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Soil microbial diversity is extremely vulnerable to fertilization, which is one of the main anthropogenic activities associated with global changes. Yet we know little about how and why soil microbial diversity responds to fertilization across contrasting local ecological contexts. This knowledge is fundamental for predicting changes in soil microbial diversity in response to ongoing global changes. We analyzed soils from ten 20-year field fertilization (organic and/or inorganic) experiments across China and found that the national-scale responses of soil bacterial diversity to fertilization are dependent on ecological context. In acidic soils from regions with high precipitation and soil fertility, inorganic fertilization can result in further acidification, resulting in negative impacts on soil bacterial diversity. In comparison, organic fertilization causes a smaller disturbance to soil bacterial diversity. Despite the overall role of environmental contexts in driving soil microbial diversity, a small group of bacterial taxa were found to respond to fertilization in a consistent way across contrasting regions throughout China. Taxa such as Nitrosospira and Nitrososphaera, which benefit from nitrogen fertilizer addition, as well as Chitinophagaceae, Bacilli, and phototrophic bacteria, which respond positively to organic fertilization, could be used as bioindicators for soil fertility in response to fertilization at the national scale. Overall, our work provides new insights into the importance of local environmental context in determining the responses of soil microbial diversity to fertilization, and identifies regions with acidic soils wherein soil microbial diversity is more vulnerable to fertilization at the national scale.

Reducing the applications of mineral phosphorus (P) fertilizers and supplementing them by organic fertilizers is becoming a necessary practice in the North China Plain due to overuse of mineral P fertilizers and improper disposal of organic wastes. Knowledge is needed about how the long-term substitution of mineral fertilizers by organic fertilizers affects soil P forms in order to understand soil P transformation and crop P uptake. In this study, we used solution 31P nuclear magnetic resonance (NMR) spectroscopy to characterize P forms in fluvo-aquic soil after 26 years of different fertilization management strategies, organic compost (OM), half compost in combination with half mineral fertilizer NPK (1/2 OM), mineral fertilizer NPK (NPK), mineral fertilizer NK (NK), and an unfertilized control (CK). Results showed that the P extraction efficiency using NaOH-EDTA varied from 13.0 to 27.7% for the soils of the treatments. 31P NMR spectra indicated that the majority of P was in the form of orthophosphate for all the treatments, which constituted 64.3–83.5% of the total extracted P. The application of P fertilizers significantly increased the concentrations of orthophosphate, monoesters and diesters regardless of the P fertilization method, although the proportions of monoesters and diesters were higher in CK. The proportions and concentrations of orthophosphate significantly decreased when all mineral fertilizers were replaced by compost. There was no significant difference in the proportions and concentrations of total organic P, corrected monoesters and diesters in NaOH-EDTA extracts of soils among NPK, 1/2OM and OM treatments. Decreasing mineral P fertilizers and partly replacing them by organic fertilizer in fluvo-aquic soil might increase soil test (Olsen) P and crop P uptake through the degradation of applied organic P forms.

Ammonia oxidizers are key players in the global nitrogen cycle, yet little is known about their ecological performances and adaptation strategies for growth in saline terrestrial ecosystems. This study combined (13)C-DNA stable-isotope probing (SIP) microcosms with amplicon and shotgun sequencing to reveal the composition and genomic adaptations of active ammonia oxidizers in a saline-sodic (solonetz) soil with high salinity and pH (20.9 cmol(c) exchangeable Na(+) kg(−1) soil and pH 9.64). Both ammonia-oxidizing archaea (AOA) and bacteria (AOB) exhibited strong nitrification activities, although AOB performed most of the ammonia oxidation observed in the solonetz soil and in the farmland soil converted from solonetz soil. Members of the Nitrosococcus, which are more often associated with aquatic habitats, were identified as the dominant ammonia oxidizers in the solonetz soil with the first direct labeling evidence, while members of the Nitrosospira were the dominant ammonia oxidizers in the farmland soil, which had much lower salinity and pH. Metagenomic analysis of “Candidatus Nitrosococcus sp. Sol14”, a new species within the Nitrosococcus lineage, revealed multiple genomic adaptations predicted to facilitate osmotic and pH homeostasis in this extreme habitat, including direct Na(+) extrusion/H(+) import and the ability to increase intracellular osmotic pressure by accumulating compatible solutes. Comparative genomic analysis revealed that variation in salt-tolerance mechanisms was the primary driver for the niche differentiation of ammonia oxidizers in saline-sodic soils. These results demonstrate how ammonia oxidizers can adapt to saline-sodic soil with excessive Na(+) content and provide new insights on the nitrogen cycle in extreme terrestrial ecosystems.

To investigate the effects of long-term fertilization on microbial decomposition of residues and priming effect (PE), 13C-labeled maize (Zea mays L.) residues were supplied to arable soils with a 20-year application of compost (COM), mineral NPK fertilizer (NPK), or without any treatments, the no-fertilizer control (NF). The soils that had been stored for 6 years were used in the present incubation experiment. The release of CO2–C and the microbial incorporation of residue-derived C determined by phospholipid fatty acids (PLFAs) analysis were monitored over a 90-day incubation period. Residue additions significantly increased cumulative CO2–C emission and induced positive PE. Cumulative residue-derived CO2-C emission and PE mainly occurred within the first 15 days. The COM soil had significantly higher cumulative residue-derived CO2–C emission but lower PE than the NF and NPK soils. Residue additions significantly increased microbial abundance and changed the composition of main microbial groups. The COM soil showed a significantly lower relative fungal abundance (mol%) but a higher relative actinomycetes abundance than the NF and NPK soils. The incorporation of residue-derived C within fungi was the highest among all the main microbial groups and decreased from 15 to 45 days, while the incorporation of residue-derived C within actinomycetes increased with time in three soils. The incorporation of residue-derived C within fungi was the highest in the COM soil over the course of incubation. The long-term compost input promoted fungal use of residue C and stimulated residue decomposition.

Field decomposition of wheat and maize straws was monitored for 20 months using litterbag method in Calcaric Fluvisol soils with three different textures (sand, sandy loam, and silty clay). Residual straw samples were collected at 0 or after 4, 6, 10, and 20 months of decomposition. The chemical structure of straw was analyzed by solid-state 13C nuclear magnetic resonance (13C-NMR) spectroscopy, and the composition of main microbial groups was evaluated by phospholipid fatty acid analysis. Regardless of the straw type and soil texture, the straw biomass and C loss increased steadily in the first 10 months and then leveled off in the following 10 months; both the chemical structure of straw and the composition of main microbial groups differed during incubation. During the first 4 months of wheat straw degradation, the decrease in di-O-alkyl and O-alkyl C and the increase in alkyl and N-alkyl/methoxyl C contents were related to the enrichment of fungi (18:1ω9c) and Gram-negative bacteria (18:1ω7c and 16:1ω7c), while the degradation of maize straw was associated with the decrease in the fungal (18:1ω9c) abundance and the increase in the abundance of Gram-negative (18:1ω7c and 16:1ω7c) and Gram-positive (a15:0) bacteria. During the 6–10-month period, the decrease in di-O-alkyl and O-alkyl C and the increase in alkyl, aryl, and carboxyl/amide C contents were related to the enrichment of arbuscular mycorrhizal fungi (AMF) (16:1ω5c) and Gram-negative (cy19:0ω8c) bacteria and the decrease in fungal (18:2ω6,9c) abundance, with AMF playing an important role in the degradation of both straw types. The altered chemical structure of wheat and maize straws had opposite links with fungal abundance during the first 4 months, while alterations occurring during the 6–10-month period mainly depended on the variation in the AMF abundance.

Humic substances (HS) are vital to soil fertility and carbon sequestration. Using multiple cross-polarization/magic-angle spinning (multiCP/MAS) NMR combined with dipolar dephasing, we quantitatively characterized humic fractions, i.e., fulvic acid (FA), humic acid (HA), and humin (HM), isolated from two representative soils (upland and paddy soils) in China under six long-term (>20 years) fertilizer treatments. Results indicate that each humic fraction showed chemical distinction between the upland and paddy soils, especially with much greater aromaticity of upland HMs than of paddy HMs. Fertilizer treatment exerted greater influence on chemical natures of upland HS than of paddy HS, although the effect was less than that of soil type. Organic manure application especially decreased the percentages of aromatic C in the upland HAs and HMs compared with the control. We concluded that humic fractions responded in chemical nature to environmental conditions, i.e., soil type/cropping system/soil aeration and fertilizer treatments.

