An adequate supply of bioavailable nitrogen (N) is critical to soil microbial communities and plants. Over the last decades, research efforts have rarely considered the importance of reactive iron (Fe) minerals in the processes that produce or consume bioavailable N in soils, compared to other factors such as soil texture, pH, and organic matter (OM). However, Fe is involved in both enzymatic and non-enzymatic reactions that influence the N cycle. More broadly, reactive Fe minerals restrict

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... organic matter (SOM) cycling through sorption processes, but also promote SOM decomposition and denitrification in anoxic conditions. By synthesizing available research, we show that Fe plays diverse roles in N bioavailability. 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. These roles can lead to different outcomes on N bioavailability, depending on environmental conditions such as soil redox shifts during wet-dry cycles. We provide examples of Fe-N interactions and discuss the possible underlying mechanisms, which can be abiotic or microbially meditated. We also discuss methodological constraints that hinder the development of mechanistic understanding of Fe in controlling N bioavailability and highlight the areas of needed research. Introduction Terrestrial ecosystem productivity is largely constrained by nitrogen (N) availability (Vitousek and Howarth, 1991) . The largest pool of N in these ecosystems is found in soils which contains 133-140 Pg of total N globally within the first top 100 cm of soil (Batjes, 1996) . A clear description of the factors controlling N bioavailability in soils is needed to design agricultural practices that meet crop demand and mitigate N loss to the environment. A large literature exists on the effects of soil texture, OM, mineral N inputs, pH, moisture, and microbial communities on N mineralization. However, geochemical factors, such as reactive Fe minerals, are rarely considered in N cycling, though they are often studied as vital components of carbon (C) cycling. Since C and N cycles are interconnected in soils (Feng et al., 2019; Gärdenäs et al., 2011) , they should be regulated