Agriculture Reference
In-Depth Information
the fumigated values are presented as an index of microbial biomass C.
Soil N and C mineralization potentials were measured as the change in
inorganic N and the release of carbon dioxide, respectively, during a 7-day
laboratory aerobic incubation at 25
C at standardized soil water content.
Soil inorganic N (ammonium + nitrate) was determined by potassium
chloride extraction (Bundy and Meisinger, 1994). Total N in soil was
determined by Kjeldahl digestion (Bremner and Mulvaney, 1982) and
automated colorimetric analysis. Organic C in soil was determined by
dichromate oxidation (Heans, 1984). Total N and C in trash were
determined using a Leco induction furnace. All soil analyses were done
without removing incorporated trash. Incorporated trash was separated
from the soil by washing the sample in water and removing floating and
suspended organic matter on a 2 mm sieve.
°
Results
Trash decomposition
After harvest, 7-12 t ha −1 of new trash dry matter (DM) was present on the
soil surface in TB plots. After 1 year, 98% of the free trash had decomposed
at Tully and 82-91% at Mackay and Harwood. The free trash returned
after harvest contained 3-5 t C ha −1 , 28-55 kg N ha −1 and had a C : N
ratio of 70-117. During the season, the C : N ratio declined to ~40 in
late-harvested crops and 23-29 in early-harvested crops (e.g. Fig. 3.1.1a).
After 1 year, 2-16% of trash C and 5-37% of trash N remained in free
trash (i.e. 50-800 kg C ha −1 and 1.3-20 kg N ha −1 ). Incorporated trash
contained 1-2.6 t DM ha −1 , 370-860 kg C ha −1 and 12-27 kg N ha −1 (e.g.
Fig. 3.1.1b). There were no apparent trends in the mass of incorporated
trash with time. By the end of the year, as much as or more C and N was in
incorporated trash than in free trash at all sites.
Soil properties
Soil organic C, total N and microbial biomass C (e.g. Fig. 3.1.1c-e) were
greater under TB than under burning in the older experiments (Mackay
and Tully), but only in the 0-20 and 20-50 mm depth layers. At Harwood,
neither organic C, total N nor microbial biomass were significantly affected
by trash management.
At most sampling dates, C mineralization potential (e.g. Fig. 3.1.1e)
was increased by TB in the 0-50 mm depth, but not at greater depths.
Unlike C mineralization, soil N mineralization potential generally did
not differ significantly between trash management treatments (data not
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