Chemical and spectroscopic characteristics of humic acids in marshes from the Iberian Peninsula

Purpose

To characterise soil humic acids (HAs) extracted from Spanish marshes formed under different vegetation types (Spartina maritima (GSp), Juncus maritimus (GJc), Phragmites australis (GPh), and Scirpus maritimus (VSc)), soil depths (0–20, 20–40 and 40–60 cm), physiographic position (low and high marshes), wetland types (salt marshes and lagoons) and environmental conditions (Atlantic and Mediterranean coast).

Material and methods

Soil samples were collected in five Spanish marshes, three on the Galicia province and two on the Valencia province. Humic acids were extracted and their elemental composition, semiquinone-type free radical (SFR) content, FTIR and CPMAS ¹³C NMR spectra determined. Total carbon (TC), total nitrogen (TN), total sulphur (TS), CaCO₃ content, and field pH and Eh (mV) in the marsh soils sampled were also measured.

Results and discussion

The field pH and Eh values were typical of coastal areas submitted to periodic inundations and the highest TC, TN and TS contents were found in the soil of lagoon marshes as an effect of physiographic position and wetland type. The HAs, in general, were highly aliphatic and exhibited a low SFR content, which suggests a low humification degree of the SOM formed in the studied areas. This is a result of the anaerobic decomposition to which SOM is submitted and the high input of plant-derived organic matter (OM) by vegetation. However, among the studied sites low salt marsh and subsurface layer of the high salt marsh showed higher SFR content, simpler FTIR spectra, higher lignin degradation and lower O-alkyl C/alkyl C ratio than the lagoon marshes, thus suggesting the presence of a more humificated SOM in these sites.

Conclusions

From the different factors analysed, only physiographic position (low versus high salt marshes) and wetland type (marshes versus lagoons) caused variations in the HAs characteristics, because as the studied soils are under anaerobic conditions, they control the exportation of plant-derived OM and the allochthonous OM contribution in the studied areas.     

Distribution of carbon and nitrogen in water-stable aggregates and soil stability under long-term manure application in solonetzic soils of the Songnen plain,northeast China

Purpose

Excessive exchangeable sodium and high pH significantly decrease soil structural stability and permeability. Long-term application of cattle manure is an important management practice that can affect water-stable aggregates (WSAs), as well as aggregate stability and distribution of soil organic carbon (SOC) and total nitrogen (TN) in solonetzic soils.

Material and methods

Experiments were carried out in a randomized complete block design comprising five treatments according to the cattle manure application history: corn (Zea mays) with manure applied for 1, 5, 12, and 17 years were used as the experimental treatments and corn without manure application was used as a control. Soil properties, including WSAs, mean weight diameter (MWD), and SOC and TN concentrations in bulk soils and WSAs, were measured across all treatments. The relationships among the measured soil attributes were determined using stepwise regression analysis.

Results and discussion

Results indicated that micro-aggregates mainly accumulated in soils without manure application, while manure application significantly increased macro-aggregates formation. MWD was highest when manure was applied to the soil for 1 year, decreased after 5 years, and increased again after 12 years. SOC and TN concentrations in bulk soils and WSAs increased with the number of years of manure application, with the highest concentrations observed for 17 years in bulk soils. Stepwise regression analysis showed that WSAs 2–5 mm, SOC in WSAs 0.25–0.5 mm, and TN in WSAs 0.1–0.25 mm were dominant independent variables affecting aggregate stability, and that SOC in WSAs 0.25–0.5 mm and TN in WSAs <0.1 mm were dominant independent variables affecting SOC and TN concentrations in bulk soils, respectively.

Conclusions

Long-term application of manure to a solonetz significantly increased macro-aggregates and aggregate stability as well as SOC and TN in bulk soils and all aggregate sizes. These results are likely related to binding agent production as well as C and N accumulation from manure application.     

Effect of agricultural land use change on community composition of bacteria and ammonia oxidizers

Purpose

Soil microbial communities can be strongly influenced by agricultural practices, but little is known about bacterial community successions as land use changes. The objective of this study was to determine microbial community shifts following major land use changes in order to improve our understanding of land use impacts on microbial community composition and functions.

