Soil Organic Matter Management: The Roles of Residue Quality in C Sequestration and N Supply - Sustainable Management of Soil Organic Matter - page 96

where k is the mineralization rate constant. Thus the remaining residue N
at equilibrium is:

N Equilibrium =N Input / k
Given the amount of residue N remaining at equilibrium, the average
mineralization rate (not including fluctuations due to very recent inputs)
can be calculated as:

N Mineralization rate =N Equilibrium × k

An interesting feature of this equation is that it indicates that the miner-
alization rate at equilibrium is the same for all residues if they are applied
at the same rate of N (Table 3.2). If we define the time to equilibrium ( t E )
as the time required for the mineralization rate to reach ~99% of the
maximum mineralization at equilibrium then t E can be expressed as (Olson,
1963):

t E =5/ k
For the investigated legume pruning materials, the time to equilibrium
ranged between 68 and 333 weeks for the fast and slow decomposing
prunings, respectively (Table 3.2). Thus a farmer would have to wait ~6
years until the average N supply rate of the slowly decomposing prunings
is equivalent to that of the fast decomposing prunings, or ~4 years if we
consider a 95% achievement of the maximum rate.
The simulated instantaneous mineralization rates, however, reveal
that there are agronomically relevant significant differences between the
prunings despite the average rate being the same. The instantaneous
mineralization rate of the fast decomposing Gliricidia residue fluctuated
widely even at equilibrium (Fig. 3.4a). Hence, synchronizing N release
with crop N demand remains important. In contrast, the instantaneous
mineralization rate of the slowly decomposing Calliandra prunings varied
little (Fig. 3.4b). This more constant N release at equilibrium poses less risk
for farmers as a more predictable N release is achieved. A further important
feature for crop production is that the N supply power from the slowly min-
eralizing materials remains high between applications. These advantages
are associated with an increased organic matter in the soils as suggested by
the much larger undecomposed residue N remaining (Fig. 3.4b). The use
of slow decomposing materials apparently provides a further advantage
in that it is more 'buffered' against withdrawl of continuous residue applica-
tion. This reduced risk for farmers is associated with the larger stock of
remaining residue N in the soil. Long-term alley cropping experiments in
Indonesia revealed that the fertility benefit of the slowly decomposing
Calliandra and Peltophorum residues was superior to that of the fast
decomposing Gliricidia in the seventh year (van Noordwijk et al ., 1997).