Rainfall and Tillage Impacts on Soil Erosion of Sloping Cropland with Subtropical Monsoon Climate- A Case Study in Hilly Purple Soil area, China

Under global warming, storm events tend to intensify, particularly in monsoon-affected regions.As an important agricultural area in China, the purple soil region in the Sichuan Basin, where it has a prevailing monsoon climate, is threatened by serious soil erosion. Tillage operations alter runoff and soil erosion processes on croplands by changing the physical properties of the soil surface. To clarify the relationship between tillage and soil erosion in the purple soil region, three different tillage practices in this region were investigated at the plot scale over 4 years: bare land with minimum tillage(BL),conventional tillage(CT) and seasonal no-tillage ridges(SNTR) which was initially designed to prevent soil erosion by contoured ridges and no-tillage techniques. The results showed that although there were no significant differences in the surface runoff and soil erosion among the three practices, BL causedrelatively high surface runoff and soil erosion,followed by CT and SNTR. Classification and comparison of the rainfall events based on cluster analysis(CA) verified that the surface runoff was not significantly different between most intensive event and long intensive events but was significantly different between most intensive and short and medium-duration events. Only the rainfall events with the highest rainfall intensity could trigger serious soil erosion, up to 1000 kg ha-1 in the region. Further detailed investigations on the effects of tillage operations on the soil erosion in a subtropical region with a monsoon climate are needed to provide a basis for modeling catchments and designing better management practices.

Modelling the Effects of Land-use Change on Runoff and Sediment Yield in the Weicheng River Watershed, Southwest China

As a major sediment area in the upper Yangtze River, Jialing River basin experienced substantial land-use changes, many water conservancy projects were constructed from the 1980 s onward to promote water and soil conservation. The water and sediment yield at the watershed outlet was strongly affected by these water conservation works, including ponds and reservoirs, which should be considered in the modelling. In this study, based on the observed data of the Weicheng River catchment, the relationships between precipitation, runoff, vegetation, topography and sediment yield were analyzed, a distributed runoff and sediment yield model(WSTD-SED) was developed, and the hydrological processes of different land-use scenarios were simulated by using the model. The main results are summarized as follows: 1) there is an alternating characteristic in river channels and reservoirs in the Jialing River hilly area, with scour occurring in wet years and deposit occurring in dry years. 2) Most of the sediment deposited in river channels and reservoirs is carried off by the largest flood in the year. 3) The model yielded plausible results for runoff and sediment yield dynamics without the need of calibration, and the WSTD-SED model could be usedto obtain qualitative estimates on the effects of land use change scenarios. 4) The modelling results suggest that a 10% increase in cropland(dry land) reforestation results in a 0.7% decrease in runoff and 1.5% decrease in sediment yield.

Impacts of soil properties on phosphorus adsorption and fractions in purple soils

Information on phosphorus(P) adsorption and its impacts on the redistribution of the P fraction in soil profiles are important for environmental management under intensive agricultural practices.To clarify the dominant factors influencing soil phosphorus adsorption in an Entisol(locally known as purple soil), P adsorption experiments were conducted in Sichuan Basin of southwestern China for cropland and woodland soils with acidic, neutral and calcareous origins throughout their profile. After various doses of P were added during incubation experiments, soil P fractions were also analyzed. The results showed that there were no significant differences in Fe-oxides and P adsorption along the vertical gradients. Agricultural practices and lower p H conditions reduced the P adsorption capacity of purple soils throughout the soil profiles. For acidic and neutral purple soil profiles, the P adsorption capability was mainly influenced by Fe-oxides and soil texture. Ca-bound P and Fe-Al-bound P represented the majority of the total inorganic P of calcareous soils.There was a saturation of adsorption capacity by sesquioxide and a high risk of dissoluble reactive P(NH_4 Cl-P) being released out of the soil profile in acidic and neutral purple soils after the greatest P addition, indicated by the higher proportions of NH_4 Cl-P(over 40%) and decreasing Fe-Al-P fraction.P fractions migrated with greater difficulty in calcareous purple soil profiles as Ca-P fraction peaked over 65% when adding a P dose at or greater than 80 g P kg^(-1), indicating the high potential of P adsorption.The X-Ray Diffraction analysis also verified the formation of brushite. Adaptive management practices should be designed to alleviate P losses for acidic and neutral purple soils.

