Soil fumigation effects on growth and phosphorus uptake by corn

Abstract

Phosphorus uptake rate, plant top weight, and grain yield were measured for corn (Zea mays L.) planted in field plots with or without fumigation, with different levels of P fertilization, and under conventional till and no till management. Plant growth and grain yield in P‐deficient, unfumigated conventional till and no till plots were significantly higher than those in the corresponding fumigated plots. The heightened responses were attributed to enhancement of P uptake by biological activity. Therefore, the effect of differential microblal activity on P uptake must be considered in estimating the lowest level of P fertilization needed for adequate growth in low P soils.     

Soil organic matter effects on plant available and water soluble phosphorus

The degree of phosphorus saturation (DPS) has been shown to be a suitable initial indicator of P loss potential from agricultural soils to surface waters. In addition, several agronomic soil tests have been evaluated as environmental predictive tools. The objectives of this study were: (1) to evaluate the modified Morgan soil test, used on acid, high-aluminum soils of the northeastern United States, as a predictor of water-soluble P and (2) to determine the effect of soil organic matter (SOM) on the ability of both DPS and soil test P to predict water-soluble P. The soils were divided into three SOM classifications depending on their loss-on-ignition contents and analyzed for water-soluble P, modified Morgan soil test P, and oxalate-extractable P, Al, and Fe. The relationship between DPS and water-soluble P showed a change point at about 15% DPS, as did the relationship between DPS and modified Morgan soil test P. A DPS of 15% corresponded to about 14 mg kg⁻¹, a threshold above which water-soluble P could be expected to increase more rapidly with additional P loading. The slopes of the regression lines of water-soluble P as a function of soil test P were 0.050, 0.036, and 0.021 (mg water-soluble P kg⁻¹ soil/mg soil test P kg⁻¹) for the low, medium, and high SOM classification groups, respectively. SOM level had a less significant effect on the relationship between DPS and water-soluble P. Higher levels of SOM were associated with higher levels of oxalate-extractable Fe and Al and, therefore, higher P sorption capacities and lower DPS values, resulting in less P in solution at all soil test levels.     

Antagonistic effects of root zone temperature and iron on phosphorus uptake by bush bean

Radiophosphorus (³²P) and hydroponically‐grown bush bean were used to study P absorption kinetics as affected by root zone temperature (RZT) and iron (Fe). Phosphorus uptake increased significantly (p < 0.05) at each successive 10°C rise in RZT from 15 to 35°C, and quadratic regression equations were highly correlated (R² = 0.98) between the uptake amount and exposure time. An estimated Q₁₀ value of 1.5, and the corresponding Arrhenius constant μ of 31 kJ/mol for P uptake were within range for a diffusion process in an aqueous solution. A concentration of 5 μmol/L Fe significantly (p < 0.05) reduced P uptake from 6 h on at the RZTs studied, and Q10 and μ values were estimated at 1.2 and 14 kJ/mol, respectively.     

Kinetics of microbial phosphorus uptake in cultivated soils

Knowledge about the role of microorganisms in P cycling at conditions of constant soil respiration rates and constant size of microbially bound P is lacking. To study the kinetics of microbial P uptake and cycling under such conditions, soils differing in biological activity were ³³PO₄ labelled by introducing a carrier-free tracer solution and incubated for 56 days. The ³³PO₄ incorporation into the fraction of microbial P releasable by chloroform treatment (P_chl) was assessed and the isotopic composition [=specific activity (SA); SA=³³PO₄/³¹PO₄] of P_chl and soil solution P compared. Soils were taken from a 20-year-old field experiment including a non-fertilised control (NON), a minerally fertilised conventional (MIN) and two organic farming systems [bio-organic (ORG); bio-dynamic (DYN)]. Tracer P incorporation continuously increased during incubation in DYN, ORG and MIN soils. It decreased in the order DYN>ORG>MIN, with differences in ³³PO₄ uptake between the farming systems being higher than suggested by the differences in the amount of P_chl. In the P-deficient NON soil, the highest initial incorporation of tracer P was found, but no additional uptake could be detected thereafter. In all soils, the SA of P_chl converged to the SA of the soil solution with increasing time. Since P_chl remained almost constant during the experiment, the findings suggest an intensive uptake of P from the soil solution into P_chl and concomitant release of P back to the soil solution and, thus, a rapid cycling through P_chl. Intensive P cycling between P_chl and the soil solution was confirmed in an additional experiment where microbial activity was stimulated by glucose and N additions.     

