Objectives Efficient utilization of agricultural organic byproducts is an pivotal strategy for advancing green agricultural development in China. This study explored the effects of straw return and organic fertilizer substitution for chemical fertilizer on soil physical properties and crop yields in loess soils, to furnish a theoretical framework for optimizing nutrient management and enhancing agricultural efficiency in the region.

Methods The research was based on a nine-year field experiment conducted in loess soil of Guanzhong Plain. The cropping system was winter wheat (Triticum aestivum L.)-summer maize (Zea mays L.) rotation. Undisturbed soil samples were collected from plots subjected to four treatments: straw return with chemical fertilizers (SNPK), 50% and 100% organic fertilizer substitution for chemical fertilizers (50%M and 100%M), and a control using only chemical fertilizers (NPK). Analyses encompassed soil bulk density, total porosity, aggregate distribution, aggregate stability, moisture retention characteristics, saturated and unsaturated hydraulic conductivity, crop root development, and yield.

Results The implementation of straw return and organic fertilizer application did not result in a significant change in soil bulk density, total porosity, aggregate distribution, or aggregate stability compared to the NPK treatment. However, the soil water retention analysis revealed that the 100%M treatment exhibited a notably higher water-holding capacity at 30 kPa compared with the NPK treatment. Across other soil water retention levels, a general trend of NPK<50%M<SNPK<100%M in soil water content was observed. The 100%M treatment significantly augmented capillary pore volume in the topsoil (0−5 cm) and increased ineffective pore volume in the subsoil (10−15 cm) compared to NPK. Both straw return and manure application generally enhanced soil saturated hydraulic conductivity relative to NPK, though no significant disparities in unsaturated hydraulic conductivity were noted among treatments. Furthermore, manure application significantly boosted root weight density and root length density of summer maize in the topsoil (0−15 cm), with no discernible impact on winter wheat root traits. Crop yields from the SNPK and 50%M treatments were comparable to those from NPK, whereas the 100%M treatment led to a significant reduction in summer maize yield and total yield (wheat plus maize).

Conclusions After nine years of double-season straw return or 50% organic fertilizer substitution for chemical fertilizer in the wheat-maize cropping system in the Guanzhong Plain of Shaanxi, soil physical properties were improved to certain extent, and crop root growth was influenced, while crop yield was comparable to that under the NPK treatment alone. Considering the trade-offs between crop yield and soil physical fertility, both straw return and manure substitution for chemical fertilizer are recommended, but the substitution rate should be controlled within 50%.