Higher resilience to repeated drought in soil with a more diverse food web

Background/Question/Methods Global warming is expected to increase the frequency of extreme weather events, including periods of drought followed by heavy rainfall. These fluctuations in moisture availability can greatly impact on soils by disrupting soil structure and killing microbes and soil fauna, resulting in a loss of microbial and faunal biomass and diversity and increased losses of nitrogen and carbon from the soil. It is often postulated that more diverse soil food webs would be less susceptible to these disturbances and thus have lower N and C losses than less diverse systems when subjected to disturbance. We tested the effects of multiple drying/rewetting (D/RW) events on soil food web dynamics and nutrient fluxes in two systems: an extensively managed grassland with a more diverse food web, and an intensively managed wheat field with a less diverse soil food web. The first drought was imposed in the field using rain shelters, after which soil was taken to the greenhouse, rewetted, and dried and rewetted again in a full-factorial set up. One, 3, 10 and 77 days after rewetting, we measured soil microbial biomass (C and N) and community composition, the abundance and functional group composition of protozoa, nematodes, microarthropods and enchytraeids, and N leaching, N₂O production and respiration.

Results/Conclusions We found that the two systems responded differently to the multiple D/RW events. Although drying in the field reduced microbial C and N more in the grassland than in the wheat field, microbial biomass N did not recover during the entire experimental period in soil taken from the wheat field and N leaching losses were greater than in the grass system. Drying/rewetting in the laboratory increased initial respiration and N₂O production more in grass than in wheat soil, which was paralleled by an increase in fungal and bacterial PLFA and protozoa. In contrast, D/RW in the laboratory decreased fungal and bacterial PLFA in the wheat soil for the whole experiment. The grassland system showed an adaptation to drought, multiple droughts impacting less on microbial biomass and nutrient fluxes than just one drought. Drought in the field affected soil fauna more than drought in the laboratory and most soil faunal groups were more abundant in the grassland soil throughout the experiment. We conclude that the grass system, with a more diverse soil food web and higher organic matter content, was more resilient and retained nutrients better than the wheat system when subjected to drought.