This study aims to investigate the presence of aggregate hierarchy during the decomposition of rice straw in a subtropical soil (Ultisol) with large quantities of Fe and Al oxides and in two temperate soils with a large amount of montmorillonite (Vertisol) or with less organic matter but a large content of CaCO3 (Inceptisol) and to explore the relationship between relative contributions of biotic and abiotic processes in soil aggregation and the presence of aggregate hierarchy. A 120-day incubation experiment with or without 13C-labeled straw was conducted in the above-mentioned three soils previously sieved at

Fertilizer management influences the accumulation of soil organic C and N in agricultural soils. The combined use of chemical and organic fertilizers is considered a good method to enhance soil fertility, but it is still unclear about the effects of substituting manure for fertilizer on aggregation and aggregate associated organic C and N. We selected a 4-yr-old soil fertility experiment to examine the aggregation and aggregate associated organic C and N in response to substituting manure for fertilizer. In comparison to control, no fertilizer added (CK); substituting manure for fertilizer significantly increased organic C and N by 11.61 to 47.97% and 21.05 to 32.63% in 0- to 20-cm bulk soil, respectively. It promoted organic C in the silt + clay fraction, microaggregates, small macroaggregates, and large macroaggregates by 3.17 to 17.15%, 12.94 to 34.09%, 10.99 to 50.20%, and 25.32 to 52.09%, respectively. Aggregate associated N also presented significant increases under substituting manure for fertilizer compared to CK. The mass distribution of small and large macroaggregates were 8.16 to 15.54% and 8.89 to 49.51% higher in the manure-substituted plots than CK, respectively. Organic C and labile C were generally greater in macro- than microaggregates in the manure-substituted plots; redundancy analysis confirmed that macroaggregates had positive effects on organic C and N sequestration in bulk soil and aggregates. Based on these results we suggested that substituting manure increases macroaggregate associated organic C and N, and the formation of small and large macroaggregates is accelerated by organic amendment in the test soil compared to CK.

Dielectric techniques are fundamental methods for measuring soil water content, and they commonly rely on the conventional laboratory calibration of the dielectric permittivity models between a dielectric constant and water content. As a non‐negligible factor, porosity has been constructed differently in some models as a calibration constant, but the systematic porosity variations during the laboratory model calibration and field applications are not yet well addressed. Based on time‐domain reflectometer laboratory calibration experiments, this study investigated this issue using three preestablished dielectric permittivity models: the Purdue calibration equation (American Society for Testing and Materials model [ASTM]), the complex refractive index model (CRIM), and a piecewise CRIM model (CRIMP). Results demonstrate that a generalized porosity constant used in the calibration would bring in additional structural bias compared with the calibration using variable porosities, and its magnitude varies with the model structure. The deviation of the generalized porosity constant can further amplify the structural bias of ASTM and CRIM for soils with low clay content, but it is insensitive for the soils with high clay content due to the overwhelming role of model structure error. Only the model CRIMP with a “perfect” model structure can effectively cope with the systematic porosity variation and keep a stable built‐in capability for estimating calibration constants from readily available soil data. These findings highlight ignoring porosity variation should not be taken for granted for calibrating and applying the preestablished models.

Long-term nitrogen field fertilization often results in significant changes in nitrifying communities that catalyze a key step in the global N cycle. However, whether microcosm studies are able to inform the dynamic changes in communities of ammonia-oxidizing bacteria (AOB) and archaea (AOA) under field conditions remains poorly understood. This study aimed to evaluate the transcriptional activities of nitrifying communities under in situ conditions, and we found that they were largely similar to those of 13C-labeled nitrifying communities in the urea-amended microcosms of soils that had received different N fertilization regimens for 22 years. High-throughput sequencing of 16S rRNA genes and transcripts suggested that Nitrosospira cluster 3-like AOB and Nitrososphaera viennensis-like AOA were significantly stimulated in N-fertilized fresh soils. Real-time quantitative PCR demonstrated that the significant increase of AOA and AOB in fresh soils upon nitrogen fertilization could be preserved in the air-dried soils. DNA-based stable-isotope probing (SIP) further revealed the greatest labeling of Nitrosospira cluster 3-like AOB and Nitrosospira viennensis-like AOA, despite the strong advantage of AOB over AOA in the N-fertilized soils. Nitrobacter-like nitrite-oxidizing bacteria (NOB) played more important roles than Nitrospira-like NOB in urea-amended SIP microcosms, while the situation was the opposite under field conditions. Our results suggest that long-term fertilization selected for physiologically versatile AOB and AOA that could have been adapted to a wide range of substrate ammonium concentrations. It also provides compelling evidence that the dominant communities of transcriptionally active nitrifiers under field conditions were largely similar to those revealed in 13C-labeled microcosms. IMPORTANCE The role of manipulated microcosms in microbial ecology has been much debated, because they cannot entirely represent the in situ situation. We collected soil samples from 20 field plots, including 5 different treatments with and without nitrogen fertilizers for 22 years, in order to assess active nitrifying communities by in situ transcriptomics and microcosm-based stable-isotope probing. The results showed that chronic N enrichment led to competitive advantages of Nitrosospira cluster 3-like AOB over N. viennensis-like AOA in soils under field conditions. Microcosm labeling revealed similar results for active AOA and AOB, although an apparent discrepancy was observed for nitrite-oxidizing bacteria. This study suggests that the soil microbiome represents a relatively stable community resulting from complex evolutionary processes over a large time scale, and microcosms can serve as powerful tools to test the theory of environmental filtering on the key functional microbial guilds.

The effects of land use and fertilization regimes on soil denitrification potential (SDP) are often investigated, but less is known about the responses of SDP to different types of land use with different fertilization regimes in soils derived from the same parent material. We conducted a study using soil samples from 2 long-term (over 30 years) fertilization experiments to determine the difference in SDP between paddy soil and upland soil derived from the same quaternary red clay parent material. The results showed that the SDP in paddy soil was 6.82 times higher than the upland soil, which was due to the higher abundances of narG, nirS, and nirK genes and nirS-denitrifying bacteria (Bradyrhizobium, Cupriavidus, and Herbaspirillum) in paddy soil. Inorganic fertilization regimes did not significantly affect the SDP of upland soil over the control group, whereas SDP in the NPK and 2NPK treatments in paddy soil were reduced relative the control group by 26.48% and 75.65%, respectively. Compared with the control, the NPKOM treatment consistently yielded the highest SDP in both soils, with 2.47 times and 2.86 times higher for paddy soil and upland soil, respectively. The SDP of paddy soil was significantly correlated with narG and nirS genes mainly regulated by amorphous aluminum oxide, whereas the treatment effect on SDP for upland soil largely depended on differences in nirS-denitrifying bacteria at the genus level (Herbaspirillum, Sulfuritalea, and Cupriavidus) and species level, which were mainly controlled by soil pH. Partial least squares path models further demonstrated that the direct effects of functional genes on SDP were greatest for paddy soil, whereas nirS-denitrifying bacterial communities played a larger role in upland soil. The results presented herein represent a key step toward understanding the mechanisms that govern SDP under land use and long-term fertilization.

Soil microorganisms are considered the most effective decomposers of applied crop residues, but it is poorly understood which communities are primarily responsible for decomposition under different conditions. A pot experiment was conducted in a greenhouse to follow the cycling of C and N derived from maize (Zea mays L.) residues labeled with both 13C and 15N to a subsequent winter wheat (Triticum aestivum L.) crop and to soil pools under planting with winter wheat (+P) or an unplanted control (–P), both in soil maintained at field moisture capacities of 40% and 80%. Soil microbes involved in residue decomposition were investigated by 13C phospholipid fatty acid (13C-PLFA) analysis technique. At wheat maturity, a total of 68% of residue N was recovered in the +P treatments, in which 26% was recovered from wheat plants and another 42% from soil total N (TN), independent of the water regimes, while only 50% was recovered from TN in the –P treatments. More residue C was recovered as soil organic carbon in +P than –P treatments (33% vs. 27%), and the trend became more significant with soil moisture. In addition, the +P soil had 35–48% larger microbial biomass carbon (MBC) than the –P soil, and more residue C was recovered as MBC in +P than –P treatments (7% vs. 4%), suggesting the induced microbial utilization of the applied residues. The distribution of the residue-derived PLFA-C showed that only 16:1ω7c and 18:1ω7c had larger relative abundances in the +P than the –P soils, suggesting that they were mainly stimulated by the presence of wheat and that they may be the most important fatty acids to define the different recoveries of residue N and C between the planted and unplanted conditions. Our results demonstrate that the enhanced recovery of residue-C and -N by the presence of wheat plants was mainly from the induced microbial utilization of applied residues by altering the activities of specific microorganisms.

