|dc.description.abstracteng||In the face of current as well as projected increased frequencies and severities of droughts the interest in responses of the belowground system to such profound changes in environmental conditions has risen. The heavily disturbed soils of agricultural fields deserve special attention since they seem especially vulnerable and are of crucial importance for future crop production. Soil organisms mediate a number of functions in soil, hence, in order to develop strategies for mitigating detrimental drought effects on agricultural systems, it is essential to better understand their response to drought and how this may be modulated by interactive factors.
In Chapter 2 we present the methodological basis of the drought experiments conducted in this thesis. We describe in great detail an adapted roof design that is suitable for experiments in agricultural context and includes the concept of a roof control in order to deal with potential unintended artifacts. We evaluated potential artifacts on microclimate by including three drought treatments: (I) A roof that intercepts precipitation, (II) a roof similar to the previous one, but without rain interception and (III) an ambient control without any manipulation. Artifacts were generally small, but soil and air temperature were little increased under high ambient temperature. The roof intercepted 64.9% of the precipitation, which is very close to the a priori calculated exclusion of 65.1%, indicating a precise prediction of experimental rain exclusion is possible with this roof design. The roofs further proofed to sustain under heavy weather in an open landscape and were suited for the use in agricultural fields where management operation require regular removal. We hope to promote this roof design for future drought simulation experiments by providing detailed technical drawings, a complete parts list and an assembly instruction.
In the study presented in Chapter 3 we investigated the effects of simulated drought on the abundance and community composition of soil microarthropods in winter wheat fields under long-term conventional and organic farming in the DOK trial, Switzerland. Drought decreased the abundance of Collembola and Oribatida more consistently in conventionally compared to organically managed fields. The abundance of Collembola as well as Meso-, Pro- and Astigmata, but not the abundance of Oribatida, increased in deeper soil layers due to simulated drought, suggesting vertical migration as drought avoidance strategy in these taxa. The species composition of Oribatida communities, but not of Collembola communities, differed significantly between drought treatments as well as between farming systems with soil carbon content being among the factors structuring Oribatida communities.
The indication, that soil organic carbon (SOC) has the potential to mitigate drought effects led to the study presented in Chapter 5, where we conducted a drought manipulation experiment in fields with high and low SOC content. Further, we extended the scope of the climate manipulation to the question if natural differences in climate influence the effect of simulated drought by establishing the same experiment at sites with temperate (Sweden and Germany) and Mediterranean climate (Spain). Of the studied microarthropod groups only Oribatida were significantly affected by drought with the direction of these effects being different in high and low SOC fields and among countries. Drought decreased abundances of Oribatida in low SOC fields and increased them in high SOC field. Negative drought effects occurred only in Sweden driven by strong negative effects in low SOC fields, where soil water content was extremely low (below 7%), indicating a threshold of soil water content below which negative drought effects prevail irrespective of the SOC content. Oribatida at the Spanish sites were only little affected indicating that communities from dry climates are adapted to drought, e.g. by living in deeper soil.
In the study presented in Chapter 4 we investigated, in addition to changes in abundances, drought induced shifts in trophic niches of Collembola and Oribatida as indicated by stable isotope analysis (13C and 15N). The study was conducted in the same fields as the first two studies (Chapter 2 and 3) under long-term conventional and organic farming in the DOK trial. Stable isotope values suggested that plant litter and root exudates were the most important resources for Collembola (Isotoma caerulea, Isotomurus maculatus and Orchesella villosa) and older plant material and microorganisms for Oribatida (Scheloribates laevigatus and Tectocepheus sarekensis). Drought treatment and farming system did not affect abundances of the studied species. However, isotope values of some species increased in organically managed fields indicating a higher proportion of microorganisms in their diet. Trophic niche size, a measure of both isotope values combined, decreased with drought and under organic farming in some species presumably due to favored use of plants as basal resource instead of algae and microorganisms.
Overall, the results from Chapter 3 and 5 indicate that the vulnerability of soil microarthropods against drought is high in soils with low SOC content and at very low soil moisture. On the other hand, in soils with high SOC content they may profit from drought, suggesting that increasing SOC levels in agricultural soils may be a useful tool to mitigate or even reverse negative drought effects. High SOC contents modulated drought effects presumably by buffering the decrease in soil moisture and providing a more complex soil structure that allowed vertical migration to deeper soil as drought avoidance strategy. Responses of soil microarthropods to simulated drought were only small at sites with Mediterranean climate, suggesting communities from dry climates being generally better adapted to drought. The results of Chapter 4 further suggest that the flexible usage of resources may buffer effects of drought and management practices on the abundance of microarthropods in agricultural systems and may be one reason for neutral responses in abundances in drought experiments. Altogether, this thesis contributes to a better understanding of the response of soil microarthropods to droughts in arable fields by identifying determinants for the direction of these responses and potential mechanisms of adaptation to drought by the different taxa.||de