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

The incorporation of organic materials (such as straw and organic fertilizers) into agricultural fields can significantly impact the nutritional quality of wheat grains. To address this, we conducted a meta-analysis using data from studies published before September 30, 2023, across various experimental sites in China. This analysis compiled 393 observations from 65 studies to assess the impact of organic material return on wheat GPC in China. The results indicated the following: 1) Straw return can enhance wheat GPC. A detailed comparison of different subgroups within straw return practices showed that a nitrogen application rate of 151−200 kg·hm⁻² was most effective for increasing wheat GPC. Moreover, straw return amounts greater than 9000 kg·hm⁻² led to the largest increase in wheat GPC, and soil with a baseline organic matter content of ≥21 g·kg⁻¹ significantly boosted wheat GPC. Additionally, returning rice and corn straw had a notable positive effect on wheat GPC, with the best improvements seen at an annual average temperature of 21−25℃ and annual rainfall over 800 mm. 2) Conversely, the return of organic fertilizers generally reduced wheat GPC, especially when 75% or 100% organic fertilizer was used to replace inorganic fertilizers, leading to a significant decrease in wheat GPC. However, replacing inorganic fertilizers with 25% or 50% organic fertilizers could mitigate this reduction in wheat GPC. Further subgroup analysis revealed that at nitrogen application rates of 151−200 kg·hm⁻² and 251−300 kg·hm⁻², replacing 25% and 50% of inorganic fertilizers with organic fertilizers could increase wheat GPC. A soil baseline organic matter of ≥21 g·kg⁻¹ could also help mitigate the loss in wheat GPC, and substituting 50% of inorganic fertilizers with organic fertilizers improved wheat GPC under an annual average temperature of 21−25℃ and rainfall exceeding 800 mm. In summary, the response of wheat GPC to straw return in China generally showed a positive effect, while the return of organic fertilizers typically had a negative effect. However, adjusting the proportion of organic fertilizer return can alleviate the negative impact on wheat GPC. These findings provide a scientific foundation for optimizing wheat grain protein content through the strategic return of organic materials in China.