Storage-Induced Changes in Phosphorus Solubility of Air-Dried Soils

(1)

Reproduced from Soil Science Society of America Journal. Published by Soil Science Society of America. All copyrights reserved.

Storage-Induced Changes in Phosphorus Solubility of Air-Dried Soils

Benjamin L. Turner*

ABSTRACT This was assessed by determining NaHCO₃–extractable inorganic and organic P in a range of temperate pasture

Soil is commonly stored in an air-dried state for extended periods

soils extracted immediately after air drying and follow-

before chemical analysis. The effect of storage on P solubility was assessed

ing air-dry storage for 3 yr at ambient laboratory tempera-

by determining NaHCO3–extractable P concentrations in a range of

pasture soils from England and Wales (total C⫽28.9–80.4 g kg^⫺¹, ture. The extracted organic P was characterized further

clay⫽219–681 g kg^⫺¹, pH⫽4.4–6.8) immediately following air drying by phosphatase hydrolysis to provide information on the

and after 3 yr of storage at ambient laboratory temperature. Following sources of organic P contributing to the observed changes.

storage, NaHCO3–extractable inorganic P concentrations decreased

by between 2 and 60% (mean decrease⫽21%), while NaHCO3– MATERIALS AND METHODS

extractable organic P concentrations increased by between 48 and

Soils under lowland permanent pasture were sampled to 156% (mean increase⫽95%). The greatest changes occurred in soils

10-cm depth during October 1998 from sites around England of pH⬍5.3. The changes appear to result from the disruption of

and Wales, selected to give a wide range of physical and organic matter coatings on mineral surfaces, continuous solid-phase

chemical properties. Some properties of the soils are presented diffusion of phosphate into soil particles, and decomposition of micro-

in Table 1. Total C (28.9–80.4 g C kg^⫺¹soil) and N (2.85–8.70 g bial cells. The results have important implications for the determina-

N kg^⫺¹soil) were determined simultaneously using a Carlo- tion of NaHCO3–extractable P in stored soils and highlight the im-

Erba NA2000 analyzer (Carlo-Erba, Milan, Italy). Total P portance of working with fresh samples to derive information with

(0.57–1.98 g P kg^⫺¹soil) was determined by ignition and H2SO4

relevance to field conditions.

extraction (Saunders and Williams, 1955). Clay content (219–

681 g kg^⫺¹soil) was determined by wet sieving followed by analysis using a Micromeritics Sedigraph 5100 with a Micro-

P

hosphorus extraction in NaHCO3 is used as an meritics Mastertech 51 automatic sampler. Soil pH (4.4–6.8) was determined in a 1:2.5 soil-to-deionized water ratio. Micro- agronomic test of plant-available P (Olsen et al.,

bial P (31–239 mg P kg^⫺¹soil) was determined on fresh soil 1954) and an environmental test to predict the potential

by CHCl3 fumigation and NaHCO3 extraction (Brookes et risk of P loss in runoff (Heckrath et al., 1995; Turner

al., 1982).

et al., 2004). It is also used to estimate labile pools of

Each soil was sieved (4 mm) and left to equilibrate for 1 wk inorganic and organic P in sequential extraction schemes at 10–15⬚C. Subsamples were then dried on shallow metal (Bowman and Cole, 1978; Hedley et al., 1982). Soil is trays for 7 d at 30⬚C and extracted within 1 wk of drying.

commonly air-dried before NaHCO3extraction, but this The soils were then stored air-dried in sealed plastic bags at can increase both inorganic and organic P concentra- approximately 22⬚C for 3 yr before reextraction. In both cases, the soils were extracted in 0.5MNaHCO3using a standard tions compared with the equivalent fresh soil (Sparling

procedure and analyzed for phosphate by molybdate colorime- et al., 1985; Turner and Haygarth, 2003). The mecha-

try (Olsen et al., 1954). Total P in the extracts was determined nisms involved are unclear, but include the disruption of

by colorimetry following acid-persulfate digestion with modifi- organic matter structures and the lysis of microbial cells.

cations for NaHCO3extracts (Turner and Haygarth, 2003).

