Effects of crop rotation and hydrogel application on soil biological properties and wheat biomass production in drylands

Background and aim: Conservation agriculture (CA) is rapidly gaining traction to promote sustainable crop production, focusing on minimum soil disturbance, permanent soil cover, and strategic crop rotation. Moreover, hydrogel is increasingly being used to improve water retention, including in combination with CA, particularly in water limited agro-ecosystems. CA tends to increase soil organic matter content—one of the main soil health indicators—but its effects, and possible interactions with hydrogel application, on soil biology remain unclear. Methods: We conducted a growth chamber experiment to evaluate how crop rotation combined with hydrogel addition impacts soil biological properties and wheat biomass production in rainfed dryland systems. The experimental design included a fallow, four crop rotations, two hydrogel and two watering treatments implemented across two growing seasons. Results: In the first growing season, crop treatment and hydrogel addition had interactive effects on soil total carbon (C), nitrogen (N), and phosphorus (P) content. In addition, the legume crop rotations had higher microbial biomarker contents compared with the fallow and wheat–wheat treatments, while hydrogel increased microbial biomarker content and enhanced soil total P and wheat biomass production in wheat compared to other cover crops. In the second (wheat only) stage, we found greater wheat biomass in the rotations with alfalfa and field pea when hydrogel was not applied (total aboveground biomass: 1.25 ± 0.07 g and 1.17 ± 0.09 g, respectively) compared with the wheat–wheat rotation (0.37 ± 0.02 g), although higher biomass was observed in the fallow–wheat rotation (1.83 ± 0.46 g). Surprisingly, hydrogel increased wheat biomass by 191.9% in the wheat–wheat rotation under ambient conditions, seemingly by alleviating negative effects of growing wheat as a monoculture. Specifically, we observed a very high microbial biomass carbon:nitrogen ratio (43.05 ± 18) in the wheat–wheat treatment without hydrogel, indicating that microbes may have been N limited. Conclusion: Overall, crop rotation, particularly alfalfa–wheat, had a positive impact in promoting wheat biomass production and soil biological properties compared to fallow and monoculture systems in drylands soil. In addition, the strong positive effect of hydrogel on biomass production and indicators of soil health was surprising but should be explored further as an approach to sustainable management. Our results suggest crop rotation can play an important role in promoting sustainable agricultural management and soil health, while hydrogel application may be effective in improving crop production in grains, especially in water-limited drylands.