Abstract:
This study aims to investigate the effects of different straw returning amounts on soil aggregate stability and carbon pool change characteristics of wheat fields in dry loess tableland. Four treatments were set, including S0 (no straw returning), S1/2 (half of the straw returning), S1 (total straw returning), and S2 (two times straw returning), in the dryland winter wheat growing area of southern Shanxi Province, China. The composition and stability of soil aggregates were determined under different straw return rates over three consecutive years. A systematic investigation was made to clarify the Total Organic Carbon (TOC), Light Fraction Organic Carbon (LFOC), Heavy Fraction Organic Carbon (HFOC), Humin Carbon (HM-C), Humic Acid Carbon (HA-C), and Fulvic Acid Carbon (FA-C) in the soil and aggregates with different grain size. A relationship was also established between the organic carbon components in the aggregates of different grain sizes and the stability of aggregates. The results showed that the content of large macroaggregates (>0.25 mm) increased gradually, with the increase of straw returning amount, while the content of macroaggregates (<0.25 mm) and silt+clay (<0.053 mm) decreased gradually. Compared with the S0 treatment, the Mean Weight Diameters (MWD), Geometric Mean Diameter (GMD), and R0.25 increased by 14.7%, 22.2%, and 13.9% (P<0.05), respectively, indicating the higher stability of aggregates. The TOC content of the soil under the S2 treatment was the highest, which increased by 28.1% compared with the S0 treatment (P<0.05). The contents of soil organic carbon components under different treatments also increased significantly, with the increase of straw returning amount. Furthermore, straw returning increased the LFOC in the 0.053-0.25mm aggregate, and the HFOC in the 0.25-2 mm aggregate. In addition, the contents of HM-C, HA-C, and FA-C in the >0.25-2mm aggregate increased by 19.0%, 25.5%, and 14.9% (P<0.05), respectively, in the S2 treatment, compared with the S0. The ratio of humic to fulvic acid carbon (HA-C/FA-C) also increased, with the increase of straw returning amount. The variation of FA-C content in the silt+clay (<0.053 mm) was the main reason for the stability change of water-stable aggregates, which was 64.1% of the variation. Therefore, straw returning promoted the transformation from the water-stable micro-aggregate to large macroaggregates in the dry loess plateau, particularly for the higher stability of the aggregate. At the same time, straw returning increased the content of organic carbon and the components in the soil and aggregates. The degree of soil humification was improved significantly, as the straw returning amount increased. Two times the amount of straw returning performed the best on the soil organic carbon content and soil structure. The finding can also provide the theoretical basis for soil fertilizer cultivation in dry farmland.