Institute of Soil and Water Conservation,Chinese Academy of Sciences and Ministry of Water Resources
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Effect of embedded-rock fragments on slope soil erosion during rainfall events under simulated laboratory conditions
No full textQuantifying the role of embedded-rock fragments in soil-water processes on a slope is crucial for the application of rock fragments in soil and water conservation practice. In this study, a laboratory runoff and erosion experiment was conducted in the soils embedded with rock fragments (including five rock fragment contents (RC) of 0, 0.1, 0.2, 0.3, and 0.4 kg kg(-1)) in a metal flume under three slopes (5 degrees, 10 degrees, and 15 degrees). Results showed that the mean steady infiltration rate in the soil without rock fragment (NULL) was lower than that in the soils containing rock fragments (low rock fragment contents of 0.1-0.2 kg kg(-1) and high rock fragment contents of 0.3-0.4 kg kg(-1), labelled as LOW and HIGH, respectively) as slope gradient = 5 degrees, but was higher than the latter two soils as slope gradients = 10 degrees and 15 degrees. Initial infiltration rate significantly increased with RC (P < 0.05). Runoff discharge was negatively related to RC as slope gradient = 5 degrees and was positively related to it as slope gradients = 10 degrees and 15 degrees. There was a steady sediment concentration (SC) after runoff generated as slope gradient = 5 degrees. Sediment discharge and soil erosion modulus for the treatment of HIGH significantly increased with slope gradient (P < 0.05). Reynolds number (Re) decreased with RC and the other hydraulic parameters of friction coefficient (f), shear stress (tau), and stream power (omega) increase with RC. In general, the embedded-rock fragment in the soil can accelerate soil erosion by changing the ratio of infiltration/runoff, surface roughness, and related hydraulic parameters, especially under higher slope gradients. Our findings suggest that using rock fragments to control soil erosion is workable only under the conditions of lower slope gradients and rock fragment contents
New design of external heat-ratio method for measuring low and reverse rates of sap flow in thin stems
No full textSap-flow techniques had limited application to thin-stemmed woody and herbaceous species and to diverse functional plant organs until the recent development of the external heat-ratio (EHR) method. Existing EHR techniques using miniature gauge configurations, however, are limited to thin stems with diameters of 2-5 mm. This study introduces a new design of an EHR gauge adapted to thin stems with diameters up to 15 mm and sapflux densities < 50 cm h(-1). The gauge was calibrated on cut stems of the shrubs Caragana korshinskii and Salix psammophila, with the measured heat-pulse velocity (V-h) compared to gravimetric measurements of sap-flux density (V-s) under controlled conditions. A validation test was also carried out by comparing MR method with the stem heat-balance (SHB) method through in situ and long-term monitoring of V-s on standing stems of C. korshinskii. Vh in the tested cut stems of both species was linearly correlated with V, up to approximately 50 cm h(-1) (R-2 up to 0.96, P < 0.001) in a range of stem diameters of 4.1-15.6 mm. An empirical multiplier for converting the measured V-h to V-s, however, varied between the two species, 2.02 and 1.15 for C. korshinskii and S. pscunmophila, respectively. The EHR technique sensitively captured the diurnal dynamics of V-s in field tests, within a range from zero to nearly 30 cm h(-1) on C. korshinskii stems, and hourly V-s was linearly correlated with the reference evapotranspiration (R-2 = 0.70, P < 0.001) over 26 successive days without drought stress. The tested SHB method, however, poorly detected the sap-flux density, especially at low densities. The gauges for the EHR method were easy to build and capable of accurate estimating bidirectional sap flow, especially at low densities. This technique, with variable EHR gauge configurations, has broader applications than SHB methods for understanding plant-water relations in understories, shrubs and ecosystems dominated by herbage
Water use strategies for two dominant tree species in pure and mixed plantations of the semiarid Chinese Loess Plateau
No full textUnderstanding the water sources and physiological responses to soil moisture pulses for plantation species, particularly in mixed plantations, are essential to assess the water use strategy and vegetation restoration in semiarid regions. We used hydrogen stable isotopes in plant and soil water to determine the potential water sources for Pinus tabuliformis and Hippophae rhamnoides in both pure and mixed plantations in the semiarid Chinese Loess Plateau during the vigorous growing season (June-August) in 2016. Stomatal conductance (g(c)), midday leaf water potential ((m)) and photosynthetic rate (P-r) were measured concurrently to analyse the physiological response. The P.tabuliformis in the pure plantation depended largely on shallow and middle soil layers regardless of precipitation amount, permitting this species to maintain stable (m) at the expense of P-r through stomatal control. In contrast, H.rhamnoides in the pure plantation shifted its water source from shallow to deep soil layers following decreases in precipitation, allowing this species to maintain stable g(c) and P-r at the expense of (m). Thus, P.tabuliformis and H.rhamnoides displayed isohydric and anisohydric behaviour, respectively. Additionally, both species in mixed plantations largely absorbed water from shallow soil layers and shifted to deep soil layers when precipitation decreased. Mixed afforestation significantly reduced (p<.05) P-r for P.tabuliformis and (m) for H.rhamnoides. Although contrasting physiological responses were adopted by these species, the major proportion of water resources were competitively obtained from similar soil depthsindicating that their mixed afforestation requires further investigation