Dried soil is assumed to be chemically stable and may

Organic P was estimated as the difference between total P be stored for long periods before extraction and analysis. and phosphate. Each soil was extracted three times. Phosphate However, substantial changes can occur during storage is termed inorganic P for simplicity, although some inorganic (Bartlett and James, 1980). In an Australian loamy sand pyrophosphate is present in these soils (Turner et al., 2003b) stored at temperatures between 5 and 61⬚C, solid-phase and may have been included in the organic P fraction.

NaHCO3–extractable organic P was characterized by phos- diffusive penetration of phosphate into soil particles

phatase hydrolysis following a method modified for NaHCO3

continued for⬎100 d, although the reaction was virtu-

extracts (Turner et al., 2003a). The hydrolyzable organic P ally halted by freezing (Bramley et al., 1992). There is no

was separated into labile monoester P (hydrolyzed by alkaline comparable information on changes in organic P during phosphatase), phospholipids (the difference between organic storage, although organic matter solubility increased dur- P hydrolyzed by phospholipase C⫹alkaline phosphatase and ing the storage of dry tropical soils (Birch, 1959). alkaline phosphatase alone), and nucleic acids (the difference Given the widespread use of NaHCO₃extraction in between organic P hydrolyzed by phosphodiesterase⫹alkaline agronomic and environmental assessment of soil P, arti- phosphatase and alkaline phosphatase alone). Phytase was not included in the current study due to its inhibition in some extracts.

facts induced by storage are of potential importance.

Concentrations are expressed on the basis of oven-dried soil (105⬚C). A correlation matrix (rvalues) was calculated to Smithsonian Tropical Research Institute, Box 2072, Balboa, Ancon, investigate relationships between soil properties and P frac- Republic of Panama. Research was conducted in the Soil and Water tions. Means differences were determined by ANOVA. All Science Dep., Univ. of Florida. Florida Exp. Stn. journal series number analyses were performed using standard procedures in Micro- R-10445. Received 1 Sept. 2004. Soil Chemistry. *Corresponding au-

soft Excel.

thor ([email protected]).

Published in Soil Sci. Soc. Am. J. 69:630–633 (2005). RESULTS

doi:10.2136/sssaj2004.0295

NaHCO₃–extractable P concentrations changed mark-

©Soil Science Society of America

677 S. Segoe Rd., Madison, WI 53711 USA edly following storage, although the changes were quan- 630

Published online April 11, 2005

(2)

TURNER: STORAGE-INDUCED CHANGES IN P SOLUBILITY 631

Table 1. Physical and chemical properties of 15 permanent pasture soils from England and Wales. The soils are ranked in order of their total C concentrations.

Soil no. USDA classification† Topsoil texture pH Clay Total C Total N Total P Organic P‡

g kg^⫺¹soil

1 Haplaquepts clay 5.0 379 28.9 3.74 1.02 0.42

2 Dystrochrepts sandy clay loam 5.5 219 30.6 3.25 0.83 0.27

3 Haplaquepts sandy clay 4.8 338 36.9 3.63 0.59 0.31

6 Udipsamments sandy clay loam 4.9 250 44.0 4.58 0.96 0.44

8 Haplaquepts clay loam 4.8 335 46.0 4.83 1.11 0.55

9 Hapludalfs silty clay 4.8 424 47.5 4.93 0.89 0.42

11 Dystrochrepts clay 4.5 445 58.7 6.35 1.54 0.90

12 Paleudalfs clay 6.8 541 60.2 5.49 1.07 0.48

13 Fluvaquents clay 5.9 547 64.4 6.52 0.78 0.25

14 Hapludalfs clay 5.2 629 68.8 7.33 1.01 0.49

15 Fluvaquents clay 5.0 681 80.4 8.70 1.98 0.88

† Soil Survey Staff, 1999.

