氮素添加对内蒙古退化草原生产力的短期影响

doi:10.17521/cjpe.2016.0141

摘要/Abstract

摘要：

不合理的土地利用方式以及气候变化导致我国草原生态系统普遍退化, 主要表现在土壤养分降低、植被覆盖度减少、生产力下降。外源氮素添加是促进退化草原尽快恢复的一项重要措施, 尤其是对那些退化较为严重的草原。该研究选取内蒙古东乌珠穆沁旗不同退化程度(轻度、中度和重度)的草原群落, 于2014-2015年开展连续两年的氮素添加实验, 设置对照(不添加)、低水平(5.0 g N·m^-2·a^-1)、中水平(10.0 g N·m^-2·a^-1)和高水平(20.0 g N·m^-2·a^-1) 4种氮素添加处理, 探讨退化草原群落生产力在恢复过程中对不同水平氮素添加的响应。结果显示: (1)高、中水平氮素添加显著提高了轻度退化群落的地上生物量, 分别比对照增加了53.1%、51.6%, 氮素各水平添加对中度、重度群落地上生物量无显著影响; (2)高、中水平氮素添加显著提高了轻度退化群落中多年生根茎型禾草地上生物量, 分别比对照增加了45.1%、47.7%, 而多年生杂类草地上生物量分别比对照减少了37.4%、42.1%, 但中度和重度退化群落各功能群生物量的响应不显著; (3)三种水平氮素添加对轻、中、重度退化群落物种丰富度在试验期间均没有显著影响。研究结果表明氮素添加有助于提高轻度退化草原中多年生根茎型禾草的生物量, 进而提高群落的生物量, 但多年生杂类草会被逐渐替代, 导致生物量降低, 可见施氮对草原恢复的影响取决于草原退化程度。

关键词: 退化草原, 氮添加, 地上生物量, 植物功能群, 物种丰富度

Abstract:

Aims Irrational utilization and global climate change have caused degradation of grassland ecosystems in northern China with low soil fertility, decreased vegetation coverage and productivity. Nitrogen addition has been suggested an effective way to enhance restoration of those degraded grasslands. In this study, we selected a typical steppe with three different degrading levels, including lightly, moderately and heavily degraded communities, in East Ujimqin, Nei Mongol. Our objectives of this study are to examine if and how nitrogen (N) addition can enhance restoration of those degraded grasslands Methods Treatments with four levels of N addition (0, 5.0, 10.0 and 20.0 g N·m^-2·a^-1) were conducted to each of the three degraded communities from 2014 to 2015. Nitrogen was applied as urea in June of both years. Aboveground biomass was collected at the species level in 1 m × 1 m plot in August each year, all species biomass was summed as net primary production, and biomass of plant functional groups was calculated by perennial rhizome grasses, perennial bunchgrasses, perennial forbs, shrubs and semi-shrubs, annuals and biennials.Important findings Our results showed that the high (20.0 g N·m^-2·a^-1) and medium level N addition (10.0 g N·m^-2·a^-1) significantly increased the aboveground biomass of the slightly degraded community by 53.1% and 51.6% compared with no N addition. N addition had no significant effects on the moderately and heavily degraded communities. N addition with high and medium levels increased aboveground biomass of perennial rhizome grasses by 45.1% and 47.7%, but decreased that of perennial forbs by 37.4% and 42.1% at the slightly degraded community. Our results indicated that N addition could increase the growth of perennial rhizome grasses, and the growth of perennial forbs was suppressed consequently. Our results suggest that even the application of N fertilizers can only be helpful to restoration of those slightly degraded grasslands. Besides, N addition had no significant effects on species richness in different degraded communities indicating the fact that the study may not last long enough. For the purpose of increasing aboveground biomass of degraded grassland, we should not only consider the type and quantity of fertilization, but also the attribute of the degraded communities. In addition, the response of degraded community in biomass may strongly be impacted by degrading level of studied grassland.

