Soil Organic Matter Management: The Roles of Residue Quality in C Sequestration and N Supply - Sustainable Management of Soil Organic Matter - page 135

From both an agricultural and environmental perspective, we need to
know what farming practices are best for retaining nitrogen and carbon in
the soil system. Ley-arable systems increase the organic nitrogen content of
the soil through fixation and thus improve soil quality. However, ploughing
out the swards releases CO 2 from the extra organic matter. This CO 2 makes
a large extra contribution to the atmosphere. Furthermore, crops may not
use the released nitrogen efficiently, compared with continuous arable
systems (Lloyd, 1992; Djuurhuus and Olsen, 1997). There is some
evidence to suggest that winter cereals may be less efficient for nitrogen
retention than spring cereals, because of early autumn cultivation and
higher fertilizer N use.
Deep tillage mixes organic matter over a greater depth of soil than
conventional tillage. This leads to immobilization and fixation of nitrogen
in mineral sub-soils but also to higher CO 2 emission rates. Shallower tillage
reduces disturbance and fossil fuel use, and so may reduce CO 2 loss. Using
published data from long-term experiments in the USA, the average soil
carbon level was 285 g m −2 higher under no-tillage than under conventional
tillage (Paustian et al ., 1997). A reduction in tillage thus may allow
significant sequestration of carbon in the soil.
We conducted a field experiment over 3 years to measure the
agronomic and environmental effects of ploughing out grass and grass-
clover swards. Our overall objective was to measure the effects of timing
of cultivations, tillage methods and fertilizer use on crop growth and the
storage of carbon and nitrogen in soil.

Methods

We sited the experiment on grass and grass-clover swards that were
~14 years old and had been part of a sheep grazing experiment. It should be
emphasized that both types of sward were managed to obtain similar gross
margins, and productivity in terms of liveweight gain was ~20% lower on
the grass-clover swards (Vipond et al ., 1997).
Average annual rainfall at the site is 870 mm. Mean monthly

temperature varies between 2.5 C in July.

There were four paddocks, two in grass and two in grass-clover. Table
3.5.1 summarizes the treatments. Within each paddock, there were four
main plots, one for each of the rotations. These main plots were split into
three sub-plots for the tillage methods, and the tillage sub-plots were split
further for nitrogen fertilizer levels. The tillage methods were conventional
ploughing to 200 mm depth and deep ploughing to 300 mm depth. We also
included a no-tillage treatment to provide the widest range for comparison
and to allow us to separate the effects of ploughing from sward destruction.

° C in January and 14.8 °