植物营养学 (8).pdf

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1、RELEASE POTENTIAL OF PHOSPHORUS IN FLORIDA SANDY SOILS IN RELATION TO PHOSPHORUS FRACTIONS AND ADSORPTION CAPACITY M. K. Zhang,1Z. L. He,1,2,*D. V. Calvert,2P. J. Stoffella,2 Y. C. Li,3and E. M. Lamb2 1College of Resource and Environmental Sciences, Zhejiang University, Huajiachi Campus, Hangzhou 31

2、0029, P. R. China 2University of Florida, Institute of Food and Agricultural Sciences, Indian River Research and Education Center, 2199 South Rock Road, Fort Pierce, FL 34945, USA 3University of Florida, Institute of Food and Agricultural Sciences, Tropical Research and Education Center, Homestead,

3、FL 33031, USA ABSTRACT Information on P release potential in relation to labile P and P fractions in sandy soils is limited. In this study, P release potential was determined by leaching, and labile P, soil P fractionation, and P adsorption capacity were measured in the laboratory using 96 Florida s

4、andy soil samples to evaluate the relationship between P release in water and soil P status. The sandy soils had a very low P adsorption capacity. The adsorption maximum, as calculated from the Langmuir equation, averaged 40.4mg Pkg?1. More than 10% of the soil P was water soluble, indicating a high

5、 risk of P leaching from soil to water. Successive leaching using deionized water released, on average, 7.7% of total P (144.5mgkg?1 ) in diff erent soils, whereas labile P recovered by successive 793 Copyright#2002 by Marcel Dekker, I *Corresponding author. E-mail: zhemail.ifas.ufl .edu J. ENVIRON.

6、 SCI. HEALTH, A37(5), 793809 (2002) water extraction accounted for 39.2% of the total P. Variation in P release potential among the diff erent soils could be explained more by the diff erence in amounts of extractable P than the adsorption capacity. Total amounts of P released by successive leaching

7、 were signifi cantly correlated with all labile P indices measured by diff erent methods and all soil P fractions except for residual P. The correlation coeffi cients (r) were 0.97* for water-soluble P, 0.96* for 0.01M CaCl2-P, 0.94* for Olsen P, 0.86* for Mehlich 1-P, 0.77* for Mehlich 3-P, and 0.6

8、4* for Bray 1-P. There were no obvious turning points in the relationships between Olsen-P, water-soluble P, or CaCl2-P and the amounts of P released from the sandy soils. The release of P from the sandy soils appeared to be controlled by a precipitationdissolution reaction rather than a P sorptiond

9、esorption process. Furthermore, the sequential extraction of soils using deionized water indicated that P released was not limited to the labile P (H2O-P, NaHCO3-IP) and potentially labile P (NaOH-P) pools, but also from the HCl-P, indicating that all of P fractions except for residual P in the sand

10、y soils can contribute to P release. Key Words:Phosphorusreleasepotential;Phosphorusfractions; Adsorption; Sandy soil INTRODUCTION Phosphorus (P) is generally considered to be the key element that limits freshwater quality and causes eutrophication in many lakes and other water bodies (13). Accelera

11、ted eutrophication of streams and lakes is one of the greatest challenges facing water quality management. This problem is generally related to increases in the annual input of nutrients to surface water. Consequently, the control of nutrients, especially P, in surface runoff and leaching (subsurfac

12、e drainage) is considered as the best way to minimize the eutrophication risk (46). Increased concentration of dissolved reactive P (DRP) in surface runoff is highly correlated with increased soil test P (STP) levels (710). Soils that contain high levels of P due to excessive fertilization can becom

13、e the primary source of P in runoff (8). As a result of the continuous rise in soil P levels, agricultural and urban areas increasingly contribute to the eutrophication of surface water by surface runoff and leaching. The loss of P from agriculture to water- courses has increased over the last few d

14、ecades as a result of increased intensive farming and the development of a more industry-based type of agriculture (1114). Even though many studies have been conducted on P in the last 20 years, information is still lacking regarding the use of soil P availability indices for environmental risk asse

15、ssment. 794ZHANG ET AL. Soil P tests provide farmers with an indicator of how much plant- available P is present in a soil and, consequently, the quantity of nutrients tobeappliedtoobtaindesiredcropyields.Severalstudieshave demonstrated that the losses of P from soil vary with soil type, tillage, so

16、il moisture content, season, and crop management (2,5,7,1518). Phosphorus losses by erosion, surface runoff , and leaching or lateral subsurface fl ow are substantial when soil test P values are above the optimal ranges (7,19,21). Soil P testing is currently used in several countries, including the

17、USA, as a means of identifying areas where applications of P fertilizers and manures should be reduced or eliminated in order to protect water quality (2124). Although the total amount of P loaded in surface runoff and stream fl ow is important to water quality, the forms or fractions of P in soils

18、that are released into the waters are probably more critical. This infor- mation is essential if agriculture is to adopt nutrient management plans and recommendations designed to reduce the P load to the surface waters. To predict the transport of P from soils to the environment, key soil chemical p

