Effects of Tillage Intensity, Cover Crop Species and Cover Crop Biomass on N-Fluxes, Weeds and Oat Yields in an Organic Field Experiment in Germany

: The non-turning or only superﬁcial turning of soil is considered to be a gentle tillage method. Nevertheless, conventional ploughs are widely used in organic farming for crop production reasons. For the further development of reduced tillage, and up to no tillage, the effects of three cover crop species and their incorporation with different tillage intensities on nitrogen (N) dynamics, weed emergence and the yield of the subsequent main crop, oats, were examined in a repeated organic one-year trial. Sinapis alba , Trifolium resupinatum, Vicia sativa and bare fallow were tested and incorporated using (1) a plough (PL), (2) reduced tillage (RT), (3) mulching + drilling (MD) and (4) direct drilling (DD). V. sativa was the most promising cover crop in combination with RT, MD and DD. In Trial 1, the soil mineral N content and oat yields after the introduction of V. sativa were on a similar level as those in the PL treatments, and weeds were not yield-limiting there. In Trial 2, the biomass production of V. sativa was only about half of that of Trial 1 and did not offer sufﬁcient weed control, but V. sativa was still successful in the RT treatments. In both trials, the yield differences were more pronounced between the cover crop treatments after RT than after PL. RT, therefore, was more dependent on an adequate cover crop species than PL. The no-till method was not only dependent on an adequate cover crop species but also on its proper biomass production for sufﬁcient weed control.


Introduction
Reduced tillage (RT) methods such as non-turning or superficial turning are considered to be environmentally friendly alternatives to conventional ploughing (PL). Compared to tillage systems that involve PL, continuously applied RT can improve soil biological and physical parameters [1][2][3][4][5][6][7][8][9]. In addition, in relation to climate change, the better structure in the topsoil from RT due to the presence of more organic carbon (C org ) in the topsoil may be suitable for providing better resilience [10].
In conventional agriculture, RT is only considered practicable with full herbicide use [11,12] because there can be a higher level of weed infestation without PL [12][13][14]. In addition, RT can lead to the delayed warming and reduced aeration of soil in spring and thus to delayed or reduced nitrogen (N) mineralization [12]. In organic farming it is therefore more difficult to avoid using a plough [15][16][17]. RT in organic farming can lead to yield losses [4,7,18].
While RT has been studied in organic farming research for some decades [7], studies on the complete avoidance of tillage ("no-till", NT) in organic farming are less widespread [19][20][21]. Organic NT is often combined with Vicia villosa and roller crimpers [22]. These systems rely

•
Which of the tested cover crop species leads to the highest mineralized N (N min ) content in spring before tillage? • How does N min develop after the cover crops under the main crop, oats, in the different tillage treatments? • Which of the tested cover crop species have the best weed-suppressing effect in spring before tillage? How do weed density, cover and biomass develop during oat growth after tillage? • How do the different cover crop-tillage combinations affect the yield of the main crop (oats)?

Site Description
The repeated one-year field trial was carried out in the trial years 2011/12 and 2012/13 on two different sites, hereafter referred to as Trial 1 and Trial 2. The location was the teaching and experimental farm of the University of Kassel, the "Hessian State Domain Frankenhausen" (51.412 N, 9.440 E; 231 m above sea level). The soil type was haplic luvisol. The soil texture in the Ap horizon was a strong clayey silt in both fields. The previous crop in both trials was winter wheat, and the pre-pre-crop was carrots.

Experimental Design
The sowing of the cover crops was followed by the sowing of the main crop, oats, under different tillage treatments. The duration of the trials was from the August of one year to the August of the next. Trial 1 was set up as a two-factor strip split-plot experiment, with the factor of cover crops on the main plots and, from differentiation in terms of tillage in spring, with the factor of tillage in the strips above. In Trial 2, the cover crops were laid out on randomized sub-plots and spring tillage was conducted above them in the main plots.
With the exception of Trial 1, the trials were conducted in four replicates. Trial 1 was continued in eightfold repetition with differentiation according to tillage, since, due to the technical feasibility of tillage in fourfold repetition, no complete randomization and thus no proper statistical evaluation was possible.
The cover crop species, varieties and seed rates in both years were as follows: • The tillage treatments and an overview of the other arable measures are shown in Table 1. No mechanical weed control and no fertilization were carried out.

