Small straw addition enhances straw decomposition and carbon stabilized in soil aggregates over time

Straw return as a common and effective management can improve soil carbon (C) sequestration and structure by forming aggregates and aggregate associated C. Quantifying soil aggregation and aggregate associated C is key to predicting soil organic C dynamics using testable models. The relative quantity of C stabilized in soil aggregate is an indicator that can reveal the conversion efficiency of added straw to organic C in aggregate. Straw size is known to alter litter decomposition, but how straw size impacts the dynamics of soil aggregate formation and the relative quantity of C stabilized in soil aggregate after litter addition is not well understood. We conducted an 80-day laboratory incubation with wheat straw addition of two different sizes (1-2 mm and <0.25 mm) to artificial soils of two textures (3 % clay and 7 % clay). The objectives were to examine 1) whether straw size significantly modified litter decomposition and the relative quantity of C stabilized in soil aggregates over time, and 2) what soil physical and biochemical factors determined soil aggregation and the formation of aggregate-associated C over time, such as straw size, soil texture, or enzyme activity. We found that both litter decomposition and the relative quantity of C stabilized in soil aggregates were higher in the treatment with small straw addition than those with large straw addition, but they were not significantly different between two soils with 3 % vs. 7 % clay content. This was due to increased enzyme activities caused by small straw addition, which could enhance dissolved organic C generation and stimulate microbial decomposition. Additionally, the mass proportion of macroaggregate (>2 mm) increased with time while the mass proportion of microaggregate (0.25-2 mm) decreased, suggesting that microaggregate combined to form macroaggregate (>2 mm) in the late stage of incubation (after day 17). Moreover, the relative quantity of C stabilized in microaggregate (<0.053 mm) increased linearly with dissolved organic C, whereas the relative quantity of C stabilized in macroaggregate (>2 mm) increased with the aromaticity of dissolved organic matter as measured by specific ultraviolet absorbance at 254 nm. Together, our findings suggest that straw size remarkably modifies litter decomposition and the relative quantity of C stabilized in soil aggregates by changing the quantity and quality of dissolved organic C at different decomposition stages, whereas small difference in soil clay content has no effects.