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Accurate determination of evapotranspiration (ET) has tremendous potential in guiding irrigation and improving the efficiency of water resources utilization in the North China Plain. Eddy covariance (EC) method is currently a popular method for determining field-scale ET. However, due to varying foot print and unclosed energy balance, the applicability of EC in different regions needs to be tested and corrected. In present work, we compared the ET of the winter wheat – summer maize rotation cropland measured by the EC method with the ET measured by large-scale lysimeters on different time scales. The degree of energy balance closure of EC measurements in this region is 78%. After adjusted by using Bowen ratio forced closure method, the ET monitored by EC is comparable with those monitored by large-scale lysimeters. The results also indicated that the consistency of the observed ET by the EC and lysimeters got better with an increasing time scale, especially for the multi-year average ET values with a relative deviation of less than 1%. The short-time events such as irrigation and precipitation and the mismatch of the varying footprint area of the EC and the small fixed source area of the lysimeter should be responsible for the discrepancy of ET in two methods on daily scale. However, the factors of crop biomass, total available water, and local climate condition exert more effects on the observed ET on large time scale. Overall, the EC technique is responsible for ET measurement of winter wheat – summer maize rotation cropland of the North China Plain.

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.

The ability of soil microorganisms to uphold a variety of ecosystem services is of increasing interest, but our understanding of the linkage between cropland ecosystem services and the soil microbiome under long-term different fertilizer regimes is still limited. Here, we measured 15 response variables commonly used as indicators of cropland ecosystem services, and employed a shotgun metagenomic sequencing technology to determine microbiome taxonomic composition and functional composition in a sandy loam soil amended with organic and inorganic fertilizers for 28 years. Long-term application of fertilizers had overall positive effects on the variables that indicate soil C and nutrient status, organic material decomposition, nutrient transformation, and plant productivity, but the strength of these effects differed between organic and inorganic fertilization. In long term, organic fertilizers significantly increased the abundance of Sphingomonas and Acidobacteria as well as genes involved in C and N metabolism and degradation of aromatic organic compounds. The observed differences in the indicators of overall biological activity, litter decomposition, nutrient transformation, cropland fertility and productivity were associated with changes in taxonomic composition and functional composition of the soil microbiome. Network analysis revealed that soil organic C, total N, L-leucine aminopeptidase, phosphatase, microbial biomass C and N, and the multifunctionality index showed strong positive associations with Acidobacterium, Lysobacter and Nitrososphaera. These microbes are implicated in the decomposition of organic material, soil C and N transformation, and plant growth promotion. The present study deepens our understanding of the relationship between the soil microbiome and agro-ecosystem functioning, and provides support for the use of organic fertilization strategy to enhance multiple ecosystem services in croplands.

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.

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.

Although straw decomposition is important for ecosystem fertility and carbon balance, influence of ultraviolet-B (UV-B) radiation and nitrogen (N) deposition on this process is unclear. In this study, UV-B-exposed rice straw was decomposed under different N addition treatments for 15 months to investigate the indirect effects of UV-B radiation on straw chemistry and direct effects of N deposition on decomposition. UV-B exposure during rice plant growth changed the rice straw chemical composition, increasing the concentrations of acid-insoluble fraction (AIF), acid-soluble fraction, and UV-B-absorbing compounds. High N content had a negative effect on decomposition of rice straw exposed to enhanced and ambient UV-B radiation. Both AIF concentration and FTIR peak intensities indicated that lignin in rice straw was selectively preserved following N addition and UV-B radiation, reducing straw decomposition rate, which corresponded to lower activities of lignin-degrading enzymes in the later stage of decomposition. Thus, enhanced UV-B radiation during rice plant growth produced more recalcitrant substrates (lignin) and N reacted with lignin to produce more resistant compounds, further decreasing straw decomposition rate. UV-B radiation during plant growth and N deposition inhibit litter decomposition in agroecosystem, and their effects should be considered when establishing biogeochemical models in response to global changes.

