Use of 1H NMR to identify nitrogen and carbon metabolites associated with mineralizable N in grassland soil

Abstract

Improved understanding of the metabolites linked to the mineralization processes of nitrogen (N) and carbon (C) in soil is important for optimum management, and to realize the soil's production potential to minimize associated environmental risks. A novel approach for investigating soil with proton nuclear magnetic resonance spectroscopy (1H NMR) was used to detect metabolites present in 35 contrasting grassland soil types in Ireland. Soil samples were analysed for a range of chemical, biological and physical properties including mineralizable N (MN) with a 7‐day anaerobic incubation (AI‐7). We identified and quantified by multivariate and regression analyses seven soil metabolites related to MN, namely glucose, trimethylamine, glutamic acid, serine, aspartic acid, 4‐aminohippuric acid and citric acid. Glucose explained the largest percentage of the variation in MN (R2 = 0.72). The amino acids, aspartic acid, 4‐aminohippuric acid, glutamic acid and serine explained 39, 38, 27 and 19% of the variation in MN, respectively. These results suggest that a labile carbon source (seven possible metabolites) appears to be an important factor for soil microbial biomass (SMB) to regulate quantities of 7‐day mineralizable N from soil organic matter N (SOM‐N) pools in these grassland soils for potential plant uptake. The results also demonstrate the analytical ability of 1H NMR as a potential practical platform to analyse the complex metabolite profiles in soil related to short‐term C and N mineralization. However, evaluation of 1H NMR against other analytical techniques, such as high performance anion exchange chromatography and pulsed amperometric detection (HPAEC‐PAD), to profile soil metabolites is recommended.HighlightsKnowledge of metabolites linked to C and N cycling in soil would help improve nutrient use efficiency. 1H NMR was investigated as a novel approach for profiling metabolites in grassland soil. Seven soil metabolites that correlated with mineralizable N were identified. 1H NMR is effective for simultaneous soil metabolite profiling, but spectral libraries need to be developed.