Previous estimates of deep soil reservoirs of biologically available nitrogen have been limited to that of desert soils, however recent evidence suggests that these pools are far more ubiquitous across biomes and therefore may be an unaccounted for source in global nitrogen budgets. We surveyed the literature and found 92 instances of deep soil (2-86 m) inorganic nitrogen measurements, most of which were reported as nitrate, across a variety of biomes. From these studies, we collect metadata such as soil texture, depth and type; N deposition and fertilization rates; land use history; and precipitation and mean annual temperature. We use statistical and process-based approaches to evaluate major controls on soil nitrogen accumulation in the form of nitrate, use a spatial modelling approach to project other areas where deep soil nitrogen may accumulate.
Results/Conclusions
We found that deep soil nitrogen pools ranged from 5.24-3,702 kg N/ha, with a mean of 522 kg N/ha. Deep soil nitrogen was significantly higher beneath grasslands and shrublands compared to other vegetation types (p=0.0005). In croplands, deep soil nitrogen increased significantly with fertilization rate (p = 0.04). When soil orders were reported, deep soil nitrogen pools were typically found in oxisols and ultisols. Deep soil nitrogen was negatively correlated with mean annual temperature (p=0.001) and precipitation (p=0.03). As a rough approximation, if we apply our mean soil nitrogen inventory value to just highly weathered, and typically deep, oxisols (8% of soils) and ultisols (7% of soils), we get an estimate of 3,993 Tg, which increases the total soil nitrogen pool (9,500 Tg) by 30% globally. While we found evidence that nitrogen deposition, historical nitrogen fixation, and fertilizer inputs contribute to this pool, the source and fate of deep soil nitrogen likely varies by region. We examine the possible influence of nitrogen deposition, rock derived nitrogen, fertilization, denitrification, soil exchange capacity, leaching and land use on this understudied pool of nitrogen.