Intensification of the US dairy industry has driven increased reliance on maize (Zea mays L.) silage as a primary forage source in place of perennial forages such as alfalfa (Medicago sativa L.). Using 29 site-years of eddy covariance, plant, and manure measurements, we calculated net ecosystem C balances (NECB) for two silage maize-based forage cropping systems and a soybean-maize grain rotation. We found that C losses were over threefold greater from continuous silage maize (-4.9 Mg C ha⁻¹ yr⁻¹) than from the predominant grain cropping system in the region, the soybean-maize rotation (-1.3 Mg C ha⁻¹ yr⁻¹). Including alfalfa in rotation reduced C losses by 23% relative to continuous silage maize, but net losses were still observed (-3.8 Mg C ha⁻¹ yr⁻¹). For every megagram of crop residue C left in-field, net C balances increased by +0.9 Mg C ha⁻¹. A winter rye (Secale cereale L.) cover crop and applications of liquid dairy manure marginally improved C-balances but were insufficient to offset C losses in respiration and crop harvest. Increasing manure application rates could bring these systems to a net equilibrium C balance but would also result in soil N and P surpluses and unacceptable loss of nutrients to air and water. Since 1980, over 800,000 hectares of alfalfa have been lost across the Upper Midwest US, and C export in harvested maize grain and silage have increased dramatically. This shift implies a substantial reduction in SOC on forage cropped soils in the region.

美国奶业的集约化驱使人们越来越依赖于玉米（Zea mays L.）青贮饲料作为主要饲料来源，代替多年生牧草如苜蓿（Medicago sativa L.）。使用29个站点年的涡度协方差，植物和粪肥测量，我们计算了两个基于青贮玉米的草料种植系统和一个大豆玉米谷物轮作系统的净生态系统碳平衡（NECB）。我们发现，连续青贮玉米（-4.9 Mg C ha ^-1 yr ^-1）的碳损失比该地区主要的谷物种植系统（玉米-玉米轮作（-1.3 Mg C ha ^-1 yr ））高三倍以上^-1）。相对于连续青贮玉米，包括苜蓿轮作可使碳损失降低23％，但仍观察到净损失（-3.8 Mg C ha ^-1 yr ^-1）。田间每剩1毫克作物残渣C，净C平衡增加+0.9 Mg C ha ^-1。冬黑麦（Secale谷物L.）覆盖作物和使用液态奶肥略微改善了碳平衡，但不足以抵消呼吸和作物收成中的碳损失。不断增加的肥料施用量可使这些系统达到净平衡C平衡，但也会导致土壤氮和磷过剩，以及空气和水中养分的流失不可接受。自1980年以来，美国中西部地区的紫花苜蓿损失超过80万公顷，收获的玉米粒和青贮饲料中的C出口急剧增加。这种转变意味着该地区饲草作物土壤的SOC显着降低。