Response of organic material return on wheat grain protein content in China: A meta-analysis

The return of organic materials (such as straw and organic fertilizers) to the field affects the nutritional quality of wheat grains. This study conducted a meta-analysis based on literature data published by Chinese research stations before September 30, 2023. A total of 393 observations from 65 studies were synthesized, and subgroups such as nitrogen application rates, soil baseline organic matter, annual average temperature, and annual average rainfall were used to assess the impact of organic material return on wheat grain protein content (GPC) in China. The results showed that straw return could enhance wheat GPC. Among different subgroups of straw return, the improvement in GPC was most pronounced under a nitrogen rate of 151–200 kg·hm⁻². A straw return amount >9000 kg·hm⁻² had the greatest positive effect, while soil baseline organic matter ≥21 g·kg⁻¹ under straw return significantly boosted wheat GPC. The return of rice and maize straw also had a significant positive effect. Optimal improvements in GPC were observed under an average annual temperature of 21–25°C and average annual precipitation >800 mm. In contrast, organic fertilizer return reduced wheat GPC. Specifically, the complete (100%) and partial (75%) replacement of inorganic fertilizer with organic fertilizer significantly decreased GPC, while reductions were smaller with 25% and 50% organic fertilizer substitution. For organic fertilizer return subgroups, nitrogen rate of 151–200 kg·hm⁻² and 251–300 kg·hm⁻² combined with 25% or 50% organic fertilizer substitution improved wheat GPC. High soil background organic matter content baseline organic matter ≥21 g·kg⁻¹ partially mitigated the loss in GPC. Furthermore, 50% organic fertilizer substitution improved GPC under an annual average temperature of 21–25℃ and average annual precipitation >800 mm. Overall, straw return had a positive effect on wheat GPC across regions in China, whereas organic fertilizer return primarily showed a negative effect. These findings provide scientific evidence for enhancing wheat grain protein content through organic material return in China.