Mattic Epipedon Research Articles

Restoration approach for alpine extremely degraded landscapes: testing direct seeding and mulching with grass clippings

Alpine meadows are key ecosystems that are facing increasing degradation due to climate change and anthropogenic disturbances. Despite tremendous efforts in ecological restoration and diverse communities rebuilding in alpine regions, restoration of extremely degraded alpine meadows (which have lost mattic epipedon) remains a major challenge for land managers. Here, we performed a field restoration experiment that combined seeding three grass species commonly (Elymus nutans, Poa annua, and Festuca sinensis) used in alpine meadow restoration with the application of grass clippings as mulch (bare land without mulching, 50% mulch cover, and 100% mulch cover) to ameliorate the soil surface conditions of degraded bare land. Soils covered with 50% and 100% mulching increased soil moisture, seedling density, plant cover, plant density, aboveground biomass, and belowground biomass, and reduced soil temperature during strong radiation days and insulated at night. In the three grass species seeding plots, mulching with grass clippings also improved seedling density, plant cover, plant density, and biomass. Moreover, 100% mulching had stronger restoration effect than 50% mulching. We further demonstrated that the improvement of surface microclimate in degraded alpine meadows by grass clipping mulching, that is, increasing soil moisture and regulating soil temperature, is an important reason for the success of grassland restoration. Mulching with grass clippings simulates the original mattic epipedon, improving the surface hydrothermal conditions and promoting ecological restoration facilitation. We encourage the application of locally available grass clippings after seeding as this can be an efficient, low‐cost, and sustainable approach for restoring extremely degraded high‐elevation alpine grassland ecosystems.

Development of pedotransfer functions for predicting hydraulic parameters of van Genuchten model by incorporating environmental variables on the Qinghai-Tibet Plateau

The Qinghai-Tibet Plateau (QTP) region lacks large database of soil hydraulic parameters (SHPs) due to difficulty in soil sampling, and time- and labor-consuming measurement, which prevents accurate modeling of hydrological and biogeochemical processes. Pedotransfer functions (PTFs) using easily measurable soil properties may provide an alternative way to indirectly estimate SHPs. In this study, soil properties (bulk density (BD), clay, silt, sand, soil organic carbon, porosity, fitting parameter related to inverse of air entry pressure (α) and soil pore distribution (n), residual and saturated soil water content (θr and θs), and environmental parameters (altitude, slope gradient, annual average precipitation, air temperature, and evaporation) were determined from a north-south transect (ca. 500 km) in Qinghai Province and an east-west transect (ca. 1800 km) in Tibet Autonomous Region for establishing PTFs for SHPs on the QTP. The coefficient of variation of α was the largest, followed by θr, θs, and n, suggesting that α had the greatest spatial variability at the regional scale. There were significant differences in BD among 0–10, 10–20, and 20–30 cm soil layers (p < 0.05), whereas there were no significant differences in α, n, θr, and θs among these soil layers (p > 0.05). The five most important variables from the random forest analysis were selected to establish the PTFs of the SHPs and BD for the 0–30 cm soil layer on the QTP. The established PTFs for predicting SHPs and BD using multiple linear regression analysis were jointly affected by relief, climate, and soil properties. The accuracy of our newly established PTFs with the consideration of gravel content (< 10% or 10–30%) and mattic epipedon (existence or non-existence) was higher than that of the existing PTFs. We developed the first PTFs of SHPs on the QTP, providing a foundation to construct SHPs prediction models for other alpine regions in the world.

Characteristics of the soil macropore and root architecture of alpine meadows during the seasonal freezing-thawing process and their impact on water transport in the Qinghai Lake watershed, northeastern Qinghai–Tibet Plateau

