# Long Term Effects of Forest Liming on the Acid-Base Budget
## 4 Discussion  
Liming forests in Rhineland-Palatinate was performed under the assumption of rapid acidification of soils taking place by air pollutants from anthropogenic sources [43]. During the 24 years, the control plots show that the acid load noticeably exceeds the proton consumption by \(\mathrm{M_{b}}\) cations. Without limiting, \(\mathrm{M_{b}}\) cations compensated for less than 15% of the acid load (Table 3). As a result \(\mathrm{M_{a}}\) cations, especially Al and Mn, are released, which can destabilize clay minerals [15; 44], disturb plant nutrition [45; 46; 47], damage fine root systems [48], and reduce the activity of soil biota [49]. With regard to the three study plots, liming partially increased the acid load because of (1) a higher output of anions (\(\mathrm{SO_{4}}^{2-}\), \(\mathrm{NO_{3}}^{-}\), \(\mathrm{Org}^{-}\)) with the seepage water and (2) a higher proton production through the incorporation of more cations into the biomass. However, the reduction of \(\mathrm{M_{a}}\) cation release below the level of the control plot shows that the higher acid load was compensated by the carbonates of the dolomitic limestone. The clay minerals were stabilized on the plots of the liming treatments whereas on the control plots an ongoing destabilization could be observed. However, this stabilization effect started to diminish for the low-dose treatments after approximately 20 years and was limited to the upper 5 cm of the mineral soil [50; 51; 52]. On the three sites, the loss of anions is linked with a loss of metal cations, which is equivalent to quantitative acidification of the ecosystem [14]. Negative consequences are the depletion of the soil nutrient reserves if \(\mathrm{M_{b}}\) cations are leached together with anions, or the pollution of spring water and other adjacent fresh water if the anions are relocated together with \(\mathrm{M_{a}}\) cations like Al\({}^{3+}\). The high loss of \(\mathrm{SO_{4}}^{2-}\) and aluminum in AD caused by the liming treatments indicates dissolution of aluminum sulfates due to changed pH value (cf. [53]). To a lower extent, this was also observed in IO at a soil depth of 10 cm [22]. In IO the \(\mathrm{SO_{4}}^{2-}\) output of the control plot is already on a high level whereby liming induced none or only a slight increase. In HS, the missing effect of the liming treatments on the \(\mathrm{SO_{4}}^{2-}\) output is caused by the soil type. Sandy soils have a lower storage capacity for \(\mathrm{SO_{4}}^{2-}\) than more cohesive soils with a higher percentage of clay [54]. Because of the lower S pools in HS [22], there is a smaller potential of a high long-term \(\mathrm{SO_{4}}^{2-}\) release, as observed in AD and IO. Figure 2: Acid load through biomass increment when \(\mathrm{NO_{3}}^{-}\) and \(\mathrm{NH_{4}}^{+}\) are taken up with different proportions for the control plots and the 3 and 15 t h\({}^{-1}\) liming treatments of the three study areas. An uptake of N only as \(\mathrm{NO_{3}}^{-}\) (100/0) leads to a lower acid load, an uptake of N only as \(\mathrm{NH_{4}}^{+}\) (0/100) leads to a higher acid load. For the following calculations, it is assumed that \(\mathrm{NH_{4}}^{+}\) and \(\mathrm{NO_{3}}^{-}\) are taken up with the same proportion (50/50, highlighted in the figure by the grey area). Negative values in IO stand for proton consumption by biomass increment. The limiting treatment 7 with the additional sulfur bound K and Mg fertilizers (Table 2) has the highest acid load due to the high SO42\({}^{-}\) output. The loss of SO42\({}^{-}\) led to a coupled output of Mg and Al. On the other hand, K was retained in the ecosystem. Higher K fluxes compared to the control plots could only be observed in soil depth of 10 cm, but not below the rooting zone in 60 cm soil depth. Also, the forest stands show a tendency for higher K concentrations in older needles and in needle litter compared to the other high-dose limiting treatment 5 and 8 [22]. The N budget in AD is the main source for the proton production on the control plot (Table 4).