Mixing effects of three Eurasian plants on root decomposition in the existence of living plant community in a meadow steppe

Threshold response of ecosystem water use efficiency to soil water in an alpine meadow

Ecosystem water use efficiency (WUE) is a coupled index of carbon (gross ecosystem productivity, GEP) and water fluxes (transpiration, Tr or evapotranspiration, ET), reflecting how ecosystem uses water efficiently to increase its carbon uptake. Though ecosystem WUE is generally considered to decrease with increasing precipitation levels, it remains elusive whether and how it nonlinearly responds to extreme water changes. Here, we performed a 5-year precipitation halving experiment in an alpine meadow, combined with extremely interannual precipitation fluctuations, to create a large range of soil water variations. Our results showed that WUETr and WUEET consistently showed a quadratic pattern in response to soil water. Such quadratic patterns were steadily held at different stages of growing seasons, with minor changes in the optimal water thresholds (25.0-28.4 %). Below the water threshold, more soil water stimulated GEP but reduced Tr and ET by lowering soil temperature, resulting in a positive response of ecosystem WUE to soil water. Above the threshold, soil water stimulated GEP less than Tr (ET), leading to a negative response of ecosystem WUE to soil water. However, biological processes, including plant cover and belowground biomass as well as vertical root biomass distribution, had less effect on ecosystem WUE. Overall, this work is among the first to reveal the nonlinearity and optimal water thresholds of ecosystem WUE across a broad range of soil water, suggesting that future extreme precipitation events will more frequently surpass the water threshold and differently change the coupling relationships of carbon and water fluxes in alpine grasslands.

Effects of vegetation roots on the structure and hydraulic properties of soils: A perspective review

This paper aims to provide a state-of-the-art review on the effects of vegetation roots on the soil structure and soil hydraulic properties. After a thorough review of current studies, the effects of vegetation roots are summarized into four: root exudation, root penetration, root water uptake and root decay. Root exudates alter the size and stability of aggregates, the contact angle of soil, and the viscosity and surface tension of pore fluid; root exudates of crops always increase the soil water retention capacity and decrease the soil saturated hydraulic conductivity. Root penetration creates new pores or clogs existing pores during root growth, and root parameters (e.g., root biomass density, root diameter and root length density) are well correlated to soil hydraulic properties. Root water uptake can apparently increase the soil water retention capacity by providing an additional negative pressure and induce micro-fissures and macropores in the rhizosphere soil. Root decay modifies the pore structure and water repellency of soil, resulting in the increase of soil macro-porosity, soil water retention, and the saturated hydraulic conductivity or steady infiltration rate. Some of the above four effects may be difficult to be distinguished, and most importantly each is highly time-dependent and influenced by a multitude of plant-related and soil-related factors. Therefore, it remains a significant challenge to comprehend and quantify the effects of vegetation roots on the soil structure and soil hydraulic properties. Unsolved questions and disputes that require further investigations in the future are summarized in this review.

Combining controlled-release urea with potassium chloride to reduce soil N/K leaching and promote growth of Italian ryegrass

Nitrogen (N) and potassium (K) are essential nutrients for Italian ryegrass (Lolium multiflorum L.) growth. A 2-year field experiment with a split-plot design was conducted to study the effect of N fertilizer type combined with different K fertilizer rates on the soil mineral N and K availability, and growth characteristics of Italian ryegrass. The main plots were assigned to two N fertilizer types, controlled-release urea (CRU) and common urea. While low, moderate and high potassium chloride (KCl) rates (150, 300 and 450 kg ha^-1, respectively) were assigned to the subplots. Compared with the common urea treatments, the CRU significantly increased the SPAD value, plant height, leaf area, and photosynthetic index of Italian ryegrass, which significantly prolonged the green stage of Italian ryegrass and prevented premature senescence. Moreover, the dry yields of the CRU increased by 4.5-12.5% in 2019 and 10.9-25.3% in 2020 compared with the urea, respectively. At the same time, compared with the KCl150 and KCl450 treatments, the KCl300 treatment resulted in better plant growth. Overall, the CRU × KCl300 maximized the soil inorganic N and different soil K forms, and reduced the soil N/K leaching. The root length, volume, surface area, average diameter, tips and branches were also improved, and there was a significant N × K interaction effect on the tips. The CRU combined with 300 kg ha^-1 KCl fertilization enhanced crop growth by improving leaf photosynthesis, soil fertility, and yield and should be recommended as the best fertilizer ratio for Italian ryegrass production.