Materials and methods

Four agricultural land use patterns were selected for the study, including old rice paddy fields (ORP), Magnolia nursery planting (MNP), short-term vegetable (STV), and long-term vegetable (LTV) cultivation. All four systems are located in the same region with same soil parent material (alluvium), and the MNP, STV, and LTV systems had been converted from ORP for 10, 3, and 30 years, respectively. Soil bacteria and ammonia oxidizer community compositions were analyzed by 454 pyrosequencing and terminal restriction fragment length polymorphism, respectively. Quantitative PCR was used to determine 16S rRNA and amoA gene copy numbers.

Results and discussion

The results showed that when land use was changed from rice paddy to upland systems, the relative abundance of Chloroflexi increased whereas Acidobacteria decreased significantly. While LTV induced significant shifts of bacterial composition, MNP had the highest relative abundance of genera GP1, GP2, and GP3, which were mainly related to the development of soil acidity. The community composition of ammonia-oxidizing bacteria (AOB) but not ammonia-oxidizing archaea was strongly impacted by the agricultural land use patterns, with LTV inducing the growth of a single super predominant AOB group. The land use changes also induced significant shifts in the abundance of 16S rRNA and bacterial amoA genes, but no significant differences in the abundance of archaea amoA was detected among the four land use patterns. Soil total phosphorous, available phosphorous, NO₃ ^?, and soil organic carbon contents and pH were the main determinants in driving the composition of both bacteria and AOB communities.

Conclusions

These results clearly show the significant impact of land use change on soil microbial community composition and abundance and this will have major implications on the microbial ecology and nutrient cycling in these systems, some of which is unknown. Further research should be directed to studying the impacts of these microbial community shifts on nutrient dynamics in these agroecosystems so that improved nutrient management systems can be developed.     

Fractal characterization of soil particle-size distribution under different land-use patterns in the Yellow River Delta Wetland in China

Purpose

Soil particle-size distribution (PSD) is an important soil physical property. Single- and multi-fractal models are increasingly used to characterize soil properties and may provide additional information. The Yellow River Delta is one of the best representative examples of river ecosystem wetlands in the world. In this area, different land resource development patterns strongly influence soil structure and fertility. Here, the single- and multi-fractal characterizations of soil PSD were determined based on fractal theory, and the correlations between PSD and soil organic matter (SOM) across different land-use patterns were studied.

Materials and methods

The study site was located in a typical area of the Yellow River Delta Wetland in Shandong Province in China. The tested soil samples were obtained from areas with four different land-use patterns, including integrated Robinia pseudoacacia and grass cover land (RPG), well-covered European and American poplar forestland (EAP), cropland used for growing cotton (COT), and waste grassland (WAG). Soil samples were air-dried and passed through a 2-mm screen. Based on the international system of soil size fraction, the soil PSD was described according to the percentages of clay, silt, and sand. The clay, silt, and sand fractions were determined using a laser particle size analyzer. The fractal characterizations of soil PSD were determined using the single- and multi-fractal methods.

Results and discussion

The single-fractal dimension (D) of the different land-use patterns varied greatly (between 2.4657 and 2.6789). The D values of the RPG and EAP were the greatest, which corresponded to the soils with the greatest silt content and the lowest sand content. In contrast, the D value of the WAG was the smallest, which corresponded with the lowest clay content and the greatest sand content. These results indicated that D was directly proportional to clay content and inversely proportional to sand content. The multi-fractal parameters of the soil PSD, capacity dimension (D ₀), information dimension (D ₁), and information dimension?/?capacity dimension (D ₁?/?D ₀), followed a regular trend due to different land-use patterns. These parameters decreased in the following order: RPG?>?EAP?>?COT?>?WAG. In addition, the PSD of the RPG and EAP varied widely and was more heterogeneous than the PSD of the COT and WAG. Furthermore, the single- and multi-fractal parameters were significantly correlated with SOM.

Conclusions

Our findings indicated that the single- and multi-fractal parameters adequately described the scaling properties of the soil PSD and the influences of soil structure and soil nutrients for the different land-use patterns in the Yellow River Delta Wetland of China.     

Short-term effect of cultivation and crop rotation systems on soil quality indicators in a coastal newly reclaimed farming area

Purpose

Soil quality (SQ), a measure of the sustainability of land use and soil management practices, can be assessed by indicators including soil physical, chemical, and biological properties. Our primary objectives were to investigate the influence of consecutive cultivation and different crop rotation systems on individual SQ indicators and to examine the impact of rotation systems on SQ using soil quality index (SQI) model.