Data Collection
The cover crops were sampled in November (17 November 2011 (Trial 1)/24 November 2012 (Trial 2)) before freezing. For this purpose, half a square meter was harvested by hand and randomly distributed six times per plot in Trial 1 (i.e., 3 m 2 in total per plot). In Trial 2, one square with a 1.5 m side length (=2.25 m 2 ) per plot was harvested by hand. Immediately after harvesting, the green cuttings were weighed. Then, a sub-sample was obtained from each sample, which was dried at 60 • C. The samples were used for above-ground biomass yield determination and to analyze its total nitrogen (TN) and total carbon (TC) contents with a Macro C and N auto-analyzer (Elementar Analysesysteme, Hanau, Germany).
Soil samples were taken at least from a depth of 60 cm, and if possible, to 90 cm. Sampling was carried out at the beginning of the trial, in November, before and after tillage in spring and at the end of the trial. On the first two dates of Trial 1, the sample was divided into the following layers: 0-10 cm, 10-20 cm, 20-40 cm, 40-60 cm and 60-90 cm. On all other dates it was conducted in 30 cm sections. A mixed sample was obtained per plot and layer from between 3 and 8 samples depending on the size of the plot. These mixed samples were immediately packed into cooled isolation boxes at the experimental sites. All samples were frozen as soon as possible. For analysis of the N min content, the samples were taken to the Hessian State Laboratory in Kassel/Harleshausen. The analysis of the samples with regard to NO 3 -N in all three layers and additionally NH 4 -N in the uppermost layer was carried out according to DIN ISO 14255 and DIN EN ISO 11732.
With regard to weeds, the degree of weed cover (WC) was determined at a late stage of the cover crop and a late stage of the main crop, oats, (BBCH 77 to 80) in an area of one square meter per plot. Reference images were used to estimate the percentage ground cover. For the total cover, the cover percentages of the different species were added. Since the plants grew at different levels, the total cover could exceed 100%. The weed density (WD) was determined at an early stage of the main crop, oats (BBCH 10 to 11), i.e., all weed plants in the sampling area were counted. The sampling area was one tenth of a square meter and was randomly distributed four times over the plot. The total above-ground biomass was quantified at a late stage of the oats' growing process (BBCH 77 to 80) at the same time as the WC and on the same plot. For this purpose, all weeds were cut off close to the ground. The samples were dried, and, after complete drying, the dry matter was determined.
The oats were harvested by hand. For this purpose, an area of half a square meter per plot was cut by hand twice in Trial 1 and four times per plot in Trial 2.

Data Analysis
To describe the distribution of N min , C and N in the cover crop biomass, weed emergence, cover crop and oat yields, mean value and standard error were calculated. The evaluation was carried out separately by year to take account of the different weather conditions. Each data set was checked for normally distributed residuals (Kolmogorov-Smirnov test). If there was no normal distribution, the data were transformed for statistical evaluation. The type of transformation was indicated in the results. The presentation of the data in the bar charts was based on the mean values of the original data. Large plot, small plot and block were tested as fixed factors with a univariate analysis of variance for significant effects and interactions. In Trial 1, due to the experimental design, it was necessary to include horizontal "row blocks" in the analysis in addition to the vertical blocks [33]. If the analysis of variance indicated significant effects or interactions, a post hoc test (Tukey-B) was then carried out on the factor combination of cover crop x tillage or on the individual factors (alpha ≤ 0.05). In the results section, significant effects (from two factors only in the case of significant interaction) are indicated by different letters.
The statistical analyses were carried out with SPSS-21.

Weather
Data from the Frankenhausen weather station were used. If these were not available, data from other weather stations in the vicinity were used (approx. 10 km away). The 30-year mean was based on data from 1981 to 2010 from the German Weather Service (DWD) from the Kassel weather station. The mean temperature for this location and period was 9.1 • C, and the mean annual precipitation was 725 m. For Trial 1, the mean temperature was 9.4 • C, and the precipitation total was 557 mm; for Trial 2 they were 8.2 • C and 482 mm ( Table 2). The temperature course in the trial period 2011 to 2013 was similar to the course of the 30-year mean. The biggest deviation was a significantly cooler March in 2013. The precipitation was lower than the averaged totals in many months. May 2013 stood out with an above-average precipitation total.

