Fertilization using manure minimizes the trade-offs between biodiversity and forage production in agri-environment scheme grasslands

Maintaining biodiversity and livestock production under different fertilizer regimes Our results showed that FYM addition is the regime best able to optimize plant community and insect community diversity while sustaining forage production. This is consistent with the notion that organic fertilizers enhance production but with less impact on biodiversity [54]. Surprisingly, we did not find any significant differences between FYM and the no-fertilizer treatment regarding to some individual indicators related to biodiversity, although forage per- formance on average was 34% higher in FYM. Interesting, we observed that forage indicators were in average 1.6% lower in FYM than in NPK +FYM. However, the impacts of NPK+FYM on the plant community was 62.5% and 100% higher for bumblebees than FYM despite nitro- gen inputs being relatively low compared to intensive grass forage systems. In this sense, pro- ducing forage by the addition of FYM seems a more sustainable alternative than the addition of higher fertilizer amounts that have proved to be detrimental to the environment in different experiments [55]. Moreover, incorporating FYM rather than mineral fertilizer constitutes an efficient use of biomass and nutrients produced within the farming system in previous grow- ing seasons and eventually returned as animal waste to be processed below-ground [56]. It is important to highlight that the addition of FYM decreases biodiversity as observed in this study and other studies e.g., [12,57,58], with the negative impacts escalating to higher tro-

phic levels of invertebrates [33]. Moreover, in plots within Colt Park trial where seed addition was carried out to accelerate plant community reassembly and improve nectar productivity, FYM still resulted in reduced plant diversity relative to a complete lack of fertilizer [33,59] indicating that the addition of FYM could slow down the restoration of plant diversity in grass- lands [12]. Nevertheless, the fact that percent of parasitism in this treatment is higher than in NPK and NPK+FYM is an important difference. The three fertilizer addition treatments increased abundance of leaf miners and parasitoids [33], but the percentage of leaf miners attacked by parasitoids was higher for FYM suggesting better pest control under this treat- ment. Overall, while we cannot conclude that FYM is the best practice to maximize biodiver- sity in our field site, we can conclude that FYM is the optimum treatment because it sustained higher levels of forage productivity alongside simultaneously lower impacts on biodiversity. Substitution of manure by inorganic mineral fertilizers has contributed to the widespread decrease of plant diversity in grasslands [60]. Nevertheless, some studies consider that low mineral fertilization rates can increase productivity without damaging biodiversity in grass- lands [61,62]. We observed that the addition of NPK yields minor improvement in forage pro- duction as it does not increase the nutritional content of the hay, but it does have negative impacts on biodiversity as it reduces bumblebee abundance and it may have negative effects

on pest control by reducing parasitism. Our results therefore provide no support for introduc- ing low NPK to sustain livestock production in this upland grass forage system [see also 61,63– 65]. Indeed, since the critical load for nitrogen for European low and medium altitude mead- ows is 20–30 kg N ha-1 yr-1, negative effects on biodiversity are expected even at this low appli- cation rate. Finally, in our field site we did not observe increases in productivity due to higher plant diversity since the unfertilized control had the lowest yield (mean dry weight), which in combi- nation with low crude protein and ruminant metabolize energy were estimated to be insuffi- cient to support beef production. It was suitable for mixed ewes though. One solution to maintain animal production under this type of management is to consider a broader range of livestock, an approach which is been used in some restoration projects where the aim is to increase biodiversity on working farms [66] This type of management requires a shift in per- spective; from intensifying the grassland system to meet the production requirements of PLOS ONE Manure minimizes trade-offs between biodiversity and forage production in agri-environment scheme PLOS ONE | https://doi.org/10.1371/journal.pone.0290843 October 4, 2023 13 / 18