Spatial and Temporal Variation in Soil Nitrous Oxide Emissions from a Rehabilitated and Undisturbed Riparian Forest

Elucidation of the dominant factors influencing N2O emission in water-level fluctuation zones in a karst canyon reservoir, southwest China

The water-level fluctuations zones (WLFZs) are crucial transitional interfaces within river-reservoir systems, serving as hotspots for N2O emission. However, the comprehension of response patterns and mechanisms governing N2O emission under hydrological fluctuation remains limited, especially in karstic canyon reservoirs, which introduces significant uncertainty to N2O flux assessments. Soil samples were collected from the WLFZs of the Hongjiadu (HJD) Reservoir along the water flow direction from transition zone (T1 and T2) to lacustrine zone (T3, T4 and T5) at three elevations for each site. These soil columns were used to conduct simulation experiments under various water-filled pore space gradients (WFPSs) to investigate the potential N2O flux pattern and elucidate the underlying mechanism. Our results showed that nutrient distribution and N2O flux pattern differed significantly between two zones, with the highest N2O fluxes in the transition zone sites and lacustrine zone sites were found at 75 % and 95 % WFPS, respectively. Soil nutrient loss in lower elevation areas is influenced by prolonged impoundment durations. The higher N2O fluxes in the lacustrine zone can be attributed to increased nutrient levels resulting from anthropogenic activities. Furthermore, correlation analysis revealed that soil bulk density significantly impacted N2O fluxes across all sites, while NO3-and SOC facilitated N2O emissions in T1-T2 and T4-T5, respectively. It was evident that N2O production primarily contributed to nitrification in the transition zone and was constrained by the mineralization process, whereas denitrification dominated in the lacustrine zone. Notably, the annual N2O efflux from WLFZs accounted for 27 % of that from the water-air interface in HJD Reservoir, indicating a considerably lower contribution than anticipated. Nevertheless, this study highlights the significance of WLFZs as a vital potential source of N2O emission, particularly under the influence of anthropogenic activities and high WFPS gradient.

Soil N2O and CH4 emissions from fodder maize production with and without riparian buffer strips of differing vegetation

PURPOSE

Nitrous oxide (N₂O) and methane (CH₄) are some of the most important greenhouse gases in the atmosphere of the 21st century. Vegetated riparian buffers are primarily implemented for their water quality functions in agroecosystems. Their location in agricultural landscapes allows them to intercept and process pollutants from adjacent agricultural land. They recycle organic matter, which increases soil carbon (C), intercept nitrogen (N)-rich runoff from adjacent croplands, and are seasonally anoxic. Thus processes producing environmentally harmful gases including N₂O and CH₄ are promoted. Against this context, the study quantified atmospheric losses between a cropland and vegetated riparian buffers that serve it.

METHODS

Environmental variables and simultaneous N₂O and CH₄ emissions were measured for a 6-month period in a replicated plot-scale facility comprising maize (Zea mays L.). A static chamber was used to measure gas emissions. The cropping was served by three vegetated riparian buffers, namely: (i) grass riparian buffer; (ii) willow riparian buffer and; (iii) woodland riparian buffer, which were compared with a no-buffer control.

RESULTS

The no-buffer control generated the largest cumulative N₂O emissions of 18.9 kg ha^- 1 (95% confidence interval: 0.5-63.6) whilst the maize crop upslope generated the largest cumulative CH₄ emissions (5.1 ± 0.88 kg ha^- 1). Soil N₂O and CH₄-based global warming potential (GWP) were lower in the willow (1223.5 ± 362.0 and 134.7 ± 74.0 kg CO₂-eq. ha^- 1 year^- 1, respectively) and woodland (1771.3 ± 800.5 and 3.4 ± 35.9 kg CO₂-eq. ha^- 1 year^- 1, respectively) riparian buffers.

CONCLUSIONS

Our results suggest that in maize production and where no riparian buffer vegetation is introduced for water quality purposes (no buffer control), atmospheric CH₄ and N₂O concerns may result.

Vegetation Type Does not Affect Nitrous Oxide Emissions from Riparian Zones in Agricultural Landscapes

Riparian zones provide multiple benefits in agricultural landscapes, but nitrogen (N) loading can cause N₂O emissions. There is a knowledge gap on how different types of riparian vegetation influence N₂O emissions. This study quantified N₂O emissions from a rehabilitated riparian zone with deciduous trees (RH), a herbaceous (grassed) riparian zone (GRS), a natural forested riparian zone with deciduous trees (UNF-D), a natural forested riparian zone with coniferous trees (UNF-C), and an agricultural field (AGR). N₂O fluxes were not significantly different (p > 0.05) among riparian zones (11-17 µg N₂O-N m^-2 h^-1) and were not significantly different (p > 0.05) when comparing riparian zones to the AGR field (34 µg N₂O-N m^-2 h^-1). Despite high N-loading, cumulative N₂O emissions (1989 µg N₂O-N m^-2) in the riparian zones was significantly lower (p > 0.05) than AGR (13,278 µg N₂O-N m^-2). The main predictors of N₂O fluxes were soil temperature and soil NO₃^--N for the riparian zones and the AGR field. We found that environmental conditions played a greater role than the type of riparian vegetation or age in predicting N₂O emissions. We suggest that soil environmental factors created an anaerobic environment that favored N₂O consumption via complete denitrification.