The main challenger to President Muhammadu Buhari in next year's presidential poll has signed a deal committing to peaceful elections a day after missing the initial ceremony in the capital."Earlier today, I appended my signature to the Peace Accord," Atiku Abubakar of the Peoples Democratic Party (PDP) wrote on Twitter late Wednesday. Abubakar, 72, was conspicuously absent from Tuesday's ceremony in Abuja at which Buhari and dozens of other presidential candidates signed a deal to ensure non-violence during February's crucial polls. The PDP had put Abubakar's absence down to a "communication lapse between the National Peace Committee (NPC) and our party's national secretariat". On Wednesday, Abubakar signed the pact with his running mate Peter Obi.Security Local media said another presidential candidate, Oby Ezekwesili, a former education minister and ex-World Bank vice president who was also absent, had signed as well. Voters in Africa's most populous nation will choose a new president and parliament on February 16 next year, with gubernatorial and state assembly elections following two weeks later. Security around polling is a recurrent concern in Nigeria which has a history of violence sparked by identity politics, ethnicity and religion. In 2011, about 1 000 people were killed during clashes after Buhari was defeated by Goodluck Jonathan. The text of Tuesday's agreement is the same as one signed before the 2015 polls, which Buhari won in the first opposition victory in the nation's history. * Sign up to News24's top Africa news in your inbox: SUBSCRIBE TO THE HELLO AFRICA NEWSLETTERFOLLOW News24 Africa on Twitter and Facebook.