Effects of soil carbon management and drought on grassland root systems and soil microbial communities

Grasslands are often water-limited ecosystems with high belowground carbon allocation. Their root systems and soil microbial communities play an important role in regulating the soil carbon pool, and properly managed, grasslands may contribute to climate change mitigation via sequestration of carbon (C) in soils. However, it is still uncertain how roots and microbial communities are affected by drought and changes in precipitation patterns in combination with management for soil C sequestration. Expected longer dry periods and more intense precipitation events will evoke soil microbial responses that may feed-back on soil carbon storage.

We set up an experiment in southern Sweden in 2019 to investigate the response of belowground biodiversity to chronic drought (via partial rainfall exclusion) and carbon amendments in the form of a compost addition. We sampled belowground plant biomass, root traits and soil microbial communities from two grasslands with different land use history and over several depths. We extracted and sequenced fungal and bacterial DNA using metabarcoding, and carried out vegetation inventories at the sites - including plant biomass and relative abundance of plant functional types. At the same time, we monitored changes in soil moisture and soil organic matter from topsoil to deep soil, to assess the effect of the treatments throughout the soil profile.

After three years of treatment, we expected to observe changes in the root systems and soil microbiota in response to decreased precipitation, as well as interactions between soil moisture and the organic matter added through the compost amendment. In addition, we expected to see shifts in the composition of fungal functional groups involved in organic matter decomposition and mycorrhizal symbionts limited to the rooting zone. The analyses suggest that changes in soil C and soil moisture affect only the topsoil. While overall root biomass did not change significantly in the treatment plots over the course of the experiment, we observed a slight increase in rooting depth and root mass density and a decrease in fine root length under drought. The results of this study will contribute to assess ecosystem responses to drought, and to evaluate the potential for soil carbon sequestration in grasslands and its possible impacts on belowground biodiversity.