Low temperatures, freezing-thawing cycles, and short growing seasons characterize alpine soils. The mattic epipedon, a special diagnostic surface horizon with an intensive root network, is widely distributed in alpine ecosystems. Studies on the soil macropores and roots of the mattic epipedon layer in response to seasonal freezing-thawing processes on the Qinghai–Tibet Plateau are lacking. This study characterized the soil macropores and roots of alpine meadows during the seasonal freezing-thawing process using X-ray computed tomography and revealed the influence of soil macropores and roots structure on water transport in the mattic epipedon layer of the alpine meadows. The results showed that the soil pore distribution was more uniform during the unstable freezing stage (UFP) and the unstable thawing stage (UTP), whereas there was a clear mattic epipedon layer during the completely thawed stage (TP) and the completely frozen stage (FP). Soils in the TP stage had a higher total surface area density (0.1898 mm² mm⁻³), length density (225.28 mm mm⁻³), node density (1,592 no. mm⁻³), and connectivity (0.3144) of soil macropores than those in the UFP, UTP, and FP stages. In the TP stages, the density, surface area density, branch density, length density, and node density of roots had significant correlations with the macroporosity, surface area density, length density, node density, and tortuosity of soil macropores. In the FP stage, there were no correlations between the root and soil macropore characteristics. Vertical water is expected to move more readily through the mattic epipedon in the TP stage than in the UFP and UTP stages. Roots were the preferential pathways for water transport into the soil layer of the alpine meadow. Therefore, the mattic epipedon is a key layer for water and nutrient storage and plays an important role in the water-holding function of the Tibetan Plateau due to its greater root development.

Mattic epipedon fragmentation strengthened the soil infiltration capacity of a hillside alpine meadow on the Qinghai‐Tibetan Plateau

AbstractDegradation of hillside alpine meadow ecosystems is a widespread problem that usually results in the fragmentation of the mattic epipedon (ME). The mattic epipedon plays a well‐known role in the hydrological cycle of alpine ecosystems due to its high water‐holding capacity. However, little is known about how ME fragmentation alters soil infiltrability—a key soil property that influences runoff and erosion processes as well as soil moisture dynamics. To address this issue, we contrasted the infiltration rates and soil and plant characteristics of preserved meadow patches (with full ME) and patches at two stages of ME fragmentation, namely, half ME and without ME. Our results showed that mattic epipedon fragmentation reduced aboveground plant biomass (mainly of sedges) by 58.6–65.6 g m−2 and root biomass by 5,643–10,054 g m−2. Soil compressive strength (11.6 kg cm−2) and total soil porosity (52%) of fragmented meadow patches without ME were significantly lower than in those with half (17.9 cm−2 and 62%, respectively) or full ME (31.7 cm−2 and 62.5%, respectively). In contrast, non‐capillary porosity and aggregate stability slightly increased with fragmentation and were highest at patches without ME. Importantly, soil infiltration rates in hillside alpine meadows greatly increased as a result of mattic epipedon fragmentation. Initial infiltration rates were approximately 10 and 20 times higher at patches with half ME and without ME (177 mm h−1 and 326 mm h−1, respectively) than at patches with the full ME (17 mm h−1). Moreover, steady‐state infiltration rates were approximately 6 and 9 times higher at patches with half ME and without ME (60 mm h−1 and 88 mm h−1, respectively) than at patches with the full ME (17 mm h−1). Overall, mattic epipedon fragmentation improved soil infiltrability by changing the soil and plant characteristics. These findings suggest that mattic epipedon fragmentation will lower runoff maintenance and have drastic effects on the water provisioning function of hillside alpine meadows on the Qinghai‐Tibetan Plateau.

Responses of soil water dynamics to precipitation events in an alpine meadow ecosystem of the Qinghai Lake Basin based on high‐precision lysimeter measurements

AbstractIn high‐altitude cold areas, how precipitation and freeze–thaw processes affect soil water dynamics is not well understood due to a lack of high real‐time resolution measurements. This study measured soil water balance components with a high‐precision lysimeter in an alpine meadow ecosystem in the Qinghai Lake watershed from June 30, 2020, to June 30, 2021. The results showed that the total precipitation (TP), evapotranspiration (ET), soil water storage change (ΔS) and the vertical water flow (WF) of this ecosystem were 705.18, 633.21, 72.58 and −0.60 mm, respectively, during the study period. Continuous wet days with precipitation events larger than 10.00 mm led to percolation, while continuous dry days with strong ET caused an upward WF. Pearson correlation analysis suggests that a larger instantaneous precipitation intensity may be more conducive to an increase in shallow soil moisture (R2 &gt; 0.72, p &lt; 0.05). Furthermore, the TP thresholds in the complete thawing period inducing the soil water response in the mattic epipedon layer (0–20 cm) and organic layer (20–40 cm) were 3.24 mm and 16.61 mm, respectively. The difference in the average response time of soil water to a single precipitation event between the mattic epipedon layer and organic layer was 5.52 ± 3.65 h. These results deepen the understanding of the temporal pattern of water flux in alpine meadow ecosystems on the Tibetan Plateau.