Key words: degraded grassland, nitrogen addition, aboveground biomass, plant functional group, species richness

王晶, 王姗姗, 乔鲜果, 李昂, 薛建国, 哈斯木其尔, 张学耀, 黄建辉. 氮素添加对内蒙古退化草原生产力的短期影响. 植物生态学报, 2016, 40(10): 980-990. DOI: 10.17521/cjpe.2016.0141

Jing WANG, Shan-Shan WANG, Xian-Guo QIAO, Ang LI, Jian-Guo XUE, Muqier HASI, Xue-Yao ZHANG, Jian-Hui HUANG. Influence of nitrogen addition on the primary production in Nei Mongol degraded grassland. Chinese Journal of Plant Ecology, 2016, 40(10): 980-990. DOI: 10.17521/cjpe.2016.0141

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链接本文: https://www.plant-ecology.com/CN/10.17521/cjpe.2016.0141

https://www.plant-ecology.com/CN/Y2016/V40/I10/980

图/表 6

表1 未添加氮处理时不同退化群落属性表(平均值±标准误差)

Table 1 Properties of the degraded community before the N addition treatment (mean ± SE)

	轻度退化 Slightly degraded	中度退化 Moderately degraded	重度退化 Heavily degraded
地上净初级生产力 ANPP (g·m^-2)	190.04 ± 12.45^a	154.02 ± 14.60^b	108.85 ± 6.53^c
物种丰富度 Species richness	10.36 ± 0.32^a	8.95 ± 0.34^b	7.34 ± 0.34^c
多年生根茎型禾草 PR (%)	20.08 ± 1.88^a	16.49 ± 2.24^b	10.42 ± 1.75^c
多年生丛生型禾草 PB (%)	27.13 ± 2.64^a	43.91 ± 3.67^b	37.58 ± 3.5^b
多年生杂类草 PF (%)	29.92 ± 2.16^a	38.15 ± 2.85^b	52.55 ± 3.5^c
一、二年生植物 AB (%)	21.59 ± 2.07^a	2.39 ± 0.82^b	2.84 ± 1.03^b
灌木及半灌木 SS (%)	2.80 ± 0.65^ns	3.34 ± 1.32^ns	5.74 ± 2.29^ns

表2 氮素添加量(CN)、退化程度(T)、处理年限(Y)以及它们的交互作用对群落地上净初级生产力及物种丰富度的线性混合模型分析。其中氮添加量(CN)、退化类型(T)作为固定效应, 处理年限(Y)作为随机效应

Table 2 Analyses with mixed linear model for species richness and aboveground biomass using treatment (N), type (T) and their interactions as fixed effects, year (Y) as random effect

	df	地上净初级生产力 Aboveground net primary production		物种丰富度 Species richness
	df	F	p	F	p
Type (T)	2	18.02	0.000 1	3.98	0.020 5
CN	3	3.17	0.026 0	1.85	0.140 8
Y	1	36.70	0.000 1	28.91	0.000 1
CN × Y	5	3.17	0.026 0	1.85	0.140 9
CN × T	6	0.58	0.745 9	1.80	0.101 2
T × Y	2	10.87	0.000 1	3.98	0.020 5
CN × T × Y	12	0.58	0.745 8	1.81	0.101 1

图1 2015年不同浓度氮素处理对不同退化类型中0-10 cm土层土壤铵态氮、土壤硝态氮、土壤含水量的线性混合模型分析(平均值±标准误差)。采用Duncan多重比较进行处理间的差异分析, 相同字母表示处理间差异不显著, 不同字母表示差异显著(p < 0.05)。I、II、III分别表示轻度退化、中度退化、重度退化。

Fig. 1 Seasonal average soil inorganic N (NH4+-N, NO3--N) soil moisture (0-10 cm) in the 2015 from the different degraded communities (means ± SE). Bars with different letters were significantly different (p < 0.05) in Duncan’s multiple range tests reported from the linear mixed model. I, II, and III represents slightly degraded, moderately degraded and heavily degraded community, respectively.

图2 氮素添加对不同退化区年际间地上生物量的影响(平均值±标准误差)。Duncan’s多重比较进行处理间差异分析, 不同字母表示有显著性差异(p < 0.05)。I、II、III分别表示轻度退化、中度退化、重度退化。

Fig. 2 Effects of nitrogen addition on above ground biomass of the different degraded communities during the three years (mean ± SE). The different letters were significantly different in Duncan’s multiple p < 0.05. I, II, and III represents slightly degraded, moderately degraded and heavily degraded community, respectively.

图3 2014-2015年氮素添加对不同退化区物种丰富度的线性混合模型分析(平均值±标准误差)。

Fig. 3 Effects of nitrogen addition on plant species richness of the different degraded blocks between 2014 and 2015 (mean ± SE).

