Effect of rewetting air‐dried soils on pH and accumulation of mineral nitrogen

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Abstract
SUMMARY: The effect of rewetting a number of air‐dried soils on pH and on accumulation of mineral‐N was examined in a laboratory incubation study. When rewetted‐soils were incubated at 25°C three patterns of change in soil p H and in accumulation of mineral‐N were observed. Ammonification and nitrification proceeded together in soils with pH values greater than 6.0; soil pH decreased whilst concentrations of nitrate rose and those of ammonium remained low. By contrast, in soils with pH values less than 5.0, although ammonification proceeded there was no appreciable nitrification; soil pH increased whilst concentrations of ammonium rose and those of nitrate remained very low. In a third group of soils with pH values between 5.0 and 5.5, there was a delay in nitrification, but ammonification was not retarded; soil p H initially rose as concentrations of ammonium increased, but when nitrification subsequently commenced the p H decreased, concentrations of nitrate rose and those of ammonium declined. When microbial activity in rewetted soils was inhibited by incubation at 3°C, or in a chloroform atmosphere at 25°C, there was little change in concentrations of ammonium and nitrate, and soil pH remained relatively constant. Such changes in soil p H, induced by ammonification and nitrification, are likely to have important consequences to soil chemical studies where p H‐dependent reactions are being studied using rewetted soils. Changes in pH can be minimized by using field moist rather than air‐dried soils.

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