Slope sensitivity: A coefficient to represent the dependency of soil CO2 emissions to slope gradients

Abstract

Slope induced runoff and erosion processes redistribute water, soil and nutrients, thus potentially influence soil CO2 emissions and perturb the carbon balances on sloping land. However, most of the previous studies on soil CO2 emissions were conducted on flat land. In regions with complex terrains and fragmented fields with terraces, such as the Chinese Loess Plateau, it is particularly important to understand slope dependent soil CO2 emissions. This requires investigations to unravel the relationship between soil erosional responses and CO2 emissions on different slopes. In this study, erosion plots, set at angles of 0.5°, 5°, 10°, and 20°, were refilled with loess soils from local farmland on the Chinese Loess Plateau. Changes of soil properties and CO2 emission rates were measured at three positions (upper, middle, lower) on each plot from October 2014 to September 2015. Our results show that greater runoff from the steeper slopes evidently depleted soil water and nutrients, directly responsible for the less soil CO2 emissions. Furthermore, the average yearly soil CO2 emissions decreased exponentially with slope gradients, from approximately 832.7 g m−2 yr−1 on the 0.5° slope, to 380.9 g m−2 yr−1 when the slope gradient was 20°. This not only proves the dependency of soil CO2 emission to slope gradients, but also provides the fundamental dataset to develop an empirical model between slope gradients and soil CO2 emissions. A coefficient βhas been parameterized for the first time in this study to represent the sensitivity of soil CO2 emissions to slope gradients. The model of slope-dependent CO2 emissions identified in this study tentatively demonstrate non-negligible implications for global carbon fluxes from sloped and terraced field to the atmosphere. Further investigations on slope-scale carbon balances should test and integrate topography specific CO2 emissions.