Multi-years averages of crop yields, N uptake and field N surplus (supply-removal) from more than 100 long term experiments (LTEs) across Europe were analyzed. The susceptibility of results to climate, crop, soil type and duration of practice were tested to evaluate which conditions mostly affected the performance of a practice.
In more than 80% of the cases, a crop grown in a rotation out yielded the crop grown in a monoculture, and the average increase in yield was 5%. Best performances were obtained in Western Europe climate, sand or loam soils, wheat or grain maize, and in long-lasting experiments (10-20 years). N uptake was also increased and N surplus was reduced by c. 40 kg ha-1. In 60% of the cases, the use of a harvested catch crop (both leguminous and non-leguminous) resulted in a yield increase of the main crop (by 5%, on average), and best results were obtained in Eastern Europe, soils other than silt, barley, maize or minor cereals, and in long-lasting experiments. N uptake was also increased in 80% of cases, and consequently N surplus was reduced. Little or no effect of green manure on yield and N uptake was observed, in all pedo-climatic conditions explored. This means that the performance of green manuring could not be predicted on the basis of the considered factors.
A reduction in yield and N uptake is to be expected when no tillage is applied, but on average this reduction was limited to 4%. Silt soils performed best. Similar but less favourable results on yield and N indicators were obtained using minimum tillage (defined as non-inversion tillage at a shallower depth than ploughing). Furthermore, the performance of this technique on yield was not influenced by the factors here considered, whereas N uptake was increased and N surplus was reduced in Western Europe. When organic and mineral fertilizers were applied at the same N rate, similar crop yield and N uptake were observed. However, the performance of organic fertilizers depended on the soil type (best results in coarse-textured soils), climate (the colder, the better) and duration of practice (more than 5-10 years). Incorporation of crop residues caused a reduction in yield by 7% (probably because of N immobilization) especially in badly-structured soils, in all crops. Quite surprisingly, burning cereal straw positively affected yield and N uptake by 3%, while slightly reducing N surplus, although observations were mostly located in Western Europe and sandy soils.
In conclusion, when assessing farm-compatibility of good management practices several aspects have to be considered. Promoting productivity, climate change mitigation, and soil quality at the same time can sometimes be difficult to achieve. LTEs provide plenty of information that rarely is aggregated, compared and well exploited. Nitrogen management interacts with most other farm practices. In order to reach a higher N use efficiency (NUE), a simultaneous and complete analysis of many interactions must be considered. Only when this is done it is possible to identify bottlenecks and potentially overcome the social and economic barriers that slow the adoption of good practices for an efficient N use.
Response of yield, N uptake, N use efficiency and difference in N surplus in the comparison adoption vs non-adoption of a BMP
Table 1: Summary of relative response rate (RR) and difference (DIFF) of indicators for productivity when applying potential BMPs. ++: very positive effect; +: positive effect; 0: neutral effect; -: negative effect; --: very negative effect. All compared to the baseline treatment indicated in grey.