‡ Determined by NaOH–EDTA extraction and solution³¹P nuclear magnetic resonance spectroscopy (Turner et al., 2003b).

titatively and proportionally greater for organic P than Concentrations of organic P hydrolyzed by phosphatase enzymes ranged between 6.7 and 27.1 mg P kg^⫺¹ for inorganic P (Table 2). Inorganic P concentrations

decreased by between 0.2 and 11.4 mg P kg^⫺¹soil (mean soil, equivalent to between 22 and 39% of the extracted organic P (Table 3). Of this, between 4.4 and 20.7 mg 4.7 mg P kg^⫺¹soil), equivalent to proportional decreases

of between 2 and 60% (mean 21%) (Table 2). In con- P kg^⫺¹ soil was labile monoesters (17–30% extracted organic P) and between 1.3 and 7.2 mg P kg^⫺¹soil was trast, organic P concentrations increased by between 9.9

and 49.9 mg P kg^⫺¹ soil (mean 26.9 mg P kg^⫺¹ soil), nucleic acids (3–10% extracted organic P). Phospholip- ids were detected in only small quantities (⬍2.6 mg P equivalent to proportional increases of between 48 and

156% (mean 95%) (Table 2). kg^⫺¹soil and⬍5% extracted organic P). Concentrations of total hydrolyzed P and labile monoesters were nega- The decreases in inorganic P were greatest, and only

statistically significant, in soils with pH ⬍ 5.3 (Fig. 1). tively correlated with soil pH (P⬍0.01) and positively correlated with microbial P (P⬍ 0.01).

Increases in organic P were greatest in soils with pH⬍ 5.3 (Fig. 1), even though the differences were statistically significant for all soils. The more acidic soils tended to

DISCUSSION contain high initial concentrations of NaHCO₃–extract-

able organic P (Table 2). Correlation coefficients (rval- The marked changes in soil P solubility following 3 yr of air-dry storage occurred in addition to the changes ues) between soil pH and the increase in NaHCO₃–

extractable P following storage were 0.73 (P⬍0.01) for induced by the initial drying of fresh samples (Turner and Haygarth, 2003). Analysis of fresh soils is therefore inorganic P and ⫺0.67 (P ⬍ 0.01) for organic P. The

increases in organic P were also positively correlated critical to obtain information with relevance to field conditions. This has important implications for the de- with microbial P (r⫽ 0.67;P ⬍ 0.01), while all other

correlations with physical and chemical soil properties termination of NaHCO₃–extractable P in archived soils or samples stored from long-term field trials because it were not significant (P⬎0.05).

Table 2. Concentrations of NaHCO3–extractable P fractions in a range of permanent pasture soils extracted immediately after air drying at 30ⴗC for 7 d and after air-dry storage for 3 yr at ambient laboratory temperature. Values are means of three replicate extracts with SE⬍5%.

Extraction immediately after air drying Extraction after storage for 3 yr Change

Soil no. Inorganic P Organic P Inorganic P Organic P Inorganic P Organic P

mg P kg^⫺¹soil %

1 24.1 29.2 23.0 64.0 ⫺5† 119

2 34.9 22.8 34.0 44.2 ⫺3† 94

3 21.4 30.7 12.6 57.7 ⫺41 88

4 17.5 10.8 16.8 19.4 ⫺4† 79

5 17.3 27.5 13.4 58.1 ⫺23 111

6 47.6 40.6 40.0 90.5 ⫺16 123

7 19.1 45.9 7.7 80.3 ⫺60 75

8 45.5 35.9 34.9 67.2 ⫺23 87

9 14.5 24.0 9.0 44.3 ⫺38 84

10 15.4 21.3 9.6 38.4 ⫺38 81

11 23.6 45.8 15.1 77.2 ⫺36 69

12 15.6 20.6 14.0 30.5 ⫺10 48

13 9.1 13.1 8.9 33.5 ⫺2† 156

14 12.5 20.2 11.7 43.6 ⫺6† 116

15 24.3 44.4 20.5 87.4 ⫺16 97

† Differences not significant (P⬎0.05).

