REFERENCES
Adler P. B., Smull D., Beard K. H., Choi R. T., Furniss T., Kulmatiski A., et al. (2018). Competition and coexistence in plant communities: intraspecific competition is stronger than interspecific competition.Ecological Letter , 21, 1319–1329.DOI:10.1111/ele.13098
Aerts R., & Chapin F. S. III. (2000). The mineral nutrition of wild plants revisited: a re-evaluation of process and patterns.Advances in Ecological Research , 30, 1–67.DOI:10.1016/s0065-2504(08)60016-1
Bagchi R., Henrys P. A., Brown P. E., Burslem D. F. R. P., Diggle P. J., Gunatilleke C. V. S., et al. (2011). Spatial patterns reveal negative density dependence and habitat associations in tropical trees.Ecology , 92, 1723–1729.DOI:10.1890/11-0335.1
Bao S. D. (2000). Soil Agro-chemistrical Analysis . 2nd ed. China Agricultural Press, Beijing (in Chinese)
Bedford B. L., Walbridge M. R., & Aldous A. (1999). Patterns in nutrient availability and plant diversity of temperate North American wetlands. Ecology , 80, 2151–2169.DOI: 10.2307/176900
Chisholm R. A., & Fung T. (2018). Comment on: plant diversity increases with the strength of negative density dependence at the global scale. Science , 360, 16–19.DOI:10.1126/science.aar4685
Detto M., Visser M. D., Wright S. J., & Pacala S. W. (2019). Bias in the detection of negative density dependence in plant communities. Ecological Letter , DOI: 10.1111/ele.13372.
Elser J. J., & Hamilton A. (2007). Stoichiometry and the New Biology: The Future Is Now. PLoS Biology , 5, 1403–1405.DOI:10.1371/journal.pbio.0050181
Elser J. J., Sterner R. W., Gorokhova E., et al. (2000). Biological stoichiometry from genes to ecosystems. Ecological Letter , 3, 540–550.DOI: 10.1046/j.1461-0248.2000.00185.x
Forrister D. L., Endara M., Younkin G. C., Coley P. D., & Kursar T. A. (2019). Herbivores as drivers of negative density dependence in tropical forest saplings. Science , 363, 1213–1216.DOI: 10.1126/science.aau9460
Grace J. B. (1999). The factors controlling species density in herbaceous plant communities: an assessment. Perspectives in Plant Ecology, Evolution and Systematics , 2, 1–28.DOI: 10.1078/1433-8319-00063
Grime J. P. (2001). Plant Strategies, Vegetation Process, and Ecosystem Properties . John Wiley & Sons Ltd., Chichester.DOI: 10.1016/s0168-9452(01)00469-1
Güsewell S., Koerselman W., & Verhoeven J. T. A. (2003). Biomass N:P ratios as indicators of nutrient limitation for plant populations in wetlands. Ecological Monographs , 13, 372–384.DOI:10.1890/1051-0761(2003)013[0372:bnraio]2.0.co;2
Güsewell S. (2004). N:P ratios in terrestrial plants : variation and functional significance. New Phytologist , 164, 243–266.DOI: 10.1111/j.1469-8137.2004.01192.x
He X. D., You W. X., & Yu D. (2016). Ecological restoration theory and vegetation reconstruction technique in Yanchi county of the Ningxia Hui Autonomous Region . Nankai University Press, Tianjin (In Chinese).
