Does biochar combined with cover crops improve health and productivity of sandy, sloping, and semi-arid soils?

Biochar may improve the health of environmentally sensitive soils (i.e., low C, sandy, sloping) especially if combined with cover crops (CCs), but research is scant. We assessed how wood biochar (836 g C kg⁻¹) applied at 0, 6.25, 12.5, 25, and 50 Mg ha⁻¹ to sandy, sloping, and semi-arid soils with and without CCs affects soils and crop yields in the central US Great Plains for 3 years. We measured crop yields and CC biomass each year, and most soil properties in Years 1 and 3. Biochar did not interact with CCs, suggesting the combination was no better than biochar or CC alone. In the semi-arid soil, crop and CC did not establish due to persistent droughts. Biochar benefits were highly site-specific. Biochar improved some soil properties but only in the sandy and sloping soils and at the biochar application depth (0- to 15-cm soil depth). The 50 Mg biochar ha⁻¹ improved the soil's ability to sorb water (0.08 cm s^−1/2). Also, in the sandy soil, it increased soil organic matter concentration (2.5 g kg⁻¹), soil pH (0.65 units), and available water (0.07 m³ m⁻³) only in Year 1, suggesting a biochar benefit in sandy soils is short-lived. In the sloping soil, >25 Mg biochar ha⁻¹ reduced bulk density (0.16 Mg m⁻³) and increased soil mean weight diameter of water-stable aggregates (0.58 mm), organic matter concentration (11.42 g kg⁻¹), infiltration (9.35 cm), CC biomass production (0.27 Mg ha⁻¹), and some microbial biomass groups. Biochar did not affect crop yields. Overall, >25 Mg biochar ha⁻¹ improved properties in some soils without interacting with CCs.