Soil Organic Matter Management: The Roles of Residue Quality in C Sequestration and N Supply - Sustainable Management of Soil Organic Matter - page 136

Table 3.5.1. The treatments in the field experiment.

Two swards: Grass Grass-clover

Three tillage methods: Conventional ploughing

to 200 mm

Deep ploughing to
300 mm

No-tillage

P D Z

Two crops Spring barley Winter barley

Four nitrogen rates (kg ha − 1 )

Spring barley Winter barley

N0 0 0

N1 40 60

N2 80 120

N3 120 180

Four crop rotations 1996 1997 1998

SSS Spring barley Spring barley Spring barley

SWW Spring barley Winter barley Winter barley

GWW Grass/grass-clover Winter barley Winter barley

GWS Grass/grass-clover Winter barley Spring barley

We measured gaseous losses of carbon and nitrogen and nitrate leaching, as
well as crop uptake.

Results

Crop growth

In 1996, which was the first year after ploughing, spring barley grown
after grass-clover responded strongly to nitrogen fertilizer, as shown in
Fig. 3.5.1. However, there was no response to nitrogen after ploughing
out from grass. These results indicate that the potentially mineralizable
nitrogen after grass-clover is much lower than after grass, contrary to
expectations. This surprising outcome may be a result of the lower fertilizer
inputs to the grass-clover swards, so they may have had higher C : N ratios.
We are awaiting results to clarify this. In an earlier (1992) study on this site,
the macroorganic matter (mainly root material) in the 0-4 cm depth range
prior to ploughing out had an N content of 1.27

0.10%
for grass-clover and grass plots, respectively. When these swards were
ploughed out, the N input from this material was 152.1

± 0.04% and 1.44 ±

± 8.7 and

11.8 kg N ha −1

204.6 ± for grass-clover and grass plots, respectively

(Davies, 1996).
In the final year, all treatment combinations showed a response to
nitrogen fertilizer, indicating that the residual effects of the original grass