Contrasting Effects of Nitrogen and Organic Fertilizers on Iron Dynamics in Soil after 38–Year Fertilization Practice

Abstract

Various environmental factors and anthropogenic practices can affect the Fe biogeochemical cycles in soils. Nitrogen and carbon states are closely associated with Fe dynamics. However, we still have a limited understanding of the complex response of Fe biogeochemical processes to long–term nitrogen– and organic–fertilization regimes. This study investigated the Fe fraction and distribution, as well as the link between Fe and nitrogen/carbon, in bulk soil and in soil aggregates. The results showed that the long–term application of the nitrogen fertilizer increased the contents of water–soluble iron (Ws–Fe) and carbonate–bound iron (Ca–Fe) in the bulk soil and various sizes of aggregates, as well as the iron contents in soybeans. The decreased pH and enhanced Feammox reaction in response to the nitrogen–fertilizer treatments were responsible for the increase in the Ws–Fe and Ca–Fe fractions. By contrast, the long–term application of the organic fertilizer decreased the contents of Ws–Fe and Ca–Fe, while it increased the contents of Ox–Fe and Or–Fe. Moreover, the contents of Ox–Fe and Or–Fe were positively correlated with the organic–carbon contents in the micro–aggregates of 0.053–0.25 mm and <0.053 mm. These results indicated that the long–term use of the organic fertilizer encouraged Fe immobilization in organo–inorganic compounds. However, the application of the nitrogen fertilizer alleviated the Fe retention induced by the organic fertilizer. In conclusion, long–term nitrogen and organic fertilization have contrasting influences on the mineralogy and availability of Fe in soil. This study is useful for understanding the mechanism underlying the interaction between Fe and nitrogen/carbon, as well as Fe’s phytoavailability in response to different fertilization practices in brown soil.