Variations in Soil Organic Matter Content in Cultivated and Uncultivated Calcareous Soils from the Mediterranean Island of Malta after 15 Years of Cultivation

The soils of Malta are calcareous and generally undeveloped. Organic matter (OM) in these soils is low and farmers are constantly urged to increase it. The objective of this study was to evaluate any temporal variation in soil OM after 15 years of cultivation, and determine whether soil series, soil depth, and cultivation influence variation. OM was determined in the topsoil and subsoil of 7 agricultural and 4 non-agricultural sites. The sites represented 7 different soil series that are present on the island. In sampling periods 1 (t = 0 years) and 2 (t =15 years), the OM content in the collective (all soil series) bulk (topsoil and subsoil) uncultivated soil was 3.9 % and 3.8 % respectively. This was significantly greater than that of the collective bulk cultivated soil (2.4% and 2.3%). The OM in the collective uncultivated topsoil was 5.4% and 5.2% in periods 1 and 2 and was significantly higher than that of the cultivated topsoil (2.5% in both periods). The OM content in the collective uncultivated subsoil was 2.3% and 2.5% in periods 1 and 2 respectively but only that measured in period 2 was significantly higher than that of the cultivated subsoil (2.2% in both periods). On an individual soil series basis, the OM in the uncultivated topsoils was significantly higher than that of their cultivated counterparts. The differences in the subsoils were not significant. Across the uncultivated soil series, OM was significantly higher in the topsoil than in the subsoil but in the cultivated soil series the differences between topsoil and subsoil were not significant. There was no significant difference in OM between the uncultivated soils of different series, but in the cultivated the OM content was higher in soils that were more mature. After 15 years, no significant change in OM occurred in both the collective cultivated and uncultivated bulk soils, the collective topsoil and subsoil, and in most of the individual series. The OM content of each soil series was also similar to what was reported 60 and 50 years earlier by other researchers.

Phosphorus Changes and Sorption Characteristics in a Calcareous Soil Under Long-Term Fertilization

Knowledge of phosphorus （P） behavior in long-term fertilized soils is essential for programming fertilization practices and for sustaining environmental quality. The long-term （1984-1997） effects of various fertilization treatments on P changes and sorption isotherms as well as the relationship of soil properties to P sorption and P forms were evaluated in an Ustic Isohumisol, a calcareous soil, on the Loess Plateau, China. Compared to 1984, after 13 years of crop production, total soil P in the no-P treatments （control and N treatment） decreased by 5%-7%, but in the phosphorus fertilizer alone （P）, nitrogen and phosphorus fertilizers in combination （NP）, manure alone （M）, and nitrogen and phosphorus fertilizers and manure in combination （NPM） treatments, it increased by 22%, 19%, 28%, and 58%, respectively. Residual fertilizer P was found mainly in NH4Ac-soluble P （Cas-P）, followed by NaHCO3-soluble P （NaHCO3-P）, and NH4F-soluble P （Al-P）. Phosphorus sorption in the soils with different fertilization practices fit the Langmuir equations. Phosphorus sorption capacity in the no-P treatments increased, whereas it decreased in the P-included treatments （P, NP, and NPM treatments）. Phosphorus sorption maximum （Qm） was significantly and negatively correlated to inorganic P including NaHCO3-P, Cas-P, NaOH-Na2CO3-soluble P （Fe-P）, and Al-P （P ≤ 0.01）. Moreover, long-term fertilization increased soil organic carbon in the NP, M, and NPM treatments and decreased pH in the NP and NPM treatments. Thus, the ability of the soil to release sorbed P to the environment increased under long-term P fertilization.

Crop Yield and Soil Properties in the First 3 Years After Biochar Application to a Calcareous Soil

It remains unclear whether biochar applications to calcareous soils can improve soil fertility and crop yield. A long-term field experiment was established in 2009 so as to determine the effect of biochar on crop yield and soil properties in a calcareous soil. Five treatments were： 1） straw incorporation; 2） straw incorporation with inorganic fertilizer; 3）, 4） and 5） straw incorporation with inorganic fertilizer, and biochar at 30, 60, and 90 t ha-l, respectively. The annual yield of either winter wheat or summer maize was not increased significantly following biochar application, whereas the cumulative yield over the first 4 growing seasons was significantly increased. Soil pH, measured in situ, was increased by a maximum of 0.35 units after 2 yr following biochar application. After 3 yr, soil bulk density significantly decreased while soil water holding capacity increased with adding biochar of 90 t ha^-1. Alkaline hydrolysable N decreased but exchangeable K increased due to biochar addition. Olsen-P did not change compared to the treatment without biochar. The results suggested that biochar could be used in calcareous soils without yield loss or significant impacts on nutrient availability.

