Interactions between cattle grazing, plant diversity, and soil nitrogen in a northeastern US coastal grassland

Questions Can cattle grazing help maintain plant diversity in coastal grasslands? What is the relationship between soil extractable nitrogen and plant diversity in sandy and infertile soils? How does cattle grazing affect soil extractable nitrogen and how might this indirectly impact plant diversity via the N-plant diversity relationship? Location Grazed coastal grasslands on Naushon Island, off of Cape Cod, MA, USA. Methods We surveyed summer vegetation from 2014 to 2017 and analyzed soils for extractable nitrogen (N) in two grazed grasslands on Naushon Island. We also set up cattle enclosures and exclosures to manipulate grazing pressure and test how cattle grazing influenced plant diversity (species richness and Shannon's diversity) and extractable NH4+ and NO3-. In the enclosure experiment, cattle were enclosed for up to a week in the mid-summer of 2014, 2015, and 2016. For the exclosure experiment, we excluded cattle from areas from 2014 to 2017. Results In both the enclosure and exclosure experiments, higher grazing pressure corresponded to less negative change of plant species richness and Shannon's diversity from 2014 to 2017. Total extractable N had a weak positive correlation with plant diversity, but increasing extractable NO3- correlated with decreasing species richness (p = 0.001) and Shannon's diversity (p = 0.009). Neither the cattle enclosures nor exclosures were related to soil extractable N. Conclusions Cattle grazing may help prevent or slow losses of plant diversity that are occurring in many coastal grasslands, including Naushon Island. This effect does not seem to be regulated by cattle deposition of N in the soil or the N-plant diversity relationship. Our results of the N-plant diversity relationship differ from the mainstream understanding, perhaps due to Naushon's sandy and infertile soils.