Former Division of Ecotoxicology

Even in extensive use systems or with reduced nitrogen fertilization, significantly higher nitrate levels can occur in the soil after the cultivation of winter rape than after cereals. Possible causes of this effect - an increased N-net mineralization compared to other crops, nitrogen input via root excretions or harvest residues from rapeseed - should be investigated in laboratory and field trials.

According to the general view, the N-forms AHL and KAS are considered to be largely on a par in terms of their yield effect on winter wheat. Nevertheless, there are always cases in which one form of fertilizer is superior to the other. Among other things, this can be attributed to the earnings components.

Priority issues:

• How does an increasing N supply affect the yield generation of winter wheat?
• Is the production of winter wheat influenced by the form of N fertilization (AHL, KAS)?

Methodological approach

• Plot trial on the Höckelheim trial field at LWK Hannover
• Duration: spring to harvest 2000
• Previous crop: sugar beet
• N fertilization: total setpoints (incl. N_min) from 170 to 290 kg N ha^-1
• N-forms AHL or KAS

Parameters to be determined: N_min, nitrate content, straw base, N uptake, yield components
Interim results from 2000 (selection):

Averages* and (standard deviations) from 4 parallels each *green values: max. red values: min. by comparison AHL - KAS
	AHL				KAS				-
Total setpoint [kg N ha-1]	210		290		210		290		Nmin
Grain yield [dt DM ha-1]	93,3	(1,2)	97,6	(2,8)	97,9	(2,1)	97,6	(3,6)	64,1	(5,3)
Straw yield [dt DM ha-1]	77,3	(3,4)	80,9	(4,1)	80,6	(3,4)	86,2	(5,2)	58,2	(6,4)
N content grain [% i DM]	1,8	(0,1)	1,9	(0,2)	1,9	(0,1)	1,9	(0,0)	1,3	(0,0)
N content straw [% i DM]	0,4	(0,1)	0,5	(0,1)	0,6	(0,1)	0,7	(0,1)	0,3	(0,0)
N amount in the grain [kg ha-1]	168	(5)	182	(15)	184	(15)	189	(9)	86	(8)
N amount in the straw [kg ha-1]	27	(4)	41	(11)	44	(3)	64	(8)	18	(3)
Tot. N amount in Growing up EC 92 [kg ha-1]	195	(8)	223	(25)	228	(16)	253	(15)	104	(11)
Density of ears of wheat [ears m-2]	386	(14)	438	(18)	371	(16)	435	(12)	304	(16)
stems EC31 [stems m-2]	828	(31)	881	(81)	853	(52)	918	(56)	744	(16)
rel. stem reduction EC 31 vs. EC 92 [%]	53	(3)	50	(3)	56	(1)	53	(4)	59	(2)
Grain number/ear	50	(2)	48	(1)	55	(4)	48	(2)	46	(4)
TGW [g DM]	48,5	(0,9)	46,7	(1,6)	48,0	(1,7)	46,6	(1,5)	46,4	(1,2)
N balance [kg N ha-1]	12	(4)	69	(12)	-10	(13)	76	(1)	-86	(8)
Nmin at harvest time [kg ha-1 90 cm-1]	19	(3)	28	(10)	21	(3)	35	(14)	26	(3)

Summary
In 1991, a "pilot project for the introduction of reduced nitrogen fertilization in agricultural holdings" was started in order to investigate the effects of an N-quota in practical holdings on production processes, production success and environmental impacts of agricultural production. The quota was 60% of the normal N expenditure - including manure - averaged over the cultivation conditions of the farms in the 1989/90 and 1990/91 crop years. Seven farms each in the Lüchow-Dannenberg district (region LD) and in the municipalities of Krummhörn and Dollart in East Frisia (region WB) committed to comply with this fertilizer limitation as an operating quota. Two other farms in each region limited their fertilization to an average of 90 kg N / ha for the duration of the project. All companies were free to distribute the fertilizer quantities to the crops grown. The only cultivation limit in the freely selectable cultivation ratio was the maximum limit of 10% for grain legumes. The data were evaluated for analysis using precisely managed field files. The basis of the investigations was the determination of harvest quantities as well as investigations of the product quality, the mineral N content of the soils and the N balances. Four companies in each region served as a reference and operated without any restrictions. The farm surveys were accompanied by a field trial in each region. Here, the adaptation measures "crop rotation" and "N fertilizer reduction" were checked by two three-part crop rotations for yield and quality effects as well as changes in the N_min dynamics of the soil and the N balance.