19、rocesses such as P adsorptiondesorption and precipitationdissolution should also be understood. The objectives of this study were (i) to determine P transport/leaching potential from typical sandy soils under citrus and vegetable production and (ii) to evaluate the relationship between P trans- port

20、/leaching potential and soil P availability indices and adsorption capacity. MATERIALS AND METHODS The soils used in this study, totaling 96 samples, were taken from commercial vegetable farms and citrus groves in Florida. The soils were Wabasso sand (sandy, siliceous, hyperthermic alfi c haplaquods

21、), Waveland fi ne sand (sandy, siliceous, hyperthermic, ortstein arenic haplaquods), Ankona sand (sandy, siliceous, hyperthermic, ortstein arenic haplaquods), Winder variant sand (sandy, siliceous, hyperthermic typic glossaqualfs) and Nettles sand (sandy, siliceous, hyperthermic, ortstein alfi c are

22、nic haplaquods). All soil samples were air-dried and ground to pass through a 2-mm sieve. Soil pH was measured in water and KCl solution at a soil:water (solution) ratio of 1:1, and electrical conductivity (EC) was measured in water at a 1:2 soil:- water ratio using a pH/ion/conductivity meter (Accu

23、met Model 50, Fisher Scientifi c) (25). The total organic C was determined using a CNS-Analyzer (NA 1500, Fisons Instruments Inc., Dearborn, MI). Particle size distribution of soil samples was determined using a micro-pipette method (26). In general, soils were slightly acidic to slightly alkaline w

24、ith low organic C, thoughacidityandorganicCvariedwidelyamongthesoilsamples.Thesesoils contained more than 800gkg?1sand, but less than 60gkg?1silt and clay (Table 1). RELEASE POTENTIAL OF P795 Measurement of Soil P Availability Indices Soil samples were analyzed for P availability indices by six diff

25、 erent extraction methods: (i) Olsen-P (1:20 ratio of soil to 0.5M NaHCO3 (pH 8.5), 30-minute reaction time (27); (ii) Mehlich 1-P (1:4 ratio of soil to 0.05M HCl0.0125M H2SO4, 5-minute reaction time (27); (iii) water- soluble P (1:10 ratio of soil to deionized water, 60-minute reaction time (27); (

26、iv) CaCl2extractable P (1:10 ratio of soil to 0.01M CaCl2, 60- minute reaction time (27), (v) Bray 1-P (1:7 ratio of soil to 0.03M NH4F0.025M HC, 1-minute reaction time (27), and (vi) Mehlich 3-P (1:10 ratio of soil to Mehlich 3 extraction solution, 5-minute reaction time (28). After each extraction

27、, the suspension was centrifuged at 7500?g for 30min and then the supernatant was passed through a Whatman 42 fi lter paper. Phosphorus concentrations in the extracts from methods (i), (ii), (iii), and (iv) were determined colorimetrically by the molybdenum blue method (29). The P concentrations fro

28、m methods (v) and (vi) were determined by inductively coupled plasma atomic emission spectroscopy (ICPAES, J-Y Emission Division Instruments SA, Inc., New Jersey). Phosphate Sorption Eighty out of the 96 soil samples were selected for measuring P sorption capacity. Phosphorus sorption isotherms were

29、 determined as follows: Soils (each 1-g) were placed in polystyrene centrifuge tubes and 30ml of 0.02M KCl solution containing 0, 2.5, 5.0, 7.5, 10.0, 15.0, or 20mgPl?1was added 796ZHANG ET AL. Table 1.Statistical Summary of Basic Soil Properties and Extractable Phosphorus in the 96 Soil Samples Pro

30、pertiesRangeMeanSDy Basic propertiesPH (H2O)3.928.806.671.27 PH (KCl)3.808.606.341.27 EC (mscm?1)52867245160 Organic C (gkg?1)0.527.24.34.1 Sand (gkg?1)78898092836 Silt (gkg?1)11132621 Clay (gkg?1)81434724 Extractable POlsen-P (mgkg?1)1.075.219.716.2 Mehlich 1-P (mgkg?1)1.3241.264.671.9 Water-solubl

31、e P (mgkg?1)0.823.97.15.4 0.01M CaCl2-P (mgkg?1)0.110.82.42.1 Bray-P (mgkg?1)0.7243.150.45.8 Mehlich 3-P (mgkg?1)5.2334.684.491.3 ySD: Standard deviation. to each tube. The tubes were shaken on an end-on-end shaker (180 cycles/ min) for 24h at 25?C. Then the suspensions were centrifuged at 10,000?g

32、for 10min and fi ltered through a Whatman 42 fi lter paper. Phosphorus con- centrations in the fi ltered solution were colorimetrically determined using the molybdenum blue method. The simple Langmuir equation was employed to describe the P sorption isotherms. The adsorption maximum (Qm) was obtaine

33、d from the Langmuir equation. The Langmuir equation is expressed as: Q KC Qm/(1KC), where Q is amount of P adsorbed (mgPkg?1soil), C is P concentration in the equilibrium solution (mgPl?1), Qmis the adsorp- tion maximum, and K is the constant related to P binding energy. Fractionation of Soil Phosph

