Final answer:
Diocletian reformed the Roman tax system by introducing an agricultural land tax and a tax on individuals, while also including Italian properties in the taxation and standardizing a five-year census.
Step-by-step explanation:
Emperor Diocletian reformed the Roman tax system in several key ways. He introduced two main new taxes: an agricultural land tax and a tax on individuals. Importantly, for the first time, property in Italy was included in the land tax. Diocletian's reforms came at a critical time when the Roman Empire was in financial crisis due to constant wars and a stretched central treasury. To address this, he standardized a five-year census which dramatically increased revenue for the empire and helped stabilize the economy in the short term.