We've heard it before, diversity in the cropping system can improve yields and even help reduce the amount of inputs such as fertiliser - and ultimately a diverse cropping system can help reduce the impact on the environment. This is particularly true for legume crop rotations.

"It is a common belief that there are many benefits in crop rotation from the use of legumes, but this has never really been systematically studied in a global context. So, we have analysed data from all over the world, a total of 11,768 yield observations from a total of 462 field trials, to compare cropping systems with and without legumes," explains Professor and Head of Department Jørgen E. Olesen from the Department of Agroecology at Aarhus University.

Up to 20% increase in yield

The analysis shows that including legumes in your cropping system can increase yields from the main crop by up to 20%. But according to Jørgen E. Olesen, it is not the case that just because you grow legumes before a cereal crop, for example, you do not automatically get a 20% increase in cereal yield.

"There are several factors at play here. It's about how much you fertilise and how diverse the farming system already is. And then, of course, there are other factors, such as soil fertility, rainfall, temperature and so on. But our data clearly shows that the more you fertilise, the less effect legumes will have on yield," he explains.

Big impact in organic systems

In other words, legumes are most beneficial in systems where there is little or no fertilisation.

"That means legumes have a particular advantage when we're talking about organic farming, or areas where you don't have access to fertiliser. This could be in developing countries where fertiliser is simply too expensive. Because the nitrogen from the legumes and the nitrogen from the fertiliser is linked in such a way that the plants need a certain amount of nitrogen, and if they don't get enough from the fertiliser, it's an advantage if they can get it from the previous crop, which they can if it contained legumes," says Jørgen E. Olesen.

The same goes for the diversity effect, as Jørgen E. Olesen calls it. It's important in terms of sustainability. According to the researchers, a greater effect of legumes is seen in systems with very low diversity, while systems that already consist of diverse crops do not experience as great an advantage.

"We clearly see that there is a much greater effect of legumes in places where you don't fertilise than in places where you do fertilise. The same is true for diversity. If you have a monoculture, i.e. you grow the same crop year after year, and you introduce legumes into the system, you will experience a much greater effect than if you already have a large variety of crops in the cropping system," he explains.

Diversity reduces environmental impact

The diversity effect increases sustainability. And cropping systems with high diversity can help reduce fertiliser use, and they can help reduce the need for plant protection, i.e. pesticides.

"Enhancing biodiversity in cropping systems can help promote ecosystem services such as preventing diseases and pests, as well as increasing soil carbon sequestration and soil fertility. This has the potential to reduce dependence on fertilisers and pesticides, while at the same time maintaining high yields and stable production," says Jørgen E. Olesen.

So, there are several advantages to using legumes. And these is created by the interaction of the nitrogen effect and the diversity effect. Because if you have a monoculture, you need to fertilise more than if you have a high diversity in your cropping system. But if you have the diversity from legumes, then you don't need to fertilise as much to get the same yield.

"That's why we see a bigger impact in the simpler systems when you introduce the legumes. They add nitrogen to the subsequent crop, which therefore doesn't need as much fertiliser," says Jørgen E. Olesen.

Legumes are important for future crop production

In other words, introducing legumes into cropping systems can be a strategy for sustainable agriculture. This is why scientists are concerned that recent years have seen a decline in legume cultivation globally, a plunge caused by low and unstable yields in legumes. But the researchers believe that building crop systems based on legumes is likely to improve local and global crop production while minimising negative environmental impacts.

"Our analysis shows that legumes in crop rotations can help increase global production, especially if legumes become a larger part of organic and other low fertility or low diversity farming systems," says Jørgen E. Olesen.