C:N:P stoichiometries explain soil organic carbon accumulation during afforestation

Carbon accumulation in agroecosystems is regulated to a major extent by the variability of C:N:P stoichiometry. To understand the impact of C:N:P ratios and the cycling of limiting nutrients in the leaf–litter–aggregate continuum on soil C sequestration following afforesxtation, we assessed the contribution of aggregate-associated organic carbon (OC) to bulk soil OC accumulation, and examined the C, N, and P concentrations in leaves, litter, aggregates, and their stoichiometric couplings along an afforestation chronosequence. Averaged across the entire chronosequence, bulk soil OC stocks increased at the rate of 12.3 g m−2 year−1 at the 0–0.4-m depth, and significantly correlated with macroaggregates-associated OC stocks which increased on average by 114.5% with increasing stand age at soil depth of 0.4 m. Moreover, the C:P and N:P ratios of litter decreased by 19.7–33.0% and 7.6–26.8%, respectively, with increasing stand age, and were significantly associated with aggregate-associated C:P and N:P ratios. Both C:P and N:P ratios in microaggregates and clay-plus-silt fractions were, respectively, 18.0%, 13.3%, and 23.0%, 19.7% lower than those in macroaggregates, indicating a higher P availability in fine fractions. Macroaggregate-associated C:P and N:P ratios increased by 29.5 and 26.3%, respectively, at the 0–0.4-m depth with plantation age and were linked to the increased macroaggregate-associated OC stocks. Collectively, our results suggest that C:P and N:P ratios are good indicators of P cycling in ecosystem, and that P status under the control of aggregates may be an important factor for the sustainable accumulation of OC during afforestation.