Chloroform fumigation and the release of soil nitrogen: The effects of fumigation time and temperature

Fumigation with CHC1₃ (24 h, 25°C) increased the amount of NH₄-N and total N extracted by 0.5 M K₂SO₄ from two soils (one arable, one grassland). The amount of N released by CHC1₃ increased with the duration of fumigation up to 5 days, when it levelled off. Between about 10–34% of the total N released by CHC1₃ was in the form of NH₄-N, the proportion increasing with duration of exposure.When a grassland soil that had received a field application of ¹⁵N-labelled fertilizer 1 yr previously was fumigated, the N released by CHC1₃ was 4 times more heavily labelled than the soil N as a whole. Prolonging the exposure of this soil to CHC1₃ increased the amount of total N released, but hardly altered the proportion of labelled N in the CHC1₃-released N, suggesting that N is being released from a single soil fraction. The most likely soil fraction is the soil microbial biomass. It is suggested that CHC1₃ does not alter the K₂SO₄-extractability of soil-N fractions other than microbial N and that the extra N released by CHC1₃ and extracted by K₂SO₄ gives a direct measure of soil microbial biomass N.In contrast to fumigation done at lower temperatures, less total N was released by soil fumigated at 60°C, or above, than was released from unfumigated soil held at the same temperature. The greater release of N in the non-fumigated soils above 60°C could have been due to soil enzymic processes which were inhibited by CHC1₃ in the fumigated soil.     

Soil properties and N application effects on microbial activities in two winter wheat cropping systems

The application of mineral N fertilizers may influence biologically mediated processes that are important in nutrient transformations and availability. This study was conducted to assess the effect of N application on microbial activities in irrigated and non-irrigated winter wheat systems. Carbon decomposition and microbial biomass C in soils with three N application rates (0, 150, and 300 kg N ha^–1 as urea) were measured over 40 days in a laboratory incubation experiment. Carbon, N, and P contents in the soil under the irrigated wheat were higher than those in the soil under the non-irrigated wheat. The reverse trend was observed for soil pH and Ca and Mg contents. However, soils from the two systems had similar C/N ratios. Carbon decomposition and microbial biomass C in the soil under the irrigated wheat increased significantly (p <0.05). Increasing rates of N fertilizer resulted in higher C decomposition and microbial biomass C levels in both soil systems. Results indicate that different wheat cropping systems affect soil properties that will then have an impact on C turnover in the soil. Moreover, the irrigated wheat system favors soil conditions required for a faster C turnover. In conclusion, it is likely that due to positive effects on microbial activity, N fertilization will increase nutrient cycling and, subsequently, crop productivity will improve in N-poor soils.     

Magnesium influence on plant uptake of phosphorus in a calcareous soil

A pot experiment was conducted to investigate the role of magnesium (Mg) in plant utilization of ³²phoshorus (P)‐labelled P in a calcareous soil (Typic Hapluquent). Results with two successive harvests of ryegrass shoots indicated that Mg had no obvious influence on plant uptake of the P applied. In a separate incubation study with the same soil, labelled P along with different levels of Mg was introduced to the soil at two time intervals and soil inorganic P was fractionated. The isotope data of P in various fractions indicated that Mg at the test levels had no obvious influence on P transformation either. It is suggested from the experiments that Mg is unlikely to be able to promote plant utilization of fertilizer P in calcareous soils.     

