A review of carbon and nitrogen balances in switchgrass grown for energy
Introduction
Switchgrass is the herbaceous energy crop species chosen by the Biofuels Feedstock Development Program of the United States Department of Energy for major research focus in the immediate future.1, 2 Among the many reasons for this choice are the expected positive impacts that switchgrass could have on C and N balances when grown for energy.[3] More specifically, switchgrass could:
- 1.
reduce CO2 emissions and the associated risk of global warming by replacing fossil fuels and sequestering C;
- 2.
improve soil productivity by C sequestration; and
- 3.
reduce NO−3 contamination of surface runoff and groundwater by absorbing N lost from fertilizer and agricultural wastes if it is planted in filter strips on adjacent land.
However, the extent of these benefits has not been well documented. Therefore, the objective of this study was to assess potential impacts of switchgrass on C and N balances by reviewing and synthesizing information from current literature, unpublished data and on-going research. This synthesis will document the current state of knowledge and help focus future research.
Section snippets
Carbon accumulation
Two important concerns about current trends in the C cycle are:
- 1.
increased atmospheric CO2 (caused largely by burning fossil fuels) and the associated risk of global warming; and
- 2.
loss of C from soils (caused mainly by annual cropping) and the associated loss in soil productivity.
Nitrogen accumulation
Although relatively little information is available on the ability of switchgrass to accumulate N from the soil, this species is already being promoted as a suitable candidate to plant in filter strips aimed at preventing chemicals from contaminating surface runoff and groundwater.[18] The traditional approach for measuring percent recovery of applied N is by the difference method, in which total N recovered in above ground biomass of unfertilized plots is subtracted from that in biomass of
Conclusions
Studies presented and reviewed in this paper suggest the following:
- 1.
C sequestration benefits from switchgrass will depend mainly on what enterprise is replaced by switchgrass;
- 2.
switchgrass will likely be superior in C sequestration to annual crops, but no better than grazed pasture;
- 3.
since profits from pasture-based beef production are lower than for row crops, switchgrass will likely replace pasture first, providing little or no net gain in C sequestration;
- 4.
C sequestration is relatively unimportant
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