Regulation of nutrient addition-induced priming effect by both soil C accessibility and nutrient limitation in afforested ecosystem

Changes in plant carbon (C) inputs to soil due to afforestation can affect the decomposition rate of the old soil C, with this so-called priming effect being dependent on the soil nitrogen (N) and phosphorus (P) availability following the establishment of the new forest. However, the patterns and drivers of the priming effect in afforested soils as well as its responses to nutrient (i.e., N and P) addition remain unclear. In this study, soil samples were collected from various sites along a 45-year Robinia pseudoacacia afforestation chronosequence on the Loess Plateau in China, and incubation experiment with the addition of 13C-labeled glucose and nutrients (N or P) was conducted. The results showed that, regardless of N or P addition, the pattern of temporal changes in the soil priming effect was the same along the chronosequence: initially increasing and then decreasing, with a peak at the 20-year site. The lower C accessibility (indicated by a high mineral associated organic C (MOC) content determined the priming effect patterns after afforestation. Moreover, the soil priming effect after afforestation differed depending on the type of nutrient added; that is, it decreased with N addition but increased with P supplied. This suggests that the decomposition of old soil C at the late stage was inhibited by N addition but stimulated after P addition. Aside from soil and microbial properties, aggravated microbial nutrient limitation was a primary reason for the decreased priming effect response induced by N addition, whereas the increased response caused by P addition was due to the alleviation of microbial P limitation. Collectively, our findings shed light on the contrasting impacts of nutrient addition on the priming effect in afforested ecosystems and highlight the importance of mineral protection and microbial nutrient limitation in regulating this phenomenon under changing nutrient availability conditions.