Variation in soil organic C (%OC) concentration has been associated with the concentration of reactive Fe- and Al-oxyhydroxide phases and exchangeable Ca, with the relative importance of these two stabilizing components shifting as soil pH moves from acid to alkaline. However, it is currently unknown if this pattern is similar or different with regard to measures of soil C persistence. We sampled soils from 3 horizons (uppermost A, uppermost B, C or lowest B horizons) across a pH gradient of 11 grass-dominated and 13 deciduous/mixed forest-dominated NEON sites to examine similarities and differences in the drivers of C concentration and persistence. Variation in C concentrations in all soils could be linked to abundances of Fe, Al and Ca, but were not significantly linked to variation in soil C persistence. Though pH was related to variation in Δ14OC, higher persistence was associated with more alkaline pH values. In forested soils, depth explained 75% of the variation in Δ14OC (p < 0.0001), with no significant additional correlations with extractable metal phases. In grasslands, soil organic C persistence was not associated with exchangeable Ca concentrations, but instead was explained by depth and inorganic C concentrations (R2 = 0.76, p < 0.0001), implying stabilization of organic C through association with carbonate precipitation. In grasslands, measures of substrate quality suggested greater persistence is also associated with a more advanced degree of decomposition. Results suggest that explanatory variables associated with C concentrations differ from those associated with persistence, and that reactive Fe- and Al-oxyhydroxide phases may not be present in high enough concentrations in most soils to offer any significant protective capacity. These results have significant implications for our understanding of how to model the soil C cycle and may suggest previously unrecognized stabilization mechanisms associated with carbonates and forms of extractable Si.

中文翻译：

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美国本土森林和草原之间碳浓度和持久性的不同控制

土壤有机碳 (%OC) 浓度的变化与反应性 Fe-和 Al-羟基氧化物相和可交换 Ca 的浓度有关，随着土壤 pH 值从酸性变为碱性，这两种稳定组分的相对重要性发生变化。然而，目前尚不清楚这种模式在土壤碳持久性测量方面是相似还是不同。我们在 11 个以草为主和 13 个以落叶/混交林为主的 NEON 站点的 pH 梯度中从 3 个层位（最高 A、最高 B、C 或最低 B 层）采样土壤，以检查 C 浓度和坚持。所有土壤中 C 浓度的变化可能与 Fe、Al 和 Ca 的丰度有关，但与土壤 C 持久性的变化没有显着联系。尽管 pH 值与 Δ14OC 的变化有关，但较高的持久性与更多的碱性 pH 值相关。在森林覆盖的土壤中，深度解释了 Δ14OC 变化的 75% (p < 0.0001)，与可提取金属相没有显着的额外相关性。在草原中，土壤有机碳的持久性与可交换的 Ca 浓度无关，而是通过深度和无机 C 浓度来解释（R2 = 0.76，p < 0.0001），这意味着通过与碳酸盐沉淀相关的有机碳的稳定。在草原中，基质质量​​的测量表明更大的持久性也与更高级的分解程度有关。结果表明，与 C 浓度相关的解释变量不同于与持久性相关的变量，并且在大多数土壤中，反应性 Fe- 和 Al-羟基氧化物相的浓度可能不够高，无法提供任何显着的保护能力。这些结果对我们理解如何模拟土壤碳循环具有重要意义，并且可能暗示了以前未被认识的与碳酸盐和可提取硅形式相关的稳定机制。