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

Abstract

In undisturbed tropical forest ecosystems, the phosphorus (P) cycle is essentially “closed” 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 31P nuclear magnetic 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 were collected from surface soils (0–10 cm) of natural forest, tea plantations and cultivated fields (25 years) at Wushwush and from Podocarpus dominated natural forest, Cupressus plantations and cultivated fields (30 years) at Munesa sites. Significantly lower ( P<0.05) amounts of total P were found following clear-cutting and long-term cultivation (31% and 39%), and establishment of plantations (21% and 22%) at Wushwush and Munesa, respectively. The largest depletion of sodium bicarbonate-extractable organic P (NaHCO 3–P o) (74% and 77%) and sodium hydroxide-extractable P o (NaOH–P o) (67% and 67%) due to cultivation occurred in the sand, followed by the silt (52% and 56%, NaHCO 3–P o and 32% and 53%, NaOH–P o) and the clay (33% and 42%, NaHCO 3–P o and 28% and 35%, NaOH–P o) size separates from the two sites, respectively. 31P NMR spectroscopy revealed that orthophosphate monoesters were the major organic P compounds (27–66%) followed by orthophosphate diesters (9–27%) and teichoic acids (7–11%). Unknown organic P species accounted for 3–8% (unknown A) and 3–5% (unknown B), whereas phosphonates made up 2–3%. The proportion of diester-P decreased in the order: natural forests (24% and 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 and/or sesquioxides. The results of sequential extraction and 31P 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.