Elevated CO2 and nitrogen addition can increase rice yield and stimulate rice to absorb more nutrients from the soil. The aim of this study was to investigate how soil organic carbon and aggregates change under such high-output conditions. Rice cultivars IIY 084 and WYJ 21 (strong and weak response to elevated CO2, respectively) were planted in free-air CO2 enrichment (FACE) platforms. Treatments included: 1) elevated CO2 without nitrogen addition (EC); 2) nitrogen addition and ambient CO2 (EN); 3) combined elevated CO2 and nitrogen addition (ECEN); and 4): ambient CO2 without N addition (CK). Soil organic carbon (SOC), readily-oxidizable organic carbon (ROC) and soil aggregate components were measured. Treatment EC increased SOC and ROC, while EN decreased SOC and ROC compared to CK in soil under either rice cultivar. EC and EN decreased the percentage of large macroaggregates (LM) and the mean weight diameter (MWD) of aggregates compared to CK in ‘WYJ 21’ soil, while they increased LM and MWD in ‘IIY 084’ soil. ECEN increased LM and MWD compared to CK in soil under either rice cultivar. High productivity of ‘IIY 084’ through long-term cultivation under current N fertilization in future climatic conditions of elevated CO2 could be achieved due to increased soil aggregate stability and SOC.

A 2-year field experiment was conducted on the North China Plain to assess the effects of two straw management practices and different nitrogen fertilizer addition levels on soil organic matter (SOM) fractions and microbial properties under a winter wheat-summer maize cropping system. Eight treatments (two straw management practices and four nitrogen fertilizer input levels) were established using a randomized complete block design. The straw management practices were no straw return and straw return. Each straw management practice received nitrogen fertilizer rates at 0 (N0), 270 (N270), 360 (N360), or 450 (N450) kg N ha−1 year−1. The soil properties measured included SOM fractionation, enzyme activities, community level physiology profile (CLPP), and microbial community abundance and composition, which were represented by phospholipid fatty acid (PLFA). Compared to no straw return, the addition of straw significantly increased soil total organic carbon and nitrogen, particulate organic carbon and nitrogen, and occluded particulate organic carbon and hot water extractable organic carbon by 7–29%. Invertase, protease, urease, and dehydrogenase activities increased by 22–40%. The biomasses of Gram-positive bacteria and fungi also increased. The increase in SOM fractions may be attributed to the decomposition of the added straw by more active enzymes, which may result from the shift of microbial community composition. In addition, hot-water extractable organic carbon, NO3-N, urease activity, and fungal biomass in the N270, N360, and N450 treatments significantly increased, independent of the nitrogen application rate, which indicated that N input level had a non-linear effect on soil properties in this area. Short-term straw return combined with a modest nitrogen fertilizer input (270 kg N ha−1 year−1) can increase SOM contents and microbial activity, which may help improve soil fertility in this region.

Structural changes in fulvic acid (FA) in response to fertilization strategies, especially P-deficiency fertilization, have critical implications for soil carbon dynamics. This study examined how long-term fertilization influenced chemical compositions of soil FA and its humification degree. Advanced solid-state 13C nuclear magnetic resonance techniques were used to evaluate structural changes of FAs from two soils (a Fluvisol and an Anthrosol) under four fertilizer treatments: organic fertilizer combined with or without NPK fertilizer (OF and NPKOF), deficient in phosphorus only (NK) and Control. The greater aromaticity, hydrophobicity and alkyl/O-alkyl (A/O-A) ratios of FAs were observed in the Anthrosol than in the Fluvisol. The Anthrosol FAs contained more alkyl and aromatic C but fewer O-alkyls (mostly being protonated) than did the Fluvisol FAs. Both FAs comprised some nonprotonated aromatics, which decreased with applications of organic and NK fertilizers. This trend was less obvious for the Anthrosol than for the Fluvisol. Organic fertilizer treatments (OF and NPKOF) in the Fluvisol increased the A/O-A ratio and aromaticity compared with P-deficient soils (NK and Control), indicating organic fertilization promoted the humification degree of its FA. The NK treatment in the Fluvisol increased H/C ratio of FA. Compared with the Fluvisol FAs, the Anthrosol FAs displayed similar chemical natures, implying their formation followed similar processes with a similar humification degree. The findings suggest that long-term fertilization differently influenced the FAs from the Fluvisol and Anthrosol. Clearly, the Anthrosol FAs shared more common features among the treatments compared with the Fluvisol FAs, whereas P-deficiency rendered the Fluvisol FAs simpler.

