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.

在草原上進行了水肥加氮放牧試驗。

?No evidence for grazing?optimization?on primary production was found in this study.

本研究未發(fā)現(xiàn)放牧優(yōu)化對初級生產的影響。

?Grazing decreased the belowground fraction of net primary production (BNPP/NPP).

放牧降低了凈初級生產的地下部分（BNPP/NPP）。

?Water and N additions significantly increased aboveground net primary production.

加水加氮顯著提高了地上凈初級產量。

?The “whole-plant”?perspective?in studying plant–herbivore interactions was highlighted.

強調了研究植物與食草動物相互作用的“全植物”視角。

Abstract

Herbivory and resource availability interactively regulate plant growth, biomass allocation, and production.

草食性和資源可利用性相互作用調節(jié)植物生長、生物量分配和生產。

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，植物在放牧強度和受控制的環(huán)境條件下的補償性生長還不是很清楚。本研究在內蒙古進行了為期2年的試驗，在每年輪牧制度下，在4個放牧強度（0 ~ 7 sheep ha?1）的場地中，分別添加水（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，放牧對地上凈初級生產力(ANPP)和凈初級生產力(NPP)均無顯著影響。

However, high grazing intensity strongly reduced the?fraction?of belowground net primary production to NPP.

而高放牧強度顯著降低了地下凈初級生產力占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分別顯著提高39%和23%，但對地下凈初級生產量無顯著影響。

ANPP showed lower response to nitrogen addition at high grazing intensity sites than at low grazing intensity sites.

高放牧強度場區(qū)ANPP對氮添加的響應低于低放牧強度場區(qū)。

We found no evidence for grazing optimization on primary production of semiarid steppe, regardless of resource supplementations.

我們沒有發(fā)現(xiàn)半干旱草原初級生產的放牧優(yōu)化的證據(jù)，無論資源補充。

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.

放牧植物通過改變分配優(yōu)先級和形態(tài)性狀來最大限度地減少ANPP。我們的研究在研究植物-食草動物的相互作用時強調了“全植物”的觀點。

Keywords

Precipitation; N fertilizer; Compensatory growth; Morphological traits; Sheep grazing; Inner Mongolian steppe

Introduction

Introduction

????天然草地初級生產對草食的響應已經在許多關于植物-動物相互作用的研究中進行了調查。補償性生長，被稱為對傷害的積極反應，會導致三種后果：過度補償（放牧增加初級產量），精確補償（放牧不改變產量）和補償不足（放牧減少產量）。一些研究已經證明了過度補償，其機制被稱為放牧優(yōu)化假說。然而，許多研究沒有發(fā)現(xiàn)過度補償?shù)淖C據(jù)，放牧優(yōu)化理論的有效性和生物學合理性受到了質疑。盡管存在爭議，但放牧優(yōu)化理論已被用來證明北美西部牧場的重度放牧是合理的，一些作者建議謹慎應用該理論，特別是在過度放牧風險較高的生態(tài)系統(tǒng)中。

??其他的植物-草食動物理論，包括代償連續(xù)體模型和有限資源模型，認為代償性生長是由資源可用性調節(jié)的。這一說法得到了許多實驗研究的支持，表明植物脫葉后的再生受水氮有效性的調節(jié)。因此，在適當?shù)沫h(huán)境因素組合下，在某些物種和系統(tǒng)中，放牧優(yōu)化偶爾會發(fā)生。補償性生長是一個被廣泛研究的課題；然而，關于放牧優(yōu)化的研究大多集中在地上凈產量(ANPP)上，對“全株”水平的響應研究較少。地下凈生產（BNPP）是草地凈初級生產（NPP）的重要組成部分，約占草地凈初級生產的40 ~ 90%。植物能夠調整碳分配的優(yōu)先次序，以應對碳源和碳匯的不平衡。因此，在許多情況下，ANPP的增強應歸因于C分配的變化，而不是整個工廠的過度補償。13C標記實驗表明，落葉和施氮促進了C在莖葉生長中的分配。因此，為了驗證放牧優(yōu)化理論，ANPP和BNPP都需要在資源可用性控制良好的條件下進行研究。此外，正如Poorter等人(2012)所述，植物在應對生物和非生物影響時具有相當大的形態(tài)可塑性。因此，對植物性狀的評價可以讓我們更深入地了解植物與食草動物相互作用的潛在機制。

????本研究旨在了解半干旱草原在控制資源可得性條件下綿羊放牧下植物的補償性生長。作為典型的華北半干旱草原，由于牲畜數(shù)量的迅速增加和過度放牧，錫林郭勒草原已經出現(xiàn)了深度退化。在這片草地上，牧草生產的水分限制是眾所周知的，氮和水相互制約初級生產。在這片半干旱草原上進行了4種放牧強度的放牧試驗，分別以主樣地和次樣地添加水肥和氮肥。我們假設：（1）放牧對NPP具有負向或中性影響；（2）放牧ANPP（放牧ANPP>未放牧ANPP）的過度補償是以犧牲BNPP為代價的；（3）放牧強度越低，氮對ANPP的影響越顯著。

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.