Saturday, 15 July 2006

Humic Acids and Aggregate Stability in Organically Amended Semiarid Soils.

Emanuela Margherita1, Gennaro Brunetti2, Carlos G. Izquierdo3, Nadia Vignozzi4, Sergio Pellegrini4, Marcello Pagliai4, and Nicola Senesi1. (1) DIBCA - Univ of Bari, Via Amendola, 165/a, Bari, 70126, Italy, (2) DIBCA - Univ of Bari, Via Amendola, 165/a, Bari, 70126, Italy, (3) Centro de Edafologia y Biologia Aplicada del Segura - CEBAS-CSIC, Avda. de la Fama, 1, Murcia, 30080, Spain, (4) CRA-ISSDS, Piazza D'Azeglio 30, Florence, 50121, Italy

Mediterranean semiarid regions, such as Southern Italy and Spain, soil erosion is a severe problem. A suitable method for recovering degraded soils is the amendment with adequately-treated organic wastes such as municipal solid residues and sewage sludges. Soil organic matter (SOM), and in particular its humic fraction, is an important factor that controls aggregate stability by promoting the binding of primary soil particles into stable aggregates. The objective of this work was to evaluate the effect of various soil organic amendment in restoring the quality of a degraded soil after seven years from biomass application. Changes of soil humic acid (HA) structure, and aggregate stability were measured as a function of the type of organic amendment used and the position in respect to soil slope. The field experiment was conducted in an abandoned agricultural area located in the Murcia region, Southeastern Spain. A total of 25 Kgm-2 of each of three organic amendment, raw sewage sludge (FS), composted sewage sludges (CS), a mixture of FS and a commercial humic substance (FS+HS), and a mixture of FS and barley straw (FS+P), was added in a single application to 15 x 2 m2 plots with a slope of 15% from the top to the bottom. An unamended plot was used as a control (C). Seven years after amendment soil samples were collected from the surface layer (0 to 20 cm) of the tophill (t) and the bottomhill (b) portion of each plot, and were labelled: FSt and FSb, CSt and CSb, FS+HSt and FS+HSb, FS+Pt and FS+Pb, Ct and Cb. Soil samples were air-dried, crushed, passed through a 2-mm sieve, and HAs were isolated according to the procedure suggested by the International Humic Substances Society (IHSS). The HAs were analysed for their elemental composition, atomic ratios and ash content, and by visible, Fourier trasform infrared and fluorescence spectroscopies. Aggregates (2<f>1) were separated from the bulk soil by passing through a 2-mm and a 1-mm sieve, and their stability was analysed according to the method of Cavazza (1981). The HA yields of all amended soil samples are higher than those of the corresponding control soil HAs, which indicates that organic amendment increases the humified organic fraction content in soil. The HA yield of soil Cb is lower than that of soil Ct, whereas the opposite generally occurs for Cb an Ct HAs of amended soils. These results suggest that the control soil plot, which is subjected to more intense loss and runoff processes, contain a higher amount of HAs in the tophill portion, possibly due to the presence of more stable aggregates. The chemical and spectroscopic characteristics of HAs are generally similar for all samples. In particular, the 3-D fluorescence spectra of HAs are characterized by a unique main fluorophore at a λex/ λem = 450-455/535-540. The significantly higher fluorescence intensity (I) of C-HAs, and expecially of the Cb -HA, than that of the other HAs suggests a decreased humification level of HAs in amended soils, with respect to HAs of the control soil. The lower humification level of Ct-HA with respect to Cb -HA is possibly a consequence of runoff processes that affect the tophill soil of the control plot. Table 1 shows the aggregate stability as water-stable aggregates (SA%) of all soil samples. The SA% of amended soils is higher than that of the control soil. The sample Cb shows the lowest value of SA, thus confirming the hypothesis that the control soil plot, which is subjected to more intense loss and runoff processes, contains a higher amount of HA and more stable aggregates in the tophill portion. The samples CS, FS+HS and FS+P have the highest SA values, and no significant difference is apparent between the tophill and the bottomhill soils of these plots. SOM contents and HA yields are positively related to SA values. Thus, plots amended with more stable organic matter (CS and FS+HS) and with a greater plant cover feature a greater aggregate stability and a reduced loss and runoff, even if their HAs have a lower humification level with respect to the control soil plot.


Table 1. Water stable aggregates of amended soil samples in comparison with control soil.

Values with a different superscript letter are significantly different at a p value = 0.05.

<>Sample Stable aggregates %





























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