Impacts of alpine meadow degradation on the soil moisture and hydrological processes in the three river source region on the Qinghai Tibetan plateau

AbstractTo explore the effects of mattic epipedon (ME) on soil moisture and hydraulic properties in the alpine meadow of three‐river source region, the soil moisture, water infiltration, evapotranspiration, soil bulk density and soil water holding capacity of original vegetation (OV), light degradation (LD), moderate degradation (MD) and severe degradation (SD) was conducted in this study, respectively. The results showed that: (1) the alpine meadow degradation reduced the soil moisture in the shallow layer (0–10 cm) and had no significant effects on the soil moisture in the deep layer (20–30 cm). (2) The effects of alpine meadow degradation on infiltration was depend on the presence of ME or not, when the ME existed on the land surface (from OV treatment to MD treatment), the alpine meadow degradation had no significant effects on infiltration. Once the ME disappeared on the land surface (from MD treatment to SD treatment), the alpine meadow degradation mainly increased the infiltration. (3) With the aggravation of alpine meadow degradation, the daily evapotranspiration first decreased and then significantly increased when the gravimetric soil water content at 0–5 cm in SD treatment (GWC5) was exceeded 19.5%, the daily evapotranspiration gradually decreased when GWC5 ranged from 9.3% to 19.5%, and had no significant changes on the evapotranspiration when GWC5 was less than 9.3%. Considering the characteristics of precipitation in alpine meadow, it was concluded that the alpine meadow degradation accelerated the evapotranspiration during the plant‐growing season. (4) The effect of alpine meadow degradation on soil bulk density and saturated water capacity was concentrated at 0–10 cm. With the aggravation of alpine meadow degradation, the bulk density at 0–10 cm was first stable and then significantly increased and the saturated water capacity at 0–10 cm was first gradually increased and then significantly decreased. Our results suggested that the ME is vital for water conservation of alpine meadow and the protection of ME should be emphasized to promote the sustainable development of the ecosystem and the water supply of water towers in China.

OSL ages and pedogenic mode of Kobresia mattic epipedon on the northeastern Qinghai-Tibetan Plateau

Kobresia mattic epipedon (ME) is widely distributed on the Qinghai-Tibetan Plateau (QTP) and alpine areas worldwide, which plays a pivotal role in maintaining regional ecological stability. However, the time of ME formation and development is still unclear, which limits the understanding of pedogenic mode of alpine soils on the QTP. In this study, the optically stimulated luminescence (OSL) dating method was used to determine 54 dating samples from 45 MEs on the northeastern QTP (NE-QTP), the deposition rate (DR) and mass deposition rate (MDR) of MEs were calculated based on the OSL sampling depth, OSL age and bulk density (BD). The provenance of ME parent materials was identified by the soil grain-size composition, elemental geochemistry characteristics and bleachability of luminescence signals analysis. The results show: (1) aeolian dust deposition played an important role of ME parent materials; (2) modern ME formed mainly in the Late Holocene (∼ 3 ka) and concentrated during ∼ 1.4–0.2 ka. It took average 0.9 ka for a majority thickness of ∼ 20 cm ME formation; (3) the average DR of MEs was 0.3 ± 0.04 mm·a−1, and MDR was 346.3 ± 45.9 g·m−2·a−1, which was close to the flux of modern atmospheric fallen dust; (4) the formation of most MEs followed an aeolian dust aggradation mode.

