A recent study in Iowa found that nitrogen (N) fertilizer provided a boost to carbon and organic matter in the soil across the state of Iowa.
Longtime convention held that N, particularly anhydrous ammonia, had a negative impact on soil health. Researchers at Iowa State University collected samples for four research farms in 1999, 2000 and then again in 2015. Each research farm received specific applications of fertilizer.
The studies observed that N had an impact on maintaining and improving soil health, due to the increase of the crop residues in the soil. Testing looked at fertilizer levels that were either too high, too low or optimum. Up until the optimum level, the fertilizer improved soil health. Beyond that, there were no improvements.