34、orus Ninety-sixsoilsamplesalongwith20soilsamplesthathadbeensubjected to eight successive extractions with deionized water were used for soil P frac- tionation. A modifi ed method of Hedley et al. (30) was selected in this study to determine soil P fractions. Soils (each 1-g) were placed into 50-ml c

35、entrifuge tubes and were sequentially extracted with 30-ml of deionized water, 0.5M NaHCO3(pH 8.2), 0.1M NaOH, and 1M HCl. Each extraction lasted for 16h onanend-to-endshaker(180 cycles/min).Aftereach extraction, thetubes were centrifuged at 7500?g for 30min, the supernatant was then passed through

36、a Whatman 42 fi lter paper. The P concentrations in the fi ltrates from deionized water and HCl extractions, and the inorganic P (IP) in the bicarbonate and hydroxide extracts were colorimetrically determined by the molybdenum-blue method. Total P in the bicarbonate and hydroxide extracts was also d

37、etermined by the molybdenum-blue method after digestion with acidifi ed ammonium persulfate (31). The organic P (OP) concentrations in the bicarbonate and hydroxide extracts were calculated from the diff erence between the total P and the IP contents in the extracts. Soil total P was deter- mined by

38、 the perchloric acid digestion method (27). Residual-P was calculated bysubtractingthesumoftheabovefourtypesofextractablePfromthetotalP content in thesoil. The residual Pwas also measuredseparately to evaluate the P budget. Based on the sequence of extractions, soil P fractions were referred to as w

39、ater-solubleP (H2O-P), bioavailable inorganicP (NaHCO3-IP), readily mineralizable organic P (NaHCO3-OP), potentially bioavailable inorganic P (NaOH-IP), potentially bioavailable organic P (NaOH-OP), acid-soluble P (Ca-bound P/HCl-P), and residual P. Measurement of Phosphorus Release Potential in Soi

40、ls Both successive leaching and extraction were conducted to evaluate the P release potential from the soils. Successive leaching measured the slowly RELEASE POTENTIAL OF P797 released P, whereas successive extractions would be quantifi ed as rapidly released P from the soils. Successive Leaching Al

41、lofthe96soilsampleswereusedforthesuccessiveleachingexperiment. A 50-g sample for each soil was placed onto a Whatman 42 fi lter paper, then fi tted in a funnel, moistened to fi eld capacity, and incubated at room tempera- ture for 2 days. During each leaching event, 50-ml of deionized water were app

42、lied. This leaching process was repeated daily for a total of 15 sequential leachings (lasting 15 days). Leachates were collected in 125-ml polyethylene bottles. Phosphorus concentration in each leachate sample was determined using the molybdenum blue method. The amount of P leached was calculated b

43、ased on the leachate volumes and P concentrations of the leachates. Successive Extraction Representative soil samples (20) were selected for the successive extrac- tion experiment. Portions of soil, each containing 1.0-g soil (oven dry basis), were placed into 50-ml centrifuge tubes and extracted wi

44、th 30-ml of deionized water. This extraction was successively repeated for a total of eight sequential extractions. Each extraction lasted for 16 h on an end-to-end shaker (180 cycles/min). After each extraction, the tubes were centrifuged at 7500?g for 30min, and the supernatant was passed through

45、a Whatman 42 fi lter paper. The P concentrations in the fi ltrates were determined colorimetrically using the molybdenum-blue method. Statistical Analyses Correlations between extractable P, P fractions and the amounts of P released by leaching (or extraction), mean and standard deviation of the ext

46、ractable P, P fractions, and linear regressions between P released in the leaching and the availability indices of P were conducted using the SAS computer programs (32). RESULTS AND DISCUSSION Soil P Availability Indices The amounts of available P from the 96 soil samples measured by six extraction

47、methods varied greatly (Table 1). The mean value of extractable 798ZHANG ET AL. P in the soils decreased in the following order for the six diff erent extraction methods: Mehlich 3-P (84.4mgkg?1)Mehlich 1-P (64.4mgkg?1) Bray- P (50.4mgkg?1)Olsen-P (19.7mgkg?1)water-soluble P (7.1mgkg?1) 0.01M CaCl2-

48、P (2.4mgkg?1). Olsen-P, water-soluble P and 0.01M CaCl2-P accounted for only a small proportion of the total P and were the readily released P pools, whereas Mehlich 1-P, Bray 1-P, and Mehlich 3-P accounted for a large proportion of the total P, and likely represented boththereadilyandslowlyreleased

49、Ppools.However,there were very signifi cant correlations (r) among all the extractable P indices (Table 2). The correlation coeffi cients (r) among water-soluble P, 0.01 MCaCl2-P, and Olsen-P were 0.87 or higher. Water-soluble P had a signifi cantcorrelationwithotherPavailabilityindices,particularly with0.01M CaCl2-Pand Olsen-P. Therefore,based on these high correlation coeffi cients,eachofthePavailabilityindicescould be mutually predicted with a reasonable amount of precision in the sandy soils. Phosphorus Sorption Capacity The P adsorption maximum (Qm) obtained from the