氮磷养分共同添加促进退化典型草原恢复的机制

Mechanism for the restoration of degraded typical steppe by nitrogen and phosphorus co-addition

土壤养分含量降低是我国草原退化的主要原因之一,养分添加是退化草原恢复的有效措施,但过量养分添加会导致物种多样性降低。为了探讨适宜的养分添加量以及养分添加促进退化草原恢复的机制,本研究选择内蒙古典型草原的退化群落,通过氮(N)磷(P)养分共同添加梯度试验,研究了退化典型草原在群落、功能群和物种3个组织水平上对养分添加的响应。结果表明:在群落水平,养分添加显著促进了退化典型草原生物量,但没有降低物种多样性;群落生物量随养分添加水平表现为饱和曲线响应,在12.0 g N·m^(-2)、3.8 g P·m^(-2)水平趋于饱和;物种多样性在低养分添加水平(N<9.6 g·m^(-2)、P<3.0 g·m^(-2))较对照显著增加,在其余养分添加水平未发生显著变化。在功能群水平,随着养分添加量的增加,多年生根茎禾草在群落中优势度增加,生物量和密度均显著提高;一年生植物生物量和密度在高养分水平添加下显著增加,多年生丛生禾草和杂类草无显著变化。在物种水平,6个物种对养分添加响应不同,羊草通过增加种群密度和个体大小显著增加了种群生物量;大针茅、冰草和糙隐子草种群生物量没有显著变化;星毛委陵菜和黄囊苔草分别因为降低个体大小和种群密度减少了种群生物量。养分添加作为草原恢复的措施,可以显著增加退化草原生物量和物种多样性,降低植物群落中退化指示种,增加多年生根茎禾草。

The reduction of soil nutrient content is one of the major reasons caused grassland degradation in China.Nutrient addition is thus considered as an effective measure for the restoration of degraded grasslands.However,over-fertilization can lead to decrease in plant diversity.To clarify the appropriate amount of nutrient addition and the underlying mechanism that promotes grassland restoration,we set up a nitrogen and phosphorus co-addition experiment in a degraded typical steppe of Inner Mongolia,and examined the responses at community,functional group and species levels to nutrient addition.The results showed that nutrient addition enhanced biomass while did not reduce species richness at the community level.The biomass showed a saturation response with the increases of nutrient addition,which approached saturation under the 12.0 g N·m^(-2),3.8 g P·m^(-2) treatment.Species richness increased significantly under the lower nutrient treatments(N<9.6 g·m^(-2),P<3.0 g·m^(-2))compared with the control,while the two high nutrient treatments did not alter species richness.At the functional group level,biomass and abundance of perennial rhizome grasses increased significantly with the increases of nutrient addition levels.Biomass and density of annuals increased significantly under high nutrient addition levels.However,the abundance and biomass of perennial bunchgrasses and perennial forbs were rarely affected.At the species level,six target species responded differently to nutrient addition.Biomass of Leymus chinensis was significantly increased due to the increase of population density and individual biomass.Biomass of Stipa grandis,Agropyron cristatum and Cleistogenes squarrosa change little.Biomass of Potentilla acaulis and Carex korshinskyi were reduced due to the decreases in individual biomass and population density,respectively.As a measure of restoring degraded grassland,nutrient addition could significantly increase biomass and species diversity,decrease biomass of the degradation indicator species,and increase biomass of perennial rhizomes grasses.