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Title:   Author  item
                                            Sainju, Upendraitem
                                            Caesar, Thecanitem
                                            Lenssen, Andrewitem
                                            Evans, Robertitem
                                            Kolberg, Robert
  Submitted to: Soil & Tillage Research
                                    Publication Type: Peer Reviewed Journal
                                    Publication Acceptance Date: 10/30/2008
                                    Publication  Date: 4/8/2009
                                    Citation: Sainju, U.M., Caesar, T., Lenssen, A.W., Evans, R.G., Kolberg, R.L. 2009. Tillage and cropping sequence impacts on nitrogen cycling in dryland farming in eastern Montana, USA. Soil & Tillage Research. 103(2):332-341.Interpretive Summary: Long-term traditional farming system, such as conventional tillage with wheat-fallow, has resulted in the decline of soil organic matter and total nitrogen content by 30 to 50% of their original levels in the last 50 to 100 yr in the northern Great Plains. As a result, the traditional farming system has become inefficient, uneconomical, and unsustainable due to reduced soil productivity, increased soil erosion, and increased dependence of producers on federal aids. Improved soil and crop management practices are needed to increase soil nitrogen conservation and mineralization so that the cost and rate of N fertilization to crops can be reduced and environmental quality [reduced potentials for N leaching and nitrous oxide (a greenhouse gas) emission] can be improved.	
A long-term (21-yr) experiment was conducted to examine the effects of combinations of tillage and cropping sequences on dryland crop biomass returned to the soil, residue N, and soil N fractions 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 (2000-2004) (FSTW-B/P), and spring till spring wheat-fallow (STW-F). Nitrogen fractions were soil total N (STN), particulate organic N (PON), microbial biomass N (MBN), and potential N mineralization (PNM), NH4-N, and NO3-N. 
Mean crop biomass was 53 to 66% greater in NTCW, STCW, FSTCW, and FSTW-B/P than in STW-F. As a result, soil surface residue amount and N content in 2004 were 30 to 127% greater in other treatments than in STW-F. The STN, PON, and PNM at the 0- to 20-cm depth were 24 to 107% greater in other treatments than in STW-F. At 0 to 5 cm, STN, PON, and MBN were greater in STCW than in FSTW-B/P and STW-F. At 5 to 20 cm, STN and PON were greater in NTCW and STCW than in STW-F, PNM and MBN were greater in STCW than in NTCW and STW-F, and NO3-N was greater in FSTW-B/P than in NTCW and FSTCW. 
Long-term reduced tillage with continuous cropping increased dryland crop biomass, residue N, and soil N conservation and mineralization compared with the conventional system, such as STW-F. Increased tillage frequency with spring wheat-pea rotation, however, increased N availability compared with other treatments at the subsurface soil.
                                Technical Abstract: Information on N cycling in dryland crops and soils as influenced by long-term tillage and cropping sequence is needed to quantify soil N sequestration, mineralization, and N balance to reduce N fertilization rate and N losses through soil processes. We evaluated the 21-yr effects of combinations of tillage and cropping sequences on dryland crop grain and biomass (stems + leaves) N, soil surface residue N, soil N fractions, and N balance at the 0-20 cm depth in Dooley sandy loam (fine-loamy, mixed, frigid, Typic Argiboroll) in eastern Montana, USA. Treatments were no-tilled continuous spring wheat (Triticum aestivum L.) (NTCW), spring-tilled continuous spring wheat (STCW), fall- and spring-tilled continuous spring wheat (FSTCW), fall- and spring-tilled spring wheat-barley (Hordeum vulgare L) (1984-1999) followed by spring wheat-pea (Pisum sativum L.) (2000-2004) (FSTW-B/P), and spring-tilled spring wheat-fallow (STW-F). Nitrogen fractions were soil total N (STN), particulate organic N (PON), microbial biomass N (MBN), potential N mineralization (PNM), NH4-N, and NO3-N. Annualized crop grain and biomass N varied with treatments and years and mean grain and biomass N from 1984 to 2004 were 14.3 to 21.2 kg N ha-1 greater in NTCW, STCW, FSTCW, and FSTW-B/P than in STW-F. Soil surface residue N was 9.1 to 15.2 kg N ha-1 greater in other treatments than in STW-F in 2004. The STN at 0-20 cm was 0.39 to 0.96 Mg N ha-1, PON 0.10 to 0.30 Mg N ha-1, and PNM 4.6 to 9.4 kg N ha-1 greater in other treatments than in STW-F. At 0-5 cm, STN, PON, and MBN were greater in STCW than in FSTW-B/P and STW-F. At 5-20 cm, STN and PON were greater in NTCW and STCW than in STW-F, PNM and MBN were greater in STCW than in NTCW and STW-F, and NO3-N was greater in FSTW-B/P than in NTCW and FSTCW. Estimated N loss through leaching, volatilization, or denitrification increased with increasing tillage frequency or greater with fallow than with continuous cropping and ranged from 9 kg N ha-1 yr-1 in NTCW to 46 kg N ha-1 yr-1 in STW-F.  Long-term no-till or spring till with continuous cropping increased dryland crop grain and biomass N, soil surface residue N, N storage, and potential N mineralization, and reduced N loss compared with the conventional system, such as STW-F. Greater tillage frequency with spring wheat-pea rotation, however, increased N availability at the subsurface layer.


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