The formation of aggregates plays a key role in shaping soil microenvironment, which in turn influences microbial community structure and organic carbon (C) dynamics in soil. The objective of this study was to identify the linkages between soil macroaggregation and microbial community as well as organic C accumulation using field samples following 9 years of conservation tillage. A wet-sieving method was used to fractionate soil aggregates and soil microbial abundance and community composition were determined using phospholipid fatty acid (PLFA) analysis. Compared to continuous tillage, reduced/no-tillage and straw returning significantly promoted soil macroaggregation and consequently increased organic C, C/N ratio and soil moisture but decreased the porosity and computed effective oxygen diffusion coefficients. The changes in these soil characteristics were closely related to the abundance and composition of microbial community. The abundance of microbial PLFAs in reduced/no-tillage soils was 30.0% higher in Gram-positive bacteria, 11.6% higher in Gram-negative bacteria, 71.7% higher in fungi and 45.4% lower in actinobacteria, whereas straw returning significantly increased all microbial PLFA abundances relative to straw removing. The ratios of bacteria/fungi (B/F) and monounsaturated/branched (M/B) PLFAs were significantly correlated with the volumetric soil water content, porosity, or computed effective oxygen diffusion coefficients. Soil macroaggregation and microbial community composition collectively explained 82.4% variation in organic C accumulation and their interaction made the largest contribution. Overall, soil macroaggregation under conservation tillage might cause a shift in microbial community to more fungi and anaerobes through primarily influencing soil moisture and aeration, which could effectively promote organic C accumulation in soil.

The Calci-Aquic Vertisols comprise one of the largest areas with low crop yields in China normally characterized with black color, high smectite contents but low organic matter contents. This study aims to establish a physical method for extracting black matter from the Calci-Aquic Vertisols, and explore the formation mechanism of black nano-organo-mineral complexes of the soils. The alternation of freezing-thawing and adding dispersant followed by oscillation and multi-level ultrasound(HOMU) method were proposed and tested to disperse soil and extract black matters from the topsoil layers (0−40 cm) and black soil layers (40−70 cm) of three Calci-Aquic Vertisols. Six different components including POM, white, light white, light black, black and nano-black were extracted successively and analyzed for blackness, organic matter content, particle gradation, mineral composition and elemental oxides. The nano-organo-mineral complexes are found be responsible for the Calci-Aquic Vertisols appearing black. Smectite (SM) absorbing organic matters mainly through cation bridges of Fe3+ and Al3+, hydrogen bond, van der Waals force formed the black nano-organo-mineral complexes which consists the main parts of the black matters in the Calci-Aquic Vertisols. Agricultural activities increased the organic matter contents in the topsoil layers, but the difference of formation environment resulted in the poorer crystallinity and higher content smectites and thus higher nano-organo-mineral complex contents and blackness in the black soil layers in the Calci-Aquic Vertisols. The result in this research is of great value for understanding the organic matter turnover and improving the productivity of the Calci-Aquic Vertisols.

This paper was primarily devoted to reveal the stock of soil organic carbon (C) as well as its lability and to compare their differences existing among tillage and residue practices, aiming to identify the effects on the accumulation process of organic C and its association with macroaggregation. Arable soils following 8-year contrasting managements were collected to determine aggregate size distribution, organic C content and its lability. A wet-sieving method was used to fractionate aggregate fractions including > 2000 μm large macroaggregates, 2000–250 μm small macroaggregates, 250–53 μm microaggregates, and

Humic substances are recalcitrant and might act as persistent binding agents to form macroaggregates. The focus of this study is in investigating the contribution of humic carbon (HC) to soil aggregation in response to various tillage and residue managements. Arable soils following 8-year contrasting managements were collected to determine aggregate size distribution and stability and HC fractions including humic acid (HA) and fulvic acid (FA). The contribution of HC to aggregation was divided into three special effects including positive effect (PE), negative effect (NE), and combined effect (CE), and these effects were measured using aggregate fractionation techniques. As well as to promote structural stability, HC bounds predominantly with the silt + clay fraction and secondarily with microaggregates to form larger aggregates. The PE increased with increasing aggregate size, whereas the NE followed the opposite pattern. A positive CE was observed for large and small macroaggregates, whereas the CE for microaggregates and the silt + clay fraction was negative. Compared to continuous tillage, reduced- and no-tillage decreased the PE for large and small macroaggregates by 1.58–30.98% at the 0–20 cm depth, and straw returning also slightly decreased the corresponding PE relative to straw removing. By contrast, a significantly higher NE for small macroaggregates at the 0–10 cm depth while 6.33–81.11% decreases in CE for large and small macroaggregates at the 0–10 cm depth as well as for large macroaggregates at the 10–20 cm depth, were observed under reduced- and no-tillage. The extraction of HC significantly reduced the aggregate stability and reduced- and no-tillage effectively limited its decrease magnitude. Small macroaggregates and microaggregates made larger contributions to soil HC accumulation than did other fractions. An averagely increased contribution from large or small macroaggregates was observed under both reduced-/no-tillage and straw returning at the 0–20 cm depth. A significant and positive relationship was found between the mass proportion of macroaggregates and the HC accumulation in 0–20 cm soil. Large macroaggregates had significantly higher HA/FA ratios than small macroaggregates, and reduced- and no-tillage significantly increased these ratios both in large and in small macroaggregates. The CE for large or small macroaggregates was also significantly negatively correlated with their HA/FA ratios. Overall, the HC accumulation in soil is likely to play a key role in macroaggregation, but conservation tillage might decrease the contribution magnitude of HC to large or small macroaggregation through increasing the corresponding HA/FA ratios.

Despite our current understanding of soil C and N interactions, less is known about the response and feedback of C mineralization to soil P availability driven by microorganisms. To better understand these interactions, soils with long-term P-sufficient NPK fertilization (available P: 13.4 mg kg −1 ) and P-deficient NK fertilization (available P: 0.96 mg kg −1 ) were incubated with and without glucose. CO 2 emissions were monitored to characterize C mineralization during the incubation. The soil bacterial community structure, quantity and metabolic activity were evaluated using high-throughput sequencing, qPCR and microcalorimetric dynamics. Compared with P-sufficient soils, P-deficient soils had significantly lower basal respiration, but a significantly higher net mineralization of added glucose, probably due to higher energy cost of soil microorganisms. Glucose addition promoted microbial biomass and activity, particularly in P-deficient soils, and this improvement was maintained for at least 70 days. Shifts in bacterial community composition were induced by a predominance of several specific taxonomic groups, all of which were capable of solubilizing P in soils. P-deficiency decreases the retention of exogenous labile C into soil. Adding labile C to P-deficient soils may shift P from relatively unavailable soil-bound pools into microbial biomass pools through pool cycling. Our results indicated negative effects of P-deficiency on soil C retention, as well as positive effects of labile C on soil P availability in arable soils.

Investigating the effects of residue chemical composition on soil labile organic carbon (LOC) will improve our understanding of soil carbon sequestration. The effects of maize residue chemical composition and soil water content on soil LOC fractions and microbial properties were investigated in a laboratory incubation experiment. Maize shoot and root residues were incorporated into soil at 40% and 70% field capacity. The soils were incubated at 20 °C for 150 d and destructive sampling was conducted after 15, 75, and 150 d. Respiration, dissolved organic carbon (DOC), hot-water extractable organic carbon (HEOC), and microbial biomass carbon (MBC) were recorded, along with cellulase and β-glucosidase activities and community-level physiological profiles. The results showed that the cumulative respiration was lower in root-amended soils than in shoot-amended soils, indicating that root amendment may be beneficial to C retention in soil. No significant differences in the contents of DOC, HEOC and MBC, enzyme activities, and microbial functional diversity were observed between shoot- and root-amended soils. The high soil water content treatment significantly increased the cumulative respiration, DOC and HEOC contents, and enzyme activities compared to the low soil water content treatment. However, the soil water content treatments had little influence on the MBC content and microbial functional diversity. There were significantly positive correlations between LOC fractions and soil microbial properties. These results indicated that the chemical composition of maize residues had little influence on the DOC, HEOC, and MBC contents, enzyme activities, and microbial functional diversity, while soil water content could significantly influence DOC and HEOC contents and enzyme activities.