Response of Tomato on Calcareous Soils to Different Seedbed Phosphorus Application Rates

Field experiments were conducted with five rates （0, 75, 150, 225, and 450 kg P205 ha^-1） of seedbed P fertilizer application to investigate the yield of tomato in response to fertilizer P rate on calcareous soils with widely different levels of Olsen P （13-142 mg kg^-1） at 15 sites in some suburban counties of Beijing in 1999. Under the condition of no P fertilizer application, tomato yield generally increased with an increase in soil test P levels, and the agronomic level for soil testing P measured with Olsen method was 50 or 82 mg kg^-1 soil to achieve 85% or 95% of maximum tomato yield, respectively. With regard to marketable yield, in the fields where Olsen-P levels were 〈 50 mg kg^-1, noticeable responses to applied P were observed. On the basis of a linear plateau regression, the optimum seedbed P application rate in the P-insufflcient fields was 125 kg P205 ha^-1 or about 1.5-2 times the P removal from harvested tomato plants. In contrast, in fields with moderate （50 〈 Olsen P 〈 90 mg kg^-1） or high （Olsen P 〉 90 mg kg^-1） available P, there was no marked effect on tomato fruit yield. Field survey data indicated that in most fields with conventional P management, a P surplus typically occurred. Thus, once the soil test P level reached the optimum for crop yield, it was recommended that P fertilizer application be restricted or eliminated to minimize negative environmental effects.

Vesicular Arbuscular Mycorrhizae-Mediated Uptake and Thanslocation of P and Zn by Wheat in Calcareous Soil

Vesicular-arbuscular mycorrhizal (VAM) fungi have been credited with improving the groWth and mineral nutrition of many host plants but these effects are moderated by soil factors and nutrient balance. The combined effects of VAM, Zn and P application on the growth and translocation of nutrients in wheat were investigated using a calcareous soil marginal in P and Zn concentrations. Wheat was grown in a growth chamber under various combinations of VAM, P and Zn with measurements done at heading stage and maturity.Vegetative dry matter accumulation was increased by P application and reduced by VAM treatments. Both P and VAM increased grain yield. Zinc concentration and uptake were generally reduced by P addition and VAM infection. There were no antagonistic effects of Zn on P acquisition in the plant. The role of VAM in enhancing the translocation of Zn and P from root to grain would be beneficial to seed setting and yield.

Effects of moisture and carbonate additions on CO_2 emission from calcareous soil during closed–jar incubation

Calcareous soil contains organic and inorganic carbon(C) pools,which both contribute to CO2 emission during closed-jar incubation. The mineralization of organic C and dissolution of inorganic C are both related to soil moisture,but the exact effect of water content on CO2 emission from calcareous soil is unclear. The objective of this experiment was to determine the effect of soil water content(air-dried,30%,70%,and 100% water-holding capacity(WHC)),carbonate type(CaCO3 or MgCO3),and carbonate amount(0.0,1.0%,and 2.0%) on CO2 emission from calcareous soil during closed-jar incubation. Soil CO2 emission increased significantly as the water content increased to 70% WHC,regardless of whether or not the soil was amended with carbonates. Soil CO2 emission remained the same or increased slowly as the soil water content increased from 70% WHC to 100% WHC. When the water content was ≤30% WHC,soil CO2 emission from soil amended with 1.0% inorganic C was greater than that from unamended soil. When the soil water content was 70% or 100% WHC,CO2 emission from CaCO3 amended soil was greater than that from the control. Furthermore,CO2 emission from soil amended with 2.0% CaCO3 was greater than that from soil amended with 1.0% CaCO3. Soil CO2 emission was higher in the MgCO3 amended soil than from the unamended soil. Soil CO2 emission decreased as the MgCO3 content increased. Cumulative CO2 emission was 3-6 times higher from MgCO3 amended soil than from CaCO3 amended soil. There was significant interaction effect between soil moisture and carbonates on CO2 emission. Soil moisture plays an important role in CO2 emission from calcareous soil because it affects both biotic and abiotic processes during the closed-jar incubation.

