|Abstract or Summary
- Total organic carbon, total nitrogen, microbial respiration and enzyme
activity (β-glucosidase) were measured on several horizons of a Dayton silt loam
(fine, montmorillonitic, mesic, Typic Albaqualf) soil cropped to annual ryegrass
under two straw residue management systems. The study evaluated the effects
of annual burning of straw residues or annual incorporation of straw residues on
the content, distribution and bioavailability of soil organic carbon.
Four fields were selected to represent the burn management system which
have been annually burned for a minimum of 40 years. Four fields were selected
to represent the straw incorportated system (mold board plow) which had been
annually burned for approximately 30 years, followed by incorporation of straw
residues into soil for a minimum of 10 years. One native site was selected to
represent non-cultivation conditions.
Straw management system strongly influenced both the total organic C and
N and microbial activity the surface soil horizon. Soil organic C and N content
were significantly greater (p < 0.05) in the Ap horizons of soils under the plowed
management system than soils under the burned management system. The
collective evidence suggests, however, that the significant differences observed
between the two residue management systems are not due to greater losses of
soil organic C and N as a result of burning but rather that organic C levels have
increased as a result of a change in management. Soil C:N ratios are slightly
higher in the Ap horizons of soils as a result of straw residue incorporation in
comparison to burning of straw residues.
When expressed on a mass soil basis, both C0₂ evolution from microbial
respiration, during a 32 day incubation period, and enzyme activity were
significantly greater (p < 0.10) for the Ap horizon of soils where straw residue had
been incorporated than in soils where residues had been annually burned. When
expressed on a per gram C basis, neither C0₂ evolution from microbial respiration
or enzyme activity were significantly different between the two management
systems. These results indicate that long-term annual burning of straw residues
has not decreased the bioavailability of soil organic C.