that total GRSP (by 80.19%) and soil organic matter content (by 80.15%) were the greatest contributing factors accounting for the variations. Edaphic factors such as soil organic matter, total nitrogen, total phosphorus, and acid phosphatase were significantly positively correlated with GRSP, while urease was negatively correlated with GRSP. Mantel tests also showed that soil organic matter, total nitrogen, urease, and acid phosphatase were positively correlated with GRSP. The results reflect the soil fertility characteristics of the black soil region of northeast China and reveal the relationship among edaphic factors. These findings could be used to inform agricultural production and provide new insight into the role of GRSP in soil quality.

Assessment of the pollution levels of potential toxic elements in urban vegetable gardens in southwest China

Vegetable gardens are increasingly common in urban areas and can provide numerous societal benefits; however, contamination with potential toxic elements (PTEs) due to urbanization and industrialization is cause for concern. The present study aimed to assess the source of contamination and pollution levels in urban garden soils, as well as the health risks for adults and children consuming vegetables grown in such environments. Various types of vegetable samples and their corresponding soils from 26 community gardens were collected throughout Chengdu City in southwestern China. The results showed that leafy vegetables, particularly lettuce leaves and Chinese cabbage, had relatively higher levels of Cd (0.04 mg/kg FW) and Pb (0.05 mg/kg FW), while higher levels of As (0.07 mg/kg FW), Cr (0.07 mg/kg FW), and Hg (0.003 mg/kg FW) were found in amaranths, tomatoes, and Houttuynia cordatas, respectively. The pollution indices revealed that the vegetable purplish soils were relatively more polluted by Cd and As, and the concentrations of these metals in vegetables were correlated with their concentrations in the soils. Principal component analysis grouped the PTEs in two dimensions that cumulatively explained 62.3% of their variation, and hierarchical clustering identified two distinct clusters indicating that Cr originated from a unique source. The health risk assessment revealed that exposure to As and Cd induced the greatest non-carcinogenic risk, whereas Cr was most likely to cause cancer risks. Furthermore, contaminated vegetable consumption was riskier for children than adults. The critical factors contributing to PTE contamination in vegetable gardens were determined to be vegetable species, total soil element content, soil pH, and soil organic matter content. Overall, Cr and As pollution present the greatest concern, and community health care services must enact more effective regulatory and preventative measures for urban gardens in terms of PTEs.

EFFECTS OF PLASTIC MULCHING AND STRAW INCORPORATION ON RICE YIELD AND NITROGEN USE EFFICIENCY IN A COLD REGION

For the synergistic improvement of soil fertility and rice yields in a cold region, we investigated the effects of plastic mulching with different straw incorporation rates on the rice yield and nitrogen use efficiency in a rice growing area of a cold region. The field experiment was conducted using the Jijing 88 rice cultivar, and set up four straw incorporated treatment: 20% (S1), 40% (S2), 60% (S3) and 100% (S4) under two conditions: mulching (M) and without mulching (NM). The results showed that under the same straw incorporation rates: (1) Plastic mulching increased the total soil nitrogen and organic matter contents while improving the grain yield; the yield increases were 2.8–14.9% in 2016 and 1.4–12.3% in 2017. (2) The mulched treatment improved the net photosynthetic rate, the maximum tiller number, the crop growth rate of the later growth stages, and the shoot dry weight. (3) The recovery efficiency, agronomic efficiency, and partial factor productivity of the nitrogen applications were all higher under the mulched treatment than under the non-mulched treatment. In a comparison of the different straw incorporation treatments, above three indicators consistently increased before they decreased, and significantly higher levels were reached under the 40% straw incorporation treatment with mulching than under the other treatments.

Effects of no-tillage sowing on soil properties and forage wheat and Italian ryegrass yields in winter fallow paddy fields.

