Changes in Inorganic and Organic Soil Phosphorus Fractions in the LTER Soils

Daroub, S.

Presented at the All Scientist Meeting (1996-07-16 to 1996-07-17 )

Changes in management practices, e.g. no-tillage and use of winter cover crops, alter the dynamics of organic matter turnover in soils and subsequently may alter the dynamics of phosphorus (P) in soils and its availability to crops. The objective of this work was to investigate if the adoption of organic based management systems with little or no chemical inputs and no-tillage affected the distribution of P within the inorganic and organic pools.Soil samples from all LTER plots were sampled at a 0-25 cm depth in July 1995. Subsamples of 1.0 g were extracted sequentially according to the procedure in Fig. 1. Resin plus bicarbonate inorganic P (Pi) are labile soil-P fractions and are recognized as the most available for plant growth. Bicarbonate extractable organic P (Po) is easily mineralizable and contributes to plant available P. Hydroxide extractable Pi has lower plant availability and is thought to be associated with amorphous Al and Fe phosphates by chemisorption reactions. More stable forms of Po are extracted with hydroxide. Ultrasonification of the soil residue in 0.1 M NaOH enables extraction of Pi and Po held at the internal surfaces of soil aggregates. Acid extractable Pi is largely Ca bound P. Oxidation and acid digestion with H2SO4 and H2O2 dissolve more chemically stable Po forms and relatively insoluble Pi forms.I present here results from treatments 1 through 5. Extraction of other treatments is still underway. Preliminary results (Fig. 1) show that treatment 3 (low input organic based wheat/ corn/ soybean), and treatment 4 (zero input organic based wheat/ corn/ soybean) have the highest levels of organic P ( extracted by NaHCO3 and NaOH) . Both treatments maintain comparable levels of labile Pi to treatment 1 (High input wheat/ corn/ soybean). Treatment 5 (perennial biomass, poplar) had the highest levels of inorganic P (extracted by resin and NaHCO3).Sequential Extraction Procedure for P AnalysisDuplicate 1.0 g soil sample in 50 ml centrifuge tube. Add 30 ml of deionized water plus 0.4 g of 1 × 8-50 Dowex anion exchange resin in the HCO3 form, shake for 16 hours, remove resin bag, centrifuge and discard supernatant. Elute resin bag with 0.5 N HClAdd 30 ml 0.5 M NaHCO3 (pH 8.5), shake 16 h, centrifuge and collect supernatantAdd 30 ml of 0.1 N NaOH, shake 16 h, centrifuge and collect supernatant; Repeat.Add 20 ml of NaOH, sonicate at 75 watts for 2 min, shake for 16 h, centrifuge and collect supernatant.Add 30 ml of 1.0 M HCl, shake 16 h, centrifuge and collect supernatant.Digest with 5 ml of H2SO4 and H2O2

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