PLANT SOIL ENVIRON., 57, 2011 (11): 513–518 513 All over the world, land application of organ- ics produced by urban population and farmers (compost, sewage sludge, farmyard manure) is a widespread practice that allows a reuse of the increasing amounts of biosolids. They provide organic matter and therefore improve soil physical parameters such as structure and water holding capacity. Furthermore, biosolids are a source of different plant nutrients like nitrogen and phos- phorus as well as micronutrients (Scherer and Sharma 2002). Therefore land application of or- ganic wastes is an effective means of the reuse of these by-products. Long-term application of sewage sludge, compost and farmyard manure resulted in an increase the total C and N content of the soil (Werner et al. 1988). This increase is the most pronounced in plots treated with compost and therefore the ap- plication of compost to agricultural soils should be especially recommended. The effects of biosolids on soil mircoorganisms have also received particular attention. Following application of biosolids the size and activity of soil microbial biomass, which is widely recognized as an important agent in soil organic matter turno- ver, might be generally expected to increase in response to added C and N as well as other nutri- ents (Madsen et al. 1994). According to Werner et al. (1988) soil respiration is positively influenced by continuous sewage sludge and compost supply. Even thirteen years after a single sewage sludge application Barbarick et al. (2004) found higher microbial activity in comparison with mineral ferti- lizer application. However, a decrease in microbial biomass as well as its activity might be expected if biosolids contain toxic heavy metals. Chandler and Brookes (1991) found that biomass size generally decreased with increasing soil metal concentrations. These conflicting reports emphasize the need for more research on long-term effects of biosolids application on soil quality and productivity. The effects of management practices on soil fertil- ity criteria are best evaluated using long-term field experiments. Therefore we collected soil samples from plots of a field experiment, established in 1962, supplied with compost, sewage sludge, farm- yard manure and mineral fertilizer, respectively in two increments. Our objective was to compare the effects of continuous application of different organic residues with those of mineral fertilizer on chemical soil parameters as well as on the size and activity of the soil microbial community. Effect of long-term organic amendments on chemical and microbial properties of a luvisol H.W. Scherer 1 , D.J. Metker 1 , G. Welp 2 1 INRES-Plant Nutrition, Bonn, Germany 2 INRES-Soil Science, Bonn, Germany ABSTRACT We studied the long-term efect (about 45 years) of farmyard manure, sewage sludge and compost application in two increments on organic carbon (C org ), the amount (C mic ) and activity of the microbial biomass (soil respiration, dehydrogenase activity), total N content and N delivery of soils as compared to manuring with mineral fertilizers. Te application of both increments of compost and the high sewage sludge application rate resulted in an increase in C org while soils treated with both compost application rates and the high farmyard manure application rate showed a signifcant increase in C mic . C mic /C org ranged between 1.7 and 3.3. Dehydrogenase activity and soil res- piration were the greatest in the soil with the highest compost and farmyard manure application rates. Total soil N content was signifcantly higher in both compost treatments and in the treatment with the high sewage sludge ap- plication rate. Tis was accompanied by the highest N uptake of ryegrass. Keywords: long-term feld experiment; enzyme activity; microbial activity; N delivery; organic manure