In South China, it is common practice for the late rice (Oryza sativa) that is planted during the summer in the paddy fields after harvest to be used for fallowing or to plant winter forage crops. The land is ploughed before early rice planting. Both forage wheat (Triticum aestivum) and Italian ryegrass (Lolium multiflorum) have relatively high nutritional value, and planting them in winter fallow paddy fields could potentially address food shortages and provide quality forage for livestock. In this study, we examined the effects of no-tillage sowing 5 days before rice harvest (NB5), no-tillage sowing 1 day after rice harvest (NA1), and conventional tillage sowing (CK) 1 day after rice harvest on forage wheat and Italian ryegrass soil properties, dry matter (DM), and crude protein (CP) yields. Soil and plant samples were collected after three months of crop growth. The results showed that the NB5 and NA1 soil bulk density (0-20 cm soil layer) tended to increase when compared to that of the CK field. The NA1 treatment increased the total soil nitrogen and organic matter content. The enzyme activities and total soil porosity in the no-tillage forage wheat and Italian ryegrass fields tended to decrease, while the no-tillage water content and soil capillary porosity tended to increase when compared to that of the CK field. Overall, planting year significantly influenced soil chemical properties (except for total nitrogen) and enzyme activity, but crop type had no significant effect on soil physical-chemical properties (except for capillary moisture capacity) and enzyme activity. Sowing methods had no significant effects on the crop DM and CP yields. The DM yield was affected by the interaction between planting year and sowing methods, or between sowing methods and crop type. No-tillage also increased the number of species and aboveground weed biomass. We concluded that the best sowing method for forage wheat and Italian ryegrass in winter fallow paddy fields was no-tillage sowing following rice harvest.

EFFECT OF INTEGRATED NUTRIENT MANAGEMENT PRACTICES ON YIELD AND PASSIVE POOLS OF SOIL ORGANIC CARBON IN LONG TERM FERTILIZER EXPERIMENT

Soil organic carbon (SOC) plays a vital role order to improve soil properties and sustainable production. The different fractions of SOM are more effective in indicating changes in soil use than total soil organic matter content. The present study was conducted to investigate the effect of integrated nutrient management (INM) on yields and passive pools of soil organic carbon (SOC) under groundnut-wheat cropping sequence of a Haplustepts soil. Such studies were for the first time initiated in long term field experiments initiated during kharif 1999 at Junagadh, Gujarat. Effect on varying doses of N, NP, NPK, NPK with FYM, Zn, S and Rhizobium on yields and passive pools of SOC viz., Humic acid, Fulvic acid and Humin was conducted after 16 year of groundnut-wheat crop sequence was studied. The result of the two was compared and conclusion deduced. The result revealed that application of 50% NPK + FYM @ 10 t ha-1 to groundnut and 100% NPK to wheat significantly increased the groundnut pod and haulm yield, wheat grain and straw yield. The highest and significant increase passive pools of soil organic carbon viz., Humic acid, Fulvic acid and Humin was also observed under combine application of 50% NPK + FYM @ 10 t ha-1 to groundnut and 100% NPK to wheat. These results indicate that long-term integrated use of FYM with chemical fertilizers or use of FYM alone exerted significant effect on the passive pools of soil organic carbon.

Freeze–thaw processes degrade post‐fire water repellency in wet soils

AbstractWildfires are a cause of soil water repellency (hydrophobicity), which reduces infiltration whilst increasing erosion and flooding from post‐fire rainfall. Post‐fire soil water repellency degrades over time, often in response to repeated wetting and drying of the soil. However, in mountainous fire‐prone forests such as those in the Western USA, the fire season often terminates in a cold and wet winter, during which soils not only wet and dry, but also freeze and thaw. Little is known about the effect of repeated freezing and thawing of soil on the breakdown of post‐fire hydrophobicity. This study characterized the changes in hydrophobicity of Sierra Nevada mountain soils exposed to different combinations of wet–dry and freeze–thaw cycling. Following each cycle, hydrophobicity was measured using the Molarity of Ethanol test. Hydrophobicity declined similarly across all experiments that included a wetting cycle. Repeated freezing and thawing of dry soil did not degrade soil water repellency, but freeze–thaw cycles decreased hydrophobicity in wet soils. Total soil organic matter content was not different between soils of contrasting hydrophobicity. Macroscopic changes such as fissures and cracks were observed to form as soil hydrophobicity decayed. Microscopic changes revealed by scanning electron microscope imagery suggest different levels of soil aggregation occurred in samples with distinct hydrophobicities, although the size of aggregates was not clearly correlated to the change in water repellency due to wet–dry and freeze–thaw cycling. A 9‐year climate and soil moisture record from Providence Critical Zone Observatory was combined with the laboratory results to estimate that hydrophobicity would persist an average of 144 days post‐fire at this well‐characterized, typical mid‐elevation Sierra Nevada site. Most of the breakdown in soil water repellency (79%) under these climate conditions would be attributable to freeze–thaw cycling, underscoring the importance of this process in soil recovery from fire in the Sierra Nevada.

