Management Intensity Modifies Plant Diversity Effects on N Yield and Mineral N in Soil

The effect of species loss on productivity were of comparable magnitude to changes in agricultural management. However, not only productivity but also the N cycle is relevant for food production comprising beneficial as well as threatening effects of agricultural management. Our objectives were to test whether grassland management (mowing two or four times a year; 0, 100, or 200 kg N/ha) (i) modifies N yield or soil N-min concentrations and changes (ii) the effect of plant functional groups (legumes, grasses, non-leguminous small and tall herbs) on N yield or soil N-min concentrations or (iii) the relationships between plant diversity and N yield or soil Nmin concentrations. We observed highest productivity under moderate management intensity which can be attributed to the growth-stimulating fertilizer effect associated with enough time for regrowth. With increasing management intensity, the positive effect of the presence of legumes on N yield and soil N-min concentrations disappeared. Plant species richness was positively related to N yield with the most pronounced effect in the highly fertilized and frequently mown treatment. In the control (mown twice a year, non-fertilized), competition for nutrients very likely underlie plant species richness effects. Under higher management intensity, a combination of competition for light and adaption to mowing frequency seemed to be responsible for plant species richness effects. The concurrence of highest N yield in high-diversity mixtures under most intensive management indicates that some plant species must have more than compensated for the N uptake of non-adapted species that did not survive frequent mowing, highlighting the value of plant diversity as an insurance against anthropogenic disturbances including management measures.