Soil organic carbon sequestration benefit and influencing factors in terraces with different cover types of soil in the loess hilly region

Background Terraces and vegetative measures significantly enhance soil organic carbon (SOC) levels and improve the efficiency of SOC sequestration, serving as crucial soil and water conservation strategies. However, there is limited research on the differences and influencing factors of SOC sequestration effects when these two measures are combined. Thus, our study analyzed the variations in SOC and its primary influencing factors across different vegetation cover types in terraces and evaluated the SOC sequestration benefits of terraces.

Methods We selected 96 sample plots in Wuqi county, Shaanxi province, including 37 terraced croplands (TC), 23 Terraced grasslands (TG), 18 terraced forestlands (TF), 10 terraced orchards (OL), and 8 slope croplands (SC) on hillsides. Soil samples were collected from soil layers at depths of 0−10, 10−20, 20−40, 40−60, 60−80, and 80−100 cm, totaling 576 soil samples. In the laboratory, SOC was determined by potassium dichromate volumetric method. Bulk density and soil water content were measured using oven drying method. Soil mechanical composition was analyzed with a laser particle size analyzer. Then, we analyzed the significant differences and correlations among the data using the Games-Howell method and Pearson correlation analysis.

Results 1) The SOC in the 0-100 cm soil layer of the four types of land cover under the terrace ranges from 2.34 to 3.42 g/kg, with the order of TF (3.42 ± 1.85) g/kg > OL (2.89 ± 0.99) g/kg > TG (2.84 ± 1.20) g/kg > TC (2.34 ± 0.95) g/kg. 2) After SC is convert into TF, TG, TO, and TC, it has improved the carbon sequestration benefits of soil. The carbon sequestration of TF, TO, TG, and TC is 12.01, 8.78, 8.13, and 2.13 t/hm², respectively. 3) The vertical distribution of SOC sequestration benefits differs among various land cover types. The SOC sequestration benefit of TC, TG, TO, and TF is 0.40,3.30, 4.29, and 7.03 t/hm² and 1.48, 2.99, 3.07, and 3.05 t/hm² in the 0−40 cm and 40−100 cm soil layer, respectively. The SOC sequestration benefit of terraced fields is higher in the 60−100 cm soil layer than the 0−40 cm soil layer. However, when terracing is combined with vegetation measures, the trend is reversed. 4) The SOC of TF, TG, TO TC, and SC have a significant negative correlation with soil bulk density and an extremely significant positive correlation with soil moisture content, respectively. However, compared to SC, only the soil moisture content of TC and TO shows a significant increase.

Conclusions The implementation of terrace measures influences SOC sequestration benefits by increasing soil moisture, especially enhancing the sequestration in deep soil layers. When terraces are combined with vegetation measures, the SOC sequestration benefits are further enhanced, with a particularly greater impact on the sequestration benefits of surface soil. The results of our study could provide strong support for achieving the effects of relevant soil and water conservation measures and developing SOC sequestration methodology.