By Paul Carsten BEIJING (Reuters) – Tencent Holdings Ltd will buy a 15 percent stake in e-commerce firm JD.com for $214.7 million, as the two seek to challenge Alibaba Group Holdings dominant position in online shopping in China. Tencent also plans take an additional 5 percent of JD.com on a post-IPO basis, and Tencent President Martin Lau will take a seat on JD.coms board of directors. Although Alibaba is by far Chinas most dominant e-commerce firm, it has been losing ground to Tencent on mobile as smartphone and tablet usage has surged over recent years. Under the agreement, JD.com will take control of Tencents own, unsuccessful e-commerce businesses, which will be 100 percent owned by JD.com.