Grasslands and savannahs are suffering heavy losses from degradation and conversion. The UN Decade on Ecosystem Restoration offers important opportunities to address these losses through a range of restoration techniques. However, if poorly planned, the Decade could undermine some remaining natural and semi-natural grassland and savannah ecosystems by encouraging afforestation on these areas, thus acting as a perverse incentive. This article outlines the main issues and steps needed to ensure that the Decade creates positive outcomes for these important and highly biodiverse ecosystems: (1) better understanding of status and trends in degraded and converted grasslands and savannahs; (2) making the case for grassland and savannah restoration at both national and international levels; (3) ensuring post-2020 biodiversity conservation targets address all natural ecosystems; (4) improving selection tools for restoration to avoid displacing valuable ecosystems; and (5) identifying successful grassland and savannah restoration approaches that address ecological, cultural, and social needs.

Synergistic interactions between multiple limiting resources are common, highlighting the importance of co-limitation as a constraint on primary production. Our concept of resource limitation has shifted over the past two decades from an earlier paradigm of single-resource limitation towards concepts of co-limitation by multiple resources, which are predicted by various theories. Herein, we summarise multiple-resource limitation responses in plant communities using a dataset of 641 studies that applied factorial addition of nitrogen (N) and phosphorus (P) in freshwater, marine and terrestrial systems. We found that more than half of the studies displayed some type of synergistic response to N and P addition. We found support for strict definitions of co-limitation in 28% of the studies: i.e. community biomass responded to only combined N and P addition, or to both N and P when added separately. Our results highlight the importance of interactions between N and P in regulating primary producer community biomass and point to the need for future studies that address the multiple mechanisms that could lead to different types of co-limitation.© 2011 Blackwell Publishing Ltd/CNRS.

分析了退化高寒草地恢复的主要制约因素，包括植物种源、土壤微生物、土壤养分和人文因素；提出了针对这些制约因素的退化高寒草地恢复的主要途径：（1）研发乡土草种子采集、扩繁、包衣等技术，不同乡土草种种子组配及免耕补播技术，解决种源制约；（2）筛选适用于退化草地恢复的复合微生物菌种并研发菌剂，解决退化草地恢复的微生物制约； （3）研发以土壤养分调控为基础的植被恢复技术，解决退化草地恢复的土壤制约；（4）构建基于牧民新技术应用的草地适应性管理模式。分析认为，基于乡土草种、微生物、养分调控为主的物源途径的“近自然恢复”，有潜力成为青藏高原退化草地恢复的有效措施。

Exogenous sulfur input is one of the most important factors that acidify grassland soils, which may inhibit mineralization of soil organic matter and facilitate organic carbon accumulation, but may contribute to the decomposition of carbonates in calcareous soils. Sulfur inputs enhance the availability of nitrogen and micronutrients (iron, manganese, copper, zinc and boron), decrease the availability of molybdenum, accelerate the leaching of exchangeable base cations, and reduce the availability of calcium, magnesium and potassium. Sulfur inputs increase the availability of phosphorus in calcareous and neutral soils, but decrease it in acid or strong acid soils. Exogenous sulfur inputs can improve the synergetic uptake of nitrogen and phosphorus by plants, and generally increase plant uptake of metallic micronutrients. Soil acidification induced by sulfur inputs might lead to plant manganese toxicity, which could increase plant manganese uptake but inhibit plant iron uptake. High inputs of exogenous sulfur and soil acidification can decline aboveground productivity and plant species diversity in grasslands. Low dose of sulfur input tends to improve aboveground productivity, but its effects on plant species diversity and community stability remain unclear.

外源硫输入增加是草地生态系统土壤酸化的一种重要诱因，在一定程度上抑制了土壤有机质的矿化从而有利于有机碳积累，但可导致石灰性土壤中碳酸盐的分解；硫输入可提高土壤中氮及微量元素铁、锰、铜、锌、硼的有效性，降低钼的有效性；导致交换性盐基离子淋失并降低钙、镁、钾的有效性；增加石灰性和中性土壤磷的有效性，而降低酸性或强酸性土壤磷的有效性。外源硫输入可促进草地植物对氮和磷的协同吸收，总体上可以促进草地植物对金属微量元素的吸收，但土壤酸化可能引起植物锰毒害，增加植物锰吸收的同时抑制了对铁的吸收。高量硫输入及土壤重度酸化可导致草地生产力和物种多样性降低；低剂量硫输入对草地生产力有一定的促进作用，但对物种多样性及群落稳定性的影响目前尚缺乏系统的研究资料。

