What evidence would support the claim that farmers would like to slow or stop the process of denitrification?

De-nitrification is the process that removes the useful nitrogen containing compounds from soil. This process is performed by soil bacteria that convert nitrates (NO3) into gaseous nitrogen (N2). When nitrogen is lost from the soil it slows down the proper development of plants growing in that soil.

Farmers are taking important steps to minimize de-nitrification and improve crop production , such as:  

Usage of fertilizers:

They use suitable nitrogen containing fertilizers that maintain soil nitrogen as urea or ammonium and slow down de-nitrification. The selection of fertilizer is very important to lower de-nitrification, for example, the usage of urea containing fertilizers is better for controlling de-nitrification than urea containing fertilizers.

De-nitrification inhibitors:

Farmers also use some de-nitrification inhibitors -the chemical compounds which slow down the rate of de-nitrification. These compounds belong to a specific group of chemicals called "Procyanidins" that restrict denitrifying bacteria from converting nitrates (NO3) into gaseous nitrogen (N2). Therefore, they help in improving the growth of crops.

Cover Crops:

Farmers also grow some cover crops after they harvest the main cash crops from land. The cover crops protect and add nutrients to the soil. The cover crops also capture the nitrates from soil and help in slowing down the process of de-nitrification. Therefore, farmers prefer to grow cover crops and increase the growth and yield of crops.

"d.) carbon dioxide and hydrogen sulfide"