Impacts of elemental S applied under various temperature and moisture regimes on pH and available P in acidic, neutral and alkaline soils
We evaluated the effect of elemental S (S0) under three moisture (40, 60, 120% water-filled pore space, WFPS) and three temperature regimes (12, 24, 36°C) on changes in pH and available P (0.5 N NaHCO3-extractable P) concentrations in acidic (pH 4.9), neutral (pH 7.1) and alkaline (pH 10.2) soils. Repacked soil cores were incubated for 0, 14, 28 and 42 days. Application of S0 did not alter the trends of pH in acidic and neutral soils at all moisture regimes but promoted a decrease in the pH of alkaline soil under aerobic conditions (40%, 60% WFPS). Moisture and temperature had profound effects on the available P concentrations in all three soils, accumulation of available P being greatest under flooded conditions (120% WFPS) at 36°C. Application of S0 in acidic, neutral and alkaline soils resulted in the net accumulation of 16.5, 14.5 and 13 g P g–1 soil after 42 days at 60% WFPS, but had no effect under flooded conditions. The greatest available P accumulations in the respective soils were 19, 19.5 and 20 g P g–1 soil (equivalent to 38, 41, 45 kg P ha–1) with the combined effects of 36°C, 60% WFPS and applied S0. The results of our study revealed that oxidation of S0 lowered the pH of alkaline soil (r=–0.88, P<0.01), which in turn enhanced available P concentrations. Also, considering the significant relationship between the release of sulphate and accumulation of P, even in acidic soil (r=0.92, P<0.01) and neutral soil (r=0.85, P<0.01) where the decrease in pH was smaller, it is possible that the stimulatory effect of sulphate on the availability of P was due to its concurrent desorption from the colloidal surface, release from fixation sites and/or mineralization of organic P. Thus, in the humid tropics and irrigated subtropics where high moisture and temperature regimes are prevalent, the application of S0 could be beneficial not only in alleviating S deficiency in soils but also for enhancing the availability of P in arable soils, irrespective of their initial pH.