Soil phosphorus availability and rice phosphorus uptake in paddy fields under various agronomic practices

Agronomic practices affect soil phosphorus(P) availability, P uptake by plants, and subsequently the efficiency of P use. A field experiment was carried out to investigate the effects of various agronomic practices(straw incorporation, paddy water management, nitrogen(N) fertilizer dose, manure application,and biochar addition) on soil P availability(e.g., soil total P(STP), soil available P(SAP), soil microbial biomass P(SMBP), and rice P uptake as well as P use efficiency(PUE)) over four cropping seasons in a rice-rice cropping system, in subtropical central China. Compared to the non-straw treatment(control,using full dose of chemical N fertilizer), straw incorporation increased SAP and SMBP by 9.3%–18.5% and 15.5%–35.4%, respectively;substituting half the chemical N fertilizer dose with pig manure and the biochar application increased STP, SAP, and SMBP by 10.5%–48.3%, 30.2%–236.0%, and 19.8%–72.4%,respectively, mainly owing to increased soil P and organic carbon inputs;adding a half dose of N and no N input(reduced N treatments) increased STP and SAP by 2.6%–7.5% and 19.8%–33.7%, respectively, due to decreased soil P outputs. Thus, soil P availability was greatly affected by soil P input and use. The continuous flooding water regime without straw addition significantly decreased SMBP by 11.4% compared to corresponding treatments under a mid-season drainage water regime. Total P uptake by rice grains and straws at the harvest stage increased under straw incorporation and under pig manure application, but decreased under the reduced N treatments and under biochar application at a rate of 48 t ha-1, compared to the control. Rice P uptake was significantly positively correlated with rice biomass, and both were positively correlated with N fertilizer application rates, SAP, SMBP, and STP. Phosphorus use efficiency generally increased under straw incorporation but decreased under the reduced N treatments and under the manure application(with excessive P input), compared to the control. These results showed that straw incorporation can be used to increase soil P availability and PUE while decreasing the use of chemical P fertilizers. When substituting chemical fertilizers with pig manure, excess P inputs should be avoided in order to reduce P accumulation in the soil as well as the environmental risks from non-point source pollution.     

Effects of methyl bromide fumigation,chloropicrin fumigation and steam sterilization on soil nitrogen dynamics and microbial properties in a pot culture experiment

Abstract

The effects of steam sterilization (SS), methyl bromide (MeBr) fumigation and chloropicrin (CP) fumigation on soil N dynamics and microbial properties were evaluated in a pot experiment. All disinfection treatments increased the NH⁺ ₄-N level and inhibited nitrification. The additional NH⁺ ₄-N in the CP treatment probably originated from the decomposition of microbial debris by surviving microbes, while that in the SS treatment was attributable to deamination processes of soil organic N occurring in a less labile fraction in addition to the decomposition of microbial debris. The MeBr fumigation increased the level of NH⁺ ₄-N without changing the soil microbial biomass. Based on the determinations of soil microbial biomass, substrate utilization activity (Biolog method) and microbial community structure (phospholipid fatty acid method), the effects of the MeBr, CP and SS treatments on the microbial community were compared. The MeBr fumigation had relatively mild and short-term effects on microbial biomass and activity, but altered the community structure drastically by promoting the growth of gram-positive bacteria. The CP fumigation had large and long-term impacts on microbial biomass and activity; the community structure remained unaffected except for the gram-negative bacteria. Steam sterilization had severe and persistent effects on all parameters. The severity of the effects decreased in the order SS ≥ CP > MeBr.     

Soil temperature affects element uptake by sorghum

The influence of soil temperature on nutrient accumulation in aerial portions of sorghum plants was evaluated in a greenhouse experiment. Plants were grown in 20‐liter containers at cooled and ambient soil temperatures of 20 and 25C, respectively, and were harvested at the 8‐ and 12‐leaf stages of development for yield and nutrient analysis.