Effects of long-term fertilisation on aggregates and dynamics of soil organic carbon in a semi-arid agro-ecosystem in China
No full textBackground. Long-term fertilisation has a large influence on soil physical and chemical properties in agro-ecosystems. The effects on the distribution of aggregates, however, are not fully understood. We determined the dynamic change of the distribution of aggregates and soil organic carbon (SOC) content over time in a long-term field experiment established in 1998 on the Loess Plateau of China and illustrated the relationship between them. Methods. We determined SOC content and the distribution of aggregates in nine fertiliser treatments: manure (M); nitrogen (N); phosphorus (P); M and N; M, N, and P; M and P; N and P; bare land; and an unfertilised control. These parameters were then used for a path analysis and to analyse the fractal dimension (D-v). Results. The organic fertiliser increased SOC content. The proportions of 0.1-0.25 mm microaggregates and 0.25-0.5 mm macroaggregates were higher and the proportion of the 0.01-0.05 mm size class of the silt + clay fraction was lower in the treatments receiving organic fertiliser (M, MN, MNP, and MP) than that in the control, indicating that the addition of organic fertiliser promoted aggregation. The distribution of aggregates characterised by their fractal dimension (D-v), however, did not differ among the treatments. Discussion. D-v was strongly correlated with the proportion of the <0.002 mm size class of the silt + clay fraction that did not differ significantly among the treatments. The change in the distribution of aggregates was strongly correlated with SOC content, which could produce organic polymer binding agents to increase the proportion of larger particles. Long-term application of organic fertiliser is thus necessary for the improvement and maintenance of soil quality in semi-arid agricultural land when residues are removed
Effects of vegetation rehabilitation on soil organic and inorganic carbon stocks in the Mu Us Desert, northwest China
No full textIn arid and semiarid areas, the importance of soil inorganic carbon (SIC) is at least as high as that of soil organic carbon (SOC) in affecting the regional carbon budget following vegetation rehabilitation. However, variations in SIC have been uncertain, and few studies have analyzed the interactions between the SOC and SIC pools. We measured SIC, SOC, C-13-SIC, and C-13-SOC after planting Mongolian pine (MP) and Artemisia ordosica (AO) on shifting sand land (SL) over 10years in the Mu Us Desert, northwest China. The results showed that, compared to SL, SIC stocks at 0-100cm in MP and AO lands significantly increased by 12.6 and 25.8Mgha(-1), respectively; SOC stocks in MP and AO lands significantly increased by 24.0 and 38.4Mgha(-1), respectively. Both C-13-SIC and C-13-SOC in the 2 plantation lands were significantly lower than those in SL were. All 315 samples exhibited a negatively linear relationship between SIC content and C-13-SIC (R-2=.70, p<.01) and showed positively linear relationships between SIC content and SOC content (R-2=.69, p<.01) and between C-13-SIC and C-13-SOC (R-2=.61, p<.01). The results demonstrated that vegetation rehabilitation on SL has a high potential to sequester SIC and SOC in semiarid deserts. The reduction in C-13-SIC and the relationship of SIC with C-13-SIC following vegetation rehabilitation suggested that SIC sequestration is likely caused by the formation of pedogenic inorganic carbon. The relationships between SIC and SOC and between C-13-SIC and C-13-SOC implied that the pedogenic inorganic carbon formation may be closely related to the SOC accumulation
Joint Control of Net Primary Productivity by Climate and Soil Nitrogen in the Forests of Eastern China
No full textThe nature and extent of climate and soil nutrient controls in Chinese forests remain poorly resolved. Here, we synthesized the data on carbon-climate-soil in eastern China, and litter N was firstly taken into consideration, to examine the variation of net primary productivity (NPP) and its driving forces. Results showed that NPP had significant latitude pattern and varied substantially across climate zones. Bivariate analyses indicated that mean annual temperature (MAT), mean annual precipitation (MAP), soil N content (N-soil), and annual litter N (N-re) were the main controlling factors in spatial pattern of forest NPP. Notably, partial general linear model analysis revealed that MAT, MAP, and N-re jointly explained 84.8% of the spatial variation of NPP. Among the three major factors, N-re explained more variation of forest NPP than the other two factors, and MAT and MAP affected NPP mainly through the change of litter N rather than via themselves, highlighting the importance of litter N in estimating forest NPP. However, to accurately describe the pattern of forest NPP in China, more detailed field measurements and methodologies on NPP and relevant confounding factors should be addressed in future studies
Quantitative study of the crop production water footprint using the SWAT model
