The impacts of organic amendments: Do they confer stability against drought on the soil microbial community?

- We evaluate if organic amendment shapes the responses of microbiome to drought.
- The stability of microbial biomass, diversity and functionality was tested.
- The PLFA content of drought soils was greater in amended samples after 45 days.
- The responses of microbial diversity to drought was not affected by amendments.
- The response of microbial populations to drought was modulated by amendments.

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 CO2 after addition of 13C-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 CO2 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.