Soil organic nitrogen is the biggest terrestrial pool of nitrogen and mineralization is the source of nitrogen for plants in most ecosystems (Batjes, 1996; 2014). Even in agricultural systems with fertilizer applied it is important. Various assays of mineralizable nitrogen have been shown to predict growth and response to fertilizers (Ros et al., 2011; Curtin et al., 2017), but they are time-consuming and expensive. Also, better management of the nitrogen cycle is crucial for global sustainability for reasons of agricultural production, greenhouse gas emissions, soil acidification and impacts on aquatic ecosystems (Suddick et al., 2013; San Martín, 2021). So, we need a better understanding of the mechanisms of nitrogen cycling, in particular the factors influencing stability and mineralization of organic nitrogen.
Soil microorganisms are responsible for 85-90% of organic matter decomposition (Lavelle et al., 1993), and microbial biomass supplies about 80% of soil organic matter (Liang & Balser, 2011; Schmidt et al., 2011; Miltner et al., 2012). Organic compounds may be resistant to microbial degradation and have a long residence time in soil. This could be due to inherent chemical recalcitrance, inaccessibility of microbial population to organic materials due to physical protection, or stabilization by reaction with clays, metal ions and oxides and other organic molecules. For example, decomposition of organic macromolecules to smaller molecules can lead to reaction with mineral particles and stabilization of organic matter in the soil (Cotrufo et al., 2013; Lehmann & Kleber, 2015; Basile-Doelsch et al., 2020). However, the relative importance of these mechanisms in the stabilization of organic nitrogen compounds is not clear (Kögel-Knabner et al., 2008).
Adsorption of organic matter to clay and oxide surfaces is an important chemical protection mechanism in carbon and nitrogen cycles. Several factors such as pH, redox conditions, and characteristics of organic materials and mineral surfaces affect the complexation and the degree of sorption of organic materials by mineral surfaces (Zhu, 1997; Jastrow et al., 2007) and hence the mineralization/immobilization of organic nitrogen (Nikolaidis & Bidoglio, 2013). In comparison to clay minerals, Al- and Fe-oxides have a greater affinity for soil organic matter due to their higher specific surface area (SSA) and reactivity (Sanjay & Sugunan, 2008; Wallenstein et al., 2010). For example, goethite has a strong association with dissolved organic matter through ligand exchange reactions resulting in Fe-carboxylate bonding (Kaiser & Zech, 1999; Chorover & Amistadi, 2001). In soils, organo-mineral complexes are also formed through the binding of organic ligands with Fe3+ and Al3+ on the exchange sites of clay minerals (Higashi, 1983; Oades, 1988; Boudot et al., 1989). The formation of these complexes stabilizes soil organic matter.
Non-crystalline materials such as allophane have exceptionally large SSA and porosity that results in preservation of organic matter via a combination of retention mechanisms (Mikutta et al., 2009; Kramer et al., 2017). In allophanic soils, organic matter reacts slowly with allophane, and the formation of organic-allophane complexes causes organic matter to decompose more slowly than in other soils. In allophanic soils enzymatic activities per unit of organic carbon are less than in other soils (Ross et al., 1982). Mulder et al. (2001) showed that increasing the concentration of Al in the forest floor caused a 30-40% decrease in the decomposition rate of soil organic matter.
Amino sugars are an important component of soil organic nitrogen. Some studies have suggested that minerals influence the turnover of amino sugars, whereas others have shown that minerals affected the proportion of nitrogen sequestered within microbial biomass but not the cycling of nitrogen through the amino sugar pool (Amelung et al., 2001; Amelung & Zhang, 2001). So, while abiotic processes influence amino sugar cycling, the mechanisms remain obscure.
Enzymes have a strong affinity for clays and oxides (Burns, 1986; Naidja et al., 2000), which can affect their activity. Clays can affect enzymes in two ways. One is by stabilizing and protecting enzymes, preventing them from being degraded and denatured (Nasseau et al., 2001). As a result, Allison (2006) found that clay minerals, especially allophane, intensified enzymatic activities in young volcanic soils. On the other hand, clays can inhibit the activity of enzymes due to blockage of active sites upon adsorption. Kobayashi and Aomine (1967) and Tietjen and Wetzel (2003) reported a decrease in enzyme activities after the addition of allophanic and montmorillonic clays. Rakhsh and Golchin (2018) showed that enzyme activities decreased significantly as the clay content increased. Marx et al. (2005) and Yan et al. (2010), observed that enzyme activities decreased in a clay-sized fraction of soils. Since adsorption of enzymes on clay surfaces occurs by different mechanisms, its effect on enzyme activities may differ accordingly.
Previous studies have focused on the effect of pure clays or pure minerals on organic matter mineralization and enzyme activity, and our knowledge of these processes in the soil matrix is lacking. Of course, there are some studies on enzyme activity and stabilization of organic matter in natural soils, which are mostly interpreted based on differences in soil texture. Studying the effect of metal oxides on enzyme activity and stabilization of organic nitrogen in natural soils is difficult due to confounding of factors including the type and amount of clay and metal oxides. This research aimed to study the effects of clay content and composition on organic nitrogen mineralization, activities of acid and alkaline phosphatases and CM-cellulase, and microbial biomass nitrogen formation. We did this by studying the decomposition of alfalfa residues in artificial soils with different contents of the kaolinite and the non-layered colloids (NLCs)s goethite, manganese oxide and imogolite.