Impact of freeze-thaw cycles on greenhouse gas emissions in marginally productive agricultural land under different perennial bioenergy crops.

Knowledge of freeze-thaw-induced carbon (C) and nitrogen (N) cycling and concomitant nitrous oxide (N2 O) and carbon dioxide (CO2 ) emissions in perennial bioenergy crops is crucial to understanding the contribution of these crops in mitigating climate change through reduced greenhouse gas (GHG) emissions. In this study, a 49-day laboratory incubation experiment was conducted to compare the impact of freeze-thaw cycles on N2 O and CO2 emissions in different perennial bioenergy crops [miscanthus (Miscanthus giganteus L.), switchgrass (Panicum virgatum L.), and willow (Salix miyabeana L.)] to a successional site and to understand the processes controlling the N2 O and CO2 emissions in these crops. The results showed that freeze-thaw cycles caused a decline in dissolved organic C (DOC) and dissolved inorganic N (DIN) concentrations but enhanced the dissolved organic N (DON) and nitrate (NO3 - ). Although, freeze-thaw decreased water stable soil aggregates in all the bioenergy crops and successional site, this did not have any significant impact on N2 O and CO2 emissions, suggesting that the N2 O and CO2 emitted during the freeze-thaw cycles may have originated mostly from cellular materials released from lysis and death of microbial biomass rather than from soil aggregate disruption. Cumulative N2 O emissions measured over the 49-day incubation period ranged from 148 mg N2 O-N m-2 to 17 mg N2 O-N m-2 and were highest in miscanthus followed by willow, switchgrass, and successional site. Cumulative CO2 on the other hand was highest in the successional site than any of the bioenergy crops and ranged from 25,262 mg CO2 -C m-2 to 15,403 mg CO2 -C m-2 after the 49 days. Higher N2 O emissions in the miscanthus and willow than switchgrass and successional site were attributed to accelerated N losses as N2 O. Results from our study indicate that managing perennial bioenergy crops on low productive agricultural lands to reduce freeze-thaw related GHG emissions and climate change mitigation is dependent on the crop species grown.