President Barack Obama's income fell to just over $436,000 in 2015, the lowest of his presidency, and he and first lady Michelle Obama paid nearly $81,500 in taxes, according to tax returns the White House released on Friday.

The president's more modest adjusted gross income reflected book sales that have continued to fall off in the years since he last published in 2006. The first family paid an effective tax rate of 18.7 percent.

The Obamas also reported donating $64,000 to charity, or about 15 percent of their income. Like last year, their largest charitable donation, $9,000, went to the Fisher House Foundation, which assists wounded military members and their families.

The 45 pages of documentation, which also includes an Illinois state tax return, shows a presidential salary of about $395,000, plus another $61,000 in payments stemming from Obama's two best-selling books. Last year, the couple reported making $94,889 from book sales -- a sharp decline from the millions Obama made when book sales surged after his 2009 inauguration.

Obama and the first lady lowered their tax bill by claiming more than $145,000 in itemized deductions, including tax-deductible donations. Overpaid on their taxes, the Obamas will be receiving a refund of nearly $23,000.

In addition to Fisher House Foundation, long the Obamas' favored charity, the couple donated $5,000 to the Beau Biden Foundation, established in honor of Vice President Joe Biden's son who died of brain cancer last year. They contributed $5,000 to Sidwell Friends School, which daughters Sasha and Malia attend, while giving smaller amounts to nearly three dozen charities including AIDS United, Habitat for Humanity, United Negro College Fund and the Sandy Hook Promise Foundation.

The Obamas reported paying just over $16,000 in state taxes in Illinois, where they own a home.

Issues of tax fairness and politicians' wealth have been at the center of this year's presidential campaign, driven largely by Bernie Sanders' calls for making the wealthy pay more and intense public interest in Donald Trump's personal wealth. Hillary Clinton has released 30 years of her tax returns, while Trump has refused to release any. Sanders said he also planned to release returns on Friday.

"While we've made progress toward ensuring that the wealthiest Americans pay their fair share, there is more work to do," White House spokesman Josh Earnest wrote in a blog post accompanying the release of Obama's taxes. "We need to close special tax loopholes for millionaires and billionaires, and invest in the middle class."

The vice president and his wife, Jill Biden, paid more than $91,500 in federal taxes last year on adjusted gross income of more than $392,000, for an effective rate of 23.3 percent. They paid smaller amounts in state taxes in Delaware, where they own a home, and in Virginia, where Jill Biden works as a community college professor.

The Bidens reported more than $6,500 in donations to charity, including $2,400 to the Catholic Diocese of Wilmington, Delaware. They also gave $100 to "Mother Emanuel" African Methodist Episcopal Church in Charleston, South Carolina, where nine parishioners were gunned down last year. They claimed $300 in deductions for clothing donated to a Delaware clothing bank.

The Bidens also brought in about $19,000 in rental income, after mortgage and tax expenses, from a cottage on their property that they rent to Secret Service officers assigned to protect their Delaware home. The Bidens will get a federal refund of less than $200.