Materials and methods

A site-specific selection of 17 potential SQ indicators representing soil chemical and physical attributes (0–10 cm) and groundwater features on 60 sampling locations and 10 representative soil profiles was chosen in a typical coastal newly reclaimed farmland of north Jiangsu Province, China. Using ANOVA analysis, the crop rotation effect was analyzed by comparing SQ indicators between rice/rape rotation soil and cotton/barley rotation soil. An overall SQI was calculated to examine whether plant biomass indices exactly responded to the SQI value.

Results and discussion

Results indicated that cultivation had significant effect on some soil profile characteristics, including SOM, SOCD, AP, CEC, AK, EC_e, and I _f. Crop rotation systems also had significant influence on some SQ indicators. Compared with cotton/barley rotation soil, rice/rape rotation soil was characterized by higher organic matter, undifferentiated nutrient storage and salinity/alkalinity, lower water accommodation and infiltration, and adverse groundwater conditions. Cotton/barley rotation had higher SQI values over rice/rape rotation (0.523 vs. 0.422). SQ indicators of SAR_e, EC_e, and WT_g contributed the most to the overall SQI value for each rotation system. Plant biomass indices significantly correlated with the SQI values for both rotation systems, suggesting that the SQI values essentially reflected the status of SQ.

Conclusions

Such results allowed us to conclude that cotton/barley rotation system contributed more to the improvement of SQ than rice/rape rotation system in coastal farming area. Also, we suggested SOCD, AK, ρ _b, WT_g, and EC_g as the minimum data set for SQ assessment, as they had potential in discriminating the effect of rotation systems on SQ between the rotation systems used here.     

Stability of soil organic matter in two northeastern German fen soils: the influence of site and soil development

Purpose

Peatland soils play an important role in the global carbon (C) cycle due to their high organic carbon content. Lowering of the water table e.g. for agricultural use accelerates aerobic secondary peat decomposition and processes of earthification. Peatlands change from C sinks to C sources. We characterized soil organic matter (SOM) with special attention to human impact through drainage. Our aim was to gain knowledge of SOM quality and soil-forming processes in drained fen soils in northeastern Germany.

Materials and methods

Through techniques of representative landscape analysis, we identified two typical and representative sampling sites in different stages of land use, representing the most important hydrogenetic mire types in northeastern Germany. We adapted chemical fractionation procedures which include hot water extraction (C_hwe and N_hwe) for determination of the labile fraction. Furthermore, a stepwise acid hydrolysis procedure was performed to measure the chemical recalcitrant part of SOM as it is more resistant to biodegradability.

Results and discussion

Total organic C decreased with increasing human impact and intensity of drainage. Conversely, C_hwe and N_hwe concentrations increased with increasing drainage and human impact. In contrast, the more recalcitrant fractions increased with soil depth.

Conclusions

Generally, there is a lack of existing data about SOM quality and the factors controlling its stability and decomposition in fen soils. For northeastern German fen soils, the data are even more inadequate. Influence of drainage seems to overlap natural influences of site on SOM quality. The used extraction scheme was suitable for the chemical fractionation of SOM into labile and more recalcitrant parts.     

Land use change exerts a strong impact on deep soil C stabilization in subtropical forests

Purpose

It has been widely recognized that land use changes can cause significant alterations of soil organic matter (SOM) of various ecosystems. Forest conversion, a common land use change, and its effects on SOM have been a hot research topic during the past two decades. However, the mechanisms of the effects of forest conversion on SOM dynamics, particularly in deep soils, largely remain uncertain. This study aimed to examine the impacts of forest conversion on SOM stabilization through the analysis of soil aggregate and density fractionation, microbial composition, and functions in deep soils.

Materials and methods

Soil C and microbes were sampled in soil layers of 0–20 and 60–80 cm under broadleaved secondary forest and two coniferous plantations (Cunninghamia lanceolata and Pinus massoniana). Aggregate and density fractionation techniques were used to analyze C accumulation in non-protected, physically, chemically, and biochemically protected C fractions. A 90-day laboratory mineralization incubation experiment with and without 400-mg C kg^?1 soil glucose and phenol was conducted to determine the potential mineralizable C, utilization of substrate capacity, and metabolic quotient (qCO₂).

Results and discussion

Conversion of secondary forests into coniferous plantations significantly decreased bulk soil C, especially in the deep soils. Forest conversion significantly decreased non-protected, physically, and chemically protected C fractions in both topsoil and deep soil and biochemically protected C fraction in deep soils. The soil organic carbon (SOC) of topsoils was dominated by non-protected fraction while in deep soil which was dominated by protected fraction. Compared with the topsoils, soil microbes in the deep soils tend to preferentially use labile soil organic matter with lower substrate use efficiency (higher values of qCO₂), which indicates that a r-strategy dominates of microbes. The increased respiration rate in the deep soils caused by forest conversion, when normalized to soil C, indicates that deep SOM may be more prone to decomposition and destabilization than top SOM.

