High Ca and P homeostasis ensure stable forage Ca:P following historical nitrogen inputs in a temperate steppe

Calcium (Ca) and phosphorus (P) are essential nutrients for plant growth, with consequences on ruminant diet and health. Their balance in forage, as indicated by Ca:P ratio, is important for the absorption and utilisation of both elements by livestock. While nitrogen (N) deposition has been globally observed, it is declining or is projected to decline due to controls on N emissions in many countries. It is an open question whether N inputs would have legacy effects on forage Ca and P nutrition in grasslands. Here, we examine the changes of Ca and P concentrations and Ca:P ratio in plants and soils in a temperate steppe for three continuous years (2016 ~ 2018) following historically 6 years N inputs (2008 ~ 2013) with a gradient ranging from 0 to 20 g N m⁻² year⁻¹. Soil extractable Ca concentrations significantly decreased with increasing N addition rates in all the 3 years, but soil available P concentrations increased and Ca:P ratio decreased only in 2016. Ca and P nutrition varied greatly amongst the four plant functional types, but they were generally conservative across the historical N addition rates, due to the high stoichiometric homeostasis. Furthermore, plant Ca and P concentrations and Ca:P ratios at the community level showed no significant variation along the N addition gradient. Our results highlighted the role of plant Ca and P homeostasis in ensuring stable Ca and P nutrition in forage following historical N inputs in temperate grasslands.