Grasslands occupying the most terrestrial land surface display multiple functions. The long-term overgrazing of livestock often occur, and additional climate changes brings about the negative effects on the ecological stability, and therefore, grassland degradation is worldwide prevailing, which reduce the multiple functions of grasslands, and how to restore those degraded grasslands remains a crucial challenge for the human beings. For the past several decades, most researches on grassland restoration have focused on restoration practice rather than underlying theoretical basis, and the general restoration theory is lacking. Consequently, it is hard to have a practical solution for degraded grasslands due to lack of the available techniques. In this paper, the authors summarize the previous grassland restoration theories or models from restoration succession trajectory to the threshold model, the alternative state model and the filter model. A new concept, named as “systematic restoration for degraded grasslands” is put forward, which emphasizes three dimensions as key structure components (trophic species and dominant species of plant-animal-microbe), self-organized processes (water-nutrient coupling, linkage between aboveground and belowground), and multi-functionality (synergy and stability of grassland multiple functions), and also give the further explanations for contexts and mechanisms of the grassland systematic restoration (GSR): system structure integration, self-organization of ecological processes, and multi-functionality. Generally, the target of grassland restoration is approaching to the climax community or primordial state of ecosystem. The authors here emphasize the restoration of system self-organization, resulting from the interactions among the species like plant-soil feedback, as well as grassland multiple functions (goods and ecological services)characterized by their synergism and coupling. At last, the authors discuss the potential restoration practices like natural restoration by fencing and resting, intervening restoration with artificial inputs, and stimulating restoration with utilizing such as grazing and cutting, and perhaps the latter is the feasible mean for practical grassland restoration. The restoration mechanisms and practices for degraded grasslands are more complicated than thought, and here we attempt to establish a comprehensive conceptual framework to provide a new insight into grassland restoration theory. Certainly, it needs more evidence and experiments to enrich this new concept, GSR.

草地是地球陆地面积最大、分布最广泛的多功能生态系统,长期以来过高的家畜数量使草地超载过牧,同时气候变化也对草地的生态稳定性产生负向作用,导致世界及我国的草地出现普遍性退化。草地退化是制约草地实现生产、生态功能的世界性环境问题之一,如何有效地使退化草地恢复是人类面临的巨大科学与技术挑战。迄今对草地恢复的研究已有几十年的历史,而这些研究总体上还处于初步发展阶段,并且实践多于理论研究,尚未形成有共识的草地恢复理论。只有深刻认识草地恢复的过程、机制及途径等理论问题,才能更有效地建立其相关的技术基础,进而研发出行之有效的草地恢复技术。本文作者针对草地恢复问题,分析总结了以往建立的草地恢复模型,即恢复演替理论、阈值模型、选择状态模型及过滤模型等,在此基础之上提出了草地的系统性恢复概念,即通过构建基本的草地关键组分（植被-动物-微生物的营养级物种与优势种）激发草地生态（跨营养级的养分-水分联系、地上-地下耦合等）的自组织过程,实现以系统稳定平衡和多功能协同为目标的恢复方式。草地的系统性恢复强调:恢复目标是接近或达到新的稳定平衡状态,这种状态能够保证维持草地主体功能即可。草地的恢复方式是从营养结构到生态过程和多功能的“系统化”,实际上是从“系统”角度定义、实施草地恢复;草地结构的恢复,即需要恢复草地的优势种或营养级物种,以此构建的生物多样性、食物网或生态网络之框架;草地过程的恢复是实现其自组织性,即依赖草地的内源动力,促发草地过程“自然地”达到稳定状态,这种内源动力源于草地系统存在的生物组分,由于生物可以进行新陈代谢而产生对环境的适应,以及环境对生物的反馈;草地功能的恢复主要是恢复草地的多功能（产品生产与生态服务）,并使多功能之间形成协同与耦合,最终体现的是草地恢复的结构整体性、过程自组织性和功能完整性。文中还阐释了草地系统性恢复的相关机制,同时,对草地系统性恢复的主要方式或途径——封育式自然恢复、人工辅助式干预恢复、适度利用式激发恢复等,进行了深刻解析。鉴于我国牧区的草地面积、家畜数量基数、以及社会生产状况,草地的适度利用恢复可能是一种现实而有效的恢复途径。尽管草地恢复与草地退化密切关联,然而,草地恢复的过程与机制远比想象得更加复杂。本文构建了草地系统性恢复的概念框架,试图丰富、推动发展草地恢复的一般性理论。今后,仍然需要在不同的退化草地上,开展更多的长期试验与技术实践来检验、修正及发展这一概念。

