Using loss-on-ignition to estimate total nitrogen content of mangrove soils

Abstract

Loss-on-ignition (LOI) has been widely used to estimate soil organic carbon (OC) content for coastal wetland soils, owing to recent interest in ‘blue carbon’ systems. Comparatively less attention has been paid to soil nutrient retention, specifically total nitrogen (TN), despite being a historically limited resource that influences C cycling and aquatic ecosystem health. A single conversion equation is available to estimate soil TN content from LOI, derived from salt marshes. This study investigated the utility of creating TN conversion equations from LOI data for application in mangrove soils to improve global understanding of their role in TN interception and sequestration. A dataset of 1275 mangrove soil samples from 17 regions in the western hemisphere was used to create one general equation, two equations pertaining to sedimentary settings, and equations for each of seven coastal environmental settings (CES) that differed in mineral sediment provision, geomorphology, and hydrology. Soil mean LOI and TN ranged from respective lows of 9.5% and 0.18% in terrigenous estuaries to respective highs of 51.4% and 1.25% in carbonate estuaries. The ability of LOI to predict TN in mangrove soils was strong, with an R2 for the general equation of 0.88, and several regional equations exceeding 0.90. Total N represented between 1.1 and 3.7% of LOI. Differences in equations were minimal between mangrove soils and a previous equation for salt marsh soils, and between carbonate and terrigenous sedimentary settings. Significant differences were observed among CES. Deltas had the lowest TN:LOI (0.014), followed by terrigenous estuaries and open coasts (0.017 and 0.019, respectively). Terrigenous lagoons, carbonate estuaries, and carbonate open coasts had the highest TN:LOI (0.024 for all three). The study provides a decision tree to select an appropriate approach for estimating TN content of mangrove soils, offering an important tool for improving the understanding of the role of coastal wetlands in global TN sequestration.