**Background:** Drought is a growing threat to soil ecosystem functions including nutrient cycling, water retention, and plant productivity. The rhizosphere is a critical zone where plant-root interactions with soil microorganisms govern nutrient availability and soil health. This study aimed to evaluate the direct effects of drought stress on the development of Deschampsia caespitosa L., microbial activity (basal respiration and dehydrogenase activity) in rhizosphere and non-rhizosphere soil, soil hydrophobicity, and mineral nitrogen (Nmin) leaching. The authors hypothesized that drought would adversely affect plant development, microbial activity, and nutrient retention, and that fertilization could mitigate some negative effects.
**Methods:** A lysimetric experiment was established in September 2012 in Březová nad Svitavou, Czech Republic (annual climatic averages 1962–2012: precipitation 588.47 mm, air temperature 7.9 °C). Fifteen PVC lysimeters (each 0.07 m² surface area) were filled with 25 kg of topsoil and 25 kg of subsoil (fluvisol, sandy loam). Five experimental variants (3 replicates each) were tested: V1 (control, no roof), V2 (84 kg N/ha, no roof), V3 (control, drought-stressed via roof), V4 (84 kg N/ha, drought-stressed), and V5 (84 kg N/ha + 1.25 L/ha Lignohumate B, drought-stressed). Drought was simulated annually for 30 days during the growing season using plastic roofs. Basal respiration (BR) was measured using soda lime granules as cumulative CO2 production over 24 h in 7-day cycles, three times before and after drought simulation. Dehydrogenase activity (DHA) was determined using the triphenyltetrazolium chloride (TTC) method, expressed as µg TPF/g dry soil. Nmin leaching was quantified in soil eluate using distillation-titration. Soil water repellence was assessed via unsaturated hydraulic conductance (Kr) calculated from cumulative water infiltration measured with a Mini Disk Infiltrometer at 2 cm tension. Statistical analysis used one-way ANOVA with Tukey HSD test (p < 0.05) and regression analysis.
**Key Results:** Plant biomass production was highest in V2 (no drought, fertilized) across all years: 76.72 g (2013), 59.93 g (2014), and 47.53 g (2015). Drought-stressed variants (V3, V4, V5) showed significantly lower biomass than V2 in all years. In 2013 and 2015, BR values decreased significantly after drought simulation in all variants. For example, V3(R) showed BR of 4.53 g CO2-C/m²·24h before drought vs. 1.32 after drought in 2013. V1(R) (no drought) showed BR of 5.40 before vs. 3.02 after in 2013. Across all three years, drought-stressed variants exhibited BR values over 60% lower after drought simulation compared to before. In 2014, no significant drought effect on BR was detected. DHA in 2015 showed the highest values in V2(R) at 20.53 µg TPF/g·h before drought, with drought-stressed variants showing decreases of over 60% after simulation. Nmin leaching was significantly higher in drought-stressed variants: in 2013 after drought, V1 had 1.90 mg/L Nmin vs. V3 at approximately 5 mg/L. Regression analysis showed a negative correlation between BR and Nmin leaching in 2013 (R = −0.67, p < 0.05) and 2015 (R = −0.69, p < 0.05). V5 (with lignohumate) showed demonstrably lower Nmin leaching than V4 (mineral N only) in drought conditions. Kr values showed no consistent significant differences between drought-stressed and non-stressed variants, though drought-stressed variants tended toward lower Kr (indicating increased hydrophobicity) in 2014 and 2015.
**Clinical Implications:** This study provides experimental evidence that drought significantly impairs soil microbial activity and plant productivity while increasing nutrient losses from soil systems. The negative correlation between microbial activity and Nmin leaching suggests that maintaining healthy soil microbial communities is critical for nutrient retention under drought conditions. The partial mitigation of drought effects by combined mineral nitrogen and lignohumate application indicates potential management strategies for maintaining grassland productivity and soil function under climate change scenarios. These findings are particularly relevant for protecting drinking water sources in areas where grassland ecosystems are used for water resource protection, as increased Nmin leaching under drought could compromise water quality.