Sulfur fertilization effects on protein concentration and yield of wheat: A meta-analysis

Wheat (Triticum aestivum L.) has a low sulfur (S) requirement compared with other macronutrients, such as nitrogen (N) and potassium (K), and similar to that of magnesium (Mg), phosphorus (P) or calcium (Ca). However, a lack of this nutrient can affect yield and protein concentration and composition, determining the characteristics of the flour. Sulfur deficiency in cropping systems is increasingly reported worldwide, but the effects of S fertilizer are inconsistent among studies depending upon genotype, environment, and management factors. Therefore, our goals were to analyze the results from published studies through a meta-analysis to quantify the effect of S fertilizer on protein concentration and yield of wheat, and to identify environmental, genotypic and management factors behind the response of protein concentration and yield to S fertilizer. This meta-analysis summarizes 22 studies reporting protein concentration, from which 15 also reported yield, most of them published in the 2000's. We calculated the effect size as the natural log of the response ratio, with the response ratio determined as protein concentration (or yield) with S fertilizer application relative to protein concentration (or yield) without S fertilizer application. On average, S fertilizer increased protein concentration by 5.4 % and yield by 16.2 %. Sulfur fertilizer impact on protein concentration and yield increased with the relatively low soil S levels of our dataset (under 7.20 mg S kg-1), under wet weather conditions (above historical precipitation average), no-till practices, or following legumes. Furthermore, ensuring a balanced nutrition by the co-application of N was a determining factor in achieving a response to S fertilizer, which increased with higher N rates. Interestingly, we found a consistent positive response to S fertilizer for yield among all bread-making quality categories, and only a positive response for protein in high and low breadmaking quality, but not in the medium quality category. Overall, this meta-analysis provides an extensive evaluation of protein and yield response to S fertilization in wheat, and how these responses are influenced by genotypic, environmental, and management factors.