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.

Conservation tillage has been gaining increasing recognition for its role in improving soil quality and maintaining agricultural sustainability. This is the first in a series of papers describing the impacts of reduced/no-tillage with and without residue based on the field experiment in the North China Plain. The experiment was established in 2006 on a sandy loam soil and involved a winter wheat-summer maize rotation system per year. The objective of this study was to investigate the impacts of different conservation tillage systems on crop yield and soil fertility that was quantified by a minimum data set and integrated index. Soil samples were collected since 2011, and the stocks of soil organic matter (SOM), total nitrogen (TN), alkali-hydrolyzale nitrogen (AN), total phosphorus (TP), available phosphorus (AP), total potassium (TK) and available potassium (AK) were measured for each year as well as soil aggregates were fractionated for 2016. Compared to continuous tillage, the reduced/no-tillage, regardless of residue, significantly increased the macroaggregate mass and soil nutrient stocks at the 0–10 cm depth, while further improvements in these soil attributes apart from TK were observed at the 0–10 and 10–20 cm depths for residue returning relative to residue removing. The accumulations of soil nutrients were closely related to soil macroaggregation. The path analysis revealed that TN was the most important soil attribute to directly determine wheat and maize yields while other soil attributes apart for TK primarily made indirect contributions to the yields. The first two factors extracted using 8 soil attributes through factor analysis were selected as the integrated indicators for the minimum data set, and their integrated score was calculated to quantify soil fertility. It was found that reduced/no-tillage did not improved soil fertility at the 0–20 cm depth. Consequently, an average 6.9% decrease in wheat yield across all years was observed under no-tillage while reduced tillage only increased the yield in the first two years in a periodic reduced tillage event. No significant difference was observed for the mean maize yield among the three tillage regimes averaged across all years and residue managements. Wheat and maize yields were significantly correlated with the integrated score for soil fertility, and thus significant increases in grain yields of wheat and maize were observed for residue returning. It can be concluded that grain yields of wheat and maize within a given residue management practice were not significantly higher for reduced/no-tillage than continuous tillage, regardless of the effects of tillage on aggregates and soil nutrients.

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.

Microorganisms mobilize phosphorus (P) from soil and make it available for plants. However, the role of microbial activity in soil P dynamics especially among different P pools is poorly understood largely due to methodological limitations. In this study, we analyzed the oxygen isotope ratios in phosphate (δ18OP) of sequentially extracted inorganic P (Pi) pools (H2O-Pi, NaHCO3-Pi, NaOH-Pi, and HCl-Pi) in a long-term agricultural research field in Henan, China with different fertilization histories and coupled with soil enzyme activity and P-cycling bacteria gene abundance studies. Results showed the dominant enzymes were alkaline phosphatase (APase) and phosphodiesterase (PDE), and the functional genes for P-cycling were bpp, cphy, phoX and pqqC. After long-term P fertilization, the δ18OP values of H2O-Pi, NaHCO3-Pi and NaOH-Pi pools approached to or achieved equilibrium, suggesting that the externally applied P was actively mobilized and cycled by soil microorganisms and speciated into different P pools. Based on the extent of isotope excursion among different Pi pools, the equilibrium fractionation oriented the source signature in the NaHCO3-Pi and NaOH-Pi pools, and HCl-Pi pool could be derived from P fertilizer but through multistep reactions, and mixed with the HCl-Pi from rock weathering product, which constituted HCl-Pi pool in the non-P treatments. Overall, the long-term P fertilization especially the balanced fertilization with nitrogen (N), P and potassium (K) was found to be beneficial for extensive utilization of soil P with abundant biological uptake and cell-internal Pi cycling. Variation partitioning analysis (VPA) indicated that the expression of functional genes may be stimulated to mobilize soil P under specific P pools distribution in this alkaline environment. Overall, our findings provide new insights to understand the roles of microbial activities in soil P biogeochemistry that are useful for agricultural P sustainability.

