72 Srđan Šeremešić, Ivica Đalović and Dragiša Milošev
Considering the fact that the unfertilized plots were established in
1946/47, a significant SOC loss was likely to take place after 60 years of
cropping. Earlier studies had confirmed that the soil samples collected from
the unfertilized plots during the 1980s had a significantly lower content of
SOC than those of the initial period (Molnar, 1999). Crop rotation is an
important subject in SOC dynamic due to soil ecological interactions and
processes that occur with time which are followed with the improvement of
soil structure, increased water use efficiency weed suppression, balanced
nutrient utilization (Carter et al., 2003). Beare et al., (1994) indicated that
tillage enhanced short-term CO 2 evolution and microbial biomass turnover,
and accelerated organic C oxidation to CO 2 not only by improving soil
aeration, but also by increasing contact between soil and crop residues, and by
exposing aggregate-protected organic matter to microbial attack. Manojlovic
et al., (2008) evaluated SOC content in the several long-term experiments
from the Rimski Šančevi and found that organic fertilizers rather than mineral
fertilizers could display beneficial effects on SOC content in soil. Generally,
SOC preservation coupled with proper management such as crop rotation and
fertilization is important for preserving soil productivity, and when SOC
increases it would benefit winter wheat yield.
Winter Wheat Yield
The analysis of the achieved yield of winter wheat points to a significant
difference between the fertilized and the unfertilized treatments (Figure 1). In
a long-term, the highest yield was achieved on the fertilized 3-year rotation,
followed in most of the years by fertilized 2-year rotation. According to
Milošev et al., (2014), a 3-year rotation of winter wheat has higher
performance-based index compared with other winter wheat cropping systems.
Increased productivity of 3-year winter wheat cropping compared with other
cropping system could be based upon a multi-cropping that in the repeated
sequence increases yields. When comparing the fertilized rotations, wheat
monoculture had the lowest yield. However, despite the unfavorable crop
rotation, it was shown that monoculture can reach the yield between 3000 and
4000 kg ha-1. The lowest yield was obtained on unfertilized rotations with
1000 kg ha-1 on unfertilized 2-year rotation and 1500 kg ha-1 on unfertilized
3 - year rotation. Current yields on the unfertilized plots therefore reflects an
initial agroecological potential after 60 years of continuous cropping compared
with the fertilized treatments (Stojkovic, 1972). Wheat yields in the
unfertilized treatments (WMS and WM) showed a steep decline after their
establishment (1946/47) until a new equilibrium was reached (1970-1980)