Soil Organic Matter Management: The Roles of Residue Quality in C Sequestration and N Supply - Sustainable Management of Soil Organic Matter - page 168

Table 3.9.1) was scheduled within the model on a biennial basis, with
measurement of the input made on composition of manures and semi-
quantitative evidence for manure addition. A range (100-300 gC m −2 per
manuring) of manure addition input values have also been modelled to
allow discussion of various cultural scenarios.
Testing the model
Soil samples were collected during August 1998 on a volumetric basis from
15-23 cm fixed depths at four profiles within the rigged land. Organic
carbon content was measured using a Strohlein Instruments Coulomat 702
carbon analyser adapted to analyse CO 2 liberated from H 3 PO 4 digestion.
Each sample was analysed a total of four times and a mean value calculated.
All these values are expressed initially in percentage terms and subsequently
have been converted, using the bulk density values obtained from replicate
samples, to a mass per unit area basis to be consistent with CENTURY
model outputs.
Results
Soil organic C values output from the CENTURY model for a 250-year
period demonstrate rising soil organic matter levels from ~6000 g m −2 to
final values of 14,880 g m −2 (organic matter addition 500 g m −2 ) and
12,760 g m −2 (organic matter addition 400 g m −2 ), approaching equilib-
rium after 150 years (Fig. 3.9.1, 250 years). These predictions relate well to
the mean measured soil organic carbon (1998) value of 14,526 g m −2 , and
suggest that the soil predictions based on ethnographic and historical data
are valid. Issues that remain to be resolved include the depth resolution
limits of the model (top 20 cm), which is of particular significance in
deepened soils. Short-term scenarios modelled demonstrate the biennial
rise and decline of organic matter as cropping and manuring interact
(Fig. 3.9.1, 14 years).
Conclusion
The validity of the model opens up the possibility of constructing
alternative manuring strategies as a way of assessing the sustainability of
the study area. One critical question of the study area relates to the use of
a finite turf resource on Papa Stour for fuel and animal bedding. If lower
levels of turf were applied as manure to the rig land, as a way of conserving
the outfield turf resource for future generations, how would this affect