Exploring nitrogen management methods for depressing the decline of wheat grain protein in plastic film mulch via 15N-labelling technique

Sustainably meeting the increasing food and nutrition demand for humanity is a great challenge for the coming decades; for this purpose, improving the yield and quality of cereal crops in drylands is a crucial strategy. Plastic film mulch (PM) systems are widely used in drylands to increase wheat yield; however, they generally reduce wheat grain protein. Here, we address the challenge by optimizing the N fertilizer application methods of PM system. Compared with the traditional flat system, PM3 (striped fertilization on the underside of the ridges) increased mean grain yield by 17% without reducing the grain protein concentration in 2019–2022. Moreover, PM3 increased grain yield by 10% and grain protein concentration by 13% compared with PM1 (broadcasting fertilizers). Results showed that PM3 increased the supply of 15N-labelled fertilizer in Wheat Row of the 0–20 cm soil layer after flowering by 146%, thus increasing the contribution of fertilizer N to grain N by 51–89% and grain N accumulation by 14–27% at 14–48 days after flowering, respectively. Notably, although soil N contributed 54–71% of plant N uptake, the difference in grain N accumulation among PM systems was mainly attributed to differences in plant N uptake after flowering due to changes in the fertilizer N supply. This study provided new insights into improving grain protein concentrations in PM systems through optimized N management, and this may be a starting point for further discussion on achieving collective improvements in yield and quality of grain in PM systems.