Organic amendments change soil organic C structure and microbial community but not total organic matter on sub-decadal scales

Organic C has many benefits for soil, but it is depleted by tillage and crop harvest, and especially so for biofuel crops. Accordingly, strategies such as partially retaining stover or planting a cover crop can help ameliorate the negative effect of C removal. We used a long-term field experiment to study the impacts of stover retention and planting a cover crop on soil organic matter (SOM), its extractable components, and the soil microbial community. SOM chemical composition characterization was determined by electrospray ionization (ESI) coupled with Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) in sequential water, methanol (MeOH), and chloroform (CHCl3) extracts. The characteristics of the soil bacterial community were measured by phospholipid fatty acid (PLFA), real-time quantitative PCR, and 16S rRNA gene sequence. The variations in total SOM content, total microbial biomass, and bacterial population were slight among treatments, but SOM chemical compounds, arbuscular mycorrhizal fungi (AMF) biomass, and bacterial structure changed significantly, and especially so in the coupled application of stover retention and cover crop. Specifically, stover retention enriched more lignin-like compounds in soil, whereas cover crop enriched more condensed hydrocarbons, and had more compounds with an aromaticity index (AI) > 0.5. The bacterial community was not altered by the cover crop, but the corn stover retention increased the relative abundances of Myxococcales (Deltaproteobacteria) and decreased that of Actinobacteria. Redundancy analysis (RDA) further revealed that the bacterial community in the stover treatments had a significant positive association with CHCl3-extracted chemical classes, i.e. unsaturated hydrocarbons and lipids, with the coupled application (stover and cover crop), and lignin and proteins with the corn stover only treatment. Taken together, our study shows how different C addition practices influence the molecular composition of SOM and the structure of soil microbial communities.

Bradford quantification of Glomalin-Related Soil Protein in coloured extracts of forest soils

Glomalin-related soil protein (GRSP) is thought to represent a fraction of recalcitrant organic matter in soil. But it is recognized that the autoclaved-citrate extraction procedure causes the co-extraction of humic substances which interfere (directly and indirectly) with the Bradford colorimetric assay. The aim of this work was to propose a reliable quantification method of GRSP from forest soil, very rich in organic matter and therefore in colour. We estimated the quantities of GRSP in the topsoil (0–10 cm) of 102 French forests using five methods: i) direct calibration, reasoned dilution with colour correction, ii) direct calibration, reasoned dilution but no colour correction, iii) direct calibration, 1:2 dilution, no colour correction, iv) indirect calibration and v) dilution method. Our results concur that the interference caused by the co-extracted compounds is not related simply to either the colour of the extracts or total soil organic matter content. These findings suggest that for improved accuracy of GRSP estimates using the Bradford method, extracts should be diluted, and the pH-specific absorbance of coloured extracts should be subtracted.