【Objective】 Overgrazing and mowing of grassland in north China caused serious vegetation degradation and soil nutrient deficiency. The purpose of this study was to investigate the effects of nitrogen and phosphorus fertilizers on plant community diversity and productivity in the degraded grassland. 【Method】 A four-year nitrogen and phosphorus addition experiment was conducted with two kinds of fertilizers, including calcium superphosphate and ammonium nitrate, scheduled 2 nitrogen rates (0, and 10 g N·m-2·a-1) and 6 phosphorus rates (0, 2, 4, 6, 8 and 10 g P·m-2·a-1) with interaction between the both, a total of 12 treatments, and each treatment repeated 5 times. Fertilizers were applied when grassland was turning green in late May, and the community was investigated in the period of maximum biomass in the middle of August every year. In the process of research, the community productivity, species diversity and relative biomass of four functional groups were analyzed. 【Result】 From 2014 to 2017, it was found that the species richness decreased by year, and the average species of the control treatment (neither nitrogen nor phosphorus) was 20.2, 17.1, 14.7 and 15.2, respectively. Based on plant community α diversity index (richness and Simpson dominance index), the study showed that nitrogen addition significantly decreased the species diversity of the community, but phosphorus addition and cumulative effects of phosphorus did not affect the species diversity of the community. There was a significant difference in diversity index among years, and the interaction between nitrogen and year had a significant effect on it. The aboveground net primary productivity (ANPP) of grassland community varied significantly among years, with the highest in 2014 and the lowest in 2015, the range was from 400 g·m -2 to 100 g·m-2, and which in dry years was significantly lower than that in normal precipitation years. Phosphorus addition had little effect on ANPP, while nitrogen addition significantly increased ANPP, and the effect of nitrogen and phosphorus addition together was greater than that of phosphorus alone. In the normal precipitation growing season, nitrogen was the main limiting factor of Hulunber meadow steppe. Nitrogen addition significantly affected the relative biomass of functional groups, while phosphorus addition had little effect on it, and there was a significant difference in the relative biomass of each functional group between years. Nitrogen addition alone significantly increased the relative biomass of the perennial rhizomatous grass (PR), but reduced the relative biomass of the perennial fobs (PF) and perennial bunch grass (PB). Phosphorus addition alone increased relative biomass of PB and perennial leguminous (LE), while had no effect on relative biomass of PR and PB. Combined addition of nitrogen and phosphorus significantly increased the relative biomass of PR, but significantly decreased the relative biomass of the other functional groups. In addition, the nitrogen and phosphorus combined addition partly reduced the plant community stability. 【Conclusion】 The differentiation of different functional types’ responses caused by nitrogen and phosphorus addition led to the changes of the original hierarchical level between different functional groups, especially nitrogen addition could cause a rapid increase in PR relative biomass, and thus led to the plant community structure developing towards the direction of increasing dominance of PR, and decreasing species diversity and community stability.

【目的】 北方草地过度放牧和刈割造成植被严重退化和土壤养分缺乏。本研究探讨施用氮、磷肥料对退化草地植物群落多样性和生产力的影响。【方法】 在呼伦贝尔退化草地连续进行4年氮、磷添加试验,设置2个施氮水平（0、10 g N·m^-2·a^-1）和6个施磷水平（0、2、4、6、8和10 g P·m^-2·a^-1）,共计12个处理,5次重复。每年5月下旬返青时施肥,8月中旬生物量最大时期进行群落调查,主要进行群落生产力、物种多样性及4种功能群相对生物量的分析。【结果】 2014—2017年物种丰富度降低明显,完全对照处理（既不施氮也不施磷）4年的物种丰富度平均分别是20.2、17.1、14.7、15.2种;对植物群落α多样性（丰富度和优势度指数）研究表明,氮添加降低了群落物种多样性,而磷添加及磷的累积效应均对群落物种多样性无影响。多样性指数年际间差异显著,氮和年交互作用对其具有显著影响。草地群落地上生产力（ANPP）年际间变化显著,2014年最高,2015年最低,变化范围从400 g·m^-2降到100 g·m^-2左右,干旱年份显著低于正常降水年份,磷对群落生产力影响较小,氮添加显著提高了群落地上生产力,而且氮和磷共同添加的影响大于磷单独添加;生长季正常降水年份,氮素是草甸草原植物生长的主要限制因子。氮添加显著影响了各功能群相对生物量,磷添加对各功能群相对生物量影响较小,各功能群相对生物量年际间具有显著差异。氮单独添加显著增加根茎型禾草的相对生物量,但降低了多年生杂类草和丛生禾草的相对生物量,而磷单独添加增加了多年生杂类草和豆科植物功能群的相对生物量,但没有影响根茎型禾草和丛生禾草的相对生物量;氮磷复合添加显著增加了多年生根茎型禾草的相对生物量,但显著降低了其他功能群的相对生物量。此外,氮、磷添加在一定程度上降低植物群落的稳定性。【结论】 氮、磷添加引起不同功能群植物的差异化响应导致群落间原有的等级关系发生变化,尤其是氮添加引起喜氮的根茎型禾草相对生物量迅速增加,最终导致植物群落组成向着根茎型禾草优势度增加的方向发展,物种多样性降低,导致群落稳定性下降。

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.

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。

Humans are both intentionally (fertilization) and unintentionally (atmospheric nutrient deposition) adding nutrients worldwide. Increasing availability of biologically reactive nitrogen (N) is one of the major drivers of plant species loss. It remains unclear, however, whether plant diversity will be equally reduced by inputs of reactive N coming from either small and frequent N deposition events or large and infrequent N fertilization events. By independently manipulating the rate and frequency of reactive N inputs, our study teases apart these potentially contrasting effects. Plant species richness decreased more quickly at high rates and at low frequency of N addition, which suggests that previous fertilization studies have likely over-estimated the effects of N deposition on plant species loss. N-induced species loss resulted from both acidification and ammonium toxicity. Further study of small and frequent N additions will be necessary to project future rates of plant species loss under increasing aerial N deposition. © 2014 John Wiley & Sons Ltd.