Chemical and spectral characterization of soil phosphorus under three land uses from an Andic Palehumult in West Cameroon

Phosphorus (P) is one of the main limiting plant nutrients in most tropical soils. Acquiring quantitative information on soil P status is essential for evaluating its sustainable management in agroecosystems. The objective of this study was to evaluate how land-use shifts from semi-permanent food crop systems (CF) to plantations of tea (Camellia sinensis) (TP) and Eucalyptus grandis (EP) impact on both organic and inorganic P species. Determination of phosphorus status combined a P sequential fractionation procedure and 31 P nuclear magnetic resonance (NMR) spectroscopy. Sequentially extracted pools included available P by 0.5 M NaHCO3, Al/Fe-P by 0.1 M NaOH, Ca-P by 0.5 M HCl and residual P by 0.5 M H2SO4 after ignition at 550degreesC. Soil total P (STP) varied significantly across land uses (P < 0.05) with the lowest mean value (1025.6 +/- 23.1 mg kg(-1)) occurring under CF and the highest (1698.0 +/- 86.1 mg kg(-1)) under TP. The largest P-pools were NaOH-P (47-51% of total soil P) and H2SO4-P (25-32%). NaHCO3-P-i under fertilized land uses (CF and TP) was greater than 12 mg kg(-1) indicating that these systems were sustainable. Unfertilized EP was P-deficient, probably as the result of organic-matter accretion and subsequent P immobilization in organic forms. P-31 NMR revealed that 88-89% of P compounds in NaOH extract were organic with monoester-P accounting for 59.1-60.8%. This was followed by diester-P (9.8-12.4%), teochoic acid (8.4-10.1%), orthophosphate (8.8-9.7%), unknown compounds (7.4-8.4%), pyrophosphate (1.1-4.6%) and phosphonate (0-1.3%). EP had higher diester-P and no phosphonate compound whereas CF had substantial amount of pyrophosphate (4.6%) and less orthophosphate and teochoic acid. These results indicate that these last P compounds are easily mineralizable P forms participating actively in plant P nutrition. (C) 2003 Elsevier B.V. All rights reserved.