Agriculture Reference
In-Depth Information
following harvest, and 50% of this trash N and 60% of this trash C is
retained in subsequent years. The effects on soil C and N were calculated
(no allowance made for fallow effects), until the systems had reached
equilibrium. At the start, the Mackay and Tully soils contained
40,000 kg C ha
−1
and 2400 kg N ha
−1
, and the Harwood soils contained
76,000 kg C ha
−1
and 6500 kg N ha
−1
(to 250 mm depth).
At equilibrium under the 'retentive' scheme, soil C would have
increased by 7-14% after 25 years and soil N (Fig. 3.1.3) would have
increased by 8-21% after 35 years. Under the 'non-retentive' scheme at
equilibrium, soil C would have increased by 1.4-2.6% after 6 years and soil
N would have increased by 1.3-3% after 6 years. Thus, our expectations
are for very small to modest increases in soil C and N under long-term TB
at these sites.
When equilibrium is attained, maximum mineralization of trash-
derived soil N has been reached, and equals annual returns (50 kg ha
−1
year
−1
). This is the maximum amount by which N fertilizer rates could be
reduced. In these sugarcane production systems, this point would be
achieved after 6 years in the 'non-retentive' system and after 25-35 years in
the 'retentive' system. It is possible that fertilizer applications may not ever
be able to be reduced to this extent if N loss is greater in TB than in burnt
systems. It has been suggested that leaching and denitrification may be
promoted under TB due to the increased C and water contents.
The suggestion from this study that trash retention will result in a slight
or modest increase in total soil C and N, and a somewhat greater increase in
mineralizable soil N in the long term, accords with the findings of Powlson
et al
. (1987), who measured an increase of only 5% in organic C and 10%
Fig. 3.1.3.
Calculated cumulative increase in total soil N during 50 years of TB management, in a
retentive and non-retentive system. See text (Discussion) for more explanation.
Search WWH ::
Custom Search