Acoustic-Physical Properties of Calcareous Seafloor Soils and Their Significance in Engineering Geology

The basic features and acoustic-physical properties of calcareous seafloor soils in the tropic sea area are obviously different from those of sediments mainly composed of terrigenous materials in the South China Sea. Generally calcareous soils, composed of carbonate particles of marine organism remains. have the characteristics of high water content, high porosity, low wet density, high sound velocity and greatly varied comprehensive strength. Recognizing the differences between calcareous soils and terrigenous sediments and engineering geologic significance of calcareous soils is crucial for seafloor geologic research and geotechnical survey for pile-jacket platform foundation design.

Organic matter fractions within macroaggregates in response to long-term fertilization in calcareous soil after reclamation

Soil organic carbon(SOC)plays a key role in improving soil quality and optimizing crop yield.Yet little is known about the fate of macroaggregates(>0.25 mm)under long-term fertilization and their relative importance in SOC sequestration in reclaimed calcareous soil.Therefore,the effects of mineral fertilizers and organic manure on the mechanisms of organic carbon(OC)stabilization in macroaggregates were investigated in this study.Four treatments were used:unfertilized control(CK),mineral fertilizer(NPK),compost chicken manure alone(M),and mineral fertilizers plus manure(MNPK).Samples from the 0–20 cm layer of soil receiving 11-year-long fertilization were separated into four fractions based on the macroaggregates present(unprotected coarse and fine particulate organic matter,cPOM and fPOM;physically protected intra-microaggregate POM,i POM;and biochemically protected mineral associated OM,MOM)by the physical fractionation method.Compared with the control,the long-term application of NPK had little effect on SOC content,total nitrogen(TN)content,and OC and TN contents of macroaggregate fractions.In contrast,incorporation of organic manure(MNPK)significantly increased SOC(45.7%)and TN(24.3%)contents.Application of MNPK increased OC contents within macroaggregate-extracted fractions of cPOM(292.2%),fPOM(136.0%)and iPOM(124.0%),and TN contents within cPOM(607.1%),fPOM(242.5%)and iPOM(127.6%),but not the mineral associated organic carbon(MOM-C)and nitrogen(MOM-N)contents.Unprotected C fractions were more strongly and positively correlated with SOC increase than protected C fractions,especially for cPOM-C,indicating that SOC sequestration mainly occurred via cPOM-C in the studied calcareous soil.In conclusion,MNPK increased the quantity and stability of SOC by increasing the contents of cPOM-C and cPOM-N,suggesting that this management practice(MNPK)is an effective strategy to develop sustainable agriculture.

Effects of different long-term crop straw management practices on ammonia volatilization from subtropical calcareous agricultural soil

Ammonia(NH3)volatilized from agricultural production and its secondary aerosols contribute greatly to air pollution.Different long-term crop straw management practices may significantly affect the soil fertility and soil nitrogen cycle,however,the effect on NH3 volatilization has not been well studied.Therefore,a one-year field experiment was conducted to evaluate the effect of straw incorporation on NH3 volatilization from subtropical calcareous agricultural soil from a longterm perspective,including four treatments:synthetic fertilizer(CK);synthetic fertilizer incorporation with 100%or 50%of the previous season’s crop straw(SI1 and SI2,respectively);and synthetic fertilizer incorporation with 50%burned crop straw(SI2B).Soil NH3 volatilizations were monitored through a wheat–maize rotation year by using a dynamic chamber method.The results demonstrated that NH3 volatilization primarily occurred within 38 days and 7–10 days following nitrogen fertilization events for the wheat and maize seasons,respectively.Different crop straw management practices mainly impacted the NH3 flux of the basal fertilization rather than the topdressing fertilization;long-term crop straw incorporation effectively lowered NH3 loss(35.1%for SI1 and 16.1%for SI2 compared to CK;and the inhibiting effect increased with increasing straw amount,possibly contributed by the high straw carbon/nitrogen ratio,and enhanced microbial activity,which contributed to inorganic nitrogen immobilization and lower ammonium content in the topsoil.However,SI2B significantly increased(29.9%)the annual NH3 flux compared with SI2,indicating that long-term 100%straw incorporation could be a promising straw management practice for mitigating NH3 loss and increasing soil fertility.

