Euro J Agro | 半干旱草地初级生产和生物量分配对氮和水补充的放牧强度梯度响应
Response of primary production and biomass allocation to nitrogen and water supplementation along a grazing intensity gradient in semiarid grasslandXiaoYing Gong a b 1, Nicole Fanselow c, Klaus Dittert c 2, Friedhelm Taube b, Shan Lin aaDepartment of Plant Nutrition, China Agricultural University, 100193 Beijing, PR ChinabInstitute of Crop Science and Plant Breeding – Grass and Forage Science/Organic Agriculture, Christian-Albrechts-University, Hermann-Rodewald-Str. 2, 24118 Kiel, GermanycInstitute of Plant Nutrition and Soil Science, Christian-Albrechts-University, Hermann-Rodewald-Str. 2, 24118 Kiel, Germany
Highlights
•We performed a grazing experiment with water and N fertilizer additions in a steppe.
在草原上进行了水肥加氮放牧试验。
•No evidence for grazing optimization on primary production was found in this study.
本研究未发现放牧优化对初级生产的影响。
•Grazing decreased the belowground fraction of net primary production (BNPP/NPP).
放牧降低了净初级生产的地下部分（BNPP/NPP）。
•Water and N additions significantly increased aboveground net primary production.
加水加氮显著提高了地上净初级产量。
•The “whole-plant” perspective in studying plant–herbivore interactions was highlighted.
强调了研究植物与食草动物相互作用的“全植物”视角。
Abstract
Herbivory and resource availability interactively regulate plant growth, biomass allocation, and production.
草食性和资源可利用性相互作用调节植物生长、生物量分配和生产。
However, the compensatory growth of plants undergrazing intensities and manipulated environmental conditions is not well understood. A 2-year experiment with water (unirrigated and irrigated) and nitrogen fertilizer (0 and 75 kg N ha−1 year−1) addition was conducted at sites with 4 grazing intensities (0–7 sheep ha−1) in an annually rotational grazing system in Inner Mongolia.
然而，植物在放牧强度和受控制的环境条件下的补偿性生长还不是很清楚。本研究在内蒙古进行了为期2年的试验，在每年轮牧制度下，在4个放牧强度（0 ~ 7 sheep ha−1）的场地中，分别添加水（未灌溉和灌溉）和氮肥（0和75 75 kg N ha−1 year−1）。
In this study, grazing had no significant effect on aboveground net primary production (ANPP) and net primary production (NPP).
本研究中，放牧对地上净初级生产力(ANPP)和净初级生产力(NPP)均无显著影响。
However, high grazing intensity strongly reduced the fraction of belowground net primary production to NPP.
而高放牧强度显著降低了地下净初级生产力占NPP的比例。
Water and nitrogen additions significantly increased ANPP by 39% and by 23%, respectively, but had no effect on belowground net primary production.
水氮添加使ANPP分别显著提高39%和23%，但对地下净初级生产量无显著影响。
ANPP showed lower response to nitrogen addition at high grazing intensity sites than at low grazing intensity sites.
高放牧强度场区ANPP对氮添加的响应低于低放牧强度场区。
We found no evidence for grazing optimization on primary production of semiarid steppe, regardless of resource supplementations.
我们没有发现半干旱草原初级生产的放牧优化的证据，无论资源补充。
Grazed plants minimized the reduction of ANPP by altering allocation priority and morphological traits. Our study highlights the “whole-plant” perspective when studying plant–herbivore interactions.
