million, which surpassed that under the Jin. After Xiaowen's rule, Northern Wei's economy began to deteriorate, and famines and droughts undermined Wei rule. From 523 onwards, conservative Xianbei noblemen dominated the north and reversed much of Xiaowen's reforms, setting up their own regimes and warring against each other. It was not until 577 that North China was again reunited by Northern Zhou, which was soon usurped by Han Chinese Yang Jian, who restored native rule over North China. Sui dynasty (581–618) The Sui dynasty was established over the Northern Zhou, whose throne was usurped by Yang Jian in 581. Yang Jian quickly enacted a series of policies to restore China's economy. His reunification of China marked the creation of what some historians call the 'Second Chinese Empire', spanning the Sui, Tang and Northern Song dynasties. Despite its brevity, the Sui reunified China, and its laws and administration formed the basis of the later Tang, Song and Ming dynasties. Sui dynasty China had a population of about 46 million at its peak. One of Yang Jian's first priorities was the reunification of China. Chen, which ruled the south, was weak compared to Sui and its ruler was incompetent and pleasure-loving. South China also had a smaller population than the north. After eight years of preparation, Sui armies marched on and defeated Chen in 589, reunifying China and prompting a recovery in the Chinese economy. The registered population increased by fifty percent from 30 to 46 million in twenty years. To encourage economic growth, the Sui government issued a new currency to replace those issued by its predecessors, Northern Zhou and Chen. They also encouraged foreign merchants to travel to China and import goods, and built numerous hotels to house them. The Sui continued to use the equal-field system introduced by Northern Wei. Every able-bodied male received 40 mou of freehold land and a lifetime lease of 80 mou of land, which was returned to the state when the recipient died. Even women could receive a lifetime lease of 40 mou of land which was returned to the state upon her death. The Sui government charged three "Shi" of grain each year. Peasants were required to perform 20 days of labour for the state per year, but those over 50 could instead pay a small fee. In 604, Emperor Wen was most likely assassinated by his son Yang Guang, who became Emperor