Biochar-mediated enhancement of soil quality and nutrient availability to spinach under water-limited conditions

Biochar as a soil amendment can mitigate plant water stress. But its effects on soil quality in calcareous soils, particularly under varying moisture conditions, have not been studied. Therefore, this three replicated factorial greenhouse study evaluated the effects of cattle manure-derived biochar (0 (B0), 25 (B25), 50 (B50), and 100 (B100) t ha− 1 which are equivalent to 0, 1.25, 2.5, and 5%wt, respectively) on spinach yield and the soil quality indices (SQI) of the postharvest soils under three soil moisture levels (100% (W100), 70% (W70), and 55% (W55) of field capacity, FC). Soil quality indices and their grades were assessed using a total of 27 soil indicators (TDS) and minimum (MDS) data sets derived from principal component analysis (PCA), with the Nemoro (NQI) and integrated (IQI) soil quality indices. PCA identified six principal components (PCs) with eigenvalues > 1, accounting for 94.77% of cumulative variance. The most influential indicators (MDS) comprised soil iron, copper, and nitrogen concentrations, mean weight diameter (MWDd), and geometric mean diameter (GMDd) from dry sieving, and ∆GMD (with eigenvectors of 0.745, 0.565, 0.958, 0.801, 0.863, and 0.743, respectively). Correlations between IQI-TDS, IQI-MDS, NQI-MDS, and most plant and soil properties were positive, except for electrical conductivity, sodium adsorption ratio, and soil sodium concentration. PCA revealed that B50W55 and B100W55 were the most effective treatments, showing higher PC1 and lower PC2 scores, and were associated with improved greenness index, water repellency, penetration resistance, plant nitrogen, phosphorus, and sodium, as well as soil potassium, magnesium, and iron. Quality grades of the biochar-treated soil varied from III to IV under drought conditions. The highest soil property scores were achieved with 100 t ha⁻¹ biochar, whereas the best plant property scores occurred with 50 t ha⁻¹ biochar at 55% FC. The lowest scores across all properties were observed in the control (no biochar). Based on the sensitivity index and efficiency ratio, IQI-MDS was identified as the optimal model for representing soil quality in relation to crop yield. Overall, biochar application enhanced soil quality indices and crop yield by supplying nutrients and improving soil attributes. These findings highlight biochar’s potential to sustain soil health and productivity under drought conditions, which are increasingly common in arid and semi-arid regions.