Drivers of legacy soil organic matter decomposition after fire in boreal forests in Northwest Territories, Canada, 2014

Boreal forests harbor as much carbon (C) as the atmosphere and significant amounts of organic nitrogen (N), the nutrient most likely to limit plant productivity in high-latitude ecosystems. In the boreal biome, the primary disturbance is wildfire, which consumes plant biomass and soil material, emits greenhouse gasses, and influences long-term C and N cycling. Climate warming and drying is increasing wildfire severity and frequency and is combusting more soil organic matter (SOM). Combustion of surface SOM exposes deeper older layers of accumulated soil material that previously escaped combustion during past fires, here termed legacy SOM. Postfire SOM decomposition and nutrient availability are determined by these layers, but the drivers of legacy SOM decomposition are unknown. We collected soils from plots after the largest fire year on record in the Northwest Territories, Canada, in 2014. We used radiocarbon dating to measure Δ14C (soil age index), soil extractions to quantify N pools and microbial biomass, and a 90-day laboratory incubation to measure the potential rate of element mineralization and understand patterns and drivers of legacy SOM C decomposition and N availability. We discovered that bulk soil C age predicted C decomposition, where cumulatively, older soil (approximately −450.0‰) produced 230% less C during the incubation than younger soil (~0.0‰). Soil age also predicted C turnover times, with old soil turnover 10 times slower than young soil. We found respired C was younger than bulk soil C, indicating most C enters and leaves relatively quickly, while the older portion remains a stable C sink. Soil age and other indices were unrelated to N availability, but microbial biomass influenced N availability, with more microbial biomass immobilizing soil N pools. Our results stress the importance of legacy SOM as a stable C sink and highlight that soil age drives the pace and magnitude of soil C contributions to the atmosphere between wildfires. Publication that originally utilized these data: https://doi.org/10.1002/ecs2.4672.北方针叶林蕴含的碳（C）总量与大气碳库相当，同时还储有大量有机氮（N）——后者是高纬度生态系统中最易限制植物生产力的营养元素。在北方生物群区，野火是主要干扰因子：其会消耗植物生物量与土壤物质、排放温室气体，并对长期碳氮循环产生深远影响。气候变暖变干正加剧野火的发生频率与烈度，且燃烧更多土壤有机质（soil organic matter, SOM）。地表土壤有机质的燃烧会暴露深层的古老累积土壤层，这类土层在过往野火中未被燃烧，本文将其称为遗留土壤有机质（legacy SOM）。火后土壤有机质的分解过程与养分有效性由这些土层决定，但遗留土壤有机质分解的驱动机制尚不明确。本研究采集了加拿大西北地区2014年（有记录以来最大野火年份）的样地土壤样品。我们通过放射性碳定年法测定Δ14C（土壤年龄指数），采用土壤浸提法定量氮库与微生物生物量，并开展为期90天的实验室培养实验，以测定元素矿化的潜在速率，进而解析遗留土壤有机质碳分解与氮有效性的模式及驱动因素。研究发现，整体土壤碳年龄可预测碳分解速率：累积来看，较老土壤（约-450.0‰）在培养过程中释放的碳量比年轻土壤（约0.0‰）少230%。土壤年龄还可预测碳周转时间，古老土壤的碳周转速率较年轻土壤慢10倍。我们观察到，呼吸释放的碳比整体土壤碳更年轻，这表明大部分碳的周转过程相对快速，而较老的碳组分则成为稳定碳汇。土壤年龄及其他指标与氮有效性无显著关联，但微生物生物量会影响氮有效性：更高的微生物生物量会固持土壤氮库。本研究结果强调了遗留土壤有机质作为稳定碳汇的重要性，并指出土壤年龄决定了两次野火之间土壤碳向大气排放的速率与规模。使用本数据集的原始文献：https://doi.org/10.1002/ecs2.4672。