Transformation of Phosphate Fertilizers with Different Solubilities in Calcareous Soils

In this research work, the authors, using the recently developed method of fractionating the forms of inorganic phosphorus in calcareous soils, have studied the transformation processes of inorgnic phosphorus in three different phosphate fertilizers, i. e., superphosphate, diammonium phosphate and calcium magnesium phosphate, being commonly used in China, during a period of 3 years after their application to calcareous soils, and based on the experimental results obtained, some problems in current use of phosphate fertilizers are discussed.

Effect of Different Fertilizer Treatments on Quantity of Soil Microbes and Structure of Ammonium Oxidizing Bacterial Community in a Calcareous Purple Paddy Soil

The quantity of soil microbes and the structure of ammonium oxidizing bacterial （AOB） community were analyzed using the dilution plate counting and most probable number method （MPN）, and denaturing gradient gel electrophoresis （DGGE）, respectively. Fertilizer application tended to increase the number of soil microbes and alter the AOB community compared to the control with no fertilizer application （CK）. Among the eight fertilizer treatments, soil samples from the treatments of mineral fertilizers （e.g., N, P, K） in combination with farmyard manure （M） had greater number.s of soil microbes and more complex structure of AOB community than those receiving mineral fertilizers alone. The principal component analyses （PCA） for ammonium oxidizing bacterial community structure showed that the eight fertilizer treatments could be divided into two PCA groups （PCA1 and PCA2）. For the soil sampled after rice harvest, PCA1 included NP, NM, NPM and NPKM fertilizer treatments, while PCA2 was consisted of CK, N, M and NPK fertilizer treatments. For soil samples collected after wheat harvest, PCA1 was consisted of M, NM, NPM and NPKM fertilizer treatments, while PCA2 was composed of CK, N, NP and NPK fertilizer treatments. For a given rotation, the richness of AOB community in PCA1 was greater than that in PCA2. In addition, AOB community structure was more complex in the soil after rice harvest than that after wheat harvest. The results indicated that different fertilizer treatments resulted in substantial changes of soil microbe number and AOB community. Furthermore, mineral fertilizers （N, NP, NPK） combined with farmyard manure were effective for increasing the quantity of soil microbes, enriching AOB community, and improving the soil biofertility.

Effects of topography and vegetation on distribution of rare earth elements in calcareous soils

This study investigated the impact of topography and vegetation on distribution of rare earth elements(REEs)in calcareous soils using methods of single extraction and mass balance calculation. The purposes of the study were to set a basis for further research on the biogeochemical REE cycle and to provide references for soil–water conservation and REE-containing fertilizer amendments. The results show a generally flat Post-Archean Average Australian Shale—normalized REE pattern for the studied calcareous soils. REE enrichment varied widely. The proportion of acidsoluble phases of heavy REEs was higher than that of light REEs. From top to bottom of the studied hills, dominant REE sources transitioned from limestone in-situ weathering to input from REE-containing phases(e.g., clay minerals,amorphous iron, REE-containing fluids). Our results indicate that the REE content of calcareous soils is mainly controlled by slope aspect, while the enrichment degree of REEs is related to geomorphological position and vegetation type.Furthermore, the proportion of acid-soluble phases of REEs is mainly controlled by geomorphological position.

Free Oxide Distribution in Poorly and Well Drained Soils Developed on Calcareous Alluvial Deposits

A study on the distribution of free iron and manganese oxides was conducted in soils developed on calcareous alluvial deposits under subhumid climatic conditions, in Western Greece. Soil samples from two well drained soils and from two poorly drained soils, classified as Alfisols, were collected and used in this study. After certification of soil homogeneity the acid ammonium oxalate and dithionite-citrate-bicarbonate methods were used to extract free iron and manganese oxides from the samples. Iron oxides extracted by the dithionite-citrate-bicarbonate method (Fed) were significantly higher than the iron oxides extracted by the ammonium oxalate method (Feo), indicating that a considerable fraction is present in crystalline forms,independent of drainage status. A confirmation of free iron oxides and fine clay was detected. The ratios Feo/Fed and (Fed-Feo)/total Fe (Fet) could not be used to distinguish the well drained soils from the poorly drained soils. Manganese movement in a soluble form is independent of the fine clay.

