Litter quality controls tradeoffs in soil carbon decomposition and replenishment in a subtropical forest

1. Species-rich forests can produce litter of varying carbon (C) and nitrogen (N) composition (i.e. quality), which can affect decomposition and play a central role in long-term soil organic carbon (SOC) accumulation. However, how differences in litter quality affect SOC decomposition and formation remains unclear over the full litter decomposition trajectory. 2. We followed the in situ complete decomposition of added 13C-labelled high-(low C:N) and low-quality (high C:N) leaf-litter and its effect on particulate organic matter (POM) and mineral-associated organic matter (MAOM) fractions over 2 years in a natural subtropical forest. 3. We found that during early stages of decomposition, low-quality litter inputs decreased SOC via a positive priming effect (i.e. new C inputs favoured decomposition of native SOC), but these SOC losses were offset by SOC gains observed via a negative priming effect during decomposition of high-quality litter. In contrast, this pattern reversed during late stages of decomposition-SOC losses via a positive priming effect induced by high-quality litter were offset by SOC gains via a negative priming effect induced by low-quality litter. Over the full decomposition of litter, both high-and low-quality litter stimulated microbial breakdown of SOC tied to POM, but also replenished more persistent SOC that associated with soil minerals (MAOM). Altogether, we observed that low-quality litter formed twice as much new SOC as high-quality litter (24% vs. 12% of added litter-C). We extend the notion of the priming effect from primarily a negative role promoting losses of native SOC, to a functional role that can replenish persistent SOC. 4. Synthesis. Our measurements raise the possibility that, in species-rich forests, high-and low-quality litter decomposition play opposite but dynamically complementary roles in renewing POM-both by inducing its decomposition and formation-while exclusively favouring MAOM formation, which can help explain how differences in litter quality favour SOC accumulation and persistence. Global change factors that shift plant community composition may ultimately affect the fate of soil C, as changes in litter quality may force soil transitions from sinks to sources or sources to sinks of atmospheric CO2.