Sedum alfredii Hance has been identified as a new zinc (Zn) hyperaccumulating plant species. In this study, the effects of cadmium (Cd) supply levels (control, 12.5, 25, 50, 100, 200, 400, 800 μmol ...Cd L-1) on the growth and cadmium accumulation and Zn supply on Cd accumulation in S. alfredii Hance were studied. The results showed that no reduction in shoot and root dry matter yields were noted when the plants were grown at Cd supply levels up to 200 μmol L-1 in nutrient solution. Slight stimulation on shoot growth was noted at relatively low Cd levels (25 to 100 μmol L-1). Cadmium concentrations in leaves and stems increased with increasing Cd supply levels, and reached a maximum of approximately 9000 and 6500 mg kg-1 (DW) at 400 μmol Cd L-1, respectively. Root Cd concentration increased sharply only at relatively high Cd levels. Cadmium distribution in different parts of the plant was in the order: leaf > stem >> root. The amount of Cd accumulated in the shoots reached 2.9 and 3.2 mg plant-1 at external Cd levels of 200 and 400 μmol L-1, respectively. The shoot/root Cd ratios were greater than 2 and more than 95% of the total Cd taken up by S. alfredii was translocated to the shoots at the external Cd levels ≤200 μmol L-1. The concentrations of P, Ca, Mg, B, Fe, Mn, Cu, and in the shoots and roots were influenced differentially by Cd treatments. High Zn supply (500 μmol L-1) enhanced Cd concentrations in the leaves and stems at the Cd levels ≤100 μmol L-1, and root Cd concentration at the Cd levels ≤50 μmol L-1. These results indicate that S. alfredii has an extraordinary ability to tolerate and hyperaccumulate Cd and this is the first report of the new Cd hyperaccumulator S. alfredii Hance. The finding of Cd/Zn hyperaccumulation in S. alfredii Hance provides an important plant material for understanding the mechanisms of Cd/Zn co-hyperaccumulation and for phytoremediation of the heavy metal contaminated soils.
Phosphorus losses by surface runoff from agricultural lands have been of public concern due to increasing P contamination to surface waters. Five representative commercial citrus groves (C1–C5) ...located in South Florida were studied to evaluate the relationships between P fractions in soils, surface runoff P, and soil phosphatase activity. A modified Hedley P sequential fractionation procedure was employed to fractionate soil P. Soil P consisted of mainly organically- and Ca/Mg-bound P fractions. The organically-bound P (biological P, sum of organic P in the water, NaHCO
3 and NaOH extracts) was dominant in the acidic sandy soils from the C2 and C3 sites (18% and 24% of total soil P), whereas the Ca/Mg-bound P (HCl-extractable P) accounted for 45–60% of soil total P in the neutral and alkaline soils (C1, C4 and C5 soils). Plant-available P (sum of water and NaHCO
3 extractable P fractions) ranged from 27 to 61
mg
P
kg
−1 and decreased in the order of C3>C4>C1>C2>C5. The mean total
P concentrations (TP) in surface runoff water samples ranged from 0.51 to 2.64
mg
L
−1. Total P, total dissolved P (TDP), and PO
4
3−-P in surface runoff were significantly correlated with soil biological P and plant-available P forms (
p<0.01), suggesting that surface runoff P was directly derived from soil available P pools, including H
2O– and NaHCO
3– extractable inorganic P, water-soluble organic P, and NaHCO
3- and NaOH-extractable organic P fractions, which are readily mineralized by soil microorganisms and/or enzyme mediated processes. Soil neutral (55–190
mg phenol kg
−1
3
h
−1) and natural (measured at soil pH) phosphatase activities (77–295
mg phenol kg
−1 3
h
−1) were related to TP, TDP, and PO
4
3−-P in surface runoff, and plant-available P and biological P forms in soils. These results indicate that there is a potential relationship between soil P availability and phosphatase activities, relating to P loss by surface runoff. Therefore, the neutral and natural phosphatase activities, especially the natural phosphatase activity, may serve as an index of surface runoff P loss potential and soil P availability.
