Soil Organic Matter Management: The Roles of Residue Quality in C Sequestration and N Supply - Sustainable Management of Soil Organic Matter - page 167

carry turves, seaweed and manure. The manure application is expressed on
a per area basis of management areas (Table 3.9.1).

CENTURY Modelling

The CENTURY model developed by Parton et al . (1987) has been applied
successfully to a variety of long-term experiments in a variety of climatic
and cropping situations worldwide (Kelly et al ., 1997). Because of this wide
applicability, transparency and peer-reviewed acceptance (Smith et al .,
1997), CENTURY was selected for this study. Since CENTURY models
only the top 20 cm of the soil, a uniform bulk density of 1.35 g cm −3 , based
on the mean of five measurements from fixed depth 15-23 cm samples was
used throughout. CENTURY model runs were for periods of 14 years
(1946-1960) and 250 years; in this case the climatic data (e.g. Hulme,
1997) were rewound several times to simulate short-term variations. The
starting values of soil C and N were derived from mean values of organic
carbon measurements of three Walls series soil samples obtained from
profiles close to the rigged land at Bragasetter (mean = 2.04%), and an
assumed C : N ratio of 15.6. This ratio was obtained from the C : N ratio
of the total C and total N measurements made on the same soils. The
allocation of initial soil C and N to 'active surface', 'active soil', 'slow' and
'passive' pools is defined arbitrarily (respectively, 5, 5, 40 and 50%) but, by
considering data generated from the model only after allowing the pool
distributions to equilibrate in the model over a period of 10 years, this effect
is minimized. This problem is also found in long-term experiments and is
overcome by similar practices (e.g. Kelly et al ., 1997). Parameters, where
available, relating to the oat crop Avenia strigosa were obtained from field
trial data. Addition of organic manures (400-500 gC m −2

per manuring;

Table 3.9.1. Quantification of traditional manuring practice of Papa Stour infield.

Proportion
Bragasetter
farm (%)

Quantity per
application
(kg m − 2 )

Mass C per
application
(kg m − 2 )

Farming
practice Land area Manure type Frequency

Rigged Cropping of

oats (Avena
strigosa)

9 Compost

from midden

Biennial ~4.14 ~0.44

Kailyard Brassica and

potato crops

4 Compost

from midden

Annual > 4.14 > 0.44

Planticrue Plant nursery < 1 Seaweed

(Tang)

Annual ~5.75 ~3 .44

Grazing Tethered

grazing

86 Grazing

animal

N/A Random Random