The study looked at how soil carbon affects maize yield with fertilizer in Tanzania. They focused on active soil carbon, which helps with nutrient availability. They found that active soil carbon predicts how well maize responds to nitrogen fertilizer. The profitability of using fertilizer depends on the prices of maize and fertilizer. When the maize-nitrogen price ratio is low, fewer farmers benefit from using fertilizer. Increasing active carbon in soil could encourage more farmers to use fertilizer and improve agriculture sustainably.