Tropical forests contain some of the largest soil carbon (C) stocks on Earth, making them broadly relevant to terrestrial-climate feedbacks, yet our understanding of how their soil organic C (SOC) fractions vary over space and time is limited. We studied effects of season, fertility, and mean annual precipitation (MAP) on the C contents of soil fractions across 14 lowland forests in Panama. We measured free-debris, occluded-debris, and mineral-associated SOC fractions, as well as soluble C associated with each fraction. We hypothesized that mineral-associated SOC would be greatest in infertile, strongly weathered soils with large reactive mineral contents. We also hypothesized that the debris SOC fractions would accumulate during the dry season, reflecting seasonal increases in litterfall. To address this, we compared soil fractions in wet and dry seasons from fertile and infertile soils across a range of 1809–2864 mm MAP. The C content (mg C/g soil) of all soil fractions varied with fertility and MAP: specifically, free-debris SOC was greatest in wet, high-fertility soils, and occluded-debris SOC was greater in high-fertility than low-fertility soils. The mineral-associated SOC fraction, which contained the majority of bulk soil C, showed increasing C content with greater MAP in infertile sites, presumably driving similar spatial patterns in the bulk soil. Only the free-debris SOC fraction showed strong seasonal variation, increasing in mass during the dry season. Nitrogen behaved similarly to C. In summary, soil C contents increased with MAP in infertile sites but not fertile sites, driven by the mineral-associated SOC fraction. The dry season had greater free-debris SOC, but this seasonal trend was not apparent in bulk soil C, likely because of the small size of the free-debris SOC fraction. Thus, changes in the quantity and seasonality of precipitation, which are projected for tropical forests, might shift spatial and temporal patterns of soil C storage, which would in turn influence forest-climate feedbacks for this C-rich biome.

热带森林包含地球上一些最大的土壤碳 (C) 储量，使它们与陆地气候反馈广泛相关，但我们对它们的土壤有机碳 (SOC) 分数如何随空间和时间变化的了解有限。我们研究了季节、肥力和年平均降水量 (MAP) 对巴拿马 14 个低地森林土壤组分碳含量的影响。我们测量了游离碎片、闭塞碎片和矿物相关 SOC 部分，以及与每个部分相关的可溶性 C。我们假设与矿物质相关的 SOC 在具有大量活性矿物质含量的贫瘠、强烈风化的土壤中最大。我们还假设碎片 SOC 部分会在旱季积累，反映凋落物的季节性增加。为了解决这个问题，我们比较了 1809-2864 毫米 MAP 范围内肥沃和不肥沃土壤在雨季和旱季的土壤成分。所有土壤组分的 C 含量（mg C/g 土壤）随肥力和 MAP 的变化而变化：具体而言，自由碎片 SOC 在潮湿、高肥力土壤中最大，而在高肥力土壤中的闭塞碎片 SOC 大于低肥力土壤。肥沃的土壤。与矿物质相关的 SOC 部分包含大部分的大块土壤 C，在不肥沃的地点随着 MAP 的增加，C 含量增加，可能在大块土壤中驱动了类似的空间模式。只有游离碎片 SOC 部分表现出强烈的季节性变化，在旱季质量增加。氮的表现与 C 相似。总而言之，在与矿物质相关的 SOC 部分的驱动下，土壤 C 含量随 MAP 在不育地点而不是肥沃地点中增加。旱季具有更大的游离碎片 SOC，但这种季节性趋势在大块土壤 C 中并不明显，可能是因为游离碎片 SOC 部分较小。因此，为热带森林预测的降水量和季节性变化可能会改变土壤碳储存的空间和时间模式，这反过来又会影响这个富含碳的生物群落的森林气候反馈。