Conclusions

Forest conversion can cause a significant alteration of SOC stabilization through the changes of physically, chemically, and biochemically protected SOC fractions. The mechanisms for the changes in non-protected or/and protected SOC fractions may be associated with the redistribution of r-strategy- and K-strategy-dominated microbes due to changes in litter inputs and priming effects.

     

Does the compositional change of soil organic matter in the rhizosphere and bulk soil of tea plants induced by tea polyphenols correlate with Pb bioavailability?

Purpose

Soil organic matter (SOM) plays a vital role in controlling metal bioavailability. However, the relationship between SOM and its fractions, including water-soluble substances (WSS), fulvic acid (FA), humic acid (HA), and soil microbial biomass (SMB), to metal bioavailability in plants has not been thoroughly investigated. This study examined the compositional change of SOM after tea polyphenols (TPs) were added to the soil and its correlation with Pb bioavailability.

Materials and methods

Ultisol samples were collected from Fuyang, spiked with two levels (0 and 300 mg kg^?1 DW) of Pb, and aged for 30 days. Four uniform seedlings were transplanted to each plastic pot, which were filled with 3 kg of air-dried soil. After successful transplantation, three levels (0, 300, and 600 mg kg^?1 DW) of TPs were amended as irrigation solution for the pots. The Pb concentrations in different tissues of the tea plants were determined after 6 months. SOM, WSS, FA, HA, and SMB were extracted and quantified using a Multi N/C Total Organic Carbon Analyser.

Results and discussion

Adding TPs to Pb-polluted soils alleviated Pb toxicity to microorganisms and increased SMB and the rhizosphere effect. The rhizosphere SOM was lower than bulk SOM in Pb-unspiked soils, while the opposite results were observed in Pb-spiked soils. A similar inconsistency for HA in the rhizosphere and bulk soil between Pb-unspiked and Pb-spiked soils might explain the difference in SOM. FA increased with the addition of TPs in both the rhizosphere and bulk soils, which might be the result of TP transformation. Positive correlations are present between the compositions of rhizosphere SOM and Pb in different tissues of the tea plant. SMB correlated negatively with Pb in young leaves and stems. Compared to rhizosphere soil, SOM components in bulk soil were less strongly correlated with Pb in tea plants.

Conclusions

Addition of TPs to soil changes the components of SOM and Pb bioavailability. SOM and its fractions, including WSS, FA, HA, and SMB, show a close relationship to Pb in different tissues of the tea plants.     

桂西北典型环境移民迁入区土壤养分特征分析

在桂西北典型环境移民迁入区,分析了5种土地利用方式表层(0-20 cm)土壤有机质(SOM)、全量养分和速效养分的变化特征.结果表明,根据变异系数(Cv)大小,除速效磷(AP)为强变异外Cv＞1.0),其余土壤养分都为中等变异(0.1＜Cv＜1.0).整体而言,移民迁入区SOM、全氮(TN)含量处于较高水平,速效氮(AN)、速效磷(AP)含量处于中等水平,全磷(TP)、速效钾(AK)处于偏低水平,而全钾(TK)处于低水平.土地利用方式是影响土壤养分的主要因素.灌丛和荒草地的SOM,TN,AN处于较高水平,果园和旱地则处于中等水平;次生林SOM处于较高水平,TN处于中等水平,而AN处于偏低水平.除旱地和果园AP含量处于中等水平外,不同利用方式土壤的TP,TK,AP,AK含量处于低或偏低水平.由于采取了较好的水土保持措施,移民后较大规模的土地利用变化没有导致明显的土壤退化,但需要增加磷钾肥的施用量.     

Acidification and salinization of soils with different initial pH under greenhouse vegetable cultivation

Purpose

Field survey and sampling of vegetable greenhouse soils were conducted in Shouguang, Shandong Province, and Ningbo, Zhejiang Province to study the acidification and salinization characteristics of soils with different initial soil pH values and greenhouse cultivation time.

Materials and methods

The pH, electrical conductivity (EC), and ion composition of 74 composite soil samples were analyzed to evaluate their relation to soil acidification and salinization.