Yield, N-Uptake and C/N Ratio of the Cover Crops
In Trial 1, the above-ground biomass yield of the cover crops averaged at 2.1 t dry matter (DM) ha −1 for S. alba and 3.0 t DM ha −1 for V. sativa in the four replicates. T. resupinatum emerged so poorly that it could not be sampled. In Trial 2, the above-ground biomass yield of S. alba was 1.7 t DM ha −1 , that of T. resupinatum was 0.6 t DM ha −1 and that of V. sativa was 1.5 t DM ha −1 . The yields of S. alba and V. sativa were significantly higher than the yield of T. resupinatum (Figure 1a The temperature course in the trial period 2011 to 2013 was similar to the course of the 30-year mean. The biggest deviation was a significantly cooler March in 2013. The precipitation was lower than the averaged totals in many months. May 2013 stood out with an above-average precipitation total.

Yield, N-Uptake and C/N Ratio of the Cover Crops
In Trial 1, the above-ground biomass yield of the cover crops averaged at 2.1 t dry matter (DM) ha −1 for S. alba and 3.0 t DM ha −1 for V. sativa in the four replicates. T. resupinatum emerged so poorly that it could not be sampled. In Trial 2, the above-ground biomass yield of S. alba was 1.7 t DM ha −1 , that of T. resupinatum was 0.6 t DM ha −1 and that of V. sativa was 1.5 t DM ha −1 . The yields of S. alba and V. sativa were significantly higher than the yield of T. resupinatum (Figure 1a,b). In Trial 1, the N uptake in the above-ground biomass of S. alba averaged at 34.9 kg N ha −1 and that of V. sativa averaged at 124.4 kg N ha −1 over the four replicates. In Trial 2, the N uptake in the above-ground biomass of S. alba was 43.4 kg N ha −1 , that of T. resupinatum was 22.9 kg N ha −1 and that of V. sativa was 72.7 kg N ha −1 (Figure 2a In Trial 1, the N uptake in the above-ground biomass of S. alba averaged at 34.9 kg N ha −1 and that of V. sativa averaged at 124.4 kg N ha −1 over the four replicates. In Trial 2, the N uptake in the above-ground biomass of S. alba was 43.4 kg N ha −1 , that of T. resupinatum was 22.9 kg N ha −1 and that of V. sativa was 72.7 kg N ha −1 (Figure 2a  Total N uptake of cover crops in the above-ground biomass in Trial 1 and Trial 2. * T. resupinatum could not be sampled and therefore no ANOVA could be carried out. Different letters indicate significant differences (p ≤ 0.05).The C/N ratio of S. alba was 16.7 and 25.9, that of T. resupinatum was 10.9 and that of V. sativa was 9.9 and 10 ( Table 3). Table 3. C/N ratio of cover crops in the two trials.

Species
Trial 1 Trial 2 S. alba 25.9 ± 0.72 16.7 ± 0.47 c T. resupinatum * n.a. 10.9 ± 0.10 b V. sativa 10.0 ± 0.19 9.9 ± 0.12 a * T. resupinatum could not be sampled. Therefore, no analysis of variance could be carried out for Trial 1. Different letters indicate significant differences (p ≤ 0.05). n.a. = not available. The C/N ratio of S. alba was 16.7 and 25.9, that of T. resupinatum was 10.9 and that of V. sativa was 9.9 and 10 ( Table 3). Table 3. C/N ratio of cover crops in the two trials.

