Soil Organic Matter Management: The Roles of Residue Quality in C Sequestration and N Supply - Sustainable Management of Soil Organic Matter - page 153

Discussion and Conclusions

Many authors (Johnson, 1991; Jenkinson et al ., 1994) have reported rapid
changes in soil organic matter contents after major land use changes (e.g.
ploughing out of pasture). The effects of ploughing out grassland on soil
organic matter levels were particularly evident at sites A and B, with a rapid
decline over the first 5 years. Sites H and L, where the cropping history was
uncertain, exhibited a similar decline from relatively high initial organic
matter levels and may well have been in pasture before the study

commenced.
The estimated 'stabilized' soil organic matter levels were well related
( P < 0.01) to the soil structural stability measurements. However, there
was only a weak ( P = 0.05) relationship between total soil organic matter
levels and structural stability. Results from this work are in agreement with
the findings of other workers (Wander and Traina, 1996) which suggested
that there was a rapidly decomposable organic matter fraction, which does
not contribute to soil stability. However, as previous work has suggested,
other soil properties can affect structural stability, such as clay content
(Sorenson, 1981), soil water regimes and microbial activity (Scott et al .,
1996).
At three of the long-term arable sites, soil organic matter levels declined
by 0.5-1.7% over the monitoring period. At two of these arable sites,
the decreases may have been related to deeper soil cultivations, as the
'apparent' changes occurred over a short period of time with steady-state
organic matter levels before and after. At the six ley-arable sites and two of
the arable sites, there were no changes in soil organic matter levels over the
monitoring period.
The data presented here suggest that all sites will reach stable or very
slowly changing organic matter levels under continuous arable cropping
regimes. It is concluded from the study that modern farming systems are
not causing a decline in the organic matter content of fen silt soils.

References

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Soils and their use in Eastern England. Soil Survey of England and Wales,
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