How does biochar impact soils and crops in a semi-arid environment? A 5-yr assessment

Context Long-term field data on the implications of biochar use are scant. Also, biochar research on semi-arid environments where biochar use might be most beneficial is rare. Objective We assessed soil and crop response to wood biochar in a no-till millet-pea-winter wheat-sunflower rotation in a semi-arid environment for 5 yr. Methods An ongoing experiment under five levels (0, 3.125, 6.25, 12.5, and 25 Mg ha−1) of biochar (56% C) and three levels of N fertilizer (0, 84, and 168 kg N ha−1) application in a silt loam in the western U.S. Great Plains was studied. Nitrogen fertilizer was applied only in 3 of the 5 yr. It was not applied when a legume crop was planted or when a crop followed a legume for the study rotation. Thus, N fertilizer was applied to millet in 2018 (year 1), sunflower in 2021 (year 4), and millet in 2022 (year 5) but not in 2019 (year 2) when peas were planted and 2020 when winter wheat followed peas (year 3). Crop yields were measured every year, while soil properties were measured in years 1, 3, and 5. Results Biochar had some effects, but biochar × N rate interaction was not significant, suggesting limited synergism between biochar and N fertilization in this study, although N was not applied every year. It is hypothesized that biochar combined with N fertilizer could be better than biochar alone in cropping systems where N fertilizer is applied every year. Across N levels, biochar had a larger effect on soil chemical and fertility properties than on soil physical properties, but the effect was mostly significant between the highest biochar rate (25 Mg ha−1) and no biochar. Biochar application at 25 Mg ha−1 increased soil pH by 0.2-0.7 units, increased soil organic matter concentration by 4.0-6.0 g kg−1 and available P concentration by 10.0-14.0 mg kg−1. Biochar impacted soil properties more in year 1 than in years 3 and 5, indicating soil benefits of biochar may be short-lived. For example, biochar applied at 12.5 and 25 Mg ha−1 reduced soil bulk density in year 1, but not in years 3 and 5. Also, biochar at 25 Mg ha−1 increased soil pH by 0.7 units in year 1, whereas it increased pH by < 0.5 in years 3 and 5. It increased soil organic matter concentration by 6.0 g kg−1 in year 1 and by 4 g kg−1 in years 3 and 5. Application of 12.5 and 25 Mg biochar ha−1 increased pea yield by 0.65 Mg ha−1 in year 2, while application of 6.25 and 25 Mg biochar ha−1 increased sunflower yield by 0.30 Mg ha−1 in year 4, and had no effect in other years. Conclusions Biochar improved soil fertility properties and increased crop yields in some years. Biochar effects on soil properties were greater in year 1 than in years 3 and 5. Implications Wood biochar (>6.25 Mg ha−1) improved some soil properties and crop yields in this semi-arid environment, but its soil benefits generally decreased with time after application.