Leaching of dissolved organic nitrogen in long-term organic and conventional crop rotations in Denmark

Nitrate leaching has been widely studied in agricultural cropping systems, whereas there are few studies of dissolved organic nitrogen (DON) leaching. Moreover, the factors determining variation of DON leaching in arable cropping systems remain unclear. Here, we examined variations in DON leaching from two organic crop rotations (Organic with Green Manure (OGM) and Organic with Grain Legume (OGL)) and one conventional rotation (Conventional with Grain Legume (CGL)) with and without the use of cover crops and animal manure application. These treatments were part of long-term experiments conducted at three sites in Denmark with different soil types and precipitation. Measurements were conducted in 2008 at all three sites (Jyndevad, Foulum and Flakkebjerg), and 2015 and 2017 at Foulum. We found that the total annual nitrogen (N) leaching ranged from 6.0 to 113.1 kg ha−1 yr−1 at the three rotations. DON leaching ranged from 1.6 to 11.6 kg ha−1 yr−1 under different crop rotations, accounting for 4.5–37.0 % of the total N leaching. The largest DON leaching (about 10 kg N ha−1 yr−1) was found on coarse sandy soil with high rainfall (Jyndevad) and the lowest DON leaching (about 1 kg N ha−1 yr−1) on sandy loam with low rainfall (Flakkebjerg). In the long-term experiment with loamy sand and medium rainfall (Foulum) in 2015 and 2017, cover crops reduced DON leaching in most cases, while manure had no significant effect on DON leaching. On average across different treatments in 2015 and 2017 at Foulum, grass-clover showed the lowest DON leaching (3.3 kg ha−1 in OGM rotation), whereas faba bean had the highest DON leaching (6.5 kg ha−1 in OGL rotation and 6.2 kg ha−1 in CGL rotation). The results demonstrate that DON leaching should be considered when assessing N leaching losses from agricultural cropping systems, in particular when assessing the effectiveness of N mitigation measures that largely affect nitrate leaching and not the losses of DON.