Soil aggregate stability is an important indicator for controlling soil losses and can improve soil quality, particularly in an area such as the Loess Plateau. The objective of this study was to estimate the differences in soil aggregates across six different land-use systems (grassland, apple orchard, abandoned apple orchard, cropland maize, cropland wheat, and shrub-grassland). For this purpose, dry and wet sieving techniques were employed to assess aggregate content and aggregate stability. Higher percentages of water stable aggregates were observed in the abandoned apple orchard and shrub-grassland at 63% and 61%, respectively. The maximum dry aggregate stability (%) was recorded at 78% and 77% in both wheat cropland and common apple orchard, and the abandoned apple orchard was only 74%. Both mean weight diameters and geometric mean diameters of aggregate were recorded as higher in grassland, shrub-grassland, and the abandoned apple orchard, than the other land uses. The formation of soil aggregates and their stability were positively correlated with soil organic carbon content and root biomass of different plant communities. Higher amounts of soil organic carbon content were noted in the abandoned apple orchard, common apple orchard, and natural grassland at the 0-20 cm soil layer. The results of the correlation coefficient showed a positive significant correlation between the mean weight diameter, geometric diameter, root biomass, and soil organic carbon content. Conclusively, the type of land use affected the soil aggregation and distribution of size fractions; the small fractions of the aggregates formed large fractions by combining with fresh organic matter, and increased soil organic carbon concentrations were closely linked with the formation of macro-aggregates. Thus, converting slope farmland to forestland and grassland could improve water-stable aggregate and reduce soil disturbances in areas (like the Loess Plateau) with the highest erosion risk.
Soil cracking is an important process and feature of degradation in terrestrial ecosystems. However, the patterns and causes of cracks related to alpine rangeland overgrazing have not been reported. In this study, we used a multiscale approach to investigate the distribution of soil-cracking areas at a landscape scale (217 survey sites), selected the grazing-induced parameter that was closely related to the cracks at a small scale (2013)(2014)(2015)(2016)(2017), and evaluated the water infiltration due to mosaic cracking at a microscale. The results showed that cracks only formed in the alpine meadow after overstocking. Soil compaction and water content explained 89.0% of the total variance in the crack characteristics, and soil compaction was closely related to cracking. The initial infiltration rate and steady infiltration rate of the healed cracks were significantly higher than those in the raised areas in the centre of the mosaic crack patches (p < .001). We propose a new model of alpine meadow degradation considering the cracking phenomenon due to overgrazing and provide key and easy-to-measure indicators to prevent overgrazing and cracking: a residual biomass greater than 65 g m −2 and a height greater than 6 cm, and the soil compaction should be lower than 1,044.26 ± 188.88 kPa. These safeguard thresholds warn rangeland managers to reduce the stocking rate or improve grazing management.We suggest that the cracking stage of alpine rangelands is the most critical turning point in the process of rangeland degradation and that rangeland managers should pay more attention to crack phenomena to prevent severe degradation.
Ligularia virgaurea and Ligularia sagitta are two species of poisonous plants with strong invasiveness in natural grasslands in China that have caused considerable harm to animal husbandry and the ecological environment. However, little is known about their suitable habitats and the key environmental factors affecting their distribution. Although some studies have reported the distributions of poisonous plants on the Qinghai–Tibet Plateau (QTP) and predicted their potential distributions at local scales in some regions under climate change, there have been few studies on the widespread distributions of L. virgaurea and L. sagitta. In this study, we recorded 276 and 118 occurrence points of L. virgaurea and L. sagitta on the QTP using GPS, and then used the MaxEnt model to predict the distribution of suitable habitats. Results showed that (1) under current climate conditions, L. virgaurea and L. sagitta are mainly distributed in southern Gansu, eastern Qinghai, northwestern Sichuan, eastern Tibet, and southwestern Yunnan, accounting for approximately 34.9% and 39.8% of the total area of the QTP, respectively; (2) the main environmental variables affecting the distribution of suitable habitats for L. virgaurea and L. sagitta are the Human Footprint Index (52.8%, 42.2%), elevation (11%, 4.4%), soil total nitrogen (18.9%, 4.2%), and precipitation seasonality (5.1%, 7.3%); and (3) in the future, in the 2050s and 2070s, the area of habitat of intermediate suitability for L. virgaurea will spread considerably in northwest Sichuan, while that of high suitability for L. sagitta will spread to eastern Tibet and western Sichuan.
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