Effect of Organic Nitrogen Management on Soil Enzyme Activities and Microbial Functional Capacity in an Organic Farming System.

Organic farming systems use organic amendments to maintain soil fertility and supply nutrients for plant growth. However, there is little information about the response of soil nitrogen (N) cycling functions and functional genes to different quality and quantity of organic amendments. This study investigated the effect of organic fertilizers (control, compost, and manure), and their interaction with cover crops (milet, buckwheat, and legume) on soil enzyme activities, N transformation rates and functional gene abundances under an organic production system. Organic N fertilizers had a stronger effect than cover crops on soil functions and functional gene abundances. Soil enzyme activities were increased by both compost and manure, but there was no difference between compost and manure. Nitrification potential, nitrite oxidation potential, and denitrification potential in manure treatment were significantly higher than those in control and compost treatment, indicating the application of manure had a higher N loss potential than compost application in this organic farming system. Organic N fertilizers showed no effect on the abundance of archaeal amoA, npr, chiA, but significantly increased the abundance of bacteria amoA, ureC, and sub. The activity of ammonia oxidizing bacteria and archaea were both increased by organic N fertilizers, and their activities were higher in manure treatment than in compost treatment. Overall, abundance of functional gene was significantly correlated with the corresponding enzyme activity. However, functional gene abundances were less important than soil geochemical properties to explain the variation in corresponding enzyme activities.