After the introduction of the N quota, crop rotations were changed in the farms. The cultivation of fruits with low demands on N fertilization, e.g. Legumes or oil flax and also the cultivation of summering were taken up or expanded. Economically important crops such as winter wheat in WB and sugar beets and starch potatoes in LD were less affected by adjustments to crop rotation or fertilization.

The drop in yield caused by the reduction in fertilization was estimated in WB on the field trial and in the farms at around 9% on average for all crops. In LD, the average drop in yields (operating data) was 10% higher, while on the trial field with a low yield level, about 2% less was harvested due to the lack of irrigation. The loss of yield on the trial in WB was statistically secured. In LD as well as in WB, yield losses were reduced through crop rotation changes.

The reduced N-fertilization reduced the quality where the protein content was used as a criterion for assessing the product quality, e.g. for baked wheat and feed grain. On the test fields, an N reduction of 40% for winter wheat (WB) resulted in a significant reduction in the protein content of 2-3% on average and for feed rye (LD) of approx. 2.5%. In the farms, the protein levels behaved very differently, depending on the choice of variety made by the farm managers (especially wheat) and the actual fertilization of the individual fields. No changes in the oil, sugar or starch content were found in rapeseed, sugar beet and starch potatoes.

The N_min levels reacted very differently to the N fertilizer restriction. The reduced fertilizer quantities had no effect on the test field in WB due to the location, while in LD there was a clear reduction in the values ​​compared to the properly fertilized plots. In WB in particular, the cultivation of summer manual crops caused increased values ​​during the fallow season in autumn before sowing. The amounts of N_min after legumes as well as after rapeseed were also increased. Despite these opposing effects of fertilizer reduction and crop rotation adjustment, the N_min levels in the farms decreased slightly in autumn and spring.

As a result of reduced fertilization, the N balances were lower than after proper fertilization. On the test field in WB, the values ​​for proper and reduced fertilization were in the tolerable range (-50 to +50 kg N / ha), while in LD (test field) only the N balances of the reduced fertilization were within defined tolerance limits. The areas of the quota holdings have also shown reduced N balances since the start of the project. The decreases in the N surpluses averaged around 40 kg N / ha in WB and around 30 kg N / ha in LD. The forage farms had the greatest difficulty in achieving tolerable N balances. In terms of crops, rapeseed was the most unfavorable with consistently high N surpluses. However, the highest N overhangs were usually the result of over-fertilization, especially with high N-doses from farm manure (> 80 kg / ha).

The N-discharge from the soil profile with the percolation water, which could only be calculated on the test field in LD, was similar to the N_min values ​​with properly fertilized compared to the reduced fertilizer greatly increased, while without fertilizer and with reduced fertilizer in approximately the same washout losses occurred.

In a field trial, in which the conventional crop rotation W-wheat / W-wheat / sugar beet is compared with W-wheat / rotation fallow / sugar beet, the effects of different greening of the rotation fallow (rye grass, phacelia, oil radish, phacelia clover and oil radish -Clover mixture) was examined for the N mineralization activity of the soil and for the amount and activity of the microbial biomass. N_min, N_org and the potential N mineralization are determined as characteristic values ​​of the N dynamics. The quantity and activity of the microbial biomass are characterized by measuring the microbially bound N and C and the activity parameters of dehydrogenase activity, specific activity, basic respiration and metabolic quotient. Since the amount and type of organic matter input into the soil is expected to influence N dynamics and microbial biomass, the amount and most important ingredients (C, N, raw cellulose, hemicellulose and lignin content) are also determined Fallow growth and its root mass.

Summary
When introducing one-year rotation wasteland into a crop rotation that has been used intensively to date, knowledge of possible changes in the primarily microbially controlled N dynamics and subsequent delivery is of particular interest. On the one hand, ecologically questionable N losses, e.g. by washing out, both under the rotating fallow itself and under its subsequent crops. On the other hand, it may be necessary for economic and ecological reasons to adapt the N fertilization strategy for the new crops. Therefore, in two-year field trials on alluvial clay soils near Göttingen and in accompanying laboratory breeding experiments, the effects of differently greened annual fallow fallows (German ryegrass, phacelia, oil radish and phacelia / clover and oil radish / clover mixture) before sugar beet on the amount and activity of the microbial biomass as well tested for other elements of the N budget (mineral nitrogen, soluble organic N, potential N mineralization, N deprivation through the sugar beet and calculated N net mineralization). Winter wheat fertilized in the usual way was used as a comparison variant, which in practice is a common sugar beet crop.

