Nitrogen mineralization in grazed BSC subsoil is mediated by itself and vegetation in the Loess Plateau, China

Soil moisture determines effects of climates and soil properties on nitrogen cycling: Examination of arid and humid soils

While soil moisture has a significant effect on nitrogen (N) cycling, how it influences the dependence of this important biological process on environmental factors is unknown. Specifically, it is unclear how the relationships of net N mineralization (Nm) and soil moisture vary with soil properties and climates. In turn, how the relationships of Nm vs. soil properties and climates vary with soil moisture is also unknown. Therefore, soil samples from the 26 sites were collected within two climatic regions (i.e., arid and humid) across China. Then a four-week microcosmic incubation experiment was conducted at five soil moisture levels (20, 40, 60, 80, and 100% field water holding capacity (FWHC)) at 25 °C to measure the dynamics of Nm. The results showed that increasing soil moisture significantly increased Nm (+212%) and the N mineralization rate constant (k) (+0.26%), and that the effects of soil moisture were greater in humid soils (+250%) than arid soils (+178%). The slopes of the relationship between Nm vs. soil moisture increased with soil organic carbon (SOC) (+50.6%) and total N (TN) (+65.3%) concentrations, and decreased with pH (-43.0%) and clay content (-0.09%), especially in arid regions. Additionally, Nm was significantly correlated with soil properties and mean annual precipitation (MAP), and the slopes of most of these relationships increased with soil moisture in arid soils (+59.2-3805%), but decreased in humid soils (-1.96-140%). The results indicated that increasing soil moisture strengthened the dependence of Nm on soil properties and climates in arid soils, and that increasing soil pH and clay content reduced, but SOC and TN concentrations enhanced the dependence of Nm on soil moisture. Therefore, with changes in rainfall distribution patterns and an increase in extreme rainfall events, there is enormous potential for Nm in agricultural soils in arid regions, which is regulated by soil moisture and properties. On the contrary, in humid regions, the decoupling of the effects of soil moisture and soil properties on N mineralization could be due to microbial adaptation. Moreover, the coupled effects of soil environment and properties on N cycling in different climatic regions merit great consideration in experimental research as well as in biogeochemical model development and prediction.

Principle, technique and application of grassland improvement

Global grassland degradation poses a significant threat to the sustainable socio-economic development of humanity. However, this trend can be effectively mitigated through scientifically sound and rational grassland improvement measures. Grassland improvement utilizes the theory of pratacultural science, especially the theory of the four production levels of grassland agro-ecosystems, to solve the fundamental contradiction between the seasonal imbalance of grassland supply and livestock demand through integrates a number of improvement techniques. In order to clarify the implementation subject, target and specific measures of grassland improvement and to improve the science and efficiency of management and conservation, we classify grassland improvement into four types according to the target, scale and attributes of grassland and livestock feedback mechanism. Grassland improvement is generally based on one key technology, with multiple technologies used in combination, and synergy or superposition formed between integrated technologies. Individual technologies mainly include enclosure of livestock, reclamation, ripping, overseeding, fertilization, irrigation, fire work and grazing management, while integrated technologies are a combination of two or more technologies. Compared to individual techniques, the integrated approach resulted in a significant enhancement of community aboveground biomass by 17-38% and species richness by 2-24%, with no discernible impact on soil properties in the short term. The establishment of a standardized grazing-based improvement process while adhering to the principles of improvement after utilization, comprehensiveness, standardization and scale consistence to improve the structure and function of grassland ecosystems. Strategy of grassland improvement reassesses "nuisance" species as "citizens" of the ecosystem because they supply productivity, species biodiversity and other ecosystem services, and they can be managed at an unharmful and even benefitful level through identifying the ecological and economic thresholds.

Effects of grazing and precipitation addition induced by functional groups on the relationship between aboveground biomass and species richness of a typical steppe

Several studies have explored the influence of grazing or precipitation addition (PA), two important components of human activities and global climate change on the structure and function of communities. However, the response of communities to a combination of grazing and PA remains largely unexplored. We investigated the impact of grazing and PA on the relationship between aboveground biomass (AGB) and species richness (SR) of communities in three-year field experiments conducted in a typical steppe in the Loess Plateau, using a split-plot design with grazing as the main-plot factor and PA as the split-plot factor. AGB and SR have response threshold value to PA, which was decreased by grazing for AGB, but increased for SR. This indicates that implementing grazing management strategies is conducive to strengthening the protection of biodiversity in arid and semi-arid grasslands. Grazing promoted the AGB-SR coupling of the community by increasing the SR of medium drought tolerance (MD), low drought tolerance, and grazing tolerant functional groups. Grazing also accelerated the AGB-SR decoupling of the community by changing the AGB of high drought tolerance, MD, high grazing tolerance, and medium grazing tolerance functional groups. PA mediated changes in MD and SR of both drought and grazing tolerant functional groups and AGB of low grazing tolerance promoted the coupling of AGB-SR of the community. The Two-dimension functional groups classification method reflects the changes of AGB and SR in communities more reasonable than the division of single-factor functional groups.