May agricultural terraces induce gully erosion? A case study from the Black Soil Region of Northeast China

Terracing of hillslopes is usually regarded as an effective measure to control soil erosion. Although it is often stressed that proper terrace construction and regular terrace maintenance are of great significance to prevent erosion on terraced hillslopes, examples of terrace-induced gully erosion are scarce. Field observations on terraced and partly abandoned hillslopes in the Black Soil Region of Northeast China, a region heavily affected by gully erosion since the middle of the 20th century, indicated that gully formation might be caused by terraces. In order to understand the impact of terracing on gully erosion, we selected several gullies to investigate the cause and timing of their triggering. We used a combination of field mapping, high-resolution digital terrain models, multi-temporal aerial photograph interpretation and interviews with local farmers. Our results showed that several gullies developed after terracing. Improper terrace design caused runoff concentration along terraces and ridges with mean inclination of 3.8%, which resulted in gully incision due to overtopping of terraces at low spots or due to the uncontrolled release of concentrated flow to adjoining unterraced hillslopes. The same processes are responsible for the persistent gully activity after abandonment and vegetation recovery. Furthermore, we showed how terraces affected gully morphology. Finally, we suggested appropriate countermeasures to stop further soil loss and land degradation on abandoned terraced hillslopes in NE China. Our findings are important as they demonstrate how poorly designed terraces may not only be ineffective but may even aggravate gully erosion.

forest and soil properties in the Loess Plateau of China

Terracing is one of the most effective ecological engineering practices that improve soil anti-erosivity properties and support plant growth in the dryland loess hilly area and other similar regions. The objective of the study was to understand the vertical distribution of root in different terraces and their relationships with soil environmental factors in the Loess Plateau of China. The vertical root distribution in the 0-400 cm soil profile, fine root distribution and soil moisture, soil nutrients (soil organic carbon, total nitrogen, and total phosphorus) and soil anti-erosivity in 0-160 cm soil profiles (every 20 cm for one layer) were investigated using the ground-penetrating radar and soil coring methods in a Pinus tabulaeformis Carr. forest under three terrace types during the growing season of 2018. We highlight several key findings here. First, level benches had the highest root density (18.14 kg m^-2), followed by fish-scale pits (13.95 kg m^-2) and reverse-slope terraces (9.84 kg m^-2), as well as the highest soil water content, nutrients and soil stability. Second, terracing caused significant differences in root distribution (P < 0.05), leading to the variation of soil moisture, nutrients, anti-erosivity (explained over 80% variation) and reduced spatial heterogeneities of soil water content and nutrients. Third, fine root density parameters attained the highest values in the topsoil (0-40 cm soil layer) and decreased with increasing soil depth in all the three terrace types (P < 0.05). Finally, fine roots contributed to soil water improvement, nutrient promotion and soil stabilization, while higher density of coarse roots might consume soil nutrients and reduce soil anti-erosivity. We thus suggest that level benches could be a more suitable terracing measure for plantation of P. tabulaeformis Carr., and to achieve soil melioration and fixation during the ecosystem restoration process in the Loess Plateau and other arid and semiarid regions.

The effects of land use and topographic changes on sediment connectivity in mountain catchments

