Simulated N deposition enhances recalcitrant POM occlusion in microaggregates within macroaggregates

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.