Phosphorus fertility regulates microbial carbon use efficiency and SOM decomposition in non-allophanic Andosols

Phosphorus (P) availability affects soil carbon (C) cycling such as microbial carbon use efficiency (CUE) and priming effects (PEs). While non-allophanic Andosols are characterized by high organic C content and strong P retention, the effects of different soil P fertility levels on C dynamics in these soils remain poorly understood. In this study, we conducted a 20-day incubation experiment using ¹³ C-enriched glucose to investigate how different soil P levels (Truog-P: 157 mg P kg ⁻¹ and 12 mg P kg ⁻¹ ) impacted microbial C dynamics in non-allophanic Andosols under different field management practices. Our results showed that soil organic matter (SOM) priming strongly depends on soil P levels, although total primed CO ₂ -C emissions were low in these soils. The addition of glucose alone promoted positive PEs compared to the addition of glucose with nitrogen (N). In high P soils, glucose and N addition resulted in negative PEs, whereas in low P soils, the same treatment stimulated microbial SOM mining, resulting in positive PEs. Additionally, higher CUE was found in high P soils than in low P soils after 20 days of incubation. These findings suggest that soil P levels influence both substrate-induced microbial assimilation and SOM decomposition. In P-limited soils, microbial P requirements drive SOM decomposition, which in turn regulates CUE. This study contributes to an understanding of the role of P in soil C cycling and provides insights into soil fertility management strategies for improving C sequestration in non-allophanic Andosols.