Monday, November 13, 2006

Characterizing Bioactive Phosphorus Fractions in Cerrado Oxisols under Conventional- and No-tillage.

Paulo S. Pavinato1, Thanh H. Dao1, and Ciro Rosolem2. (1) USDA-ARS, Bldg. 306 Powder Mill Rd, BARC-East, Beltsville, MD 20705, (2) Dep. Crop Science, FCA-UNESP, Botucatu, CP237, Brazil

The capacity of a soil to supply essential nutrients for crop growth and its productivity can be inferred from biochemical indices of nutrient availability. Oxisols are low in available phosphorus (P), particularly in Cerrado savanna-like regions of Brazil, where the soil under native vegetation is very low in plant-available nutrients.  This study was conducted to evaluate P bioavailability in two Brazilian Cerrado clay soils from the States of Goias (soil PA) and Mato Grosso do Sul (soil PL), using the ligand-based enzyme-hydrolyzable phosphorus fractionation method (Dao, 2004).  These soils are characterized by high clay content (60 – 70%) and oxalate extractable Fe (2 – 3%). Soil organic matter was about 4 – 5% in the 0-10 cm layer of both soils. The soils had been conventionally-tilled and no-till for the last 10 years.  Phosphorus distribution with soil depth showed a distinct stratification. There was a significant interaction between soil depth and tillage method in the bioactive P distributions. The enzymatic P assay showed that water-extractable P concentrations ranged 3.6 to 7.3, and 3.5 to 13.0 mg kg-1 in the PA and PL soils, respectively. More ligand-exchangeable inorganic P (LEPi) was released in no-till than conventionally tilled soils, and the LEPi fraction was largest in the upper soil depths, up to 15 cm. The enzyme-hydrolyzable P fraction was a relatively larger pool of organic P and showed a similar depth distribution to LEPi in both soils. No-till management resulted in higher accumulation of these organic P species, representing an increase in potentially available P for crop uptake in the cultivation of these Cerrado soils.