Heavy metal accumulation in agricultural soils increases export potential of the metals to the environment. The concentrations of dissolved heavy metals (Cd, Co, Cr, Cu, Fe, Ni, Pb, Zn, Mn, and Mo) ...in surface runoff were monitored over a 2-yr period at 11 sites of vegetable farms and citrus groves in Florida. A total of 1277 surface runoff samples were analyzed for dissolved metals and extractable metals in the surface soils of each field site were determined. Concentrations of the metals in the runoff ranged widely from nondetectable level to 2.80, 18.5, 14.1, 1475, 9227, 39.3, 30.4, 1401, 2118, and 15.0 microgram L-1 for Cd, Co, Cr, Cu, Fe, Ni, Pb, Zn, Mn, and Mo, respectively. Spatial and temporal variations in the concentrations of heavy metals and runoff discharge were noted among the different sites. Ninety-four, 96, 55, 32, 93, and 61% of the samples had metal concentrations below the detection limits for Cd, Co, Cr, Ni, Pb, and Mo, respectively, whereas 0.62, 30, and 23% of the samples had Cu, Fe, and Mn higher than their drinking water standards. Annual loads of dissolved metals in the runoff varied widely among monitoring sites and were different between the year 2001 and 2002. The concentrations of heavy metals in the surface runoff were associated with the accumulation of the metals in the soils. The 0.01 M CaCl2 extractable Cu, Fe, Zn, and Mn in soil were found to significantly correlate with Cu, Fe, Zn, and Mn concentrations in the surface runoff.
The impact of heavy metal contamination on crop growth and water quality has become a public concern in southern China where variable charge soils are widespread. A laboratory experiment was designed ...with contaminated levels of copper (Cu) to estimate the distribution of anthropogenic Cu in different constituents of two variable charge soils using a sequential extraction procedure (SEP) and to evaluate Cu extractability with a single extraction procedure. Soil Cu was chemically fractionated into water-soluble, exchangeable, weakly specifically adsorbed, Fe/Mn oxide-bound, organically bound, and residual fractions using the SEP. One mole NH
4Ac l
−1 (ammonium acetate, pH 5.0), 0.1 mol HCl l
−1, and Mehlich 3 were employed as the single extractant to evaluate the mobility or availability of anthropogenic Cu added in the variable charge soils. The two soils were an inceptisol (clayey mixed siliceous thermic typic Dystrochrept) and an ultisol (clayey kaolinitic thermic Plinthudults), and the Cu levels were treated at 250, 500, 1000 or 1500 mg Cu kg
−1 soil as Cu(NO
3)
2. During a 6-week incubation, most of the anthropogenic Cu was associated with the mobile fractions, i.e. water-soluble, exchangeable, and weakly specifically adsorbed fractions in the inceptisol, whereas in the ultisol the mobile fractions became dominant only at the Cu amendments of 1000 and 1500 mg Cu kg
−1 soil. The Fe/Mn oxide-bound fractions that are relatively stable prevailed in both soils except for the high Cu level treatments. Soil organic matter had a minimal effect on the anthropogenic Cu distribution in the soils because of its low concentration range. Transformations between the mobile fractions and Fe/Mn oxide-bound fraction occurred during the 6-week incubation in both soils. The 1 mol NH
4Ac l
−1 (pH 5.0) and Mehlich 3 extractable Cu in both Cu-amended soils were statistically associated with the mobile fractions according to the stepwise multiple regressions. However, the 0.1 mol HCl l
−1 extractions resulted in an overestimation of Cu availability in these soils due to the dissolution of Fe/Mn oxides, which released part of the adsorbed Cu that may not be available to plants under normal soil conditions.