Multi-scale soil architectures in shallow subsurface are widespread in natural and anthropogenic depositional environments, and acquisition of the surface stratal structure and hydrological properties are essential in quantifying water cycling. Geophysical methods like ground-penetrating radar (GPR) can provide quantitative information like soil architecture and spatiotemporal soil water content distribution for the shallow layer. Concerning the informative multi-dimensional water flow in the surface layer with an undulating bottom at the plot scale, this study assesses the feasibility of efficiently estimating soil hydraulic properties using a few time-lapse multi-channel GPR observations, namely soil water storage and layer thickness of the surface layer, at reclamation land near an old river channel. We show that effective hydraulic properties of the surface layer can be obtained with a small number of time-lapse GPR measurements during a rainfall event. Additionally, we analyze the effect of some key factors controlling the informative lateral water redistribution on the results of the proposed approach using synthetic simulations.

The importance of soil saprotrophic fungi in agroecosystems is receiving increasing attention. However, the linkages between saprotrophic fungal communities and soil fertility and stoichiometry in agroecosystems are poorly understood. Here, four long-term field fertilization experiments with three treatments (i.e., no fertilizer (control), chemical nitrogen (N), phosphorus (P), and potassium (K) fertilizer (NPK), chemical combined with organic fertilizer (NPKM)) were performed to evaluate the associations of saprotrophic fungal communities with the soil fertility index (SFI) and stoichiometric carbon (C):N:P ratios. Long-term NPKM application increased the SFI by 57.66%–129.50% at the four sites, and the SFI was strongly correlated with the saprotrophic fungal community composition, which indicated that exogenous organic matter improved soil fertility to a large extent and stimulated the growth of saprotrophic fungi in arable soils. The soil C:P and N:P ratios were notably decreased in the fertilization treatment, indicated that P was sufficient. Further, the composition of saprotrophic fungi was significantly related to the C:N and N:P ratios, suggested that N status is an important factor for soil saprotrophic fungal communities. Our study provides empirical evidence that the saprotrophic fungal communities in arable soils are strongly associated with soil fertility and stoichiometry and offers guidance for improving soil fertility by regulating the saprophytic fungal community.

The supposition that cropping system might affect fertilization-induced changes in chemical composition of soil organic carbon (SOC) and microbial community composition has mostly derived from field experiments in different sites, which may have been disturbed by different climate conditions and parent materials. Here, two adjacent long-term field experiments were used, which contained similar four fertilization regimes of NPK, 2NPK, NPKOM, and an unamended control (Control), while contrasting annual cropping system of rice–rice and maize–maize, to investigate how fertilization-induced changes in SOC chemistry differ in paddy soil from upland soil, and whether they correlate with different bacterial taxa. The bacterial community composition of the paddy soil was mainly determined by NO3–-N and total N, and that of the upland was mostly explained by SOC. The NPK, 2NPK, and NPKOM treatments from the paddy soil increased CCH3, decreased OCH, and reduced CH/CH2, while those from the upland soil decreased CH/CH2, increased OCH, and raised CH/CH2, respectively, indicating opposite shifts of OCH and alkyl C (i.e., CCH3 and CH/CH2) in each paired fertilizer treatment of the two soils. In the paddy soil, the CCH3 from the NPK treatment showed no association with any bacterial taxa, the OCH from the 2NPK showed a negative association with NO3–-related species, and the CH/CH2 from the NPKOM showed a negative association with Proteobacteria and Bacteroidetes. In the upland soil, the CH/CH2 from the NPK negatively associated with Thermogemmatisporales and Acidobacteriaceae subgroup 1, the OCH from the 2NPK positively associated with Thermogemmatisporales, and the CH/CH2 from the NPKOM positively associated with copiotrophic bacterial taxa. Our results provided direct evidence that cropping system mediated the shift direction of specific functional group under specific fertilization regimes, and implied that completely opposite shifts of OCH and alkyl C in each paired fertilizer treatment of the paddy soil and upland soil can likely be attributed to different fertilization-induced shifts in the microbial community composition resulting from soil N change in the paddy soil and soil C change in the upland soil.

Soil microbes play a key role in terrestrial ecosystems. In the saline-sodic environment, the microbial communities and their community assembly processes, especially throughout soil profiles, however, are not well understood. Here, we characterized the soil microbial communities across three sodicity/salinity gradients to 1.0 m depth in the western Songnen Plain, Northeast China. Soil sodicity (exchangeable sodium percentage) and salinity (electrical conductivity) exerted a strong influence on bacterial community compositions compared with other soil properties. These filtering effects of sodicity/salinity were strong on the microbial communities in the layer of 0–60 cm across the three sites, with more contributions of deterministic processes over stochastic processes. As sodicity/salinity decreased, some potential biomarkers for a high community tolerance to salt or soda, such as KSA1, Halomonas, Euzebya and Bacillus, were depleted; meanwhile, some taxa associated with decomposition of organic matter and nutrient transformations, such as Arthrobacter, Mesorhizobium, Pseudomonas and Nitrospira, were enriched. Within each soil profile, the turnover in community composition was lower than between sites, and the stochastic processes (homogenizing dispersal) dominated the microbial assembly. Yet, the surface and deepest depth (80–100 cm) of saline-sodic soils still harbored some distinct microbial communities. The near-surface (20–40 cm) may serve as a gating role for limiting surface microbial penetrance into the deeper depth. Taken together, our results show that sodicity/salinity can strongly structure soil microbial communities, and emphasize the importance of soil depth as an environmental gradient on the assembly processes in saline-sodic soils.

The changes of chemical structure and decomposer community during straw degradation have been inconsistent in previous reports, showing either convergence or divergence of chemical structures or microbial communities among different straw types. Hence the strength of their relationship remains unclear. Here, we directly measured the chemical structures and bacterial and fungal communities of wheat (Triticum aestivum L.) and maize (Zea mays L.) residues after four-year decomposition in three Calcaric Fluvisols that varied in soil textures. Separation of the chemical structures among the six soil-straw combinations (two straw types × three soil textures) were generally coupled with trends in the communities of bacterial and fungal decomposers, but the nature of their coupling differed. In the sand soil, soil texture was the central feature that shaped bacterial and fungal communities for both straw types, driving in turn the convergence of chemical structures of both decomposing straws. In contrast, in the sandy loam and silty clay soils, straw type was the primary regulator of the colonizing bacterial and fungal communities, which resulted in the divergence of chemical structures according to straw type.

Determining soil pore size distribution is difficult and time-consuming using traditional methods. Additionally, for swelling and shrinking soil, the specific volume of soil changes with soil moisture conditions. Nuclear magnetic resonance (NMR) relaxometry allows observation of pore size distribution changes during the process of dehydration and soil shrinkage. Naturally structured soil cores of a Vertisol with characteristic swelling and shrinking behavior were collected from the Huang-Huai-Hai Plain of China. The samples were saturated with water and dehydrated gradually at room temperature and relaxometry tests conducted at different moisture contents as they air-dried. Then, the soil cores were oven-dried at 105 °C and saturated with n-octane, which prevented clay swelling. The pore size distribution of soil cores at the end of the shrinking process was determined through the description of n-octane-filled pores. The shrinkage characteristic curves were determined as naturally structured soil cores were air-dried from moisture content at full expansion to constant volume, indicating the relationship between the volume of bulk soil and moisture content. The transverse relaxation time (T 2) distribution deduced from NMR relaxometry gives a good description of the size distribution of pores filled with protons (contained in water or n-octane). The T 2 distribution curves of soil cores at saturation were trimodal, due to the presence of interlayer, interparticular, and interaggregate pores. Combined with the shrinkage characteristic curves, it was deduced that the structural pores were evacuated during the structural shrinkage period. The normal and residual shrinkage was accompanied by the narrowing and closure of the interlayer spaces. During the residual shrinkage period, the frame structure of the soil particles prevented further shrinkage of the bulk soil. The shrinkage process was accompanied by the closure of interlayer spaces and the formation of large cracks between aggregates. 1H NMR relaxometry was especially suited to studying the changing pore size distribution of swelling and shrinking soils. When the soil cores began shrinking, almost all remaining water was retained in interlayer spaces. The volume change of the interlayer space was the main cause of swelling and shrinking. The swelling limit could be estimated from the T 2 distributions of soils at full expansion.

