Soil Carbonate Dominates Calcium‐Bound Organic Carbon Storage at the Continental Scale

ABSTRACT Multi‐pool soil organic carbon (SOC), consisting of particulate OC and mineral‐associated OC, has complex functionality and stability. Discriminating between these different carbon pools can constrain the uncertainty in predicting SOC dynamics under global change. Calcium (Ca)‐bound organic carbon (OC‐Ca) is considered an important subset of stable and persistent mineral‐associated OC. However, the estimates and drivers of OC‐Ca at large geographic scales are poorly understood. Here, we synthesized field‐collected and published observations to assess OC‐Ca stocks in Chinese terrestrial ecosystems. We found that OC‐Ca accounted for ~8% of SOC storage. Soil pH‐controlled carbonate, rather than exchangeable Ca (Ca ex ), dominated OC‐Ca accumulation. The limits of moisture availability at the continental scale drove the distribution of OC‐Ca by increasing soil pH and carbonate precipitation. These mechanisms explain the large OC‐Ca pool found in drylands with a lower aridity index and in anthropogenically disturbed croplands. Additionally, compared to the unfertilized control, carbonate reduction due to acidification induced by nitrogen enrichment appeared to decrease OC‐Ca concentration by 11% and its contribution to SOC by 15%. This highlights the negative effect of nitrogen enrichment on SOC stability. Collectively, our results demonstrate that OC‐Ca is an essential stable pool of SOC. These findings linking carbonate to OC‐Ca provide clues for understanding OC‐Ca responses to multiple global changes and theoretical bases for enhancing OC‐Ca storage in multi‐pool SOC management.