Long-term ecosystem carbon losses from silage maize-based forage cropping systems

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.

Automated summary

The intensification of the US dairy industry has led to increased reliance on maize silage as a primary forage source, resulting in higher carbon losses compared to perennial forages like alfalfa. Including alfalfa in crop rotation can reduce carbon losses, but net losses are still observed. Winter rye cover crops and applications of liquid dairy manure marginally improved carbon balances but were insufficient to offset losses in respiration and crop harvest.