Effects of biochar on farmland soil aggregate stability and phosphorus desorption

Soil aggregates, as the basic functional units of soil, control soil structure and carbon and phosphorus cycling. However, the long-term effects of biochar on the distribution and stability of soil aggregates, carbon and phosphorus contents within these aggregates, and the soil phosphorus release potential remain unclear. Therefore, a three-year field plot experiment was conducted in farmland of Northeast China with three treatments: no biochar (C0), 15 t of biochar input at one time (C1) and 5 t of biochar input at three times (C2). The distribution of aggregates and their contributions to the carbon and phosphorus contents of the bulk soil were explored, the key driving factors for improving aggregate stability were identified, and the relationship between soil aggregate stability and phosphorus release was clarified. The results revealed that biochar increased the proportion of large macro-aggregates (>2 mm), small macro-aggregates (0.25-2 mm) and aggregate stability. Compared with that of C0, the mean weight diameter (MWD) of C1 and C2 increased by 17.11 % and 25.73 %, respectively. The aggregate distribution was closely related to the soil carbon and phosphorus contents, and compared with the other aggregates, the small macro-aggregates (0.25-2 mm) in the C1 and C2 contributed more to the soil carbon and phosphorus contents. Moreover, biochar significantly increased the aromaticity and hydrophobicity of soil organic carbon in the C1 and C2, and the biochar content, aromaticity and hydrophobicity were significantly positively correlated with MWD. Furthermore, the improvement in aggregate stability increased the phosphorus reserves in farmland soil and indirectly increased the soil phosphorus release potential. The amount of phosphorus desorbed following the C2 increased by 8.86 % compared with that of C0. Taken together, these results indicate that biochar increases carbon and phosphorus contents by increasing aggregate stability and that continuous low-dose application of biochar is more effective at improving phosphorus desorption than a single high-dose application. These results are helpful for evaluating the long-term effects of biochar on aggregate stability and developing biochar application patterns suitable for farmland soils.