Impact of subsoil melioration on water use of arable crops: case studies from Germany and South Africa

Subsoils represent an important yet poorly studied component of terrestrial ecosystems. By storing large quantities of water, carbon and nutrients, the subsoil has the potential to support plant productivity. Especially during dry spells, which are assumed to intensify with climate change, subsoil water resources provide a valuable buffer to reduce water stress. However, deep root growth is frequently hampered by the presence of root-restricting layers, such as dense subsoil horizons. Hence, subsoil management options should be established to support plant growth.

We conducted field experiments in arable regions in Germany and South Africa to test whether soil water storage and crop water use efficiency (WUE) could be enhanced through subsoil amelioration by biological and mechanical deep loosening in combination with the incorporation of organic material.

We analyzed stable oxygen isotope (δ¹⁸O) values at different soil depths to determine water uptake depth using the Bayesian statistical model MIXSIAR. A dual-isotope approach using carbon (δ¹³C) and oxygen isotopes in plant biomolecules was also applied to investigate crop water use efficiency of.

The findings demonstrate that the success of subsoil management depends on soil type. In sandy soils, mechanical deep-loosening promoted root water uptake from deeper soil layers and improved biomass production. In contrast, in silty soils, only biological deep-loosening showed positive effects. However, the associated increase in biomass production intensified water stress in the crops. This effect can be mitigated by compost applications, which enhanced soil water retention and promoted root growth into deeper layers, leading to an improved water supply for crops.