Soil Organic Matter Management: The Roles of Residue Quality in C Sequestration and N Supply - Sustainable Management of Soil Organic Matter - page 109

mineralization) in TB than in burnt soils. This difference between the
treatments is likely to have been even greater under field conditions, where
surface trash (which was excluded from the laboratory incubations) would
have contributed additional C to the soil, thereby increasing the ratio in
TB soils. Our results are consistent with observations of reduced soil N
availability in the first few years of residue conservation (Thompson, 1992).
The magnitude of the trash effect on soil properties in the different
experiments depended on the cumulative amount of trash returned to the
soil. Cumulative returns of trash DM were estimated as 11% of the fresh
sugarcane yield from past years, this percentage being derived from the
regression of trash DM against sugarcane yield in our five experiments.
Cumulative returns of C and N in trash were estimated by assuming the
same trash C and N concentrations as measured in our experiments.
Regression of the increase in soil C or N due to TB against the cumulative
C or N returns gave coefficients of determination ( r 2 ) of 0.94 for soil
organic C, 0.80 for microbial biomass C, 0.97 for C mineralization
potential and 0.94 for total soil N ( n = 5, overall mean from each
experiment). Retention of trash C and N in the soil varied among
experiments, however, with 10-20% (mean 13%) of cumulative C returns
measured as soil organic C and 40-100% (mean 75%) of cumulative N
returns measured as total soil N.
The rates of accumulation and mineralization of C and N under TB
measured in these experiments can only be considered indicative of the first
crop cycle (5-6 years) after conversion from a burnt to a TB system. At the
end of a crop cycle, sugarcane soils are normally cultivated several times
to 150-200 mm depth, the effect of which may be to reduce differences
in soil C and N between burnt and TB treatments (e.g. Thorburn et al .,
1999). Furthermore, rates of C and N accumulation in TB systems must be
expected to decrease with time and reach an equilibrium level, as measured
and simulated in sugarcane systems (Thorburn et al ., 1999), and is
known to happen in other crop and pasture systems (e.g. Jenkinson, 1991).
Accumulation rates decrease because mineralization and loss of C and N
from the soil (through respiration, leaching, denitrification and plant
uptake) increase.
In order to explore a range of possible responses of soil C and N
to trash blanketing, we combined measured rates of decomposition and
accumulation with assumptions about the mineralization of C and N from
trash left from previous years, and calculated equilibrium C and N balances
for the top 250 mm of soil at each site. We chose two decomposition
scenarios: (i) a 'retentive' system, where for each crop, 100% of trash N and
20% of trash C is retained in the soil in the year following harvest, and 90%
of the remaining trash N and 85% of the remaining trash C is retained
in subsequent years; and (ii) a 'non-retentive' system, where 80% of trash
N and 10% of trash C from each crop is retained in the soil in the year