Major Type of Mattic-Epipedon Ruptures in Alpine Meadow Ecosystems on the Qinghai–Tibet Plateau

Alpine meadow degradation is an important issue for ecosystem stability in the Qinghai–Tibet Plateau (QTP). As an essential part of alpine meadow ecosystems, mattic epipedon (ME) refers to the soil surface horizons rich in organic matter and roots, which plays an important role in nutrient cycling, water retention, and ecological stability of QTP. The rupture of ME has caused serious alpine meadow degradation, biodiversity accidents, and ecosystem service decline, as well as attracted substantial concerns. However, the type of ME ruptures and their distribution environment are not well studied. In this perspective, to identify the major type of ME ruptures in QTP, 257 areas were investigated and 24 dominant plant species were collected from 2019 to 2023. Results indicated that ME ruptures exist in 234 investigated areas and can be broadly divided into 5 types based on visual observations and expert knowledge: collapsed ruptures, polygon ruptures, sheddable ruptures, cavernous ruptures, and hummock ruptures. The dominant plant species and coverage also present great differences between different rupture types. This study provides insights into alpine meadow degradation from the ME rupture types and distribution perspectives. Identifying the ME rupture types and their distribution will contribute to enhance alpine meadow restoration success for protecting the ecological benefits.

Fragmentation alters the soil water conservation capacity of hillside alpine meadows on the Qinghai-Tibetan Plateau

The Qinghai-Tibetan Plateau—known as the roof of the world and the water tower of Asia—is facing serious degradation of its ecosystems, with alpine meadows being particularly threatened. However, little is known about how the loss of mattic epipedon (the characteristic topsoil of alpine meadows) alters the water conservation capacity of the region. Here we examined the effects of different mattic epipedon coverages on surface runoff, evapotranspiration, and soil water storage under different rainfall conditions. Our results showed that the surface runoff was significantly higher under moderate rain conditions (P > 10 mm day−1) than under light rain conditions (P < 10 mm day−1), and that declining coverage significantly increased surface runoff, especially under moderate rain conditions. The average evapotranspiration at 90 %, 60 %, and 30 % mattic epipedon coverages were 5.38 mm, 5.88 mm, and 6.38 mm under non-rainfall conditions, and 0.97 mm, 1.60 mm, and 2.24 mm under moderate rain conditions. Meanwhile, mattic epipedon coverage had a positive effect on the supplementation of soil water under moderate rainfall and on the conservation of soil water under non-rainfall conditions. Overall, alpine meadows with higher mattic epipedon coverage were more effective at maintaining surface runoff and improving soil water conservation. These findings highlight the importance of conserving and restoring alpine meadows to improve the water security in the Qinghai-Tibetan Plateau and other downstream regions.

The Process of Soil Nutrient Stabilization in Micro-Patches in Alpine Kobresia Meadows

Micro-patches are the basic unit of grazing ecosystems; the characteristics of these micro-patches are relatively stable in species under different grazing intensities in the same vegetation, but obviously different in terms of the distribution pattern. This leads to differentiation of plant community numerical characteristics under different grazing intensities. Understanding the driving force of soil nutrient variation in micro-patches under grazing disturbances will help us comprehend the regulation strategy and adaptation mechanisms of the ecosystem against over-disturbance. We designed four scales: spatial (three typical micro-patches), temporal serial (6 years), a degradation succession process (four key degradation stages), and recovery treatment (three treatments: the original grazing intensity based on herder preferences, half of the original grazing intensity, and zero grazing). The soil nutrient characteristics used to estimate stabilization were the typical soil total nutrient content (soil organic matter [SOM], total nitrogen [TN], total carbon [TC], inorganic carbon [IC], total phosphorus [STP], total potassium [TK], and pH), and available soil nutrients (NH4+, NO3−, phosphorous [avP], and potassium [avK]). Variations in the SOM, TC, IC, TN, STP, avK, and NO3− levels in the main root distribution layers (0–20 cm) on the spatial scale were 69.8–79.7%, 61.4–80.35%, 49.8–79.58%, 60.52–76.34%, 46.44–89.89%, 45.5–71.36%, and 59.21–65.38%, respectively, which accounted for the largest variation in the four scales, based on multivariable analysis. The variations in the avP and NH4+ content of the main root distribution layers (0–20 cm) at the temporal scale were 46.42–67.93% and 48.11–64.55%, respectively, which accounted for the greatest variation in the four scales, based on a multivariable analysis. Upon comparing the degradation succession stages and recovery treatment in each stage, we found that the variation in avP, avK, STP, TN, TC, SOM, TC, and TN content was greater at the degradation succession scale than at the recovery treatment scale. The soil nutrient content of the micro-patches exhibited the smallest decrease in the Gramineae-Kobresia transformation (G-KP) micro-patch, followed by the Gramineae micro-patches (G) and Kobresia micro-patches (KP). The number of G micro-patches decreased with increasing grazing intensity whereas the number of KP micro-patches increased. When the number of KP micro-patches increased to a certain degree, the number of G-KP micro-patches then increased as well. G-KP micro-patches, characterized by cracking in the mattic epipedon in alpine meadows, increased with the grazing intensity increasing in a certain degree in K. pygmaea meadows with mattic epipedon cracking (CP); the latter buffered the nutrient variation and maintained the soil nutrients’ relative stability in the ecosystem. Thus, CP formed the buffer stage for maintaining self-stabilization during a regime shift and was considered the withstanding stage during the alpine Kobresia meadow degradation process.

