68 Srđan Šeremešić, Ivica Đalović and Dragiša Milošev
clustering of the soil aggregates and disrupted aggregation. Higher content of
the WSA was found at the unfertilized treatment compared to the fertilized
ones. Moreover, it could be estimated that loss of SOC and a reduced amount
of biomass incorporation of the unfertilized treatments resulted with in partial
Ca governance of aggregation processes (Tisdal and Oades, 1982). The
fractionation study was conducted in wheat-based rotations with methodology
adapted from Cambardella and Elliot (1993) and Six et al., (1998) to determine
long-term changes in the soil structure. Crop residue was found to be
important in the preservation of SOC since biomass could provide the
substrate for microorganisms that produce substances for binding soil
particles. The soil samples taken after winter wheat showed that smaller soil
aggregates (<53μm) were lower in C compared with large macroaggerate
(>2000 μm) were accumulation of C occurred. Accordingly, it was confirmed
that soil structure is affected with SOC concentration and management
practices that control the soil structure could help in preservation of SOC.
Kong et al., (2005) also found that a majority of the accumulated SOC was
preferentially sequestered in the macroaggregates. In addition to that, we also
observed that the smaller number and disrupted drying and wetting cycles over
the winter season contributed to the soil aggregates dispersion (Denef et al.,
2001). Šeremešić (2005) on the same experiment found that the concentration
of the water stable aggregates was linked with the soil texture arrangement.
The stability of aggregates in the fertilized treatments indicates that the
addition of the fertilizers with crop residues was not sufficient for soil
structure preservation. Likewise, concentration of smaller aggregates 53- 250
μm and <53 μm is prerequisite for formation of the surface crusts. Generally,
soil crust under winter wheat is not likely to occurred as winter wheat covers
the soil during the period of intensive rainfall appearance (April-Jun) that
prevent compaction of the dispersed soil.
Bulk density measurements were repeatedly performed to access the soil
physical condition of the regularly tilled plots. Over the observed period we
detected decrease in bulk density values as compared to Vučić (1960) and
Belić et al., (1986) study. This trend could be explained with the fact that the
powerful machinery was introduced for primary tillage and advanced seedbed
preparation with lower number of passes that consequently led to a lower soil
compaction. Winter wheat monoculture showed lower values of the bulk
density compared with maize and lower compared with soybean. Higher bulk
density for both, winter wheat and maize, was observed at the 2-year rotation.
Compacted soil in the plow layer had lower SOM content. A statistically
significant regression was found between winter wheat C input and SOC stock
at the 0-100 cm depth (Figure 2).