High doses of fine biochar in sandy subsoils increase water retention, but also cause first-year yield depressions for drought-stressed barley

Context

Coarse sandy subsoils often suffer from low water retention and rooting depth, limiting crop yields, especially under drought conditions.

Aims

To determine whether high doses of fine-grained biochar (150 or 300 Mg/ha) incorporated into sandy subsoil could improve water retention, nutrient uptake, root development and yield of spring barley under drought conditions.

Methods

A two-year mesocosm study (2022-2023) using soil columns containing subsoil layers of ground biochar (<80 µm, at 1%, 2% and 4%) or biochar pellets (2%) compared to unamended controls in two sandy soils.

Key results

Ground biochar increased available water capacity while intact pellets showed no measurable effect. First-year (2022) biochar applications negatively affected plant growth and nitrogen (N) uptake, likely due to N immobilization. Second-year (2023) results showed neutral to positive yield effects following winter irrigation. Biochar addition did not improve phosphorus uptake, despite the creation of wetter subsoils enriched with P. Root density decreased with increasing biochar concentration especially in one soil type, possibly due to increased potassium levels. Rooting depth was unaffected and greater than under field conditions.

Conclusions

Biochar incorporation into sandy subsoils can improve water retention, but may cause temporary N immobilization, affecting first-year crop performance. Overall benefits may emerge in subsequent growing seasons.

Implications

Long-term field trials are needed to assess nutrient management strategies, including additional N-application the first year. Incorporating biochar in autumn may help avoid N immobilization before spring sowing. Field-scale methods for applying fine-grained biochar to sandy subsoil must be developed.

Highlights