Biochar application does not improve crop growth and yield in a semi-humid region in the HuangHuaiHai Plain of China: A 7-year consecutive field experiment

Biochar (BC) has gained worldwide attention as a soil amendment to improve soil fertility and crop yield. However, long-term field data are still lacking to evaluate the effectiveness of BC application in increasing crop yield under various specific site conditions. A 7-year field experiment of BC application with consecutive corn-wheat rotations was carried out in a semi-humid region in the HuangHuaiHai Plain in north China. Straw-derived BC was mixed with the sandy loam soil in 9 m² (3 m × 3 m) plots to a depth of 20 cm with the application rates (w/w) of 0 t ha⁻¹, 12 t ha⁻¹, 24 t ha⁻¹, 36 t ha⁻¹, 48 t ha ⁻¹, and 60 t ha ⁻¹, respectively. Physiological indexes, e.g. plant height, spike number, number of ears, spike length, straw dry matter and root dry matter, and the crop yield, i.e. 1000-grain weight, and grain yield, were measured for each growing season under different BC treatments. Disturbed and undisturbed soil samples were collected 4 years after BC application. Laboratory measurements were conducted to determine soil properties, such as bulk density, saturated hydraulic conductivity (K_s), aggregate size distribution, soil water retention characteristics, pore size distribution, and soil nutrient contents under different BC treatments. The results showed that plant physiological indexes and grain yield of both corn and wheat were not significantly affected by BC application, being independent of the application rate, climate variation and time. Besides, the BC application significantly improved soil physio-chemical properties, indicated by increases in soil macro-aggregate, aggregate stability, K_s, plant available water capacity and soil available nutrients. The long-term field results demonstrate that the soil fertility improved by BC application may not always lead to significant increase in crop yield. Apart from soil properties, genetic merits of the specific crop cultivar and field managements of irrigation and fertilization could be the dominating factors affecting crop growth and yield under particular soil type and climate conditions. Thus, applying BC as a soil amendment to improve crop yield should be approached with comprehensive consideration of the local environmental conditions, crop cultivars and field managements.