N-Dynamic
Since in Trial 1 the proportion of NH 4 -N in the total N min was low on all the sampling dates (generally less than 1 kg ha −1 layer −1 , with a maximum of 1.6 kg ha −1 layer −1 ), separate presentation of NO 3 -N and NH 4 -N was omitted in the following section. At the start of Trial 1 on 29 August 2011 the N min contents were at similar levels across the trial area and there were no significant differences. On 24 November 2011, there was a significant influence of the cover crop factor. The N min contents in all the soil layers were significantly the highest in the bare fallow treatment compared to the other treatments. In the 20-40 cm layer, the T. resupinatum and V. sativa treatments occupied an intermediate position, i.e., the N min content was significantly higher than in the S. alba treatment, but significantly lower than in the bare fallow treatment. On 26 March 2012 there was still a significant influence of the cover crop factor on the N min content of all the investigated layers. In the V. sativa treatment, the N min content was significantly the highest in the upper two layers. In the 60-90 cm layer, the N min content was at the same level in the V. sativa and bare fallow treatments. The three soil layers of the T. resupinatum treatment had medium values, as did the 30-60 cm layer in the bare fallow treatment. The S. alba treatment had significantly the lowest values in all three layers (Figure 3a).
After differentiation in terms of tillage on 2 May 2012 there was a significant influence of the cover crop factor on the N min content of all three soil layers and a significant influence of the tillage factor on the N min content of the top two soil layers in Trial 1. The N min content was significantly the highest in the 0-30 cm layer and in the 30-60 cm layer in the V. sativa x plough and V. sativa x chisel treatments. The second highest N min content in these two layers was in the T. resupinatum x plough and T. resupinatum x chisel and bare fallow x plough and bare fallow x chisel treatments. The N min content was the lowest in the S. alba treatments. The mulch and no-till treatments had a lower N min content in the upper two soil layers in each cover crop than in the plough and chisel treatments, although the N min content for the mulch and no-till treatments in the V. sativa plots was still at a high level. In the soil layer at 60-90 cm, there were no significant differences between the treatments. There was no significant interaction (Figure 3b). At the end of the trial on 29 August 2012 there were only minor differences between the remaining treatments in terms of quantity. Nevertheless, the treatments differed significantly. There was a significant influence of both factors and a significant interaction in the 0-30 cm layer. In the 30-60 cm layer, the cover crop factor had a significant influence. In the V. sativa x plough treatment, the N min content in the 0-30 cm layer was significantly higher than in all the other treatments. In the 30-60 cm layer, the N min content in the V. sativa x chisel treatment was significantly higher than in the S. alba x plough and S. alba x chisel treatments.  In Trial 2, the proportion of NH 4 -N in the total N min was higher on all sampling dates than in Trial 1, but the proportion was also only low, so no separate presentation also is given here. The highest NH 4 -N values were achieved in the V. sativa treatments in general and in the V. sativa x plough treatments in particular (in the 0-30 cm layer up to a maximum of 6 kg NH 4 -N ha −1 ).
In addition, in this trial, the N min content at the start of the trial on 24 August 2012 was at a similar level in all the treatments, and there were no significant differences. On 6 December 2012 there was a significant effect of the cover crops on the N min content of each of the three soil layers. The N min content in the 0-30 cm layer in the V. sativa treatment was significantly the highest, and in the bare fallow treatment it was significantly the lowest. The N min content of the T. resupinatum treatment was in between that of the other treatments. In the 30-60 cm layer, the N min content of the V. sativa treatment was significantly the highest, although the differences in quantity were only slight from a plant cultivation point of view. The situation was similar in the 60-90 cm layer; the N min content was low overall, but there were significant differences. The N min content ranged from the highest values being obtained in the V. sativa treatment to medium values being obtained in the T. resupinatum treatment to the lowest values being obtained in the S. alba and bare fallow treatments. On 11 April 2013 there was a significant influence of the cover crop factor. The differences in quantity increased. Again, the highest N min content was found in the V. sativa treatment in all three layers. In the top layer, the N min content after the bare fallow, S. alba and T. resupinatum treatments was similarly low, and in the lower two layers the N min content after the fallow treatment was significantly lowest (Figure 3c).
On 16 May 2013, after differentiation by tillage, both the cover crop factor and the tillage factor had a significant influence on the N min content of all three soil layers, and there was a significant interaction with respect to the N min content of the topsoil layer. The N min content in the 0-30 cm layer was significantly the highest in the T. resupinatum x plough treatment. The N min content was similarly high in the V. sativa x plough and V. sativa x disc harrow treatments. In the 30-60 cm layer, the V. sativa x disc harrow treatment had significantly the highest N min content. There were no significant differences in the 60-90 cm layer. The mulch and no-till treatments after the V. sativa treatment had significantly higher N min contents in all three soil layers than the mulch and no-till treatments after the other cover crops and the fallow treatments (Figure 3d). On 25 June 2013, the differences in terms of quantity were again only slight. However, there was a significant interaction in the 0-30 cm layer; the N min content was highest in the fallow x mulch, S. alba x plough, V. sativa x mulch and V. sativa x no-till treatments, and it was the lowest in the fallow x plough treatment. In the 30-60 cm layer, there were no significant differences between the treatments. In the 60-90 cm layer there was a significant influence of the cover crop factor and a significant interaction as well. The V. sativa x mulch seed treatment had significantly the highest N min content, while the mulch and no-till treatments had significantly the lowest values in the other cover crops and the bare fallow treatments. At the end of the trial on 3 September 2013, there were no significant differences in any soil layer in the remaining treatments.

