The appropriate biochar amendment rate varies with targets for reducing runoff N and P losses and improving soil properties in bamboo forests

Biochar amendment rate can regulate soil properties and runoff-associated nutrient losses in bamboo forests. However, the magnitudes and directions of these effects remain elusive. Here, five treatments, namely an untreated control (CK), a treatment with organic fertilizer (OF), and a treatment with biochar supplied at 10, 20, and 50 t ha⁻¹ (B10, B20, and B50), were established in a yellow-soil bamboo forest to investigate how biochar rate regulates runoff-associated nitrogen (N) and phosphorus (P) losses and soil properties under natural rainfall. Runoff losses of total nitrogen (TN), dissolved nitrogen (DN), total phosphorus (TP), and dissolved phosphorus (DP) were recorded across six natural rainfall events at event and seasonal cumulative scales, whereas temporal changes in soil physicochemical properties and dissolved organic matter (DOM) were simultaneously evaluated. Compared with CK, B10 increased cumulative runoff depth by 4.6% and TP loss by 75.9%, indicating no cumulative mitigation advantage at the tested low rate under the monitored conditions. At the seasonal cumulative scale, treatment B20 had the lowest observed runoff depth and TN and DN losses, which were 17.3%, 10.3%, and 12.2% lower than those of CK, respectively; it also resulted in the lowest soil bulk density and highest pH. Treatment B50 had the lowest observed cumulative DP loss, 25.7% lower than that of CK, and higher soil organic carbon (SOC) and available phosphorus (AP) contents than B20 at the final July sampling; however, its cumulative runoff depth was 12.8% higher than that of CK, and its TN and DN losses were only 3.1% and 0.9% lower, respectively. Overall, B20 and B50 differed in their dominant response features under the present field conditions: B20 corresponded to lower cumulative runoff and N losses, whereas B50 corresponded to lower DP loss and higher late-season SOC and AP contents.