Dryland Soil Carbon and Nitrogen after Thirty Years of Tillage and Cropping Sequence.

Little information is available about the long-term impact of tillage and cropping sequence on dryland soil C and N levels in the northern Great Plains. This study provided an opportunity to evaluate the effect of thirty years (1983-2013) of tillage and cropping sequence combination on dryland crop residue and soil organic (SOC) and inorganic C (SIC), soil total N (STN), NH₄-N, and NO₃-N contents at the 0- to 120-cm depth in eastern Montana. Treatments were no-till continuous spring wheat (NTCW), spring till continuous spring wheat (STCW), fall and spring till continuous spring wheat (FSTCW), fall and spring till spring wheat-barley (1984-1999) followed by spring wheat-pea (2000-2013) (FSTW-B/P), and spring till spring wheat-fallow (STW-F). Mean annualized crop biomass returned to the soil was lower in STW-F than in other treatments. At 0 to 7.5 cm, soil bulk density was greater in STW-F but SOC, SIC, and STN were greater in STCW than in other treatments, except NTCW, in 2013. The SIC at 90 to 120 cm was greater in NTCW and STCW than FSTCW and FSTW-B/P and STN at 30 to 60 cm was greater in NTCW and STW-F than in STCW and FSTW-B/P. The NH₄-N content at 0 to 30 cm and NO₃-N content at 0 to 120 cm were greater in FSTCW than in other treatments, except STCW. From 1983 to 2013, SOC at 0 to 7.5 cm declined from 104 kg C ha^-1 yr^-1 in STCW to 214 kg C ha^-1 yr^-1 in FSTW-B/P. Soil total (organic + inorganic) C was strongly related to SOC (R² = 0.82, P ≤ 0.001, n = 168). Reduced tillage, followed by increased amount of crop residue returned to the soil, probably increased C and N storage and reduced the rate of decline in NTCW and STCW, but increased tillage intensity increased available N in FSTCW. Soil total C can be used as a substitute for SOC in dryland soils, which reduce the need to measure SIC