Shrub encroachment enhances the infiltration capacity of alpine meadows by changing the community composition and soil conditions

Due to global changes and anthropogenic disturbances shrubs are widely encroaching into alpine meadows, which may have profound impacts on the ecohydrological functioning of these key ecosystems. This phenomenon is particularly alarming in the alpine meadows of the Qinghai-Tibetan Plateau, the headwaters for many of Asia’s largest rivers. However, little is known about the extant in which shrub encroachment alters the infiltration capacity of those soils. Here we examined the effects of shrub encroachment on the plant community and soil infiltration processes by contrasting vegetation and infiltration variables between three areas: a reference alpine meadow, an alpine meadow lightly encroached by smaller shrubs, and an alpine meadow heavily encroached by larger shrubs. We found that shrub encroachment reduced herbaceous plants aboveground biomass by 10–24 g m−2 and root biomass by 1346–2239 g m−2. The sedge functional group was gradually replaced by forbs. Initial infiltration rates gradually increased with shrub encroachment, and were approximately doubled and tripled at the earlier and later stages of encroachment, respectively. Overall, shrub encroachment improved the soil infiltrability by changing soil conditions and inhibiting herbaceous species due to the reduction of surface mattic epipedon and the formation of dead root channels. These findings suggest that the rapid transformation of alpine meadows into shrublands could have drastic effects on the surface runoff and streamflow dynamics of the Qinghai-Tibetan Plateau.

Lateral flow between bald and vegetation patches induces the degradation of alpine meadow in Qinghai-Tibetan Plateau

Bald patches (BPs) are known to accelerate and simultaneously mitigate the process of desertification. However, the mechanisms of these two synchronous actions are little studied in high desert and cold systems; and the incidence of BPs on alpine meadows degradation in Qinghai-Tibetan Plateau (QTP) of China is still unavailable. This study first aims to investigate the soil properties and the erodibility of the system BPs-VPs at the Beiluhe basin in QTP. Then, we adopted dye tracer and HYDRUS-2/3D methods to interpret the water infiltration patterns from point scale to slope scale. The results show that the mattic epipedon layer on the top soil (0–20 cm) of VPs directly reduced the impact of raindrops on alpine meadow; and the adhesion of root system in VPs prevented the soil particles from stripping and washing away by runoff. The soil particles in BPs were easily eroded by rainfall, lowering the ground level of BPs relative to the ground level of VPs. The two patches therefore alternated to form an erosion interface where marginal meadow was likely detached by raindrops, and washed away through runoff. The saturated hydraulic conductivity (Ks) of surface soil (0–10 cm) was 124% higher in BPs than the VPs. Thereby, BPs caused a high spatial variation of infiltration and runoff in QTP. Moreover, this difference in Ks between the two patches conducted to a lateral flow from BPs to VPs, and to soil layers with different water contents. These findings highlight that the water flow features can potentially disturb the processes of freezing-thawing, frost heaves, and thaw slump; and accelerate the alpine meadow degradation. Therefore, land cover such as crop and vegetation should be applied over the bare soil surface to prevent the degradation of alpine meadow.

