Phosphorus forms and dynamics as influenced by land use changes in the sub-humid Ethiopian highlands

Ž . In undisturbed tropical forest ecosystems, the phosphorus P cycle is essentially AclosedB with minimal short-term losses or gains of P. The forms and dynamics of soil P, however, can be greatly affected by land use changes, which often involve changes in vegetation cover, biomass production and nutrient cycling in the ecosystem. Sequential extraction and P nuclear magnetic Ž . resonance NMR spectroscopy were used to investigate the influence of land use changes on the amount and structural composition of P in the sub-humid highlands of southern Ethiopia. Samples Ž . were collected from surface soils 0–10 cm of natural forest, tea plantations and cultivated fields Ž . 25 years at Wushwush and from Podocarpus dominated natural forest, Cupressus plantations Ž . Ž . and cultivated fields 30 years at Munesa sites. Significantly lower P-0.05 amounts of total P Ž . were found following clear-cutting and long-term cultivation 31% and 39% , and establishment Ž . of plantations 21% and 22% at Wushwush and Munesa, respectively. The largest depletion of Ž . Ž . sodium bicarbonate-extractable organic P NaHCO –P 74% and 77% and sodium hydroxide3 o Ž . Ž . extractable P NaOH–P 67% and 67% due to cultivation occurred in the sand, followed by the o o Ž . Ž silt 52% and 56%, NaHCO –P and 32% and 53%, NaOH–P and the clay 33% and 42%, 3 o o ) Corresponding author. Current address: Department of Crop and Soil Sciences, Bradfield and Emerson Hall, Cornell University, Ithaca, NY 14853, USA. Tel.: q1-607-254-1236; fax: q1-607255-8615. Ž . E-mail address: [email protected] D. Solomon . 1 Current address: Department of Crop and Soil Sciences, Bradfield and Emerson Hall, Cornell University, Ithaca, NY 14853, USA. 0016-7061r02r$ see front matter q2002 Published by Elsevier Science B.V. Ž . PII: S0016-7061 01 00090-8 ( ) D. Solomon et al.rGeoderma 105 2002 21–48 22 . 31 NaHCO –P and 28% and 35%, NaOH–P size separates from the two sites, respectively. P 3 o o NMR spectroscopy revealed that orthophosphate monoesters were the major organic P compounds Ž . Ž . Ž . 27–66% followed by orthophosphate diesters 9–27% and teichoic acids 7–11% . Unknown Ž . Ž . organic P species accounted for 3–8% unknown A and 3–5% unknown B , whereas phosphoŽ nates made up 2–3%. The proportion of diester-P decreased in the order: natural forests 24% and . Ž . Ž . 27% )plantations 15% and 13% )cultivated fields 10% and 9% at Wushwush and Munesa, respectively. The percentages of teichoic acid, unknown A, unknown B and phosphonates also decreased, whereas the proportions of orthophosphate monoesters increased following land use changes. Greater decline in proportions of diester-P and teichoic acids were found in the silt than in the clay size separates, which may be attributed to stabilization of microbially derived organic P structures by closer association with clay minerals andror sesquioxides. The results of sequential extraction and P NMR spectroscopy indicate that continuous cultivation with little or no P input not only decreased the amount, but also influenced the structural composition and bioavailability of P in these tropical soils. Thus measures have to be designed for replenishment and subsequent maintenance of soil P stocks, to ensure sustainable crop production in sub-humid highland agroecosystems of southern Ethiopia. q 2002 Published by Elsevier Science B.V.