Soil Organic Matter Management: The Roles of Residue Quality in C Sequestration and N Supply - Sustainable Management of Soil Organic Matter - page 208

The distribution of organic matter contents in the whole soil and in the
0.5-7.0 mm water-stable aggregates is presented in Table 3.15.1. At the
end of the 2-year growing period of the grass-clover mixture, the organic
matter content of the whole soil was higher than that after the spring wheat
and was higher on the plots with the grass-clover mixture after minimum
tillage than after conventional tillage.
The data for the spring wheat plot (Table 3.15.1) show that there was
a significant ( P < 0.05) decrease in organic matter content of the whole
soil during the 5-year period of the crop rotation which followed the
grass-clover mixture.
The above trends were also observed in most of the fractions of
0.5-7.0 mm water-stable aggregates. However, after the first year of the
grass-clover mixture, there was no greater increase in the organic matter of
the aggregates compared with that of the whole soil. Only after 2 years of
grass-clover mixture was there a higher accumulation of organic matter
in the aggregates than in the whole soil. Despite such positive changes in
organic matter content in the soil after 2 years of the perennial grass-clover
mixture, the increase of organic matter in the aggregates was not large.
Subsequent crops did not lead to any accumulation of organic matter in
soil aggregates. In contrast, a more or less pronounced decomposition of
organic matter in the water-stable aggregates was observed during the

Table 3.15.1. Organic carbon content in the whole soil (C), in 0.5-7.0 mm water-stable
aggregates (C * ) and total amount of 0.5-7.0 mm water-stable aggregates in the podzolic loamy
sand soil affected by crops and tillage systems.

T
(% by
weight)

Average diameter
of the water-stable
aggregates (mm)

Date of soil
sampling

C
(g kg − 1 )

C *
(g kg − 1 )

Type of tillage Crop

Conventional
tillage

SW May 1998 21.2 7.8 36.6 2.1

September 1998 23.9 11.4 48.9 4.1

PG1 May 1998 22.3 11.2 42.5 3.3

September 1998 24.1 12.2 54.8 4.4

PG2 May 1998 26.8 22.1 60.4 4.0

September 1998 28.1 14.9 49.1 4.0

Minimum
tillage

SW May 1998 19.5 7.7 29.5 3.3

September 1998 21.7 10.9 49.7 3.7

PG1 May 1998 23.9 8.2 38.0 3.2

September 1998 25.7 11.8 48.9 4.0

PG2 May 1998 30.9 16.0 52.8 3.2

September 1998 32.9 20.8 58.5 3.7

SW = spring wheat; PG1 = perennial grass-clover mixture (first year); PG2 = perennial grass-
clover mixture (second year).