Net Carbon Balance between Priming and Replenishment of Soil Organic Carbon with Biochar Addition Regulated by N Addition Differing in Contrasting Forest Ecosystems

The replenishment and priming effect (PE) are two decisive processes that determine the carbon (C) sequestration potential of biochar. However, how increased nitrogen (N) availability affect these two processes and the consequent net C balance remains poorly understood. By collecting soils from three forest ecosystems (deciduous broad-leaf forest (DBF), evergreen coniferous forest (ECF), and evergreen broad-leaf forest (EBF)), we conducted a 365-day incubation experiment by adding C-13-labelled biochar plus five rates of inorganic N (0 to 15% N of soil total N). The -results showed that N addition significantly stimulated the early period (0-48 days) but did not affect the late period (49-365 days) of biochar decomposition. The effect of N addition on PE varied largely with the forest type and decomposition period; N addition significantly enhanced the negative PE -in both periods in DBF and at the late period in EBF, whereas it stimulated positive PE in the early period in EBF and ECF. At the end of incubation, the addition of biochar caused net C accumulation across all treatments due to the huge proportion of biochar (98.1%-98.9% of added biochar) retained in soils and the negative or neutral cumulative PE (-11.25-0.35 g C kg(-1) SOC), and the magnitude of net C balance increased linearly with the N addition rate in DBF and EBF. Collectively, the results of this study indicate that biochar input can contribute to soil C sequestration and that N addition can enhance the C sequestration potential of biochar.