Effects on the number of soil microorganisms and tree nutrition of groundcover management systems

This study investigated the effects of different groundcover materials on soil microorganisms and plant nutrition. Trees of apple cv. 'Florina'/M.26 (Malus domestica Borkh.)were planted into acidic sandy soil in the spring of 1994. Soil strips of 120 cm width were covered either with straw, livestock manure, black plastic foil, pine bark mulch or were without cover i.e. clean cultivation as a check. Every year during the 4-year experimental period (1994-1997) leaf and soil samples were collected for chemical analysis. Mycorrhizal infection rate of the roots was measured in 1997. Population size of nitrifying, aerobic nitrogen fixing and aerobic cellulose decomposing bacteria was determined eight times during the experiment. The mycorrhizal colonisation rate under various groundcover materials differed significantly with the lowest value found under livestock manure (9 %) and the highest in the clean cultivated soil (62 %). The infection rate correlated negatively with the soil nitrate, P2O5 and K2O content (r=-0.65, -0.71 and -0.76, respectively) and correlated positively with the concentrations of Cu and Zn in the leaf (r=0.53 and r=0.61, respectively). Soil nitrate, P2O5 and K2O concentrations were much higher under livestock manure and straw treatments than under other materials (pine bark, black plastic foil and clean cultivation), while the Cu and Zn concentrations did not differ significantly. Trees of clean cultivated, black plastic foil and pine bark mulch plots had higher concentrations of Zn and Cu in their leaves than trees of straw or livestock manure treatments. These results suggest that high nutrient release from the livestock manure and straw covers adversely affects the apple root colonization by arbuscular mycorrhizal fungi and the low infection level of the roots results in less effective uptake of Zn and Cu. The effect of groundcover on some physiological groups of soil bacteria are discussed in the text.