Daroub, S. 1994. The effect of tillage on phosphorus transformations in soils. Dissertation, Michigan State University, East Lansing, Michigan, USA.

Conservation tillage is becoming more widely accepted as an alternative tillage system in crop production. Organic matter and microbial activity may increase with no-tillage (NT) compared to conventional tillage (CT). Accumulation of organic matter and phosphorus (P) near the surface of soil occurs in NT soils which may affect the distribution of P pools in soils. The effect of NT and CT on soil P fractions was investigated, and the turnover rates of 33P added in soybean residues to soils from three experimental sites in Michigan was determined. The role of microorganisms in the turnover of 33P added in residues was also studied in a never tilled soil. The effect of microorganisms was separated by sterilizing the soil with gamma irradiation before adding the residues. Phosphorus transformations were compared in the sterilized and non-sterilized soil.

Soybean plants labeled with 33P were added to soils, incubated at field capacity, extracted periodically (0, 6, 12, 18, 26, and 34 days), and P analyzed in the different P fractions. The fractions extracted were resin, NaHC03, microbial biomass, NaOH, NaOH after sonication, HCI, and residue P fractions.

With NT inorganic P accumulated in the calcareous soil which received P fertilizers. NaOH extractable organic P (Po) concentrations were higher in the NT treatments in the two non-calcareous soils. There was no effect of tillage on the labile Po pool.

The largest fraction of the applied 33P was found in the inorganic labile form. This fraction decreased with time with an increase in the NaOH fraction in all three soils. An increase in the HC1 fraction occurred in the calcareous soil. Phosphorus cycling through the microbial pool was evident before more of the 33P ended up in the NaOH fraction. The effect of tillage on the 33P turnover was minimal in all the P fractions extracted including the microbial fraction.

Twenty percent of the 33P was found in the microbial pool at day 12 in the nonsterilized never tilled soil. About 6% to 11% of the 33P found in the NaOH fraction is suspected to be organic and have cycled through the microbial pool.

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