Influence of Pinus pinaster age on aluminium fractions in acidic soils
Authors: Eimil-Fraga Cristina , Rodríguez-Soalleiro Roque , Fernández-Sanjurjo María José , Álvarez-Rodríguez Esperanza 
Published: September 2021

The influence of plantation age on the chemical properties of acidic soils was studied in 16 plots in adult <em>Pinus pinaster</em> stands established in Galicia (NW Spain). The Al fractions in the soil solid phase and the total Al in soil solution were determined in the upper soil layer (0-20 cm) and the lower soil layer (20-40 cm) in each plot. The pH, total C and N, exchangeable Ca, Mg, Na, K, and Al and Al saturation (% Al) were determined in the solid fraction. Aluminium was extracted from the solid phase with the following solutions: ammonium oxalate (Al<sub>o</sub>), sodium pyrophosphate (Al<sub>p</sub>), copper chloride (Al<sub>cu</sub>) and ammonium chloride (Al<sub>NH4</sub>). The total Al in the liquid phase was also determined. All soil chemical parameters, except total N, C/N ratio and % Al, were significantly influenced by soil depth. The mean pH was lower in the upper than in the lower layer (4.57 vs. 4.97), but the opposite was observed for the organic C (77.2 vs. 50.4 g kg<sup>-1</sup>), the effective cation exchange capacity (eCEC) (9.43 vs. 6.25 cmol<sub>(+)</sub> kg<sup>-1</sup>), P (8.95 vs. 4.65 mg kg<sup>-1</sup>) and the exchangeable cations. Organic matter, total N and eCEC were significantly and positively correlated with plantation age (r = 0.69 in the upper layer and r = 0.82 in the lower layer, p &lt; 0.01; r = 0.62, p &lt; 0.05 in the upper layer and r = 0.78, p &lt; 0.01 in the lower layer; r = 0.77, p &lt; 0.01 in the upper layer and r = 0.85, p &lt; 0.0001 in the lower layer, respectively), and pH<sub>KCl</sub> was negatively correlated with plantation age (r = -0.55 in the upper soil layer and r = -0.61 in the lower soil layer, p &lt; 0.05). The concentrations of the different Al forms in all soils decreased in the order Al<sub>p </sub>&gt; Al<sub>o </sub>&gt; Al<sub>cu </sub>&gt; Al<sub>NH4</sub>. Highly stable organo-aluminium complexes (Al<sub>p-cu</sub>) predominated over moderate and low stability complexes (Al<sub>cu</sub>) in all soil plots. The highly stable organo-Al complexes were significantly more abundant in the lower layer, whereas the opposite was observed for the exchangeable Al and the total Al in soil solution. The concentrations of all Al forms (except Al<sub>p-cu</sub>) were significantly and positively correlated with plantation age (Al<sub>o</sub> r = 0.50, p &lt; 0.05 for the upper layer and r = 0.67, p &lt; 0.01 for the lower layer; Al<sub>p</sub> r = 0.64, p &lt; 0.01 for the lower layer; Al<sub>cu </sub>r = 0.84 for the upper layer and r = 0.83 for the lower layer, p &lt; 0.0001; Al<sub>cu-NH4</sub> r = 0.65 for the upper layer and r = 0.78 for the lower layer, p &lt; 0.01; Al<sub>NH4</sub> r = 0.76, p &lt; 0.01 for the upper layer and r = 0.84, p &lt; 0.0001 for the lower layer; total Al in soil solution r = 0.61 for the upper layer and r = 0.60 for the lower layer, p &lt; 0.05). Stepwise linear regression analysis showed that plantation age, pH and total C explained between 67% and 93% of the variance in the Al forms. In all regression models, plantation age was a significant predictor variable for the different Al fractions, except total soluble Al, which is an important variable to consider in the study of chemical properties in forest soils.