As tropical savannas are undergoing rapid conversion to other land uses, native C₃‐C₄ vegetation mixtures are often transformed to C₃‐ or C₄‐dominant systems, resulting in poorly understood changes to the soil carbon (C) cycle. Conventional models of the soil C cycle are based on assumptions that more labile components of the heterogenous soil organic C (SOC) pool decompose at faster rates. Meanwhile, previous work has suggested that the C₄‐derived component of SOC is more labile than C₃‐derived SOC. Here we report on long‐term (18 months) soil incubations from native and transformed tropical savannas of northern Australia. We test the hypothesis that, regardless of the type of land conversion, the C₄ component of SOC will be preferentially decomposed. We measured changes in the SOC and pyrogenic carbon (PyC) pools, as well as the carbon isotope composition of SOC, PyC and respired CO₂, from 63 soil cores collected intact from different land use change scenarios. Our results show that land use change had no consistent effect on the size of the SOC pool, but strong effects on SOC decomposition rates, with slower decomposition rates at C₄‐invaded sites. While we confirm that native savanna soils preferentially decomposed C₄‐derived SOC, we also show that transformed savanna soils preferentially decomposed the newly added pool of labile SOC, regardless of whether it was C₄‐derived (grass) or C₃‐derived (forestry) biomass. Furthermore, we provide evidence that in these fire‐prone landscapes, the nature of the PyC pool can shed light on past vegetation composition: while the PyC pool in C₄‐dominant sites was mainly derived from C₃ biomass, PyC in C3‐dominant sites and native savannas was mainly derived from C₄ biomass. We develop a framework to systematically assess the effects of recent land use change vs. prior vegetation composition.

中文翻译：

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热带稀树草原的土地转化优先分解新添加的生物质，无论是C3还是C4衍生。

随着热带稀树草原正迅速转变为其他土地利用方式，本地的C _3- C ₄植被混合物通常被转换为C _3-或C _4-优势系统，导致人们对土壤碳（C）循环的了解甚少。土壤碳循环的常规模型基于以下假设：异质土壤有机碳（SOC）库中较不稳定的组分以更快的速率分解。同时，先前的工作表明，SOC的C ₄衍生成分比C ₃更加不稳定。派生的SOC。在这里，我们报告了澳大利亚北部原生和转化热带稀树草原的长期（18个月）土壤培养情况。我们检验了以下假设：无论土地转换类型如何，SOC的C ₄成分都会优先分解。我们测量了63种土壤核心的SOC和热解碳（PyC）库的变化，以及SOC，PyC和呼吸的CO ₂的碳同位素组成，这些土壤核心是从不同土地利用变化情景中收集的。我们的结果表明，土地利用变化对SOC池的大小没有一致的影响，但是对SOC分解速率的影响却很大，而C ₄入侵地点的分解速率却较慢。虽然我们确认原生稀树草原土壤优先分解C ₄衍生自SOC，我们还表明，无论是C ₄衍生（草）还是C ₃衍生（林业）生物质，转化的稀树草原土壤都会优先分解新添加的不稳定SOC 。此外，我们提供的证据表明，在这些易发火灾的景观中，PyC池的性质可以揭示过去的植被组成：而C ₄占优位置的PyC池主要来源于C ₃生物量，而C ₃占优的PyC位点和天然稀树草原主要来自C ₄生物量。我们开发了一个框架来系统地评估近期土地利用变化与先前植被组成的影响。