Low-cost, simple, rapid and accurate approaches for measuring soil hydraulic properties are of great importance in the application of hydrologic models of the vadose zone. A recently proposed method of constant-saturation absorption (MCSA) for determining Brooks-Corey (BC) model parameters is promising. However, this method has not been tested using experimental data. In this paper, measured soil-water diffusivity or soil-water retention curves (SWRCs) of 20 soil samples were used to test the reliability of MCSA. The results indicate that MCSA consistently estimated soil-water diffusivities by the Bruce and Klute method, but it substantially overestimated air-entry suctions (h(d)) and thereby soil suction. A new method (MCPA) is proposed herein to overcome the problems associated with MCSA. Using the improved method, estimated SWRCs are consistent with observation. With the estimated soil hydraulic parameters, HYDRUS-1D generated very accurate simulations of the cumulative absorption curves. Compared with direct or other indirect methods, MCPA is a more accurate, low-cost, and simple method, and it is especially useful for rapid determination of SWRC for soil-water movement simulations. However, if unsaturated soil-water diffusivity is of greater concern, our results also indicate that MCSA more accurately predicts this quantity and thus should be preferred. (C) 2015 Elsevier B.V. All rights reserved

Solonetz soils are typically characterized with excessive saline-alkali content, and physiochemical methods are often employed for solonetz reclamation. However, increasing lines of evidences have suggested that microbiomes play a pivotal role in sustaining soil fertility than previously appreciated, and it remains poorly understood about the taxonomic identity of microbiomes in solonetz soils and the underlying mechanisms that govern the community assembly of solonetz restoration. In this study, we show that significant increase of rare phylotypes occurred after solonetz restoration by natural grassland and reclaimed farmland, and prokaryotic community assembly was dominated by deterministic rather than stochastic processes. The exchangeable sodium decreased significantly from 27.04% in solonetz, to 4.35% in grassland and 0.52% in farmland. Soil organic carbon (SOC) increased significantly with 4.17- and 2.18- fold in grassland and farmland soils relative to solonetz, respectively. Real-time quantitative PCR indicated drastic increases in the abundance of functional genes responsible for nitrogen fixation and nitrification. High-throughput sequencing of 16S rRNA genes further revealed that 180 and 215 phylotypes could be considered as key taxa showing significant increase after solonetz restoration by grassland and farmland. Intriguingly, more than 96% of these key phylotypes were originally rare phylotypes with

Agricultural fertilization plays a crucial role in crop production, and the fungal communities catalyze transformation of soil nutrients in support of crop production. However, it remains controversial about the optimal strategy for fertilizer inputs and the adaptive mechanisms of fungal communities across China. By using seven long-term field fertilization experiments in China, we analyzed crop yields, soil properties and fungal communities in soils that were treated for > 25 years with no fertilizer (control), inorganic fertilizers (NPK) and organic-inorganic fertilizers (NPKM). Long-term NPK resulted in significant acidification up to a decline by 1.20 pH units, while NPKM prevented acidification and increased pH up to 6.39 in three acidic soils with pH 8.11, both NPK and NPKM led to only marginal decline of soil pH, and NPK and NPKM showed comparable crop yields. Some members of Ascomycota in alkaline soils were both enriched by NPKM and NPK. Soil available P and C:N ratio, rather than pH, directly or indirectly affect crop yield in alkaline soils. High crop yield can be achieved by the sole use of inorganic fertilizers in alkaline soils, but acidic soil productivity should be maintained by organic amendment to counteract acidification by inorganic fertilization. Our study advances a mechanistic understanding for optimizing fertilization strategies towards sustainable agriculture under increasingly intensified fertilizer inputs.

Soil bacteria and fungi are essential constituents of arable ecosystems. Long-term phosphorus (P) deficiency has been proven to significantly affect microbial community diversity and composition. However, how P deficiency impacts the interplay between bacteria and fungi remains unclear. The objective was to explore the bacterial-fungal co-occurrence patterns in soil as influenced by long-term P deficiency, using network analysis. Bacterial and fungal community datasets were obtained from soils which have consistently and continuously been fertilized with P (+P) or without P (−P) for >20 years in three long-term field experiments in China. Our results showed that the −P network was less complex and less efficient as indicated by fewer edges, lower average degree and higher average path length. Chronic P deficiency simultaneously decreased positive and negative bacterial-fungal edges, implying that both synergistic and antagonistic interactions were enhanced by P application. The keystone species differed between +P and −P network, and those from the +P network showed significant correlations with less soil properties, indicating the −P network was more sensitive to environmental disturbances. Four major modules were observed in both +P and −P networks, and they related with different soil properties, suggesting distinct habitat heterogeneity or niche partitioning. All the modules had similar ecological roles, while the module composition was altered by long-term P deficiency, indicating a possible ecological redundancy. Most of the fungi from the −P treatment were distributed in a special module negatively correlated with available P, indicating that fungi may prefer niches with low P concentration. Taken together, our results illustrated the crucial roles of P availability in microbial community assembly and investigated the responses of co-occurrence patterns to long-term P deficiency by regulating the topological properties, keystone species and module composition of the bacterial-fungal network.

Soil organic matter (SOM) and interparticle forces, including the electrostatic repulsive force (ERF), surface-hydration repulsive force (SHRF) and van der Waals attractive force (vDWAF), play crucial roles in aggregate stability. However, few studies investigated their coupled effects on aggregate stability in variably charged soils, in which soil particle interactions are more complex than in permanently charged soils due to the coexistence of positive and negative charges and the variable surface charge characteristics. Therefore, this study aims to: 1) investigate the combined effects of soil solution pH and the overlapping of electric double layers (EDLs) between positively charged Fe/Al (hydro)oxides and negatively charged SOM on the ERF and aggregate stability before (control) and after SOM removal and after straw incubation for 240 days; and 2) evaluate the importance of soil interparticle forces and SOM for aggregate stability in a permanently charged temperate soil (Vertisol) and a variably charged subtropical soil (Ultisol). Soil aggregate stability was determined by measuring the release of small particles (w(

The North China Plain (NCP) is a strategic grain production base in China with a wild distribution of fertile soils. During the past 20 years, high-input intensive agriculture with excess chemical fertilizer application has sustained high grain yields, but may have resulted in contamination of some elements in farmland. In this study, the accumulation and transfer of arsenic (As) and mercury (Hg) in typical Calcaric Fluvisols with long-term different fertilization practices were investigated. Field experiments with seven treatments were launched in 1989, and soil and plant samples were collected and analyzed periodically. The treatments include OM (organic manure), OM + NPK (50 % organic manure + 50 % chemical fertilizer), NPK, NP, PK, NK, and CK (the control experiment with no fertilizer). With over 20 years (1989–2009) of cultivation, various extents of As and Hg accumulations were really observed in the soil. The higher As and Hg contents were found in P fertilizers than those in N, K, or OM fertilizers. As a result, the long-term P fertilization slightly promoted Hg accumulations with decreased soil Hg concentrations in the order of NPK ≈ NP ≈ PK ≈ OM + NPK > OM > NK ≈ CK, which was similar to the order of crop yields. At the tillage layer (0–20 cm), Hg accumulation in the soil was enhanced by crop production, due to the highly accumulated Hg in plant roots finally remained in the soil. However, no significant differences of soil As concentrations can be found between treatments with and without P fertilizers probably due to water leaching and plant uptake. Soil As and Hg were mainly contributed by fertilizers, irrigation, and atmospheric deposition in recent years, but they did not exhibit in significant accumulations in the soil. The contents of As and Hg were not above the critical safe levels of soils for crop production (As

Sustainable agricultural production is of vital importance to food supply security. This study aimed to investigate crop yield response to spatial variability of soil quality at a county scale in the North China Plain (NCP) and subsequently derive key soil quality indicators. Soil samples were geo-referenced and taken in 2008 from both surface (0–20 cm) and subsurface (20–40 cm) layers in 132 fields throughout the Fengqiu County, located in the centre of the NCP, for subsequent soil properties' analyses. Annual crop yields were obtained from the same fields where soil samples were collected. Soil quality was evaluated based on a fuzzy set with 13 soil properties, and its spatial distributions were investigated by integrating geostatistical analysis and geographic information system (GIS) techniques. Soil quality indices were classified into five grades, and their spatial distributions were mapped within the county. The surface soil qualities were about one to two grades higher than the subsurface soil. The quality indices for surface and subsurface soils were positively associated with the annual crop yields, suggesting the importance of both. Soil organic matter, total nitrogen, available P, and available K contributed 50% of the combined weight to the soil quality index and were identified as key indicators of soil quality status in the area in terms of sustainability.

