Effect of margin strips on soil mineral nitrogen and plant biodiversity

We studied the effects of two- to three-year-old unfertilized field margin strips, installed between the pre-existing field boundary and the field crop, on soil ammonium N and nitrate N, and on the botanical composition of the adjacent semi-natural vegetation in the field boundary. Margin plots were regenerated spontaneously or were sown to grass/forb mixtures and were managed under a cutting regime with removal of cuttings. In general, soil nitrate N, soil ammonium N and soil mineral N losses were significantly affected by distance from the field crop edge and not by plant community type. The further away from the crop edge, the lower soil nitrate ( up to fivefold lower than in the crop) was found in the margin strip, but soil ammonium N was approximately 50% higher close to nearby trees and shrubs. Inside the margin strip, total soil mineral N as well as N loss during winter was minimal at a distance of 5 m from the crop edge. The reduction of soil nitrate N near the boundary by the presence of a margin strip was responsible for the increase in abundance of less competitive species and for an up to 20% higher species-richness within the field boundaries. In summary, our results show clearly that margin strips both decrease N pollution of groundwater and increase botanical diversity. A minimal margin width of 5 m is recommended.