Limited Advantages of Green Manure Planting on Soil Nutrients and Productivity in Intensive Agriculture: A Case Study of Wheat-Maize-Sunflower Rotation in Hetao Irrigation District

Crop diversification has been proposed as a promising strategy for transitioning towards sustainable agricultural systems. The Hetao Irrigation District faces multiple challenges in ensuring food security and farmer livelihood. A diversified green manure inclusive cropping system was designed to evaluate the influences of hairy vetch (Vicia villosa Roth) incorporation on soil nutrients, yield, and economic benefit compared to the existing wheat-maize-sunflower rotation system in the Hetao Irrigation District. The specific method was as follows: spring wheat (Triticum-durum L.)-vetch rotation in the first year, maize (Zea mays L.)/vetch intercropping in the second year, and sunflower (Helianthus annuus L.)/vetch relay intercropping in the third year. Over a 6-year experimental observation, green manure incorporation significantly increased soil alkaline hydrolyzed nitrogen and exchangeable potassium levels. Although the effect of green manure incorporation on the yield of rotated spring wheat and intercropped maize was not prominent, it improved sunflower yield in the second rotation cycle by 11%. Consequently, only the sunflower led to an increase in income of 235 CNY ha-1 in the second rotation cycle. This indicates that the soil-property-building and yield-enhancing effects of green manure incorporation require time to accumulate. However, the benefits derived from increased yield were not sufficient to offset the production costs associated with green manure cultivation, resulting in less-than-desirable economic returns.