Soil nutrient variations among different land use types after reclamation in the Pingshuo opencast coal mine on the Loess Plateau, China

Soil nutrients are critical indicators of land reclamation effects and play significant roles in soil restoration in degraded mining areas; however, few studies have analyzed the characteristics and variations in soil nutrients after reclamation in ecologically fragile regions on the Loess Plateau. To analyze the spatial and temporal variations in reclaimed soil nutrients, an experimental field was established in an inner reclaimed dump consisting of four land use types (cultivated land (CL), forestland (FL), grassland (GL), and barren land (BL)) in the Antaibao mine on the Loess Plateau of China. Sampling was conducted in 2013 and 2017, with 78 and 81 sampling points, respectively. The soil available phosphorus (SAP), soil available potassium (SAK), soil total nitrogen (STN) and soil organic matter (SOM) contents at depths of 0–20 cm were measured at each sampling point, and the variations in soil nutrients were analyzed using two-way multivariate analysis of variance, geostatistical analysis, and correspondence analysis. This study revealed the following: (1) All the reclaimed soil nutrients tested increased between 2013 and 2017. (2) There were significant differences in the SAP, SAK, STN and SOM contents among land use types and reclamation ages, and land use type and reclamation age had an interaction effect on the reclaimed soil nutrients. (3) The spatial distributions of the SAP, SAK, STN and SOM contents in 2017 were significantly different from those in 2013 but were almost consistent with the distributions of soil nutrient changes between 2013 and 2017. (4) The most improved SAP, SAK, STN and SOM (SAP-VI, SAK-VI, STN-VI, and SOM-VI) were predominantly distributed in the GL, and there was no obvious trend for the other land use types. These findings are significant for the restoration of reclaimed soil nutrients and provide reasonable land use type options for land reclamation in coal mining areas, especially in ecologically fragile areas on the Loess Plateau.

Effects of maize-soybean relay intercropping on crop nutrient uptake and soil bacterial community

Maize-soybean relay intercropping is an effective approach to improve the crop yield and nutrient use efficiency, which is widely practiced by farmers in southwest of China. To elucidate the characteristics of different planting patterns on crop nutrient uptake, soil chemical properties, and soil bacteria community in maize-soybean relay intercropping systems, we conducted a field experiment in 2015–2016 with single factor treatments, including monoculture maize (MM), monoculture soybean (MS), maize-soybean relay intercropping (IMS), and fallow (CK). The results showed that the N uptake of maize grain increased in IMS compared with MM. Compared with MS, the yield and uptake of N, P, and K of soybean grain were increased by 25.5, 24.4, 9.6, and 22.4% in IMS, respectively, while the N and K uptakes in soybean straw were decreased in IMS. The soil total nitrogen, available phosphorus, and soil organic matter contents were significantly higher in IMS than those of the corresponding monocultures and CK. Moreover, the soil protease, soil urease, and soil nitrate reductase activities in IMS were higher than those of the corresponding monocultures and CK. The phyla Proteobacteria, Acidobacteria, Chloroflexi, and Actinobacteria dominated in all treatments. Shannon's index in IMS was higher than that of the corresponding monocultures and CK. The phylum Proteobacteria proportion was positively correlated with maize soil organic matter and soybean soil total nitrogen content, respectively. These results indicated that the belowground interactions increased the crop nutrient (N and P) uptake and soil bacterial community diversity, both of which contributed to improved soil nutrient management for legume-cereal relay intercropping systems.

Humic Acid Fertilizer Improved Soil Properties and Soil Microbial Diversity of Continuous Cropping Peanut: A Three-Year Experiment

Although humic acid has been demonstrated to improve the quality of some soil types, the long-term effects of humic acid on soil under continuous cropping peanut are not fully understood. This study aimed to investigate the continuous effects of humic acid on the physicochemical properties, microbial diversity, and enzyme activities of soil under continuous cropping peanut. In this study, a three-year consecutive experiment of cropping peanut was conducted in the North China Plain. In addition to the equal nitrogen, phosphorus, and potassium inputs, humic acid treatment was applied with inorganic fertilizers. Compared with control experiments, humic acid increased the yield and quality of continuous cropping peanut. To elucidate the mechanism of humic acid affecting the soil quality, various soil quality indicators were evaluated and compared in this study. It was found that humic acid increased soil nutrient contents, including the total soil nitrogen, total phosphorus, total potassium, available nitrogen, available phosphorus, available potassium, and organic matter contents, which exhibited the maximum effect in the third year. Meanwhile, the urease, sucrase, and phosphatase activities in the soil significantly increased after treated with humic acid, of which the maturity period increased most significantly. The same results were observed for three consecutive years. Microbial diversity varied considerably according to the high throughput sequencing analysis. Specifically, the number of bacteria decreased while that of fungi increased after humic acid treatment. The abundance of Firmicutes in bacteria, Basidiomycota, and Mortierellomycota in fungi all increased, which have been reported as being beneficial to plant growth. In contrast, the abundance of Ascomycota in fungi was reduced, and most of the related genera identified are pathogenic to plants. In conclusion, humic acid improved the yield and quality of continuous cropping peanut because of improved physicochemical properties, enzymatic activities, and microbial diversity of soil, which is beneficial for alleviating the obstacles of continuous cropping peanut.

