On a global scale, nitrate contamination of surface and groundwaters is a growing problem, resulting in reduced water quality. Agricultural land use is considered to be the main source for nitrate pollution of groundwater. To protect the waterbodies from contamination with nitrates from agricultural land, the European Union established the Nitrate Directive 30 years ago in 1991, it has since been followed by the Water Framework Directive from 2000, and more recently the Groundwater Directive in 2006. 

”Groundwater bodies in for example Northern Germany and Denmark, which are both intensively managed agricultural regions, are characterised by continuously high NO₃ concentrations. Currently, approximately 28% of groundwater bodies in Germany and 16% in Denmark exceed the EU quality standard of 11.3 mg NO₃-N 1^-1, and this is alarming,” says Senior Researcher Iris Vogeler Cronin from Department of Agroecology at Aarhus University. 

Along with co-researchers from Christian Albrechts University she has been involved in an on-farm study to determine if stockless organic farming can reduce nitrate leaching compared to a stockless conventional system on high productive sites in Northern Germany. 

On-farm study

”The study is based in Northern Germany; however, the climate and soil are very similar to Danish conditions, making our study transferable to Denmark and other, likewise, comparable areas,” says Iris Vogeler Cronin. 

Different systems were compared, three in total:  

1. A specialised organic system with arable crop rotation which is  low N-intensive
2. A specialised organic system with arable crop rotation which is  semi-N-intensive
3. A typical N-intensive conventional crop rotation

“We conducted this study under commercial farm scale conditions covering close to 400 ha in total, and we measured nitrate leaching losses by using ceramic suction cups. We chose to conduct our studies in an on-farm study, as well-managed experimental plots do not accurately represent the same conditions as on-farm settings,” say Iris Vogeler Cronin. 

Apart from the three systems above, the researchers compared the results to two different reference systems in order to understand the additional environmental pollution coming from agricultural systems. 

Cannot live up to the EU standards

The researchers found that area-scaled losses tend to be lower for organically managed crop rotations compared to the conventional ones. 

“But this was in fact only significant in one of the two study periods, and we found that the critical N-load was exceeded in both organic and conventional systems,” says Iris Vogeler Cronin. 

Area-scaled losses are lower in organic systems, but looking at the product related nitrate losses, the researchers found it to be the other way around. 

“Due to lower yields in organic systems, product-based nitrate losses were significantly higher than in the conventional systems. This tells us, that under high productive northwest European conditions, stockless organic farming does not agree with the concept of sustainable intensification (SI). Our study revealed that both organic and conventional cropping systems are, under current management practices, not sufficient to meet the requirement of environmental standards for water protection in the EU,” says Iris Vogeler Cronin. 

SI is defined as a process or system where agricultural yields are increased without adverse environmental impact and without the conversion of additional non-agricultural land. 

Integrated instead of specialised systems

According to the researchers there is an urgent need to improve N-management of farming systems to protect the groundwater quality. And there are ways to do so. 

“Traditionally, organic farming has been characterised by mixed farming systems, which combined crop production with livestock, and they had a high percentage of forage legumes in the crop rotation. More recently, there has been a trend toward specialisation and simplification with stockless rotations in both organic and conventional systems,” says Iris Vogeler Cronin. 

The reintegration of livestock into these all-arable stockless organic farming systems could, according to the researchers, be a way of reducing the leaching of nitrate. Another means for sustainable intensification is to have integrated systems with strategic inclusion of crops which ensure high N uptake efficiency during late autumn; this could be grass or grass-clover mixes. They will increase the nitrogen use efficiency as well as increase the plant diversity

“Organic systems need to increase yields, but the potentials for this in specialized arable organic systems are limited. Increased efficiency by using organic manure from animal husbandry can be better realised in organic mixed farming,” says Iris Vogeler Cronin.

According to the researchers there are also some problematic crops within organic rotations. Grass-clover leys, which are often used as the main crop to accumulate and provide N for subsequent crops, are beneficial, when ploughed in spring, but have a high leaching risk when ploughed during the autumn. 

“Organic systems could benefit and get a more effective conversion of grass-clover leys by shifting the date of ploughing from autumn to spring and implement catch crops instead of having bare soils during winter,” says Iris Vogeler Cronin. 

The study suggests that integrated approaches based on a balanced combination of forages, catch crops and cash crops are the way forward for organic and conventional systems to better meet the requirements of environmental water protection.