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

It is well established that in the moderately acidic soils of southern Aust ralia, 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 contrad ictory or speculative reports in the literature on how soil moisture fluctu ation affects pH and processes which influence pH. Therefore, the aim of th is study was to determine the effect of moist-dry cycles on pH, and on proc esses involving H+ transformations, in 3 surface soils (0-2 cm) sampled fro m southern New South Wales. Following a pre-incubation, the 3 surface soils were incubated for 28 days at 30 degrees C and were: (i) maintained continuously dry, (ii) subjected t o 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 b y 0.03-0.10 units, while the pH of continuously moist soil decreased by 0.1 6-0.39 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 moi st, exposure to moist-dry cycles suppressed acidification by 0.05-0.26 pH u nits. In dry soils the pH increased, since some of the NH4+-N produced by net N m ineralisation was not subsequently nitrified, and there was a net reduction of Mn. In soils which received water, acidification was predominately attr ibuted 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 nitrific ation on exposure to moist-dry cycles. In acidic surface soils (pH <5.5), a cidification 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 contin uously moist soils.