High Capacity of Nutrient Accumulation by Invasive Solidago canadensis in a Coastal Grassland.

BackgroundSolidago canadensis is a notorious invasive species from North America that is spreading across East China. It is invading some coastal grasslands and replacing native grass species. The effects of the S. canadensis invasion on soil nutrient cycling in the grasslands remain unclear. This study examined the effects of the invasion of S. canadensis on macronutrient accumulation in species aboveground part and soil.MethodsAboveground biomass, macronutrient (N, P, and K) pools in biomass, litter mass and decomposition rates, soil macronutrient availability and soil microbial biomass and enzyme activity that were related to nutrient transformation were compared between plots invaded by S. canadensis and uninvaded plots dominated by three different native grass species: Phacelurus latifolius, Phragmites australis, and Imperata cylindrica.ResultsS. canadensis had higher aboveground biomass, higher leaf N, P, and K concentrations, and consequently, a larger macronutrient pool size in the standing biomass. S. canadensis also produced more litter with higher N, P, and K concentrations and faster decomposition rates. The S. canadensis invasion did not change the total N, P, and K concentration in the topsoil (0–10 cm), but the invasion did increase their availability. The S. canadensis invasion did not increase the total soil organic matter (TSOM) content but did increase the soil microbial biomass and the activities of urease, alkaline phosphatase, invertase, amylase, and glucosidase in the topsoil.ConclusionThe invasion of S. canadensis accelerates the macronutrient cycling rate via increases in aboveground productivity and nutrient accumulation in standing biomass, faster nutrient release from litter and higher soil microbial activity. An enhanced nutrient cycling rate may further enhance its invasiveness through a positive feedback on soil processes.

Soil suppressiveness to Pythium ultimum in ten European long-term field experiments and its relation with soil parameters

Soil suppressiveness to pathogens is defined as the capacity of soil to regulate soil-borne pathogens. It can be managed by agricultural practices, but the effects reported so far remain inconsistent. Soil suppressiveness is difficult to predict and for this reason different soil properties have been linked to it with the aim to find informative indicators, but these relationships are not conclusive. The objectives of this study were i) to test if soil suppressiveness is affected by long-term agricultural management such as tillage and organic matter (OM) addition; ii) to understand the direct and indirect relationships between soil suppressiveness and labile organic carbon fractions; and iii) to understand the relationship between soil suppressiveness and other chemical, physical and biological soil quality indicators. We measured soil suppressiveness with a bioassay using Pythium ultimum - Lepidium sativum (cress) as a model system. The bioassay was performed in soils from 10 European long-term field experiments (LTEs) which had as main soil management practices tillage and/or organic matter addition. We found that the site had a stronger influence on soil suppressiveness than agricultural practices. Reduced tillage had a positive effect on the suppressive capacity of the soil across sites using an overall model. Organic farming and mineral fertilization increased soil suppressiveness in some LTEs, but no overall effect of OM was found when aggregating the LTEs. Soil suppressiveness across LTEs was linked mainly to microbial biomass and labile carbon in the soil, but not to total soil organic matter content. From structural equation modelling (SEM) we conclude that labile carbon is important for the maintenance of an abundant and active soil microbial community, which is essential for the expression of soil suppressiveness. However, soil suppressiveness could only partly (25%) be explained by the soil parameters measured, suggesting that other mechanisms contribute to soil suppressiveness such as the presence and the activity of specific bacterial and fungal taxa with high biocontrol activity.

