&amp

M Kelm

R Loges

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16th IFOAM Organic World Congress, Modena, Italy, June 16-20, 2008 
Archived at http://orgprints.org/view/projects/conference.html 
 
 
Comparative analysis of conventional and organic farming 
systems: Nitrogen surpluses and nitrogen losses 
Kelm, M.1, Loges, R.1 & Taube, F.1 
Key words: Nitrogen, nitrate, leaching, farming systems, nitrogen fixation 
Abstract  
Nutrient management is a key factor for both economic viability and environmental 
performance of farming systems. On 32 representative conventional and organic 
farms in Northern Germany, nutrient management was analyzed in the 
interdisciplinary monitoring project “COMPASS”. Organic farms had significantly lower 
nitrogen (N) surpluses compared with conventional farms. The majority of organic 
farms had very low or even negative N surpluses, indicating insufficient N supply in 
the cropping system. Nitrogen leaching, however, was too high in many cases on both 
conventional and organic farms. Strategies for a more targeted nutrient supply in 
organic farming need to be developed and implemented. 
Introduction 
Nutrient-efficient farming is characterized by the minimization of nutrient losses to the 
environment while ensuring the necessary nutrient supply to crops and livestock. 
Organic farming is generally associated with sustainable nutrient management. 
Representative data from typical farms is scarce, however. Symbiotic N2 fixation of 
legumes represents the most significant N input in organic farming. Even though both 
own and purchased organic fertilizers are applied on most organic farms, a sufficient 
N supply to crops cannot always be realized. On the other hand, N losses to the 
environment, e.g. through leaching, should be minimized.  
In the present study, N fluxes and N leaching on 32 commercial farms in Northern 
Germany were analyzed in a 3-year monitoring project that allowed for a direct 
comparison of conventional and organic faming systems. 
Materials and methods 
The project “COMPASS” was carried out over the period 2004-2006 on commercial 
farms in Northern Germany. 16 pairs of farms were selected by two main criteria:  
a. Farm type: Specialized arable farms, specialized dairy farms. 
b. Intensity: Conventional farms, organic farms.  
 
Within a pair of farms, a conventional farm and a comparable organic farm were 
located at the same site. This project set-up allowed for a direct comparison of 
conventional and organic farms under similar soil and environmental conditions. We 
analyzed 8 pairs of arable farms and 8 pairs of dairy farms. The comparability of farms 
within a pair was assured by comparable farm size, and similar specialization under 
the prevailing soil conditions. In dairy farming, the stocking rate was generally lower 
on organic farms (mean: 0.86 LSU ha-1 vs. 1.46 LSU ha-1 on conventional dairy 
farms).  
Farm management was documented accurately, covering all aspects of crop and 
animal production. Additionally, we sampled forage yields, forage quality, botanical 
diversity of grassland, and symbiotic N2 fixation of legumes on representative fields.  
16th IFOAM Organic World Congress, Modena, Italy, June 16-20, 2008 
Archived at http://orgprints.org/view/projects/conference.html 
 
 
Symbiotic N2 fixation was determined according to Høgh-Jensen et al. (2004). N 
balances were calculated for all 32 farms. N input included purchased fertilizers, 
feedstuffs and livestock, and N2 fixation. N output was the sum of sold crops, animals, 
milk, and manure. From the difference between N input and N output at the farm 
scale, NH3 emissions during manure storage and application were subtracted.  
N leaching was determined on 8 selected farms. We used suction cups during the 
leaching periods (November-April) of 2004-2005 and 2005-2006 in representative 
fields of selected crops (see Table 1). 
18 (arable crops) or 24 (permanent grassland) suction cups were installed per field. N 
concentrations (NO3-N + NH4-N + Norganic) in the leaching water (sampled at weekly 
intervals) were measured photometrically with an autoanalyzer. Total N leaching per 
winter [kg N/ha] was calculated as the sum-product of N concentrations and amounts 
of leaching water over the sampling period.  
In a separate experiment (split-plot with four replicates, large plots of >600 m²), N 
turnover and N leaching were measured after the renewal of an organically managed 
permanent grassland sward on a sandy soil (ploughed in spring or autumn).  
Results and discussion 
Farm-gate N surpluses (indicating the total potential N loss to the environment) were 
significantly higher in conventional farming compared with organic farming, with 
generally higher N surpluses on dairy farms compared wiith arable farms (Figure 1). 
This is in line with some former studies (e.g., Dalgaard et al., 1998). Other 
investigations, however, did not find systematic differences in N surpluses between 
conventional and organic farms (e.g., Hansen et al., 2000).  
The majority of analyzed conventional farms – both arable and dairy farms – need to 
improve their N management in order to fulfil future requirements. The large variation 
among individual farms clearly shows the potential for improvement. Some 
conventional farms already show that efficient nutrient management is possible 
without yield losses. Amongst conventional farms, there was no significant correlation 
between N surpluses and crop yields or milk yields, respectively. Weak points in 
conventional agriculture were mainly associated with N fertilization. In many cases, 
mineral N application to crops was far too high, or nutrients in slurry were not 
accounted for. Replacement rates frequently exceeded 35%, and the feeding ration 
was not always well adjusted.  
In contrast, the majority of organic farms suffered from low or even negative N 
surpluses at the farm scale, which indicates a lack of nitrogen (Figure 1). The main 
reasons for variation in N surpluses were the proportion of legumes in the crop 
rotation and N2 fixation rates of legume crops. For instance, the proportion of 
grass/clover in the crop rotation of organic arable farms varied from 0% to 53% (mean: 
19%). N2 fixation rates ranged between 45 kg N/ha (Persian clover [Trifolium 
resupinatum], mulched) and 339 kg N/ha (red clover [Trifolium pratense], mixed 
cutting/mulching). N2 fixation rates of red clover/perennial ryegrass (Lolium perenne) 
swards were in the range 150-250 kg N/ha, depending on the proportion of red clover, 
soil type, and cutting, mulching or grazing treatment. Some organic arable farms tried 
to maximize cash crop production by eliminating grass/clover from the crop rotation. N 
input was realized only through N2-fixing cover crops, undersown clover in cereals, or 
purchased organic fertilizers. Even though this strategy could be opportune from an 
economic point of view, declining soil fertility and increasing weed levels might limit 
cash crop yields of these systems in the long run. Another weak point on organic 
16th IFOAM Organic World Congress, Modena, Italy, June 16-20, 2008 
Archived at http://orgprints.org/view/projects/conference.html 
 
