Field-aged biochar improves soil aggregation stability and phosphorus use efficiency in paddy field

Objectives Biochar has been widely used as soil amendment because of its large surface area and stable structures. However, the long-term effects of field-aged biochar on soil fertility and nutrient availability are relatively lacking. Therefore, we compared the effects of field-aged biochars at different years on rice crop yield and nutrient use efficiencies through in situ long-term field experiments.

Methods The long-term field experiment was established in Nanjing, Jiangsu, which belongs to clayey paddy soil. Five treatments were carried out in triplicates as following: control without phosphate fertilizer (CK), phosphate fertilizer (P), P with fresh biochar amended in 2017 (PB_0y), P with 2-year field-aged biochar amended in 2015 (PB_2y) and with 5-year field-aged biochar amended in 2012 (PB_5y). Soil samples were collected after rice harvest, and the contents of soil aggregate size classes of macro-aggregate (MacroA, 250–2000 μm), micro-aggregate (MicroA, 53–250 μm), silt fraction (SiltF, 2–53 μm) and clay fraction (ClayF, < 2 μm), available P content and basic physicochemical properties were measured. The rice yields and P uptake by rice were determined.

Results The PB_2y and PB_5y treatments significantly increased the content of MacroA (250–2000 μm) by 69.2%–107.8%, decreased that of SiltF (2–53 μm) by 13.1%–14.7% and ClayF (<2 μm) by 6.9%–41.9%, while PB_0y did not, compared with the P treatment. The PB_0y, PB_2y and PB_5y treatments significantly increased SOC by 18.5%–58.5% and total P by 5.7%–17.1%, but did not significantly increased in SOC content of 2–53 μm fraction, compared to the P treatment. Similarly, both the PB_2y and PB_5y treatments significantly increased rice yield by 13.7% and 16.3% and phosphorus use efficiency by 35.4% and 45.5%, respectively, while PB_0y did not, compared to the P treatment. Based on the structural equation modeling, the field-aged biochar (PB_2y and PB_5y) increased the rice yield and P storage by improving soil nutrient status (SOC, TP, SOC/TP, etc.) and structure (increasing the ratio of soil MacroA).

Conclusions As compared to the fresh biochar amendment, the field-aged biochar at different years effectively increased the ratio of soil macro-aggregates of 250–2000 μm and the protection of soil available phosphorus and total phosphorus, thus facilitating the absorption and utilization of phosphorus by plants and improving yield and phosphorus use efficiency. The improvement effects of field-aged biochar at 5-year is greater than field-aged biochar at 2-year.