Soil Organic Matter Management: The Roles of Residue Quality in C Sequestration and N Supply - Sustainable Management of Soil Organic Matter - page 131

would tend to cancel out any difference between cultivated and undis-
turbed treatments in a sandy soil (whilst exaggerating those in clay-rich
soils). These results lend support to this conclusion. In practical terms, it is
unlikely that the cultivation method has importance in influencing fertilizer
N advice for sandy soils, since it is common practice to plough these soils.
The effect of residue type
The SMN immediately before residue incorporation (0-90 cm) in autumn
1995, and the calculated contributions of above-ground and below-ground
residues to mineralized N at harvest of the winter barley test crop (autumn
1996) are shown in Fig. 3.4.2. The main N contribution to the winter
barley test crop (autumn 1995 to autumn 1996) from the crop residues
came from a below-ground component. Much of this was already in
SMN form when the barley was sown. It therefore derived from fertilizer
and/or rapidly mineralized residues arising from root turnover or leaf drop
during the growing season. The impact of fertilizer rate on SMN was
demonstrated by the three wheat crops which received different N rates
(Fig. 3.4.2), agreeing with other results (e.g. Chaney, 1990). With the
exception of sugarbeet, the majority of mineralized N following incorpora-
tion was derived from below-ground residues. In general, above-ground
residues made a negative contribution to the mineral N pool.
Although the general trend of increasing N supply with N index
(MAFF, 1994) was apparent, there were significant differences between the
Fig. 3.4.2. Pre-incorporation soil mineral nitrogen and subsequent mineralization of above-
ground and below-ground residues. Numbers in parentheses indicate fertilizer N applied (kg ha − 1 ).