The Effects of Warming and Short-Term Drought on Soil Nutrients Are Dependent on Microbial Biomass and Mycorrhizal Inoculation

Soil nutrient cycling and availability in pastures are affected by warming and drought. These effects may be further enhanced in intensively managed/degraded systems due to altered microbial community size and composition. Arbuscular mycorrhizal (AM) fungi may help compensate for this climate-related disruption in nutrient cycling and availability by facilitating access to nutrients. However, the extent to which the interaction between warming and drought may affect AM fungal mediation of soil nutrient availability in degraded soil systems is unknown. To investigate this, we grew lucerne (Medicago sativa) and tall fescue (Festuca arundinacea), with and without AM fungal inoculation (Rhizophagus irregularis), under ambient (26°C—aT) and elevated (30°C—eT) temperatures, and well-watered (100% soil water holding capacity (WHC)) and drought (40% WHC) conditions, in intact soil (non-degraded) and in gamma-irradiated sterilised soil (degraded soil). Soil microbial biomass C, N and P, nutrients (NO₃⁻, NH₄⁺ and PO₄³⁻) and enzyme activities were measured after 4 months of plant growth. Soil microbial biomass C, N and phosphorus decreased in degraded soils. Warming treatments decreased NO₃⁻ and PO₄³⁻ availability in degraded soil under lucerne, with these effects further intensified by AM fungi inoculation. In contrast, drought increased NH₄⁺ in degraded soils under lucerne and increased PO₄³⁻ in non-degraded soils under tall fescue. In non-degraded soils, arbuscular mycorrhizal fungi increased NH₄⁺ under lucerne and NO₃⁻ and PO₄³⁻ under tall fescue in warmed + drought soils, suggesting that AM fungi can enhance nutrient cycling under specific plant species and climate conditions when soil biota have not been degraded. In contrast, altered biological communities in degraded soils may have limited the ability of AM fungi to support nutrient availability. These results underscore the pivotal role of soil biological communities in modulating nutrient dynamics under climate stress. Overall, our findings suggest that AM fungal inoculation holds potential to improve nutrient cycling and plant performance under extreme climate conditions, but its effectiveness likely depends on both plant species and the composition of the soil biotic community.