Soil aggregation accounts for the mineral soil organic carbon and nitrogen accrual in broadleaved forests as compared to that of coniferous forests in Northeast China: Cross-sites and multiple species comparisons

Differences in soil organic carbon (SOC) and total nitrogen (TN) between broadleaved and coniferous forests are not well-defined, hindering the evaluation of the influence of forest-type on land degradation and development. For these two foresttypes, we determined differences in soil aggregation, SOC, and TN by comparing 202 plots at six sites with 14 common afforestation tree species. Bulk soil was separated to macroaggregates (0.25-2 mm), microaggregates (0.053-0.25 mm), and silt and clay fractions (<0.053 mm). Inter-site variations were excluded by MANOVA (multiple analysis of variance) and MANCOVA (multiple analysis of covariance). Broadleaved forests exhibited 30-50% higher SOC and TN in bulk soils than that of coniferous forests, and aggregates contributed to 75-77% of the SOC and TN accrual. The high accrual was a result of SOC and TN concentration increases in aggregates (30-50%), macroaggregates fraction increases (50%, with a 14% decrease in the non-aggregate fractions), aggregates diameter increases (one-third), and non-significant differences in C/N. Uncertainty tests showed a low bias (4.2%), and the accrual of SOC and TN in broadleaved forests were larger at sites with less clay and at higher altitude and more precipitation and afforested with more fraxinus and less poplar. Our results highlight that SOC and TN accrual in broadleaved forests is mainly due to the stronger aggregates protection, which should be fully considered during precise land evaluation and species selection for afforestation. Future studies identifying the mechanisms underlying these changes should consider interactions among leaf functional traits, aggregation-disaggregation processes, and microbial rhizosphere deposition.

Automated summary

Differences in soil organic carbon and total nitrogen between broadleaved and coniferous forests were investigated by comparing 202 plots at six sites with 14 common afforestation tree species. Broadleaved forests exhibited significantly higher SOC and TN in bulk soils compared to coniferous forests, with aggregates contributing to the majority of the accrual. The high accrual was attributed to increases in SOC and TN concentration, macroaggregates fraction, aggregates diameter, and no significant differences in C/N ratio.