Weed Cover, Density and Biomass
The data for weed cover in the late stage of cover crops in Trial 1 were not normally distributed. For the statistical evaluation, the data were log-transformed. The weed cover was significantly the lowest in the V. sativa treatment and significantly the highest in the T. resupinatum treatment. In the S. alba treatment, the weed cover was at a medium level (Figure 4a). In Trial 2, the weed cover in the late stage of cover crops was again significantly the lowest in the V. sativa treatment. This was followed by the S. alba treatment and the T. resupinatum treatment, each with a significantly higher degree of weed cover. The weed cover was significantly highest in the bare fallow treatment. Overall, the weed cover was seven times higher than in Trial 1 (Figure 4b).
Crops 2022, 2, FOR PEER REVIEW 10 plough treatment. In the 30-60 cm layer, there were no significant differences between the treatments. In the 60-90 cm layer there was a significant influence of the cover crop factor and a significant interaction as well. The V. sativa x mulch seed treatment had significantly the highest Nmin content, while the mulch and no-till treatments had significantly the lowest values in the other cover crops and the bare fallow treatments. At the end of the trial on 3 September 2013, there were no significant differences in any soil layer in the remaining treatments.

Weed Cover, Density and Biomass
The data for weed cover in the late stage of cover crops in Trial 1 were not normally distributed. For the statistical evaluation, the data were log-transformed. The weed cover was significantly the lowest in the V. sativa treatment and significantly the highest in the T. resupinatum treatment. In the S. alba treatment, the weed cover was at a medium level (Figure 4a). In Trial 2, the weed cover in the late stage of cover crops was again significantly the lowest in the V. sativa treatment. This was followed by the S. alba treatment and the T. resupinatum treatment, each with a significantly higher degree of weed cover. The weed cover was significantly highest in the bare fallow treatment. Overall, the weed cover was seven times higher than in Trial 1 (Figure 4b). There was a significant interaction between the cover crop and tillage factors on the weed density in Trial 1. In all the mulch and no-till treatments, the weed density was significantly lower than in the other tillage treatments (with the exception of V. sativa x mulch sowing). The no-till treatment did not disturb or hardly disturbed the development of the existing weeds and no or only few new weed seeds were brought to the surface. Thus, there were fewer but larger weeds in these treatments. Significantly, the most weeds were found in the S. alba x plough treatment (Figure 5a). There was a significant interaction between the cover crop and tillage factors on the weed density in Trial 1. In all the mulch and no-till treatments, the weed density was significantly lower than in the other tillage treatments (with the exception of V. sativa x mulch sowing). The no-till treatment did not disturb or hardly disturbed the development of the existing weeds and no or only few new weed seeds were brought to the surface. Thus, there were fewer but larger weeds in these treatments. Significantly, the most weeds were found in the S. alba x plough treatment (Figure 5a). In Trial 2, there was a great deal of volunteer growth of the preceding crop, winter wheat. In order to enable the continuation of the mulch and no-till plots, one half of each of these treatments was flamed. In the following section, the results of the flamed plot halves are always shown. There was no significant interaction between the cover crop and tillage factors. The tillage factor had a significant effect. The no-till treatments had significantly the lowest weed density due to the flaming. The ploughed treatments had a medium weed density. The disc harrow treatments had significantly the highest weed density ( Figure 5b). Overall, the weed density in Trial 2 was nearly three times higher than in Trial 1.