Responses of N_2O reductase gene(nosZ)-denitrifier communities to long-term fertilization follow a depth pattern in calcareous purplish paddy soil

The effect of long-term fertilization on soil denitrifying communities was analysed by measuring the abundance and diversity of the nitrous oxide （N2O） reductase gene, nosZ. Soil samples were collected from plots of a long-term fertilization experiment established in 1982 in Suining City, China. The fertilizer treatments were no fertilizer （CK）, three chemical fertilizer （CF） treatments （N, NP, NPK）, manure （M） alone, and manure with chemical fertilizers （NM, NPM, NPKM）. The abundance and diversity of the denitrifying bacteria were assessed by real-time quantitative PCR, terminal restriction fragment length polymorphism （T-RFLP）, and cloning and sequencing of nosZ genes. The diversity and abundance of nosZ-denitrifiers was higher in soil amended with manure and chemical fertilizers （CFM） than in soil amended with CF alone, and the highest in topsoil （0-20 cm）. The nosZ-denitrifier community composition was more complex in CFM soil than in CF soil： Specific species were detected only in the CFM soil. The abundance of nosZ-denitrifier in the NPKM treatment was approximately two times higher than that in the CK, N, and NPK treatments. Most of the cloned nosZ sequences were closely related to nosZ sequences from Bradyrhizobiaceae and Rhodospirillaceae in Alphaproteobacteria. Of the measured abiotic factors, soil organic matter correlated significantly with the abundance （P〈0.01）; available phosphorus correlated significantly with the topsoil community composition （P〈0.01）, whereas soil organic matter correlated significantly with the subsoil （20-90 cm） community composition （P〈0.01）. This study demonstrated that long-term CFM fertilization affected both the abundance and composition of the nosZ-denitrifier community.

The effects of calcium combined with chitosan amendment on the bioavailability of exogenous Pb in calcareous soil

Lead(Pb) in soil may accumulate in crops and enter the human body. This study aimed to understand the speciation transformation and accumulation characteristics of exogenous Pb in calcareous soil with or without the application of soil amendments. Field experiments with continuous maize cultivation have been carried out for two years. The results showed that the contents of total Pb were slightly lower in 2016 than in 2015 for the same treatments;however, no significant difference between the years was observed. Soil Pb existed mainly in the residual fraction without exogenous Pb addition, and its proportion was more than 33% of the total soil Pb in the control and Ca treatments. When Pb was added to calcareous soil, Pb existed largely in the oxidizable and reducible fractions during the two-year experimental period, and those fractions made up over 83% of the total Pb. The proportion of the water-soluble Pb, regardless of the addition of Pb, was the lowest and was less than 0.0019% in all treatments, but the addition of Ca and chitosan amendments reduced the water-soluble and exchangeable Pb contents. The Pb content in different parts of maize followed the order root>leaf>stem>grain during the experimental period. Although maize had low transfer and enrichment factors in calcareous soil, which make the Pb content in the grain show no significant difference among the five treatments in the same year, adding Ca and chitosan to calcareous soil can reduce the Pb contents of maize, especially reduce the Pb contents of root, stem and leaf. Therefore, the addition of calcium and chitosan is an effective strategy for reducing Pb availability in calcareous soils.

