|Follett, Ronald - Ron|
Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/1/2008
Publication Date: 3/1/2009
Citation: Follett, R.F., Varvel, G.E., Kimble, J., Vogel, K.P. 2009. No-Till Corn after Bromegrass: Effect on Soil C and Soil Aggregates. Agronomy Journal. Vol. 101: 261-268. Interpretive Summary: Formation and breakdown of soil aggregates is a dynamic process intimately associated with SOC, especially when a grass sod is cultivated. This study evaluated the dynamics of loss and gain of SOC in a 6½ yr field-experiment conducted near Mead, NE by studying amount and rate of replacement of C3-C from a killed-bromegrass sod with C4-C from continuous no-till corn in soil aggregates on a Mollisol and to estimate amount of crop-residue C to maintain SOC. Periodic collection of soil samples to three depths, soil aggregate size separation, and use of stable C isotope analyses distinguished changes to the remaining C from the bromegrass (C3-C) sod, that originating from no-till continuous corn (C4-C), and changes in amounts of C3-C and C4-C associated with individual soil aggregate fractions. Gains of C4-C averaged 680, 210, and 70 kg ha-1 yr-1 in the 0-5, 5-10, and 10- to 30 cm depths. By comparison changes of C3-C averaged -730, -210, and -10 kg C3-C ha-1 yr-1 for these same three depths. No-till corn planted into bromegrass sod resulted in a loss of both C4-C and C3-C from the 2 mm aggregates and a gain of both in the 1 mm aggregate fraction. Gains in C4-C also occurred in the all smaller aggregate sizes within the 0- to 5 cm depth with the additions almost offsetting C3-C losses. A significant decrease in total SOC, C3-C, and C4-C occurred by 52mo (September, 2003). Timing of this decrease followed a drought beginning in 2001 and most severe in 2003 when no aboveground biomass was harvested. After the drought was less severe, the SOC increased. Amount of crop-residue C needed to sustain SOC while producing corn grain under no-till on productive cropland is likely 2100- to 2300 kg ha-1 yr-1.
Technical Abstract: A different 13C/12C isotope signature is imparted to SOC by C4 plants versus C3 plants. We measured changes in C isotope ratios in SOC during ~ 6½ yrs of no-till corn (Zea mays L) planted into 13yr old bromegrass (Bromus inermis Leyss) sod for a C3 ' C4 plant switch to identify SOC isotope changes from addition of C4-C from corn and C3-C losses from bromegrass sod. Three replications of no-till corn as main treatments and N-rate (60 and 120 kg ha-1) as subplots were used. Soil was collected 8 times during the study from the 0-5 and 5-10 cm depths. At 4 of these times soil was also collected from the 10-30 cm depth. Clod samples were collected for soil bulk density. Samples were separated into 6 aggregate sizes by wet sieving. Drought reduced crop yields in years 3 to 5. Soil aggregates were analyzed for total SOC and 13C/12C ratio. Our objectives were to determine change in C4 versus C3 derived C in different soil aggregate sizes over time and to estimate how much corn stover was required to maintain SOC under continuous no-till corn. Total SOC did not change significantly at any depth during the study. However, total C4-C (corn) increased over this time while C3-C (bromegrass sod) decreased in the 0-5 and 5-10 cm depth, while neither changed in the 10-30 cm depth. In the 0-5 and 5-10 cm depths, largest loss of C3-C was from 2 mm aggregates while largest increases in C4-C were in the 1, 0.5, 0.25, and 0.125 mm aggregates. Corn stover return averaged ~5500 kg ha-1 yr-1 and is likely the amount of above-ground no-till corn residue required to maintain SOC. [GRACEnet publication]