Nitrogen fertilization-induced acidity and suitability of δ13C to study the dynamics of soil inorganic carbon in agroecosystems

Abstract

Neutralization of soil inorganic carbon (SIC), i.e., dissolution of carbonate minerals caused by N fertilization-induced acidity, is an ongoing reaction in agroecosystems that leads to significant contribution of SIC to soil CO₂ emission. Analyzing δ¹³C natural abundance of the emitted CO₂ is commonly used to quantify the contribution of SIC in total CO₂ emission. However, carbonates recrystallization in isotopic equilibrium with soil respiration can lead to miscalculation, where despite detecting δ¹³C signal from carbonates in the emitted CO₂, the SIC stock may not necessarily change. We tested the effects of ammonium sulfate, urea, chicken manure, liquid pig manure and no fertilization (i.e. control) on soil carbonates neutralization and the contribution of SIC in total CO₂ emission over a 21 day incubation experiment. The alkali trap method was used to measure the total CO₂ emission and the isotopic composition of carbon (C) in the emitted CO₂ from the soil. Liquid pig manure followed by ammonium sulfate apparently led to the highest cumulative SIC-originated CO₂ emissions, while chicken manure and urea showed equally the smallest amount. Nitrate can support the estimation of SIC-originated CO₂ emission when nitrification is the only source of soil acidification. Dissolved Ca²⁺ concentration was a more robust proxy than ${NO}_3^{-}$ for quantification of SIC-originated CO₂ emission in our batch experiment, however, more evidences have to be collected before using Ca²⁺ as a proxy in open soil systems. Verification of the δ¹³C values by dissolved Ca²⁺ showed that the contribution of SIC to the total CO₂ emission is overestimated after organic N fertilization, whereas an underestimation in the contribution of SIC to the soil CO₂ emission was observed after inorganic N fertilization. In summary, the δ¹³C analysis could yield inaccurate results regarding the effects of N fertilization on the contribution of SIC in soil CO₂ emission which should be considered in the future studies. From an environmental point of view, it can be concluded that when the amounts of N, especially in ammonium form e.g. amino groups are relatively high in organic fertilizers as it was the case in liquid pig manure compared to inorganic fertilizers, organic and inorganic N fertilizers lead to comparable amounts of SIC loss. This should be considered in applying organic fertilizers as a sustainable strategy to slow down the rate of soil acidification.