图4 氮素添加对不同退化区年际间群落中植物功能群所占比例的影响。I、II、III 分别表示轻度退化、中度退化、重度退化。AB, 一、二年生植物; PB, 多年生丛生型禾草; PF, 多年生杂类草; PR, 多年生根茎型禾草; SS, 灌木及半灌木。

Fig. 4 Effects of nitrogen addition on relative aboveground biomass of the compositional plant functional groups in the different degraded communities during the three years. I, II, and III represents slightly degraded, moderately degraded and heavily degraded community, respectively. AB, annuals and biennials; PB, perennial bunchgrasses; PF, perennial forbs; PR, perennial rhizome grasses; SS, shrubs and semi-shrubs.

参考文献 55

[1]	Ba L, Wang DL, Liu Y, Wang X, Sun W (2002). Analysis on competition and coexistence patterns of Aneurolepidium chinense and its main companion species.Chinese Journal of Applied Ecology, 13, 50-54.(in Chinese with English abstract)[巴雷, 王德利, 刘颖, 王旭, 孙伟 (2002). 羊草与其主要伴生种竞争与其共存的格局分析. 应用生态学报,13, 50-54.]
[2]	Bai YF, Han XG, Wu JG, Chen ZZ, Li LH (2004). Ecosystem stability and compensatory effects in the Inner Mongolia grassland.Nature, 431, 181-184.
[3]	Bai YF, Wu JG, Clark CM, Naeem S, Pan QM, Huang JH, Zhang LX, Han XG (2010). Tradeoffs and thresholds in the effects of nitrogen addition on biodiversity and ecosystem functioning: Evidence from Inner Mongolia grasslands.Global Change Biology, 16, 358-372.
[4]	Bai YF, Wu JG, Xing Q, Pan QM, Huang JH, Yang DL, Han XG (2008). Primary production and rain use efficiency across a precipitation gradient on the Mongolia Plateau.Ecology, 89, 2140-2153.
[5]	Borer ET, Seabloom EW, Gruner DS, Harpole WS, Hillebrand H, Lind EM, Adler PB, Alberti J, Anderson TM, Bakker JD, Biederman L, Blumenthal D, Brown CS, Brudvig LA, Buckley YM, Cadotte M, Chu CJ, Cleland EE, Crawley MJ, Daleo P, Damschen EI, Davies KF, DeCrappeo NM, Du GZ, Firn J, Hautier Y, Heckman RW, Hector A, HilleRisLambers J, Iribarne O, Klein JA, Knops JMH, La Pierre KJ, Leakey ADB, Li W, MacDougall AS, McCulley RL, Melbourne BA, Mitchell CE, Moore JL, Mortensen B, O’Halloran LR, Orrock JL, Pascual J, Prober SM, Pyke DA, Risch AC, Schuetz M, Smith MD, Stevens CJ, Sullivan LL, Williams RJ, Wragg PD, Wright JP, Yang LH (2014). Herbivores and nutrients control grassland plant diversity via light limitation.Nature, 508, 517-520.
[6]	Chen SP, Bai YF, Zhang LX, Han XG (2005). Comparing physiological responses of two dominant grass species tonitrogen addition in Xilin River Basin of China.Environmental and Experimental Botany, 53, 65-75.
[7]	Clark CM, Tilman D (2008). Loss of plant species after chronic low-level nitrogen deposition to prairie grasslands.Nature, 451, 712-715.
[8]	Cleland EE, Collins SL, Dickson TL, Farrer EC, Gross KL, Gherardi LA, Hallett LM, Hobbs RJ, Hsu JS, Turnbull L, Suding KN (2013). Sensitivity of grassland plant community composition to spatial vs. temporal variation in precipitation.Ecology, 94, 1687-1696.
[9]	Clough TJ, Sherlock RR, Mautner MN, Milligan DB, Wilson PF, Freeman CG, McEwan MJ (2003). Emission of nitro- gen oxides and ammonia from varying rates of applied synthetic urine and correlations with soil chemistry.Australian Journal of Soil Research, 41, 421-438.
[10]	de Schrijver A, de Frenne P, Ampoorter E, van Nevel L, Demey A, Wuyts K, Verheyen K (2011). Cumulative nitrogen input drives species loss in terrestrial ecosystems.Global Ecology and Biogeography, 20, 803-816.