At the 8‐leaf stage, sorghum plants subjected to 25C were significantly higher in concentration of N, P, K, Mg, and Cu, but were significantly lower in Ca. Soil temperature did not significantly affect concentration of Zn, Fe, and Mn. At the 12‐leaf stage, sorghum plants grown in the warm soil temperature treatment were lower in concentration of N, K, Ca, Mg, Zn, Fe, Mn, and Cu than plants grown in the cooled‐soil treatment. Phosphorus showed a negative response to increased temperature.

It was concluded that further research relating element uptake and translocation to temperature is needed. Element accumulation in the roots, stems, leaves, and floral and seed portions of the plant should be included. In addition, the interaction between plant age and element concentration should be studied more thoroughly. Both this study and the published literature indicate that this interaction is significant for many of the elements.     

Soil microbial biomass phosphorus as an indicator of phosphorus availability in a Gleyic Andosol

Abstract

The relationship between plant phosphorus (P) uptake and soil microbial biomass phosphorus (biomass P) or available phosphorus (Truog P) was estimated in a Gleyic Andosol in Sapporo, Hokkaido, in a 4-year field trial (2004–2007). Every year, the soil was treated in duplicate (each plot 36?m²) or triplicate (each plot 24?m²) with chemical fertilizer, cow manure compost or sewage sludge compost, and then kidney beans (Phaseolus vulgaris) were planted. Pooled data of the shoot content of P at harvest over the 4?years was significantly correlated with biomass P determined 1?month after the application of fertilizer (P?<?0.01). A multivariate analysis revealed that the grain yield was significantly positively correlated with the shoot content of P (P?<?0.01) and significantly negatively correlated with the shoot content of calcium (P?<?0.05), but not correlated with the shoot content of either nitrogen or potassium. These results suggest that P is the most limiting element to affect the productivity of kidney bean plants in this trial and that biomass P is an important P source that explains the differences in P availability among soil amendments. Biomass P is a better indicator of P availability for kidney beans grown in Gleyic Andosols compared with Truog P, which is widely used in Japan.     

Soil amelioration effects on boron uptake by sunflowers

Abstract

Results of field trials with sunflowers grown on an Ultisol over three seasons indicated that applications of lime and gypsum had little effect on the boron concentration in plant tissues. Only when 30 kg borax/ha/annum had been applied did soil amelioration slightly but significantly decrease the B concentration in 1‐month‐old sunflower seedlings. Liming for three seasons significantly decreased the B concentration in the topmost mature leaf at flowering only when 30 kg borax/ha/annum had been applied and the pH (N KCl) was 4.4 and above. A pot experiment with the same soil was designed to test whether high pH levels affected B uptake. The reduction of toxic aluminum markedly increased top and root growth but higher rates of line had no further benefit. Liming significantly decreased the B concentration in seedling tope but the total B content of the tops was increased by liming. It appeared that liming to pH (N KCl) 7.0 did not adversely affect the uptake and translocation of B by sunflower roots, and that the decreased B concentration in seedling tops resulted from a dilution effect due to the benefit from liming.     

Soil microbial biomass C:N:P stoichiometry and microbial use of organic phosphorus

Microbial mineralization and immobilization of nutrients strongly influence soil fertility. We studied microbial biomass stoichiometry, microbial community composition, and microbial use of carbon (C) and phosphorus (P) derived from glucose-6-phosphate in the A and B horizons of two temperate Cambisols with contrasting P availability. In a first incubation experiment, C, nitrogen (N) and P were added to the soils in a full factorial design. Microbial biomass C, N and P concentrations were analyzed by the fumigation-extraction method and microbial community composition was analyzed by a community fingerprinting method (automated ribosomal intergenic spacer analysis, ARISA). In a second experiment, we compared microbial use of C and P from glucose-6-phosphate by adding ¹⁴C or ³³P labeled glucose-6-phosphate to soil. In the first incubation experiment, the microbial biomass increased up to 30-fold due to addition of C, indicating that microbial growth was mainly C limited. Microbial biomass C:N:P stoichiometry changed more strongly due to element addition in the P-poor soils, than in the P-rich soils. The microbial community composition analysis showed that element additions led to stronger changes in the microbial community in the P-poor than in the P-rich soils. Therefore, the changed microbial biomass stoichiometry in the P-poor soils was likely caused by a shift in the microbial community composition. The total recovery of ¹⁴C derived from glucose-6-phosphate in the soil microbial biomass and in the respired CO₂ ranged between 28.2 and 37.1% 66 h after addition of the tracer, while the recovery of ³³P in the soil microbial biomass was 1.4–6.1%. This indicates that even in the P-poor soils microorganisms mineralized organic P and took up more C than P from the organic compound. Thus, microbial mineralization of organic P was driven by microbial need for C rather than for P. In conclusion, our experiments showed that (i) the microbial biomass stoichiometry in the P-poor soils was more susceptible to additions of C, N and P than in the P-rich soils and that (ii) even in the P-poor soils, microorganisms were C-limited and the mineralization of organic P was mainly driven by microbial C demand.     

