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The application of organic residues has been presented as an adequate strategy against soil degradation in semiarid environments. However, the interactions between organic amendments and drought are not fully known. Here, we evaluate whether sludge and compost amendment in semiarid areas influences the stability of the soil microbial community and microbially-mediated processes against drought. Sludge-amended, compost-amended and control (without amendment) soils were collected from a long-term restoration trial in southeastern Spain. A drought-induction model was initiated by first pre-incubating soil samples from each treatment at an optimum water-holding capacity (WHC) of 60%. One subset of samples was then partly dried and subsequently maintained at 20% of the WHC (induced-drought samples) and another subset was maintained at 60% of the WHC. The responses of the microbial biomass (through the analysis of phospholipid fatty acids, PLFAs), enzyme activities related to the C, N and P cycles, microbial diversity and microbial populations (through 16S rRNA and ITS amplicon sequencing) were analysed after 2, 9, and 45 days of incubation. In parallel samples, the mineralisation of soil organic matter (SOM) and fresh-organic matter (FOM) were evaluated by tracking the isotope signature of CO 2 after addition of 13 C-enriched plant tissue (97 atom %). Overall, we found that: i) the soil microbial biomass was greater in amended soils at 20% WHC than in the corresponding soils at 60% WHC after 45 days of incubation; ii) changes in the soil microbial biomass were accompanied by changes in the relative abundances of microbial populations; iii) the release of CO 2 from SOM and FOM was diminished in soils at 20% WHC, in comparison to soils at 60% WHC, while β-glucosidase and urease activities were higher in amended soils at 20% WHC than in soils at 60% WHC; and iv) bacterial and fungal diversity did not change as a consequence of drought. A multi-level characterisation of the soil microbial community provided a better understanding of the responses of amended soils to drought. [ABSTRACT FROM AUTHOR]

Sustainable agriculture in Mediterranean areas is compromised by the structural deficit of water resources. Under this situation, the impacts of alternative water managements on the microbial community, as a critical component of the soil quality, need to be properly understood. We evaluated the long-term impacts of irrigation systems differing on the quantity and quality of water, and their interactions, on the biomass (phospholipid fatty acid analysis), diversity and composition (16S rRNA gene profiling), and enzyme activities of the soil microbial community of an orchard cultivated with grapefruit trees in South-East Spain. The impact of water quantity was evaluated by irrigation with optimal amount of water or by irrigation with a reduced volume of water in the temporal frame when the crop is less sensitive, so-called regulated deficit irrigation (RDI). The impact of water quality was evaluated attending to the source of the irrigation water: water from a river channel-transfer (TW) or reclaimed water from a wastewater treatment plant (RW). Electrical conductivity was higher in soils irrigated with RW than in soils irrigated with TW. The content of total organic C in the soil was affected by water quality but not by water quantity. Soils irrigated with TW showed higher total organic C than soils irrigated with RW. As in the case of plant productivity, RDI had a negative impact on plant productivity, soil microbial biomass and enzyme activities in summer. This finding indicates a slow-down of organic matter decomposition under restricted irrigation. Bacterial biomass was more sensitive to RDI when RW was used, whereas the fungal biomass was more sensitive to RDI when TW was used. Bacterial diversity and plant productivity were more sensitive to water quantity than to water quality. The increase of the abundance of Proteobacteria and Bacteroidetes in soils irrigated with RW in summer suggested a higher resilience of this treatment mediated by copiotrophic organisms. A recovery of the enzyme activity and microbial biomass of soils irrigated with RW and RDI was observed in January and June. The resilience of biogeochemical and the microbial biomass processes after RDI coursed through changes in the structure of the microbial community as revealed by the multivariate analyses of fatty acids. The utilisation of reclaimed water during RDI promoted a more-resilient community that translated into a recovery of microbial biomass and enzyme activities after the water restriction ended. These results imply potential ecological benefits of the irrigation with reclaimed water that should be considered under the water limitation predicted in climate change models in Mediterranean areas. [ABSTRACT FROM AUTHOR]

