Data from: Plant functional composition affects soil processes in novel successional grasslands

1. Secondary succession may lead to novel, exotic-dominated community states differing in structure and function from the original native counterparts. We hypothesized that grassland soil processes associated with C and N cycling decelerate with community turnover from short-lived forbs and grasses to long-lived native grasses, whereas invasion by exotic perennial grasses maintains fast cycling rates. 2. We measured litter C and N turnover during decomposition, soil respiration, and soil N dynamics in synthetic plant communities resembling four successional stages, established on abandoned farmland in the Inland Pampa, Argentina. We also compared litter chemistry and decay rates of dominant species from each community stage in a common garden, and assessed mass loss for a standard litter type incubated in all communities. 3. Litter decomposition and soil respiration decreased, while litter N retention increased from early, through mid to late community stages dominated by forbs, short-lived grasses and native perennial grasses, respectively. Soil process rates in exotic perennial grass communities were faster than in native grass communities, but similar to annual grass communities. Further, the standard litter decomposed more slowly in the native perennial than in the exotic perennial grass community. In the common garden, short-lived forbs and grasses decomposed faster than native or exotic perennial grasses, with species’ decay rates being negatively related to initial litter C:N ratio. 4. Our results show that changes in soil processes across old-field communities arise chiefly through differences in the quality of litter produced by dominant functional groups. A dominance shift from native to exotic perennial grasses prevented the deceleration of C and N cycling expected with plant successional turnover. Thus invasion by fast-growing exotic grasses may fundamentally alter ecosystem functioning in novel grasslands.

1. 次生演替（secondary succession）可能会形成新颖的、以外来物种为主的群落状态，其结构与功能均与原始本地群落存在显著差异。我们提出如下假说：当草地群落从短命杂类草与草本植物向多年生本地草本植物发生演替更替时，与碳（C）、氮（N）循环相关的土壤过程会逐渐放缓；而外来多年生草本植物的入侵则会维持较快的碳氮循环速率。
2. 我们在阿根廷内陆潘帕斯（Inland Pampa）地区的弃耕农田上，构建了模拟4个演替阶段的人工植物群落，并在此基础上测定了分解过程中的凋落物碳氮周转速率、土壤呼吸速率以及土壤氮动态。此外，我们还在同质园（common garden）实验中，比较了各群落优势种的凋落物化学组成与分解速率，并针对在所有群落中培养的标准凋落物类型，评估了其干质量损失情况。
3. 从以杂类草、短命草本为主的早期演替阶段，经以短命草本为主的中期阶段，到以多年生本地草本为主的晚期演替阶段，凋落物分解速率与土壤呼吸速率均呈逐步下降趋势，而凋落物氮保留量则逐渐升高。外来多年生草本群落的土壤过程速率快于多年生本地草本群落，但与一年生草本群落相近。进一步分析显示，标准凋落物在多年生本地草本群落中的分解速率显著慢于外来多年生草本群落。在同质园实验中，短命杂类草与草本植物的分解速率快于本地或外来多年生草本植物，且物种的分解速率与凋落物初始碳氮比呈显著负相关关系。
4. 本研究结果表明，弃耕农田群落间的土壤过程差异主要源于优势功能群所产生的凋落物质量差异。从本地多年生草本向外来多年生草本的优势度转变，阻断了植物演替更替本应带来的碳氮循环放缓效应。由此可见，快速生长的外来草本植物的入侵，可能会从根本上改变新型草地生态系统的生态系统功能。