Results and discussion

Compared with their corresponding open-field soils, acidification and salinization of the greenhouse soils occurred in both 0-20 cm and 20-40 cm soil layers for the Shouguang and Ningbo soils. The soil pH decreased gradually at different rates as greenhouse cultivation time increased in the two surveyed regions, but the opposite trend was observed for soil EC. For the Shouguang soils, while the percentages of K⁺ and NO₃ ^? increased dramatically and Ca²⁺ and HCO₃ ^- decreased significantly after the soils were converted to greenhouse use, the correlation between soil pH and EC was significant, and the stepwise multiple regression analysis further showed that there was a significant correlation between pH and the percent of Ca²⁺ and HCO₃ ^?.

Conclusions

Soil acidification and salinization are common in greenhouse soils with different initial soil pH. Soil acidification in the Shouguang soils is a result of decrease in the percent of Ca²⁺, HCO₃ ^? due to over application of N and K fertilizers. Future research should be devoted to understanding the relevant mechanisms in greenhouse soils with lower initial soil pH values to assess if there are correlations between soil acidification and salinization under greenhouse cultivation.     

Soil pH,organic matter,and nutrient content change with the continuous cropping of <Emphasis Type="Italic">Cunninghamia lanceolata</Emphasis> plantations in South China

Purpose

Chinese fir (Cunninghamia lanceolata (Lamb.) Hook) is an important native tree species in China. Consecutive cropping traditionally occurs in Chinese fir plantations (CFPs), but this practice has resulted in productivity declines in subsequent rotations. This study was designed to better understand the change of soil properties in the continuous cropping CFPs.

Materials and methods

We investigated soil pH, soil organic matter (SOM), and nutrient contents in different soil layers and in rhizosphere soil (RS) and non-rhizosphere soil (NRS) under CFPs of different ages and in different rotations.

Results and discussion

In the upper (0–20 cm) soil layer, soil pH decreased, while SOM increased, beneath mature CFPs with consecutive rotations. Total nitrogen (TN), available potassium, and available phosphorus contents in the upper soil layers did not differ significantly with consecutive rotations. Soil pH in RS was significantly lower than in NRS under mature plantations of the third rotation. Soil organic matter, TN, and available nitrogen did not differ between RS and NRS. Available phosphorus in RS was consistently lower than in NRS, and was highly deficient in the third rotation.

Conclusions

We conclude that no severe soil nutrient degradation occurred in the continuous cropping CFPs examined in this study, with soil acidification and phosphorus deficiency being two primary problems observed.

     

Major controlling factors and prediction models for mercury transfer from soil to carrot

Purpose

Soil-plant transfer models are needed to predict levels of mercury (Hg) in vegetables when evaluating food chain risks of Hg contamination in agricultural soils.

Materials and methods

A total of 21 soils covering a wide range of soil properties were spiked with HgCl₂ to investigate the transfer characteristics of Hg from soil to carrot in a greenhouse experiment. The major controlling factors and prediction models were identified and developed using path analysis and stepwise multiple linear regression analysis.

Results and discussion

Carrot Hg concentration was positively correlated with soil total Hg concentration (R ²?=?0.54, P?<?0.001), and the log-transformation greatly improved the correlation (R ²?=?0.76, P?<?0.001). Acidic soil exhibited the highest bioconcentration factor (BCF) (ratio of Hg concentration in carrot to that in soil), while calcareous soil showed the lowest BCF among the 21 soil types. The significant direct effects of soil total Hg (Hg_soil), pH, and free Al oxide (Al_OX) on the carrot Hg concentration (Hg_carrot) as revealed by path analysis were consistent with the result from stepwise multiple linear regression that yielded a three-term regression model: log [Hg_carrot]?=?0.52log [Hg_soil]???0.06pH???0.64log [Al_OX]???1.05 (R ²?=?0.81, P?<?0.001).

Conclusions

Soil Hg concentration, pH, and Al_OX content were the three most important variables associated with carrot Hg concentration. The extended Freundlich-type function could well describe Hg transfer from soil to carrot.     

Short-term evolution of hydration effects on soil organic matter properties and resulting implications for sorption of naphthalene-2-ol

Purpose

Sorption of xenobiotics in soils and especially to soil organic matter (SOM) determines their mobility and bioavailability in ecosystems. However, SOM as the major sorbent may be altered in its physicochemical properties upon changes in boundary conditions such as hydration. Hence, the goal of this study was to determine the influence of soil hydration on physicochemical properties of SOM and the resulting effects on sorption of xenobiotics.

