The acidification due to organic matter accumulation (O[A.sub.ac]) was estimated on the soil layer 0-0.175 m depth interval because there are no available C stock data for deeper layers at the beginning of the experiment and because in no till systems the accumulation of organic matter occurs predominantly in surface layers.
The loss of ammonium and organic anions in runoff and leaching was neglected, and the effect of inorganic C pool was not accounted for (input of HC[O.sub.3.sup.-] by rainfall and its loss by leaching or run-off) on soil acidification. Lime was not applied to the soil since the establishment of the experiment, and acid rainfall was not detected in the locale.
By difference between the total net acidification and the acidification attributed to the aforementioned components, the effect of nitrate leaching on soil acidification was estimated.
The effect of clover on soil acidification is highlighted in the literature (Ridley et al.
In the C cycle, while acids exudated by roots due to an excess of cation uptake over anions during plant growth are distributed in all soil layers explored by roots, plant residues are mainly oxidised at the soil surface, thus promoting acidification in subsoil and alkalinisation in topsoil.
Nitrogen mineral fertilisation and soil acidification
According to Fenton and Helyar (2002), although urea hydrolysis generates [H.sup.+] and, therefore, is acid in the short-term (Eqn 4), N added to the soil as urea or through biological fixation causes net acidification in medium- and long-terms only if there is N loss as nitrate leaching.
Mineral N application, in affecting soil acidification, also affected exchangeable A1 (Fig.
The net soil acidification is a function of the change in pH and the pH buffer capacity of the soil.
Afterwards, the net soil acidification (kmol [H.sup.+]/ha) in the layer 0-0.30 m was assessed for the period of 19 years using Eqn 1, in addition to the data for pH-[H.sub.2]O and bulk density from Table 2, and the results were converted to equivalent CaC[O.sub.3] (Table 3).
Net acidification occurred in all treatments, ranging from 20.4 to 50.4 kmol [H.sup.+]/ha (equivalent to 1.02 Mg CaC[O.sub.3]/ha and 2.52 Mg CaC[O.sub.3]/ha, respectively) for the period of 19 years, which corresponded to rates from 1.07 to 2.65 kmol [H.sup.+]/ha.year.
The acidification rates estimated in the present study are consistent with those usually observed in agricultural ecosystems, from 2 to 5 kmol [H.sup.+]/ha.year, according to Helyar (2003).