Fallow Contributions to Greenhouse Gas Flux in Dryland Cropping Systems: Observations from a GRACEnet Site in Central North Dakota.

Fallow is often used in dryland cropping systems to mitigate production risks by increasing storage of soil water.  Increased soil water and a lack of vegetation cover during fallow, however, may contribute to greater greenhouse gas emissions.  An investigation was conducted to quantify the effects of fallow on carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) flux within spring wheat (Triticum aestivum L.) – fallow (SW-F; chemical-fallow) and spring wheat – safflower (Carthamus tinctorius L.) – rye (Secale cereale L.) (SW-S-R; green-fallow) cropping systems in west-central North Dakota.  Using static chamber methodology, flux measurements were taken approximately every week over 21 months during the fallow period of each cropping system.  To facilitate analyses, data were partitioned into five phases based on management of the rye cover crop: 1) first winter, 2) spring growth of rye, 3) termination of rye until soil freeze-up, 4) second winter, and 5) spring thaw until planting.  Differences in gas flux between chemical- and green-fallow were few.  Emission of CO₂ was greater under green-fallow than chemical-fallow during spring growth of rye (P < 0.01).  Methane flux ranged from -2.8 to -0.3 g CH₄-C ha^-1 d^-1 over the course of the measurement period, and only differed between chemical- and green-fallow phases following termination of rye (P < 0.05).  Nitrous oxide emission did not differ between chemical- and green-fallow during any of the partitioned phases.  Furthermore, cumulative fluxes of CO₂, CH₄, and N₂O did not differ between the chemical- and green-fallow phases over the 21 month period.  Results suggest fallow types investigated in this study have little effect on greenhouse gas emissions in dryland cropping systems.