Middle-Late Pleistocene Paleo-Climate and Paleo-Altimetry of the Centre of Tibetan Plateau Indicated by the Sporopollen Record of Well QZ-4

The core sample from well QZ-4 is an important climate archive for the central Tibetan Plateau in the middle-late Pleistocene. In this work, a detailed pollen analysis of it is carried out to provide a preliminary insight into the paleo-climate and paleo-altimetry change in the central Tibetan Plateau. It can be concluded that the pollen assemblage can be obviously divided into two pollen zones, Pollen zone I (251.1 - 314 m in depth, 120.0 - 345.8 ka BP.) and Pollen zone II (200 - 251.1 m in depth, 105.4 - 120 ka BP.). The paleo-climate during pollen zone I deposition period was comparatively colder and wetter than it during the pollen zone II deposition period. After Gonghe Movement, the center of Tibetan Plateau was uplifted about 300 m (from 3500 - 3700 m to 3800 - 4000 m in elevation). The wind was changed from horizontal or downward direction to upward direction, in the study area. In the central of Tibetan Plateau, the climate change seems to be mainly driven by global climate change, and that tectonic uplift may have been a subordinate influence at the middle-late Pleistocene.

Freeze-thaw process induced by increased precipitation affects root growth of alpine steppe on the Tibetan Plateau

The response of vegetation productivity to precipitation is becoming a worldwide concern.Most reports on responses of vegetation to precipitation trends are based on the growth season.In the soil freeze/thaw process,the soil water phase and heat transport change can affect root growth,especially during the thawing process in early spring.A field experiment with increased precipitation(control,increased 25%and increased 50%)was conducted to measure the effects of soil water in early spring on above-and below-ground productivity in an alpine steppe over two growing seasons from June 2017 to September 2018.The increased 50%treatment significantly increased the soil moisture at the 10 cm depth,there was no difference in soil moisture between the increased 25%treatment and the control in the growing season,which was not consistent in the freeze/thaw process.Increased soil moisture during the non-growing season retarded root growth.Increased precipitation in the freezing-thawing period can partially offset the difference between the control and increased precipitation plots in both above-and below-ground biomass.

Decline in tree-ring growth of Picea mongolica and its intra-annual eco-physiological responses to drought and CO_(2) enrichment in semi-arid China

Episodes of drought-induced decline in tree growth and mortality are becoming more frequent as a result of climate warming and enhanced water stress in semi-arid areas.However,the ecophysiological mechanisms under-lying the impact of drought on tree growth remains unre-solved.In this study,earlywood and latewood tree-ring growth,δ^(13)C,andδ^(18)O chronologies of Picea mongolica from 1900 to 2013 were developed to clarify the intra-and inter-annual tree-ring growth responses to increasingly fre-quent droughts.The results indicate that annual basal area increment residuals(BAI_(res)),which removed tree age and size effects,have significantly decreased since 1960.How-ever,the decreasing trend of earlywood BAI_(res) was higher than that of latewood.Climate response analysis suggests that the dominant parameters for earlywood and latewood proxies(BAI_(res),δ^(13)C andδ^(18)O)were drought-related climate variables(Palmer drought severity index,temperature,rela-tive humidity,and vapor pressure deficit).The most signifi-cant period of earlywood and latewood proxies’responses to climate variables were focused on June-July and July-August,respectively.BAI_(res),andδ^(13)C were significantly affected by temperature and moisture conditions,whereasδ^(18)O was slightly affected.Decreasing stomatal conduct-ance due to drought outweighed the influence of increasing CO_(2) on intrinsic water use efficiency(iWUE),and ultimately led to a decline in BAI_(res).Compared to latewood,the faster decreasing BAI_(res) and smaller increasing iWUE of early-wood suggested trees were more vulnerable to water stress in the early growing season.Our study provides insights into the inter-and intra-annual mechanisms of tree-ring growth in semi-arid regions under rising CO_(2) and climate change.

