The effect of fertilization and crop rotation on soil chemical and biological properties. Field trials on a clay soil in Southern Finland

The effects of prolonged fertilizer use and crop rotation on microbial biomass and activity and nitrogen content in a clay soil in Southern Finland were investigated. Neither fertilization nor crop rotation had any lasting effect on the level of inorganic nitrogen: the low level observed in the spring before fertilization was found again in the autumn in both a monoculture and a rotation, irrespective of the fertilization level. The clover-grass ley nevertheless had a residual effect on yield, as the average yield over the total trial period was higher in the rotation. Even in the second year after the clover-grass ley, nearly double the quantity of nitrogen was mobilized in unfertilized plots as compared with the monoculture: 36.8 kg/ha and 21.0 kg/ha, respectively. Fertilization resulted in a slight increase in the abundance and activity of microbes. An increasing effect on straw decomposition was found up to the highest level of fertilization. Decomposition was equally effective in monoculture and rotation. At different levels of fertilization (0, 400, 800 and 1200 kg/ ha NPK containing 15% N), an average of 24, 29, 34 and 38 percent of the straw was decomposed during the growing season in the monoculture, and 25, 28, 32 and 37 per cent in the rotation. Carbon dioxide evolved during the growing seasons in unfertilized plots at the rate of 4.3 mmol/m2/h in monoculture and 4.9 mmol/m2/h in rotation. At a fertilizer level of 400 kg/ha the rates were 4.7 and 5.3 mmol/m2/h in monoculture and rotation, respectively. Maximum carbon dioxide was evolved at fertilization levels of 800 and 1200 kg/ha: 5.0 mmol/m2/h the monoculture and 5.8 mmol/m2/h in the rotation. Maximum ATP content and dehydrogenase activity were observed at the fertilization levels of 400 and 800 kg/ha and were slightly greater in the rotation than in the monoculture. Fertilization and crop rotation increased the soil microbial activity and biomass by a few ten per cent. In this trial, favourable conditions for plant growth were also found to favour microbial activity