Background and aims

Rhizodeposition is organic matter released by living plant roots that can be transformed by microbes into particulate organic matter (POM), but that can also become more stable through the adsorption of organic matter onto soil minerals (mineral-associated organic matter, MAOM), thereby playing an important role in mitigating climate change. We examined how root-derived carbon (C) as a proxy for rhizodeposition contributed to POM and MAOM formation in a grassland affected by nitrogen (N) fertilisation and defoliation frequency, and to what degree rhizodeposition was incorporated into microbial biomass.

Methods

We applied N fertiliser (0 vs. 40 kg N ha⁻¹ yr⁻¹) and defoliation frequencies (3–4 vs. 6–8 clipping events year⁻¹, simulating low and high grazing intensity) for three years, then used a ¹³CO₂ pulse labelling technique to examine the incorporation of rhizodeposition into microbial biomass, POM and MAOM fractions.

Results

With N fertilisation, rhizodeposition contributed less to the formation of MAOM compared to the formation of POM, while defoliation frequency decreased the contribution of rhizodeposition into both POM and MAOM, particularly with N fertilisation. Although the MAOM fraction was relatively rich in N (C: N ratio of 10.5 vs. 13.5 for POM), our results suggest that adding inorganic N promoted the formation of POM more than of MAOM from rhizodeposition.

Conclusion

A large proportion of rhizodeposition was taken up by microbes that eventually could contribute to POM and MAOM formation. Our results provide insightful information regarding the stabilisation of rhizodeposition into different soil organic matter pools.

中文翻译：

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在草地土壤中低落叶频率和高落叶频率下，氮肥减少了根源碳对矿物质相关有机物形成的贡献

 背景和目标

根际沉积是活植物根部释放的有机物，可以被微生物转化为颗粒有机物（POM），但也可以通过将有机物吸附到土壤矿物质（矿物相关有机物，MAOM）上而变得更加稳定，从而在减缓气候变化方面发挥着重要作用。我们研究了在受氮 (N) 施肥和落叶频率影响的草地中，根源碳 (C) 作为根际沉积的代表如何促进 POM 和 MAOM 的形成，以及根际沉积在多大程度上融入微生物生物量。

 方法

我们施用氮肥（0 vs. 40 kg N ha ⁻¹ 年 ⁻¹ ）和落叶频率（3–4 vs. 6–8 修剪事件年 ⁻¹ ，模拟低和高放牧强度）三年，然后使用 ¹³ CO ₂ 脉冲标记技术来检查根际沉积与微生物生物量、POM 和 MAOM 组分的结合情况。

 结果

施氮肥时，与 POM 的形成相比，根际沉积对 MAOM 形成的贡献较小，而落叶频率降低了根际沉积对 POM 和 MAOM 的贡献，特别是施氮肥时。尽管 MAOM 部分的 N 含量相对丰富（POM 的 C:N 比率为 10.5，而 POM 的 C:N 比率为 13.5），但我们的结果表明，添加无机 N 比根际沉积中的 MAOM 更能促进 POM 的形成。

 结论

大部分根际沉积被微生物占据，最终有助于 POM 和 MAOM 的形成。我们的结果提供了有关不同土壤有机质库根际沉积稳定性的深刻信息。