(3)

632 SOIL SCI. SOC. AM. J., VOL. 69, MAY–JUNE 2005

of organic P in fresh soils (Ron Vaz et al., 1994), but is probably acceptable for air-dried samples.

Changes in chemical properties of stored soils are well known (Bartlett and James, 1980), although the mechanisms involved remain unclear. Soil drying dis- rupts organic matter coatings on mineral surfaces, which is exacerbated during storage. The effect is analogous to an elastic band (Bartlett and James, 1980) which remains sound when relaxed (analogous to organic matter in moist soil), but eventually fails if held continuously or too long under tension (analogous to organic matter in dry soil). This process almost certainly explains observed increases in organic matter solubility during storage of dried tropical soils (Birch, 1959) and the changes in organic P solubility in the pasture soils analyzed here.

It is likely that decomposition of microbial cells also contributed to the observed changes because fresh soils contained large microbial biomass concentrations. Many cells survive initial air drying (Sparling et al., 1985), but would be susceptible to prolonged storage.

The decreases in inorganic P are probably due largely to continuous solid-phase penetration of sorbed phosphate into soil particles (Bramley et al., 1992), or diffusion into the interior of small aggregates. However, the trans- formation of phosphate minerals to more crystallized (lower energy) states may also have contributed (Haynes and Swift, 1985). Such processes appear to depend in part on ambient humidity, as demonstrated by changes in phosphate solubility during storage of air-dried cal- careous soils (Castro and Torrent, 1993). This effect could not be assessed for the samples analyzed here because humidity was not determined during storage.

Disruption of organic matter coatings would be ex- pected to further increase P sorption by exposing fresh

Fig. 1. Scatter plots of the relationships between the changes in mineral surfaces (Haynes and Swift, 1985). Both in-

NaHCO3–extractable inorganic and organic P fractions after air- organic and organic P would be affected, although sorp-

dry storage for 3 yr at ambient laboratory temperature and the

tion of organic P would depend in part on its composi-

soil pH (determined in a soil/water ratio of 1:2.5). Note the different

tion. For example, extracts of stored soils in the current

scales on theYaxes.

study contained large proportions of phosphate diesters cannot be assumed that air drying will prevent further and simple phosphate monoesters that sorb weakly in changes in P solubility. In this respect, a previous recom- soil (Frossard et al., 1989). In contrast, the presence of mendation that stored soils should be frozen to prevent phytic acid (not determined here) would have indicated changes in inorganic P solubility (Bramley et al., 1992) the potential for strong sorption to freshly exposed sur-

faces (Celi and Barbaris, 2004).

seems appropriate. Freezing can increase the solubility

Table 3. Concentrations of phosphatase hydrolyzable P fractions in NaHCO3extracts of 15 permanent pasture soils from England and Wales extracted after air-dry storage for 3 yr at ambient laboratory temperature. Values are mean⫾SE of three replicate extracts.

Values in parentheses are the proportion (%) of the NaHCO3organic P.

Soil no. Labile monoesters Phospholipids Nucleic acids Total hydrolyzed P

mg P kg^⫺¹soil

1 11.9⫾0.2 (19) 0.0⫾0.0 (0.0) 2.1⫾0.4 (3.3) 14.0 (22)

2 8.3⫾0.9 (19) 0.4⫾0.3 (1.0) 1.3⫾0.9 (2.9) 10.1 (23)

3 14.7⫾0.5 (26) 2.6⫾2.4 (4.5) 2.4⫾2.1 (4.1) 19.7 (34)

