Soil Organic Matter Management: The Roles of Residue Quality in C Sequestration and N Supply - Sustainable Management of Soil Organic Matter - page 95

Consequences of Increased C Sequestration for N
Cycling

Fisher et al . (1997) clearly demonstrated that the large increases in SOM
under improved tropical pastures (up to 70 t ha −1 ) were only possible with a
substantial increase in the C : N ratio of the SOM. The amounts of N that
otherwise would have to be immobilized at the commonly found soil C : N
ratio of ~10 or 12 : 1 would have exceeded by far the soil N supply and
would have caused severe immobilization. To increase the C sequestration
potential or to improve synchrony of nutrient release from residues with
crop N demand, incorporation of 'lower' quality residues (or mixtures) is
often promoted. These residues decompose slowly, thereby conserving
nutrients, but nutrient release is lower during a given crop cycle. There is
little nutrient release over subsequent cycles regardless of initial residue
quality, indicating that there is little compensation for initial lower N
release from slow decomposing residues (Cadisch et al ., 1998). Repeated
applications of residues of 'low' quality (high N, lignin and/or polyphenols)
should, however, result in build up of soil organic matter and associated
increased nutrient release over time. We tested this hypothesis using
experimental and modelling approaches to evaluate the time frame within
which such a potential compensation effect occurs. Decomposition data
were taken from Handayanto et al . (1995) who derived N mineralization
rate constants for G. sepium and C. calothyrsus from single exponential
decay functions (Table 3.2). Using a single exponential decay model, the
change in remaining residue N at equilibrium is (Jenkinson, 1981):

dN/d t =0

or:

N Input − N Equilibrium × k =0

Table 3.2. Single exponential decay model output values at equilibrium level of repeated
applications of legume pruning at a rate of 75 kg N ha − 1 3 months − 1 .

N
Mineralization
rate constant
(week − 1 ) a

Time
to
equilibrium
(weeks)

Litter
N
equilibrium
(kg N ha − 1 )

Accumulated
mineral N
(385 weeks)
(kg N ha − 1 )

Mineralization
rate at
equilibrium
(kg N week − 1 ) Species

Gliricidia sepium 0.074 68 78 1982 5.8 ± 3.0

Calliandracalothyrsus 0.040 125 144 1922 5.8 ± 1.3

Calliandracalothyrsus 0.015 333 385 1698 5.8 ± 0.6

a From Handayanto et al. (1995).