The extent of changes in the N household under the rotating fallow itself, but also under the subsequent crop of sugar beet, was dependent on the type of fallow greening and the quality of the plant material introduced into the soil with the above and below ground residues. This was expressed on the one hand in the soil's mineral nitrogen content (N_min), which was in some cases considerably higher than that of the control wheat winter among all rotating fallows with the exception of ryegrass greening. In the fallow period there were maximum values ​​of up to 75 kg N ha^-1 30 cm^-1, in the sugar beet period even up to 209 kg N ha^-1 30 cm^-1. On the other hand, the changed N-supply behavior was shown in higher yields and increased nitrogen removal of the N-unfertilized sugar beet after rotation fallow. Again ryegrass was an exception with slightly reduced yields compared to the control variant. The highest greenhouse gas deprivation of the following crop was caused by the fallow greenery with Phacelia and Phacelia / Klee mixture (up to 48% more deprivation compared to winter wheat). In these variants, almost the same beet and leaf yields of the subsequent N-unfertilized sugar beet were achieved as in the variant of winter wheat fertilized with 110 kg N ha^-1.

In contrast, the amount and activity of the microbial biomass, measured as microbially bound nitrogen (N_mik) and carbon (C_mik) or as dehydrogenase activity and basic respiration, were slightly reduced in the fallow fallow variants in both the fallow and sugar beet periods. This is attributed to a reduced substrate and especially C supply as a result of the comparatively short vegetation period of the fallow wastes compared to winter wheat, which was comparatively short with only about 5 months. The increased N-supply under and after the rotating fallow land could not be explained by increased amounts or increased activity of the microbial biomass. When calculating the N and C sales of the microbial biomass, however, it was found that those variants with the strongest N subsequent deliveries simultaneously had the highest sales activity and also the highest nutrient flux due to the microbial biomass.

The accompanying short-term incubation experiments in the laboratory did not reflect the differentiated N subsequent deliveries that occurred in the field between the fallow and control variants. They therefore did not provide any additional information about the possible causes of the changes in the nitrogen balance observed in the field.

One-year rotation fallow as a sugar beet crop can therefore, compared to winter wheat as a conventional crop rotation link, be reduced (ryegrass), but also significantly increased (phacelia, oil radish, phacelia / clover or oil radish / clover mixture), depending on the type of greening. Carry out subsequent delivery in the floor. These changes must be taken into account when selecting the appropriate fallow management and when measuring nitrogen fertilization for the next crop.

After rapeseed harvesting and the following autumn/winter, modern grain rape cultivation is often accompanied by high nitrogen emissions (nitrate shift with the leachate, release of the climate-relevant trace gas N₂O). This is undesirable, particularly with regard to the production of rapeseed as a renewable raw material. According to the current state of knowledge, the mineralization of organic nitrogen compounds from vegetation and crop residues is of particular importance both for nitrate leaching and for N₂O emissions after rape cultivation.

Intention:
The aim of this project is to examine an alternative cultivation concept for rapeseed with regard to its potential for reducing environmentally harmful nitrogen emissions.

Trial duration: July 1998 to February 2000
Investigations:

• Calculation of the N₂O emissions from the N₂O content in an event-related manner, but at least once a week from gas samples obtained using floor hoods ( „Closed - Chamber - Method")
  
• N_min contents of field-moist soil samples (0-15 and 15-30 cm soil depth), parallel to the trace gas measurements
  
• N-net mineralization of the soil in-situ using the "tube method" using the differential approach according to RAISON et al. (1987) and STENGER et al. (1996)
  
• Grain yield surveys using a plot combine.

Summary of results:
A central problem of current rapeseed production - also with regard to sustainable production as a renewable raw material - are environmentally harmful nitrogen emissions, which among other things in the form of increased nitrate levels in the leachate and an increased emission of the climate-relevant trace gas N₂O. The results so far allow the conclusion that the nitrogen mineralization processes in the breakdown of the crop and root residues are decisive for the nitrate leaching as well as for the N₂O emission in rapeseed cultivation. With the aim of reducing environmentally harmful nitrogen emissions from rapeseed cultivation, an alternative cultivation concept was investigated in the field trial, the basis of which is to forego tillage after the rapeseed harvest and to increase vegetable N uptake before winter through the use of rapeseed as a catch crop.

