Tuesday, November 14, 2006

Influence of Low Molecular Weight Organic Acids on the Release of P and Soil Organic Matter from highly weathered acid soil.

Kiran Rana1, G. S. Premachandra1, Cliff T. Jhonston1, Darrell G. Schulze1, J. R. Okalebo2, Caleb Othieno3, Antonio Marcus Coehlo4, and Pamela Obura1. (1) Purdue University, 914 W State Street, West Lafayette, IN 47907, United States of America, (2) Soil Science Dept., Moi Univ., "PO Box 1125, Chepkoilel Campus", Eldoret, KENYA, (3) PO Box 1125, KENYA, Moi Univ., Moi University Chepkoilel Campus, Soil Science Department, Eldoret, KENYA, (4) Embrapa Maize and Sorghum, Sete Lagoas, MG, Brazil, Sete Lagoas, Brazil

Acid soils comprise over 40% of the world’s potentially arable land. Two of the main soil constraints for these types of soils are aluminum (Al) toxicity and phosphorus (P) deficiency.  The exudation of low molecular weight organic acids (LMWOAs), such as oxalic and citric acid, by maize and sorghum roots has been identified as one of the interventions plants have developed to decrease Al-toxicity.  Recently, the presence of LMWOAs has been proposed as a means for plants to improve their P efficiency in addition to decreasing Al-toxicity.  P-fixation by highly weathered acid soils is known to involve chemisorbed P resulting from inner-sphere surface complexes between inorganic P and Fe-, Al-oxides as well as edge sites of clay minerals.  LMWOAs have the ability to displace chemisorbed P through a ligand-exchange reaction.  Although the role of LMWOAs has been studied in other soils (e.g., Spodosols), relatively little is known about their behavior in highly weathered acid soils.  Our objective was to investigate the influence of low concentrations of LMWOAs on the desorption of P, release of soil organic matter, metal chemistry and other changes occurring in the soil solution of highly weathered acid soils.  P sorption and release, under the influence of LMWOAs, were studied on a dark red latasol (Rhodic Hapludox).  This soil was incubated with P and the LMWOAs-induced release of P was investigated.  Our data show that very low concentrations LMWOAs, representative of concentrations measured in the rhizosphere, increase the bioavailability of P.  In addition, the soil solution concentrations of Al and Fe increased along with a net release of organic matter from the soil.