The data for weed cover in the late stage of the main crop, oats, for Trial 1 were not normally distributed. Log-transformed data were used for the analysis of variance. There was a significant interaction between cover crop and tillage. The T. resupinatum x plough and the V. sativa x plough treatments had significantly the lowest weed cover. The bare fallow x chisel, T. resupinatum x chisel and V. sativa x no-till treatments had significantly the highest weed cover (Figure 6a). The mulch and no-till treatments in the bare fallow, S. alba and T. resupinatum treatments had to be abandoned due to there being too much weed pressure. The mulch and no-till treatments in the V. sativa plots could be maintained. In Trial 2, there was a great deal of volunteer growth of the preceding crop, winter wheat. In order to enable the continuation of the mulch and no-till plots, one half of each of these treatments was flamed. In the following section, the results of the flamed plot halves are always shown. There was no significant interaction between the cover crop and tillage factors. The tillage factor had a significant effect. The no-till treatments had significantly the lowest weed density due to the flaming. The ploughed treatments had a medium weed density. The disc harrow treatments had significantly the highest weed density (Figure 5b). Overall, the weed density in Trial 2 was nearly three times higher than in Trial 1.
The data for weed cover in the late stage of the main crop, oats, for Trial 1 were not normally distributed. Log-transformed data were used for the analysis of variance. There was a significant interaction between cover crop and tillage. The T. resupinatum x plough and the V. sativa x plough treatments had significantly the lowest weed cover. The bare fallow x chisel, T. resupinatum x chisel and V. sativa x no-till treatments had significantly the highest weed cover (Figure 6a). The mulch and no-till treatments in the bare fallow, S. alba and T. resupinatum treatments had to be abandoned due to there being too much weed pressure. The mulch and no-till treatments in the V. sativa plots could be maintained. In Trial 2, there was no significant interaction between the cover crop and tillage factors. The tillage had a significant effect. The plough treatments had a significantly lower weed cover than the disc harrow treatments. The weed emergence was overall significantly higher than in Trial 1. All the mulch and no-till treatments had to be abandoned (Figure 6b).
The no-till treatments, besides that after V. sativa in Trial 1, were not included in the analyses due to there being too much weed pressure In Trial 2, there was no significant interaction between the cover crop and tillage factors. The tillage had a significant effect. The plough treatments had a significantly lower weed cover than the disc harrow treatments. The weed emergence was overall significantly higher than in Trial 1. All the mulch and no-till treatments had to be abandoned (Figure 6b). The no-till treatments, besides that after V. sativa in Trial 1, were not included in the analyses due to there being too much weed pressure The weed biomass data in Trial 1 were not normally distributed. Log-transformed data were used for the analysis of variance. There was no significant interaction between the factors cover crop and tillage. Tillage had a significant effect on weed biomass. The plough treatments had a significantly lower weed biomass than the chisel treatments in each cover crop treatment. The weed biomass in the V. sativa x mulch treatment was at a similar level to the chisel treatments. The V. sativa x no-till treatment had a significantly higher weed biomass (Figure 7a).
In Trial 2, there was no significant interaction between the cover crop and tillage factors. The tillage had a significant effect. The plough treatments had a significantly lower weed cover than the disc harrow treatments. The weed emergence was overall significantly higher than in Trial 1. All the mulch and no-till treatments had to be abandoned (Figure 6b).
The no-till treatments, besides that after V. sativa in Trial 1, were not included in the analyses due to there being too much weed pressure The weed biomass data in Trial 1 were not normally distributed. Log-transformed data were used for the analysis of variance. There was no significant interaction between the factors cover crop and tillage. Tillage had a significant effect on weed biomass. The plough treatments had a significantly lower weed biomass than the chisel treatments in each cover crop treatment. The weed biomass in the V. sativa x mulch treatment was at a similar level to the chisel treatments. The V. sativa x no-till treatment had a significantly higher weed biomass (Figure 7a).  In Trial 2, tillage had a significant effect. In the plough treatments of the bare fallow, S. alba and T. resupinatum plots, there was a lower weed biomass than in the disc harrow treatments. For V. sativa, however, there was no significant difference in weed biomass between the plough and disc harrow treatments (Figure 7b).
The no-till treatments, besides that after V. sativa in Trial 1, were not included in the analyses due to there being too much weed pressure.