Materials and methods

Samples of a Histosol with 51?% SOM were adjusted to five water contents from 10 to 75?% (w/w based on dry soil mass) and aged for water contact times of 0?weeks to 3?years. The hydrated samples were characterized with respect to thermal properties of SOM and of the incorporated water via differential scanning calorimetry and with respect to hydration-induced swelling via ¹H-NMR relaxometry, and the sessile drop method was applied to determine their soil?Cwater contact angle. Sorption kinetics and isotherms of naphthalene-2-ol in the pre-treated peat samples were determined in batch experiments.

Results and discussion

SOM matrix rigidity varied with the water content and increased with water contact time. An initial minimum in SOM rigidity at ~30?% water content became maximum after ~20?weeks, also resulting in the strongest resistance towards water infiltration. We argue that the anomalies at 30?% water content are related to the critical water content for the formation of freezable water w _crit in the peat samples, which was 26.2?±?0.3?%. Conditions for water-assisted molecular bridging were assumably optimal at 30?% water content. Whereas parameters of naphthalene-2-ol sorption reflecting the sorbed amount were mainly altered by the wetting properties of SOM, sorption linearity and hysteresis were influenced by the anomalies in peat matrix properties at a water content around 30?%.

Conclusions

The study revealed that the interplay of SOM and water led to highly variable and complex changes in SOM physicochemical properties. These properties may serve as a predictor for sorption of xenobiotics in soil at varying hydration conditions enabling a more precise assessment of the environmental fate of xenobiotics.     

黄河三角洲土地利用对土壤氮素及其转化的影响

以黄河三角洲垦利县轻度盐渍化土壤为研究对象,选取菜地、果园、粮田和新淤未利用地共4种土地利用类型,通过实地采样分析,探讨了土地利用对土壤氮素及其转化的影响。结果表明,全氮含量在0—20 cm土壤中以粮田最高,平均含量为1.42 g/kg,其次是果园、菜地和新淤未利用地土壤,其平均含量分别为1.17,0.97和0.57 g/kg,而20—40 cm土壤中,菜地全氮平均含量为0.86 g/kg,明显高于其它3种用地土壤;硝态氮在菜地0—20 cm土壤中的平均含量为27.25 mg/kg,远高于果园、粮田和新淤未利用地土壤,铵态氮在4种用地类型土壤中含量范围为2.65~4.09 mg/kg,不同用地类型间差异不大,二者在20—40cm土壤中的变化规律与0—20 cm基本相同;通过铵态氮、硝态氮与全氮的相关分析,表明菜地土壤中有效氮含量主要与外源氮素的补充有关,果园和粮田土壤中的有效氮与土壤全氮关系密切而受环境变化情况较小,新淤未利用地土壤氮素反映了研究区土壤氮素及其转化的初始状况。     

Land preparation and vegetation type jointly determine soil conditions after long-term land stabilization measures in a typical hilly catchment,Loess Plateau of China

Purpose

Land preparation (e.g., leveled ditches, leveled benches, adversely graded tableland, and fish-scale pits) is one of the most effective ecological engineering practices to reduce water erosion in the Loess Plateau, China. Land preparation greatly affects soil physicochemical properties. This study investigated the influence of different land preparation techniques during vegetation restoration on soil conditions, which remained poorly understood to date.

Materials and methods

Soil samples were collected from depths of 0–10, 10–20, 20–40, 40–60, 60–80, and 80–100 cm, in the typical hilly watershed of Dingxi City, Loess Plateau. Soil bulk density (BD), soil organic matter (SOM), and total nitrogen (TN) were determined for different land preparations and vegetation type combinations. Fractal theory was used to analyze soil particle size distribution (PSD).

Results and discussion

(1) The effect of land preparation on soil properties and PSD varied with soil depth. For each land preparation category, SOM and TN values showed a significant difference between the top soil layer and the underlying soil depths. (2) The fractal dimension of PSD showed a significant positive correlation with clay and silt content, but a significant negative correlation with sand content. (3) The 20 cm soil layer was a boundary that distinguished the explanatory factors, with land preparation and vegetation type as the controlling factors in the 0–20- and 20–100-cm soil layers, respectively.

Conclusions

Land preparation and vegetation type significantly influenced soil properties, with 20 cm soil depth being the boundary for these two factors. This study provided a foundation for developing techniques for vegetation restoration in water-limited ecosystems.

