Biogeochemistry and sustainable management of the subsoil

More than half of the soil water and nutrients are allocated below a 30 cm soil depth. Yet, this reservoir is hardly included in soil management strategies and is sometimes not even accessible to plants due to root-restricting layers. Here, we present an overview of different research projects on (i) the coupling and decoupling of subsoil biogeochemistry from topsoil processes under different management practices, (ii) the option to manipulate subsoil access through biopores and deep-rooting plants, and (iii) the success of subsoil management through compost injection and burial of straw for the cropping of rainfed (barley, maize) and flooded cereals (paddy rice), respectively. We show that plants are key to connecting top- and subsoil processes, but that it takes decades to centuries for subsoil processes to reach new steady-state equilibria. The interactions between sub- and topsoils, however, can be disentangled using stable and geogenic isotope tracing techniques, such as δ¹⁸O and ⁸⁷Sr/⁸⁶Sr, and can be utilized for management via biological or mechanical techniques to lower the physical resistance of soil to plant growth. Intelligent management of subsoil offers new options for making land use more resilient to climate change and for maintaining high productivity and sustainability with lower long-term fertilizer requirements.