Soil Organic Matter Management: The Roles of Residue Quality in C Sequestration and N Supply - Sustainable Management of Soil Organic Matter - page 150

Methods

The sites were located on typical silt-land soils of the Romney and Blacktoft
Association, with one Downholland Association site (Hodge et al ., 1984).
The sites had been ploughed out of pasture land pre-1960 (higher organic
matter status) or were long-term arable land (lower organic matter status).
Crop rotations following sampling were either arable with vegetables, or
ley-arable rotations.
At each site, two points were selected where soil samples consisting of
ten cores were taken to 15 cm depth, within a 5 m radius of each of these
points. Samples were analysed for soil organic matter content (MAFF,
1986). In the 1960s, measurements had been made on an annual basis, but
less frequently thereafter until 1989 (Johnson and Prince, 1991). The soil
sampling strategy was repeated in spring 1996 at 13 sites, labelled A-N
(access to the Downholland Series site (D) was not granted). The soils were
again analysed for organic matter content but, in addition, particle size
distribution (MAFF, 1986) and soil structural stability measurements were
made. Structural stability was assessed using a dispersion ratio technique
adapted from the standard method outlined in MAFF Reference Book 441
(MAFF, 1985), where the stability of 'natural' soil aggregates (size range
5-30 mm) was measured, rather than the < 2 mm ground fraction.
Regression analysis using Genstat 5 (Payne et al ., 1987) was carried out
on the soil organic matter data over the ~30-year measurement period at
each of the 13 fen silt sites, to assess if soil organic matter levels had
increased, decreased or stayed the same. The models applied were simple
linear and four-parameter logistic functions. It is appreciated that many
of the logistic fits approach a simple exponential function, however, use of
the exponential function on the data imposed an excessive bias on early
sampling points.

Results

Results of the soil organic matter and structural stability measurements,
and estimates of 'stabilized' organic matter levels inferred from the
regression analyses are summarized in Table 3.7.1.
At eight sites (C, E, F, I, J, K, M and N), there were no changes over
time in soil organic matter levels ( P > 0.05). Soil organic matter decline
was evident at five sites, with the lower asymptote having minimum values
between 2.5 and 3.4%. This minimum value was taken to represent the
'stabilized' soil organic matter content.
'Fitted'

regressions of soil organic matter change over time for

ex-pasture sites (Fig. 3.7.1) and arable sites (Fig. 3.7.2).