More variation in the field reduces nitrogen losses

Reducing nitrogen loss from arable land is challenging, but the benefits are significant for the climate, the environment, and farmers’ economies. In soil, microorganisms are constantly at work converting nitrogen. Sometimes harmless nitrogen gas is produced, but at other times the potent greenhouse gas nitrous oxide is formed. This process, called denitrification, is natural but can increase significantly in fertilized soils. This leads to both major nitrogen losses from the soil and nitrous oxide emissions. Therefore, it is important to find cultivation methods that reduce unwanted losses. A European study led by SLU researchers has investigated this.

– Our results show that practices promoting soil biodiversity can involve both benefits and trade-offs. But the really positive finding is that more diverse crop rotations appear to reduce the amount of nitrogen lost to the air relative to the size of the harvest, says Aurélien Saghaï, SLU researcher and one of the lead authors of the study.

Crop rotation diversity pays off

The study shows that fields that are frequently tilled or covered with crops for long periods, such as with cover crops, have a greater capacity for denitrification. But when researchers compared nitrogen losses with field yields, a different pattern emerged: fields with more varied crop rotations emitted less nitrogen per unit of harvested grain. This suggests that crop rotation diversity can deliver both good yields and lower nitrogen losses.

– This is a crucial distinction. If you only look at nitrogen losses alone, it is easy to miss the bigger picture. But when comparing losses to harvest size, it becomes clear that a more diverse crop rotation provides benefits, says Monique Smith, SLU researcher and another lead author of the study.

Soil microorganisms play a vital role

The researchers’ results show that the soil’s capacity for denitrification is not governed by short-term fertilization efforts, but by long-term factors. It depends on how much nitrogen is present in the soil and the composition of microbial communities, i.e., which species and functions, and how many microorganisms live there. The fact that researchers could identify a threshold for soil nitrogen content, but not for most microbial factors, indicates that the microorganisms themselves play a key role in regulating denitrification in arable soil.

– This shows that management practices that affect soil life over the long term can have lasting effects on how nitrogen is cycled in arable land, says Sara Hallin, professor of soil microbiology at SLU and author of the study.

Implications for sustainable agriculture

We already know that more diverse crop rotations can positively affect yield and increase resilience to climate variability. The new results show that it is also possible to reduce nitrogen losses without sacrificing yield.

– There is no simple solution for sustainable agriculture. Different practices often involve trade-offs, but more diverse crop rotations appear to provide multiple benefits simultaneously, concludes Sara Hallin.

Read more

Read the scientific publication in the journal Communications Earth & Environment:
Diverse crop rotations off-set yield-scaled nitrogen losses via denitrification