Spatial variability and influential factors are important to evaluate soil organic C (SOC) and the C pool in large areas. In the present study, sampling was conducted from May to November 2011 in Henan Province, a typical agricultural region of Central China, to study the effects of soil properties and anthropogenic factors on SOC variability in cropland. Physicochemical properties of soil samples collected at 280 sites from the surface layer (at a depth of 0–20 cm) were analyzed, and related data about the sampling sites were also collected from the Second State Soil Survey of China, conducted in 1981. Firstly, we found increasing trends in SOC density (SOCD) and SOC pool (SOCP) were obvious from 1981 to 2011, and we concluded that cropland presents great C sequestration potential for the future. Carbon pool ability varied with soil properties: the order of fixed Camount in different soil types was found to be Inceptisols > Luvisols > semihydromorphic soil > Anthrosols, and the average SOCP increased significantly from 1981 to 2011. Secondly, soil bulk density, pH, and returning straw were the key influential factors for SOCD in the past 30 yr. Thirdly, although random factors (returning straw) only explain 29.1% of SOCD variability, more attention should be paid to the factors, because application of returning straw was the most dominant anthropogenic factor, which can be used to improve cropland productivity and C sink capacity within a short period if they are properly managed in the future.

Irrigation and nitrogen (N) fertilization in excess of crop requirements are responsible for substantial nitrate accumulation in the soil profile and contamination of groundwater by nitrate leaching during intensive agricultural production. In this on-farm field trial, we compared 16 different water and N treatments on nitrate accumulation and its distribution in the soil profile (0-180cm), nitrate leaching potential, and groundwater nitrate concentration within a summer-maize (Zea mays L.) and winter-wheat (Triticum aestivum L.) rotation system in the Huang-Huai-Hai Plain over five cropping cycles (2006-2010). The results indicated that nitrate remaining in the soil profile after crop harvest and nitrate concentration of soil solutions at two depths (80cm and 180cm) declined with increasing irrigation amounts and increased greatly with increasing N application rates, especially for seasonal N application rates higher than 190kgNha-1. During the experimental period, continuous torrential rainfall was the main cause for nitrate leaching beyond the root zone (180cm), which could pose potential risks for contamination of groundwater. Nitrate concentration of groundwater varied from 0.2 to 2.9mgL-1, which was lower than the limit of 10mgL-1 as the maximum safe level for drinking water. In view of the balance between grain production and environmental consequences, seasonal N application rates of 190kgNha-1 and 150kgNha-1 were recommended for winter wheat and summer maize, respectively. Irrigation to the field capacity of 0-40cm and 0-60cm soil depth could be appropriate for maize and wheat, respectively. Therefore, taking grain yields, mineral N accumulation in the soil profile, nitrate leaching potential, and groundwater quality into account, coupled water and N management could provide an opportunity to promote grain production while reducing negative environmental impacts in this region.

Fertilization-induced changes in soil properties from the paddy soil and upland soil may directly regulate bacterial community composition, which usually coincide with shifts in molecular composition of soil organic carbon (SOC) in the upland soil. However, systematical comparisons lack on how regulators vary with cropping systems under the same weather conditions and soil parent materials. Here, we simultaneously investigated the changes of soil physicochemical parameters and shifts in the bacterial community and SOC molecular composition in two adjacent rice and maize fields that have received five fertilization regimes for more than 30 years. The separation of the bacterial community composition among the treatments from the paddy soil was mainly determined by soil nitrate-N, and that from the upland soil was mainly determined by soil available P (AP) and pH. The SOC molecular composition from the paddy soil was separated by the treatments with N application or not, with those treatments with N application being enriched with CCH3 and aromatic C C, and those without N application being enriched with aromatic C H. These changed C functional groups showed close association with amorphous Fe2O3. For the upland soil, the SOC molecular composition was separated by the treatments with P application or not, with those treatments with P application being enriched with OCH, and those without P application being enriched with CH/CH2. These changed C functional groups had close association with AP and total P. Our results indicated inconsistent separation patterns and regulators of the bacterial community and SOC molecular composition among the treatments of the paddy soil and upland soil, and suggested that the relatively dominant role of the fertilization-induced changes in soil properties or other soil microbes that controlled the SOC molecular composition over the bacteria measured in the present study.

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.

