Moderate topsoil erosion rates constrain the magnitude of the erosion-induced carbon sink and agricultural productivity losses on the Chinese Loess Plateau

Abstract

Abstract. Despite a multitude of studies, overall erosion rates as well as the contribution of different erosion processes on Chinese Loess Plateau (CLP) remain uncertain, which hampers a correct assessment of the impact of soil erosion on carbon and nutrient cycling as well as on crop productivity. In this paper we used a novel approach, based on field evidence, to reassess erosion rates on the CLP before and after conservation measures were implemented (1950 vs. 2005). We found that current average topsoil erosion rates are 3 to 9 times lower than earlier estimates suggested. Under 2005 conditions, more sediment was produced by non-topsoil erosion (gully erosion (0.23 ± 0.28 Gt yr−1) and landsliding (0.28 ± 0.23 Gt yr−1) combined) than by topsoil erosion (ca. 0.30 ± 0.08 Gt yr−1). Overall, these erosion processes mobilized ca. 4.77 ± 1.96 Tg yr−1 of soil organic carbon (SOC): the latter number sets the maximum magnitude of the erosion-induced carbon sink, which is ca. 4 times lower than one other recent estimate suggests. The programs implemented from the 1950s onwards reduced topsoil erosion from 0.51 ± 0.13 to 0.30 ± 0.08 Gt yr−1 while SOC mobilization was reduced from 7.63 ± 3.52 to 4.77 ± 1.96 Tg C yr−1. Conservation efforts and reservoir construction have disrupted the equilibrium that previously existed between sediment and SOC mobilization on the one hand and sediment and SOC export to the Bohai sea on the other hand: nowadays, most eroded sediments and carbon are stored on land. Despite the fact that average topsoil losses on the CLP are still relatively high, a major increase in agricultural productivity has occurred since 1980. Fertilizer application rates nowadays more than compensate for the nutrient losses by (topsoil) erosion: this was likely not the case before the dramatic rise of fertilizer use that started around 1980. Hence, erosion is currently not a direct threat to agricultural productivity on the CLP but the long-term effects of erosion on soil quality remain important.

Vegetation cover and topography rather than human disturbance control gully density and sediment production on the Chinese Loess Plateau

“…It also implies that it remains difficult to assess the overall impact of human activities on overall sediment mobilisation. While several studies have clearly shown that human activities may increase topsoil erosion rates by over two orders of magnitude (Montgomery, 2007;Zhao et al, 2016), the impact of human activities on non-topsoil erosion is much less clear. Human activities have a much smaller effect on the overall sediment transfer from the land to the ocean than on topsoil erosion: studies suggest that the land-ocean sediment transfer would have increased from ca.…”

“…1.37 Gt of sediments to the Bohai Sea, which was ca. 9% of the total global land to sea export: over 90% of these sediments were coming from the CLP (Shi and Shao, 2000;Syvitski et al, 2005;Zhao et al, 2016).…”

“…Recently, Zhao et al (2016) developed a modelling procedure to estimate topsoil erosion rates on the CLP based on erosion plot data documenting the variation of topsoil erosion rates with topography, land use and land management practices. Validation of the model using 137 Cs inventories showed that topsoil erosion rates were predicted with an acceptable accuracy (b 25% uncertainty).…”

Global soil organic carbon removal by water erosion under climate change and land use change during AD 1850–2005

“…Some studies coupled process-oriented soil erosion models with carbon turnover models calibrated for specific micro-catchments on timescales of a few decades to a millennium, (Billings et al, 2010;Van Oost et al, 2012;Nadeu et al, 2015;Wang et al, 2015a;Zhao et al, 2016;Bouchoms et al, 2017). Other studies focused on the application of parsimonious erosion-SOC-dynamics models using the Revised Universal Soil Loss Equation (RUSLE) approach together with sediment transport methods at regional or continental spatial scales (Chappell et al, 2015;Lugato et al, 2016;Yue et al, 2016;Zhang et al, 2014).…”

Soil conservation in the 21st century: why we need smart agricultural intensification

“…One may therefore assume that in these societies erosion rates can be reduced to tolerable levels provided that the necessary policies are maintained and/or strengthened. Countries with a strong central government that can impose decisions on land use and soil conservation, as is the case in China, can successfully reduce erosion: the excessive erosion rates on the Chinese Loess Plateau were strongly reduced through massive government programmes implementing erosion control measures (Chen et al, 2007;Zhao et al, 2016) These approaches are, at present, not possible in most countries of the Global South. Many governments in the Global South are not able to implement a successful soil conservation policy as they do not have at their disposal the necessary data and/or the necessary political and societal instruments to do so.…”