The extent of the net nitrogen mineralization in the Ap after harvest until the next spring was decisively influenced by the period between harvest and early December. Up to this date, the average NNM after winter rape was 55 kg ha^-1 with intensive tillage, plowing and the usual sowing time of winter wheat. With decreasing cultivation intensity, it decreased to 34 kg ha^-1 while resting on the ground, but was still significantly higher than that after pre-crop winter wheat. The largest NNM was measured after the rape straw was removed. In the test medium, the top 15 cm of the soil had by far the largest share of the total nitrogen release within the Ap.

As a result of the nitrogen net mineralization, after the winter rape harvest, the N_min quantities in the soil rose significantly in the first two, but not in the third year of the trial, compared to the comparison areas for winter barley and winter wheat. The maximum values ​​in 1997 reached 120 and 85 kg N ha^-1 90 cm^-1 in 1998. Compared to the variants with tillage after the rapeseed harvest, the plots with quiet soil and covered by rapeseed had the lowest N_min values ​​in autumn in the two years. They were of the same order of magnitude as the variants with cereal crop. Replacing the plow with the cultivator resulted in slightly lower N_min in autumn.

The N₂O trace gas measurements showed that there was crop-specific N₂O emissions from the soil. The annual oilseed rape emissions in 1998/99 were 2.9 kg N₂O-N ha^-1, around 1 kg N₂O-N ha^-1 higher than those of the other crops. The difference was independent of the fertilization intensity. Based on the amount of fertilizer applied, the N₂O emission of all types of fruit was between 1 and 1.7%. During the vegetation period from March to July, the highest N₂O emissions and the greatest differences between crop types in crop rotation were recorded for all crop types. The N₂O emissions of the winter rape vegetation periods (1998 2.0 kg N₂O-N ha^-1; 1999 6.5 kg N₂O-N ha^-1) exceeded those of the other crops by 0.6 to 6.2 kg N₂O-N ha^-1. These increased N₂O emissions were mainly related to the high N_min levels after fertilization and short-term rainfall events. The differences between the types of fruit and in the type of fruit between the years were significantly influenced by the different distribution of fertilizer. A pre-fruit effect of the winter rape on the N₂O emissions after the harvest in autumn and winter could not be proven. After the winter rapeseed harvest into the winter, the N_min contents were barely above 50 kg N ha^-1 30 cm^-1 and, accordingly, there were no increased N₂O emissions. The emissions in autumn - winter represented a maximum of 35% of the annual emission. The tillage variants did not make any difference for the N₂O emission at the Marienstein site.

In addition to the field investigations, there was an intensive discussion of the management strategies with regard to their degree of target achievement and their feasibility in agricultural practice with cooperation partners from public administration as well as from public and private agricultural consultancy. The focus of interest was on avoiding nitrate emissions, while N₂O emissions have so far been of little importance from an agronomic point of view. In the discussions it became clear that, in particular, the quietness of the soil after the rapeseed harvest on a large number of practice areas, even more than in the field trial, was the most effective measure to avoid high nitrate levels in the soil in autumn. However, the introduction of summering as winter rape crop was associated with economic problems, which are mitigated by compensation payments in Lower Saxony on soils at high to very high risk of discharge from water protection areas.

The N-pilot project has been carried out since 1991 in cooperation with the Lower Saxony Chamber of Agriculture, the advisory boards of the research regions and the University of Göttingen (Research and Study Center Agriculture and Environment, Institute for Agricultural Economics, Institute for Agricultural Chemistry) and under the Ministry of Food, Agriculture and Forestry of Lower Saxony EU participation funded under Objective 5b funding. The first phase of the project came to an end with the 1996 harvest, and an extension to the 1999 harvest was decided.

As part of this pilot project, the use of nitrogen is reduced by an average of 40% in 18 farms, spread across the regions of East Frisia (Landkreis Leer and Aurich) and Lüchow (Landkreis Lüchow-Dannenberg). Another 8 companies provide data for comparison purposes.

The adaptation behavior of farmers, the short and long-term yield and quality developments of agricultural products (arable and grassland), the individual farm income effects and the effects on the N-leaching potential of the usable areas are examined.