Soil phosphorus and tree cover modify the effects of livestock grazing on plant species richness in Australian grassy woodland

The effects of grazing on the richness of understorey plant communities are predicted to vary along gradients of resources and tree cover. In temperate Australia livestock management has involved phosphorus addition and tree removal but little research has examined how the effects of grazing on plant species richness may vary with these management regimes. Patterns of understorey plant species richness were examined in 519, 0.09 ha quadrats in grazed pastures and remnant grassy forests and woodlands in southern Australia. Sheep grazing was the primary land use and sites varied widely in grazing frequency and density, tree cover and phosphorus fertiliser history. Using an information theoretic approach the available data provides strong evidence that the effect of grazing on total species richness varies according to available phosphorus and tree cover. Intermittent grazing and no grazing were associated with high total and native plant richness, but only at low phosphorus concentrations. Phosphorus was strongly negatively correlated with richness, particularly at low grazing frequency. Total species richness was positively correlated with tree cover except under frequent grazing at high stocking rates, suggesting that heavy grazing eliminates spatial and temporal heterogeneity imposed by trees. Native plant species richness was negatively correlated with a history of cultivation, positively correlated with tree cover and varied according to landscape position and geological substrate. Frequent high density grazing, particularly when associated with clearing, cultivation and fertiliser addition, was associated with the persistence of very few native plant species. In contrast, the richness of exotic plant species was relatively invariant and performance of the best model was low. While several studies have highlighted the importance of the grazed and cleared matrix for the conservation of native plant species, this benefit may be limited in landscapes where intensive grazing management systems dominate. Strong evidence for interactions between grazing, phosphorus and tree cover suggest that failure to consider other land use practices associated with grazing management systems could lead to erroneous conclusions regarding vegetation responses to livestock grazing.     

Sorghum genotype differences in phosphorus uptake rate and distribution in plant parts

Relatively low amounts of the phosphorus (P) added to soils is recovered by plants. Many plants show differences in their ability to take up and use P, but the mechanisms for these differences are not fully understood. The purpose of this study was to determine differences among sorghum [Sorghum bicolor (L.) Moench] genotypes for P uptake rates and distribution in plant parts.

Differences in P uptake rates were determined for six sorghum genotypes at 24, 38, and 52 days of age at three P levels. Larger differences were noted among genotypes in 24‐day‐old plants than for older plants. Uptake rates were 6‐ to 14‐times higher (dependent on genotype) in 24‐day‐old plants than in 52‐day‐old plants. NB9040 which had the highest dry matter yield at each age had the lowest rate of P uptake, and CK60‐Korgi which had the lowest dry matter yield at each age had the highest rate of P uptake.

Only small differences were noted among genotypes for distribution of P within plant parts for younger plants. Older plants showed differences in P distribution, and NB9040 translocated more P from lower to upper leaves, had higher efficiency ratios (dry matter produced/unit P), and had a larger root system than CK60‐Korgi.

The sorghum genotypes that produced more dry matter under low P conditions had lower uptake rates of P and had the ability to distribute P from older to younger developing tissues. When grown in soils, plants that have lower P uptake rates, greater ability to distribute P, and larger root systems may not deplete P from soil solutions as rapidly, could explore more soil, and possibly use P more efficiently than plants that do not possess these traits.     