     

Previous exposure advances the degradation of an anthropogenic s-triazine regardless of soil origin

Purpose

Previous investigations—field samplings and laboratory experiments—support the hypothesis that the degradation of s-triazines is enhanced in previously exposed as compared to pristine soils in terrestrial environments. Despite this, bottlenecks of soil sampling and various soil modification practices in microcosm studies have made it difficult to guarantee that previous contamination history enhances contaminant degradation regardless of soil origin in terrestrial ecosystems. We test the hypothesis that the degradation of simazine (2-chloro-4,6-bis(ethylamino)-s-triazine) is enhanced in previously exposed soils as compared to pristine soils in 10 l buckets at the mesocosm scale.

Materials and methods

We collected soil at three separate sites consisting of a previously exposed and a pristine field. At every field, soil was collected at three separate plots and simazine degradation (days 0 and 65) and the response to atzB degrader gene primers (days 0 and 110) were followed. We analyzed the results using analysis of covariance (ANCOVA). Previous exposure and field site were assessed as fixed factors and initial simazine concentration and abiotic soil conditions as covariates.

Results and discussion

After the 65-day exposure, remaining simazine concentrations depended on previous exposure but not on collection site. The response to atzB gene primers was positive in all mesocosms where simazine degradation had been rapid. Soil moisture, pH, and organic matter content were insignificant. If soil moisture was not included in the ANCOVA model, previous exposure did not appear as a significant factor.

Conclusions

The results support the hypothesis that simazine is degraded more rapidly in previously exposed soils as compared to pristine environments, provided that degradation genes are available. Previously exposed soil might be used to enhance the degradation of simazine in recently contaminated terrestrial soils, supposing that the central requirements for microbial growth are adequate.     

Landscape and Land‐Use Effects on the Spatial Variation of Soil Chemical Properties

The current study addressed the spatial variation of soil organic matter (SOM), total nitrogen (TN), extractable phosphorus (EP), and extractable potassium (EK) in agricultural soils of a representative region, northeast China. Soil cation exchange capacity (CEC) and the effects of landscape attributes and land use were also investigated. The techniques used included conventional statistics, geostatistics, and geographic information systems (GIS). Our study demonstrated that EP had the greatest coefficient of variation (CV), and CEC had the least CV. The experimental semivariograms of the five soil chemical properties included in this study were all fitted with exponential models. The five soil variables all showed moderate spatial dependence. The SOM, EK, and CEC decreased with increasing altitude. Significant negative relationships were found between the slope gradient and EP, EK, and CEC. Relatively steeper slopes might result in greater soil erosion, which leads to a decline in soil nutrients. Soil types had significant impacts on all soil chemical properties, which reflect the effect of the parent soil material. In general, the mean values of soil variables for vegetable land were statistically greater than those for upland and paddy fields. After being divided into two parts along the Yinma River, soil samples of the western part have statistically greater SOM, EP, EK, and CEC values than those collected from the eastern part.     

Immobilization of Cu and Cd in a contaminated soil: one- and four-year field effects

Purpose

Many amendments have been applied to immobilize heavy metals in soil. However, little information is available on the changes of immobilization efficiencies of heavy metals in contaminated soils over time. This work investigated the immobilization efficiencies of copper (Cu) and cadmium (Cd) in contaminated soils in situ remediated with one-time application of three amendments for 1 year and 4 years.

Materials and methods

Apatite, lime, and charcoal were mixed with the topsoil of each plot with the amounts of 22.3, 4.45, and 66.8 t/ha, respectively. Soil chemical properties and fractions of Cu and Cd were examined after in situ remediation for 1 year and 4 years. Soil sorption and retention capacities and desorption proportions for Cu and Cd were investigated by batch experiments.

Results and discussion

The addition of amendments significantly increased soil pH, but decreased exchange acid and aluminum (Al). The amendments significantly decreased the CaCl₂ extractable Cu and Cd and transformed them from active to inactive fractions. After the application of amendments for 1 year, the maximum sorption capacities ranged from 35.6 to 38.8 mmol/kg for Cu and from 14.4 to 17.0 mmol/kg for Cd, which were markedly higher than those of the application of amendments for 4 years (Cu, 29.6–34.7 mmol/kg; Cd, 10.9–16.4 mmol/kg). Desorption proportions (D) of Cu and Cd using three extractants followed the order of \( {D}_{{\mathrm{NaNO}}_3}<{D}_{{\mathrm{CaCI}}_2}<{D}_{{\mathrm{MgCI}}_2} \) . Moreover, the retention capacities (R) of Cu and Cd both increased and followed the order of R _apatite?>?R _lime?>?R _charcoal, resulting in higher Cu and Cd in the amended soils than the untreated soil.