The rate of ammonia emissions from a small circular plot of maize was estimated by three procedures: (i) by inverse dispersion (based on upwind and downwind laser concentration-detectors interpreted using a backward Lagrangian stochastic model); (ii) by the height-integrated horizontal flux measured by point flux-detectors arrayed along the axis of the plot (i.e., the integrated horizontal flux method, IHF); and (iii) by extrapolation from static flux chambers (SC). The results indicated that the estimates made by the open-path tunable diode laser (OPTDL) system combined with the backward Lagrangian stochastic (BLS) model were statistically equivalent to those made by the IHF method. The ammonia fluxes estimated by the OPTDL-BLS technique were only 2.3% higher than those from the IHF method. Although the OPTDL technique failed to monitor concentration differences at low ammonia fluxes due to its detection limit, the OPTDL-BLS technique estimated the total ammonia loss to be only 10.9% less than the IHF results. The SC method was found to underestimate ammonia emissions and cumulative ammonia loss significantly compared with both the OPTDL-BLS and IHF methods Although the ammonia emissions estimated by the OPTDL-BLS technique showed a similar emission pattern to those estimated by the IHF method, the former provided an opportunity to estimate the diurnal pattern of ammonia emissions and to understanding the primary driving factors. A clear diurnal cycle and a dominant net solar radiation dependence in ammonia emissions were found.

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.

This paper was primarily devoted to reveal the composition of soil organic carbon (C) at a molecular level and to compare the differences existing among tillage practices after 8 years of contrasting managements, aiming to identify the effects of conservation tillage on the accumulation process of soil organic C and its association with macroaggregation. A sandy loam soil following 8-year conservation tillage was collected to identify the dynamics of organic components and their associations with macroaggregation. The content of particulate organic matter within macroaggregates was determined using aggregate fractionation techniques. Conservation tillage improved the accumulation efficiency of different C functional groups in large and small macroaggregates, and resulted in a higher soil organic C level. The content of C functional groups was significantly and positively correlated with the C amount of subfractions in macroaggregates. The accumulation of organic components, particularly the methoxyl/N-alkyl C in the 0–5 cm layer and the O-alkyl C in the 5–10 cm layer, in turn, accelerated soil macroaggregation. Overall, organic component accumulation and soil macroaggregation were interacted on each other. Our results suggested that the enrichment of methoxyl/N-alkyl C and O-alkyl C in the test soil following conservation tillage could effectively promote organic C accumulation and macroaggregation.

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.

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.

Fertilisation plays key roles in soil organic carbon (SOC) formation and stabilization by regulating a range of microbial traits. However, little is known about the relationships between SOC composition and microbial traits under long-term fertilisation at aggregate scales. Here, we selected four long-term fertilisation field experiments in China to evaluate the potential associations between SOC composition and microbial traits within aggregates. The four experiments have been treated for more than 25 years with inorganic nitrogen (N), phosphorus (P) and potassium (K) fertilisers (NPK), organic manure (M), and NPK plus M (NPKM). After aggregate isolation, SOC physical fractions including free particulate organic carbon (fPOC), occluded particulate organic carbon (oPOC) and mineral organic carbon (MOC) was measured via density fractionation. SOC chemical structure was determined by 13C nuclear magnetic resonance, and the bacterial community composition was analysed using amplicon sequencing of the 16S rRNA genes. The results showed that fertilisation increased the contents of free particulate organic carbon (fPOC) and occluded particulate organic carbon (oPOC) while decreasing the relative abundance of aromatic C in macroaggregates. Irrespective of fertilisation regime, the relative abundance of cellulose-degrading genes (cbhI and GH48 genes) was higher in the clay and silt fractions than in the macro- and microaggregates. Structural equation modeling indicated that SOC chemical structure was associated with pH-driving shifts in bacterial community composition, and its physical fractions were associated with soil nutrients-induced (mainly total N and P) changes in cellulose-degrading genes and specific taxa in macroaggregates. Finally, we conclude that fertilisation changed soil pH and total N, which were the major drivers affecting the SOC physical fractions and chemical structure, and the effects were caused by altering the bacterial community composition and fungal genes involved in C cycling within soil aggregates following long-term fertilisation.

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.

