Functional traits of Zea mays L. varieties determine drought effects on soil structure and carbon allocation in the rhizosheath

The spatial arrangement of the soil surrounding the root can improve plant resource acquisition under drought and is closely related to the fate of soil organic carbon (SOC). Thus, the formation of soil structure and the establishment of a stable rhizosheath can potentially improve plant drought resistance and contribute to maintained crop yields during drought events. Yet, soil structure formation is a complex process determined by the interaction between various functional plant and soil properties, such as the soil (micro)biome, root exudation, or root morphological characteristics. To date, it is not understood how water scarcity affects soil aggregation in the vicinity of roots, by which functional traits these drought effects can be modified, and how this feedbacks on the cycling of SOC. 

Thus, we investigated drought effects on rhizosheath properties and their link with functional plant traits. We conducted a greenhouse experiment with 38 maize varieties where half of the plants were grown under optimum moisture, while the second half of replicates were subjected to drought stress after an initial establishment phase. For each plant, the rhizosheath soil was sampled and its aggregate size distribution, carbon (C) and nitrogen (N) content, and the proportion of newly maize-derived C were analysed via natural abundance ¹³C. In addition, we recorded functional plant and rhizosphere traits, such as morphological and chemical root properties, microbial enzyme activities, and plant biomass.

Drought-stressed plants formed lower amounts of rhizosheath, with a decreased physical aggregate stability and increased concentrations of SOC, N, and newly maize-derived C. Furthermore, under drought larger proportions of the elements were allocated into the microaggregate fractions. In particular, maize-derived C, along with N, accumulated under drought stress in the smaller aggregate size classes of the rhizosheath. Maize varieties forming larger amounts of roots under drought stress tended to maintain higher macroaggregate stability in the rhizosheath. In contrast, cultivars that invested little in root growth but promoted higher microbial enzyme activities in the rhizosheath and maintained root N contents under drought were associated with a strong accumulation of maize-C and N in the smaller aggregate size classes. 

Trait-based experimental approaches, such as the one presented here, are deepening our mechanistic understanding of drought effects in the crop rhizosheath and can thus help to guide future crop selection for improved drought resistance.