Data from: Ecosystem carbon exchange on conversion of Conservation Reserve Program grasslands to annual and perennial cropping systems

Land use changes into and out of agricultural production may substantially influence ecosystem carbon (C) balance for many years. We examined ecosystem C balances for eight years after the conversion of 22 year-old Conservation Reserve Program (CRP) grasslands and formerly tilled agricultural fields (AGR) to annual (continuous no-till corn) and perennial (switchgrass and restored prairie) cropland. An unconverted CRP field (CRP-Ref) was maintained as a historical reference. Ecosystem C balance was assessed using adjusted net ecosystem carbon exchange (NEEadj) calculated by adding C removed in harvested biomass to NEE measured using eddy covariance method. The cumulative NEEadj of the corn and perennial systems on former CRP fields showed that these systems were a net C source to the atmosphere over the 8-year period while on former AGR fields, the perennial systems were net C sinks and the corn system near-neutral. The CRP-Ref was near neutral until a drought year when it became a net source. The corn system on the CRP field will likely reach a new lower soil C equilibrium at least 14 years after conversion but will never regain the C lost upon conversion under current no-till management with residue partially removed. On the other hand, the perennial systems could fully regain in ~14 years the C lost following conversion. The cumulative NEEadj of the corn systems exhibited a higher C emission than did the perennial systems within the same land use histories, reflecting the dominant role of crop type and management in agricultural ecosystem C balance. Results suggest that converting croplands to grasslands results in immediate C gains whereas converting grasslands to croplands results in permanent (no-till corn with partial residue removal) or temporary (perennial herbaceous crops) net C loss to the atmosphere. This has a significant implications for global climate change mitigation where biomass production from annual and perennial crops is promoted to avoid fossil-fuel C emissions (biofuel) or to remove CO2 from the atmosphere (bioenergy C capture and storage).

农业生产用地的转入与转出变化，会在多年内对生态系统碳（C）平衡产生显著影响。本研究针对22年生保护储备计划（Conservation Reserve Program, CRP）草地与既往耕作农田（formerly tilled agricultural fields, AGR），将其转换为一年生（连续免耕玉米）及多年生（柳枝稷与恢复草原）耕地后，开展了为期8年的生态系统碳平衡观测；同时设置一块未转换的CRP地块（CRP-Ref）作为历史对照样地。本研究采用修正后的生态系统碳交换量（adjusted net ecosystem carbon exchange, NEEadj）评估生态系统碳平衡：该指标通过将收获生物质移除的碳量与涡度协方差法（eddy covariance method）测得的生态系统碳交换量（net ecosystem carbon exchange, NEE）相加计算得到。对原CRP地块上的玉米与多年生种植系统的累积NEEadj分析显示：8年观测期内，此类系统均为大气碳源；而原AGR地块上的多年生系统为大气碳汇，玉米系统则接近碳平衡中性状态。CRP-Ref样地除遭遇干旱的年份转为大气碳源外，其余时段均维持碳平衡中性状态。原CRP地块上的玉米系统，在转换后至少14年才可能达到新的更低土壤碳平衡水平，且在当前部分移除秸秆的免耕管理模式下，永远无法恢复转换过程中损失的碳储量。与之相对，多年生种植系统可在约14年内完全恢复转换过程中损失的碳储量。在相同土地利用历史背景下，玉米系统的累积NEEadj表现出比多年生系统更高的碳排放量，这反映了作物类型与管理模式在农业生态系统碳平衡中的主导作用。研究结果表明：将耕地转换为草地可快速实现碳固存增益，而将草地转换为耕地则会造成永久性（部分移除秸秆的免耕玉米模式）或暂时性（多年生草本作物模式）的大气净碳损失。这一发现对于全球气候变化减缓具有重要意义——当前推广一年生与多年生作物的生物质生产，既可规避化石燃料碳排放（生物燃料, biofuel），也可实现大气CO₂的移除（生物能源碳捕获与封存, bioenergy C capture and storage）。