Search

The rhizosphere of Brachypodium retusum was surveyed in eight semiarid soils from southeastern Spain. The percentages of stable aggregates, microbial populations, organic matter content, and carbohydrate contents were assessed in order to establish correlations between these parameters. The percentages of stable aggregates were highly correlated with total hyphal length in soil (R = 0.9444, P < 0.001) and. to a lesser extent, with total bacteria (R = 0.8714. P < 0.01) and culturable fungal propagules (R = 0.7256, P < 0.005). No correlation was found between aggregation levels and carbohydrate contents in soils. The estimation of total fungi and bacteria was a more accurate method than the assessment of culturable populations when the relative importance of micro-organisms were considered as factors in soil aggregation processes.

Plants of Hedysarum confertum Desf. and Hedysarum spinosissimum L. inoculated with five Glomus species (G. epigaeum, G. etunicatum, G. fasciculatum, G. macrocarpum and G. mosseae) were grown in a marl soil for 14 weeks. Both legumes became colonized with VAM fungi, although colonization percentages were lower than those observed in nature. For H. confertum, G. epigaeum, G. fasciculatum, and G. macrocarpum increased growth significantly when compared to controls. For H. spinosissimum the most efficient fungi for growth improvement were G. macrocarpum, G. mosseae, and G. etunicatum. A clear growth-depressing effect of G. fasciculatum on H. spinosissimum was observed. Root/shoot ratio increases with inoculation in H. confertum but can decrease in H. spinosissimum. P uptake was not markedly improved by mycorrhizal inoculation. The greatest amounts of P occurred in both legumes colonized by G. macrocarpum.

Erosion mapping is a useful tool for soil conservation planning. The possible correlation between soil type or lithology with soil erodibility has been tested with the aim of facilitating the accomplishment of erosion maps from soil or lithology maps. Soil erodibility (K factor of the universal soil loss equation [USLE]) was determined from 340 samples taken from 4 soil types, on which approximately 95% of agriculture in the semiarid Mediterranean area is located. Calcaric regosols (K = 0.47) showed the highest erodibility and are significantly different from the other soil types. It was also possible to differentiate between calcic xerosols (K = 0.41) and petrocalcic xerosols (K = 0.37). No significant differences were established between the other soil types. The statistical analysis of the soil samples, grouped according to lithology, showed a better discrimination than is evident from those grouped as a function of soil type. It was possible to establish significant differences among all lithologies with the exception of alluvial (K = 0.38) and colluvial (K = 0.38) sediments, which showed a 30% probability that the mean values of K were equal. Marls (K = 0.46) were the most erodible materials. Variance analysis, taking into consideration both soil type and lithology, did not improve the discrimination. The results from this study suggest that lithological maps may be used for delineating zones with different erodibility.

When soils of semiarid areas are used for agriculture, their arbuscular mycorrhizal (AM) fungal propagules undergo a series of changes, and when agricultural land is abandoned it is very difficult for these populations and the plant cover to recover. We studied soils that have been abandoned for different lengths of time (3-45 years) in a homogeneous semiarid area in order to observe changes in their physical and chemical properties and in the AM fungi propagules, comparing them with soils that continue to be cultivated and a soil that has never been cultivated. The data we collected clearly showed that agricultural use reduces soil fertility and lowers AM fungi populations compared to the soil kept in its natural state. After abandonment, there is a 5-year period when the soils undergo a greater degree of degradation after which they slowly recover, with AM fungi propagules reaching values similar to those of the virgin soil after 45 years or so. Although the physical and chemical properties followed a similar pattern of recovery, it was not possible to establish significant correlations between these parameters and the recovery of the AM fungi. Except in isolated cases, the distribution of AM fungi did not seem to be influenced by position on the landscape, with their recovery appearing to be more influenced by the presence of host plants. In this sense, the rhizosphere o/Anthyllis cytisoides showed normal values of root infection and spore numbers even during the initial stages following abandonment.

Structural stability of soils is one of the most important characteristics related to soil degradation in semiarid areas. Organic additions can considerably improve soil structure and prevent soil degradation. In this experiment the effectiveness was compared of four types of organic amendments on the improvement of soil structure. The four treatments are sewage sludge, fresh uncomposted urban refuse, composted urban refuse, and horse manure. The uncomposted urban refuse was the most effective in increasing soil stable aggregates. The higher doses of sewage sludge and compost also improved aggregation of soil particles. Horse manure did not significantly improve soil aggregation. A significant correlation (r = 0.955, ρ < 0.01) was found between fungal populations and stable aggregates. No significant correlations were found between stable aggregates in soil and the other parameters assessed. The organic treatments increasing soil microbial populations and promoting their activity appear to be very effective in improving soil structure.