Conclusions

Apatite, lime, and charcoal increased the soil sorption and retention capacities of Cu and Cd and resulted in higher immobilization efficiencies in the amended soils than the untreated soil. However, the immobilization efficiencies of Cu and Cd decreased with the decrease of sorption capacities after 4 years. It was concluded that apatite had the best effect on the long-term stability of immobilized Cu and Cd and can be applied to immobilize heavy metals in contaminated soils.     

Defining soil geochemical baselines at small scales using geochemical common factors and soil organic matter as normalizers

Purpose

The establishment of geochemical baselines is essential for accurate evaluation of the present state of surface environments. In this study, normalization procedures (NP), which can improve the explanation of the natural variation of elements, were conducted using geochemical common factors (GCF) and soil organic matter (SOM) as normalizers to define the geochemical baselines of soil trace elements.

Materials and methods

Soil samples (n?=?345) were collected in Luhe County, Jiangsu, China, a county with a complex geologic setting and intensive anthropogenic influence. Conservative elements, Al, Ca, Fe, K, Mg, Mn, Na, P, and Ti; trace elements, As, Cd, Cr, Cu, Hg, Ni, Pb, and Zn; and SOM were measured. Normalization procedures were conducted using multiple linear regressions between soil trace elements and SOM and GCFs, acquired from factor analysis of the soil major elements. Normalization procedures using univariate linear regressions between soil trace elements and conservative elements Al, Fe, and Ti were also conducted for comparison.

Results and discussion

Comparison of NPs using GCFs and SOM as normalizers with NPs, which use single conservative elements as normalizers, shows that the former is more accurate than the latter for As, Pb, and Zn and is as accurate for Cd, Cr, Cu, Hg, and Ni, when the most appropriate single conservative element is chosen. Small-scale geochemical baselines in the county are significantly different from regional-scale geochemical baselines for Jiangsu Province, China.

Conclusions

The application of regional-scale geochemical baselines at small scales may lead to estimation errors in determining anomalies and assessing environments. Baselines obtained from the NPs using GCFs and SOM as normalizers are more accurate.     

Electron transfer capacity of soil dissolved organic matter and its potential impact on soil respiration

Purpose

Soil dissolved organic matter (DOM) as the labile fraction of soil organic carbon (SOC) is able to facilitate biogeochemical redox reactions effecting soil respiration and carbon sequestration. In this study, we took soil samples from 20 sites differing in land use (forest and agriculture) to investigate the electron transfer capacity of soil DOM and its potential relationship with soil respiration.

Materials and methods

DOM was extracted from 20 soil samples representing different land uses: forest (nos. 1–12) and agriculture (nos. 13–20) in Guangdong Province, China. Chronoamperometry was employed to quantify the electron transfer capacity (ETC) of the DOM, including electron acceptor capacity (EAC) and electron donor capacity (EDC), by applying fixed positive or negative potentials to a working electrode in a conventional three-electrode cell. The reversibility of electron accepting from or donating to DOM was measured by applying switchable potentials to the working electrode in the electrochemical system with the multiple-step potential technique. Carbon dioxide produced by soil respiration was measured with a gas chromatograph.

Results and discussion

Forest soil DOM samples showed higher ETC and electron reversible rate (ERR) than agricultural soil DOM samples, which may be indicative of higher humification rate and microbial activity in forest soils. The average soil respiration of forest soil (nos. 1–12) and agricultural soil (nos. 13–10) was 26.34 and 18.58 mg C g^?1 SOC, respectively. Both EDC and EAC of soil DOM had close relationship with soil respiration (p?<?0.01). The results implied that soil respiration might be accelerated by the electroactive moieties contained in soil DOM, which serve as electron shuttles and facilitate electron transfer reactions in soil respiration and SOC mineralization.

Conclusions

DOM of forest soils showed higher ETC and ERR than DOM of agricultural soils. As soil represents one of the largest reservoirs of organic carbon, soil respiration affects C cycle and subsequently CO₂ concentration in the atmosphere. As one of the important characteristics of soil DOM related to soil respiration, ETC has a significant impact on greenhouse gas emission and soil carbon sequestration but has not been paid attention to.