Soil microbial, fungal, and nematode responses to soil fumigation and cover crops under potato production

Sodium N-methyldithiocarbamate (metam sodium) and 1,3 dichloropropene are widely used in potato production for the control of soil-borne pathogens, weeds, and plant parasitic nematodes that reduce crop yield and quality. Soil fumigation with metam sodium has been shown in microcosm studies to significantly reduce soil microbial populations and important soil processes such as C and N mineralization. However, few published data report the impact of metam sodium on microbial populations and activities in potato production systems under field conditions. Fall-planted white mustard (Brassica hirta) and sudangrass (Sorghum sudanense) cover crops may serve as an alternative to soil fumigation. The effect of metam sodium and cover crops was determined on soil microbial populations, soil-borne pathogens (Verticillium dahliae, Pythium spp., and Fusarium spp.), free-living and plant-parasitic nematodes, and C and N mineralization potentials under potato production on five soil types in the Columbia Basin of Eastern Washington. Microbial biomass C was 8–23% greater in cover crop treatments compared to those fumigated with metam sodium among the soil types tested. Replacing fumigation with cover crops did not significantly affect C or N mineralization potentials. Cumulative N mineralized over a 49-day laboratory incubation averaged 18 mg NO₃-N kg⁻¹ soil across all soil types and treatments. There was a general trend for N mineralized from fumigated treatments to be lower than cover-cropped treatments. Soil fungal populations and free-living nematode levels were significantly lowered in fumigated field trials compared to cover-cropped treatments. Fumigation among the five soil types significantly reduced Pythium spp. by 97%, Fusarium spp. by 84%, and V. dahliae by 56% compared to the mustard cover crop treatment. The percentage of bacteria and fungi surviving fumigation was greater for fine- than coarse-textured soils, suggesting physical protection of organisms within the soil matrix or a reduced penetration and distribution of the fumigants. This suggests the potential need for a higher rate of fumigant to be used in fine-textured soils to obtain comparable reductions in soil-borne pathogens.     

A mass-balance approach to measuring microbial uptake and pools of phosphorus in nutrient-amended soils

Soil microbial biomass P is usually determined through fumigation-extraction (FE), in which partially extractable P from lysed biomass is converted to biomass P using a conversion factor (K_p). Estimation of K_p has been usually based on cultured microorganisms, which may not adequately represent the soil microbial community in either nutrient-poor or in altered carbon and nutrient conditions following fertilisation. We report an alternative approach in which changes in microbial P storage are determined as the residual in a mass balance of extractable P before and after incubation. This approach was applied in three low-fertility sandy soils of southwestern Australia, to determine microbial P immobilisation during 5-day incubations in response to the amendment by 2.323 mg C g⁻¹, 100 μg N g⁻¹ and 20 μg P g⁻¹. The net P immobilisation during the amended incubations determined to be 18.1, 14.1 and 16.3 μg P g⁻¹ in the three soils, accounting for 70.6-90.5% of P added through amendment. Such estimates do not rely on fumigation and K_p values, but for comparison with the FE method we estimated ‘nominal’ K_p values to be 0.20-0.31 for the soils under the amended conditions. Our results showed that microbial P immobilisation was a dominant process regulating P concentration in soil water following the CNP amendment. The mass-balance approach provides information not only about changes in the microbial P compartment, but also about other major P-pools and their fluxes in regulating soil-water P concentrations under substrate- and nutrient-amended conditions.     