The microbiomes inhabiting plant roots and their adjacent soil environment play significant roles in plant growth, health, and ecological services. Recent studies have provided detailed insights into many phytomicrobiomes, but our understanding of the root-associated microbiomes of perennial ryegrass is still very limited. Here we carried out a stratified rhizobox experiment involving a fluvo-aquic soil and red soil under elevated (750 ppm) and ambient (350–400 ppm) CO2, and used deep amplicon sequencing to investigate the ryegrass root-associated bacterial microbiomes across compartments (the bulk soil, 2-mm outer rhizosphere, rhizosphere, rhizoplane, and endosphere). We observed increased relative abundances of Proteobacteria (mainly Rhizobiaceae and Enterobacteriaceae) in the rhizosphere, rhizoplane, and endosphere compared with the bulk soil and outer rhizosphere. The dominant bacterial genera (mainly Methylobacterium, Rhizobium, Pseudomonas, Stenotrophomonas, and Enterobacter) which facilitate plant growth and soil nutrient cycling were enriched in the rhizosphere, rhizoplane, and endosphere compared with outer rhizosphere. PICRUSt predicted moderate functional differentiation of bacterial microbiomes across compartments. The bacterial α-diversity showed a decreased trend along a spatial gradient from the bulk soil to the endosphere. The β-diversity and network analyses indicated significant differences in the root-associated bacterial microbiomes between the fluvo-aquic soil and red soil. In conclusion, our study suggests that the filtration and acquisition of the ryegrass root-associated bacterial microbiomes are largely controlled by indigenous soil microbial communities, rather than the changes in root rhizodeposition caused by elevated CO2.

Variations in temperature and moisture play an important role in soil organic matter (SOM) decomposition. However, relationships between changes in microbial community composition induced by increasing temperature and SOM decomposition are still unclear. The present study was conducted to investigate the effects of temperature and moisture levels on soil respiration and microbial communities involved in straw decomposition and elucidate the impact of microbial communities on straw mass loss. A 120-d litterbag experiment was conducted using wheat and maize straw at three levels of soil moisture (40%, 70%, and 90% of water-holding capacity) and temperature (15, 25, and 35 °C). The microbial communities were then assessed by phospholipid fatty acid (PLFA) analysis. With the exception of fungal PLFAs in maize straw at day 120, the PLFAs indicative of Gram-negative bacteria and fungi decreased with increasing temperatures. Temperature and straw C/N ratio significantly affected the microbial PLFA composition at the early stage, while soil microbial biomass carbon (C) had a stronger effect than straw C/N ratio at the later stage. Soil moisture levels exhibited no significant effect on microbial PLFA composition. Total PLFAs significantly influenced straw mass loss at the early stage of decomposition, but not at the later stage. In addition, the ratio of Gram-negative and Gram-positive bacterial PLFAs was negatively correlated with the straw mass loss. These results indicated that shifts in microbial PLFA composition induced by temperature, straw quality, and microbial C sources could lead to changes in straw decomposition.

The influence of compost and mineral fertilization on soil organic carbon (SOC) and physical properties varied greatly in previous studies because of differences in site conditions such as climate and soil. This study was conducted as a long-term (1989–2012) field experiment to evaluate the effects of mineral fertilization and compost application on SOC content and some physical properties of an intensively cultivated sandy loam soil in the North China Plain. The experiment consisted of seven treatments: organic compost (OM); half organic compost plus half mineral fertilizer NPK (1/2OM1/2NPK); mineral fertilizer NPK (NPK); mineral fertilizer NP (NP); mineral fertilizer PK (PK); mineral fertilizer NK (NK); and unfertilized control (CK), each with four replicates. Relative to CK, the SOC content was higher in all of the compost and mineral fertilizer treatments, but increments in SOC under the compost application treatments were higher than mineral fertilization application. Compost application (OM, 1/2OM1/2NPK) decreased soil bulk density and increased total porosity significantly in comparison with that in the CK plots. Soil bulk density and total porosity for the mineral fertilization treatments, except NP, did not significantly differ from CK. The lowest penetration resistance at 0–20 cm soil depth was observed in the NK plots, and the highest penetration resistance was found in the CK plots. Compost application increased the total amount of water-stable macro-aggregates (>0.25 mm); however, MWD was not significantly affected by compost application. The MWD in the NK treatment was lower than in CK by 0.2 mm, while its variation among NPK, OM, 1/2OM1/2NPK, NP, PK, and CK was not significant. The compost- and mineral fertilizer-treated soil had 34.6–91.7% higher volume of macropores than the CK soil. The OM and 1/2OM1/2NPK-treated soil had a significantly higher proportion of small pores (

