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Purpose: Nitrogen (N) and phosphorus (P) additions are the widely used restoration management for degraded grasslands. However, soil microbial community responses to N and P additions are still not well understood, especially multiple levels or/and combined N and P additions in high-altitude grasslands. In this study, the single and interactive effects of N and P additions on the soil microbes were explored. Methods: A field experiment with two factors (N 0, 7.5, and 15 g N m−2 year−1; P 0, 3.27, and 6.55 g P m−2 year−1) was conducted by a completely randomized block design in a Tibetan plateau steppe. Total and specific microbial biomass (bacteria, fungi, G+/G− bacteria, saprotrophs, actinomycetes, and arbuscular mycorrhizal fungi) were analyzed by using phospholipid fatty acids (PLFAs). Additionally, based on their fixed weight composites, we used a composite variable (phenotype) to represent the microbial community. Results: The results showed that N addition decreased the microbe phenotype, total microbial biomass, and specific microbial biomass (total bacteria, G+, and G− bacteria). P application and its interaction with N application all increased the microbial phenotype, total microbial biomass, and specific microbial biomass (total bacteria, G+, G−, saprotrophs, and actinomycetes). Besides, the microbial phenotype was positively correlated with P application rates and not correlated with N application rates. Conclusion: In conclusion, P was the key limiting factor for soil microbes, and its interaction with N is also important for some specific microbes. In addition, long-term responses of soil microbes after N and P amendments need further investigation in the Tibetan alpine steppe. [ABSTRACT FROM AUTHOR]

In the alpine steppe zone on the Central Tibetan Plateau, a large amount of area has been degraded due to natural and artificial factors. N & P fertilization is widely accepted to recover degraded pastures in other regions all over the world. However, it is not clear how alpine steppe communities respond to N & P fertilization, and what is the optimal application rate, in the perspective of forage production. To attempt to explore these questions, in July 2013, two fencing sites were designed in Baingoin County with 12 treatments of different levels of nitrogen (N0: 0; N1: 7.5 g m-2 yr-1; N2: 15 g m-2 yr-1) & phosphate (P0: 0; P1: 7.5 gP2O5 m-2 yr-1; P2: 15 gP2O5 m-2 yr-1; P3: 30 gP2O5 m-2 yr-1). The results indicated N&P addition was capable to ameliorate the quality of the two sites in the Tibetan Plateau steppe. Increasing N application level resulted in significant increment in Gramineae and total biomass in the two sites. P addition significantly improved the quantity of Compositae, total biomass and the biomasss of other species in site II, while it only significantly improved the total biomass in site I. Gramineae was much more sensitive to N-induced changes than P-induced changes, and this indicated N addition was better to ameliorate the quality of plateau steppe than P-induced changes. No strong evidence was found for critical threshold within 15 g N m-2 yr-1, and there was decreasing tendency when P addition rate was above 15 g m-2 yr-1. N&P has the potential to accelerate soil acidification, which improved the content of available K, likely as a result of nonsignificant correlation between biomass and soil moisture. This work highlights the the tradeoffs that exist in N and P addition in recovering degraded steppe. [ABSTRACT FROM AUTHOR]

To study how grazing affects the uptake of inorganic and organic N forms, three focal plant species (i.e., the graminoid species Kobresia pygmaea, which decreases with grazing, and the forbs Potentilla bifurca and Potentilla multifida, which increase with grazing) were selected in ungrazed and grazed plots in an alpine meadow on the Tibetan Plateau. Three times during the growing season (i.e., June, July, and September), these plots were injected with N-labeled NO-N, NH-N, or glycine-N, or with only water as a control. Two hours after N injection, exchangeable NH-N, glycine-N, and NO-N as well as plant and soil samples were collected and analyzed for N/N and total N content. Our result showed that all three plant species took up glycine-N, but uptake of inorganic N was generally predominant. The graminoid K. pygmaea took up all three N forms equally in June but preferred NO-N in July (particularly under grazing) and exchangeable NH-N in September. The forbs P. bifurca and P. multifida preferentially took up exchangeable NH-N in July (particularly under grazing), while NO-N was the dominant form of N uptake in September. P. bifurca generally preferred exchangeable NH-N, but preference shifted toward NO-N under grazing in June. P. multifida preferred glycine-N in ungrazed plots and shifted its preference to NO-N under grazing in June. In conclusion, the three plant species showed niche partitioning for uptake of three forms of N across the season, which was modified by grazing. These findings indicate that plant N uptake patterns should be considered for better understanding the mechanisms of grazing effects on plant diversity and species coexistence. [ABSTRACT FROM AUTHOR]

[Display omitted] Structural information of grassland changes on the Tibetan Plateau is essential for understanding alterations in critical ecosystem functioning and their underlying drivers that may reflect environmental changes. However, such information at the regional scale is still lacking due to methodological limitations. Beyond remote sensing indicators only recognizing vegetation productivity, we utilized multivariate data fusion and deep learning to characterize formation-based plant community structure in alpine grasslands at the regional scale of the Tibetan Plateau for the first time and compared it with the earlier version of Vegetation Map of China for historical changes. Over the past 40 years, we revealed that (1) the proportion of alpine meadows in alpine grasslands increased from 50% to 69%, well-reflecting the warming and wetting trend; (2) dominances of Kobresia pygmaea and Stipa purpurea formations in alpine meadows and steppes were strengthened to 76% and 92%, respectively; (3) the climate factor mainly drove the distribution of Stipa purpurea formation, but not the recent distribution of Kobresia pygmaea formation that was likely shaped by human activities. Therefore, the underlying mechanisms of grassland changes over the past 40 years were considered to be formation dependent. Overall, the first exploration for structural information of plant community changes in this study not only provides a new perspective to understand drivers of grassland changes and their spatial heterogeneity at the regional scale of the Tibetan Plateau, but also innovates large-scale vegetation study paradigm. [ABSTRACT FROM AUTHOR]