Jirui Gong

The responses of soil respiration to nitrogen addition in a temperate grassland in northern China.

Anthropogenic activities have increased nitrogen (N) inputs to grassland ecosystems. Knowledge of the impact of soil N availability on soil respiration (RS) is critical to understand soil carbon balances and their responses to global climate change. A 2-year field experiment was conducted to evaluate the response of RS to soil mineral N in a temperate grassland in northern China. RS, abiotic and biotic factors, and N mineralization were measured in the grassland, at rates of N addition ranging from 0 to 25gNm(-2)yr(-1). Annual and dormant-season RS ranged from 241.34 to 283.64g C m(-2) and from 61.34 to 83.84g C m(-2) respectively. High N application significantly increased RS, possibly due to increased root biomass and increased microbial biomass. High N treatment significantly increased soil NO3-N and inorganic N content compared with the control. The ratio of NO3-N to NH4-N and the N mineralization rate were significantly positively correlated with RS, but NH4-N was not correlated or negatively correlated with RS during the growing season. The temperature sensitivity of RS (Q10) was not significantly affected by N levels, and ranged from 1.90 to 2.20, but decreased marginally significantly at high N. RS outside the growing season is an important component of annual RS, accounting for 25.0 to 29.6% of the total. High N application indirectly stimulated RS by increasing soil NO3-N and net nitrification, thereby eliminating soil N limitations, promoting ecosystem productivity, and increasing soil CO2 efflux. Our results show the importance of distinguishing between NO3-N and NH4-N, as their impact on soil CO2 efflux differed.

Effects of grass–legume mixtures on the production and photosynthetic capacity of constructed grasslands in Inner Mongolia, China

Abstract. Constructed grasslands are primary restoration measures in areas with degraded natural grasslands. Grass–legume mixtures are chosen to obtain high production and forage quality; however, the photosynthetic and other traits of such mixtures are not well understood. In this study, we evaluated the effects of grass–legume mixtures on the growth and photosynthetic capacity of three forage crops over two growing seasons. Bromus inermis and Elymus nutans were grown as monocultures or in mixtures with Medicago sativa. We analysed forage yields, quality, gas exchange and chlorophyll fluorescence parameters. The grass–legume mixtures improved the forage yield, root : shoot ratio, and contents of crude protein and lignin. Compared with the monoculture, grasses in the mixtures had higher net photosynthesis, water-use efficiency (WUE), and leaf nitrogen (N) content, but lower carbon : N ratio, and distributed more absorbed light to photosynthetic electron transport and thermal dissipation. In the mixture, B. inermis had a higher light-saturation point, indicating high light-use efficiency. Elymus nutans had a lower light-compensation point and dark respiration rate, suggesting good shade tolerance. However, water deficits decreased biomass and photosynthetic capacity in the E. nutans–M. sativa mixture, suggesting that E. nutans was sensitive to soil moisture. The B. inermis–M. sativa mixture had greater and more consistent biomass and WUE.The grass—legume mixture is an important way to construct grasslands. We analysed forage yields, quality, gas exchange and chlorophyll fluorescence in the mixtures, finding that grass—legume mixtures improved forage production and photosynthetic capacity. The study provides a scientific basis for establishment of constructed grasslands with high yield and quality.

Growth-defense trade-off regulated by hormones in grass plants growing under different grazing intensities

Herbivory creates conflicts between a plants need to allocate resources for growth and defense. It is not yet clear how plants rebalance resource utilization between growth and defense in response to increasing grazing intensity. We measured characteristics of the primary and secondary metabolism of Leymus chinensis at five levels of grazing intensity (control, light, moderate, heavy and extremely heavy). Furthermore, we evaluated hormone signaling by quantifying the impact of key hormones on plant growth and defense. Under light grazing intensity, indole-3-acetic acid and jasmonates appeared to promote the growth of L. chinensis through a high photosynthetic rate, high water-use efficiency and high soluble protein contents, whereas abscisic acid decreased these properties. Under moderate grazing intensity, L. chinensis had a low photosynthetic capacity but greater production of secondary metabolites (tannins, total flavonoids and total phenols), possibly induced by salicylic acid. When the grazing pressure further intensified, L. chinensis translocated more carbohydrates to its roots in order to survive and regrow. Leymus chinensis therefore exhibited a trade-off between growth and defense in order to survive and reproduce under herbivory. Plants developed different mechanisms to enhance their grazing tolerance by means of hormonal regulation.

Response of dominant grassland species in the temperate steppe of Inner Mongolia to different land uses at leaf and ecosystem levels

In order to study the responses of dominant species to different land uses in the semiarid temperate grassland of Inner Mongolia, we tested the physiological responses of Stipa grandis, Leymus chinensis, and Artemisia frigida to mowing, grazing exclusion, and grazing land uses at the leaf and ecosystem levels. The grazing-exclusion and mowing sites released CO2, but the grazing site was a net carbon sink. L. chinensis and S. grandis contributed more to the ecosystem CO2 exchange than A. frigida. At the grazing-exclusion and mowing sites, Leymus chinensis and Stipa grandis both exhibited a higher light-saturation point and higher maximum photosynthetic rate than that at the grazing site, which increased photosynthesis and growth compared to those at the grazing site. In contrast, A. frigida possessed a higher nitrogen content than the other species, and more of the light energy used for photosynthesis, particularly at the grazing site.