Deep-rooting cover crops enhance maize subsoil resource uptake under drought: links to soil and microbial carbon within legacy root channels

The increasing frequency and intensity of drought exacerbate topsoil desiccation, limiting water and nutrient availability for crops. Deep-rooting cover crops can access subsoil resources and create persistent legacy root channels that may form microbial hotspots in subsoils that promote nutrient mobilization. However, whether these carbon (C)-enriched channels are reused by subsequent crops to enhance subsoil water and nutrient uptake under drought remains poorly understood.

We conducted a field experiment to investigate whether deep-rooting cover crop mixtures (legume/grass; brassica/grass) enhance maize (Zea mays L.) uptake of water and nitrogen (N) under drought across three contrasting soil types (Phaeozem, Luvisol and Podsol) in Germany. Drought was simulated using rainout shelters during maize growth. Soil layers (0-30 cm; 30-60 cm and 60-90 cm) were labeled with ¹⁵N (¹⁵N-NH₄⁺ and ¹⁵N-NO₃^-) and D₂O using the “pipe in tube” method at maize growth stage BBCH 50 to trace depth-specific resource uptake. Soil organic carbon (SOC) and microbial biomass carbon (MBC) were quantified in the legacy root channels.

Maize following deep-rooting cover crop mixtures showed a higher recovery of ¹⁵N and D₂O from subsoil layers under drought compared to fallow, with particularly pronounced responses after the brassica/grass mixture. Across treatments and soil depths, tracer recovery was related to SOC and MBC, suggesting that C-enriched legacy root channels support subsoil water and N uptake and enhance drought resilience.