Leaching of nitrate ($\mathrm{N}{\mathrm{O}}_{3}^{-}$) below the root zone and gaseous losses of nitrogen (N) such as ammonia (NH3) volatilization, are major mechanisms of N loss from agricultural ...soils. New techniques to minimize such losses are needed to maximize N uptake efficiency and minimize production costs and the risk of potential N contamination of ground and surface waters. The effects of cellulose (C), clinoptilolite zeolite (CZ), or a combination of both (C+CZ) on NH3 volatilization and N transformation in a calcareous Riviera fine sand (loamy, siliceous, hyperthermic, Arenic Glossaqualf) from a citrus grove were investigated in a laboratory incubation study. Ammonia volatilization from NH4NO3 (AN), (NH4)2SO4(AS), and urea (U) applied at 200 mg N kg-1 soil decreased by 2.5-, 2.1- and 0.9-fold, respectively, with cellulose application at 15 g kg-1 and by 4.4-, 2.9- and 3.0-fold, respectively, with CZ application at 15 g kg-1 as compared with that from the respective sources without the amendments. Application of cellulose plus CZ (each at 15 g kg-1) was the most effective in decreasing NH3 volatilization. Application of cellulose increased the microbial biomass, which was responsible for immobilization of N, and thus decreased volatilization loss of NH3–N. The effect of CZ, on the other hand, may be due to increased retention of NH4 in the ion-exchange sites. The positive effect of interaction between cellulose and CZ amendment on microbial biomass was probably due to improved nutrient retention and availability to microorganisms in the soil. Thus, the amendments provide favorable conditions for microbial growth. These results indicate that soil amendment of CZ or CZ plus organic materials such as cellulose has great potential in reducing fertilizer N loss in sandy soils.
The presence of P and heavy metals in different forms or in association with different size fractions influences availability and discharge of these elements from watersheds. Understanding the ...association of P and heavy metals with size fractions can improve evaluation of leaching potential of P and heavy metals from soils. In this study, five aggregate‐size fractions, ranged from 1.00 to 0.50 to <0.053 mm, were separated from seven Florida sandy soils by dry sieving. Each aggregate fraction was characterized by phosphate sorption, sequential fractionation of P, total, water‐ and Mehlich III‐extractable concentrations of P and heavy metals. Size differences in sand, silt, and clay aggregates influence the amount and strength of element binding. Elemental attachment (particularly heavy metals) increased with decreasing aggregate sizes. Phosphorus and heavy metals in the sandy soils are readily transported to surface waters with suspended fine particles. Higher percentages of water‐extractable, Mehlich III‐extractable P, and heavy metals were found in both the 0.50‐ to 0.25‐ and 0.25‐ to 0.125‐mm aggregate fractions, suggesting that P and heavy metals in these two fractions had higher release potential. The sequential fractionation of P suggested that the 1.00‐ to 0.50‐mm fraction contained a larger percentage of Ca‐bound P, whereas the 0.50‐ to 0.25‐, 0.25‐ to 0.125‐, and 0.125‐ to 0.053‐mm fractions had higher ratios of labile P (H2O‐P and NaHCO3–P). Phosphorus release from smaller aggregate fractions is faster with a higher P1/P168 ratio than from larger aggregate fractions because of larger amounts of water soluble P attached in the smaller aggregate fractions.
ABSTRACT
Adsorption–desorption of copper (Cu2+) at contaminated levels in two red soils was investigated. The red soil derived from the Quaternary red earths (clayey, kaolinitic thermic plinthite ...Aquult) (REQ) adsorbed more Cu2+ than the red soil developed on the Arenaceous rock (clayey, mixed siliceous thermic typic Dystrochrept) (RAR). The maximum adsorption values (MA) that are obtained from the simple Langmuir model were 25.90 and 20.17 mmol Cu2+ kg−1 soil, respectively, for REQ and RAR. Adsorption of Cu2+ decreased soil pH, by 0.8 unit for the REQ soil and 0.6 unit for the RAR soil at the highest loadings. The number of protons released per Cu2+ adsorbed increased sigmoidally with increasing initial Cu2+ concentration for the RAR soil, but the relationship was almost linear for the REQ soil. The RAR soil released about 2.57 moles of proton per mole of Cu2+ adsorbed at the highest Cu2+ loading and the corresponding value for the REQ soil was 1.12. The distribution coefficient (Kd) decreased exponentially with increasing Cu2+ loading. Most of the adsorbed Cu2+ in the soils was readily desorbed in the NH4Ac. After five successive extractions with 1 mol L−1 NH4Ac (pH 5.0), 61 to 95% of the total adsorbed Cu2+ in the RAR soil was desorbed and the corresponding value for the REQ soil was 85 to 92%, indicating that the RAR soil had a greater affinity for Cu2+ than the REQ soil at low levels of adsorbed Cu2+
