Biochar's impact on dissolved organic matter (DOM) export from a cropland soil during natural rainfalls

Although biochar amendment to soil has achieved widely recognized benefits such as plant growth improvement and carbon sequestration, what impact it would have on soil carbon cycling, especially on the behavior of the active dissolved organic matter (DOM), is still unclear. This study evaluated biochar's effects on soil properties and the characteristics of DOM exporting from a cropland Entisol. The soil is the major arable land resource in the upper reaches of the Yangtze River, China and noted for its low soil organic matter and prevalence of preferential flow. Surface runoff, soil pore water and leachate were collected from the field upon a series of natural rainfall. Concentration of dissolved organic carbon (DOC) of soil pore water was found to respond strongly toward the rainfalls. Biochar application led to relatively higher DOC levels within the soil (p > 0.05). Despite apparent increase of DOC and EEM intensities for the freshly-amended soil, PARAFAC analyses indicated no changes in DOM's fluorophore compositions after two years of ageing. The identified DOM components (C1 and C2) showed similar dynamics with the DOCs in responding to the rainfalls. On the other hand, total flux of DOM leaching from biochar-amended plots during three monitored storms was on average 59% higher than the control (p > 0.05), whereas DOM export via surface runoff was slightly diminished. Noticeably, biochar amendment had caused a significant increase of >1000 μm macropores and thus the enhanced infiltration of soil water. Such changes led to increased flow discharge, which in turn resulted in elevated leaching of organic carbon during rainfalls. The results therefore implies that changes of soil structure and hydraulic properties that will take place after biochar application merits attention as they may play a continuous role in influencing the transport of DOM and possibly other solutes via runoff processes.