 
farms was the adjustment of feeding rations to lactating cows. Both milk yields and 
animal health could be improved significantly by constantly providing a well-adjusted 
ration.  
Figure 1: Net N surplus of conventional and organic farms, mean of 2004-2006.  
 
a, b significantly different for P<0.05 
 
Significant N leaching losses were observed in both conventional and organic farming 
(Table 1). In organic farming, the ‘critical N load’, which corresponds to a mean 
concentration of 50 mg/l nitrate in leaching water, was exceeded if grass/clover was 
ploughed in autumn (followed by winter wheat), a finding supported by a number of 
other authors (e.g., Dreymann, 2005). The same occurred after silage maize harvest 
on sandy soils. Organic silage maize was grown after grass/clover ploughed in spring. 
Additionally, high amounts of slurry and manure were applied, which is not necessary, 
since the N supply to silage maize is already ensured by the mineralization of 
grass/clover residues. Neither conventional nor organic maize fields had a winter 
cover crop. On grassland and in cash crop production, N leaching losses were 
significantly affected by N input (mainly N fertilizer, slurry, and excrements) and N 
surpluses at the field scale. In silage maize production, however, this relationship was 
absent. Mineralization of soil organic matter seemed to have a much stronger effect 
16th IFOAM Organic World Congress, Modena, Italy, June 16-20, 2008 
Archived at http://orgprints.org/view/projects/conference.html 
 
 
on N leaching since these fields have been fertilized with manure and slurry for many 
years, which accumulated a large pool of soil nitrogen.  
Extremely high N leaching losses of 115-125 kg N/ha were observed in the first winter 
after the renewal of organically managed permanent grassland (data not shown). It 
made no significant difference if the grass sward was ploughed in spring or autumn. 
Mineralization of organic matter from the grass sward released very high amounts of N 
that could hardly be utilized until the following leaching period. As a consequence, 
grassland should be maintained in good condition as long as possible in order to 
postpone the renewal of grassland swards.  
Tab. 1: N leaching (kg N ha-1 yr-1; sum of NO3-N + NH4-N + organic N) in 
representative crops on conventional and organic farms. Mean of 18-24 suction 
cups per field, averaged over leaching periods 2004-2005 and 2005-2006. 
              
Farm   Winter wheat Oilseed rape After  Permanent
   after oilseed  (c) or grass/  silage maize  grassland
   rape (c) or   clover (o)    (1 cutting +  
   grass/clover (o)  after cereals     grazing) 
          
   Arable farms Arable farms Dairy farms  Dairy farms
     
      
conventional   32.1 L  28.7 L  22.1 L  40.1 L 
organic   45.4 L  13.2 L  27.8 L  26.2 L 
              
conventional   45.5 L  42.2 L  52.4 S  38.2 S 
organic    37.5 L  15.2 L  65.5 S  19.5 S 
              
              
L: loamy soils, S: sandy soils 
Conclusions 
Organic farms were characterized by lower N surpluses and – in most cases – lower N 
leaching losses compared with conventional farms. N leaching on organic farms, 
however, still exceeded a ‘critical N load’ in most crops. Options for improved N 
management in organic farming include the layout of crop rotations, grass/clover 
management, more efficient utilization of manure, and better adjustment of feeding 
rations (Taube et al., 2007).  
Acknowledgments 
The project “COMPASS” was supported by the Ministry of Agriculture, Environment 
and Rural Areas (MLUR) of Schleswig-Holstein, the Rentenbank Frankfurt/Main, and 
the foundation Schleswig-Holsteinische Landschaft.  
References  
Dalgaard T., Halberg N., Kristensen I.S. (1998): Can organic farming help to reduce N-losses? 
Nutr. Cycl. Agroecosyst. 52:277-287.  
Dreymann S. (2005): N-Haushalt unterschiedlich bewirtschafteter Rotklee-Bestände und deren 
Bedeutung für die Folgefrucht Weizen im Ökologischen Landbau. Ph.D. Thesis, University of 
Kiel, Germany (in German, with English summary).  
Hansen B., Kristensen E.S., Grant R., Høgh-Jensen H., Simmelsgaard S.E., Olesen J.E. (2000): 
Nitrogen leaching from conventional versus organic farming systems: a systems approach. 
Eur. J. Agron. 13:65-82. 
16th IFOAM Organic World Congress, Modena, Italy, June 16-20, 2008 
Archived at http://orgprints.org/view/projects/conference.html 
 
 
Høgh-Jensen H., Loges R., Jørgensen F., Vinther F., Jensen E. (2004): An empirical model for 
quantification of symbiotic nitrogen fixation in grass-clover mixtures. Agric. Syst. 82:181-194. 
Taube F., Kelm M., Verreet J.-A. (Eds.) (2007): Ergebnisse des Projektes “COMPASS“. University 
of Kiel (Germany), 82 pp. (in German). 


Comparative analysis of conventional and organic farming systems: Nitrogen surpluses and nitrogen losses

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Comparative analysis of conventional and organic farming systems: Nitrogen surpluses and nitrogen losses

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