Tillage System Comparison in Organic Farming: Effects on N Mineralization, Soil Microbial Biomass, and Yield

The nitrogen supply can be a yield-limiting factor in organic farming, especially when reduced tillage is applied. An organic field experiment was conducted from 2007 to 2013 to analyse the potential of the nitrogen supply through the efficient use of green manure crops in different tillage systems. Three farming systems were compared: a stubble cleaner system (SC) and a plough system (PL), both in a cereal-based crop rotation, and another plough system in a crop rotation that included alfalfa grass ley (PLALF). In the fifth year of the experiment, the experimental design was extended into a split plot design, and seven green manure treatments (Lolium perenne, Phacelia tanacetifolia, Sinapis alba, a mixture of Sinapis alba and Trifolium resupinatum, Trifolium resupinatum, Vicia sativa, and bare fallow as the control) were integrated into each of the three systems. The effects of the three systems and the green manure treatments on N mineralization, the soil microbial biomass and the yield of the main crops of oats and field beans in the sixth and seventh years of the experiment were analysed. The results showed that the choice of green manure species was of minor importance in the PLALF system. This system generally success- fully supplied N to the oats with oat yields from 3.6 to 5.1 t per ha.Vicia sativa was the most promising green manure crop in the SC and PL systems, with the Nmin values and oat yields (4.0 and 4.6 t per ha) being similar to those in the PLALF system. In the subsequent year, the PLALF system again was more successful in most of the Nmin assessments than the PL and SC systems, which often had rather similar results. In addition, a main crop of field beans was able to compensate for the differences in the Nmin content, and the yields were similar in all three systems (3.1 to 3.7 t per ha). The microbial biomass in the top soil was significantly increased in the reduced tillage system compared to the plough systems. In conclusion, reduced tillage in organic farming can promote soil microorganisms and be competitive if the nitrogen supply is improved through the efficient use of green manure or an adequate leguminous main crop.