Clarifying the fate of different nitrogen (N) species in different pools of terrestrial ecosystems is a prerequisite for a comprehensive understanding of the influence of human activities on the N cycle. Grazing has always been an important way of grassland management for centuries in the temperate grasslands of North China. However, how grazing regimes affect the N fate of urea derived from the livestock in the plant–soil systems of grazed grasslands remains poorly understood. Therefore, an in situ three-factor (grazing regime, soil depth, and sampling time) ¹⁵N-labeling experiment in a temperate steppe was conducted to answer this question. After 48 days of ¹⁵N labeling, the ¹⁵N recovered in shoots under no grazing (7.3% ± 1.8%) was approximately 2.3 times of that under rotational (3.2% ± 0.4%) and 2.5 times of that under continuous overgrazing (2.9% ± 0.5%). More ¹⁵N was recovered in roots under rotational than continuous overgrazing (19.8% ± 2.4% vs. 10.4% ± 0.5%, respectively), indicating that rotational overgrazing could promote more N retention in the roots. However, the ¹⁵N recovered in the soil was lower under continuous (23.7% ± 2.0%) than that of no grazing (42.0% ± 5.6%). Additionally, overgrazing reduced the magnitude of the soil active N pool in microbial biomass N and soluble N relative to no grazing. The grazing regimes would have significantly influenced both the soil and plants. That is to say, grazing regimes have directly impacted plant growth, and subsequently indirectly affected soil properties. Overgrazing often led to excessive vegetation consumption, resulting in decreased soil water content (SWC) and reduced soil organic carbon (SOC), ultimately caused alterations in plant species composition. The retention of ¹⁵N within the plant–soil system under continuous overgrazing was notably lower compared to that of no grazing. Continuous overgrazing has led to a shift in the dominant plant species from Leymus chinensis to Stipa grandis, by decreasing the proportion of perennial grasses by 10%, and increasing the annual and biennial plants by 8%. The fate of ¹⁵N was also altered in response to the variations in grazing regimes. Consequently, the recovery of ¹⁵N within the plant–soil system under continuous overgrazing was significantly lower compared to that of no grazing. In conclusion, overgrazing reduces the recovery of ¹⁵N within the plant–soil system in the temperate steppe, and rotational grazing is more preferable over continuous grazing as it could promote higher N retention in grassland ecosystems.

中文翻译：

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放牧制度改变了温带草原中 15N 标记尿素的命运


阐明不同陆地生态系统库中不同氮（N）物种的命运是全面了解人类活动对氮循环影响的先决条件。几个世纪以来，放牧一直是华北温带草原重要的草原管理方式。然而，放牧制度如何影响放牧草原植物-土壤系统中牲畜产生的尿素的氮归宿仍知之甚少。因此，在温带草原进行了原位三因素（放牧制度、土壤深度和采样时间） ¹⁵ N标记实验来回答这个问题。 ¹⁵ N标记48天后，不放牧条件下地上部回收的¹⁵ N（7.3%±1.8%）约为轮作条件下（3.2%±0.4%）的2.3倍和连续过度放牧条件下（2.9%）的2.5倍。 ±0.5%）。与连续过度放牧相比，轮作下根部回收了更多的¹⁵ N（分别为 19.8% ± 2.4% 和 10.4% ± 0.5%），表明轮作过度放牧可以促进根部保留更多的氮。然而，连续放牧下土壤中¹⁵ N的回收量（23.7%±2.0%）低于不放牧（42.0%±5.6%）。此外，相对于不放牧，过度放牧降低了微生物生物量氮和可溶性氮中土壤活性氮库的数量。放牧制度会对土壤和植物产生重大影响。也就是说，放牧制度直接影响植物生长，进而间接影响土壤性质。 过度放牧常常导致植被消耗过多，导致土壤含水量（SWC）下降和土壤有机碳（SOC）减少，最终导致植物物种组成的改变。连续过度放牧下植物-土壤系统中¹⁵ N 的保留量明显低于不放牧的情况。持续的过度放牧导致优势植物种类从羊草转向大针茅，多年生草本植物比例减少了10%，一年生和二年生植物增加了8%。 ¹⁵ N 的命运也因放牧制度的变化而改变。因此，连续过度放牧下植物-土壤系统内¹⁵ N 的恢复量明显低于不放牧的情况。总之，过度放牧减少了温带草原植物-土壤系统中¹⁵ N 的恢复，轮牧比连续放牧更可取，因为它可以促进草原生态系统更高的 N 保留。