Soil water retention in alpine meadows under different degradation stages on the northeastern Qinghai-Tibet Plateau

Alpine meadow degradation has become serious in recent decades due to the combined effects of overgrazing, human activities and rodents, but very few studies focus on the possible impact of alpine meadow degradation on soil water retention on the Qinghai-Tibet Plateau. In this study, four degradation stages (non-degradation, ND; light degradation, LD; moderate degradation, MD; heavy degradation, HD) were selected. Results showed that (1) the aboveground biomass decreased with increasing degradation, while the belowground biomass first increased and then decreased with a maximum value in MD. (2) As the degree of degradation increased from ND to MD, the 0–10 cm soil bulk density (BD) and soil compaction (SC) decreased with the minimum values in MD (0.61 g cm−3 for BD and 1619.51 kPa for SC), while soil organic matter (SOM) increased with maximum value in MD (247.36 g kg−1), the 0–10 cm SOM in MD was the 1.5 times of that in ND. However, the 0–10 cm particle size distribution did not change significantly with increasing degradation, while the 20–40 cm clay content in HD was significantly higher than those of the other three degradation stages, (3) The MD stage displayed a strong water holding capacity in 0–10 cm soil layer due to its higher soil organic matter induced by the thick mattic epipedon. Meanwhile, the water holding capacity in the HD stage was higher than that of the other three degradation stages in the 20–40 cm soil layer owing to its higher capillary porosity. Furthermore, the soil water retention was greatly affected by the mattic epipdon through its effects on soil properties such as reducing BD and increasing SOM. Our results suggested that the mattic epipdon may play a crucial role in water retention in alpine ecosystems.

Physicochemical properties and micromorphology of degraded alpine meadow soils in the Eastern Qinghai-Tibet Plateau

The Qinghai-Tibet Plateau is currently suffering from severe ecological problems. Soil degradation is a major environmental problem, and there is strong evidence that such processes present an immediate threat to both biomass and economic yields. The aim of the present study is to clarify the response mechanism of mattic epipedon during soil degradation process in alpine meadow, we investigated (1) morphological characteristics and physicochemical properties of soils, (2) soil micromorphology under different degradation stage. According to plant and land use coverage, three sites were chosen from the Hequ horse farm: lightly degraded meadow (HQ1-L); moderately degraded meadow (HQ2-M); and heavily degraded meadow (HQ3-H). A vegetation survey and soil profile morphology showed that Kobresia kansuensis was the dominant species in HQ1-L and HQ2-M, featuring in topsoil horizons rich in rhizogenic organic matter which creates turfs. There are strong relationships between the presence of the mattic epipedon (OA horizon) and soil physicochemical properties. Thus, exchangeable cations, cation exchangeable capacity (CEC), total nitrogen (TN), and organic carbon (OC) decrease during the degradation of grassland; however, the pH value shows an opposite trend. Exchangeable cations, CEC, TN, and OC provide an index of soil fertility to measure the degree of grassland degeneration. Soils were classified as Luvic Phaeozems, Haplic Phaeozems, and Calcaric Phaeozems. From the micromorphology of soils in thin section, considering soil degradation, the porosity and fractal dimension decrease, and the soil microstructure evolves to an intergrain micro-aggregate structure. Also, by counting the number of excrements in the soil thin sections, and combining this with the fractal dimension of the porosity, we found the HQ1-L is more conducive to soil fauna survival, by comparison to HQ2-M, and HQ3-H which follow HQ1-L in condicivity. These results suggest that the existence of the OA horizon (mattic epipedon) is directly related to soil degradation.

The geomorphology – sediment distribution – soil formation nexus on the northeastern Qinghai-Tibetan Plateau: Implications for landscape evolution

The interactions between geomorphology, sediment distribution and soil development in alpine regions are poorly understood. In this study, we analyzed the spatial variations of different sediment types and corresponding soils in 15 pedosedimentary profiles across three major geomorphic units in an alpine ridge-valley basin of the Qilian Mountains, northeastern Qinghai-Tibetan Plateau. Particle size distribution was used to trace the sediment types. Inorganic and organic carbon concentrations were included as metrics of soil processes. Aeolian silts from both distant and proximal sources cover floodplains, hillslopes and some positions on alluvial fans, while the distribution of fluvial sediments and periglacial slope deposits are much more limited due to the coverage of loess. The soils vary in response to sediment types and climatic conditions under the influence of their geomorphic settings. The vertical distribution patterns of carbonates in soil profiles show considerable differences among different geomorphic units, reflecting their contrasting water balance and migration pathways. Soil organic carbon accumulates most extensively in aeolian silts with sufficient water supply. A special pedogenic soil horizon – the mattic epipedon is formed primarily in loessial sediments in Kobresia ecosystems. The complex entanglement of fine earths and massive grassroots make the mattic epipedon extremely resilient, which helps to preserve the soils and to shape the landscapes. By integrating geomorphology, sedimentology, ecology and pedology, the outcomes of this study promote the understanding of the evolution of typical landscapes and ecosystems of the northeastern Qinghai-Tibetan Plateau and many other periglacial settings.