Effect of Arbuscular Mycorrhiza Fungal Inoculation with Compost on Yield and Phosphorous Uptake of Berseem in Alkaline Calcareous Soil

An experiment was conducted in pots under natural conditions in alkaline calcareous soil to determine berseem (Trifolium alexandrium) yield and P uptake as affected by Arbuscular mycorrhizal fungi (AMF) inoculation with compost prepared from fresh animal dung and rock phosphate. Data indicated that berseem shoot and roots yields increased significantly (P ≤ 0.05) by inoculation of indigenous mycorrhiza (AMF-I) and half dose of compost. Shoot yield increased as 98% and 76% roots yield as 60% and 52% over control and N and K fertilizers. Maximum and significantly (P ≤ 0.05) increased plant N and P uptake by berseem was observed in the treatment inoculated by commercial mycorrhiza (AMF-II) with full dose of compost followed by the inoculation of AMF-II with half dose of compost. Plants uptake of Cu, Mn and Fe was improved significantly (P ≤ 0.05) by the inoculation of AMF-II with half dose of compost, while Zn uptake was increased in the treatment of AMF-II inoculation with full dose of compost. Maximum and significantly (P ≤ 0.05) increased soil spores density of AMF as 27 spores per 20 g soil was noted by inoculation of AMF-I with half dose of compost, while maximum roots infection intensity in berseem was observed by the inoculation of AMF-I with full dose of compost. Results suggest that inoculation of AMF with compost has potential to improve berseem yields and plants nutrients uptake under given soil conditions.

A Comparative Study of Element Cycling in the Soil-Plant System: A Case Study of Shaly and Calcareous Soils, Southern Benue Trough, Nigeria

This study focused on the cycling of major and trace elements in the soil-plant system in parts of Southern Benue Trough, Nigeria. Surface soil samples and cassava crop samples were collected from cultivated farmlands underlined by shaly and calcareous soils and were analysed using standard techniques. The results show that shaly soils are relatively acidic (pH, 4.8 - 6.6) with high level of organic matter content (OM, 3.2% - 8.7%) compared to calcareous soils (pH, 5.6 - 7.2;OM 1.6% - 7.0%). The soils are enriched in elemental composition relative to the world average abundances in soil. The maximum levels of K, Al, and Zn were obtained from shaly soils. The computed accumulation factors are generally <1. Elemental levels decreased in the plant parts in the order tuber > leaf > stem. Significant correlation was obtained between elemental associations of calcareous surface soils compared to that of shaly soils. R-mode factor analysis revealed the controls of soil geochemistry to include lithology, anthropogenic and environmental factors. A stepwise linear regression analysis identified soil elemental component, pH and organic matter as some of the factors influencing soil-plant metal uptake.

Effect of Long-Term Fertilization on Organic Nitrogen Forms in a Calcareous Alluvial Soil on the North China Plain

In order to illustrate the change of nitrogen （N） supply capacity after long-term application of manure and chemical fertilizer, as well as to properly manage soil fertility through fertilizer application under the soil-climatic conditions of the North China Plain, organic N forms were quantified in the topsoil with different manure and chemical fertilizer treatments in a 15-year fertilizer experiment in a Chinese calcareous alluvial soil. Soil total N （TN） and various organic N forms were significantly influenced by long-term application of chemical fertilizer and manure. TN, total hydrolysable N, acid-lnsoluble N, amino acid N and ammonium N in the soil increased significantly （P 〈 0.05） with increasing manure and fertilizer N rates, but were not influenced by increasing P rates. Also, application of manure or N fertilizer or P fertilizer did not significantly influence either the quantity of amino sugar N or its proportion of TN. Application of manure significantly increased （P 〈 0.05） hydrolysable unknown N, but adding N or P did not. In addition, application of manure or N fertilizer or P fertilizer did not significantly influence the proportions of different soil organic N forms.

Effect of Submergence on Iron Transformation and Phosphorus Availability in Calcareous Soils

A thermostatic incubation experiment was carried out to estimate the effects of flooding periods, stalk application and P addition on Fe transformation and P availability in calcareous soils. Submergence increased amorphous Fe, especially in the case of stalk application. The newly formed amorphous Fe with a great surface area played an important role in P sorption; and submergence also stimulated the dissolution of inorganic P, thus increasing the availability of soil P in calcareous soils. Meanwhile, a part of soluble P was adsorbed and fixed again on the surface of newly formed amorphous Fe, thus resulting in a decrease of P availability. Soil rapidly available P increased after ISO-day incubation. There existed significantly negative correlations between soil amorphous Fe content and soil Fe-P and rapidly available P contents. Submerged conditions promoted the transformation of inorganic P added to-ward Fe-P in calcareous soils, especially in the case of stalk application.