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

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.