From Depletion to Restoration: Lessons From Long‐Term Monitoring of Carbon Gains and Losses in Cropping Systems Global Change Biology

As global atmospheric CO2 rapidly approaches a key tipping point, there is an urgent need to implement strategies to reverse this pattern. A generally accepted understanding of carbon (C) in agricultural fields includes: (H1) substantial C loss occurs when natural vegetation is converted to crops, (H2) soils typically reach a steady‐state C concentration under contemporary practices, and (H3) improved management or crop selection can enhance soil C stocks over time. Significant variability exists, but studies consistently show large C losses from agricultural ecosystems, supporting H1. Although steady‐state C levels (H2) are commonly assumed, measuring C gains or losses in mature agroecosystems is challenging. Efforts to increase soil C storage (H3) have limited data due to the diversity of potential practices, compounded by substantial variability in soil C measurements. Here, long‐term (7–17 year) ecosystem C flux data from diverse cropping systems revealed that conventionally tilled annual row crops (maize and soybean) act as significant long‐term atmospheric C sources, challenging H2. Furthermore, conservation tillage practices reduced C losses compared with conventional tillage but showed minimal evidence for long‐term ecosystem C storage, even after 20+ years. This indicates that no‐till practices reduce C losses but imply that no soil C is added, challenging H3. By contrast, perennial Miscanthus × giganteus, <styled-content style="fixed-case">Panicum virgatum</styled-content>, and restored tallgrass prairie systems store C at the ecosystem scale more effectively than minimally tilled annual row crops. Analysis over multiple years demonstrates significant ecosystem C storage with perennial crops, varying by species, starting in the first year of transition. These findings, although focused on one region, suggest that the assumptions of steady‐state C levels and increased storage from conservation practices do not universally apply and that significant changes to agroecosystems are required to increase C storage.

随着全球大气二氧化碳（CO₂）快速逼近关键临界点，亟需制定相关策略以扭转这一趋势。学界对农田碳（C）的普遍认知包含以下三项假设：（H1）当自然植被被改造为农田种植作物时，会发生大量碳流失；（H2）在现有耕作模式下，土壤碳浓度通常会达到稳态；（H3）通过优化管理措施或筛选作物品种，可随时间推移提升土壤碳库储量。尽管不同研究间存在显著异质性，但已有研究一致证实农业生态系统会出现大规模碳流失，这一结果支持了假设H1。尽管稳态碳水平（H2）是学界普遍采用的假设，但在成熟农业生态系统中量化碳增减量仍极具挑战。由于潜在耕作措施种类繁多，加之土壤碳测量本身存在显著误差，针对提升土壤碳储量（H3）的相关研究数据仍较为匮乏。本研究通过分析来自多种种植系统的长期（7~17年）生态系统碳通量观测数据，发现传统耕作的一年生条播作物（玉米与大豆）属于长期显著的大气碳源，这一结论对假设H2提出了挑战。此外，相较于传统耕作，保护性耕作可减少碳流失，但即便经过20年以上的耕作，相关研究也几乎未发现其能长期提升生态系统碳储量的证据。这表明免耕措施仅能减少碳流失，却无法实现土壤碳的新增积累，进而对假设H3构成挑战。与之形成鲜明对比的是，多年生巨芒草（Miscanthus × giganteus）、<styled-content style="fixed-case">Panicum virgatum</styled-content>以及恢复型高草草原生态系统，在生态系统尺度上的碳固存效率远高于轻耕一年生条播作物。多年数据分析表明，多年生作物从转型第一年起即可实现显著的生态系统碳固存，且固存效率因物种而异。尽管本研究结果仅聚焦于单一区域，但仍表明稳态碳水平假设以及‘保护性耕作可提升碳储量’的结论并非普遍适用，同时也指出，若要提升农业生态系统的碳储量，需对现有农作系统进行重大变革。