Yield of the Main Crop, Oats
There was a statistically significant interaction between the cover crop and tillage for oat yield in Trial 1. The grain yield of oats was significantly the lowest in the S. alba plough and chisel treatments. It was significantly the highest in the V. sativa chisel treatment (Figure 8a).
Crops 2022, 2, FOR PEER REVIEW 13 In Trial 2, tillage had a significant effect. In the plough treatments of the bare fallow, S. alba and T. resupinatum plots, there was a lower weed biomass than in the disc harrow treatments. For V. sativa, however, there was no significant difference in weed biomass between the plough and disc harrow treatments (Figure 7b).
The no-till treatments, besides that after V. sativa in Trial 1, were not included in the analyses due to there being too much weed pressure.

Yield of the Main Crop, Oats
There was a statistically significant interaction between the cover crop and tillage for oat yield in Trial 1. The grain yield of oats was significantly the lowest in the S. alba plough and chisel treatments. It was significantly the highest in the V. sativa chisel treatment (Figure 8a). There was a statistically significant interaction between cover crop and tillage in Trial 2. The grain yield in Trial 2 was significantly the lowest in the disc harrow treatments after bare fallow and S. alba. It was significantly the highest in the plough treatments after T. resupinatum and V. sativa as well as in the disc harrow treatment after V. sativa (Figure 8b).  There was a statistically significant interaction between cover crop and tillage in Trial 2. The grain yield in Trial 2 was significantly the lowest in the disc harrow treatments after bare fallow and S. alba. It was significantly the highest in the plough treatments after T. resupinatum and V. sativa as well as in the disc harrow treatment after V. sativa (Figure 8b).
The no-till treatments, besides that after V. sativa in Trial 1, were not included in the analyses due to there being too much weed pressure.

Discussion
In Trial 1, the biomass yield of S. alba was approximately 0.4 t DM ha −1 , and that of V. sativa was approximately 1. T. resupinatum had a very poor emergence in Trial 1. In the literature, a poor emergence or total failure of T. resupinatum is mentioned several times [34][35][36]. In Trial 1, silting due to heavy rainfall shortly after sowing was probably decisive for the unfavorable emergence conditions in 2011. In Trial 2, T. resupinatum emerged better and yielded 0.6 t DM ha −1 , which corresponded to the average value given in the literature [34].
S. alba was more yield-stable than the other two cover crop species and was more tolerant to the late sowing date. This confirmed various studies [30,31,36,37]. S. alba had a higher C/N ratio in Trial 1 than in Trial 2. S. alba was in a more advanced stage of vegetation in Trial 1 than in Trial 2, and there was woodier stem material in the samples than in the samples from Trial 2. In the literature, values of 12.9 [36], 17.8-30.3 [38] and 11-28 [39] are given for the C/N ratio of S. alba. The high value of 25.9 was therefore not unusual and was within the ranges of these data.
In both trials, the cover crops had a clear influence on the N min content. In spring, before tillage, V. sativa resulted in the highest N min content in the 0-30 cm and 30-60 cm layers in both years. In addition, after tillage, V. sativa resulted in a higher N min content than the other cover crops. The biomass yield of V. sativa was only about half as high in Trial 2 as in Trial 1. Nevertheless, the N min content after the V. sativa treatment in Trial 2 was at a similar level as in Trial 1. The better emergence of T. resupinatum in Trial 2 was visible in the results for the N min content of the ploughed system. In the RT system, however, the T. resupinatum treatment performed significantly worse than the V. sativa treatment, even in Trial 2. The N min content was the lowest in both trials in the S. alba treatments. As desired, the N uptake of S. alba in the growing season led to a low N min content in autumn. However, the N uptake was rather low, with values of 34.9 kg N ha −1 in Trial 1 and 43.4 kg N ha −1 in Trial 2. Presumably, the N uptake could have been higher with a higher N min content. The initial N min content of both plots was around 34 kg N ha −1 in the 0-60 cm layer. In spring, after tillage, the N min overall content of the S. alba treatments did not reach the N min content after the other cover crops and bare fallow treatments.
Soil cultivation with a chisel or disc harrow did not lead to a significantly lower N min content in spring compared to the ploughed treatment of the respective cover crop. The mulch and no-till treatments, on the other hand, led to significantly lower N min levels, especially after the bare fallow, S. alba and T. resupinatum treatments. In the mulch and no-till treatments, the N min content was the highest after V. sativa in Trial 1. The higher yield of V. sativa compared to Trial 2 was probably noticeable here.
Regarding weed cover in the late stage of the cover crops, it was significantly the lowest in both trials after V. sativa. S. alba had a medium weed suppression potential. In Trial 2, weed emergence in all the treatments was significantly higher than in Trial 1.
The weed density was significantly the highest in Trial 1 in the S. alba x plough treatment. This higher number of weeds was put into perspective again during the growing season of the oats. The weed density was counted when the weeds were young and thus only said something about the existing weed potential and not about further development. The weed density was lowest in the mulch and no-till treatments, where there were significantly fewer but larger weeds. In these plots, the weeds were hardly or not at all disturbed in terms of their development due to the lack of tillage and were thus able to continue growing undisturbed, in some cases from the previous autumn.
