TABLE 1.

Summary of 13CO2 field respiration assays conducted in the agricultural field test plots

SubstrateaTreatmentbTotal evolved 13CO2 (μm)13CO2 blankc (μm)Net 13CO2 (μm)% of added 13C respiredInitial rate (μm/h)
GlucoseSoil cover3.4 ± 0.32.3 ± 0.371.16.80.25
Sealed chamber
    Intact ped7.4 ± 104.5 ± 6.32.9d20d0.28d
    Core4.8 ± 1.91.7 ± 0.393.1220.27
    Crushed soil16 ± 3.75.7 ± 1.29.8681.2
PhenolSoil cover8.1 ± 0.85.7 ± 1.02.59.10.2
Sealed chamber
    Intact ped7.2 ± 4.11.8 ± 0.635.4200.2
    Core5.5 ± 3.71.2 ± 1.14.2160.1
    Crushed soil13.5 ± 1.23.5 ± 0.569.5370.5
NaphthaleneSoil cover2.6 ± 0.452.4 ± 0.430.11d
Sealed chamber
    Intact ped0.87 ± 0.730.87 ± 0.690d
    Core0.81 ± 0.340.79 ± 0.340.02d
    Crushed soil2.0 ± 0.261.9 ± 0.250.1d
CaffeineSoil cover3.2 ± 1.63.0 ± 1.50.13d
Sealed chamber
    Intact ped1.4 ± 0.841.4 ± 0.810.03d
    Core1.1 ± 0.451.0 ± 0.420.09d
    Crushed soil2.5 ± 0.512.4 ± 0.490.08d
  • a Aqueous substrates (glucose, phenol, and caffeine) were added as 450 μg in 0.1 ml. Naphthalene was added as 0.5 ml of a 20-ppm aqueous solution.

  • b Four treatments were prepared. “Soil cover” designates field release with open-bottom chamber cover installed to trap 13CO2. The three sealed chamber treatments were incubated in the field alongside the soil cover experiment. All treatments were prepared in triplicate.

  • c Blank was interpolated from 12CO2 evolved simultaneously with 13CO2.

  • d Net was not significantly different from zero as judged by Student's t test comparing final total and background 13CO2.