rmined for Chinese cabbage (Brassica chinensis L.), pakchoi (Brassica chinensis L.), and celery (Apiumg graveolens L.). Different Zn levels (0, 100, 200, 300, 400mg kg21 soil, supplied as ZnSO4•7H2O) were added to the soil samples, based on Zn adsorption-desorption characteristics of the soil, the availability of added Zn in the soil decreased with time, with minimal change after 10-12 weeks of incubation. The results from sand and soil culture experiments showed that shoot growth was significantly inhibited at Zn concentrations above 25mg L21 in nutrient solution or at DTPA-Zn above 170mg kg21 in the soil. The sensitivity to Zn toxicity differed among the three vegetable crops, changing in the order: celery . Chinese cabbage. pakchoi. Zinc concentration in shoots and edible parts varied with Zn supply levels and type of the vegetables. Negative correlations were noted between Zn concentrations in shoots and dry matter yields (r=0.90-0.98, P, 0.01), the critical Zn concentrations in plant tissues at 10% reduction of biomass yield (PT10) were 173.1, 167.5, 144.2 and 222.2mg kg21 (DW) for Chinese cabbage, pakchoi, celery (stem) and celery (leaf), respectively. Zinc concentrations in the edible parts were positively correlated with available and total Zn in the soil (r=0.91-0.99, P, 0.01). Based on the threshold of human dietary toxicity for Zn (20mg kg21), the critical concentrations of total and available Zn in the soil were 413 and 244mg kg21 for Chinese cabbage, 224 and 75mg kg21 for pakchoi, and 272 and 101mg kg21 for celery, respectively. These results indicate that some vegetable species like pakchoi might accumulate Zn in edible parts over human dietary toxic threshold before the dry matter yield reduction was observed.

It is well known that the availability of nutrients in red soils (equivalent to Ultisols and some of the Alfisols and Oxisols in the soil taxonomy of USA) changes after conversion of upland to irrigated rice (Oryza sativa L.) production, but long-term changes in carbon (C) and nutrients are not well documented. To characterize changes in C and nutrients in paddy fields on a Quaternary red clay (clayey, kaolinitic thermic typic plinthudults) during long-term rice cropping, we measured total C, nitrogen (N), phosphorus (P) and potassium (K), particulate organic matter (POM), N in the POM, potential mineralized N, available P, as well as other properties (pH, exchangeable cations, effective cation exchange capacity (ECEC), aggregate stability) in the plow layer (0-15cm) of 66 rice fields with rice-cultivation time ranging from 2 to 100 years. Total C, N, and P distributions were also determined in six soil profiles with rice-cultivation times of 2, 5, 19, 48, 68, and 100 years, respectively. Significant increases in organic C, total N, and P concentrations in plow layer were found in the first 30-40 years of rice cropping, accompanied by increases in available P and potential mineralized N, exchangeable Ca, Mg, Na, base saturation, and waterstable aggregates, and decreases in total K and clay content. The C/N ratio of organic matter tended to decrease in the first 20 years of rice cropping, and remained constant at approximately 10, whereas the ratio of humic acid to fulvic acid (H/F ratio) increased gradually to about 1 after 50 years of rice cropping. Long-term rice cropping elevated C, N, and P in the plow layer and increased accumulation of C, N, and P in the subsurface soils. The results indicate: (i) long-term rice cropping improved soil fertility as evidenced by neutralization of soil acidity, and increases in ECEC, organic C content, and H/F ratio; (ii) imbalance of fertilization by high N and low K, as revealed by decreased soil K and increased soil N; (iii) long-term rice cropping caused downward movement of organic C, N, and P, which may result in environmental impacts.

The microbial biomass and community structure of eight Chinese red soils with different fertility and land use history was investigated. Two community based microbiological measurements, namely, community level physiological profiling (CLPP) using Biolog sole C source utilization tests and phospholipid fatty acid (PLFA) profiles, were used to investigate the microbial ecology of these soils and to determine how land use alters microbial community structure. Microbial biomass-C and total PLFAs were closely correlated to organic carbon and total nitrogen, indicating that these soil microbial measures are potentially good indices of soil fertility in these highly weathered soils. Metabolic quotients and C source utilization were not correlated with organic carbon or microbial biomass. Multivariate analysis of sole carbon source utilization patterns and PLFAs demonstrated that land use history and plant cover type had a significant impact on microbial community structure. PLFAs showed these differences more than CLPP methods. Consequently, PLFA analysis was a better method for assessing broad-spectrum community differences and at the same time attempting to correlate changes with soil fertility. Soils from tea orchards were particularly distinctive in their CLPP. A modified CLPP method, using absorbance readings at 405nm and different culture media at pH values of 4.7 and 7.0, showed that the discrimination obtained can be influenced by the culture conditions. This method was used to show that the distinctive microbial community structure in tea orchard soils was not, however, due to differences in pH alone.

To evaluate muck sediments as a potential soil amendment, total and Mehlich III-extractable concentrations of Cd, Cu, Cr, Ni, Pb, Zn, and Co in 59 muck sediment samples from the St. Lucie Estuary were analyzed. A seven-step chemical fractionation procedure was used to assess the potential mobility of heavy metals. Except for Cd, the average total concentrations of the metals are lower than the reported average concentrations of these elements in municipal composts in the U.S.A. The concentrations were also below critical levels for the safe use of wastes and byproducts in agriculture, as established by the United States Environmental Protection Agency. The Cd, Cu, Cr, Ni, Pb, Zn and Co in the sediments were predominantly associated with silicate minerals in the residual form. Most metals in the muck sediments occur predominantly in weakly mobile or nonbioavailable forms. Use of mucks in neutral pH upland soils should not pose any significant hazards or risk to the environment. However, Cd, Cu, Cr, Ni, Pb, Zn, and Co, especially Zn, Cu, and Pb, could be more readily released from the muck sediments under acidic soil conditions.

Information on phosphorus (P)-solubilizing microorganisms in variable charge soil is lacking. We screened soil microorganisms that can effectively utilize P adsorbed on variable charge minerals using a series of synthetic media of decreased P availability. Rhizospheric soil (Orthic plintaqualt) from a 30-y old tea plant was diluted and aseptically inoculated to a series of media containing P adsorbed on goethite at 0, 255, 50, 75, and 100% saturation. Microorganisms, which survived in the 25% P sorption saturation medium, were each by colony isolates transferred to another freshly prepared medium of the same type for growth stability test. Microbial species, which could grow and reproduce for more than 15 generations in the 25% P-saturation medium were considered as P-solubilizing microbes (PSMs) and used for identification tests. By this procedure, we screened one PSM population which was identified as the bacterium Moraxella sp by Gram staining, Gamma culture and staining, optical and electron microscopic observation, and enzyme-oxidizing reaction. The PSMs have higher maximal transport rates to P than the ubiquitous soil microorganisms and have an optimal temperature of 378C and optimal pH of 5.5-7.5 for growth.