[11]	Delabays N, Slacanin I, Bohren C (2008). Herbicidal potential of artemisinin and allelopathic properties of Artemisia annua L.: From the laboratory to the field.Journal of Plant Diseases and Protection, 21, 317-322.
[12]	Eckstein RL, Karlsson PS (2001). Variation in nitrogen-use efficiency among and within subarctic graminoids and herbs.New Phytologist, 150, 641-651.
[13]	Elser JJ, Bracken MES, Cleland EE, Gruner DS, Harpole WS, Hillebrand H, Ngai JT, Seabloom EW, Shurin JB, Smith JE (2007). Global analysis of nitrogen and phosphorus limitation of primary producers in freshwater, marine and terrestrial ecosystems.Ecology Letters, 10, 1135-1142.
[14]	Fang Y, Xun F, Bai WM, Zhang WH, Li LH (2012). Long-term nitrogen addition leads to loss of species richness due to litter accumulation and soil acidification in a temperate steppe.PLOS ONE, 7, e47369. doi:10.1371/journal.pone. 0047369.
[15]	Ge J, Xing F (2012). A review of adaptive strategies of clonal plants to interspecific competition.Chinese Journal of Plant Ecology, 36, 587-596.(in Chinese with English abstract)[葛俊, 邢福 (2012). 克隆植物对种间竞争的适应策略.植物生态学报,36,587-596.]
[16]	Gough L, Osenberg CW, Gross KL, Collins SL (2000). Fertili- zation effects on species density and primary productivity in herbaceous plant communities.Oikos, 89, 428-439.
[17]	Grime JP (1979). Plant strategies and vegetation processes.Wiley, 23, 197-200.
[18]	Hautier Y, Seabloom EW, Borer ET, Adler PB, Harpole WS, Hillebrand H, Lind EM, MacDougall AS, Stevens CJ, Bakker JD, Buckley YM, Chu CJ, Collins SL, Daleo P, Damschen EI, Davies KF, Fay PA, Firn J, Gruner DS, Jin VL, Klein JA, Knops JMH, La Pierre KJ, Li W, McCulley RL, Melbourne BA, Moore JL, O’Halloran LR, Prober SM, Risch AC, Sankaran M, Schuetz M, Hector A (2014). Eutrophication weakens stabilizing effects of diversity in natural grasslands.Nature, 508, 521-525.
[19]	Hill JO, Simpson RJ, Moore AD, Chapman DF (2006). Morphology and response of roots of pasture species to phosphorus and nitrogen nutrition.Plant and Soil, 286, 7-19.
[20]	Hu ZM, Yu GR, Fan JW, Zhong HP, Wang SQ, Li SG (2010). Precipitation-use efficiency along a 4500-km grassland transect.Global Ecology and Biogeography, 19, 842-851.
[21]	Huang JY, Yu HL, Yuan ZY, Li LH (2012). Effects of long-term increased soil N on leaf traits of several species in typical Inner Mongolia grassland.Acta Ecologica Sinica, 32, 1419-1427. (in Chinese with English abstract)[黄菊莹, 余海龙, 袁志友, 李凌浩 (2012). 长期N添加对典型草原几个物种叶片性状的影响. 生态学报, 32, 1419-1427.]
[22]	Huff LM, Potts DL, Hamerlynck EP (2015). Ecosystem CO2 exchange in response to nitrogen and phosphorus addition in a restored, temperate grassland.American Midland Naturalist, 173, 73-87.
[23]	Jessing KK, Cedergreen N, Mayer P, Libous-Bailey L, Strobel BW, Rimando A, Duke SO (2013). Loss of artemisinin produced by Artemisia annua L. to the soil environment.Industrial Crops and Products, 43, 132-140.
[24]	Jiang Y, Bi XL, Huang JH, Bai YF (2010). Patterns and drivers of vegetation degradation in Xilin River Basin, Inner Mongolia, China.Chinese Journal of Plant Ecology, 34, 1132-1141.(in Chinese with English abstract)[姜晔, 毕晓丽, 黄建辉, 白永飞 (2010). 内蒙古锡林河流域植被退化的格局及驱动力分析. 植物生态学报,34, 1132-1141.]
[25]	Kang L, Han XG, Zhang ZB, Sun OJ (2007). Grassland eco- systems in China: Review of current knowledge and research advancement.Philosophical Transactions of the Royal Society B-Biological Sciences, 362, 997-1008.
