UC Davis

Attempts at understanding controls and drivers of soil nitrification, the process by which ammonium (NH4+) is converted to nitrate (NO3-), often omit the role of iron (Fe) minerals. However, these minerals are widespread components of soils and sediments and their involvement in both the enzymatic and non-enzymatic reactions that influence the N cycle, including nitrification, is well-known. In soils and sediments experiencing fluctuation in pH or redox conditions, Fe often coprecipitates with organic carbon (OC), which yields iron-organic coprecipitates (Fe-OC) or flocs. These soil components are critical to stabilizing carbon (C) against microbial decay and determining Fe reactivity, which may limit Fe participation in nitrification. Moreover, Fe-OC flocs may affect nitrification by controlling the availability of trace metals (e.g., Fe, molybdenum (Mo), etc.) and nutrients that are required for microbial growth, metabolism, and activity. In this dissertation, I explored these possible interactions with the goal of providing mechanistic descriptions of how Fe-OC affect nitrification in agricultural soils. The dissertation document is structured around 3 chapters:  In the first chapter, I have taken a general approach and reviewed the processes by which Fe affects N bioavailability in soils. To do so, I categorized these processes into four different categories/roles. In fact, Fe affects N bioavailability directly by acting as a sorbent, catalyst, and electron transfer agent, or indirectly by promoting certain soil features, such as aggregate formation and stability, which affect N turnover processes. Then, I explored the possible outcomes of these roles on N bioavailability as influenced by soil environmental conditions, such as redox status. Finally, I highlighted research needs for each category of roles and detailed the analytical framework needed for a complete understanding of Fe-N interactions in soils.  In the second chapter, I researched the mechanisms by which Fe-OC flocs affect nitrification in agricultural soils. To do so, I used flocs of different chemistry (aromatic and aliphatic) and known Fe and C composition to investigate their effects on nitrification in soils along a soil C gradient. I found that in mineral soils (< 3% C soil), Fe-OC flocs dramatically reduce net nitrification by restricting the availability of molybdenum (Mo) to the nitrifying communities. In fact, these microbes use Mo as cofactor to oxidize nitrite (NO2-) to NO3-. This reduction in Mo bioavailability possibly originates from its incorporation into or adsorption to flocs or their decomposition products. In contrast to mineral soils, Fe-OC flocs reduced net nitrification to a lesser extent in organic soils (>3% C), likely because organic matter limited floc adsorption capacity and/or released Mo through mineralization.  In the third chapter, I was intrigued that, even though Fe-OC flocs decreased net nitrification by restricting Mo bioavailability, supplying Mo to a soil did not reverse their effects on nitrification. I found that beside affecting Mo bioavailability, Fe-OC flocs changed soil nutrient status. Generally, flocs increased water-soluble Fe, copper (Cu), nickel (Ni), phosphorus (P), zinc (Zn), manganese (Mn), magnesium (Mg), aluminium (Al), cobalt (Co), calcium (Ca) and potassium (K)) while decreasing water-soluble Mo in mineral soils. We interpreted these results in the light of the current knowledge on the influence of nutrient on microbial processes. We apply the principle of the Liebig’s Law of the Minimum and knowledge on nutrient toxicity to microbial communities to build a conceptual framework of the possible links among Fe-OC flocs, soil nutrient status and nitrification in soils.

The results of this dissertation give new insight into mechanisms by which Fe affects nitrification in soils. Also, because flocs originate from engineering systems like wastewater treatment and their retention in wetlands was proposed as a strategy to build soil C stock and reverse land subsidence in wetlands, it is important to address their effects on N cycling processes. Our results suggest that Fe-OC flocs can be used as nitrification inhibitors in mineral soils, which can alleviate other environmental issues such as eutrophication of lakes and water pollution. However, more research on their effects on other N processes, such as denitrification and N immobilization are needed.