Interactive effects of soil temperature and moisture on soil N mineralization in a Stipa krylovii grassland in Inner Mongolia, China

Determining soil N mineralization response to soil temperature and moisture changes is challenging in the field due to complicated effects from other factors. In the laboratory, N mineralization is highly dependent on temperature, moisture and sample size. In this study, a laboratory incubation experiment was carefully designed and conducted under controlled conditions to examine the effects of soil temperature and moisture on soil N mineralization using soil samples obtained from the Stipa krylovii grassland in Inner Mongolia, China. Five temperature（i.e. 9℃, 14℃, 22℃, 30℃ and 40℃） and five moisture levels（i.e. 20%, 40%, 60%, 80% and 100% WHC, where WHC is the soil water holding capacity） were included in a full-factorial design. During the 71-day incubation period, microbial biomass carbon（MBC）, ammonium nitrogen（NH4 ^＋-N） and nitrate nitrogen（NO3^--N） were measured approximately every 18 days; soil basal respiration for qCO2 index was measured once every 2 days（once a week near the end of the incubation period）. The results showed that the mineral N production and net N mineralization rates were positively correlated with temperature; the strongest correlation was observed for temperatures between 30℃ and 40℃. The relationships between moisture levels and both the mineral N production and net N mineralization rates were quadratic. The interaction between soil temperature and moisture was significant on N mineralization, i.e. increasing temperatures（moisture） enhanced the sensitivity of N mineralization to moisture（temperature）. Our results also showed a positive correlation between the net nitrification rate and temperature, while the correlation between the NH4 ^＋-N content and temperature was insignificant. The net nitrification rate was negatively correlated with high NH4 ^＋-N contents at 80%–100% WHC, suggesting an active denitrification in moist conditions. Moreover, qCO2 index was positively correlated with temperature, especially at 80% WHC. With a low net nitrification rate and high soil basal respiration rate, it was likely that the denitrification concealed the microbial gross mineralization activity; therefore, active soil N mineralization occurred in 60%–80% WHC conditions.

Shrubs proliferated within a six-year exclosure in a temperate grassland—Spatiotemporal relationships between vegetation and soil variables

Overgrazing has been considered one of the maj or causes that trigger shrub encroachment of grassland. Proliferation of shrubs in grassland is recognized as an important indicator of grassland degradation and desertification. In China, various conservation measures, including enclosures to reduce livestock grazing, have been taken to reverse the trend of grassland desertification, yet shrubs have been reported to increase in the grasslands over the past decades. In late 2007, we set up a 400-m-by-50-m exclosure in a long-term overgrazed temperate grassland in Inner Mongolia, with the ob- jective to quantify the spatiotemporal relationship between vegetation dynamics, soil variables, and grazing exclusion. Soil moisture was continuously monitored within the exclosure, and cover and aboveground biomass of the shrubs were measured inside the exclosure in 2007, 2009, 2010, 2012, and 2013, and outside the exclosure in 2012 and 2013. We found the average shrub cover and biomass significantly increased in the six years by 103 % and 120%, respectively. The result supported the hypothesis that releasing grazing pressure following long-term overgrazing tends to trigger shrub invasion into grassland. Our results, limited to a single gradient, suggest that any conservation measures with quick release of overgrazing pressure by enclosure or other similar means might do just the opposite to accelerate shrub en- croachment in grassland. The changes in vegetation cover and biomass were regressed on the temporal average of the soil moisture content by means of the generalized least square technique to quantify the effect of the spatial autocor- relation. The result indicates that the grass cover and biomass significantly increased with the top, but decreased with the bottom layer soil moisture. The shrub cover and biomass, on the other hand, decreased with the top, but increased with bottom soil moisture, although the regression coefficients for the shrubs were not statistically significant. Hence this study supports the two-layered soil model which assumes grasses and shrubs use belowground resources in dif- ferent depths.

Plant community dynamics during the growing season of typical ecosystems on the Tibetan Plateau

The relationships between vegetation and environmental factors have always been a core concern of ecologists.The dynamic characteristics of plant communities during the growing season can directly reflect these relation-ships,so we examined this issue for three typical ecosystems on the Tibetan Plateau.During the growing season,the dominant species remained stable while non-dominant species changed significantly in the alpine meadow and alpine steppe and a mono-dominant community was found in the temperate desert shrub.Due to the seasonal variations of temperature and soil water content,plant species diversity varied significantly during the growing season.Patrick richness,Pielou evenness and Simpson diversity indices differed significantly in the alpine meadow and alpine steppe.The total biomass of these three ecosystems was the largest during the middle growing season.Biomass was greater in the alpine meadow than the alpine steeps or temperature desert.The root-to-shoot ratio was the lowest during the middle growing season for the alpine meadow and alpine steppe and largest during the early growing season for temperate desert shrub.RDA showed the belowground and total biomass were greatly affected by soil physicochemical factors.Multiple linear stepwise regression showed the above ground biomass was greatly affected by relative atmospheric humidity and belowground and total biomass were greatly affected by soil organic carbon,total nitrogen at 0-20 cm soil depth and pH at 10-20 cm soil depth.These findings pro-vide insights into understanding the relationships between vegetation and environmental factors and promote the sustainable utilization of local grasslands on the Tibetan Plateau.

Seasonal patterns of root and shoot interactions in an alpine meadow on the Tibetan Plateau

Aims Numerous studies have showed that the balance between negative and positive plant–plant interactions shifted along environmental gradients.But little is known how the positive or negative plant–plant interactions varied with temporal fluctuating habitat conditions and plant ontogenetic phases.Methods In a 2-year experiment,the four perennial grasses(Kobresia humilis,Stipa aliena,Elymus nutans and Saussurea superba)were grown under four interaction treatments(no root or shoot interaction,only shoot interaction,only root interaction,root and shoot interaction).Intensity of above-and belowground interactions is proposed to vary with the fluctuation of seasonal climatic conditions and soil available nutrients.Here we report measurements of above-and belowground interactions during entire growing season.Correlation between plant interaction intensity and seasonal soil available N as well as habitat climate conditions was also performed.Important findings Our experiment found that root interactions had negative effect on plant growth for the four species during growing season.However,both negative and positive shoot interactions occurred among the four species.Despite there being shoot facilitative effect for E.nutans and S.superba,the full interaction was negative,suggested that root interaction take more important role on plant growth than that of shoot interaction.The interaction between root and shoot effect varied as a function of species identity and growth phases.The weak correlation of plant interaction intensity to habitat environmental factors suggested that plant ontogenetic characteristics may be primary factors causing temporal variation in plant interaction.