Alterations of the nutrient uptake depth as inferred from radiogenic strontium isotopes in two case studies

Abstract

Climate change and its associated effects, such as higher temperatures and atmospheric CO₂ concentration, pose increasing challenges to both forest and agricultural ecosystems. Nutrient uptake from deeper soil might be a promising adaptation strategy of the plants, as the subsoil contains additional water and nutrient resources. Yet, we know little on how forests adapt their nutrient acquisition strategies to different climate conditions, or how the nutrient uptake depth of crops can be manipulated by different management strategies, particularly for sandy soil. My thesis aimed to 1) investigate whether and how Norway spruce responds to global changes by changing its mean nutrient uptake depth, and 2) to evaluate the role of subsoil management for the nutrient uptake from sandy soils. <br/> To address these objectives, radiogenic strontium isotope ratios (⁸⁷Sr/⁸⁶Sr) were used as a geochemical proxy to trace nutrient sources and uptake depth in forest and agricultural systems. In a long-term forest monitoring site at Mitterfels forest (Bavaria, Germany), I tracked changes in needle biomass, nutrient concentrations and ⁸⁷Sr/⁸⁶Sr ratios in Norway spruce (Picea abies L.) from 1992 to 2014. In an agricultural experiment trial on sandy soil (Retisol) at Thyrow (near Berlin, Germany), I investigated the effects of subsoil management strategies including strip-wise deep loosening with and without the incorporation of biowaste compost on nutrient uptake depth in crops. <br/> Needle biomass significantly increased over the decades, indicating enhanced plant growth under elevated CO₂. Nutrient concentrations of K and P declined, but Mg largely increased by ~ 40%, often beyond requirement levels. Simultaneously, the ⁸⁷Sr/⁸⁶Sr ratios in spruce needles increased from 0.7147 to 0.7177, suggesting a shift in nutrient uptake toward older organic layers and deeper mineral soil horizons with higher ⁸⁷Sr/⁸⁶Sr ratios. These findings indicate that spruce trees adapted to climate change by deepening their nutrient uptake depth to satisfy increased nutrient demand. <br/> The nutrient status and ⁸⁷Sr/⁸⁶Sr ratios in the arable subsoil were largely altered by the additional nutrients from composts compared to the control and deep loosening plots; however, Sr isotope ratios in crop tissues remained unchanged. Hence, the crop's primary nutrient source remained in the topsoil despite the elevated nutrient levels in the subsoil. These findings underscore the importance of soil water retention in sustaining and improving crop performance in the sandy soil. <br/> Overall, my data demonstrates that radiogenic strontium isotopes provide a valuable approach to identify plant nutrient sources and trace nutrient uptake depth in different ecosystems. Both systems illustrate how plants regulate nutrient uptake depth in response to environmental drivers such as climate change or agricultural management, highlighting the interaction between plant plasticity and external environment changes in controlling nutrient acquisition from varying soil depths.