放牧植物通过改变分配优先级和形态性状来最大限度地减少ANPP。我们的研究在研究植物-食草动物的相互作用时强调了“全植物”的观点。
Keywords
Precipitation; N fertilizer; Compensatory growth; Morphological traits; Sheep grazing; Inner Mongolian steppe
Introduction
Introduction
天然草地初级生产对草食的响应已经在许多关于植物-动物相互作用的研究中进行了调查。补偿性生长，被称为对伤害的积极反应，会导致三种后果：过度补偿（放牧增加初级产量），精确补偿（放牧不改变产量）和补偿不足（放牧减少产量）。一些研究已经证明了过度补偿，其机制被称为放牧优化假说。然而，许多研究没有发现过度补偿的证据，放牧优化理论的有效性和生物学合理性受到了质疑。尽管存在争议，但放牧优化理论已被用来证明北美西部牧场的重度放牧是合理的，一些作者建议谨慎应用该理论，特别是在过度放牧风险较高的生态系统中。
其他的植物-草食动物理论，包括代偿连续体模型和有限资源模型，认为代偿性生长是由资源可用性调节的。这一说法得到了许多实验研究的支持，表明植物脱叶后的再生受水氮有效性的调节。因此，在适当的环境因素组合下，在某些物种和系统中，放牧优化偶尔会发生。补偿性生长是一个被广泛研究的课题；然而，关于放牧优化的研究大多集中在地上净产量(ANPP)上，对“全株”水平的响应研究较少。地下净生产（BNPP）是草地净初级生产（NPP）的重要组成部分，约占草地净初级生产的40 ~ 90%。植物能够调整碳分配的优先次序，以应对碳源和碳汇的不平衡。因此，在许多情况下，ANPP的增强应归因于C分配的变化，而不是整个工厂的过度补偿。13C标记实验表明，落叶和施氮促进了C在茎叶生长中的分配。因此，为了验证放牧优化理论，ANPP和BNPP都需要在资源可用性控制良好的条件下进行研究。此外，正如Poorter等人(2012)所述，植物在应对生物和非生物影响时具有相当大的形态可塑性。因此，对植物性状的评价可以让我们更深入地了解植物与食草动物相互作用的潜在机制。
本研究旨在了解半干旱草原在控制资源可得性条件下绵羊放牧下植物的补偿性生长。作为典型的华北半干旱草原，由于牲畜数量的迅速增加和过度放牧，锡林郭勒草原已经出现了深度退化。在这片草地上，牧草生产的水分限制是众所周知的，氮和水相互制约初级生产。在这片半干旱草原上进行了4种放牧强度的放牧试验，分别以主样地和次样地添加水肥和氮肥。我们假设：（1）放牧对NPP具有负向或中性影响；（2）放牧ANPP（放牧ANPP>未放牧ANPP）的过度补偿是以牺牲BNPP为代价的；（3）放牧强度越低，氮对ANPP的影响越显著。
Fig. 1.Accumulated precipitation (black bars) and irrigation (grey bars) of each 10 days, and air temperature in growing season 2007 (a) and 2008 (b). The total precipitation from May to September was 178 mm in 2007 and 277 mm in 2008. The total amount of irrigation water was 185 mm in 2007 and 120 mm in 2008.
Fig. 2.Illustration of the layout for the main plots of grazing intensity and subplots in a rotational grazing system. In this rotational grazing system, same land was used for grazing in one year and for hay production in the next year. G0–G3 represent no grazing, low, moderate, and high grazing intensities. Notably, the locations of the main plots of grazing intensities were randomly selected in the field. Nested panels represent the arrangement of subplots within a movable exclosure.W0 and W1 represent unirrigated and irrigated treatments, respectively. N0 and N1 represent no fertilization and fertilizer N supplementation (75 kg N ha-1 y-1) treatments, respectively. Lowercase letters a–c indicate the changing of locations of exclosures in each month. For the size of main plots and mobile exclusures see Section 2.2.
Fig. 3. Peak plant aboveground green biomass outside exclosure (PAGB, a), plant aboveground net primary production (ANPP, b), belowground net primary production (BNPP, c), and fraction of BNPP to NPP (fBNPP, d), as affected by grazing intensities, water and nitrogen additions. PAGB, ANPP were shown as the means of two sampling years. BNPP and fBNPP were shown based on data of 2007. For abbreviations of water and N treatments see Fig. 2. Error bars represent standard error of the means.
Fig. 4.Relative response of ANPP to N addition (ANPPN1/ANPPN0, water treatments were pooled) and to water addition (ANPPW1/ANPPW0, N treatments were pooled) along the grazing intensity gradient. Data of two years were pooled. Error bars represent standard error of the means.
Fig. 5. Litter dry mass (litter DM) as affected by water addition a) and stocking rate b). For abbreviations of water and N treatments please see Fig. 2. Error bars represent standard error of the means.