The Green–Ampt model (GAM) is a widely used water infiltration model. However, it lacks a reliable theoretical basis to independently and simultaneously determine the GAM parameters of effective hydraulic conductivity (Kₑ) and the average matric pressure head at the wetting front (Hf) from those commonly measured soil hydraulic properties. In this paper, we derived an approximate analytical solution similar to GAM for one-dimensional vertical infiltration into soils with initially uniform soil moisture distribution, with Kₑ and Hf being simultaneously related to the Brooks–Corey (BC) model parameters. The new relationships are not restricted to the piston-type moisture profile or delta-type water diffusivity like in the traditional GAM (TGAM). Infiltration experiments on three soils of various textures were conducted to validate the new approximate analytical solution and the corresponding GAM parameters. It was demonstrated that the predicted cumulative infiltrations and soil moisture profiles by the GAM with Kₑ and Hf calculated by the new expressions (new GAM, NGAM) generally agreed well with the numerical solutions, especially for initially dry soils. In contrast, cumulative infiltrations were considerably over-predicted by the TGAM using Kₑ = Kₛ and Hf calculated by Neuman’s equation. Our theoretical analysis further indicated that even without air entrapped during infiltration, the effective hydraulic conductivity can still be less than the saturated hydraulic conductivity Kₛ. The relative effective hydraulic conductivity ζ (Kₑ/Kₛ) and the relative average matric pressure head at the wetting front η (-Hf/hd where hd is absolute value of the air-entry suction) show a dependency only on soil pore structure index (n) and effective initial soil-water saturation (Sᵢ). The values of ζ increased but η decreased with Sᵢ and n. The theoretical ranges of ζ and η were from 0.5 and 2 for dry heavy-texture soils to 1 and 0 for saturated soils, respectively, which are consistent with the experimental values reported in literatures.

It is of ecological significance to investigate microbial communities in response to straw amendment and moisture in arable soils. However, in Chinese fluvo-aquic soils, these responses are still poorly understood. We designed an incubation experiment involving two soils with and without the addition of maize stubble at two moisture levels, and bacterial community structure at days 20, 80, and 200 after the start of incubation was assessed via bar-coded pyrosequencing of the 16S rDNA amplicons. In the presence of stubble with identical moisture level, we observed higher bacterial diversity and richness in long-term organic manure-fertilized soil compared with the unfertilized soil at days 20 and 80, which we attributed to the different quality and quantity of organic matter between the two soils. However, there was no significant difference in bacterial diversity and richness between the two soils at day 200, indicating that long-term straw amendment probably lessens the difference in bacterial community structure between the two soils. In the amended soils bacterial diversity, richness, and community composition at 25% of the water-holding capacity distinctly differed from those at 55% of the water-holding capacity, indicating that moisture strongly affects bacterial distribution in the amended soils. As stubble-C availability declined over time, the dominance of copiotrophic population weakened, and oligotrophic population was moderately abundant. Finally, our study suggests that dissolved organic carbon, which drives redistribution in copiotrophic and oligotrophic categories in response to the varying water and stubble-C availability, is a determinant of bacterial community composition in the amended soils. (C) 2014 Elsevier B.V. All rights reserved.

Gaseous nitrogen (N) losses are mainly responsible for low N use efficiency of applied fertilizers and their derivative environmental hazards, while N application rates and soil water regime are important factors regulating these N processes. In this study, gaseous N losses via ammonia volatilization (AV) and denitrification, determined by sponge trapping-KCl extraction and acetylene inhibition-soil core incubation, respectively, were investigated under different coupled water-N management practices with a winter wheat (Triticum aestivum L.) and summer maize (Zea mays L.) rotation system in the Huang-Huai-Hai Plain. Ammonia volatilization after urea application, particularly after topdressing during maize growing seasons, was the dominant gaseous N loss, amounting up to 53.2 kg N ha−1 (38.6 % of the applied N). Ammonia volatilization generally decreased by increasing irrigation rate applied immediately after fertilization. Denitrification N losses were lower than ammonia-N losses, accounting for 1.3 to 18.7 kg N ha−1 year−1, with more than 90 % occurring during the maize growing seasons. Total gaseous N losses showed a significant (P

A field experiment was conducted to study the effect of biochar amendment (0, 2.5, 5, 10, 20, 30 and 40 t·ha− 1) on rapeseed and sweet potato yields, aggregate particle size distribution, aggregate stability, soil organic carbon, total N content and C:N ratio in soil water stable aggregate under a rapeseed–sweet potato rotation system in upland red soil in Jiangxi Province of southern, China. The results were as follows: compared with control treatment (CK), when the amount of biochar was 40 t·ha− 1, rapeseed and sweet potato yields were increased by 36.02% and 53.77% respectively; the soil water stable aggregate (> 0.25 mm) in the 0–15 cm soil layer had a remarkable increase than other treatments, especially the macroaggregate with particle size larger than > 2 mm. In the rapeseed harvest season, the mean weight diameter of soil water stable aggregate was enhanced by 28.02% compared to CK with the application rate of biochar being 40 t·ha− 1. A significant increment in soil organic carbon, total N and C:N ratio was observed in the > 2 mm, 2–0.5 mm, 0.5–0.25 mm and

Although increasing evidences have emerged for responses of soil microorganisms to fertilizations, the knowledge regarding community assemblages that cause variations in composition is still lacking, as well as the possible feedback to soil fertility. Phylogenetic conservatism of species indicates their similar environmental preferences and/or function traits and phylogenetic signals further can infer community assemblages and influenced ecological processes. Here, we calculated the mean pairwise phylogenetic distance (MPD) and nearest relative index (NRI), characterizing phylogenetic signal and the undergone ecological process to evaluate the community assembly of soil bacterial phylotypes in 20-year fertilized soils. The bacterial community assembly is structured by environmental filtering, regardless of fertilization regime. Soil phosphorous (P) availability imposes selection on community assemblage and influences their community turnover among fertilizations. When P nutrient lacks, the effect of environmental filtering becomes stronger, hence bacterial functional traits become more coherent; this process results into increased intraspecific interactions characterized by co-occurrence network analysis. In contrast, when P nutrient becomes abundant, the environmental selection is mitigated; function traits are evened. This process reduces intraspecific interactions and increases carbon sequestration efficiency, which is finally of great favor to the increases in soil fertility. This study has made the first attempt, at the bacterial level, to understand how fertilization affects agroecosystems. When more phylogenetic information on how nutrient cycling-related microbes respond to fertilization becomes available, the systematic knowledge will eventually provide guidance to optimal fertilization strategies.

Estimation of hydraulic properties of active layers is challenging due to the freeze-thaw effect in space and time. Provided an active layer with an undulating thawing table, monitoring of spatial soil water dynamics could provide significant information for commonly-used inverse estimation of soil hydraulic properties. In this study, we assess the feasibility of efficiently estimating soil hydraulic properties using Ground-Penetrating Radar (GPR) observations – soil water storage and layer thickness in conjunction with a two-dimensional hydrological model. The results of this study conceptually demonstrate that spatial and temporal observations of soil moisture in the active layer during a rain event are sufficient for inverse estimation of soil hydraulic parameters. The proposed method depends on the lateral water redistribution controlled by the undulating frost table and/or the intensity and duration of the precipitation. We suggest that this method could be used for seasonal-scale estimation of soil hydraulic properties.

