Influence of moist–dry cycles on pH changes in surface soils

Abstract

It is well established that in the moderately acidic soils of southern Australia, the 0–2 cm layer commonly has a higher pH than soil layers between 2 and 10 cm depth. The surface 2 cm of soil is also exposed to much greater fluctuations of moisture content than deeper soil layers. There are contradictory or speculative reports in the literature on how soil moisture fluctuation affects pH and processes which influence pH. Therefore, the aim of this study was to determine the effect of moist–dry cycles on pH, and on processes involving H⁺ transformations, in 3 surface soils (0–2 cm) sampled from southern New South Wales.

Following a pre-incubation, the 3 surface soils were incubated for 28 days at 30°C and were: (i) maintained continuously dry, (ii) subjected to short (2 days dry, 5 days moist) or long (7 days dry, 7 days moist) moist–dry cycles, or (iii) maintained continuously moist.

During the incubation, the pH of continuously dry soil slightly increased by 0.03–0.10 units, while the pH of continuously moist soil decreased by 0.16–0.39 units. In soils subject to both short and long moist–dry cycles, the pH decreased by 0.06–0.34 units. However, relative to soils maintained moist, exposure to moist–dry cycles suppressed acidification by 0.05–0.26 pH units.

In dry soils the pH increased, since some of the NH₄⁺-N produced by net N mineralisation was not subsequently nitrified, and there was a net reduction of Mn. In soils which received water, acidification was predominately attributed to nitrification. Relative to soils maintained moist, acidification was suppressed by 1.6–6.5 mmol H⁺/kg due to the 11–35% decrease of nitrification on exposure to moist–dry cycles. In acidic surface soils (pH <5.5), acidification rates were further suppressed by 0.1–1.0 mmol H⁺/kg due to the 1.06–2.06 times greater net Mn reduction in moist–dry soils than in continuously moist soils.