Capturing the Scale Dependency of Erosion-Induced Variation in CO2 Emissions on Terraced Slopes

Yaxian Hu,Shengli Guo,Nikolaus J Kuhn,Brigitte Kuhn,Vincent Schneider

doi:10.3389/fenvs.2021.688286

Abstract

Net soil CO2 emissions are not independent of topography but tend to decline with increasing slope gradients. Such decline has been attributed to increased runoff and greater soil loss on steep slopes, leaving the soil less habitable for microorganisms. However, the specific variations of slope gradients and thus the associated soil properties relevant for CO2 emissions, especially from terraced slopes, are often disguised by the coarse resolution of digital terrain models (DTMs) based on commonly available open-source data. Such misrepresentation of the relationship between topography and soil CO2 emissions carries the risk of a wrong assessment of soil-atmosphere interaction. By applying a slope dependent soil CO2 emission model developed from erosion plots to nearby sloping and partially terraced cropland using two DTMs of different spatial resolutions, this study tested the significance of these resolution-induced errors on CO2 emission estimates. The results show that the coarser-resolution Shuttle Radar Topography Mission (SRTM) underestimated CO2-C emission by 27% compared to the higher-resolution DTM derived from Unmanned Aerial Vehicles (UAV) imagery. Such difference can be mostly attributed to a better representation of the proportion of flat slopes in the high-resolution DTM. Although the observations from erosion plots cannot be directly extrapolated to a larger scale, the 27% underestimation using the coarser-resolution SRTM DTM emphasizes that it is essential to represent microreliefs and their impact on runoff and erosion-induced soil heterogeneity at an appropriate scale. The widespread impact of topography on erosion and deposition on cropland, and the associated slope-dependent heterogeneity of soil properties, may lead to even greater differences than those observed in this study. The greatly improved estimation on CO2 emissions by the UAV-derived DTM also demonstrates that UAVs have a great potential to fill the gap between conventional field investigations and commonly applied coarse-resolution remote sensing when assessing the impact of soil erosion on global soil-atmosphere interaction.

Highlights

A substantial number of studies have utilized a combination of field data and remote sensing to assess the potential impacts of soil erosion on atmospheric CO2 at various spatiotemporal scales (Nadeu et al, 2012; Yue et al, 2016; Borrelli et al, 2017; Wilken et al, 2017; Lugato et al, 2018)
The slope-dependent CO2 emissions can vary with pixel size when the Unmanned Aerial Vehicles (UAV)-derived digital terrain models (DTMs) was of different resolutions, the difference between the CO2 emission values between Shuttle Radar Topography Mission (SRTM)- and UAV-derived DTMs can be considered as indicative of the sensitivity of emission calculations to the spatial patterns of soil along slopes affected by erosion and deposition
We must acknowledge that the comparison of CO2 emissions derived from the two DTMs in this study is obviously biased by the terraced topography, leading to a larger area with intermediate slope gradients and in turn lower emission totals

Summary

Introduction

A substantial number of studies have utilized a combination of field data and remote sensing to assess the potential impacts of soil erosion on atmospheric CO2 at various spatiotemporal scales (Nadeu et al, 2012; Yue et al, 2016; Borrelli et al, 2017; Wilken et al, 2017; Lugato et al, 2018). The rates of erosion by water and gravity, including tillage and intentional soil redistribution, are all slopedependent and much more sensitive to minor topographic variations than a coarse resolution grid cell DTM would reflect (Kirkby, 2010; Balaguer-Puig et al, 2018; He et al, 2020). This in turn introduces potentially significant uncertainties when accounting for erosion-induced spatial redistribution of soil water and nutrients (Li et al, 2018), and the vertical CO2 exchanges with the atmosphere in a landscape characterized by small-scale soil redistribution (Du et al, 2020b)

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Conclusion