Soil Aggregate C-Isotope Signatures in No-Till Corn after Bromegrass (GRACEnet).

This field study was located near Ithaca, Nebraska, USA on a Filbert silt loam.  The site was planted to no-till of corn that was seeded into a 13 yr old bromegrass sod.  A randomized complete block with three replicates was used with crop as main treatment and nitrogen (N) rate as the subplots (60 and 120 kg N/ha).  Predetermined areas from each plot were harvested annually to measure aboveground biomass.  Soil samples were collected eight times between May 1999 and Oct. 2005 from the 0 – 5, 5 – 10, and at four of the 8 harvests also from the 10 – 30 cm depth.  Soil bulk densities were determined on clods from each soil layer.  Moist soil was sieved (8 mm), air dried, and stored for separation into aggregate size fractions with a Yoder wet-aggregate separation method.  Soil aggregated size fractions studied were larger than 2, 1, 0.5, 0.25, 0.125, 0.045, less than 0.045 mm.  Aggregate fractions were dried at 55°C, weighed, sub-sampled, ground, and analyzed for total C and N and their stable isotopes.  The bromegrass (C3 plant) isotope signature allowed determination of re-labeling rates of each aggregate fraction with the corn (C4 plant) isotope signature during this 77 month study.  During nearly 6.5 years total SOC decreased from 21.1, 17.0, and 55.8 t/ha in the 0 – 5, 5 – 10, and 10 – 30 cm depth increments to 20.1, 16.7, and 55.5 t/ha, respectively.  The SOC from C4 plants was 34.8, 49.8, and 73.2 percent of total SOC in the 0 – 5, 5 – 10, and 0 – 30 cm depths at the beginning of the study and 47.3, 59.0, and 71.8 percent, respectively after 77 months.  Use of the stable isotopes provides a tool so that the rates of change associated with various soil fractions can be evaluated.