Dennis Friesen

Phosphorus transformations in an oxisol under contrasting land-use systems: The role of the soil microbial biomass

It is generally assumed that phosphorus (P) availability for plant growth on highly weathered and P-deficient tropical soils may depend more on biologically mediated organic P (Po) turnover processes than on the release of adsorbed inorganic P (Pi). However, experimental evidence showing the linkages between Po, microbial activity, P cycling and soil P availability is scarce. To test whether land-use systems with higher soil Po are characterized by greater soil biological activity and increased P mineralization, we analyzed the partitioning of P among various organic and inorganic P fractions in soils of contrasting agricultural land-use systems and related it to biological soil properties. Isotopic labeling was used to obtain information on the turnover of P held in the microbial biomass. Soil samples were taken from grass–legume pasture (GL), continuous rice (CR) and native savanna (SAV) which served as reference. In agreement with estimated P budgets (+277, +70 and 0 kg P ha−1 for CR, GL and SAV, respectively), available P estimated using Bray-2 and resin extraction declined in the order CR > GL > SAV. Increases in Bray-2 and resin Pi were greater in CR than GL relative to total soil P increase. Organic P fractions were significantly less affected by P inputs than inorganic fractions, but were a more important sink in GL than CR soils. Extractable microbial P (Pchl) was slightly higher in GL (6.6 mg P kg−1) than SAV soils (5.4 mg P kg−1), and significantly lowest in CR (2.6 mg P kg−1). Two days after labeling the soil with carrier free 33P, 25, 10 and 2% of the added 33P were found in Pchl in GL, SAV and CR soils, respectively, suggesting a high and rapid microbial P turnover that was highest in GL soils. Indicators of P mineralization were higher in GL than CR soils, suggesting a greater transformation potential to render Po available. Legume-based pastures (GL) can be considered as an important land-use option as they stimulate P cycling. However, it remains to be investigated whether crops planted in pasture–crop rotations could benefit from the enhanced Po cycling in grass–legume soils. Furthermore, there is need to develop and test a direct method to quantify Po mineralization in these systems.

Development of a soil-plant phosphorus simulation model for calcareous and weathered tropical soils

Phosphorus (P) is a limiting nutrient affecting crop yields in many regions of the world, in particular in areas with highly weathered acid soils and calcareous alkaline soils. Given the many factors associated with P behavior in a range of soil environments, there is a need to go beyond site-specific situations. The use of crop simulation models is a valuable tool to evaluate the efficacy of applying P fertilizers under different soil, management and climatic conditions. A computer model was developed to simulate P in the soil–plant system adapted to soils with high P limitations. The soil P module is operated with two comprehensive crop simulation models (CERES and CROPGRO) within the DSSAT software. The P module comprises inorganic and organic P pools estimated from measured P fractionation data and works on a daily time step. The rate constants for P movement between the pools follow first order kinetics. The P module was calibrated and tested using three data sets from Colombia, Syria, and Tanzania. The limited testing showed that the P module simulated accurately grain yield and P uptake by wheat grown under semi-arid conditions. The wheat crop responded little to fertilization although measured Olsen P was as low as 2.6 mg kg−1. The P module overestimated P uptake for both soybean and bean crops grown in acidic soils, but predicted with a good degree of accuracy labile P in the soil and P uptake for maize grown under the same acidic conditions. Testing with more data sets is needed to improve model predictions.

Appropriateness of biotechnology to African agriculture: Striga and maize as paradigms

This paper focuses on the impact of Striga spp. on maize production and the use of biotechnology (e.g. transgenics, tissue culture, Bt genes, and transposon-transmitted suicide (inducible) genes) to control the damage caused by Striga spp. The use of biocontrol measures to Striga control is briefly discussed.

Phosphorus fractions and dynamics in surface earthworm casts under native and improved grasslands in a Colombian savanna Oxisol

The objective of this study was to assess the effect of a native anecic species on phosphorus availability in an Oxisol characterised by a low chemical fertility. Experiments were carried out at Carimagua research station in a representative site of the isohyperthermic savannas on the Colombian Orinoco basin. One field study and two laboratory/incubation studies were performed in a natural herbaceous savanna and a Brachiaria decumbens and Pueraria phaseoloides pasture. In the laboratory, experiment pots were prepared containing soil collected from the respective field paddock’s topsoil. Total P content was higher in earthworm casts than in the surrounding soil in field samples, 50% in native savanna soil and more than 100% in pasture soil. In casts produced under laboratory conditions this increase was relatively low (10 – 20%). Under field conditions, almost without exception, all P fractions were increased in casts relative to the original soil (corresponding to the increase in total P content), being relatively greater in the labile inorganic P fractions. In addition, samples from the natural savanna showed that pH of casts was higher (5.2) than that of soil (4.6) in both field and laboratory samples. Except in the native savanna under field conditions, the phosphatase activity was reduced in casts by 16.7 to 44%. From our results we conclude that earthworms in the field incorporate P from litter or other organic sources (i.e. undecomposed plant and root material, earthworm faeces) which is not normally measured in the analysis of bulk soil. q 2003 Elsevier Science Ltd. All rights reserved.

Comparison of soluble P in soil water extracts determined by ion chromatography, colorimetric, and inductively coupled plasma techniques in PPB range

The determination of soil solution phosphate ions at trace levels is questionable with colorimetric method due to both limit of sensitivity and possible hydrolysis of organic soluble phosphate (P). A simple procedure was developed to determine orthophosphate at trace levels in soil solutions by single-column, suppressed-ion chromatography with conductivity detection without prior sample concentration. The procedure requires column capacity selection and use of a 500-μL injection loop. The method shows a detection limit of 0.05 μg P L−1 as well as a high resolution with a Dionex AS9SC column. The proposed method was tested by analyzing 45 soil samples (Colombian oxisols) differently P fertilized. Phosphorus levels in solutions were also determined by green-malachite colorimetry and inductively coupled plasma as reference methods. The linear correlation between the various methods showed that analysis of orthophosphate in the ppb range by ion chromatography are likely more acceptable that value obtained with colorimetry, which hydrolyzed organic P and caused systematic error. Examination of coefficients of variation for the IC method calculated from triplicate analyses of five randomized samples (situated between 7.4 and 16.3 μg P L−1) showed value ranged from 2.0 to 6.7%. The procedure allows precise measurements of trace amounts of orthophosphate in the presence of moderate background levels of salts such as chloride, nitrate, and sulfate ions and was adequate for routine analysis.

A new approach to Striga control

Fred Kanampiu (e-mail: f.kanampiu@africaonline.co.ke) and Dennis Friesen (email: d.friesen@cgiar.org) of the International Maize and Wheat Improvement Center (CIMMYT), based in Nairobi in Kenya, and Jonathan Gressel (e-mail: Jonathan.Gressel@weizmann.ac.il) of the Weizmann Institute of Science in Rehovot, Israel, outline new herbicide maize seed coating technology and new IR varieties for Striga control.