Xi'an is the latest Chinese city to accept Alipay on its subway system, according to local reports. The system started to accept Alipay as of Jan. 1.

As part of the initiative, riders can participate in a program meant to encourage more "green" ways of travel such as public transit.

Once an Alipay user accumulates a certain amount of green "energy" from using the mobile wallet to pay for subway fares, Alipay's partners, such as the Alxa SEE foundation, will plant a real tree in areas suffering from desertification upon the user's request.

 Alipay is now accepted on public transport in more than 30 Chinese cities, including Hangzhou, Wuhan, Tianjin, Qingdao and Guangzhou. Zhengzhou became the first Chinese city to adopt mobile payments in its subway system in September, followed by Beijing and Shanghai.