The significantly higher weed emergence in Trial 2 compared to Trial 1 was also reflected in the weed density; the number was many times higher in each treatment of Trial 2 than in Trial 1. The problem here was the strong volunteer growth of the previous crop, winter wheat, which was also counted as a weed. The weed density was the highest in the disc harrow plots. It was the lowest in the mulch and no-till treatments due to flaming.
In Trial 1, the weed cover in the late stage of oats was generally low in the plough treatments compared to the other tillage treatments and did not differ with regard to the cover crop treatment. In the mulch and no-till treatments, the oats could not be harvested after the cover crops S. alba and T. resupinatum and after the bare fallow treatments due to excessive weed growth. In comparison, V. sativa was much better at suppressing the weeds. In Trial 2, all the mulch and no-till treatments had to be abandoned due to excessive weed pressure. The one-time flaming did not achieve a sufficient effect. The weed pressure in the disc harrow treatments was significantly higher than the weed pressure in the plough treatments, and there were no significant differences between the cover crop treatments. Overall, there was a significantly higher degree of weed cover in Trial 2 than in Trial 1, including in the plough treatments.
The superiority of the plough as an instrument for weed control was shown in terms of weed biomass. In both trials, the weed biomass was overall the lowest in the ploughed treatments.
The relative excellence of V. sativa in Frankenhausen in Trial 1 with regard to weed suppression confirmed the results of [40]. The results of [41] showed a similarly good success of S. alba and V. sativa, which corresponded to the results of Trial 2. In Trial 1, it was observed that V. sativa literally formed "felt plates" when it froze, which kept the soil well covered where there was sufficient biomass and largely prevented the weeds from appearing. The lack of success of V. sativa in the mulch and no-till treatments of Trial 2 was found on the one hand in the lower biomass production of V. sativa and on the other hand in the generally higher occurrence of weeds.
Regarding oat yields, in Trial 1, the S. alba treatments showed a reduced yield in both the remaining tillage systems. In the study in [42], oat yields were also reduced after S. alba treatment. The authors concluded that the N taken up by brassica cover crops was often not available when the subsequent crop needed it. V. sativa resulted in the highest oat yields in all the tillage treatments and in the only harvestable mulch and no-till treatments. These were at a similar level of yield as the plough and chisel treatments. In Trial 2, the yield differences were greater than in Trial 1. Ploughing led to consistently good oat yields; RT resulted in yield losses, except after V. sativa, where the highest yields were harvested just after RT.
Of the cover crops presented here, V. sativa seemed particularly suitable for being combined with RT in organic farming. In this respect, the positive results of [41] were confirmed, where, using V. sativa as a cover crop in a system with RT, yields comparable to those achieved with a ploughing system were achieved.
In Trial 1, the results showed that in organic farming, even with mulch and no-till treatments after the cultivation of a suitable cover crop, in this case V. sativa, in a suitable location, oat yields comparable to those obtained after tillage with a plough can be achieved. In Trial 2, however, in contrast to Trial 1, none of the cover crop treatments were able to suppress the weeds to such an extent that a mulch or no-till treatment would have led to satisfactory yields for the main crop, oats. This may have been due to the fact that the cover crops, especially V. sativa, produced significantly less biomass than in Trial 1. Therefore, it could not provide sufficient weed control. In addition, the flaming did not show a sufficient weed-suppressing effect, so much so that the mulch and no-till treatments finally had to be abandoned in Trial 2.
In both years, the yield differences were more pronounced between the cover crop treatments after RT than after PL. RT, therefore, was more dependent on an adequate cover crop species than the PL system. The no-till treatment was not only dependent on an adequate cover crop species but also on its proper biomass production for sufficient weed control. In this regard, the results of [43] were confirmed, who found that cover crop effects increased with decreasing management intensity.

Conclusions
The results showed that in organic farming with methods of RT (here chisel and disc harrow) in combination with a suitable cover crop, in this case V. sativa, comparable oat yields could be achieved in the short term to those after tillage with a plough. In order to further reduce the intensity of tillage in the direction of no-till, it is not only important that a suitable cover crop is cultivated but that it also produces enough biomass for good ground cover and satisfactory weed suppression. However, since the biomass production of the cover crop will be subject to fluctuation, a flexible choice of tillage method could be the most promising solution. With sufficient biomass production and soil cover of the cover crop, a reduction in tillage intensity in the direction of no-till could be considered for the subsequent crop. On the other hand, less growth of the cover crop would speak for a somewhat higher tillage intensity, e.g., the use of a chisel or a disc harrow.
Of course, one-year trials cannot be used to conclude for no-ploughing in the long term. How weed growth and soil properties develop in the case of several years of exclusively using RT cannot be answered on the basis of these trials and requires long-term ongoing trials. The experiences of practitioners are also of great value for answering such questions.