Quantification of Soil Macropores at Different Slope Positions under Alpine Meadow Using Computed Tomography in the Qinghai Lake Watershed, NE Qinghai–Tibet

Soil macropores largely control fluid and solute transport and the runoff processes on the slopes. However, the characterization of soil macropores for different slope positions under alpine meadow conditions remains scarce, especially in the Qinghai Lake watershed. The objective of this study was to visualize and quantify the macropore characteristics of soils at different slope positions (i.e., the upper, middle and bottom slopes) in an alpine meadow in the Qinghai Lake watershed using X-ray computed tomography (CT). Nine soil cores (0–40 cm deep) were collected from the upper, middle and bottom slopes with three replicates per position, and cores were scanned with a GE HISPEED FX/I Medical Scanner. Macroporosity, length density, node density and tortuosity within the 40 cm soil profile were interpreted from the CT data to analyze the soil architecture. The results showed that the macroporosity decreased from the upper slope to the bottom slope. The macroporosity of soil on the upper slope was 50 times greater than that on the bottom slope, which can be attributed to the gravel in the former soil. The large number of continuous macropores and cracks concentrated in the deep soil layers (200–350 mm) of the upper slope can be attributed to the gravel layer. Most of the soil macropores under the middle and bottom slopes were distributed in the surface soil (0–150 mm), which can be attributed to the “mattic epipedon.” The soil macropores in the bottom slope, due to the well-developed root system, were more continuous and tubular than those in soils of the upper and middle slopes. It is expected that water can move preferentially through soil macropores on the upper and middle slopes compared to the bottom slope. Furthermore, due to its relatively low macroporosity the soil of the bottom slope contributes more to surface flow than those of middle and upper slopes.

Strong seasonal connectivity between shallow groundwater and soil frost in a humid alpine meadow, northeastern Qinghai-Tibetan Plateau

The freeze-thaw cycle in the active soil layer plays a crucial role in cold-region hydrological processes, yet the effect of vertical water migration caused by the soil frost on shallow groundwater remains poorly quantified. The shallow groundwater level (GWL), soil frost depth and main environmental controls were monitored in a humid alpine meadow on the northeastern Qinghai-Tibetan Plateau from 2012 to 2017. GWL fluctuated with two seasonal maxima and minima during soil frost-centric year from November to October. GWL declined remarkably from the strong peak (3.9 ± 0.2 m, Mean ± S.D.) in mid-January to the strong minimum (4.8 ± 0.1 m) in mid-May, and then increased to the second peak (4.2 ± 0.2 m) in late August. This seasonal variability in GWL was mainly attributed to the freeze-thaw processes of the lower layer soil frost from November to late June both at a ∼45-day delay, respectively. The clear increasing GWL from mid-November to mid-January was probably related to the melted upper layer soil frost, which was detained in subsoil layers because of the high thermal isolating effect in topsoil Mattic Epipedon (dense organic-rich turf) and the associated negative subsoil profile temperature gradient from mid-May to late August. The seasonal N-type in GWL was thus hysteretically determined by the soil freeze-thaw processes and the related subsoil profile temperature gradient. Annually, GWL was significantly correlated with air temperature (R2 = 0.88, P = 0.003, N = 6), suggesting the shallow groundwater recharges would be weakened under the warming scenario. Annual GWL was not directly connected with annual precipitation (P = 0.81), likely owing to the high water holding capacity of the topsoil Mattic Epipedon and the similarity between precipitation input and evapotranspiration loss. This critical buffering function of the topsoil Mattic Epipedon in vertical moisture movement should be taken into consideration when modeling regional water dynamics in humid alpine meadows.

