The molecular composition of soil organic carbon remains contentious. Microbial-, plant- and fire-derived compounds may each contribute, but whether they vary predictably among ecosystems remains unclear. Here we present carbon functional groups and molecules from a diverse spectrum of North American surface mineral soils, collected primarily from the National Ecological Observatory Network and quantified by nuclear magnetic resonance spectroscopy and a molecular mixing model. We find that soils vary widely in relative contributions of carbohydrate, lipid, protein, lignin and char-like carbon, but each compound class has similar overall abundance. Ninety percent of the variance in carbon composition can be explained by three principal component axes representing a trade-off between lignin and protein, a trade-off between carbohydrate and char, and lipids. Reactive aluminium, crystalline iron oxides and pH plus overlying organic horizon thickness—predictors that are all related to climate—best explain variation along each respective axis. Together, our data point to continental-scale trade-offs in soil carbon molecular composition that are linked to environmental and geochemical variables known to predict carbon mass concentrations. Controversies regarding the genesis of soil carbon and its potential responses to global change can be partially reconciled by considering diverse ecosystem properties that drive complementary persistence mechanisms.

土壤有机碳的分子组成仍然存在争议。微生物，植物和火源化合物可能各自起作用，但是在生态系统中它们是否可预测地变化仍不清楚。在这里，我们介绍了来自北美表层矿物土壤的各种光谱的碳官能团和分子，这些碳官能团和分子主要来自美国国家生态观测站网络，并通过核磁共振波谱法和分子混合模型进行了定量。我们发现土壤在碳水化合物，脂质，蛋白质，木质素和类似炭的碳的相对贡献方面差异很大，但是每种化合物类别的总体丰度都相似。碳成分变化的90％可以通过三个主要成分轴来解释，分别代表木质素和蛋白质之间的权衡，碳水化合物和炭之间的权衡，和脂质。活性铝，结晶性氧化铁和pH值加上上层有机层厚度（与气候有关的预测因素）最好地解释了沿每个轴的变化。总之，我们的数据指出了土壤碳分子组成的大陆尺度权衡，这些权衡与已知可预测碳质量浓度的环境和地球化学变量有关。关于土壤碳的成因及其对全球变化的潜在响应的争议可以通过考虑驱动互补性持久机制的各种生态系统特性来部分解决。我们的数据指出了土壤碳分子组成的大陆尺度权衡，这与已知的可预测碳质量浓度的环境和地球化学变量有关。关于土壤碳的成因及其对全球变化的潜在响应的争议可以通过考虑驱动互补性持久机制的各种生态系统特性来部分解决。我们的数据指出了土壤碳分子组成的大陆尺度权衡，这与已知的可预测碳质量浓度的环境和地球化学变量有关。关于土壤碳的成因及其对全球变化的潜在响应的争议可以通过考虑驱动互补性持久机制的各种生态系统特性来部分解决。