Intercropping with switchgrass improves net greenhouse gas balance in hybrid poplar plantations on a sand soil

Authors: Collins, Harold; KIMURA, EMI - Washington State University; FRANSEN, STEVE - Washington State University; HIMES, AUSTIN - Greenwood Resources, Inc; Fay, Philip

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/17/2017
Publication Date: 8/31/2017
Publication URL: https://handle.nal.usda.gov/10113/5863777
Citation: Collins, H.P., Kimura, E., Fransen, S., Himes, A., Fay, P.A. 2017. Intercropping with switchgrass improves net greenhouse gas balance in hybrid poplar plantations on a sand soil. Soil Science Society of America Journal. 81:781-795.

Interpretive Summary: Major shifts in crop production will occur as farmers gear up to supply the demand for biomass feedstocks supporting biofuel production. These shifts will change agroecosystems services related to water use, carbon storage, nutrient cycling and greenhouse gas (GHG) emissions, that have direct consequences on air, water, and soil quality. Highly productive, commercial hybrid poplar plantations are being managed in the Pacific Northwest for high-value timber production at relatively low stocking densities under irrigation. The open understory was used for the production of bioenergy feedstocks. GHG’s were measured in plantings of hybrid poplar intercropped with Kanlow switchgrass during four years of production. Greenhouse gas and N2O fluxes increased following the application of nitrogen fertilizer. N2O emissions factors were greatest in fertilized treatments, averaging 1.3% of the applied N after four years of cropping. Contributions of CO2 emissions to Global Warming Potentials (GWP) were offset by the C fixed in the crop biomass resulting in a net GWP zero balance for the monoculture poplar, where, poplar/switchgrass intercrop and switchgrass monocultures sequestered significant CO2 to reduce net GWP over 4 years by 34 and 23 Mg CO2eq ha-1, respectively. Intercropping switchgrass with hybrid poplar was shown to be a viable mitigation strategy to counter GWP.

Technical Abstract: Highly productive, commercial hybrid poplar plantations are being managed in the Pacific Northwest for high-value timber production at relatively low stocking densities under irrigation. The open understory was used to produce switchgrass (Panicum virgatum) prior to canopy closure. The objectives were to describe cropping season patterns of greenhouse gas fluxes (CH4, CO2, N2O) during intercropping of switchgrass with hybrid poplar; estimate growing season losses of N2O, and amount of fertilizer losses as N2O and; estimate global warming potentials produced by intercropping vs monocultures of poplar or switchgrass. Cumulative 4 y biomass C produced in the poplar monoculture (PM) closely balanced the 4 y CO2-C emissions (0.8 Mg C ha-1), where, biomass production for the switchgrass monoculture (SM) and intercrop (IC) exceeded CO2-C emissions by 7.5 and 10.9 Mg C ha-1, respectively. Average seasonal soil CH4-C uptake was -109, -89 and -77 g CH4-C ha-1, with a 4 y cumulative uptake of -434, -356 and -306 g CH4-C ha-1 for the PM, IC and SM, respectively. Cumulative N2O-N seasonal emissions from the PM averaged 269 g N2O-N ha-1 over 2011 – 2014. Seasonal emissions from the fertilized IC and SM increased significantly from establishment in 2011 (250 – 264 g N2O-N ha-1) and peaked in 2013 for the fertilized IC and SM averaging 2864 and 3294 g N2O-N ha-1, respectively. N2O emissions factors averaged 1.3% of the applied N over the study. Contributions of CO2 emissions to Global Warming Potentials (GWP) were offset by the C fixed in the crop biomass resulting in a net GWP zero balance for the PM, where, IC and SM sequestered significant CO2 to reduce net GWP by 34 and 23 Mg CO2eq ha-1 season-1. Intercropping switchgrass with hybrid poplar was shown to be a viable mitigation strategy to counter GWP.