高寒草甸植物化学计量比对磷添加的分层响应

Hierarchical responses of plant stoichiometry to phosphorus addition in an alpine meadow community

为了探明氮(N)限制的植物群落中物种水平和功能群水平的碳(C)、N、磷(P)含量以及C:N:P对P添加的响应是否一致,明确P添加对群落物种构成改变的内在机制。以青藏高原高寒草甸为研究对象,通过P添加试验,研究了功能群水平和物种水平生态化学计量比对P添加的响应,以及P添加对物种水平的优势度和功能群水平生物量的影响。结果表明:在青藏高原高寒草甸连续5年添加P显著改变了植物的C、N、P含量以及C:N:P,且在物种水平和功能群水平(不含典型物种)的响应规律基本一致。在禾本科、莎草科和杂类草功能群(不含典型物种)和相应物种水平上P添加对C含量影响不显著。P添加显著增加了禾本科、莎草科、豆科和杂类草4个功能群(不含典型物种)和相应物种水平的植物P含量,降低了C:P和N:P。禾本科和莎草科的N含量和C:N对P添加在物种水平和功能群水平上(不含典型物种)的响应规律一致,表现为N含量显著降低,C:N显著增加;P添加使豆科物种水平上N含量显著增加而C:N显著降低,但在功能群水平上(不含典型物种)无显著作用;杂类草的N含量和C:N对P添加在物种水平和功能群水平上(不含典型物种)的响应规律均不一致。在N限制的生境中添加P,禾本科物种在群落中逐渐占据优势跟其增高的N、P利用效率相关,而杂类草由于逐渐降低的N和P利用效率使其生物量在群落中所占的比重逐渐下降。

Aims Terrestrial carbon （C）, nitrogen （N）, phosphorus （P） stoichiometry will reflect the effects of adjustment to local growth conditions as well as species’ replacements. However, it remains unclear about the hierarchical responses of plant C：N：P to P addition at levels of species and functional groups in the N-limited alpine meadow. Methods A field experiment of P enrichment was conducted in an alpine meadow on the Qinghai-Xizang Plateau during 2009-2013. The stoichiometric patterns of four functional groups （grass, sedge, legume and forb） and five representative species, Elymus nutans （grass）, Kobresia humilis （sedge）, Oxytropis ochrocephala （legume）, Taraxacum lugubre （rosette forb）, Geranium pylzowianum （upright forb） were investigated in 2013, and the effects of P addition on species dominance and plant biomass were also analyzed. Important finding Both plant nutrition content and C：N：P varied significantly after five years’ P addition, and the responses were consistent at species- and functional group （exemplar species excluded）-levels in the alpine meadow. P addition had neutral effect on C concentrations of grasses, sedges and forbs at both species- and functional group （exemplar species excluded）-levels. P fertilization increased plant P concentrations and thus decreased C：P and N：P of the four functional groups （exemplar species excluded） and the corresponding species. N concentrations significantly decreased and C：N increased in grasses and sedges after P addition, and the species-level responses were consistent with the functional group （exemplar species excluded） level. P addition significantly increased N contents and decreased C：N in Oxytropis ochrocephala, but had neutral effect on N contents and C：N at the functional group （exemplar species excluded） level of the legumes. While N contents and C：N in forbs responded to P addition differently at species and functional group （exemplar species excluded） levels. In the N-limited alpine meadow, species dominance of grasses increased gradually after P addition due to the increased N and P use efficiencies, while the biomass proportion of forbs decreased because of the lowered nutrition use efficiency.