Continuous applications of organic and inorganic fertilizers can affect soil and food quality with respect to selenium (Se) concentrations. A long-term (over 20 years) experimental field study, started in 1989, was conducted to investigate the changes in soil Se fractions and its uptake by crops, as affected by different fertilizer practices, in the North China Plain with an annual crop rotation of winter wheat and summer maize. The long-term experiment was arranged in a complete randomized block design consisting of 4 replications with 7 fertilizer treatments: 1) organic compost (OC), 2) half organic compost plus half N-P-K chemical fertilizers (OC + NPK), 3) N-P-K fertilizers (NPK), 4) N-P fertilizers (NP), 5) P-K fertilizers (PK), 6) N-K fertilizers (NK), and 7) an un-amended control. Soil samples from the surface (20 cm) were collected in 1989, 1994, 1999, 2004 and 2009 to characterize Se and other soil properties. In 2009, the average soil Se concentrations in the treatments (149 ± 8 pg kg−1) were higher than those in the soil samples collected in 1989 at the beginning of the experiment (112 ± 4 pg kg−1), and decreased in the order of OC > OC + NPK > NPK ≈ NP > PK ≈ NK > control. Sequential extraction showed the oxidizable fraction (50.06% ± 3.94%) was the dominant form of Se in the soil, followed by the residual fraction (24.12% ± 2.89%), exchangeable fraction (15.09% ± 4.34%) and Fe-Mn oxides fraction (10.73% ± 4.04%). With an increase of soil K, the exchangeable Se concentrations in the soil increased. The Se concentrations in the soil tillage layer (0–20 cm) were mainly related to soil organic carbon (SOC), although different contributions came from atmospheric deposition, irrigation and fertilizers. With the accumulation of SOC, the uptakes of soil Se by two crops were inhibited. For the OC and OC + NPK treatments, Se concentrations in wheat grains were lower than the critical standard of Se in stable food (100 µg kg−1). Additionally, Se concentrations in grains were also decreased by the deficiencies of major soil nutrients, especially P.

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

The backward Lagrangian stochastic (BLS) model for estimating ammonia emissions from agricultural sources was assessed in an ammonia recovery experiment. The open-path tunable diode laser spectrometer was used to measure atmospheric ammonia concentrations. The results indicated that the ratio of estimated to actual emission ( Q BLS / Q ) decreased with increasing concentration measurement height, and the optimum measurement height increased as fetch ( F ) increased. The accuracy of Q BLS was insensitive to fetch with a range of 15–60 m. The optimum ratio of concentration measurement height to fetch decreased with increasing fetch. In this study, the optimum ratio of concentration measurement height to fetch was 0.083 and 0.045 for F = 15 m and F = 30 m, respectively. It is recommended that the laser sensor should be placed as close to the source as possible to reduce concentration measurement uncertainty for measurement of ammonia emission from farmland. The wheat canopy had a significant effect on Q BLS / Q when the concentration measurement height above the wheat canopy was no more than 0.38 m. The laser path should be placed at least 0.58 m above the wheat canopy to minimize the effect of wheat canopy on the accuracy of the BLS model.

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

Improvement of Shajiang black soil in the Huanghuai plain is crucial for sustainable agricultural production; however, the effects of intensified fertilizer input on microbial communities in this soil type remain uncertain. Here, we conducted a long-term field trial using a wheat-soybean-maize cropping system, which had five treatments: organic material (soybean cake) (OM), mineral fertilizers (NPK), standard nitrogen (N) (half from OM and half from NPK) (MNPK), high N (1.6-fold higher than other treatments, with half from OM and half from NPK) (HMNPK), and unfertilized control. The organic carbon (C) composition of the soil was analyzed by 13C nuclear magnetic resonance spectroscopy, and bacterial community structure was determined based on 16S rRNA gene sequencing analysis. Addition of NPK resulted in soil acidification, which was effectively alleviated by organic amendment. NPK significantly (P

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.

