The aim of this study was to clarify the effects of long-term optimized fertilization (nitrogen reduction) and nitrogen reduction plus film mulching on the characteristics of soil organic carbon (SOC) and its components content, and their contribution to winter wheat grain yield, so as to provide the theoretical support for dryland fertility and crop productivity in the Loess Plateau.

Based on a 7-year location experiment, the undisturbed soil samples from 0-20 cm under different management measures were collected. The variation characteristics of winter wheat grain yield, SOC and its components content were explored, including particulate organic carbon (POC), mineral organic carbon (MOC), light-particulate organic carbon (Light-POC) and heavy-particulate organic carbon (Heavy-POC), and the relationship between SOC and its component content as well as their quantity contribution to the winter wheat grain yield were clarified. The organic carbon components were grouped by continuous physical grouping method, and their contribution for wheat yield was analyzed by redundancy analysis. The experiment set four treatments, including farmer’s practice fertilization (FP), monitoring fertilization (MF), monitoring fertilization plus ridge mulching-furrow planting (RF), and monitoring fertilization plus whole field filming (FH).

The average yield of winter wheat was the lowest under FP treatment, which was 3 070 kg·hm^-2. Compared with FP, the wheat yield under MF showed no significant difference, while wheat grain yield under RF and FH could achieve a significant increase by 27.0% and 46.4%, respectively; compared with the initial year of the experiment (2012), the SOC content showed a significant increase under different management measures during seven consecutive years, and the increase rate from low to high was 11.8% (MF), 22.4% (RF), 25.5% (FP), and 36.1% (FH), respectively. The improvement of SOC under MF was the slowest in calcareous cinnamon soil, when MF combining with plastic film mulching showed a significant improvement. The observed SOC components under different treatments showed, compared with FP and MF, RF and FH significantly increased the content of POC, Light-POC and the proportion of Light-POC in SOC. Based on the cooperation of the redundancy analysis, sensitivity index analysis and correlation analysis between SOC and its component, POC contributed the most to the improvement of wheat yield (up to 71.0%) and was the most sensitive to SOC content change and different management measures. Therefore, the improvement of crop yield and soil fertility by FH was mainly achieved by increasing POC content in soil organic carbon.

The monitoring fertilization plus whole field filming treatment was benefit to increase soil organic carbon and active organic carbon content in the cinnamon soil in the southeastern of the Loess Plateau, and achieve a sustainable increase in winter wheat yield.