Soil aggregation is one of the key properties affecting the productivity of soils and the environmental side effects of agricultural soils. In this study, we aimed to identify whether biochar could be used to improve aggregate stability. A 2-year field experiment was conducted to investigate the effect of biochar application (0, 2.5, 5, 10, 20, 30 and 40 t ha−1) on aggregate characteristics of upland red soil under a rapeseed–sweet potato rotation in subtropical China. Percentage of aggregate destruction (PAD0.25), mean weight diameter (MWD), geometric mean diameter (GMD) and fractal characteristics of soil aggregates were measured using both wet and dry sieving methods. Results showed that applying biochar significantly decreased the percentage of aggregate destruction and soil fractal dimension and increased the MWD and GMD. The optimal amelioration was observed when biochar was applied at a rate of 40 t ha−1. The decline of the fractal dimension of dry aggregates was 2–9 times as much as that of water-stable aggregates in the 0–15 soil layer and 1–4 times in the 15–30 cm soil layer. These results suggested that biochar could improve the resistance of aggregates to stresses and provide scientific strategies for the agricultural production.

Returning crop straw into fields is a typical agricultural practice to resolve an oversupply of straw and improve soil fertility. Soil microorganisms, especially eukaryotic microorganisms, play a critical role in straw decomposition. To date, microbial communities in response to straw amendment at different moisture levels in Chinese fluvo-aquic soil are poorly understood. The aim of this study was to explore the effects of straw amendment and moisture on microbial communities in Chinese fluvo-aquic soil. Two soils (one was applied with organic manure, and the other was not applied with any fertilizer) from a long-term field experiment in the North China Plain were collected. Soils with and without straw amendment at 25 and 55 % of the average water-holding capacities of the two soils were incubated at 25 °C for 80 days. All treatments were sampled 20 and 80 days after the start of incubation. Microbial biomass and community structure were analyzed by phospholipid fatty acids (PLFA) assay, and the eukaryotic diversity and community composition were assessed via barcoded pyrosequencing of the 18S ribosomal RNA (rRNA) gene amplicons. PLFA analysis showed that straw amendment increased the biomass of Gram-positive bacteria, Gram-negative bacteria, actinobacteria, and fungi and shifted microbial community structure. The varied straw availability resulted in a large variation in microbial community structure. In the presence of straw, actinobacterial and fungal biomass both decreased under high moisture content. 18S rRNA gene pyrosequencing indicated that straw amendment decreased eukaryotic diversity and richness and probably restructured the eukaryotic community. Under identical moisture content, long-term organic manure-fertilized soil had higher eukaryotic diversity and richness than the unfertilized soil. In the amended soils under high moisture content, the relative abundance of dominant fungal taxa (Dikarya subkingdom, Ascomycota phylum, and Pezizomycotina subphylum) decreased. Straw amendment increases microbial biomass, shifts microbial community structure, and decreases eukaryotic diversity and richness. High moisture content probably has a negative effect on fungal growth in the amended soils. In conclusion, microbial communities in Chinese fluvo-aquic soil are significantly affected by straw amendment at different moisture levels.

An 80-d incubation experiment was conducted to investigate straw decomposition, the priming effect and microbial characteristics in a non-fertilized soil (soil 1) and a long-term organic manure-fertilized soil (soil 2) with and without 13C-labeled maize straw amendment under different moisture levels. The soil 2 showed a markedly higher priming effect, microbial biomass C (Cmic), and β-glucosidase activity, and more abundant populations of bacteria and fungi than the soil 1. Also, soil CO2 emission, Cmic, β-glucosidase activity, and bacterial and fungal population sizes were substantially enhanced by straw amendment. In the presence of straw, the amount of straw mineralization and assimilation by microbes in the soil at 55% of water holding capacity (WHC) were significantly higher by 31% and 17%, respectively, compared to those at 25% of WHC. In contrast, β-glucosidase activity and fungal population size were both enhanced as the moisture content decreased. Cmic decreased as straw availability decreased, which was mainly attributed to the reduction of straw-derived Cmic. Amended soils, except the amended soil 2 at 25% of WHC, had a more abundant fungal population as straw availability decreased, indicating that fungal decomposability of added straw was independent of straw availability. Non-metric multidimensional scaling analysis based on fungal denatured gradient gel electrophoresis band patterns showed that shifts in the fungal community structure occurred as water and straw availability varied. The results indirectly suggest that soil fungi are able to adjust their degradation activity to water and straw availability by regulating their community structure.

Different irrigation and fertilization managements can greatly affect plant potassium (K) uptake in soils. A continuous extraction experiment was used to analyze the influence of poor-quality irrigation water on K release from soils under long-term copping with different fertilizations. The results demonstrated that K release kinetics included a rapid stage and a subsequent slow one. K leached in the solution with Ca:Mg ratio 75:25 and sodium adsorption ratio (SAR) 45 was minimum (146 mg kg−1), while K leached in the solution with Ca:Mg ratio 25:75 and SAR 5 was maximum (212 mg kg−1). Due to the specific ion effect, Mg2+ played a more important role in the release of K than Ca2+. The K release rates and quantities of released K in the soils without K fertilizers were lower than others. Long-term planting without K fertilizers results in the decrease of K fertility. Power equation and parabolic equation fully simulated the process of K release, suggesting a diffusion-controlled process. The K extracted by ...

This study investigates the effects of residue incorporation coupled with plant growth and soil moisture level on wheat biomasses, soil nutrients, labile organic carbon (LOC), microbial metabolic profiles, and community composition. Four management practices were used in a 180-day pot experiment: (1) control (CON), (2) maize (Zea mays L.) residue incorporation without plants (MR), (3) wheat (Triticum aestivum L.) plants without maize residue (WP), and (4) maize residue incorporation with wheat plants (MRWPs). Each management practice included soil moisture at both 40 and 80% of field capacity. At wheat harvest, soil nutrient contents in the WP and MRWP treatments were significantly lower than in the CON and MR treatments. In comparison with the CON treatment, MR, WP, and MRWP treatments resulted in 35, 23, and 67% increases in dissolved organic carbon content; 17, 12, and 34% increases in hot-water extractable organic carbon content; and 78, 50, and 150% increases in microbial biomass carbon content. Furthermore, microbial utilizations of carboxylic acids and polymer carbon sources in the MR, WP, and MRWP treatments were 261 and 88%, 239 and 105%, and 300 and 126% higher than in the CON treatment. The MR and CON treatments had similar phospholipid fatty acid (PLFA) content but the WP and MRWP treatments had significantly increased gram-negative content and changes to community composition compared with the CON and MR treatments. The wheat biomass, LOC, and PLFA contents significantly increased with greater soil moisture. Overall, these results suggest an additive effect of residue incorporation and plant growth on LOC contents, primarily due to the changes in microbial utilization of carbon sources and community composition.

Initial soil pH has been reported to have a great impact on the decomposition of added organic materials and hence to determine the direction and magnitude of subsequent soil pH changes. However, most previous investigations have been conducted on different soil types differing in initial pH and other soil properties. Here, we investigated the effects of initial soil pH on the subsequent soil pH and N mineralization changes caused by addition of crop residues to two soils (a Paleudalf and a Plinthudult) with different pHs induced by treatment with direct electric current. This produced pH gradients of 6.50–3.20 and 6.74–3.81, respectively. Three typical field crop residues differing in C/N ratio, i.e. rice straw C/N = 42, canola residue C/N = 36 and Chinese milk vetch C/N = 14 (vetch), were incubated with the soils for 102 days. With both soils, total CO2 fluxes differed between the type of added crop residues, with vetch > canola residue > rice straw, and decreased with decreasing initial soil pH. The incorporation of crop residues into the two soils at all pHs increased soil pH except for the Paleudalf at pH 3.81 and 4.25 amended with rice straw and canola residue, where pH slightly decreased. As expected, vetch incorporation caused a greater pH increase than the two non-legume residues (rice straw and canola residue), but this effect was transient in soils of higher initial pH. The pH declined rapidly with time in soils of initial pH ≥ 4.40 treated with vetch, while addition of rice straw and canola residue maintained stable pHs. Irrespective of crop residue addition, soil pH continuously increased or remained steady over the 102-day incubation period in the highly acidic soils (pH ≤ 4.25). While NH 4 + accumulated in highly acidic soils, NO 3 − accumulated in higher pH soils. Contrasting effects on nitrification and subsequent pH changes occurred between soil pH ≤ 4.25 and pH ≥ 4.40. However, no clear relationship between net N mineralization and soil pH was found. This study demonstrated that low soil pH greatly inhibited nitrification, while net N mineralization was generally less affected. This, in turn, affected the direction and extent of soil pH changes after addition of crop residues.

