Plant phenological responses under global climate change is a hot topic in current ecological research. However, it also has comprehensive interactions with other anthropogenic environmental factors including nitrogen (N) enrichment owing to atmospheric deposition and fertilization, and grassland degradation, which attracts less attention. Continuous photographic techniques enabled us to monitor the plant cover dynamics and extract the phenophases (start, end, length and the peak cover position of growing season) in a field experimental platform with six N treatment levels on five grasslands along a degradation gradient indicated by a human disturbance index (HDI). The results showed that N addition significantly promoted seasonal plant covers of degraded grasslands, while mature grassland responding less sensitively. Moreover, increasing rates during spring green-up and decreasing rates during autumn senescence were promoted by N addition, while those of grasslands with lower degradation levels showed non-linear saturation patterns. Phenophases at the community level were generally insensitive to N addition, with the most degraded grassland (totally degraded grassland, TD) and mature grassland (MG) responding significantly. The length of growing season of TD was shortened by 8 days and that of MG lengthened by 15 days on average from N addition. The responses of extracted phenophases were regulated more by the proportions of three groups of degradation indicators (annuals, moderate grazing degradation indicator species, and climax species) than those of grasses and forbs. Our findings highlight the relationship between community phenology and grassland degradation, which can benefit the assessment and restoration of degraded grasslands in semiarid regions.

中文翻译：

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氮富集对植被生长动态的影响受草地退化状况的调节

全球气候变化下的植物物候响应是当前生态学研究的热点。然而，它也与其他人为环境因素综合相互作用，包括大气沉降和施肥导致的氮（N）富集，以及草地退化，较少受到关注。连续摄影技术使我们能够在田间实验平台上监测植物覆盖动态并提取物候期（生长季节的开始、结束、长度和峰值覆盖位置），在五个草地上沿着退化梯度由人为干扰指数（HDI）。结果表明，N添加显着促进退化草地的季节性植物覆盖，而成熟草地响应较不敏感。而且，N添加促进了春季绿化期间的增加率和秋季衰老期间的减少率，而退化程度较低的草地则表现出非线性饱和模式。群落物候期普遍对N添加不敏感，退化最严重的草地（完全退化草地，TD）和成熟草地（MG）响应显着。添加N后，TD的生长期平均缩短8天，MG的生长期平均延长15天。与禾本科和杂草相比，提取物候期的响应更受三组退化指标（一年生、中度放牧退化指标和高潮物种）的比例调节。我们的研究结果强调了群落物候与草地退化之间的关系，