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Title:   Author  item
                                            Sainju, Upendraitem
                                            Allen, Brettitem
                                            Caesar, Thecanitem
                                            LENSSEN, ANDREW - Iowa State University
  Submitted to: Agronomy Journal
                                    Publication Type: Peer Reviewed Journal
                                    Publication Acceptance Date: 5/8/2015
                                    Publication  Date: 7/6/2015
                                    Citation: Sainju, U.M., Allen, B.L., Caesar, T., Lenssen, A.W. 2015. Dryland soil carbon and nitrogen after thirty years of tillage and cropping sequence. Agronomy Journal. 107(5):1822–1830. doi:10.2134/agronj15.0106.Interpretive Summary: Soil carbon and nitrogen conservation are needed to increase carbon sequestration for carbon trading, reduce the rate of nitrogen fertilization and nitrogen losses through leaching, denitrification, and volatilization, and mitigate greenhouse gases from agroecosystems. Improved management practices are needed to increase soil carbon and nitrogen storage and optimize nitrogen mineralization for providing enough nitrogen available for crop growth. 
This study provided a unique opportunity to evaluate the effect of thirty years (1983-2013) of tillage and cropping sequence combination on dryland crop biomass (stems + leaves) yield and soil bulk density, organic carbon, inorganic carbon, total nitrogen, ammonium-nitrogen, and nitrate-nitrogen at the 0-120 cm depth in a sandy loam soil in eastern Montana, USA. 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 over 30 yr was lower in STW-F than the other treatments. At 0-7.5 cm, soil bulk density was greater in STW-F, but soil organic carbon, inorganic carbon, and total nitrogen contents were greater in STCW than the other treatments, except NTCW, in 2013. Soil inorganic carbon at 90-120 cm was greater in NTCW and STCW than FSTCW and FSTW-B/P, and soil total nitrogen at 30-60 cm was greater in NTCW than the other treatments, except FSTW-B/P. Ammonium-nitrogen content at 0-30 cm and nitrate-nitrogen content at 0-120 cm were greater in FSTCW than the other treatments, except STCW. From 1983 to 2013, soil organic carbon at 0-7.5 cm declined from 104 kg C/ha/ yr in STCW to 214 kg C/ ha/yr in FSTW-B/P. Similarly, soil total nitrogen declined from 5 kg N/ha/yr in STCW to 26 kg N/ha/yr in FSTW-B/P. Soil total (organic + inorganic) carbon was strongly related to soil organic carbon (R2 = 0.82, P = 0.001, n = 168). 
Reduced tillage, followed by increased amount of crop residue returned to the soil, probably increased soil carbon and nitrogen storage and reduced their rate of decline, but increased tillage intensity increased available nitrogen. Soil total carbon can be used to replace soil organic carbon in dryland soils where soil inorganic carbon contents are high, thereby reducing the need to measure soil inorganic carbon.
                                Technical Abstract: Little information is available about the long-term (= 30 yr) impact of tillage and cropping sequence on dryland soil C and N levels in the northern Great Plains, USA. This study provided a unique opportunity to evaluate the effect of thirty years (1983-2013) of tillage and cropping sequence combination on dryland crop biomass (stems + leaves) yield and soil bulk density, organic C (SOC), inorganic C (SIC),  total N (STN), NH4-N, and NO3-N contents at the 0-120 cm depth in a sandy loam soil in eastern Montana, USA. 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 over 30 yr was lower in STW-F than the other treatments. At 0-7.5 cm, soil bulk density was greater in STW-F, but SOC, SIC, and STN were greater in STCW than the other treatments, except NTCW, in 2013. The SIC at 90-120 cm was greater in NTCW and STCW than FSTCW and FSTW-B/P, and STN at 30-60 cm was greater in NTCW than the other treatments, except FSTW-B/P. The NH4-N content at 0-30 cm and NO3-N content at 0-120 cm were greater in FSTCW than the other treatments, except STCW. From 1983 to 2013, SOC at 0-7.5 cm declined from 104 kg C ha-1 yr-1 in STCW to 214 kg C ha-1 yr-1 in FSTW-B/P. Similarly, STN declined from 5 kg N ha-1 yr-1 in STCW to 26 kg N ha-1 yr-1 in FSTW-B/P. Soil total (organic + inorganic) C (STC) 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 soil C and N storage and reduced their rate of decline in NTCW and STCW, but increased tillage intensity increased available N in FSTCW compared to the traditional STW-F. The STC can be used to replace SOC in dryland soils where SIC contents are high, thereby reducing the need to measure SIC.


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