Soil Specific Surface Area After 40 Years of Different Organic and Mineral Fertilizer Use.
Francesco Morari and Chiara Pagliarin. DAAPV, Univ of Padova, Viale Dell'UniversitÓ, 16, Legnaro (PD), Italy
Anthropogenic activities have a fundamental role in reducing atmospheric CO2 concentrations. Long-term manure application is one of the most effective management practices for increasing the SOC pool in north-eastern Italy. Knowledge of the relationship between SOC and soil physical parameters is extremely important in order to understand the effect of RMPs on soil quality. Soil specific surface area (SSA) is an important indicator for describing the behaviour of soil organic carbon (SOC) stabilization and turnover. The effects of 40 years of organic and mineral fertilisations on SSA were evaluated in a continuous maize system. Since 1962 a long-term experiment has been underway on the University of Padova Experimental Farm (Northern Italy). We compared 4 different fertilisation treatments: only organic (L2, farmyard manure - 60 t ha-1; Lq2, liquid manure, 120 t ha-1 with crop residue incorporation), only mineral (M2, high mineral input - 300 kg ha-1 N) and a control (O, no fertilisation). The experimental layout was a randomised block with three replicates, on plots of 7.8 x 6 m. The soil was a fluvi-calcaric cambisol (CMcf), silty or sandy loam, with sub-basic pH. We investigated the SSA by N2 adsorption at 77░ K, applying the BET equation. The Thermo Electron Sorptomatic 1900 was used to perform the analysis. In each plot, about 0.5 Kg of soil was collected from the top layer (0-30 cm). The soil was dried, sieved to 2 mm and then crushed in a mortar. Prior to analysis 10 g of soil was degassed and air-dried at 105 ░C for 12 h. The blank analysis was carried out with He. The pore size distribution in the range 5-50,000 ┼ was also analysed, applying the Horvath/Kawazoe method. We defined 3 classes: 5-20 ┼, 20-500 ┼, 500-50,000 ┼. Results were analysed with ANOVA and multiple backward stepwise regression. The independent parameters considered in the multiple regression were texture, non-humic SOC and 3 humic fractions separated on the basis of apparent molecular weight: F1 (>100 kDa); F2 (10-100 kDa); F3 (<10 kDa). The treatments had no effect on SSA, which showed an average value of 5.2 m2 g-1. Significant differences (p<0.02) were observed in the values of the 20-500 ┼ class, with the highest relative volume in the L2 (27%) and Lq2 (26%). The 500-50,000 ┼ class represented 65% of the porosity and the 5-20 ┼ class only 9%. Multiple backward stepwise regression identified the humic fraction F3 as the only significant factor correlated with SSA (r = 0.73) and 5-20 ┼ pore class (r = 0.74). Instead the 500-50,000 pore class was correlated only with the clay content (r = 0.64). No correlation was observed with the humic fractions with the highest degree of polycondensation. Additional analysis are required to evaluate if the complex structure of the F1 and F2 fractions collapsed with the air dried pre-treatment or if the chemical nature of molecular sites and/or pore diameter couldn't be detected by the N2 adsorption method.