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

ContextCoarse sandy subsoils often suffer from low water retention and rooting depth, limiting crop yields, especially under drought conditions. AimsTo 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. MethodsA two-year mesocosm study (2022-2023) using soil columns containing subsoil layers of ground biochar (<80 {micro}m, at 1%, 2% and 4%) or biochar pellets (2%) compared to unamended controls in two sandy soils. Key resultsGround 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. ConclusionsBiochar 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. ImplicationsLong-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. HighlightsO_LIFine-grained biochar can increase the in-situ water retention in coarse sandy subsoils C_LIO_LIIn the first season (2022), biochar reduced barley growth and yield, likely due to initial soil nitrogen immobilization C_LIO_LIIn the second season (2023), after intensive winter irrigation, biochar effects became neutral or positive C_LIO_LIRoot density decreased with biochar use, possibly due to high potassium levels creating unfavourable subsoil conditions C_LI