Florida has more than 14 million ha of acidic soils and most of these acidic soils are very sandy. Transport of phosphorus from these soils to surface water is a great concern in Florida. One of the ...promising approaches is to develop slow release P fertilizers to replace the regular water soluble fertilizers. Dolomite phosphate rock (DPR) is an ideal material for making this type of fertilizer for acidic sandy soils, which are wide-spread in Florida. The effects of soil moisture, particle size, and source of DPR on the extent and rate of DPR dissolution in a representative acidic sandy soil were examined in an incubation study. DPR dissolution estimated by the NaOH extraction method was strongly dependent on the mineralogy and chemistry of the DPR source materials. Decreasing particle size increased the percentage dissolution of the DPR in soil. Soil moisture affected the initial dissolution rate of the DPR and the time to reach equilibrium. Soil extractable P (Olsen-P) generally increased with DPR dissolution, but the ratios of ΔOlsen-P to dissolved DPR-P (ΔNaOH-P) were significantly different between the two DPR sources. Dissolution of DPR in the acidic sandy soil was well described by the Langmuir and Elovich kinetic equations, with the Langmuir equation being better. Constant A derived from the Elovich equation and potential maximum dissolution (PMDPR) from the Langmuir equation were closely correlated with the increases in pH and exchange (Ca + Mg) after DPR application. The two parameters appeared useful in evaluating DPR dissolution and timing of DPR application in acidic sandy soils.
Heavy metal pollution in soils influences crop yield and quality, and metal accumulation in vegetables may pose a human health risk when consumed. Zinc (Zn), one of the heavy metals, is an essential ...element for plants, animals, and humans, but it is toxic at high levels. In this study, bioavailability of added Zn in a vegetable garden soil and critical Zn concentrations for phytotoxicity and potential dietary toxicity were determined for Chinese cabbage (Brassica chinensis L.), pakchoi (Brassica chinensis L.), and celery (Apiumg graveolens L.). Different Zn levels (0, 100, 200, 300, 400 mg kg(-1) 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 25 mg L(-1) in nutrient solution or at DTPA-Zn above 170 mg kg(-1) 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 222.2 mg kg(-1) (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 (20 mg kg(-1)), the critical concentrations of total and available Zn in the soil were 413 and 244 mg kg(-1) for Chinese cabbage, 224 and 75 mg kg(-1) for pakchoi, and 272 and 101 mg kg(-1) 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.
Acid soils play an important role in the production of world food and fiber. The majority of acid soils in the tropical and subtropical regions are highly weathered, subject to intensive cropping, ...and vulnerable to soil erosion. Degradation of these soils poses a great challenge to sustainable agriculture in these regions. An efficient indexing system using minimal physical, chemical, and biological parameters is immediately needed to assess and monitor the dynamics of soil quality under diversified farming systems and to improve agricultural practices and productivity.
Various physical, chemical, and biological properties can be used to characterize soil quality. However, the number, activity, and diversity of microorganisms and the related biochemical processes are the most important components of soil quality, especially for the highly weathered acid soils, in which plant productivity is closely related to biological cycling.
Several microbiological and biochemical parameters have been suggested as indicators of soil quality. They include: microbial biomass carbon (Cmic), nitrogen (Nmic), and phosphorus (Pmic) and their turnover rates; the microbial quotient (MQ) (Cmic/organic C ratio); basal respiration (qCO2); the microbial metabolic quotient (MMQ) (qCO2/Cmic); the ratio of microbial N over total Kjeldahl N; and enzyme activity. Recently, microbial diversity parameters such as community level physiological profile, phospholipid fatty acids, the ratio of Gram-negative/Gram-positive bacteria, the ratio of fungal/bacterial microorganisms, and free-living diazotrophic bacteria, etc., have been identified as important indicators of soil quality. All these microbiological and biochemical parameters have been shown to relate to soil productivity and respond to changes of land use, vegetation coverage, agricultural practices such as liming, fertilization, and tillage, as well as climate factors (temperature and rainfall).
Current progress on measurement, interpretation, and potential application of the microbiological and biochemical indices in assessing quality, fertility, and sustainability of highly weathered acid soils are reviewed in this chapter.
