Rate of soil acidification under wheat in a semi-arid environment

Abstract

The rate of acidification under wheat in south-eastern Australia was examined by measuring the fluxes of protons entering and leaving the soil, using the theoretical framework of Helyar and Porter (1989). Monthly proton budgets were estimated for the root zone (0-90 cm layer) and for the 0-25 and 25-90 cm layers. After an annual cycle, the root zone was alkalinized by 0.5 to 3.1 kmol OH-ha-1. The alkalinity originated from the mineralization of the organic anions contained in the organic matter. The budget was near neutrality in the 0-25 cm layer (range: -1 . 0 to 1.4 kmol H + ha-l), whereas there was net alkalinization in the 25-90 cm layer (1.7 to 2.3 kmol O H -ha-a). In the 0-25 cm layer, the acidity produced in autumn by mineralization of organic nitrogen was counterbalanced by the alkalinity released from crop residues. The main acidifying factor in this layer was leaching of NO~-during early winter (2.4 kmol H + ha-1). Nitrate added through leaching was the main alkalinizing factor in the 25-90 cm layer, as added NO~-was taken up by the roots or denitrified in this layer. Urea fertilization had almost no effect on the rate of acidification, as little NO 3 was leached out of the root zone. The factors acidifying the soil under wheat were limited in this environment because of the small amount of NO 3 leached and the retention of the crop residues.

Acidification of Soil in a Dry Land Winter Wheat-sorghum/corn-fallow Rotation in the Semiarid U.S. Great Plains

“…The approach outlined by Helyar and Porter (1989) determines the inputs and outputs of protons (H + ) to and from the soil. Protons are either added or removed from the soil, or produced or consumed in reactions in the soil (Poss et al, 1995). The acid addition (AA), which is equivalent to the acidification rate (AR in equation 1) can be determined by taking into account the major components influencing the inputs and outputs of protons (N cycle, C cycle, Al cycle, Mn cycle, acid additions, and other alkali additions).…”

“…Additions of NO 3 -causes alkalinity due to the consumption of H + during the con-version of NO 3 -to organic N (R-NH 3 ) in the plant and during denitrification, and by the production of OH -as NO 3 -is assimilated (Avila-Segura et al, 2000; Barak et al, 1997;Fan et al, 2002;Tang et al, 2000). -is considered acid/base neutral (Poss et al, 1995). The acidifying effect of NH 4 + and the alkaline effect of NO 3 -from additions to the soil system and production from urea balance each other if all the NO 3 -applied in and formed from fertilizer is utilized by plants and converted back to into the original input forms (Bolan et al, 1991) The net reactions of ammoniacal fertilizers in soils and associated uptake and assimilation of nitrate by plants is acid/base neutral (Barak et al, 1997):…”

“…Soil acidification is generally more pronounced in areas of higher rainfall and on soils with low buffer capacities (Poss et al, 1995). However, soil acidification can occur over longer periods of time in arid climates.…”

“…However, soil acidification can occur over longer periods of time in arid climates. Ranges of acidification rates have been reported from near 0 to 20 kmoles H + ha -1 yr -1 in research located in a variety of climates and cropping systems (Poss et al, 1995). In a study conducted by Poss et al (1995), the calculated acidification rate of a wheat cropping system in semiarid Australia was between -1.0 and 1.4 kmoles H + ha -1 .…”

“…Ranges of acidification rates have been reported from near 0 to 20 kmoles H + ha -1 yr -1 in research located in a variety of climates and cropping systems (Poss et al, 1995). In a study conducted by Poss et al (1995), the calculated acidification rate of a wheat cropping system in semiarid Australia was between -1.0 and 1.4 kmoles H + ha -1 . This rate was measured in the top 25 cm of soil, which received annual applications of 157 kg N ha -1 as diammonium phosphate and urea.…”