Effect of Spring Oilseed Rape Crop Density on Plant Root Biomass and Soil Enzymes activity/Suvirapsi Taimiku Tiheduse Moju Taimejuurte Biomassile Ja Mulla Ensuumide Aktiivsusele

INTRODUCTION Soil health is the capacity of soil to function as a vital living system within ecosystem and land-use boundaries, to sustain plant and animal productivity and water and air quality, and to promote plant and animal health. To evaluate sustainability of agricultural practices, assessment of soil health using various indicators of soil quality is needed (Doran & Zeiss, 2000). Soil enzyme activity can be used as an indicator of soil quality for assessing the sustainability of agricultural ecosystems (Gianfreda et al., 2005; Roldan et al., 2005). Soil enzymes are important in catalysing innumerable reactions involved in the decomposition of organic matter, cycling of nutrients, and formation of organic matter structure (Bandick & Dick, 1999; Kandeler et al., 1999; Liu et al., 2008; Melero et al., 2008). Enzyme activity is closely related to other important indicators of biological activity: respiration intensity, nitrification ability, total amount of microorganisms, and even more strongly to soil organic carbon content, content of available [P.sub.2][O.sub.5] and [K.sub.2]O, soil acidity, and crop yield (Schimner & Sonnleitner, 1996; Bandick & Dick, 1999; Svirskiene, 1999; Liu et al., 2008). Enzymes that catalyse a wide range of soil biological processes offer a useful assessment of soil 'function', and common enzymes, such as urease and saccharase, fit into this category (Burns & Dick, 2005). Urease catalyses the hydrolysis of urea to C[O.sub.2] and N[H.sub.3], which is of particular interest because urea is an important nitrogen fertilizer. Urease is released from living and disintegrated microbial cells, and in the soil it can exist as an extracellular enzyme absorbed on clay particles or encapsulated in humic complexes (Nannipieri, 1994; Schimner & Sonnleitner, 1996). Saccharase catalyses the hydrolysis of saccharose into glucose and fructose and characterizes change processes of organic carbon compounds (Schimner & Sonnleitner, 1996). Several studies show that enzyme activities can be used as early indicators of changes in soil properties originated by soil and crop management practices such as tillage, crop rotation, residue management, and fertilization (Bandick & Dick, 1999; Kandeler et al., 1999; Acosta-Martinez et al., 2003; Roldan et al., 2005; Melero et al., 2008; Zakarauskaite et al., 2008; Wang et al., 2011). Acosta-Martinez et al. (2003) reported that the trends of the enzyme activities as affected by management depend on the soil, but in general crop rotation and conservation tillage increase enzyme activities. According to Bandick & Dick (1999), enzyme activities are generally higher in continuous grass fields than in cultivated fields. In cultivated systems, enzyme activity is higher where organic residues have been added as compared to treatments without organic amendments. Zakarauskaite et al. (2005) found higher urease activity in the soil where cereals were grown and higher saccharase and dehydrogenase activity where perennial grasses were grown. Marcinkeviciene et al. (2011) established that with increasing spring rape crop density from 100 to 450 plants [m.sup.-2], compared with the thinnest crop (50-100 plants [m.sup.-2]), the activity of urease in the soil does not change significantly, while the activity of saccharase significantly increases (by 31-53%). According to the data of Zakarauskaite et al. (2008), long-term application of mineral fertilizers inhibits the activity of urease and saccharase. Roots are the primary tools for the uptake of all mineral elements and water required for crop growth. Due to this fact, root growth and development are highly plastic (Neumann & Martinoia, 2002) and depend on climatic factors (Kjellstrom & Kirchmann, 1994), soil properties (Barraclough, 1989), crop density (Liakas et al., 2006), and fertilization (Govahi & Saffari, 2006). Roots store not only nutrients and carbohydrate reserves for plants, but they also host microbes (Chung et al. …