Long-Term Tillage and Cropping Sequence Effect On Dryland Crop Yields and Carbon and Nitrogen Cycling.

Improved management practices are needed to increase dryland crop yields and soil organic matter compared with conventional farming practices in the northern Great Plains. We evaluated the 21-yr effect of tillage and cropping sequence on dryland grain and biomass (stems + leaves) yields and N uptake of spring wheat, barley, and pea and soil organic matter at the 0- to 20-cm depth in eastern Montana, USA. Treatments were no-tilled continuous spring wheat (NTCW), spring-tilled continuous spring wheat (STCW), fall- and spring-tilled continuous spring wheat (FSTCW), fall- and spring-tilled spring wheat-barley (1984-1999) followed by spring wheat-pea (2000-2004) (FSTW-B/P), and the conventional spring-tilled spring wheat-fallow (STW-F). Spring wheat grain and biomass yields and N uptake increased with crop growing season precipitation (GSP) and were greater in STW-F than in FSTCW and FSTW-B/P when GSP was <250 mm. Although mean grain and biomass yields and N uptake were greater, annualized yields and N uptake were lower in STW-F than in other treatments. In FSTW-B/P, barley and pea grain and biomass yields and N uptake increased with GSP. Soil organic C and total N were lower in STW-F than in other treatments and linearly related (R² = 0.64 to 0.78) with total annualized biomass residue returned to the soil from 1984 to 2004. While estimated C loss through mineralization at 0- to 20-cm remained similar among treatments, estimated N loss through leaching, volatilization, or denitrification increased with increasing tillage frequency or greater with fallow than with continuous cropping. Alternate-year summer fallowing increased spring wheat grain and biomass yields, N uptake, and N loss but reduced annualized yields and soil organic matter compared with annual cropping. For sustaining dryland crop yields and soil organic matter, no-tillage with annual cropping system can be adopted in the northern Great Plains.