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

Here, we evaluate the impact of different nutrient input practices (no fertilizer addition, farm yard manure, mineral fertilizer (N:P:K) and farm yard manure plus mineral fertilizer) on indicators supporting both biodiversity and livestock production. We used information on four components of biodiversity: plants, pollinators, herbivores and parasitoids, and forage nutritional content collected in a restored grassland, managed under each of the four nutrient input regimes for 27 years. Our research objectives are as follows: (a) compare the performance of the four different fertilizer regimes on indicators relating to plant community, insect com- munity, forage yield and forage nutrient content, (b) evaluate the trade-offs of each fertilizer practice on biodiversity and forage production, (c) investigate which fertilizer regime opti- mizes the indicators collectively, noting that this is not the same as achieving maximum values of each indicator, and (d) use our results to estimate the potential livestock production under each treatment. Materials and methods Field site and experimental design The study was carried out at the Colt Park Hay Meadow Field Trial, a long-term nutrient manipulation experiment established in 1991, based in Ingleborough National Nature Reserve (North Yorkshire, England, grid reference SD775782; 54˚12’N, 2˚21’W). At the start of the experiment the grassland was dominated by the perennial grass species Lolium perenne and Cynosurus cristatus. The topsoil was a clayey brown earth over limestone bedrock (pH ~5.8;

8.9 C%; 0.92 N% [31]). The aim of the experiment was to test different management strategies for improving the plant species diversity of grasslands within farms. Strategies included seed addition during the first 4 years (1990–1994) and different nutrient inputs, including no input [12] for 27 years. The experiment is managed as part of a working farming system typical in the region. Man- agement involves sheep grazing from March to mid-May, application of the fertilizer treat- ments in late May, hay cutting after 21 July, and sheep grazing for a further two weeks after the hay-cut, followed by cattle grazing during early winter. Fertilization application is done follow- ing the maximum loads allowed by agri-environmental schemes. It consists of the addition, once per year, of the following four treatment of nutrient inputs: (1) farmyard manure (12 t ha-1 yr-1, with an average content of 25 kg of dry matter (300 kg ha-1 yr-1) and nutrient compo- sition of 6 kg of nitrogen (72 kg ha-1 yr-1), 3.5 kg of phosphate (42 kg ha-1 yr-1) and 8 kg of pot- ash (96 kg ha-1 yr-1) per tonne of which only 20% are available for plants following application [32]), hereafter FYM; (2) low levels of mineral fertilizer (in the ratio 20 N:10 P:10 K, 25 kg ha-1 yr-1 N plus 12.5 kg ha-1 yr-1 of P2O2 and K2O) hereafter referred to as NPK; (3) both fertilizers together (FYM+NPK equivalent to 97 kg ha-1 yr-1 N, 54.5 kg ha-1 yr-1 of P2O2 and 108 kg ha-1 yr-1 K2O); and (4) a no-fertilizer control, hereafter no-fertilizer. There were six replicates of

each treatment, each 2.5 m x 6 m (15 m2) in size, arranged in three blocks. In each block, the six plots were randomly assigned to each fertilizer treatment and the same fertilizer treatment was applied once per year for 27 years. Data collection We collated data on the effect of the four treatments from the 27-year field experiment on 12 indicators covering plant communities, insect communities and forage in terms of yield and nutrient content (Table 1). These indicators proved to be independent as no correlations equal or higher to 90% were seen among variables (Table 1 in S1 File). We set a threshold of 90% due to the fact that correlations between variables are expected as they respond jointly to fertil- ization [33]. Setting such a threshold allow us to detect variables that are not independent. For 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 3 / 18