[Effects of Manure Application Rates on the Soil Carbon Fractions and Aggregate Stability]

An incubated study was conducted to explore the effect of different manure application dosages (0%, 1%, 2%, and 4%, referred to as T0, T1, T2, and T4, respectively) on dynamic changes in the organic carbon fraction and aggregate stability of soil under different incubation times (120, 180, 240, 300, and 360 days). Soil organic carbon (SOC) and its fractions, such as light fraction organic carbon (LFOC), and polysaccharides, cellulose, water-soluble substance (WSS), fulvic acid carbon (FAC), humic acid carbon (HAC) content, and aggregate stability were measured. The results showed that SOC and its fractions were increased with increasing manure application rates. The SOC, LFOC, polysaccharides, cellulose, WSS, FAC, HAC contents and the HAC/FAC ratio increased by 15.3%-83.2%, 6.8-15.9 times, 8.5%-46.4%, 39.3%-122.6%, 35.7%-112.9%, 3.3%-46.9%, 42.5%-88.3%, and 28.5%-38.6% under T1-T4 treatments, respectively, compared to the T0 treatment at the end of the incubation period. With a longer period of incubation, the contents of SOC and HAC showed a decreasing trend, the LFOC increased first and decreased. The FAC content and the HAC/FAC ratio showed a fluctuation trend, but the content of polysaccharides showed an increasing trend. The application of manure decreased the content of >2 mm mechanically stable aggregates but increased the content of > 0.25 mm water stable aggregates in the soil. The mean weight diameter of water stable aggregate (WMWD) increased by 58.6%, while by the end of the incubation period, the percentage aggregate destruction rate (PAD) decreased by 22.2% under the T4 treatment compared to the T0 treatment. Correlation analysis showed that there was a significant correlation between SOC and its fractions, and between organic carbon fractions (except polysaccharides) and aggregate stability. Path analysis showed that the content of HAC and > 2 mm mechanically stable aggregate had a significant direct impact on the mean weight diameter of mechanically stable aggregate (DMWD) (P<0.05). Furthermore, the content of > 2 mm and < 0.25 mm water stable aggregates had a significant direct impact on the WMWD (P<0.01). The content of<0.25 mm water stable aggregates had a significant direct impact on the PAD (P<0.01), while the content of SOC and WSS had a significant indirect impact on the PAD via a direct effect on the content of<0.25 mm water stable aggregate (P<0.05).