70 Srđan Šeremešić, Ivica Đalović and Dragiša Milošev
Whitney (2000) elaborated that deep tillage (mouldboard plowing) could bring
up higher pH soil from deeper depths, and slow down pH decline even under
fertilizer application.
Among all chemical soil properties examined, the most significant change
occurred in the SOC content. Agricultural practices in the investigated LTE
decreased SOC in the plow layer of winter wheat-based cropping systems. The
consequence of SOC depletion in the soil is unsustainable wheat production in
the WMS and WM cropping systems (Table 1).
According to Franzlubbers et al., (1995), SOC content changes slowly
with management practices due to the large size of the SOM pool and the
addition of crop-derived organic inputs which varies from year-to-year. Since
a significant proportion of plant biomass is removed by harvest and only 20%
of total biomass remains in the soil (Beauchamps and Voroney, 1994), plant
roots and the aboveground remains cannot instantaneously provide a large C
substrate. Therefore, quantity and quality of the residues likely affected SOC
formation and its dynamics in the fertilized and unfertilized treatments (Table
2). According to Bogdanović (1954) soil at the experimental field had 4.4% of
SOM that we use as an initial SOM content prior to the experiment set up. The
unfertilized rotation showed a considerable loss of SOC in the 0-30 cm depth
compared with the initial level observed at their establishment (1946/47),
likely due to lower yield and smaller biomass production. Depletion of SOC is
mainly induced by deficiency in a photosynthetically fixed C stored in soil
(Christensen and Johnson, 1997). An outcome of this process is soil
degradation, caused by topsoil attenuation due to permanent soil loss and
nutrient removal. The evidence of a thinner upper soil layer is higher CaCO 3
(%) content at the 0-30 cm depth, also found in Molnar et al., (1997) study
carried out at the same long-term experiment. This was especially seen in the
WM plot, since carbonates were redistributed in the topsoil layer with tillage.
Soils at the unfertilized treatments under winter wheat in the March and April,
maize and soybean throughout the April and May, might be subjected to
surface erosion by wind that has likely depleted the unprotected topsoil. Direct
field measurements carried out near the experimental field showed that the
average wind deposit for Chernozem soil is 0.25 t ha year-^1 (Savić, 2000). In
the semiarid Canadian prairies, Larney et al., (1998) also reported, that the
wind erosion could be the main form of soil degradation causing nutrient
redistribution and changes in soil productivity. Therefore, we assumed that the
loss of SOC and associated processes such as nutrient depletion, structure
deterioration, equally with insufficient inputs of fertilizers, pesticides, and
intensive technology could irreversibly affect soil productivity of such
normally productive soils as Chernozem.