Restoration techniques affect soil organic carbon, glomalin and aggregate stability in degraded soils of a semiarid Mediterranean region

The first step to restoring degraded mine soils from calcareous quarries in semiarid environments, usually without soil structure, mainly consists in creating a structured topsoil with suitable physical, chemical and biological properties. The aim of this study is to determine the effects of organic amendments and mulches on soil aggregate stability and aggregation-associated soil characteristics, six years after beginning experimental restoration in the Gádor Mountains (Almería, SE Spain). Experimental plots were set up to test two organic amendments (sludge and compost) and two mulches (gravel and woodchip) and their respective control plots. Soil samples from neighboring undisturbed soils were used as the quality reference threshold. The tested variables were total organic C (TOC), glomalin-related soil protein (GRSP), easily extractable glomalin-related soil protein (EE-GRSP) and water aggregate stability evaluated by both wet sieving (WS) and water-drop test (WDT). Relationships among the measured soil properties were checked in order to assess the best indicators for the most suited restoration practices. After 6 years, the results showed that the combination of organic amendments and mulches enhanced soil aggregate stability and the content of aggregate binding agents such as TOC and glomalin. Nevertheless, the role of organic amendments, especially compost, was more important than mulch treatments in increasing TOC and glomalin, showing the closest values to the undisturbed reference soils (over 30 g kg-1 for TOC and 3.5 g kg-1 for GRSP). Despite the considerable improvement in water stable aggregates found in sludge-amended plots (average mean weight diameter of 2.13 mm in WS, and 25-drop impacts in WDT), the reference soils provided the highest values (average mean weight diameter of 3.32 mm in WS, and 99-drop impacts in WDT). The lack of a good correlation between soil structure-related variables restricted the evaluation of the real effects of restoration treatments, and suggested considering other soil properties (e.g., hydrophobicity, hardening) associated to aggregate stability

Experimental evidence of laser diffraction accuracy for particle size analysis

Laser diffraction analysis is a fast, reliable and automated method that provides detailed and highly resolved soil and sediment particle size distribution. In recent studies, the methods were compared against independent methods based on direct observation of particles by digital imaging. The data showed that laser diffraction results were in better agreement with the digital imaging independent method than with sedimentation-based methods. However, analysis was performed over a limited number of samples. In this study, 47 soil samples with a wide range of textural properties were analyzed with Laser Diffraction, Pipette, Sieving, Sedigraph and Digital Imaging methods. Detailed statistical analysis using Altman plots and Honest Significant Difference tests demonstrated (at 95% significance) that the five methods do not show statistically significant differences for grain sizes above 100 mu m. However, in the lower end of the size range, i.e. less or equal to 50 mu m, Laser Diffraction showed much better agreement with the reference method selected for comparison, which was Digital Imaging. New regression equations were derived with slope coefficients for linear regressions between Pipette and Laser of 0.2952 (R-2 = 0.8625) for clay, 1.4261 (R-2 = 0.5746) for silt and 1.031 (R-2 = 0.6586) for sand, classified with the International Soil Science Society (ISSS) system. For the United States Department of Agriculture (USDA) classification system, the slopes were: 0.261 (R-2 = 0.8625) for clay, 1.3493 (R-2 = 0.8179) for silt and 1.063 (R-2 = 0.888) for sand. These data were consistent with previous studies. Based on regression and equivalent diameters, Laser Diffraction data were represented on textural triangles for classification, allowing for employing Laser Diffraction for soil texture classification. Two alternative for representing the Laser Diffraction data in textural triangles were employed: (1) using regression equations to convert data to be represented on the standard triangles and (2) modify the upper limit for the clay range, from 2 to 8 mu m. Finally, based on the additional evidence presented in this research, demonstrating that the Laser Diffraction was in better agreement with the optical method with respect to traditional sedimentation methods, it is suggested that the standards for particle size analysis be changed from sedimentation to Laser Diffraction methodologies