多糖微生物菌液对油菜吸收养分和土壤氮磷淋失的影响

为探究富含多糖微生物菌液和化肥混合使用对植物吸收养分和控制土壤氮磷淋失的影响,采用油菜盆栽试验方法,把一定体积微生物菌液和不同量化肥进行混合灌溉油菜,油菜收获后,测定油菜和上下两层土壤的理化性质。结果表明,T5 处理(50 mL 微生物菌液,1.2 g 尿素和0.72 g 磷酸二氢钠混合加入到4.5 kg 土壤中)油菜生物量最大,油菜对氮、磷、钾具有较高同化吸收能力;随着化肥用量的增加,上层土壤有效磷含量增加缓慢,下层土壤有效磷含量与对照没有显著性差异,磷的垂直淋失风险较小;当化肥用量低于T5 处理用量时,上层土壤铵态氮含量高于对照,而土壤硝态氮含量低于对照,下层土壤铵态氮和硝态氮含量与对照没有显著性差异,说明氮淋失量较小;化肥用量高于T5 处理用量时,与对照相比,上层土壤硝态氮和铵态氮含量显著增加,下层硝态氮增加不显著,但是土壤铵态氮显著增加,提高了氮垂直淋失风险。加入微生物菌液可以活化土壤中的钾元素,有利于油菜对钾的吸收。研究表明,微生物菌液与适量化肥混合使用,有助于化肥减量和油菜不减产,不但对土壤氮、磷淋失具有一定抑制作用,还可促进油菜对土壤中钾的吸收。     

Drying method effects on extractable phosphorus levels in plant tissue

Abstract

The level of soluble tissue phosphorus (PO₄‐P) may be correlated with the plant P nutritional status, but the amount extracted depends upon dry matter losses or the amount of enzymatic or heat—induced hydrolysis of organic P compounds during sample drying.

Alfalfa (Medieago sativa L) and sugarbeet (Beta vulgaris L) plant parts grown under low and high soil P conditions, were freeze‐dried or oven‐dried at 40, 56, 70, or 100 C. Total K, P, and 0.35 N acetic‐acid‐soluble P (measured as PO ‐P) were determined. Dry matter losses were 0, 6.5, 3.6, 5.5, and 4.9 percent for the respective drying methods. The total‐P values, once corrected for dry matter losses, were not affected by the drying methods. The corrected PO₄‐P values were 0.15, 0.17, 0.16, 0.16, and 0.19 percent, while the (PO₄‐P)/P values were 0.63, 0.69, 0.67, 0.68, and 0.78 for each of the respective drying methods.

The potential utility of extractable PO₄‐P in describing the plants’ P nutritional status will depend upon rigorous sampledrying techniques. Freeze‐drying was the most satisfactory method tested, since it resulted in the least dry‐matter loss and least organic P hydrolysis.     

Impact of soil fumigation practices on soil nematodes and microbial biomass

This study was designed to understand the impact of methyl bromide (MB) (CHaBr) and its alternatives on both free-living and root-knot nematodes in the soil. A randomized complete block experiment with six treatments and 4 replicates (each replicate in a separate greenhouse) was established in Qingzhou, Shandong Province, China. In addition to MB and untreated control (CK) treatments there were four alternative soil fumigation practices including MB virtually impermeable films (VIF), metam sodium (MS), MS VIF and soil solarization combined with selected biological control agents (SS BCA). Two tomato (Lycopersicum esculentum Mill.) cultivars, cv. Maofen-802 from the Xian Institute of Vegetable Science, China, and cv. AF179 Brillante from the Israeli Hazera Quality Seeds, were selected as test crops. The results indicated that Rhabditidae was the most dominant population with percentage abundance as high as 85% of the total number of identified free-living nematodes, followed by that of Cephalobidae. Methyl bromide and its alternatives except for the non-chemical SS BCA treatment controlled the target pest, root-knot nematodes. Also, the impact of the three chemical alternatives on free-living nematode number and functional group abundance was similar to the impact associated with a typical methyl bromide application. Chemical fumigation practices, especially that with MB, significantly reduced the number of nematodes in the soil and simultaneously significantly reduced the number of nematode genera thereby reducing nematode diversity. All the four soil chemical fumigation activities decreased soil microbial biomass and had an obvious initial impact on microorganism biomass. Furthermore, both plant-parasitic and fungivore nematodes were positively correlated with soil microbial biomass.