Soil organic carbon (SOC) accumulation occurs mostly in the top 5 cm of soil with continuous no-till (NT) while SOC losses often occur at deeper depths. We hypothesize that one-time tillage conducted once in .10 yr to mix the high SOC surface layer with deeper soil will not result in large SOC losses following tillage with a net positive gain in SOC eventually. Two experiments in long-term NT fields were installed under rainfed corn (Zea mays L.) or sorghum [Sorghum bicolor (L.)Moench.] rotated with soybean [Glycine max (L.) Merr.] in eastern Nebraska. Tillage treatments were applied in the spring or fall and included: NT, disk, chisel with 10-cm wide twisted shanks, moldboard plow (MP), and mini-moldboard plow (miniMP). A portable infrared gas analyzer was used to monitor CO 2 flux immediately following tillage. Effect of tillage on profile distribution of total and labile (particulate and oxidizable) SOC was determined. At 24 to 32 mo following tillage, SOC mass was determined for depths of 0 to 5, 5 to 20, and 20 to 30 cm. Some tillage operations effectively redistributed total J.A. Quincke, INIA La Estanzuela, CC 39173 Colonia, Uruguay; C.S. Wortmann, M. Mamo, and R. Drijber, 279 Plant Science, Univ. of Nebraska, Lincoln, NE 68583-0915; and T. Franti,

Continuous no-till (NT) has numerous benefits, including improved soil aggregate stability in the surface soil and increased rate of water infiltration, but accumulation of soil P at the soil surface with NT can increase P concentration in runoff. We hypothesized that occasional one-time tillage of NT land, conducted once in 10 or more years, can reduce P runoff and improve crop yields without reducing soil aggregation or increasing runoff. Research was conducted in long-term NT fields under rainfed corn [Zea mays (L.)] or sorghum [Sorghum bicolor (L.) Moench.] rotated with soybeans [Glycine max (L.)Merr.] at two locations in eastern Nebraska. Tillage treatments were applied in the spring or fall and included continuous NT, tandem disk (disk), chisel with 10-cm-wide twisted shanks, moldboard plow (MP), and mini-moldboard plow (miniMP). Subplots had either 0 or 87.4 kg P ha 21 applied as composted feedlot manure before tillage. Yield and yield components were measured for 2 and 3 yr after the spring and fall one-time tillage, respectively. In Year 2 or 3 after tillage, soil sorptivity, field-saturated infiltration rate, runoff volume, runoff P loss, and soil aggregate stability were determined. Yield was not affected by the tillage 3 compost interaction, but was increased by compost application at one location and sorghum yield was affected by tillage treatments at the second location. Grain yield was never significantly more or less with one-time tillage as compared with NT. Soil aggregate stability was not affected by tillage treatments. Sorptivity and infiltration were increased with MP tillage compared with NT at one location but reduced at the other. One-time MP tillage reduced dissolved P loss at both locations and total phosphorus (TP) loss at one location. The benefit of one-time MP tillage in terms of reduced dissolved reactive P loss in runoff was positive with no negative effect on soil aggregate stability but no gain in yield.

A gronomy J our n al • Volume 10 0 , I s sue 6 • 2 0 0 8 ABSTRACT Continuous no-till (NT) results in soil improvements, primarily in the surface 5 cm of soil. One-time tillage may improve NT systems by inverting surface soil with less improved deeper soil. Research was conducted to determine the change in abundance of soil microbial groups aft er a one-time tillage of NT and their recovery dynamics. Experiments were conducted under rainfed corn (Zea mays L.) or sorghum [Sorghum bicolor (L.) Moench] rotated with soybean [Glycine max (L.) Merr.] in eastern Nebraska with one-time moldboard plow (MP) and mini-moldboard plow (mini-MP) tillage compared with continuous NT. Fatty acid methyl ester (FAME) profi les were used as biomarkers of soil microbial groups. Th e biomass of microbial groups within the soil profi le was aff ected by tillage treatment, soil depth, and time aft er one-time tillage. Soil microbial biomass under NT was greatest at the 0-to 5-cm depth with 50% less in the 5-to 20-cm depth, and least in the 20-to 30-cm depth. Microbial group biomass was decreased by one-time MP tillage, and generally by mini-MP tillage, compared with NT. On an equivalent soil mass basis, the quantity of the arbuscular mycorrhizal (AM) biomarker C16:1(c11) in the second year aft er tillage was 22% less for tilled treatments compared with NT. In contrast, the fungal biomarker C18:2(c9,12) was 6% more in the second year aft er tillage for tilled compared with NT. Tillage aff ected biomass and recovery of microbial groups diff erently, with all except AM returning to the NT microbial biomass levels within 1 to 3 yr.

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