Understanding the evolution of sediment connectivity associated with different land use and topographic changes is a prerequisite for a better understanding of sediment budgets and sediment transport processes. We used the Index of Sediment Connectivity (IC) developed by Cavalli et al. (2013) based on the original approach by Borselli et al. (2008) to study the effects of decadal-scale land use and topographic changes on sediment connectivity in mountain catchments. The input variables of the IC (i.e. land cover and topography) were derived from historical aerial photos using Structure from Motion-Multi View Stereo algorithms (SfM-MVS). The method was applied in different sub-catchments of the Upper River Cinca Catchment (Central Pyrenees), representative of three scenarios: (a) Land cover changes; (b) Topographic changes in agricultural fields (terracing); and (c) Topographic changes associated with infrastructure (road construction). In terms of land cover changes, results show that although connectivity is increased in some areas due to the establishment of new field crops, for most of the study area connectivity decreased due to afforestation caused by rural abandonment. Topographic changes due to the establishment of agricultural terraces affected connectivity to a larger degree than land cover changes. Terracing generally reduced connectivity due to the formation of flat areas in step-slopes, but in certain points, an increase in connectivity caused by the topographic convergence produced by terraces was observed. Finally, topographic changes associated with road construction greatly modified surface flow directions and the drainage network, resulting in changes in connectivity that may affect erosional processes nearby. The methodology used in this paper allows to study the effects of real decadal-scale land use and topographic changes on sediment connectivity and also evaluating and disentangling those changes. Furthermore, this approach can be a useful tool to identify potential risks associated with morphological and land use changes, involving road infrastructures.

The effects of rainfall regimes and terracing on runoff and erosion in the Three Gorges area, China

Changes in natural rainfall regimes have taken place and are expected to become more pronounced in future decades. These changes are also likely to be accompanied by changes in crop management practices. The main purpose of this study was to analyze runoff and soil loss in relation to rainfall regimes and terracing in the Three Gorges area, China. Based on 10 years of field observation and k-mean clusters, 101 rainfall events were grouped into three rainfall regimes. Rainfall regime I was the group of events with strong rainfall intensity, high frequency, and short duration. Rainfall regime III consisted of events with low intensity, long duration, and high rainfall amount. Rainfall regime II was the aggregation of events of high intensity and amount, and less frequent occurrence. The results showed that event runoff coefficients were not significantly different among rainfall regimes. However, the average soil erosion rates in rainfall regimes I and II were significantly higher than that in regime III. The average erosion rates under rainfall regimes I, II, and III were 21.6, 39.7, and 9.8 g m^-2, respectively. The effect of rainfall regime on soil erosion also was changed by terracing. On unterraced cropland, soil erosion rate in rainfall regime I is significantly higher than that in regime III. However, the situation did not exist in unterraced orchard. Terracing significantly reduced runoff and soil erosion, and compensated the effects of rainfall regime on soil erosion, which indicated that runoff and erosion in terraced system may be little influenced by climate change. Based on these results, it was suggested more attention should be paid to the timing of rainfall events in relation to crop development and the high erosion on unterraced citrus orchard to control soil erosion in this area.

Soil Organic Carbon Fractions and Stocks Respond to Restoration Measures in Degraded Lands by Water Erosion

Assessing the degree to which degraded soils can be recovered is essential for evaluating the effects of adopted restoration measures. The objective of this study was to determine the restoration of soil organic carbon under the impact of terracing and reforestation. A small watershed with four typical restored plots (terracing and reforestation (four different local plants)) and two reference plots (slope land with natural forest (carbon-depleted) and abandoned depositional land (carbon-enriched)) in subtropical China was studied. The results showed that soil organic carbon, dissolved organic carbon and microbial biomass carbon concentrations in the surface soil (10 cm) of restored lands were close to that in abandoned depositional land and higher than that in natural forest land. There was no significant difference in soil organic carbon content among different topographic positions of the restored lands. Furthermore, the soil organic carbon stocks in the upper 60 cm soils of restored lands, which were varied between 50.08 and 62.21 Mg C ha^-1, were higher than 45.90 Mg C ha^-1 in natural forest land. Our results indicated that the terracing and reforestation could greatly increase carbon sequestration and accumulation and decrease carbon loss induced by water erosion. And the combination measures can accelerate the restoration of degraded soils when compared to natural forest only. Forest species almost have no impact on the total amount of soil organic carbon during restoration processes, but can significantly influence the activity and stability of soil organic carbon. Combination measures which can provide suitable topography and continuous soil organic carbon supply could be considered in treating degraded soils caused by water erosion.