Pedosphere 24(6): 763--775, 2014
ISSN 1002-0160/CN 32-1315/P
©2014 Soil Science Society of China
Published by Elsevier B.V. and Science Press
| Apatite loss in Pothwar Loess Plain (Pakistan) fits a simple linear reservoir model |
M. S. AKHTAR1, M. IMRAN1 , A. MEHMOOD1, M. MEMON2, S. RUKH1 and G. S. KIANI1 |
1Department of Soil Science and Soil Water Conservation, PMAS-Arid Agriculture University Rawalpindi, Rawalpindi 46300 (Pakistan)
2
Department of Soil Science, Sindh Agriculture University, Tando Jam, Hyderabad 70050 (Pakistan) |
| ABSTRACT |
| Apatite is the dominant phosphorus (P) mineral in early stages of soil development, and its redistribution as labile forms under pedogenesis controls terrestrial bioavailability. Quantitative distribution of labile forms of P and apatite-P was examined in Pothwar Loess Plain, Pakistan where the degree of pedogenesis varied with relief. Four soil types, Typic Ustorthents (Rajar), Typic Calciustepts (Missa), Udic Calciustepts (Basal), and Udic Haplustalfs (Guliana), were sampled from three replicated locations at genetic horizon level. With the exception of total P value at surface, the mean total and apatite-P decreased towards the surface in Udic Haplustalfs and Udic Calciustepts where dicalcium and octacalcium phosphate increased toward surface. Iron (oxides and oxyhydroxides) adsorbed and occluded P forms were also in greater quantities in Udic Haplustalfs and increased toward the surface, whereas they were lowest and uniform in the Typic Ustorthents. Aluminum- and organic P correlated with soil organic matter. Apatite-P decreased exponentially with an increase in the sum of iron adsorbed and occluded P fractions, and fitted the equation M(x) = M0[1 – exp(–wx)] with r2 = 0.996, where M(x) is the mean apatite-P concentration in solum, M0 is the apatite-P content in the loess sediments, x is the cumulative iron adsorbed and occluded P, and w is an empirical factor indicating the change rate of apatite-P in the loess. From the calculated apatite-P of 740 mg kg-1 at the time of deposition, mean apatite-P loss was 60% in Udic Haplustalfs, 33% in Udic Calciustepts, 23% in Typic Calciustepts, and 8% in Typic Ustorthents. The transformation of soil P to labile forms was faster and deeper in level or slight depressions followed by gently sloping areas in wide plains, and was the least in the gullied land. |
| Key Words: labile forms, pedogenic iron oxides, phosphorus distribution, phosphorus fractions, toposequence |
| Citation: Akhtar, S., Imran, M., Mehmood, A., Memon, M., Rukh, S. and Kiani, G. S. 2014. Apatite loss in Pothwar Loess Plain (Pakistan) fits a simple linear reservoir model. Pedosphere. 24(6): 763-775. |
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