The phrase "as the biggest source" is correct and usable in written English.
It can be used when referring to something that is the largest or most significant contributor to a particular context or situation.
Example: "The company has been recognized as the biggest source of renewable energy in the region."
Alternatives: "as the primary source" or "as the largest contributor".
But China has already supplanted India as the biggest source of foreign direct investment in Nepal.
Among overseas students studying in the UK, China surpassed the EU as the biggest source of students.
The number moving to the UK to study has grown year-on-year since 2008, with the country counting as the biggest source of international students.
GCHQ The surveillance arm of British intelligence, normally credited as the biggest source of intelligence, GCHQ first found data on Adebowale in 2011.
China is set to take over from the United States next year as the biggest source of growth in global ad spending.
Countries such as Uganda have actively courted Chinese companies, to good effect: in 2010 China replaced the UK as the biggest source of foreign direct investment.
Couples cited frivolous purchases, household budgeting and credit card debt as the biggest sources of friction.
Conversely, discussions in Viwandani revealed that industries were perceived as the biggest sources of air pollution.
Mike: Maybe not so much a surprise as it is the biggest source of joy for me, but the authors we've been lucky enough to work with and get to know have been the absolute best.
As an example, the biggest source of excitement leading up to Yellen's press conference was about the omission of three words regarding when the central bank would raise interest rates: "in considerable time".
The student spying stories come as the U.S. is trying to encourage more Americans to study in China, and as China has become the biggest source of foreign students in U.S. colleges and universities.
