The effect of non-chemical weed control on soil biological properties in a spring oilseed rape crop

The current study was aimed to establish the effects of non-chemical weed control methods on the activity of soil enzymes and abundance of earthworms in an organically grown spring oilseed rape crop in the soil with a regular (23-25 cm) and thickened (45-50 cm) humus layers. A field experiment was conducted during the 2013-2015 period at Aleksandras Stulginskis University on a Calc(ar) i-Endohypogleyic Luvisol (LVg-n-w-cc). The following three non-chemical weed control methods were explored: 1) thermal (using water steam), 2) mechanical (interrow loosening) and 3) smothering (self-regulation). In the thermal and mechanical weed control treatments, spring oilseed rape was grown with an inter-row spacing of 48 cm and in weed smothering treatments with an inter-row spacing of 12.0 cm. The highest root dry biomass of spring oilseed rape (on average 1.68 t ha(-1)) had been produced in the soil with a regular humus layer in the mechanical weed control treatment. Spring oilseed rape root dry biomass depended on the crop density (r = 0.82-0.96, P < 0.05). In the soil with a regular humus layer, the different non-chemical weed control methods tested exerted little impact on soil enzyme activity. In the soil with a thickened humus layer, a significantly stronger activity of saccharase and urease enzymes, compared with the other weed control methods applied, was recorded for the plots under the thermal weed control treatments using water steam, while in a dry year of 2015 - in the plots under smothering treatments. Compared with a regular humus layer, the activity of urease enzyme in the thickened humus layer was significantly (1.5-1.6 times) higher in the plots where in 2013 and 2015 thermal weed control had been applied, while in 2015 - in the smothering treatment (2.8 times). The activity of saccharase significantly (1.8 times) increased in 2015 in the plots under smothering treatment. Significantly the highest number of earthworms and their biomass were determined in the plots with a thickened humus layer in which in 2013 thermal weed control had been applied and in 2014 and 2015 in the plots under smothering treatment. Compared with a regular humus layer, in the thickened humus layer the number and biomass of earthworms significantly (1.5 and 1.6 times) increased in the plots in which in 2014 mechanical weed control had been applied, and in 2015 in the plots under smothering treatment (2.6 and 3.1 times, respectively). Soil enzyme activity and abundance of earthworms depended on the meteorological conditions and soil agrochemical properties. The number of earthworms in the soil correlated with the soil enzyme activity. Positive strong and very strong statistically significant correlations were established between saccharase activity and number of earthworms (r = 0.89, P < 0.05), urease activity and number of earthworms (r = 0.99, P < 0.01) as well as between urease activity and earthworm biomass (r = 0.94, P < 0.01).