A four-stage static N fertilization test in the Lüchow area enables detailed data on the N_min dynamics and nitrate shifting of a sandy soil. In the plant nutrition sub-section, the focus is on determining nitrogen efficiency, the N balance balances and some soil parameters (N_min value, carbon and nitrogen content).

The results of this research work were published in a dissertation:
Karsten Möller: Wirkungen einer Reduktion des Stickstoffaufwandes in landwirtschaftlichen Betrieben auf Pflanzenproduktion und Umwelt – Empirische Untersuchungen in West- und Ostniedersachsen. Diss. Universität Göttingen, 1999. Hainholz Verlag, Göttingen. ISBN 3-932622-53-7

Summary
The increased use of nitrogenous fertilizers has led to an intensification of the negative environmental effects in agriculture in recent decades, the perception of which has increased in the media and society in recent years. An environmentally politically motivated intervention in agricultural holdings on the main nutrient nitrogen, as has been discussed since the mid-1980s, could contribute to environmental relief. Against this background, the Lower Saxony Ministry of Food, Agriculture and Forestry selected the model of an N-quota in 1991 as a possibility of administrative intervention in agricultural holdings to reduce environmental impacts and production surpluses for an investigation in a pilot project.

In the "Niedersächsisches Pilotprojekt zur Einführung einer reduzierten Stickstoffdüngung in landwirtschaftlichen Betrieben" (Lower Saxony pilot project for the introduction of reduced nitrogen fertilization in agricultural holdings), 9 differently structured holdings (quota holdings) were used for the application of a farm-specific N-quota (N-quota) in the Lüchow-Dannenberg regions of Lower Saxony, which are extremely contrasting in terms of location (mainly light sandy soils). and Aurich-Leer (heavy maritime soils) based on the cultivation conditions of the individual farms in 1989/90 and 1990/91, the N-quota was set to 60% of the normal N-expenditure (target values ​​of the chambers of agriculture) including the manure on a farm-specific basis. The second variant, chosen by a few farmers, but with a higher compensation payment, limited the annual fertilization on the farm to 90 kg N ha^-1, in contrast to 106 to 131 kg N ha^-1 in the farm-specific variant yearly. The farm managers were able to distribute the allocated N fertilizer quantities freely among their crops. The only further restriction in the freely selectable cultivation program was, based on the total arable area, a cultivation limit of 10% for grain legumes.

Five companies were available in each region for comparison purposes. Operational records (field records) and investigations by the chambers of agriculture (N analyzes of harvested products including basic feed, manure and soil) in the project farms served as the data basis. The costs of compensating for the loss of income and the complete scientific processing were borne by the State of Lower Saxony and the EU in Brussels.

A nitrogen reduction test on a light sandy soil near Lüchow in the east of Lower Saxony was integrated into the practice-oriented project. The design of the nitrogen fertilization levels was based on the setpoint concept. The focus of the study was on the yields and qualities achievable with a 40% reduction in N fertilizer, but also the associated nitrate leaching and N_min dynamics in comparison to the 'proper' N fertilization. A 'local' crop rotation consisting of potato - winter rye - W barley and an 'adapted' crop rotation in which the N-intensive W barley was replaced by oats were examined in parallel. The nitrate N concentrations were measured in percolation water obtained regularly with the help of suction candles and from this the nitrate loads were calculated with the aid of the leachate quantities.

The main results of the study of the effects of a reduction in nitrogen expenditure are:

• N-intensive fruits, especially W rapeseed and W wheat, were replaced by N-extensive fruits, such as grain legumes, W rye, grass seed propagation and summer cereals, in a region-specific manner on around a sixth of the area. As an indirect consequence, the use of plant protection products decreased, as the N-extensive ones usually need less. The area shares of the economically strong fruit sugar beet and potato in the Lüchow-Dannenberg region and milk thistle in the Aurich-Leer region did not change due to the N ratio.
• The reduction in N fertilization was crop-specific. The W grain and sugar beet experienced a sharp reduction of 25 to 35% under the recommendation of the official advice, while there was no reduction in N fertilization for potatoes and silage maize in Lüchow-Dannenberg.
• As a result of the N quota, yield losses of an average of 10 to 20% can be determined in cereal and silage maize cultivation and on grassland, but not for sugar beet and potatoes. The depressed earnings led to a drop in sales in the companies observed.
• Due to the reduced N fertilization, depending on the type of grain, the raw protein content fell by 0.7 to 1.9 percentage points compared to the reference farms, which is also supported by the field test results. Quota farms in Lüchow-Dannenberg are in fact not able to produce quality wheat due to the N quota.
• Other revenue-influencing quality parameters of the crop products of the arable land (sugar content, starch content, thousand grain weight, hectolitre weight, protein quality), supported by the field test results, were not changed by lowering the N-fertilization to a proper amount.
• The N balance sheet surplus was drastically reduced by the N quota. The average N-area surplus of arable and grassland resulted in average values ​​of 30 to 45 kg N ha^-1 in the quota farms Lüchow-Dannenberg, in the Aurich-Leer region 10 to 20 kg N ha^-1, while these in the reference group in During the investigation years, between 50 and 90 kg N ha^-1 oscillated.
• Despite the average reduction in balance sheet surpluses, there were still individual strikes with annual N surpluses of over 100 kg N ha^-1 on average over the project years 1992 to 1997 with an N quota even after the N quota. Only on fields with a high productivity (especially the Aurich-Leer region) did negative N balance balances occur, which presumably require a slight drain from the N pool of the soil.
• In the field test, the N balances were consistently negative when the N fertilization was reduced by 40%, while here the 'proper' N fertilization according to the setpoint concept led to balanced N balances
• The apparent N-efficiency, based on N-removal and N-supply, could be improved by region and crop type by the N-quota.
• The mineral nitrogen content of the soil in autumn could not be reduced on average by reducing N fertilization within 6 years, rather there was a strong dependency on the fruit grown.
• The reduced N fertilization did not lead to any measurable changes in the nitrogen and humus content of the arable crumbs in the project farms over a period of 6 years. In the field trial, there was a tendency for the nitrogen and humus contents to decrease slightly due to the static N reduction by 40%.
• On the light sandy soil of the field trial, the nitrate load was reduced by an average of 20%, while the N effort was reduced by 40% compared to the normal variant.
• The crop diversity was increased by changes in the cultivation program, caused by the shortage of nitrogen, especially on lighter soils.
• For some managers, the N quota was the positive reason to think about their overall management intensity and to reduce it. The lowering of the N-fertilization intensity via a company-specific N-quota with an otherwise greatest possible leave of entrepreneurial freedom leads to slight yield losses, an overall lower use of pesticides and lower nitrogen emissions, but also to revenue reductions, which EHLERDING (2000) and STOYKE (1995 ) to be discribed.

It is questionable whether an environmental policy measure such as the N quota can be implemented across Germany or Europe, since the administrative effort for the introduction and control would be very high compared to the N tax. The Dutch nutrient balance concept also requires a high level of administration and control, but offers a more targeted approach to reducing nutrient emissions in agriculture.

Summary
The EC set-aside ordinance prescribes 1- to 5-year-old brown fallow land (GB), the effects of which on N-subsequent deliveries and N-fertilizer requirements for the subsequent crops are largely unknown. It is to be expected that the fallow green itself, as well as its upheaval, will cause changes in the N budget of the soil, which may i. a. can be controlled microbiologically. These depend on the growth of the GB portfolio and after the change to the mass and quality of the GB growth. For the question of the effects of multi-year GB on N mineralization / immobilization processes and on the N nutrition of subsequent crops, a 6-year exact test was therefore carried out on a floodplain near Göttingen. In comparison to a normal crop rotation (FF) and a brown fallow (SB) of several years, the influence of a permanent green fallow (GB; clover grass) on the N_min formation, on the N supply of the subsequent crops and on the amount and activity of the microbial biomass was examined . Important functions of the microbial biomass as sink and source for N during the fallow and in the turnover of the N bound in the GB growth after the GB upheaval were worked out in a vessel test with 15N-marked grass and clover grass (green mass).

Under fallow land of several years, the N household was largely determined by the fallow land form and the associated uninterrupted (GB) or missing (SB) entry of organic matter (vegetation). An expression of this are the N_min amounts of the soil, which reached up to 250 kg N ha^-1 90 cm^-1 under SB and thus represented a considerable potential for nitrate that is susceptible to leaching. In contrast, the N_min quantities under GB always only amounted to 20-50 kg N ha^-1 90 cm^-1.

In accordance with the range of organic matter, the microbial biomass and its activity increased under multi-year GB compared to the usual crop rotation and showed a pronounced depth gradient in the arable land. In contrast, the SB floor showed the lowest values ​​for all soil biological parameters with a wide C_mik / N_mik ratio.

