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Fig. 3.3.2. Net effect (W 96 -W 0 ) of PMS addition (day 165) on mineral N dynamics simulated
by MOTOR compared with field measurements; 125 kg N ha 1 applied on day 185. MOTOR
simulation excludes plant uptake.
uptake, residual soil mineral N and N 2 O-N emissions (88 kg N ha −1 ;
see Table 3.3.2). Overall, these comparisons suggest that while the size
of the immobilized pool of N was well simulated, the kinetics of both N
immobilization and remineralization during PMS decomposition were
slower than MOTOR simulates. This may be partly due to shortage of
mineral N delaying N immobilization (although MOTOR does allow for
this). The soil mineral N content in the W 96 treatment on day 179 (prior to
fertilizer N application) was very low - only 2.6
1.3 mg N kg −1 soil
±
1.3 mg N kg −1 soil in the control. The scaling in Fig.
3.3.1 is set so that C and N measurements overlay if biomass C:N=4.
Results show a wider biomass C : N before N fertilizer application (day
185) than immediately afterwards, which also suggests N limitation during
initial decomposition.
It is also likely that some physical protection of the cellulose occurs -
the PMS was applied as large pieces, which would be colonized only slowly
by soil organisms. The laboratory incubation showed that, for both soils,
C mineralization was slower for 1 cm diameter spheres than for cellulose
powder or filter paper squares (Fig. 3.3.3).
The function used to retard decomposition rates of organic additions
in MOTOR needs to account for the decomposition of cellulose in the
absence of lignin. In the MOTOR simulations, we assumed that cellulose
was 100% DPM on the basis that (i) no lignin was present and (ii) that
microorganisms present in the PMS from the waste treatment process
would ensure a ready supply of the cellulase enzyme. In the laboratory
incubation (Fig. 3.3.3), all forms of cellulose initially decomposed more
slowly in the Balmalcolm soil than the MOTOR-simulated soils. However,
compared with 9.9
±
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