COPENHAGEN - Denmark will convert 15 percent of its farmland into forest and natural habitats in an effort to reduce fertilizer usage, which has resulted in severe oxygen depletion in Danish waters as well as the loss of marine life, lawmakers said on Monday.

Denmark, among the most intensively cultivated countries in the world with almost two-thirds of its territory farmed, set aside 43 billion Danish crowns ($6.1 billion) to acquire land from farmers over the next two decades.

Under the deal, which also makes Denmark the first country to impose a carbon tax on agriculture, the Nordic country plans to plant one billion trees on farmland over the following 20 years, according to the ministry for the Green Tripartite agreement.

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The ministry was created in August to implement a green deal reached in June between farmers, industry, labor unions and environmental groups.

Reducing emissions from agriculture, Denmark's largest source of greenhouse gases, has been a major hurdle for lawmakers seeking to achieve a legally binding 2030 target of cutting greenhouse gas emissions by 70 percent from 1990 levels.

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Oxygen levels in Danish waters reached alarmingly low levels this year, due to the runoff of nutrients from fertilizers in lowlands.