The GB as a previous crop promoted long-term N-deprivation and crop yields. Around 10% of the additional withdrawals and earnings compared to FF were still found in the 4th offspring. This was caused by the change in N-turnover in the soil due to set-aside and was reflected in the spring-N_min-quantities of the soil and in particular in the N-net mineralization. The N subsequent delivery was dependent on the material composition of the fallow growth at the time of the upheaval.

The fallow land caused the biomass parameters to be shifted into deeper layers of soil, which only leveled out in the spring of the following year. Overall, the amount and activity of microbial biomass increased after the GB change. In contrast, in the crop rotation variant there was no significant redistribution of the biomass and its activity due to uniform mixing of the soil.

The positive effects of the GB on the quantity and activity of the microbial biomass in the soil were still measurable 3 years after the change. There were no variant differences for the microbial activity (specific activity) related to one unit of biomass. It followed a pronounced annual cycle with increases in phases of increased N mineralization.

With the use of 15N, the results of the vascular test were able to show the causes of the increase in N delivery after several years after the GB change. Substantial amounts of N were released from the green mass in the soil very quickly and were available to plants, but only about 40-50% were withdrawn by the wheat growth (comparable to the 1st GB crop). About half of the green mass-based N remained in the soil after the wheat harvest, was subject to long-term implementation processes and was therefore available to the soil's microorganisms in the long term. The microbial biomass accumulated only a small, constant share of the labeled N of up to 10% of the total N amount (sink), which, however, quickly transformed due to significantly increased microbial turnover rates and a high N-flux and thus as mineral N plants became available (source).

With the incorporation of the green mass into the soil, the N delivery of the soil was also stimulated. About 65% of the N quantities in wheat growth and the N_min quantities of the soil came from the green mass, the rest from the soil substance. Without the addition of green mass and without mineral N fertilization, the N delivery of the soil only made up about 40% of this amount. This is assigned to a priming effect caused by the incorporated green mass. Only 13-27% of the microbially bound N was formed from the green mass-N. The largest N share came from other sources. This is proof of the importance of microbial biomass in the N-supply from the soil.

The changes in N-turnover in the soil caused by several years of GB are crucial to the promotion of microbial biomass and its activity. Through the microbial turnover of the UK growth, but also of the soil substance, the N supply of the plant stands is improved in the long term. With regard to the N fertilization for the follow-up crops according to GB, however, no quantitative statements can currently be made about the actual microbial N subsequent supply.

After the cultivation of oilseed rape, the following autumn / winter very often leads to increased nitrate levels in the soil, which exceed the N requirement of the usually following winter wheat before the end of the growing season. From the point of view of water protection, low nitrate levels in the soil would be desirable at this point. Strategies that lead to a solution to the problem by influencing the net nitrogen mineralization (tillage intensity, catch crops, crop rotation design) are examined in several years of field trials for their effectiveness and their mechanisms of action.

The increased N_min values ​​in the soil found in autumn after the harvest of winter rape can only be used insufficiently by the winter wheat crop before the end of the growing season and are at risk of loss. Avoiding any tillage after rapeseed in autumn can help to solve the problem by using the fall rape as a "catch crop". To maximize this effect, it is necessary to add a summer crop to the crop rotation before winter wheat is the second rapeseed crop. The aim of the concept is to optimize the transfer of "rapeseed" nitrogen into the summer and thereby save mineral fertilizers.

Question: What influence does differently intensive tillage in spring have on:

• the net nitrogen mineralization?
• the N_min amounts in the soil?
• the N-uptake of the oat?
• the yield of oat?

Methodological approach
The investigations are carried out within the INTEX project Reinshof on the areas "Tönjeswinkel" and "Flöhburg" as a splitting plant with 3 tillage intensities (plow / cultivator / direct sowing or Rototiller) and 3 N fertilization levels (without / N40 / N80; as AHL) in 2 Repetitions performed. Weed control and the use of a growth regulator are the same in all variants. Accompanying the vegetation, N_min and N uptake of the oats are determined in narrow intervals. The net nitrogen mineralization is determined using balance sheet calculations. The investigation concludes with the determination of the grain yield and quality parameters.

At the same time, similar studies are being carried out on two farms in the Lower Saxony demonstration project on environmentally friendly land management.