• Energy Research

The degree of degradation of soils can be measured by reference to a variety of chemical, physical, biochemical and biological properties, the last two being particularly sensitive to the changes that take place. Among these parameters, those related with the soil carbon content, such as total organic carbon and microbial biomass carbon, are of the greatest interest although others are also important indicators of microbial activity, including ATP and phosphatase activity. This study looks at the microbial activity of various soils in SE Spain which present different degrees of plant cover. The results demonstrate that vegetation is of fundamental importance for the development of microbial populations and their activity since soils with a substantial vegetal cover showed higher values for the above parameters than those with a reduced cover, which are more degraded. A seasonal study of the soil properties showed that the time of year exercises an important influence on the parameters that are indicative of microbial activity.

Water shortage and low organic carbon content in soil limit soil fertility and crop productivity. The use of desalinated seawater is increasing as an alternative source of irrigation water. However, it has a high boron (B) content that could cause toxicity in the plant-soil microbial system. Here, we evaluated the responses of the soil microbiota and lemon trees to 3 irrigation B doses (0.3, 1, and 15 mg L-1) under two types of soil management (conventional, CS; and organic, OS) in a 180-days pot experiment. High B doses promoted B accumulation in soil, reaching harmful concentrations that affected soil biodiversity. Our results suggest a close interaction between B and organic labile fractions that increased B availability in soil solution. Besides, B addition to soil impacted on microbial biomass. The bacterial community showed sensitivity to the B dose. Organic amendment did not increase B soil adsorption but it favored B plant uptake. The highest B dose had a detrimental impact on plant physiology, finally resulting lethal for the plants. Our study provides a comprehensive assessment of the microbes-plant interactions in soils irrigated with water with high B content. This will be fundamental in the design of future fertirrigation strategies.

The increasing use of silver nanoparticles (AgNPs) due to their well-known antimicrobial activity, has led to their accumulation in soil ecosystems. However, the impact of environmental realistic concentrations of AgNPs on the soil microbial community has been scarcely studied. In this work, we have assessed the impact of AgNPs, that mimic real concentrations in nature, on tropical soils cultivated with Coffea arabica under conventional and organic management systems. We evaluated the biomass, extracellular enzyme activities, and diversity of the soil microbial community, in a microcosm experiment as a function of time. After seven days of incubation, we found an increase in microbial biomass in an AgNPs-concentration-independent manner. In contrast, after 60-day-incubation, there was a decrease in Gram+ and actinobacterial biomass, in both soils and all AgNPs concentrations. Soil physico-chemical properties and enzyme activities were not affected overall by AgNPs. Regarding the microbial community composition, only some differences in the relative abundance at phylum and genus level in the fungal community were observed. Our results suggest that environmental concentrations of AgNPs affected microbial biomass but had little impact on microbial diversity and may have little effects on the soil biogeochemical cycles mediated by extracellular enzyme activities.

Extensive green roofs are used to increase the surface area covered by vegetation in big cities, thereby reducing the urban heat-island effect, promoting CO2 sequestration, and increasing biodiversity and urban-wildlife habitats. In Mediterranean semi-arid regions, the deficiency of water necessitates the use in these roofs of overall native plants which are more adapted to drought than other species. However, such endemic plants have been used scarcely in green roofs. For this purpose, we tested two different substrates with two depths (5 and 10 cm), in order to study their suitability with regard to adequate plant development under Mediterranean conditions. A compost-soil-bricks (CSB) (1:1:3; v:v:v) mixture and another made up of compost and bricks (CB) (1:4; v:v) were arranged in two depths (5 and 10 cm), in cultivation tables. Silene vulgaris (Moench) Garcke and Lagurus ovatus L. seeds were sown in each substrate. These experimental units were subjected, on the one hand, to irrigation at 40% of the registered evapotranspiration values (ET0) and, on the other, to drought conditions, during a nine-month trial. Physichochemical and microbiological substrate characteristics were studied, along with the physiological and nutritional status of the plants. We obtained significantly greater plant coverage in CSB at 10 cm, especially for L. ovatus (80-90%), as well as a better physiological status, especially in S. vulgaris (SPAD values of 50-60), under irrigation, whereas neither species could grow in the absence of water. The carbon and nitrogen fixation by the substrate and the aboveground biomass were also higher in CSB at 10 cm, especially under L. ovatus - in which 1.32 kg C m(-2) and 209 g N m(-2) were fixed throughout the experiment. Besides, the enzymatic and biochemical parameters assayed showed that microbial activity and nutrient cycling, which fulfill a key role for plant development, were higher in CSB. Therefore, irrigation of 40% can maintain an adequate plant cover of both endemic species, particularly in a deeper and soil-containing substrate.

The high boron (B) content in desalinated seawater is a concern for crop development. However, in spite of the importance of the soil microbial community in soil fertility, the below-ground impacts of B are still unknown. Here, in a soil-ryegrass model system, the activity, biomass and diversity of the soil microbial community were evaluated in response to irrigation with: i) 0.3 mg B L-1; ii) 1 mg B L-1; and iii) 50 mg B L-1. We assessed two different compounds of boron: boric acid (H3BO3) and disodium tetraborate decahydrate (Na2B4O7·10H2O). Overall, the 1 mg B L-1 dose was identified as the threshold limit that did not irreversibly harm soil sustainability. In contrast, the highest B dose had a noticeable impact on the nitrogen (N) cycle of the soil, as demonstrated by an increase in the water-soluble N content and a decrease in urease activity. Analysis of the phospholipid fatty acids (PLFAs) revealed that the effect of B on the soil microbial biomass was dependent on the chemical form used. High B doses reduced soil microbial respiration and influenced the composition of the bacterial and fungal communities, with fungal diversity being diminished, as revealed by sequencing approaches.