^(137)Cs tracing of the spatial patterns in soil redistribution,organic carbon and total nitrogen in the southeastern Tibetan Plateau

The southeastern Tibetan Plateau,which profoundly affects East Asia by helping to maintain the stability of climate systems,biological diversity and clean water,is one of the regions most vulnerable to water erosion,wind erosion,tillage erosion,freeze-thaw erosion and overgrazing under global climate changes and intensive human activities.Spatial variations in soil erosion in terraced farmland(TL),sloping farmland(SL)and grassland(GL)were determined by the^(137)Cs tracing method and compared with spatial variations in soil organic carbon(SOC)and total nitrogen(total N).The^(137)Cs concentration in the GL was higher in the 0-0.03 m soil layer than in the other soil layers due to weak migration and diffusion under low precipitation and temperature conditions,while the^(137)Cs concentration in the soil layer of the SL was generally uniform in the 0-0.18 m soil layer due to tillage-induced mixing.Low^(137)Cs inventories appeared at the summit and toe slope positions in the SL due to soil loss by tillage erosion and water erosion,respectively,while the highest^(137)Cs inventories appeared at the middle slope posi-tions due to soil accumulation under relatively flat landform conditions.In the GL,the^(137)Cs data showed that higher soil erosion rates appeared at the summit due to freeze-thaw erosion and steep slope gradients and at the toe slope position due to wind erosion,gully erosion,freeze-thaw erosion and overgrazing.The^(137)Cs inventory generally increased from upper to lower slope positions within each terrace(except the lowest terrace).The^(137)Cs data along the terrace toposequence showed abrupt changes in soil erosion rates between the lower part of the upper terrace and the upper part of the immediate terrace over a short distance and net deposition on the lower and toe terraces.Hence,tillage erosion played an important role in the soil loss at the summit slope positions of each terrace,while water erosion dominantly transported soil from the upper terrace to the lower terrace and resulted in net soil deposition on the flat lower terrace.The SOC inventories showed similar spatial patterns to the^(137)Cs inventories in the SL,TL and GL,and significant correlations were found between the SOC and^(137)Cs inventories in these slope landscapes.The total N inventories showed similar spatial patterns to the inventories of,37Cs and SOC,and significant correlations were also found between the total N and^(137)Cs inventories in the SL,TL and GL Therefore,^(137)Cs can successfully be used for tracing soil,SOC and total N dynamics within slope landscapes in the southeastern Tibetan Plateau.

A Preliminary Study on Traditional Level-trench Method to Prevent Rill Initiation in the Three Gorges Region,China

The level-trench method is a traditional tillage operation to develop a drainage system on slope land in the Three Gorges region.Before crops are planted,farmers prepare the land with level trenches spaced 3-10 m apart,depending on the slope gradient:steeper slopes require shorter distances.Little scientific analysis has been done on the use of traditional level trenches for soil conservation.We conducted a field investigation and simulation experiments,and present a comparison between the spacing of level trenches and the slope length required for rill initiation.The results indicate that the spacing of level trenches in farmland is close to the slope length required for rill initiation in the experimental plots,and regression models of trench spacing with the gradient and the slope length required for rill initiation with the gradient have similar formats;the coefficients of determination (R2) for the two equations are 0.99 and 0.88,respectively.There were some differences between the field survey and experiments,but we conclude that the traditional level-trench method has an important role in preventing rill initiation.

Incorporating a rainfall intensity modification factorγinto the I a-S Relationship in the NRCS-CN method

The Natural Resources Conservation Service runoff curve number(NRCS-CN)method is widely used to simulate direct runoff,but the impact of rainfall intensity has not been considered.In this study,a rainfall intensity modification factor(γ)was incorporated into the Ia-S relationship of the NRCS-CN method,and the modified method(NRCS-CN-γ)was compared with the NRCS-CN method withλ=0.2 andλ=0.05 in three watersheds of the Walnut Gulch Experimental Watershed(WGEW).The results showed that for 2016-2018 period,the simulation performance of the NRCS-CN-γmethod was close to the NRCS-CN(λ=0.05)method and better than the NRCS-CN(λ=0.2)method.When the new data(2009 data with high variance)was added,the significant improvement was observed by NRCS-CN-γmethod with all the evaluation parameters being the best in the three watersheds,indicating a more adapted capa-bility of the modified method with highly uneven rainfall intensities.The covariance between rainfall intensity and the simulated runoff were 19.01,15.14,and 16.35 for the three methods,respectively.When the optimal CN changed,the relative errors representing CN sensitivity were 6.25,6.49 and 17.39 for the methods,respectively.It is suggested that the NRCS-CN-γmethod outperformed the other two methods and could contribute to a more accurate estimation of direct runoff where rainfall intensity greatly varied,especially in monsoon region or under the context of climate change.