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.
Bibliographical noteFunding Information:
Funding for this work was provided by USDA-ARS through the St. Paul, Minnesota, and Morris, Minnesota, locations.
JB, GF, and CW conceived the experiment. JB and GF designed the experiment. JB, GF, and CW performed the experiments. JG analyzed the data and wrote the manuscript in consultation with GF, TG, JB, and CW. Funding for this work was provided by USDA-ARS through the St. Paul, Minnesota, and Morris, Minnesota, locations. The authors thank the operator of the cooperating dairy farm for allowing us field access for sample collection and for performing agronomic management during the study. We also thank Todd Schumacher and William Breiter, USDA-ARS, St. Paul, Minnesota; Steve Wagner, USDA- ARS, Morris, Minnesota; and Sharon Papiernik and David Schneider, USDA-ARS Brookings, South Dakota; Finally, we thank Dr. Tyson Ochsner, now of Oklahoma State University, who assisted JMB in the original design and implementation of the project.
- Carbon balance
- Eddy flux
- Forage crops
- Silage maize