Effects of Biodegradable Film Mulching and Water-Saving Irrigation on Soil Moisture and Temperature in Paddy Fields of the Black Soil Region

Paddy cultivation in the black soil region of northeast China is faced with the problems of low irrigation water use efficiency (IWUE) and low temperature stress during sowing. Therefore, the combinations of film mulching and water-saving irrigation methods were adopted to adjust the balance between water and yield under the condition of suitable soil water and heat environment, and to quantify the relationship between irrigation water and yield formation. This study investigated the mechanisms of two kinds of biodegradable film mulching combined with two water-saving irrigation on soil hydrothermal conditions in cold-region paddy fields. The results show that film mulching improved the water retention capacity of the soil at different depths, with black film exhibiting better moisture conservation than white film. Overall, controlled irrigation resulted in higher soil moisture than ridge irrigation before the heading–flowering stage, but lower values in heading–flower stage and the later stage. Film mulching also increased soil temperature across different layers, with black film showing a more warming effect in the 0–5 cm soil layer. All combinations of biodegradable film mulching and water-saving irrigation enhanced the IWUE, with the ridge irrigation combined with black film mulching showing the most significant improvement. This research provides technical references for water-efficient rice cultivation in cold regions.