4 4.4⫾0.6 (23) 0.4⫾0.4 (2.0) 2.0⫾0.2 (10) 6.7 (35)

5 15.7⫾1.1 (27) 0.2⫾0.2 (0.3) 3.1⫾0.1 (5.3) 19.0 (33)

6 20.7⫾0.3 (23) 0.6⫾0.7 (0.6) 4.8⫾0.6 (5.3) 26.1 (29)

7 15.1⫾0.8 (19) 1.3⫾1.0 (1.6) 4.7⫾2.0 (5.9) 21.1 (26)

8 14.1⫾0.9 (21) 1.6⫾0.8 (2.3) 5.6⫾0.6 (8.3) 21.2 (32)

9 11.1⫾0.8 (25) 0.2⫾0.3 (0.5) 3.5⫾0.2 (8.0) 14.8 (33)

10 10.2⫾0.8 (27) 0.6⫾0.8 (1.6) 3.1⫾1.0 (8.0) 13.9 (36)

11 13.1⫾0.7 (17) 0.6⫾0.2 (0.8) 5.4⫾0.5 (7.0) 19.1 (25)

12 7.8⫾0.7 (25) 0.1⫾0.2 (0.4) 2.5⫾0.3 (8.3) 10.4 (34)

13 9.9⫾0.2 (30) 0.2⫾0.2 (0.5) 2.9⫾0.2 (8.6) 13.0 (39)

14 11.6⫾0.5 (27) 0.4⫾0.5 (1.0) 3.1⫾0.5 (7.2) 15.2 (35)

15 17.8⫾2.3 (20) 2.1⫾1.8 (2.4) 7.2⫾1.2 (8.3) 27.1 (31)

(4)

TURNER: STORAGE-INDUCED CHANGES IN P SOLUBILITY 633

Castro, B., and J. Torrent. 1993. Phosphate availability in soils at water

The relatively large proportions of labile monoesters

activities below one. Commun. Soil Sci. Plant Anal. 24:2085–2092.

and phosphate diesters measured here in NaHCO3ex- Celi, L., and E. Barbaris. 2004. Abiotic stabilization of organic phos-

tracts are similar to the values (6–49% extracted organic phorus in the environment. p. 113–132.InB.L. Turner et al. (ed.)

P) reported for extracts of semiarid arable soils from Organic phosphorus in the environment. CAB International, Wall- ingford, UK.

the western USA (Turner et al., 2003a), but are in direct

Frossard, E., J.W.B. Stewart, and R.J. St. Arnaud. 1989. Distribution

contrast to the small proportions detected in air-dried and mobility of phosphorus in grassland and forest soils of Saskatch-

arable soils from Australia and Japan (Otani and Ae, ewan. Can. J. Soil Sci. 69:401–416.

1999; Hayes et al., 2000). The western U.S. soils were Hayes, J.E., A.E. Richardson, and R.J. Simpson. 2000. Components of organic phosphorus in soil extracts that are hydrolysed by phy-

stored dry for 2 yr before analysis, so it seems possible

tase and acid phosphatase. Biol. Fertil. Soils 32:279–286.

that storage increases the susceptibility of NaHCO₃– Haynes, R.J., and R.S. Swift. 1985. Effects of air-drying on the adsorp-

extractable organic P to enzymatic hydrolysis, perhaps tion and desorption of phosphate and levels of extractable phos-

through degradation of high molecular weight organic phate in a group of New Zealand soils. Geoderma 35:145–157.

Heckrath, G., P.C. Brookes, P.R. Poulton, and K.W.T. Goulding. 1995.

structures. Storage time may therefore be an important

Phosphorus leaching from soils containing different phosphorus

consideration when assessing the potential bioavailabil- concentrations in the Broadbalk Experiment. J. Environ. Qual. 24:

ity of soluble organic P using phosphatase hydrolysis. _904–910.