No full textAssessment of the water use efficiency is the key to effectively manage agricultural water resource. The water footprint is a new index for water use evaluation, and its quantification is a precondition for assessment of the agricultural water use efficiency. Due to the shortage of water footprint calculation methods and computational module defects, this study aims to establish a method for calculating the water footprint of crop production based on hydrological processes. In this study, the field-scale water footprints of wheat, corn and sunflower were calculated using the SWAT model in the Hetao irrigation district (HID), China. The results showed that the average total water footprints of wheat, corn and sunflower were 1.036 m(3)/kg, 0.774 m(3)/kg and 1.510 m(3)/kg, respectively. Additionally, the proportions of green water footprints in wheat, corn and sunflower were 22.3%, 26.1% and 29.4%, respectively. Water footprint calculations based on the SWAT model can reflect the spatial differences of water footprints during the process of crop production. The overall distribution pattern of the green, blue and total water footprints for the three crops demonstrated that high values were in the east part of the HID, followed by the west and the central areas. The SWAT-based water footprint offers high spatial resolution and is effective in exploring the spatial heterogeneity of crop water footprints
Effect of straw mulch and seeding rate on the harvest index, yield and water use efficiency of winter wheat
No full textStraw mulching has been used to improve water use efficiency (WUE, yield per unit evapotranspiration) in the Loess Plateau of China, but the seeding rate may need to be adjusted from conventional practice. We tested the three-way interaction between seeding rate, soil management (conventional tillage and straw mulching) and seasons. Wheat yield ranged from 2851 to 6981 kg ha(-1) and WUE from 5.3 to 16.2 kg ha(-1) mm(-1). Generally, soil water storage was higher and soil temperature was lower under straw mulching than under conventional practice. Evapotranspiration was higher under straw mulching. Yield was significantly affected by the interaction between straw mulching and season. Conventional practice showed significantly higher mean harvest index (HI), yield, and WUE than straw mulching in favorable seasons. However, yield was significantly higher under mulching than under conventional tillage in very dry season. Seeding rate had no effect on yield, but low seeding rate increased HI compared to high seeding rate. It is concluded that low seeding rate would be suitable for straw mulching
A laboratory study on rill network development and morphological characteristics on loessial hillslope
No full textRills are basic pathways for runoff, sediment, and pollutant transport at hillslopes within agricultural watershed. The objectives of this study were to investigate the development processes of rill network and morphological characteristics and to examine their affecting factors. A soil box (10 m long, 1.5 m wide, and 0.5 m deep) was subjected to four successive simulated rains under rainfall intensity of 90 mm h(-1) with slope gradients of 15A degrees and 25A degrees. Digital elevation models (5 mm resolution) were created from the terrestrial laser scanning measurements. Total soil loss was 46.3 and 61.0 kg m(-2) at the 15A degrees and 25A degrees slope gradients, and rill erosion occupied over 75% of the total soil loss. Soil loss and rill erosion were expressed as power equations to the product of slope gradient and accumulated rainfall. Rill networks evolved in a converging way and reached maturity in the fourth rain. Main rill length and rill width, depth, and degree of contour line departure increased with increased rains, while rill width/depth ratio showed the opposite trend. Secondary rill length and rill density increased in the first two rains, and then both decreased in the latter two rains. Scour effect of lateral interfluve flow and meander cutoffs of rill flow were two sub-processes of rill piracy. Rill length and density decreased due to rill piracy specific in merging of secondary rills into main rills. Plow pan and secondary headcuts played key roles in main rill bed incision and sidewall expansion processes, while both had little impact on secondary rills. Results of this study can improve the understanding of how plow pan, rill piracy, and secondary headcut affect rill network and morphologies and provide fundamental knowledge for designing rill prevention practices
A new method to optimize root order classification based on the diameter interval of fine root
No full textPlant roots are a highly heterogeneous and hierarchical system. Although the root-order method is superior to the root diameter method for revealing differences in the morphology and physiology of fine roots, its complex partitioning limits its application. Whether root order can be determined by partitioning the main root based on its diameter remains uncertain. Four methods were employed for studying the morphological characteristics of seedling roots of two Pinus species in a natural and nitrogen-enriched environment. The intrinsic relationships among categories of roots by root order and diameter were systematically compared to explore the possibility of using the latter to describe root morphology. The normal transformation method proved superior to the other three in that the diameter intervals corresponded most closely (at least 68.3%) to the morphological characteristics. The applied methods clearly distinguished the results from the natural and nitrogen-rich environments. Considering both root diameter and order simplified the classification of fine roots, and improved the estimation of root lifespan and the data integrity of field collection, but failed to partition all roots into uniform diameter intervals