Neb. Rev. Stat. §§ 19-501
Whenever, in any city having a population of more than five thousand inhabitants as determined by the most recent federal decennial census or the most recent revised certified count by the United States Bureau of the Census, a charter convention shall have prepared and proposed any charter for the government of such city or any amendments to the charter previously in force, it shall be the duty of the city clerk to also publish and submit, at the same time and in the same manner as in the case of the submission of such proposed charter, any additional or alternative articles or sections, to the qualified voters of such city for their approval, which shall be proposed by the petition of at least ten percent of the qualified electors of such city voting for the gubernatorial candidates at the next preceding general election. The petition must be filed within thirty days after the work of such charter convention shall have been completed.
Neb. Rev. Stat. §§ 19-501