Background and aim

Root exudates are generally known to play an important role in ecosystem carbon (C) and nitrogen (N) cycling. We aimed to quantify the responses of root exudates to increased N availability in an alpine shrub-dominated ecosystem, and unravel the associated ecological consequences to N cycling.

Methods

After three consecutive years of N addition at three rates (N0, control; N5, 5 g N m⁻² year⁻¹; N10, 10 g N m⁻² year⁻¹), we measured in situ rate of root exudation for mature Sibiraea angustata shrubs, as well as soil microbial biomass, the activities of three extracellular enzymes, and net N mineralization rates (N_min) in both rhizosphere and bulk soil.

Results

Exudation rates were 13% and 45% lower in the N5 and N10 plots compared to the control plots over the primary growing season. This, together with decreasing fine root biomass and length, lead to an annual decline of 44% and 66% in root exudate C inputs to soil, respectively. These decreased exudation rates were both directly and indirectly, through its effect on the rhizosphere effect (RE) on polyphenol oxidase activity, linked to decreased the RE on N_min. As a result of N-induced decreases in both the RE on N_min and rhizosphere volume, we estimated that the ecosystem-level rhizosphere effect for N_min from 13% in the control plots to 8% in the N5 plots, and to 2% in the N10 plots.

Conclusions

Overall, our results demonstrate that N addition may decelerate the C efflux from root exudates, resulting in a slower decomposition of soil organic matter and also gradually decelerating soil N cycling in alpine shrub ecosystems.

中文翻译：

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氮素对高灌木灌丛西番莲根系渗出及相关微生物氮转化的影响

背景和目标

众所周知，根系分泌物在生态系统的碳（C）和氮（N）循环中起重要作用。我们旨在量化根系分泌物对高寒灌木为主的生态系统中氮素有效性的响应，并揭示氮素循环的相关生态后果。

方法

在连续三年以三种速率（N0，对照； N5，5 g N m - ²年^-1； N10，10 g N m - ²年^-1）添加氮后，我们测量了成熟的根系渗出速率西伯利亚ea（Sibiraea angustata）灌木以及土壤微生物量，三种胞外酶的活性以及根际和块状土壤中的净氮矿化率（N _min）。

结果

在主要生长期，N5和N10样地的渗出率分别比对照样地低13％和45％。这与减少的细根生物量和长度减少在一起，导致土壤中的根系分泌物碳输入量分别每年下降44％和66％。这些降低的渗出率分别为直接和间接地，通过其上的多酚氧化酶活性的根际效果（RE）的效果，连接于降低N个对RE_分钟。由于N引起的RE在N _min和根际上的减少，我们估计N _min的生态系统水平根际效应从对_照样区的13％降至N5样区的8％，以及N5样区的2％在N10地块。

结论

总体而言，我们的结果表明，氮的添加可能会使根系分泌物的C流出速度降低，从而导致土壤有机质分解速度变慢，并逐渐减缓高山灌木生态系统中土壤氮的循环。