To achieve the same yield, less productive agriculture increases overall GHG emissions
Simply looking at greenhouse gas emissions per hectare gives the impression that applying nitrogen (whether fertilizers or an organic source of nutrients) at the economic rate (which generally corresponds to recommended good agricultural practices) leads to a greater carbon footprint than applying half as much or no nitrogen at all (the purple bars).
However, in order to achieve stable or increased yields, using less than the economic optimum level of N would require expanding production to a greater land area because of lower yields. This land conversion would mean the release of additional carbon (the orange bars) and would lead to a greater overall carbon footprint. Thus it can be concluded that intensive crop production aiming to use resources, including agricultural land, as efficiently as possible reduces global GHG emissions.
In order to analyze which option is the most sustainable solution in the long term, the carbon dioxide release from land use change has been evenly spread over 100 years in these calculations.
Source: Brentrup and Pallière 2009