Impact of long-term N fertilisation on CO2 evolution from old and young SOM pools measured during the maize cropping season

The relationship between carbon (C) inputs and nitrogen (N) fertilisation is a key element of soil organic matter (SOM) dynamics, which remains poorly resolved. In temperate climates, it is critical to investigate the interactive effect of C and N inputs on SOM stabilisation under low or high substrate availability. We measured SOM content and in situ soil respiration in a long-term field experiment in Sweden, which started in 1956. In 2000, the previous C3 crops were replaced with C4 maize, making it possible to trace old- (C3-derived) and young-C (C4-derived) sources in CO2 and SOM under bare fallow, maize cropped with or without N-fertilisation (root C-inputs). Soil respiration and its isotopic composition were measured in the field prior to sowing, every second week during crop growth and once after harvest. During 1956–1999, the bare fallow lost 38% of its SOM, following an exponential decay trend. Despite root C inputs, total SOM content under C3 crops declined from 1.5% in 1956 to 1.4% and 1.2% C in fertilised and unfertilised treatments, respectively, in 1999. After the crop change in 2000, estimated C input increased by 5% (under fertilisation), but SOM content continued to decline (as before 2000), to 1.25% (fertilised) and 1.03% (unfertilised) in 2017. Analysis of δ13C revealed that 9 and 11% of young-C was retained in unfertilised and fertilised SOM, respectively. However, up to 70% of soil respiration derived from young-C. Comparing the contributions of old- and young-C to CO2 and SOM showed that, irrespective to the time of measurement, young-C was always more available for microbial decomposition than old-C, particularly under fertilisation. We conclude that the amount of C entering the soil through root inputs was insufficient to counterbalance SOM losses over time. Moreover, soil nutrient status and recent root-C availability appear to be important for CO2 release, and must be considered in further recommendations on maintaining/improving SOM stocks.

Conversion of Tibetan grasslands to croplands decreases accumulation of microbially synthesized compounds in soil

Conversion of grassland to cropland affects microbial transformation of plant derived organic substances and the accumulation of microbially synthesized products in soil. We investigated long-term effects of agricultural use [more than 50 years' rotation with barley (Hordeum vulgare) and rapeseed (Brassica rapa)] after conversion from grasslands (grazed in the winter season) on the contents and composition of non-cellulose sugars and amino sugars in a broad range of soils on the Tibetan Plateau. Soils from two types of croplands (barley or rapeseed) were compared to a reference grassland soil by detailed analysis of plant vs. microbial and bacterial vs. fungal biomarkers. Long-term cultivation decreased total soil organic matter (SOM), light SOM and total non-cellulose sugar contents in the 0–20 cm soil by 27%, 47–72% and 57%, respectively, reflecting decreased root biomass compared to grassland. The ratios of (galactose + mannose)/(arabinose + xylose) and (rhamnose + fucose)/(arabinose + xylose) were both 26% smaller in cropland than in grassland soils, while the ratios of hemicelluloses in shoots or roots were similar between native and cultivated plants. Consequently, net transformation of plant substances to microbially synthesized polysaccharides decreased in cultivated soils. The total amino sugars (muramic acid, glucosamine, mannosamine, galactosamine) in cropland soils decreased by 42% as did their contribution to the SOM pool by 22%, compared to grassland soils, but the ratio of glucosamine/muramic acid in cropland soils doubled when compared to the grassland soil. This shows a strong decrease in microbial residue under cultivation, with the magnitude of the decrease greater in the bacterial than in the fungal components. All the above results from the intensively sampled site were confirmed in seven other sites featuring lower sampling intensity. We concluded that the conversion of grassland to cropland strongly decreases microbial transformation of plant residues and accumulation of the resulting microbial compounds – necromass – in soil (i.e., reduction of microbial input to stable SOM). The conversion also leads to a shift in the composition of microbial compounds towards a decreasing contribution of bacterial compared to fungal necromass.