[26]	Koerselman W, Meuleman AFM (1996). The vegetation N:P ratio: A new tool to detect the nature of nutrient limitation.Journal of Applied Ecology, 33, 1441-1450.
[27]	Lan ZC, Bai YF (2012). Testing mechanisms of N-enrichment- induced species loss in a semiarid Inner Mongolia grass- land: Critical thresholds and implications for long-term ecosystem responses.Philosophical Transactions of the Royal Society B-Biological Sciences, 367, 3125-313
[28]	LeBauer DS, Treseder KK (2008). Nitrogen limitation of net primary productivity in terrestrial ecosystems is globally distributed.Ecology, 89, 371-379.
[29]	Li A, Wu JG, Huang JH (2012). Distinguishing between human-induced and climate-driven vegetation changes: A critical application of RESTREND in Inner Mongolia.Landscape Ecology, 27, 969-982.
[30]	Li B (1997). The rangeland degradation in North China and its preventive strategy.Scientia Agricultura Sinica, 30(6), 1-9.(in Chinese with English abstract)[李博 (1997). 中国北方草地退化及其防治对策. 中国农业科学,30(6), 1-9.]
[31]	Li BY, Wang JY, Zhao SJ, Zhang HW, Zhao MX (2004). The influence of fertilizers on the soil fertility, population structure and yield of herbages grown in degraded grassland.Grassland of China, 26(1), 14-17.(in Chinese with English abstract)[李本银, 汪金舫, 赵世杰, 张宏伟, 赵明旭 (2004). 施肥对退化草地土壤肥力、牧草群落结构及生物量的影响. 中国草地,26(1), 14-17.]
[32]	Li JZ, Lin SH, Taube F, Pan QM, Dittert K (2011). Above- and below-ground net primary productivity of grassland influenced by supplemental water and nitrogen in Inner Mongolia.Plant and Soil, 340, 253-264.
[33]	Liao MJ, Wang QB, Song MH, Dong M (2002). Clonal architecture and ramet population characteristics of Leymus chinesis from different habitatis in the Xilin River watershed. Acta Phytoecologica Sinica 26, 33-38.(in Chinese with English abstract)[廖明隽, 王其兵, 宋明华, 董鸣 (2002). 内蒙古锡林河流域不同生境中羊草的克隆构型和分株种群特征. 植物生态学报,26, 33-38.]
[34]	Liu ZL, Wang W, Hao DY, Liang CZ (2002). The discussion about the mechanism of degradation and restoration in Inner Mongolia grassland. Journal of Arid Land Resources and Environment, 16(1), 84-91.(in Chinese with English abstract)[刘钟龄, 王炜, 郝敦元, 梁存柱 (2002). 内蒙古草原退化与恢复演替机理的探讨. 干旱区资源与环境,16(1), 84-91.]
[35]	Mansson KF, Falkengren-Grerup U (2003). The effect of nitro- gen deposition on nitrification, carbon and nitrogen miner- alisation and litter C:N ratios in oak (Quercus robur L.) forests.Forest Ecology and Management, 179, 455-467.
[36]	Pan QM, Bai YF, Wu JG, Han XG (2011). Hierarchical plant responses and diversity loss after nitrogen addition: Testing three functionally-based hypotheses in the Inner Mongolia grassland.PLOS ONE, 6, e20078. doi:10.1371/ journal.pone. 0047369.
[37]	Quan Q, He NP, Zhang Z, Zhang YH, Gao Y (2015). Nitrogen enrichment and grazing accelerate vegetation restoration in degraded grassland patches. Ecological Engineering, 75, 172-177.
[38]	Ring E, Jacobson S, Hogbom L (2011). Long-term effects of nitrogen fertilization on soil chemistry in three Scots pine stands in Sweden.Canadian Journal of Forest Research, 41, 279-288.
[39]	Rochette P, Angers DA, Chantigny MH, Gasser MO, Mac- Donald JD, Pelster DE, Bertrand N (2013). NH3 volatilization, soil NH4+ concentration and soil pH following subsurface banding of urea at increasing rates.Canadian Journal of Soil Science, 93, 261-268.