Abstract: Although the application of organic amendments is considered a suitable tool for improving soil fertility, few studies have been conducted in semiarid climates to evaluate the joint effect of such practice on the structure and function of the soil''s microbial community. The aim of this work therefore was to make a comparative study of the effect of organic materials of differing degrees of stabilization (a sewage sludge and a compost made from the same) on the size, activity and structure of the microbial community in a semiarid soil. In samples taken in spring, summer and autumn over a 2-year period we analysed parameters that indicate the size of the microbial community [microbial biomass C (MBC)], its general activity (ATP and respiration) and specific activity related to the N, P and C cycles in the soil (urease, phosphatase and β-glucosidase, respectively). Two years after the organic amendment, the structure of the microbial community was studied by analysing phospholipid fatty acids (PLFAs). At the end of the experiment, the MBC of the compost and sludge-treated soils was 489 and 463mgCmic kg−1, respectively, while the MBC of the control soils was 247mgCmic kg−1. Over the 2-year period, the higher β-glucosidase, urease and alkaline phosphatase activities of compost and sludge-treated soils reflected higher substrate availability in these plots compared to the control plots. Indeed, plant abundance increased by about 25% with organic amendments. Whereas the bacterial to fungal ratio of signature PLFAs did not change, the ratio of monounsaturated/saturated and the ratio of Gram+/Gram− of the different treatments indicated that a different bacterial community developed 2 years after amendment with compost and sludge. Thus, factor analysis via PLFAs showed a change in microbial community structure in amended soils versus control soil. It can be concluded that microbial biomass and activity of degraded semiarid soils can be improved by the addition of organic materials of differing degrees of stabilization (compost and sewage sludge). Compost-amended soils showed the highest carbon contents, while in general no differences in activity or microbial biomass where found between compost or sludge treatments. Although phytotoxic substances in sewage sludge might negatively affect plant development, the similar density of plant cover developed in sludge and compost-treated soils suggests that any phytotoxic substances had been degraded. [Copyright &y& Elsevier]

The structural deficit of water resources in Mediterranean areas forces us to search for new sources of water for irrigation as a mandatory requirement for a sustainable agriculture. However, given their critical role in soil fertility, the impacts of irrigation in soil microbial communities must be carefully considered alongside the crop responses. Here, we evaluate the impacts of irrigation with water from different origins in the soil microbial community and on the tree physiology and fruit yield in a Mediterranean mandarin agroecosystem. Two sources of water for irrigation were considered: i) fresh water, with an electrical conductivity (EC) of 1.2 dS m−1, from the Tajo-Segura canal (transfer water, TW); and ii) reclaimed water (EC = 3.4 dS m−1) from a wastewater treatment plant (RW). Further, the two types of water were applied using two different regimes: control irrigation (C), to fully satisfy the crop water requirements (100% ET c), and regulated deficit irrigation (RDI), in which the trees received half the amount of water applied to the C trees (50% ET c) during the second stage of fruit development. In the case of TW, RDI increased bacterial biomass, and urease and β-glucosidase activities in soil. In contrast, in the case of RW, RDI did not increase bacterial biomass in comparison to control (RW-C). Irrigation with RW caused a reduction in yield in comparison to TW treatments. The combined evaluation of the plant and soil responses to different irrigation strategies is essential in water-limited Mediterranean areas used to grow citrus crops that require less water and nutrients than other crops. Further, this crop can be favored by the use of low to moderate vigorous rootstocks. Our results demonstrate that RDI does not have a drastic negative impacts on crop yield when RW is used, and that there may be some positive effects in soil microbial communities when TW is used for irrigation. • Water irrigation strategies were evaluated in a mandarin agroecosystem. • Reclaimed water reduced crop yield. • Regulated deficit irrigation did not affect crop yield when reclaimed water is used. • Regulated deficit irrigation with transfer water improved some enzyme activities. • Regulated deficit irrigation with transfer water improved bacterial biomass. [ABSTRACT FROM AUTHOR]

Pesticides are chemical compounds, mostly synthetic, which are used widely in agricultural fields to prevent and to control pests and soil-borne diseases. The synthetic nature of these compounds makes some of them non-biodegradable and they may accumulate in harmful concentrations in soils. Solarization seems to be a non-chemical strategy that could enhance pesticide degradation in soils. Here, we evaluate the combined impact of pesticides and solarization on the microbial community of a Mediterranean soil. For this purpose, enzyme activities, basal respiration, and the biomass and composition of the microbial community (through analysis of phospholipid fatty acids, PLFAs) were evaluated in solarized and non-solarized soils, in a 90-day greenhouse experiment with a combination of different pesticides. The degradation of the pesticides in the solarized soils was 30% greater than in non-solarized samples. However, solarization also affected the soil microbial community. The soil respiration was lowest in solarized samples without pesticides, while the enzyme activities were greater in non-solarized samples (with and without pesticides). Both the bacterial and fungal PLFA contents declined in solarized samples. The G+/G− ratio was highest in the solarized samples without pesticides and in the non-solarized samples with pesticides. Considering such impacts on the soil microbial community and the relationship of soil microbes with soil ecosystem services, the utilization of solarization must be carefully considered when adopting strategies for pesticide degradation in Mediterranean soils. • Solarization may enhance pesticide degradation in soil. • We evaluate the combined impact of pesticides and solarization. • Soil biochemical and microbial parameters are evaluated. • Solarization reduces microbial respiration and microbial PLFA content. • Solarization must be carefully used for pesticide degradation in semiarid soils. [ABSTRACT FROM AUTHOR]