Nitrogen isotopic signatures and fluxes of N2O in response to land-use change on naturally occurring saline-alkaline soil

The conversion of natural grassland to semi-natural or artificial ecosystems is a large-scale land-use change (LUC) commonly occurring to saline–alkaline land. Conversion of natural to artificial ecosystems, with addition of anthropogenic nitrogen (N) fertilizer, influences N availability in the soil that may result in higher N₂O emission along with depletion of ¹⁵N, while converting from natural to semi-natural the influence may be small. So, this study assesses the impact of LUC on N₂O emission and ¹⁵N in N₂O emitted from naturally occurring saline–alkaline soil when changing from natural grassland (Phragmites australis) to semi-natural [Tamarix chinensis (Tamarix)] and to cropland (Gossypium spp.). The grassland and Tamarix ecosystems were not subject to any management practice, while the cropland received fertilizer and irrigation. Overall, median N₂O flux was significantly different among the ecosystems with the highest from the cropland (25.3 N₂O-N µg m⁻² h⁻¹), intermediate (8.2 N₂O-N µg m⁻² h⁻¹) from the Tamarix and the lowest (4.0 N₂O-N µg m⁻² h⁻¹) from the grassland ecosystem. The ¹⁵N isotopic signatures in N₂O emitted from the soil were also significantly affected by the LUC with more depleted from cropland (− 25.3 ‰) and less depleted from grassland (− 0.18 ‰). Our results suggested that the conversion of native saline–alkaline grassland with low N to Tamarix or cropland is likely to result in increased soil N₂O emission and also contributes significantly to the depletion of the ¹⁵N in atmospheric N₂O, and the contribution of anthropogenic N addition was found more significant than any other processes.

Estimation of Methane Emissions from Rice Paddies in the Mekong Delta Based on Land Surface Dynamics Characterization with Remote Sensing

In paddy soils in the Mekong Delta, soil archaea emit substantial amounts of methane. Reproducing ground flux data using only satellite-observable explanatory variables is a highly transparent method for evaluating regional emissions. We hypothesized that PALSAR-2 (Phased Array type L-band Synthetic Aperture RADAR) can distinguish inundated soil from noninundated soil even if the soil is covered by rice plants. Then, we verified the reproducibility of the ground flux data with satellite-observable variables (soil inundation and cropping calendar) and with hierarchical Bayesian models. Furthermore, inundated/noninundated soils were classified with PALSAR-2. The model parameters were successfully converged using the Hamiltonian–Monte Carlo method. The cross-validation of PALSAR-2 land surface water coverage (LSWC) with several inundation indices of MODIS (Moderate Resolution Imaging Spectroradiometer) and AMSR-2 (Advanced Microwave Scanning Radiometer-2) data showed that (1) high PALSAR-2-LSWC values were detected even when MODIS and AMSR-2 inundation index values (MODIS-NDWI and AMSR-2-NDFI) were low and (2) low values of PALSAR-2-LSWC tended to be less frequently detected as the MODIS-NDWI and AMSR-2-NDFI increased. These findings indicate the potential of PALSAR-2 to detect inundated soils covered by rice plants even when MODIS and AMSR-2 cannot, and show the similarity between PALSAR-2-LSWC and the other two indices for nonvegetated areas.

Development of a field-deployable method for simultaneous, real-time measurements of the four most abundant N2O isotopocules

Understanding and quantifying the biogeochemical cycle of N₂O is essential to develop effective N₂O emission mitigation strategies. This study presents a novel, fully automated measurement technique that allows simultaneous, high-precision quantification of the four main N₂O isotopocules (¹⁴N¹⁴N¹⁶O, ¹⁴N¹⁵N¹⁶O, ¹⁵N¹⁴N¹⁶O and ¹⁴N¹⁴N¹⁸O) in ambient air. The instrumentation consists of a trace gas extractor (TREX) coupled to a quantum cascade laser absorption spectrometer, designed for autonomous operation at remote measurement sites. The main advantages this system has over its predecessors are a compact spectrometer design with improved temperature control and a more compact and powerful TREX device. The adopted TREX device enhances the flexibility of the preconcentration technique for higher adsorption volumes to target rare isotope species and lower adsorption temperatures for highly volatile substances. All system components have been integrated into a standardized instrument rack to improve portability and accessibility for maintenance. With an average sampling frequency of approximately 1 h^-1, this instrumentation achieves a repeatability of 0.09, 0.13, 0.17 and 0.12 ‰ for δ¹⁵N^α, δ¹⁵N^β, δ¹⁸O and site preference of N₂O, respectively, for pressurized ambient air. The repeatability for N₂O mole fraction measurements is better than 1 ppb (parts per billion, 10^-9 moles per mole of dry air).

Response of nitric and nitrous oxide fluxes to N fertilizer application in greenhouse vegetable cropping systems in southeast China

It is of great concern worldwide that active nitrogenous gases in the global nitrogen cycle contribute to regional and global-scale environmental issues. Nitrous oxide (N₂O) and nitric oxide (NO) are generally interrelated in soil nitrogen biogeochemical cycles, while few studies have simultaneously examined these two gases emission from typical croplands. Field experiments were conducted to measure N₂O and NO fluxes in response to chemical N fertilizer application in annual greenhouse vegetable cropping systems in southeast China. Annual N₂O and NO fluxes averaged 52.05 and 14.87 μg N m⁻² h⁻¹ for the controls without N fertilizer inputs, respectively. Both N₂O and NO emissions linearly increased with N fertilizer application. The emission factors of N fertilizer for N₂O and NO were estimated to be 1.43% and 1.15%, with an annual background emission of 5.07 kg N₂O-N ha⁻¹ and 1.58 kg NO-N ha⁻¹, respectively. The NO-N/N₂O-N ratio was significantly affected by cropping type and fertilizer application, and NO would exceed N₂O emissions when soil moisture is below 54% WFPS. Overall, local conventional input rate of chemical N fertilizer could be partially reduced to attain high yield of vegetable and low N₂O and NO emissions in greenhouse vegetable cropping systems in China.