Climate change is altering the frequency and severity of drought events. Recent evidence indicates that drought may produce legacy effects on soil microbial communities. However, it is unclear whether precedent drought events lead to ecological memory formation, i.e., the capacity of past events to influence current ecosystem response trajectories. Here, we utilize a long-term field experiment in a mountain grassland in central Austria with an experimental layout comparing 10 years of recurrent drought events to a single drought event and ambient conditions. We show that recurrent droughts increase the dissimilarity of microbial communities compared to control and single drought events, and enhance soil multifunctionality during drought (calculated via measurements of potential enzymatic activities, soil nutrients, microbial biomass stoichiometry and belowground net primary productivity). Our results indicate that soil microbial community composition changes in concert with its functioning, with consequences for soil processes. The formation of ecological memory in soil under recurrent drought may enhance the resilience of ecosystem functioning against future drought events.

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<p>Soil microbial growth, respiration and carbon use efficiency (CUE) are essential parameters to understand, describe and model the soil carbon cycle. While seasonal dynamics of microbial respiration are well studied, little is known about how microbial growth and CUE change over the course of a year, especially outside the plant growing season. In this study we measured soil microbial respiration, growth and biomass in an agricultural field and a deciduous forest 16 times over the course of two years. We sampled plots, at which harvest residues or leaf litter were either incorporated or removed. We observed strong seasonal variations of microbial respiration, growth and biomass. All microbial parameters were significantly higher at the forest site, which contained 3.5% organic C compared to the agricultural site with 0.9% organic C. CUE also varied strongly but was overall significantly higher at the agricultural site ranging from 0.1 to 0.7 compared to the forest site where CUE ranged from 0.1 to 0.6. We found that microbial respiration and to a lesser extent microbial growth followed the seasonal dynamics of soil temperature. Microbial growth was further affected by plant or foliage presence. At low temperatures in winter, both microbial respiration and growth rates were lowest. Due to higher temperature sensitivity of microbial respiration, CUE showed the highest values in the coldest months. Microbial biomass C was also strongly increased in winter. Surprisingly, this winter peak was not connected to high microbial growth or an increase in DNA content. This suggests that microorganisms accumulated osmo- or cryoprotectants but did not divide. This microbial winter bloom and following decline, where C is released and can be stabilized, could constitute the main season for C sequestration in temperate soil systems.  Highly variable CUE, and the fact that CUE is calculated from independently controlled microbial respiration and growth, ask for great caution when CUE is used to describe soil microbial physiology, soil C dynamics or C sequestration. Instead, microbial respiration, microbial growth and biomass should rather be investigated individually to better understand the soil C cycle.</p>

<p>Seasonality of soil microorganisms plays a critical role in terrestrial carbon (C) and nitrogen (N) cycling. The asynchrony of immobilization by microbes and uptake by plants may be important for N retention during winter, when plants are inactive. Meanwhile, the known warming effects on soil microbes (decreasing biomass and increasing growth rates) may affect microbial seasonal dynamics and nutrient retention during winter.</p><p>We sampled soils from a geothermal warming site in Iceland (www.forhot.is) which includes three in situ warming levels (ambient, +3 °C, +6 °C). We harvested soil samples at 9 time points over one year and measured the seasonal variation in microbial biomass carbon (Cmic) and nitrogen (Nmic) and microbial physiology (growth and carbon use efficiency) by an <sup>18</sup>O-labelling technique.</p><p>We observed that Cmic and Nmic peaked in winter, followed by a decline in spring and summer. In contrast growth and respiration rates were higher in summer than winter. The observed biomass peak at lower growth rates, suggests that microbial death rates must have declined even more than growth rates. Soil warming increased biomass-specific microbial activity (i.e., growth, respiration, and turnover rates per unit of microbial biomass), prolonging the period of higher microbial activity found in summer into autumn and winter. Microbial carbon use efficiency was unaltered by soil warming. Throughout the seasons, warming reduced Cmic and Nmic, albeit with a stronger effect in winter than summer and restrained winter biomass accumulation by up to 78% compared to ambient conditions. We estimated a reduced microbial winter N storage capacity by 45.5 and 94.6 kg ha<sup>-1</sup> at +3 °C and +6 °C warming respectively compared to ambient conditions. This reduction represents 1.57% and 3.26% of total soil N stocks, that could potentially be lost per year from these soils.</p><p>Our results clearly demonstrate that soil warming strongly decreases microbial C and N immobilization when plants are inactive, potentially leading to higher losses of C and N from warmed soils over winter. These results have important implications as increased N losses may restrict increased plant growth in a future climate.</p>

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