title: Nutrient-Access Strategies in Contrasting Top- and Subsoils: A Multi-Isotope Approach
Creators: name: Ai, Juanjuan
Publisher: name: Universität Tübingen
Year: 2026
Subjects: subject: soil science, agriculture
subject: 500
subject: 550
subject: 570
subject: subsoil
subject: nutrient cycling
subject: cover crop

description: The subsoil, which refers in cropland soils to the layer below the cultivated horizon, contains considerable reservoirs of carbon (C), water and essential nutrients such as nitrogen (N) and phosphorus (P). With increasing frequency of extreme droughts and climate change-related stresses on topsoils, subsoils are gaining recognition as valuable resource for mitigating adverse impacts on agricultural ecosystems. However, challenges such as high bulk density and unfavorable conditions - such as low temperatures and limited oxygen - make it essential to understand how to effectively utilize this drought-resistant source of nutrients to sustain crop productivity. Root traits, such as deep-rooting, are particularly important for increasing nutrient and water uptake. As a consequence, deep-rooting cover crops have been identified as a key pathway for accessing subsoil resources. This thesis aims to identify strategies for enhancing access to subsoil nutrient pools by optimizing combinations of different cover crop species based on their potential for nutrient allocation and evaluating the effect of different cover crop mixtures on nutrient uptake by subsequent cash crop maize. In addition, nutrient turnover and mobilization processes in the subsoil, as induced by the root exudates of maize, are determined using multi-isotope labelling techniques, to improve our understanding which N and P sources in subsoils can be made available by maize through its root exudates. These objectives were addressed in three studies. The first study focused on field experiments across three contrasting soil types. Various cover crop mixtures (CCM), always deep-rooting species combined with shallow-rooting ones from legumes, grasses, and brassicas, were planted as pre-crops to enhance subsequent maize root penetration into the subsoil. The 15N isotope labelling method was used to quantify the incorporation of CCM-derived N into different soil and microbial N pools, as well as its contribution to subsequent maize N nutrition. The greatest contribution of previous cover crop mixtures to the soil labile and microbial N pool was observed when the legume-based mixtures were used as winter cover crop. Conversely, CCMs containing deep-rooting brassica demonstrated a beneficial impact on the maize N uptake by either the re-use of root channels or by indirect effects of root exudates and degradation intermediates diffusing into the soil, which increased subsoil nutrient availability. Overall, the lower the soil fertility the more the main crop maize profited most from the winter cover crop mixtures by showing highest CCM-derived 15N uptake. In contrast, maize growing on high-fertile soils with nutrient-rich organic matter exhibits a preference for the uptake of nutrients released from the native soil and not derived from cover crops. The degradation of cover crop residues is essential for providing their nutrients to subsequent cash crops. However, the mechanisms governing these degradation processes making their nutrients available for organisms, particularly under nutrient- limited conditions such as in subsoil, remains largely unclear. The second and third studies, both from one controlled laboratory incubation experiment, were designed to fill these knowledge gaps. The second study specifically aimed to evaluate the impact of maize root exudates on the availability of various N sources in top- and subsoil, while also elucidating the microbial mechanisms through which root exudates influence N availability. The results revealed that root exudates significantly enhanced microbial N mining and uptake from organic sources i.e chicory residues. Interestingly, this enhancement was more significant in the subsoil than in the topsoil. This observation is linked to the stronger C limitation in the subsoil, demonstrating that the subsoil microbial biomass responds quickly to fresh inputs of organic C or N boosting their turnover. The third study aims to identify important microbial processes regulating the availability of organic and inorganic P in top- and subsoils and further identify whether rhizodeposition is a key rhizosphere strategy to access various P sources. Results showed that microbial communities in top- and subsoils used high- (inorganic phosphate in solution) and low-available (sorbed phosphate) mineral P to a similar extent, but that the subsoil communities were much more efficient in mobilizing and incorporating complex litter-derived organic P. This capability of subsoil communities was even enhanced when root exudates were present. Microbial activity and nutrient- mobilizing mechanisms (e.g., P-related enzymes) clearly increased as a consequence of root exudate addition, an effect that was generally higher in C-limited subsoils than in topsoils. In conclusion, this thesis provides valuable insights into the potential of subsoil as a reservoir for plant nutrients. The findings highlight the importance of selecting deep- rooted cover crops to enhance nutrient cycling and microbial activity in subsoil, which can be particularly beneficial for modern sustainable agriculture. By optimizing root exudation patterns and, in consequence, microbial processes, plants can tap into subsoil resources more effectively, offering a promising strategy for mitigating the effects of climate change on crop productivity.
description: Die Dissertation ist gesperrt bis zum 11. Dezember 2026 !