Application of biogas-slurry and biochar improves soil multifunctionality in a poplar plantation during afforestation processes

Aims: Management can largely influence the sustainability and functionality of forest plantations. However, the role of biogas-slurry and biochar application in supporting soil’s multifunctionality during afforestation remains poorly understood. Methods: We conducted a six-year field experiment to examine the responses of soil ecosystem multifunctionality (EMF) to the application of biogas-slurry applied annually as four treatments, and a single initial application of four treatments of biochar in a new poplar plantation based on a reclaimed coastal area. The EMF was calculated as the standardized average of multiple soil surrogates of soil functions linked with C, N, and P cycles. Results: Both biogas-slurry and biochar had significantly positive effects on EMF and individual functions, with biogas-slurry having the strongest positive effect, thus promoting carbon stocks and nutrient availability. Furthermore, interactions between biochar and biogas-slurry produced a substantial synergistic effect on soil multifunctionality. The application of biogas-slurry significantly and directly increased the C, N, and P cycle multifunctionality (CCMF, NCMF, and PCMF, respectively). In contrast, a single application of biochar profoundly increased the CCMF but decreased the NCMF, thus requiring a trade-off of multiple elements of soil multifunctionality. Conclusions: We show that organic management has the potential to boost multiple ecosystem functions during afforestation. These findings further highlight the importance of rational fertilization strategies for long-term forest management.