The Effects of Cover Crops and Reduced Tillage in Organic Cropping Systems on Nitrogen Fluxes and Weeds

Summary

The goal of this work was to test different cover crops regarding their weed suppressive potential and their contribution to the N-supply for the successive field crops within different scenarios of reduced tillage in Organic Farming. Therefore the results of two repeated one-year trials (2011/12 and 2012/13) on two different sites and the results of the last two years of a medium long-term trial (2007-2013) were examined. The sites of the one-year trials were the research farm of University of Kassel ‘Domaene Frankenhausen’ and the organic farm ‘Meyerhof’ in Belm near Osnabrueck. In these trials the main crop oat succeeded after the cover crops Sinapis alba, Trifolium resupinatum, Vicia sativa and bare fallow as a control. The oat was sown after different tillage methods. These were in Domaene Frankenhausen plough (22-24-cm depth), chisel (10-12 cm) or disc harrow (7 cm), mulching and drilling or direct drilling. In Belm the tillage methods were plough (23 cm), chisel (13-15 cm) or rotary hoe (6-8 cm).

In Domaene Frankenhausen best results regarding weed suppression and N-supply were achieved with V. sativa. 2011/12 even mulching and drilling as well as direct drilling succeeded after V. sativa without any further weed control and with comparable oat yields with the ploughed or chiselled treatments on a level from 5.5 t ha^-1 to 5.9 t ha^-1. On Meyerhof Belm ploughing after bare fallow, S. alba and T. resupinatum resulted in higher oat yields compared with the reduced tillage treatments. After V. sativa yield was not significantly different between the plough- and reduced tillage treatments in both years.

In the medium long-term trial a system comparison between plough (to a depth of 25 cm) in a crop rotation with three years of alfalfa grass ley, plough (25 cm) in a cereal-based crop rotation and stubble cleaner (maximum depth of 12 cm) in a cereal-based crop rotation was carried out. Originally the trial was established to examine if the stubble cleaner is similar efficient for the control of Cirsium arvense like perennial alfalfa grass ley. 2011 in all three systems the following green manure crops were integrated: Lolium perenne, Phacelia tanacetifolia, Sinapis alba, Trifolium resupinatum, mixture from S. alba and T. resupinatum and Vicia sativa. Bare fallow served as control. Main crops were 2012 oats and 2013 field beans. V. sativa had the highest yield and the highest N-uptake of all green manure species in all three systems. The pre-crop alfalfa-grass ley (2007-2010) had two and three years after its termination a distinct impact onto the N-content of the soil in the years 2012 and 2013 and resulted in higher N-contents compared to the cereal-based crop rotation. 2013 the soil was analysed regarding the content of microbial N and C. In the uppermost layer the stubble cleaner system had significantly more N_mic and C_mic than the two other systems. 2012 weed emergence was usually lowest in the plough system with alfalfa-grass ley and highest in the stubble cleaner system. Bare fallow normally led to high weed emergence in every system. In 2012, there was to some extent a negative impact of the stubble cleaner on the diversity of the weed community, measured in the number of species, equity and Shannon-Wiener Index. The green manures had a slightly positive effect, i.e. a higher diversity together with more equal distribution, compared to the bare fallow. C. arvense occurred more frequently in the plough system in the cereal-based crop rotation and in the stubble cleaner system than in the plough system with pre-crop alfalfa in both years. The three years of alfalfa grass ley, therefore, were more important for the control of C. arvense than the choice of the tillage method in this trial. 2012 the stubble cleaner system resulted in the lowest, the plough system with pre-crop alfalfa in the highest and the plough system in the cereal-based crop rotation in an intermediate oat yield. In the plough system with pre-crop alfalfa all green manure species resulted in highest oat yields. In contrast to that in the stubble cleaner system, the choice of the green manure crop was very important. Only V. sativa resulted in oat yields like in all treatments of the plough system with pre-crop alfalfa and in the best treatments of the plough system in the cereal-based crop rotation. 2013 there were no significant differences between the different treatments in the yield of field bean.