Background and aims This paper was primarily devoted to understand the interactions of soil aggregates, organic carbon (C) and carbon cycle enzymes in aggregates under different fertilization managements, aiming to identify the effects of organic and inorganic fertilizer amendments on soil organic C accumulation and the activities of carbon cycle enzymes within aggregates in Vertisol. Methods A Vertisol soil following 4-year compost and inorganic fertilizer amendments, i.e. no fertilizer (CK), mineral fertilizer (FR) and 60% compost N plus 40% fertilizer N (FRM), was collected to identify the dynamics of organic C, enzymes activities and their associations with macroaggregation using aggregate fractionation techniques. Results The organic C content in all FR and FRM treatments was 8.24–41.15% higher than that in CK. An increased amounts of carbon cycle enzymes in aggregates or 0–20 cm bulk soil were also observed in FRM plots. Compared to FR, FRM significantly strengthened the structural stability of macroaggregates and the intimate connection between enzyme activities and macroaggregates. Conclusions As a recommended measure, supplementation with organic manure such as compost strengthened the process of mutual promotion between carbon cycle enzymes and macroaggregates, and the synergistic effect would be highly beneficial to soil organic C sequestration.

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(

Atmospheric nitrogen (N) deposition has caused concern due to its effects on litter decomposition in subtropical regions where N-fixing tree species are widespread. However, the effect of N deposition on litter decomposition in N-fixing plantations remains unclear. We investigated the effects of a 2-year N deposition treatment on litter decomposition, microbial activity, and nutrient release in two subtropical forests containing Alnus cremastogyne (AC, N-fixing) and Liquidambar formosana (LF, non-N-fixing). The decomposition rate in AC was faster than in LF when there was no experimental N deposition. In AC, the initial decomposition rate was faster when additional N was applied and was strongly linked to higher cellulose-degrading enzyme activities during the early decomposition stage. However, N deposition reduced litter decomposition and inhibited lignin-degrading enzyme activities during the later decomposition stage. Nitrogen deposition enhanced carbohydrate and alcohol utilization, but suppressed amino acid and carboxylic acid uptake in the AC plantation. However, it did not significantly affect litter decomposition and microbial activity in the LF plantation. In conclusion, N deposition could inhibit litter decomposition by changing microbial enzyme and metabolic activities during the decomposition process and would increase carbon accumulation and nitrogen retention in subtropical forests with N-fixing tree species.

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

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.

Humin is the most recalcitrant fraction of soil organic matter (SOM). However, little is known about quantitative structural information on humin and the roles of soil mircoorganisms involved in the humin formation. We applied advanced solid-state 13C nuclear magnetic resonance (NMR) spectroscopy to provide deep insights into humin structural changes in response to long-term balanced fertilization on a Calcaric Fluvisol in the North China plain. The relationships between humin structure and microbiological properties such as microbial biomass, microbial quotient (qmic) and metabolic quotient (qCO2) were also studied. The humins had a considerable (35–44%) proportion of aromatic C being nonprotonated and the vast majority of O-alkyl and anomeric C being protonated. Alkyl (24–27% of all C), aromatic C (17–28%) and O-alkyl (13–20%) predominated in humins. Long-term fertilization promoted the aliphatic nature of humins, causing increases in O-alkyl, anomeric and NCH functional groups and decreases in aromatic C and aromatic C O groups. All these changes were more prominent for treatments of organic fertilizer (OF) and combined mineral NPK fertilizer with OF (NPKOF) relative to the Control and NPK treatments. Fertilization also decreased the alkyl/O-alkyl ratio, aromaticity and hydrophobic characteristics of humins, suggesting a more decomposed and humified state of humin in the Control soil. Moreover, the soil microbiological properties had strong correlations with functional groups of humins. Particularly, microbial biomass C was a relatively sensitive indicator, having positive correlations with oxygen-containing functional groups, i.e., COO/N C O and protonated O-alkyl C, and negative correlations with nonprotonated aromatic C. The qmic and qCO2 were also significantly positively correlated with NCH and aromatic C O, respectively. Our results deepen our understanding of how long-term fertilization impacts the structure of humin, and highlight a linkage between microbiological properties and recalcitrant fraction of SOM besides labile fraction.