Grazing-induced increases in soil moisture maintain higher productivity during droughts in alpine meadows on the Tibetan Plateau

Grazing is the primary land use practice in alpine ecosystems on the Tibetan Plateau. However, it remains unclear how grazing regulates levels of carbon and the water cycle in this ecosystem. A paired set of eddy covariance systems were set in adjacent fenced (FM) and grazed meadows (GM) to explore the grazing effects on alpine meadows. Aboveground biomass removed by grazing caused declines in grass transpiration (T), whereas the evaporation (E) was enhanced because of greater exposure to radiation, which in turn led to higher evapotranspiration (ET) in GM. However, the deep-layer soil moisture remained high because of the effects of mattic epipedon, which worked as a water-resistant layer. Therefore, the deep-layer soil moisture in GM was higher than that in FM because of decreased water consumption caused by the grazing-induced reduction in leaf area in GM. As a consequence, the deep-layer soil in GM could provide more water to help plants endure droughts. Additionally, grazing enhanced the sensitivity of productivity to soil moisture during the wet season in drought years, which could restore grassland productivity more quickly after droughts. Therefore, grazing could help this fragile ecosystem to withstand droughts to some extent and maintain higher productivity. This may provide us with an ecological approach for confronting global climate change.

Seasonal dynamics and controls of deep soil water infiltration in the seasonally-frozen region of the Qinghai-Tibet plateau

Deep soil water infiltration is a key hydrological process in seasonally frozen regions, and displays a distinct behavior from that in other unfrozen regions; however, the seasonal dynamics and key controls of deep soil water infiltration in the seasonally frozen region of the Qinghai-Tibet plateau (QTP) are still poorly understood. In this study, we examine the seasonal dynamics of deep soil water infiltration, and attempt to quantitatively assess the relative influences of several driving factors by applying a random forests statistical analysis. The following key results were obtained. Deep soil water infiltration shows marked seasonal variation. Rainfall intensity exerts little effect on the seasonal dynamics of deep soil water infiltration as almost all rainfall returns to the atmosphere via evapotranspiration. No infiltration occurred when the depth of lower layer frozen soil reached 29 cm, whereas deep infiltration increased during the thawing of soil frost, suggesting that deep infiltration is impeded by soil freezing. Overall, the observed seasonal dynamics of deep soil water infiltration and volumetric soil moisture content were similar during the freeze-thaw process. Furthermore, deep soil water infiltration was more strongly influenced by deep volumetric soil moisture than by shallower volumetric soil moisture. Our results suggest that the topsoil Mattic Epipedon, soil organic matter content, and root systems’ role in vertical moisture movement should be taken into consideration when modeling hydrological processes in alpine meadows.

Predicting mattic epipedons in the northeastern Qinghai-Tibetan Plateau using Random Forest

Mattic epipedon (ME), a special diagnostic surface horizon for soils in alpine meadow, plays an important role in carbon storage, soil water retention, and indication of alpine meadow degradation. At the global scale, it mainly distributes in the Qinghai-Tibetan Plateau and other similar alpine environments. However, its spatial patterns and the relations with environmental conditions remain unknown. This study attempts to explore key environmental variables responsible for the occurrence of the special surface soil horizon and based on those factors to predict and map digitally the spatial distribution of ME. By combining a variable selection procedure and the Random Forest (RF) algorithm, the variables extracted from Landsat 5 TM mosaic image, ASTER GDEM, and climate data were optimized and their importance was measured. The classification accuracy was compared with that obtained from binary logistic regression algorithm. In addition, a land use/land cover (LULC) map-based modification was conducted to further improve the classification accuracy. Results showed that the variable selection procedure had little effect in improving prediction accuracy. However, the number of used variables markedly reduced from 26 to 6 with a 77% decrease, which could speed up the training of the RF model (about one-third of the computation time could be saved). Analysis of variable importance showed that band 3 of TM and normalized difference vegetation index were the most important environmental variables influencing the occurrence of ME. The final overall accuracy of the ME map was predicted to be 84%. Our results demonstrate that the proposed procedure, which combined the proposed variables (derived from remote sensing, GDEM, and climate data), the variable selection approach, the RF algorithm, and the LULC map-based modification method can identify key environmental variables influencing the occurrence of ME and map the spatial distribution of ME effectively on the Qinghai-Tibetan Plateau.