P19 - Rhizosoil Aggregation

P19
Self differentiation of soil aggregation and organic carbon allocation induced by plant and microbial activity in the rhizosphere

The project aims to investigate the formation of soil aggregates in the rhizosphere as compared to the bulk soil. This ranges from the single root (lab experiments) up to the root system (field experiments), integrating quantitative and qualitative approaches from micron to profile scale.
A focus will be on the characterization and fate of soil organic matter in macro- and microaggregates developed in the rhizosphere. To follow the spatiotemporal development of aggregated soil structures in the rhizosphere, we will link classical quantitative with state-of-the-art spectromicroscopic approaches. With these results, we will provide a contribution to focal topic iv ‘Carbon sequestration/ microaggregate formation/ stability of soil structure’ of the PP. Specific aspects also take into account microbial interactions (focal topic i) and the legacy of roots that are also of major interest for soil organic matter dynamics.


Impact of soil texture and root hair characteristics on the conversion of maize-derived carbon into soil organic matter: insights from a five-year field study

Outcome:
• Soil aggregate formation and size distribution are determined by soil texture.
• Root hairs increase rhizosheath volume and the amount of OC retained in slow-cycling soil OC fractions.
• Roots create a lasting "legacy effect" on soil OC retention and SOM quality arising from rhizospheric self-organization extending beyond the rhizosphere in the bulk soil.

Our project investigated the spatial and temporal patterns of root-induced soil aggregation and organic carbon (OC) accumulation. We were able to demonstrate that the maize genotype with root hairs produced a greater mass of root-adhering soil (rhizosheath) ( Teixeira et al., 2023 ) and a larger amount of root-derived OC compared to the mutant lacking root hairs. Unexpectedly, the presence of root hairs did not influence aggregate formation or the resulting aggregate size distribution, which remains predominantly governed by soil texture.
With data from the 5-year field experiment, we explored the spatial and temporal patterns induced by rhizospheric processes using the natural 13C abundance variation approach. Over time, 13C enrichment of bulk soil increased, indicating that the conversion of root C inputs into soil C follows a characteristic spatial-temporal pattern, as proposed by Vetterlein et al. ( 2020 ). As roots explore different soil volumes during successive crop cycles, root-derived C is gradually spread beyond the seasonal rhizosphere and distributed throughout the bulk soil. Soil texture was decisive for the root-derived C proportions in bulk soil: 3.4% in loam and 14% in sand. The genotype with root hairs induced 16% more root-derived soil OC than the mutant, especially in the presumably slow-cycling soil C fractions. Mineral-associated OC increased by 25% and occluded OC by 70%, indicating that root hairs enhance the accumulation of rather persistent OC but do not alter aggregate size distribution. This demonstrates that soils exhibit a certain degree of plasticity to accommodate new carbon inputs without necessarily altering the aggregate distribution.
Our findings highlight the "root legacy effect" recently described by Mueller et al. ( 2024 ), with roots leaving a lasting imprint on soil OC retained in the soil beyond the rhizosphere. This effect is understood as an emergent property arising from the self-organization of the rhizosphere, which shapes the quantity and quality of OC retained in the bulk soil as it extends throughout the entire soil volume after a few crop cycles.


Link to English scientific abstract

Link to German scientific abstract