Humid tropical forests contain some of the largest soil organic carbon (SOC) stocks on Earth. Much of this SOC is in subsoil, yet variation in the distribution of SOC through the soil profile remains poorly characterized across tropical forests. We used a correlative approach to quantify relationships among depth distributions of SOC, fine root biomass, nutrients and texture to 1 m depths across 43 lowland tropical forests in Panama. The sites span rainfall and soil fertility gradients, and these are largely uncorrelated for these sites. We used fitted β parameters to characterize depth distributions, where β is a numerical index based on an asymptotic relationship, such that larger β values indicate greater concentrations of root biomass or SOC at depth in the profile. Root β values ranged from 0.82 to 0.95 and were best predicted by soil pH and extractable potassium (K) stocks. For example, the three most acidic (pH < 4) and K-poor (< 20 g K m⁻²) soils contained 76 ± 5% of fine root biomass from 0 to 10 cm depth, while the three least acidic (pH > 6.0) and most K-rich (> 50 g K m⁻²) soils contained only 41 ± 9% of fine root biomass at this depth. Root β and SOC β values were inversely related, such that a large fine root biomass in surface soils corresponded to large SOC stocks in subsoils (50–100 cm). SOC β values were best predicted by soil pH and base cation stocks, with the three most base-poor soils containing 34 ± 8% of SOC from 50 to 100 cm depth, and the three most base-rich soils containing just 9 ± 2% of SOC at this depth. Nutrient depth distributions were not related to Root β or SOC β values. These data show that large surface root biomass stocks are associated with large subsoil C stocks in strongly weathered tropical soils. Further studies are required to evaluate why this occurs, and whether changes in surface root biomass, as may occur with global change, could in turn influence SOC storage in tropical forest subsoils.

潮湿的热带森林包含地球上一些最大的土壤有机碳（SOC）库。大部分有机碳都存在于地下土壤中，但是整个热带森林中土壤剖面中有机碳分布的变化仍然很差。我们采用了一种相关方法来量化巴拿马43个低地热带森林中SOC深度分布，细根生物量，养分和质地至1 m深度之间的关系。这些地点跨越降雨和土壤肥力梯度，并且与这些地点在很大程度上不相关。我们使用拟合的β参数来表征深度分布，其中β是基于渐近关系的数值索引，因此较大的β值表示剖面中深度处的根生物量或SOC浓度较高。根β值范围从0.82到0.95，可以通过土壤pH值和可提取的钾（K）储量最好地预测。例如，三种酸性最高（pH <4）和钾贫乏（<20 g K m ^-2）的土壤在0到10 cm深度处含有76±5％的细根生物量，而三种酸性最低（pH> 4） 6.0）和大多数富含钾的土壤（> 50 g K m ^-2）在该深度仅含有41±9％的细根生物量。根β和SOC β值成反比关系，使得在表面污垢大的细根生物量对应于大SOC股底土（50-100厘米）。SOC β最好通过土壤pH值和碱金属阳离子储量来预测值，三种最贫碱的土壤在50到100 cm深度含SOC的34±8％，三种最富碱的土壤仅含SOC的9±2％在这个深度。营养深度分布是不相关的根β或SOC β值。这些数据表明，在强烈风化的热带土壤中，大量的表面根系生物量储量与大量的地下土壤碳储量相关。需要进行进一步的研究来评估这种情况的发生原因，以及地表根生物量的变化（如全球变化可能发生的变化）是否会反过来影响热带森林土壤中的SOC储存。