Effect of organic fertilisation on soil phosphatase activity, phosphorus availability and forage yield in mountain permanent meadows

Grassland management aims to ensure sufficient yield, forage quality and biodiversity. Robust knowledge supports sustainable management practices. In South Tyrol (NE Italy), we studied the effect of organic fertilisation on soil acid and alkaline phosphatase activity (ACP, ALP, or both APase), phosphorus (P) availability and forage yield in mountain permanent meadows. Three factors were included in the experimental design, which was arranged as a split-plot design: the initial vegetation class (C1 = moderately species-poor or C2 = moderately species-rich), being the main plot, randomised within three study areas, as well as the manure type (slurry, farmyard manure, and a combination of farmyard manure and manure effluent) and the nitrogen (N) fertilisation input (0, 55.5 and 111 kg N ha−1 yr−1), both randomised within the vegetation class. Soil samples were collected from the top 10 cm before the last cut in summer 2022. Results showed that the combined use of farmyard manure and manure effluent decreased ACP activity with increasing N input, whilst ALP activity remained unaffected. These novel findings show that organic N input does not imply an increase in APase activity. Moreover, the C2 meadow class showed higher ACP activity than C1, possibly due to higher species diversity, a lower mowing frequency and the legacy effect of more extensive management prior to the start of the trial. Both ACP and ALP activity responded to pH (negatively for ACP activity and positively for ALP activity) and both were negatively affected by soil moisture, highlighting their sensitivity to changes in the soil conditions. ALP activity was positively influenced by total organic carbon (TOC) and by the Shannon diversity index of the plant communities, possibly due to its link with the soil microbial community. Soil available P increased with pH, TOC, soil moisture, K2O content, and N organic input from farmyard manure, which provided the highest P input. The forage yield of the last growth cycle was positively affected by organic N input but negatively affected by TOC, whilst the activity of both APase had no effect on it. The annual yield increased with the N input and was higher in the C1 meadow class than in the C2. On the whole, the results suggest that organic fertilisation, rather than APase activity, was the main driver of forage yield.