Sequestration of Soil Organic Carbon is Driven by Aggregate Turnover under Straw Addition in Mollisols

Objective Soil structure and soil organic carbon (SOC) dynamics are crucial to soil quality. However, the interactions between soil structural dynamics and SOC changes remain unclear. The purpose of this study was to elucidate the coupling process of aggregate organic carbon accumulation and aggregate turnover under straw addition.

Method Rare earth oxides (REOs) were used as tracers to track aggregate formation and breakdown pathways. Two different types of straw (alfalfa and maize straw) were added to REOs-labelled Mollisols. Soil aggregate distribution, mean weight diameter (MWD), aggregate turnover, and organic carbon content were measured after the 0^th, 6^th, 9^th and 12^th months of incubation.

Results The results showed that the remaining mass of added straw significantly decreased with time, in which the higher content was observed with alfalfa application compared with maize straw. The content of macroaggregates (> 0.25 mm) and MWD reached the peak on the 9^th month and then decreased significantly with or without straw addition. Compared with the control treatment, MWD was increased by 33.7% and 37.3% under alfalfa and maize straw treatments (P < 0.05). Straw addition significantly increased > 0.25 mm aggregates proportions and reduced < 0.25 mm aggregates contents, such effects were more considerable in maize treatment compared with alfalfa treatment (P < 0.05). The proportions and relative changes of macroaggregates explained 35.9% and 32.8% to the variation of MWD, indicating that MWD is mainly determined by macroaggregates changes. The SOC and macroaggregate SOC contents reached a maximum in all three treatments on the 6^th month and then decreased significantly (P < 0.05). Compared to the control, straw addition increased carbon contents in bulk soil and aggregates by 1.5%-5.6% and 0.6%-43.9%. At the early stage of straw decomposition, the SOC content in > 0.25 mm aggregates as enhanced, while that in < 0.25 mm aggregates was accumulated afterwards (P < 0.05). Compared to aggregate distribution, the relative changes in aggregates explained the highest variation of aggregates organic carbon (51.6% vs.81.8%), suggesting that the aggregates turnover determined the sequestration of organic carbon in aggregates.

Conclusion Straw addition increased the SOC content of aggregates by promoting the formation of macroaggregates and decreasing relative breakdown.