Soil samples collected from a long-term (19-year) experimental field with seven treatments were analyzed for fatty acids methyl esters (FAMEs) to determine fertilization regime effects on microbial community structure in sandy loam soils. The amounts of FAMEs in bacteria, actinomycetes, and fungi were highest with the two organic manure (OM)-fertilized treatments (OM and 1/2 OMN – half OM plus half mineral fertilizer), lowest with the NK treatment, and fell in the middle levels with three mineral P-fertilized treatments (NPK, NP and PK) and the control with no fertilizer (CK), with the exception of fungi which showed no significant difference among the five treatments without OM fertilization. Principal component analysis of FAME patterns indicated that NPK was not significantly different from CK, but the two manure-containing treatments and the P-deficiency treatment (NK) were significantly different from CK and NPK. Redundancy analysis plot showed that FAME amounts significantly correlated to soil organic C and total N contents, while soil available P and total P contents, which were greatly decreased by the NK treatment, also had positive and substantial effects on soil microbial FAMEs. The results demonstrated the importance of P fertilization as well as organic manure in maintaining soil microbial biomass and impacting community structure.

Soil water retention curves (SWRCs) can be predicted by fractal models with soil pore fractal dimensions, which can be derived from either particle size distribution (PSD) or image analysis. The PSD curve is first converted to a pore size distribution, and fractal dimensions are then calculated to estimate the water retention function. The current study compares SWRC predicted by conventional PSD to the results of image analysis in three different textural fluvo-aquic soil samples. Images of soil pores were obtained using nondestructive computerized tomography (CT). The Brooks–Corey, Rieu–Sposito, Tyler–Wheatcraft and van Genuchten models were applied to estimate SWRC. The results indicated that image analysis improved the estimated results of the fractal models except for the Rieu–Sposito model, indicating significantly greater quantitative information provided by CT technique compared to the PSD approach. The Brooks–Corey model produced more acceptable results for all three soil samples than th...

Laser diffraction method (LDM) provides a rapid solution for obtaining soil particle size distribution (PSD) especially in the anal- ysis of a larger number of soil samples. However, there are disadvan- tages associated with the use of LDM for PSD measurements over the traditional sieve-pipette method (SPM). The present study is aimed to assess the suitability of LDM as a routine method for determining soil PSD, evaluate the precision or its repeatability of PSD parameters, and establish a simplified protocol for transforming the LDM results into SPM ones. The soil samples (n = 43) from 13 Chinese provinces were analyzed, and the results indicated that the relative errors for clay, silt, and sand fractions by the LDM were 35.34, 27.38, and 19.41%, respec- tively, compared with those by the SPM in the condition of limited sam- ple numbers and large between-sample variation; samples could best be run at least twice during the LDM analysis to reduce the error caused by a limited sample volume; and a soil particle refractive index of 1.50 and a soil particle absorption index of 0.01 were found to be optimal for the Mie theory model. With relatively limited sample numbers and apparent textural difference between the samples, the distinct incompatibilities were observed in the present work between the PSD obtained by the LDM and SPM. However, depending on the specific research purpose, the deviations between the LDM and SPM may be considerably reduced with an increase in the sample capacity or a decrease in the spatial scale.

We explore the feasibility to quantify the field-scale soil water dynamics through time series of GPR (ground-penetrating radar) measurements. They bridge the gap between accurate and well-established point measurements and the field-scale where soil hydrology issues are addressed. Working on a 40 m × 50 m area in a heterogeneous agricultural field, we obtain a time series of radargrams after a heavy rainfall event. On the one hand, these yield a three-dimensional representation of the subsurface architecture, in particular of the layer boundary that originates from paleo-sand dunes and of a number of clay inclusions in an otherwise rather uniform sand. On the other hand, the total soil water volume between the surface and the layer boundary is obtained. We assess the precision and the accuracy of these quantities and conclude that the method is sensitive enough to capture the spatial structure of the changing soil water content. While the sensitivity of the method still needs to be improved, it already produced useful information to understand the observed patterns in crop height and it yielded insight into the dynamics of soil water content at this site including the effect of evaporation.

Knowledge of the structural features of humic substances (HSs) is essential for elucidating the mechanisms of humification in different soil environments and realizing their profound roles in environmental issues. The aim of this work was to investigate the chemical structures of fulvic acid (FA), humic acid (HA) and humin (HM) fractions isolated from an upland soil (Fluvisol) and a paddy soil (Anthrosol) typical in China using advanced solid-state 13C nuclear magnetic resonance (NMR) techniques. The results revealed that there were great structural differences of HSs between the two soils. The two FAs showed distinct quantitative differences in aliphatics with more polysaccharides in the FA from the upland soil than from the paddy soil. The HM from the upland soil differed from the paddy soil HM in having more proteins/peptides (23% vs 20%), total aromatics (21% vs 12%) as well as fewer lipids (24% vs 35%) and polysaccharides (27% vs 31%). The HM fractions represented the most different components of organic matter between the two soils. The degree of difference between the two HAs fell in between that of FAs and HM fractions. In particular, the HA from the upland soil had relatively greater degree of aromaticity. Our study indicated that the upland soil exhibited a higher degree of humification compared with the paddy soil. Among the three humic fractions, the FAs featured COO/N-CO groups, and the HAs were more enriched in protonated aromatic C for both soils. In contrast, the two HM fractions contained more O-alkyl C and fewer aromatics than did the other humic fractions, being closer to the original organic materials in soils. We speculate that the evolutionary route of HSs is likely to be the transformation of original organic materials into HM, followed by increased degradation, further oxidization and conversion into HA, and then into FA.

The Fengqiu long-term field experiment was established to examine effects of organic manure and mineral fertilizers on soil total nitrogen (N) and natural 15N abundance. Fertilizer regimes include organic manure (OM), one-half N from organic manure plus one-half N from mineral N fertilizer (1/2OMN), mineral fertilizers [N–phosphorus (P)–potassium (K), NP, NK, PK], and a control. Organic manure (OM and 1/2OMN) significantly increased soil total N and δ15N, which was expected as a great amount of the N applied remained in soils. Mineral NPK fertilizer and mineral NP fertilizer significantly increased total N and slightly increaed δ15N. Phosphorus-deficient fertilization (NK) and N-deficient fertilization (PK) had no effect on soil total N. Significantly greater δ15N was observed in the NK treatment as compared to the control, suggesting that considerable N was lost by ammonia (NH3) voltalization and denitrification in this P-deficiency fertilization regime.

Soil unsaturated hydraulic properties are an important input for simulations of water and solute movement in the vadose zone. However, in the case of soils containing rock fragments, direct measurement of unsaturated hydraulic properties remains difficult. Recently, an analytical method was proposed (Ma et al., 2009) for determining soil hydraulic properties from horizontal water absorption experiments. Here, we test if this method could be used in the determination of the hydraulic properties of stony soils with a set of experiments. The results show that the method can predict accurately and quickly the hydraulic parameters of stony soils using the cumulative infiltration volume and the rate of wetting front advancement during water absorption into a horizontal soil column. The influence of rock fragment content on some soil hydraulic properties including air entry suction, saturated hydraulic conductivity and the shape coefficients of hydraulic functions was further evaluated. Both air entry suction and shape coefficient show a large range of variation with rock fragment content. Globally, the saturated hydraulic conductivities of stony soils decreased with increasing rock fragment content. However, when the volumetric rock fragment content is about 0.08 cm(3) cm(-3), the stony soil displayed a greater saturated hydraulic conductivity compared to the fine earth at 95% confidence level. The relationship between saturated hydraulic conductivity and rock fragment content is not accurately estimated by different equations relating saturated hydraulic conductivity of stony soils to that of non-stony soils. These findings imply that other factors than the reduction of cross sectional area for water flow influence the hydraulic properties of stony soils. (C) 2010 Elsevier B.V. All rights reserved.