Despite the importance of plant litter decomposition for ecosystem nutrient cycling and soil fertility, it is still largely unknown how this biogeochemical process is affected by different forms of nitrogen (N). Numerous studies have investigated the effects of exogenous N addition on leaf litter decomposition, while the response of decomposing roots and their microbial communities to externally applied N is rarely studied. Fine roots, however, represent a key input to soil organic matter and understanding their decomposition under elevated atmospheric N deposition is important for predicting soil carbon (C) dynamics in response to changes in climatic conditions. In this study, we decomposed fine roots of five dominant grassland species for two years in field plots fertilized with different forms of N in a typical temperate grassland in Inner Mongolia. Ammonium nitrate was selected as inorganic N (IN), while urea and glycine were chosen as organic N (ON). Equal amounts of N (10 g N·m⁻²·yr⁻¹) with different ratios of IN: ON (control, 10 : 0, 7 : 3, 5 : 5, 3 : 7, and 0 : 10) were added to the soil. Our results showed that all exogenous N additions, either IN or ON forms, stimulated the decomposition rates across species. Furthermore, the treatment with a mixture of IN and ON fertilizers led to the strongest responses in decomposition rate, which were, on average, 20% higher than control, and 12% higher than using just IN addition across the five studied species. Our results suggest that we need to consider the different components in N deposition when examining nitrogen deposition effects on terrestrial ecosystem carbon and nutrient cycles.

尽管植物凋落物分解对于生态系统养分循环和土壤肥力很重要，但仍然很大程度上未知这种生物地球化学过程如何受到不同形式的氮（N）的影响。大量研究调查了外源氮添加对凋落物分解的影响，而很少研究分解根及其微生物群落对外部施用氮的响应。然而，细根代表了土壤有机质的关键输入，了解大气氮沉降升高下它们的分解对于预测土壤碳（C）动态响应气候条件的变化非常重要。在这个研究中，我们在内蒙古典型的温带草原上，以不同形式的N施肥的田间地块，分解了5种优势草原种的细根两年。硝酸铵被选为无机氮（IN），尿素和甘氨酸被选为有机氮（ON）。等量的N（10 g N·m^将具有不同比例的IN：ON（对照：10：0、7：3、5：5、3：7和0:10 ）的^-2 ·yr ^-1）添加到土壤中。我们的结果表明，所有外源氮的​​添加，无论是IN还是ON形式，都刺激了物种间的分解速率。此外，在五个研究物种中，用IN和ON肥料混合处理导致分解速率最强，平均比对照高20％，比仅添加IN高12％。我们的结果表明，在检查氮沉降对陆地生态系统碳和养分循环的影响时，我们需要考虑氮沉降的不同成分。