[40]	Seagle SW, McNaughton SJ (1993). Simulated effects of precipitation and nitrogen on Serengeti grassland productivity. Biogeochemistry, 22, 157-178.
[41]	Sirulnik AG, Allen EB, Meixner T, Allen MF (2007). Impacts of anthropogenic N additions on nitrogen mineralization from plant litter in exotic annual grasslands.Soil Biology & Biochemistry, 39, 24-32.
[42]	Song L, Ba XM, Liu XJ, Zhang FS (2012). Impact of nitrogen addition on plant community in a semi-arid temperate steppe in china.Journal of Arid Land, 4, 3-10.
[43]	Standish RJ, Fontaine JB, Harris RJ, Stock WD, Hobbs RJ (2012). Interactive effects of altered rainfall and simulated nitrogen deposition on seedling establishment in a global biodiversity hotspot.Oikos, 121, 2014-2025.
[44]	Suding KN, Collins SL, Gough L, Clark C, Cleland EE, Gross KL, Milchunas DG, Pennings S (2005). Functional- and abundance-based mechanisms explain diversity loss due to N fertilization.Proceedings of the National Academy of Sciences of the United States of America, 102, 4387-4392.
[45]	Sun HL (2005). China Ecosystem. Science Press, Beijing. 489-499.(in Chinese)[孙鸿烈 (2005). 中国生态系统. 科学出版社, 北京. 489-499.]
[46]	Tilman D (1982). Resource Competition and Community Structure. Princeton University Press, Princeton, USA. 12-33.
[47]	Tilman D, Downing JA (1994). Biodiversity and stability in grasslands.Nature, 367, 363-365.
[48]	Wan HW, Yang Y, Bai SQ, Xu YH, Bai YF (2008). Variations in leaf functional traits of six species along a nitrogen addition gradient in Leymus chinensis steppe in Inner Mongolia. Journal of Plant Ecology (Chinese Version), 32, 611-621.(in Chinese with English abstract)[万宏伟, 杨阳, 白世勤, 徐云虎, 白永飞 (2008). 羊草草原群落6种植物叶片功能特性对氮素添加的响应. 植物生态学报,32, 611-621.]
[49]	Wedin DA, Tilman D (1996). Influence of nitrogen loading and species composition on the carbon balance of grasslands.Science, 274, 1720-1723.
[50]	Wilson JB, Lee WG (2000). C-S-R triangle theory: Commu- nitylevel predictions, tests, evaluation of criticisms, and relation to other theories.Oikos, 91, 77-96.
[51]	Yang HJ, Jiang L, Li LH, Li A, Wu MY, Wan SQ (2012). Diversity-dependent stability under mowing and nutrient addition: Evidence from a 7-year grassland experiment.Ecology Letters, 15, 619-626.
[52]	Yuan ZY (2004). Nutrient Use Effieieney of Main Plants in an Agro-Pastoral Ecotone of Northern China. PhD disserta- tion. Insititute of Botany, Chinese Academy of Sciences, Beijing. 15-25.(in Chinese with English abstract)[袁志友 (2004). 农牧交错区主要植物的氮素利用效率. 博士论文, 中国科学院植物研究所, 北京. 15-25.]
[53]	Zhang TH, Zhao HL, Li YL, Cui JY, Han TB, Zhang H (2008). Effect of irrigation and fertilizer on grassland productivity in Horqin Sandy Land.Acta Prataculturae Sinica, 17(1), 36-42.(in Chinese with English abstract)[张铜会, 赵哈林, 李玉霖, 崔建垣, 韩天宝, 张华 (2008). 科尔沁沙地灌溉与施肥对退化草地生产力的影响. 草业学报,17(1), 36-42.]
[54]	Zhang YH, Han X, He NP, Long M, Huang JH, Zhang GM, Wang QB, Han XG (2014a). Increase in ammonia volatilization from soil in response to N deposition in Inner Mongolia grasslands.Atmospheric Environment, 84, 156-162.
[55]	Zhang YH, Lü XT, Isbell F, Stevens C, Han X, He NP, Zhang GM, Yu Q, Huang JH, Han XG (2014b). Rapid plant species loss at high rates and at low frequency of N addition in temperate steppe.Global Change Biology, 20, 3520-5329.