Little is known about the long-term management impact on soil C and N contents in the northern Great Plains. We evaluated the 30-yr effect of tillage and cropping sequence combination on dryland crop biomass yield and soil bulk density, soil organic carbon (SOC), soil inorganic carbon (SIC), soil total nitrogen (STN), NH 4-N, and NO 3-N contents at the 0- to 120-cm depth in a Dooley sandy loam (fine loamy, mixed, frigid Typic Argiboroll) in eastern Montana. Treatments were no-till continuous spring wheat ( Triticum aestivum L.) (NTCW), spring till continuous spring wheat (STCW), fall and spring till continuous spring wheat (FSTCW), fall and spring till spring wheat-barley ( Hordeum vulgare L., 1984-1999) followed by spring wheat-pea ( Pisum sativum L., 2000-2013) (FSTW-B/P), and spring till spring wheat-fallow (STW-F, traditional system). Mean annualized crop biomass returned to the soil was 23 to 30% greater in NTCW, STCW, FSTCW, and FSTW-B/P than STW-F. At 0 to 7.5 cm, bulk density was 13 to 21% greater in STW-F, but SOC, SIC, and STN were 12 to 98% greater in STCW than other treatments. Ammonium-N and NO 3-N contents were 25 to 74% greater in FSTCW than other treatments. At other depths, SOC, SIC, STN, NH 4-N and NO 3-N contents varied among treatments. Reduced tillage with increased crop residue returned to the soil increased soil C and N storage in NTCW and STCW, but increased tillage intensity increased mineral N content in FSTCW compared with STW-F. Improved management practices, such as NTCW and STCW, may be adopted to improve dryland soil C and N stocks.
