*Article* **Intraspecific Variation in Functional Traits of** *Medicago sativa* **Determine the Effect of Plant Diversity and Nitrogen Addition on Flowering Phenology in a One-Year Common Garden Experiment**

**Yue Ma 1,2, Xiang Zhao 1,\*, Xiaona Li <sup>2</sup> , Yanxia Hu <sup>2</sup> and Chao Wang 2,\***

<sup>1</sup> College of Grassland Science, Shanxi Agricultural University, Jinzhong 030801, China

2 Institute of Grassland, Flowers and Ecology, Beijing Academy of Agriculture and Forestry Sciences, Beijing 100097, China; sunny3629@126.com (Y.H.)

**\*** Correspondence: sxndzhaox@126.com (X.Z.); wangchao@grass-env.com (C.W.)

**Abstract:** Nitrogen deposition and biodiversity alter plant flowering phenology through abiotic factors and functional traits. However, few studies have considered their combined effects on flowering phenology. A common garden experiment with two nitrogen addition levels (0 and 6 g N m−<sup>2</sup> year−<sup>1</sup> ) and five species richness levels (1, 2, 4, 6, and 8) was established. We assessed the effects of nitrogen addition and plant species richness on three flowering phenological events of *Medicago sativa* L. via changes in functional traits, soil nutrients, and soil moisture and temperature. The first flowering day was delayed, the last flowering day advanced, and the flowering duration shortened after nitrogen addition. Meanwhile, the last flowering day advanced, and flowering duration shortened along plant species richness gradients, with an average of 0.64 and 0.95 days change per plant species increase, respectively. Importantly, it was observed that plant species richness affected flowering phenology mainly through changes in plant nutrient acquisition traits (i.e., leaf nitrogen and carbon/nitrogen ratio). Our findings illustrate the non-negligible effects of intraspecific variation in functional traits on flowering phenology and highlight the importance of including functional traits in phenological models to improve predictions of plant phenology in response to nitrogen deposition and biodiversity loss.

**Keywords:** functional traits; abiotic factors; flowering phenology; nitrogen addition; plant diversity; common garden
