Background and aims

Increased N availability transforms labile soil organic carbon (SOC) to recalcitrant SOC in N-limited forest soils under atmospheric N deposition. However, N-induced variation in SOC stability within aggregates is rarely studied. Thus, the mechanism of SOC sequestration in aggregates under N deposition was studied.

Methods

Soils from N-amended and adjacent forest fields were sampled and separated into macroaggregates, free microaggregates and silt and clay (SC) fractions. The microaggregates (mM), coarse particulate organic matter (cPOM), fine particulate organic matter (fPOM-mM and fPOM-m) and SC fractions (SC-M, SC-mM and SC-m) occluded in aggregates were further separated. Their fraction masses, carbon concentrations and lignin indexes were determined.

Results

The fraction masses of 1–2 mm macroaggregates, mM, SC-M, fPOM-mM and SC-mM increased with fragmentation of 2–8 mm macroaggregates under N addition. The carbon contents in mM, SC-M and SC-mM also increased with increasing mass. Nitrogen addition caused distinct lignin loss in the occluded SC fractions. Lignin oxidation occurred in mM, cPOM, fPOM-m and fPOM-mM, while mM and cPOM exhibited increased lignin/N ratios under N enrichment. The results indicate that N deposition facilitated preservation of recalcitrant fPOM rather than carbon-rich particles in mM. The N-induced increase in mM proportion in macroaggregates and carbon stability of fPOM in mM contributed to SOC sequestration in the studied fields.

Conclusion

The quantitative and qualitative changes in mM and fPOM within macroaggregates may predict the positive response of SOC sequestration in the 300-year-old forest to long-term atmospheric N deposition in the future. 

中文翻译：

---

模拟的 N 沉积增强了大团聚体内微聚集体中的顽固 POM 遮挡

 背景和目标

在大气氮沉降下，氮有效性的增加将氮限制森林土壤中不稳定的土壤有机碳 （SOC） 转化为顽固的 SOC。然而，很少研究聚集体内 SOC 稳定性的 N 诱导变化。因此，研究了 N 沉积下聚集体中 SOC 封存的机制。

 方法

对来自 N 改良林地和相邻林地的土壤进行取样，并将其分离成大团聚体、游离微团聚体以及淤泥和粘土 （SC） 组分。进一步分离聚集体中封闭的微团聚体 （mM）、粗颗粒有机物 （cPOM）、细颗粒有机物 （fPOM-mM 和 fPOM-m） 和 SC 馏分 （SC-M、SC-mM 和 SC-m）。测定其馏分质量、碳浓度和木质素指标。

 结果

在氮添加下，1-2 mm 大聚集体、mM、SC-M、fPOM-mM 和 SC-mM 的馏分质量随着 2-8 mm 大聚集体的碎裂而增加。mM、SC-M 和 SC-mM 中的碳含量也随着质量的增加而增加。氮添加导致闭塞的 SC 组分中明显的木质素丢失。木质素氧化发生在 mM 、 cPOM 、 fPOM-m 和 fPOM-mM 中，而 mM 和 cPOM 在 N 富集下表现出更高的木质素/氮比。结果表明，N 沉积有助于在 mM 中保存顽固的 fPOM 而不是富含碳的颗粒。N 诱导的大团聚体中 mM 比例的增加和 fPOM 在 mM 中的碳稳定性有助于研究领域的 SOC 封存。

 结论

大团聚体中 mM 和 fPOM 的定量和定性变化可能预示未来 300 年生森林 SOC 封存对长期大气氮沉降的正响应。