MAOM Formation: Abiotic Drivers of Soil Organic Nitrogen Adsorption to Soil Mineral Surfaces

Organic nitrogen (ON) represents the majority of nitrogen compounds found in soil systems, although often overlooked in favour of the minority of more bioaccessible inorganic nitrogen compounds. One of the main pools of soil ON is mineral-associated organic matter (MAOM), in these complexes organic compounds chemically and physically interact with soil mineral surfaces. Understanding ON cycling in soils is paramount to ensuring sustainable management of soil resources and predicting pollutant nitrogen outgassing.  A key aspect of this, is understanding ON vulnerability to microbial breakdown. Previous work has shown how organic matter in MAOM complexes can persist in soils for centuries, protected from microbial degradation. However, more recent research has highlighted that organic matter can be mobilised from soil mineral surfaces and become accessible to microbes once more. MAOM complexes therefore can represent a far more dynamic pool of nitrogen than previously thought and the drivers of their formation need more robust characterisation. Our research focused on abiotic soil properties as potential drivers of ON adsorption to soil mineral surfaces, as well as the influence of time and the identity (chemistry and size) of investigated nitrogen compounds. Soil mineralogy data, from XRF analysis, as well as soil texture and pH data were coupled with adsorption assays and mixed effects modelling. This revealed the dominant influence of adsorbate identity on total adsorption and rate of adsorption to soil mineral surfaces. The presence of aluminium and iron, both common in soils and reactive with organic matter, and soil pH also had significant influences on nitrogen compound adsorption. These findings add to the growing body of literature on the drivers of MAOM complex formation, support the use of newer adsorption potential proxies, and highlight the importance of considering organic matter identity when predicting nitrogen recalcitrance in soils.