Hille Ris Lambers J., Clark J. S., & Beckage B. (2002). Density dependent mortality and the latitudinal gradient in species diversity.Nature , 417, 732–735.DOI: 10.1038/nature00809
Hu M. J., Penuelas J., Sardans J., et al. (2018). Stoichiometry patterns of plant organ N and P in coastal herbaceous wetlands along the East China Sea: implications for biogeochemical niche. Plant and Soil , 431, 273–288. DOI: 10.1007/s11104-018-3759-6
Johnson D. J., Beaulieu W. T., Bever J. D., & Clay K. (2012). Conspecific negative density dependence and forest diversity.Science , 336, 904–907.DOI: 10.1126/science.1220269

Johnson D. J., Condit R., Hubbell S. P., & Comita L. S. (2017). Abiotic niche partitioning and negative density dependence drive tree seedling survival in a tropical forest.Proceedings of the Royal Society B-Biological Sciences , 284, 2017-2210. DOI: 10.1098/rspb.2017.2210

Kellner J. R., & Hubbell S. P. (2018). Density-dependent adult recruitment in a low-density tropical tree. Proceedings of the National Academy of Sciences of the United States of America , 115, 11268–11273.DOI: 10.1073/pnas.1800353115
Koerselman W., & Meuleman A. F. M. (1996). The vegetation N:P ratio: a new tool to detect the nature of nutrient limitation. Journal of Applied Ecology , 33, 1441–1450.DOI: 10.2307/2404783
Kranabetter J. M., Harman-Denhoed R., & Hawkins B. J. (2019). Saprotrophic and ectomycorrhizal fungal sporocarp stoichiometry (C:N:P) across temperate rainforests as evidence of shared nutrient constraints among symbionts. New Phytologist , 221, 482–492.DOI: 10.1111/nph.15380
LaManna J. A, Mangan S. A., Alonso A., Bourg N. A., Brockelman W. Y., Bunyavejchewin S., et al. (2017). Pant diversity increases with the strength of negative density dependence at the global scale.Science , 3824, 1–5.DOI: 10.1126/science.aar4685
Liebig J. V. (1840). Organic Chemistry in Its Application to Agriculture and Physiology . Taylor and Walton, London.DOI:10.5962/bhl.title.40751
Matzek V., & Vitousek P. M. (2009). N : P stoichiometry and protein : RNA ratios in vascular plants: an evaluation of the growth-rate Hypothesis. Ecological Letter , 12, 765–771.DOI: 10.1111/j.1461-0248.2009.01310.x
Shaver G. .R, & Chapin F. S. Ⅲ. (1995). Long-term responses to factorial NPK fertilizer treatment by Alaskan wet and moist tundra sedge species. Ecography , 18, 259–275.DOI: 10.1111/j.1600-0587.1995.tb00129.x
Shelford V. E. (1913). Animal Communities in Temperate America as Illustrated by the Chicago region . University of Chicago Press, Chicago, 326. DOI: 10.5962/bhl.title.7231
Sterner R. W., & Elser J. J. (2002). Ecological stoichiometry: the biology of elements from molecules to the biosphere . Princeton University Press, Princeton. DOI: 10.1093/plankt/25.9.1183
Tian D. S., Reich P. B., Chen H. Y. H., Xiang Y. Z., Luo Y. Q., Shen Y., Meng C.., Han W. X., & Niu S. L. (2019). Global changes alter plant multi-element stoichiometric coupling. New Phytologist , 221, 807–817. DOI: 10.1111/nph.15428
Tilman D. (2000). Causes, consequences and ethics of biodiversity. Nature 405: 208–211.DOI: 10.1038/35012217
Venterink H. O., Wassen M. J., Verkroost A. W. M., et al. (2003). Species richness-production patterns differ between N-, P-, and K-limited wetlands. Ecology , 84, 2191–2199.DOI: 10.1890/01-0639
Verhulust P. F. (1838). Notice sur la loi que la population suit dans son accroissement. Correspondences Mathematical Physics , 10, 113–121. DOI: 10.1007/BF02309004
Vitousek P. M, Porder S., Houlton B. Z., et al. (2010). Terrestrial phosphorus limitation: mechanisms, implications, and nitrogen-phosphorus interactions. Ecological Applications , 20, 5–15.DOI: 10.1890/08-0127.1
Volterra V. (1926). Variations and fluctuations of the numbers of individuals in animal species living together . Reprinted in 1931. In: R. N. Chapman, Animal Ecology . McGraw Hill, New York.DOI: 10.1093/icesjms/3.1.3
Wu W., He X. D., Zhang N., Wang H. T., & Ma D. (2009). Response of plant densities to N/P ratio in soil under Artemisia ordosicacommunity in succession. Acta Pedologica Sinica , 46, 472–479 (in Chinese with English abstract). DOI: 10.1007/978-1-4020-9623-5_5
Luo W. T., Li M. H., Sardans J, Lü X. T., Wang C, Peñuelas J, Wang Z. W., Han X. & Jiang G., Y. (2017). Carbon and nitrogen allocation shifts in plants and soils along aridity and fertility gradients in grasslands of China. Ecology and Evolution , 7, 6927–6934. DOI: 10.1002/ece3.3245
Liu Z, Y., Baoyin T. G. T., Sun J, Minggagud H, & Li X. L. (2018). Plant sizes mediate mowing-induced changes in nutrient stoichiometry and allocation of a perennial grass in semi-arid grassland. Ecology and Evolution , 8, 3109–3118. DOI: 10.1002/ece3.3866
TABLE 1 N:P ratios of vegetation and soil in communities of different successional stages in a semi-arid area