Hedley, M.J., J.W.B. Stewart, and B.S. Chauhan. 1982. Changes in inorganic and organic soil phosphorus fractions induced by cultiva- tion practices and laboratory incubations. Soil Sci. Soc. Am. J.

CONCLUSIONS

46:970–976.

Storage of air-dried pasture soils for 3 yr at ambient labo- Olsen, S.R., C.V. Cole, F.S. Watanabe, and L.A. Dean. 1954. Estima- tion of available phosphorus in soils by extraction with sodium

ratory temperature caused marked changes in NaHCO3–

bicarbonate. USDA Circ. No. 939. U.S. Gov. Print. Office, Wash-

extractable P concentrations. These changes were in

ington, DC.

addition to the increases in extractable P that followed Otani, T., and N. Ae. 1999. Extraction of organic phosphorus in

initial drying of fresh soils. The results have profound Andosols by various methods. Soil Sci. Plant Nutr. (Tokyo) 45:

151–161.

implications for the determination of NaHCO3–extract-

Ron Vaz, M.D., A.C. Edwards, C.A. Shand, and M.S. Cresser. 1994.

able P in dried, stored soils. Air-dried soils should be Changes in the chemistry of soil solution and acetic-acid extractable

frozen for prolonged storage to minimize changes in P P following different types of freeze/thaw episodes. Eur. J. Soil

solubility, whereas fresh soils should be analyzed to obtain Sci. 45:353–359.

Saunders, W.M.H., and E.G. Williams. 1955. Observations on the

results with relevance to field conditions.

determination of total organic phosphorus in soils. J. Soil Sci. 6:

254–267.

Soil Survey Staff. 1999. Soil Taxonomy. A basic system of soil classifi- ACKNOWLEDGMENTS

cation for making and interpreting soil surveys. 2nd ed. USDA- I thank Dr. Dale Westermann (USDA–ARS, Kimberly) Natural Resources Conservation Service, U.S. Gov. Print. Office,

Washington, DC.

for comments on the manuscript.

Sparling, G.P., K.N. Whale, and A.J. Ramsay. 1985. Quantifying the contribution from the soil microbial biomass to the extractable P

REFERENCES levels of fresh and air-dried soils. Aust. J. Soil Res. 23:613–621.

Turner, B.L., B.J. Cade-Menun, and D.T. Westermann. 2003a. Or- Bartlett, R., and B. James. 1980. Studying dried, stored soil samples—

ganic phosphorus composition and potential bioavailability in semi- Some pitfalls. Soil Sci. Soc. Am. J. 44:721–724.

arid arable soils of the western United States. Soil Sci. Soc. Am.

Birch, H.F. 1959. Further observations on humus decomposition and J. 67:1168–1179.

nitrification. Plant Soil 11:262–286. Turner, B.L., and P.M. Haygarth. 2003. Changes in bicarbonate- Bowman, R.A., and C.V. Cole. 1978. An exploratory method for extractable inorganic and organic phosphorus following soil drying.

fractionation of organic phosphorus from grassland soils. Soil Sci. Soil Sci. Soc. Am. J. 67:344–350.

125:95–101. Turner, B.L., M.A. Kay, and D.T. Westermann. 2004. Phosphorus in

Bramley, R.G.V., N.J. Barrow, and T.C. Shaw. 1992. The reaction surface runoff from semiarid arable soils of the western United between phosphate and dry soil. I. The effect of time, temperature States. J. Environ. Qual. 33:1814–1821.

and dryness. J. Soil Sci. 43:749–758. Turner, B.L., N. Mahieu, and L.M. Condron. 2003b. The phosphorus Brookes, P.C., D.S. Powlson, and D.S. Jenkinson. 1982. Measurement composition of temperate pasture soils determined by NaOH–

of microbial biomass phosphorus in soil. Soil Biol. Biochem. 14: EDTA extraction and solution³¹P NMR spectroscopy. Org. Geo- chem. 34:1199–1210.

319–329.