Keywords: ¹⁵N, animal manure, DOC long-term field experiment, mineral fertilizer, nitrogen use efficiency, soil microbial biomass.

Global population is increasing exponentially and is forecasted to be about 9 billion in 2050. During the next years fertilizer consumption also will increase as food security has to be attained. Currently, fertilizer nitrogen (N) use efficiency is on average only 50% for mineral fertilizer and even lower for animal manure. Fertilizer N which is not recovered by crops or immobilised in the soil is lost from the soil-plant system and adversely affects the environment. Hence, one of the biggest challenges in agriculture is to increase production and at the same time minimising N losses. Organic farming generally is assumed to be environmental-friendlier than conventional farming. Due to the prohibition of synthetic fertilizer, organic farming depends on organic fertilizers whose availability to crops depends on microbial mineralization and immobilisation processes. Microbial biomass and activity is usually higher in organically than in conventionally managed cropping systems. Using the DOC long-term field experiment as a model we analyzed the impact of the soil microbial biomass on N use efficiency of animal manure and mineral fertilizer and on the fate of fertilizer N not taken up by crops in an organic and a conventional cropping system. Despite higher microbial activity in the soil of the organically than the conventionally managed cropping system the fate of fertilizer N was the same in both cropping systems, as shown by similar fertilizer N use efficiency by crops and similar recovery of fertilizer-derived N in the soil. This suggests that the two cropping systems have the same potential to emit N compounds to the environment.

C Bosshard¹, A Oberson² and J Mayer^1
1 Agroscope Reckenholz-Tänikon, Reckenholzstrasse 191, 8046 Zurich, Switzerland.
² Institute of Plant Sciences, Group of Plant Nutrition, ETH Zurich, Switzerland.

24 pages, 4 tables, 1 figure, 84 references.