Introduction
Freshwater scarcity and the increasing demand from the agricultural sector, which consumes approximately 70% of global freshwater, have posed serious challenges to global food security. This pressure is particularly severe in arid and semi-arid regions like Iran, where high evaporation rates and irregular rainfall threaten crop production sustainability. Conservation agriculture, particularly the retention of crop residues (straw mulch) on the soil surface, is an effective strategy to manage soil and water resources. Mulch conserves soil moisture by physically reducing the solar radiation reaching the soil, thus directly decreasing soil evaporation, especially during the early stages of crop growth. In Golestan province, Iran’s primary soybean (Glycine max L.) production hub, soybean is mostly cultivated in rotation with wheat (Triticum aestivum L.). However, farmers commonly burn or remove wheat residues to accelerate land preparation, exposing the soil to severe evaporation. Despite the importance of residue retention, there is a lack of quantitative knowledge regarding the effects of different mulch rates on soybean water-saving in this region. Therefore, this study aimed to quantitatively evaluate the impact of various rates of wheat straw mulch on soil evaporation, irrigation requirements, and the water use efficiency (WUE) of soybean using an integrated field experiment and simulation modeling approach.
Materials and Methods
This study utilized a two-step integrated approach comprising a field experiment and long-term simulation modeling. First, a field experiment was conducted at the research farm of Gorgan University of Agricultural Sciences and Natural Resources, Iran, to quantify the effect of mulch on soybean phenology. The experiment was laid out in a randomized complete block design with four replications. Treatments consisted of six levels of wheat straw mulch: 0 (control), 750, 1500, 2250, 3000, and 3750 kg.ha⁻¹. Soybean (cv. DPX) was sown on July 1. Phenological stages, particularly days to emergence and flowering, were meticulously recorded to extract linear correction factors. In the second step, the SSM-iLegume model was employed for long-term simulations (2000-2020) across two distinct climates in Golestan province: Gorgan and Minodasht. The model’s genetic coefficients for the DPX cultivar had been previously parameterized and evaluated for the region in previous studies. The primary novelty in the modeling phase was incorporating the mulch effect. Using the field data, correction factors were applied to simulate the delay in phenology caused by mulch. Furthermore, the physical effect of mulch on reducing potential soil evaporation (Ep) was dynamically simulated based on the soil cover fraction. The model simulated crop responses, water balance components, and WUE under the specified mulch treatments and automated irrigation scenarios. Output data were statistically analyzed using Analysis of Variance (ANOVA) and Duncan’s multiple range test (P<0.05) in SAS 9.4 software.
Results and Discussion
Field observations indicated that wheat straw mulch significantly delayed soybean phenology by shading the soil and reducing received thermal time. Specifically, for every one ton per hectare of applied mulch, emergence and flowering were delayed by 0.7 and 1.03 days, respectively. These correction factors were successfully integrated into the SSM-iLegume model. Long-term simulation results revealed that while straw mulch did not significantly alter the total above-ground dry matter and grain yield in either Gorgan or Minodasht, it profoundly impacted the soil water balance. Mulch cover significantly reduced cumulative evaporation (CE) from the soil surface, especially during the early growth stages before canopy closure. The application of 3.75 t.ha⁻¹ of mulch decreased CE by 51% (equivalent to 68 mm) in Gorgan and by 50% (equivalent to 92 mm) in Minodasht over the growing season. Consequently, the ratio of soil evaporation to total evapotranspiration (E/ET) decreased to its lowest levels (0.16 and 0.18 in Gorgan and Minodasht, respectively). The significant reduction in non-productive evaporation translated directly into decreased irrigation water requirements. Applying 3.75 t.ha⁻¹ of mulch reduced the required irrigation volume by 15% (from 329 mm to 280 mm) in Gorgan and by 21% (from 366 mm to 288 mm) in Minodasht, eliminating the need for approximately one irrigation event per season. Because grain yield was maintained while water consumption decreased, Water Use Efficiency (WUE) improved significantly, reaching maximum values of 14.9 and 16.8 kg.ha⁻¹.mm⁻¹ in Gorgan and Minodasht, respectively, under the highest mulch rate. Furthermore, scaling these findings to the 57,462 hectares of soybean cultivated area in Golestan province demonstrated that implementing this residue management strategy could potentially save up to 34.41 million cubic meters of agricultural water annually.
Conclusion
In conclusion, the integrated field and modeling assessment demonstrated that utilizing wheat straw mulch is a highly efficient agronomic strategy for soybean production in semi-arid regions. Retaining up to 3.75 t.ha⁻¹ of crop residues significantly suppresses soil evaporation and improves water use efficiency without incurring any yield penalty. This practice not only ensures the sustainability of crop production but also plays a vital role in conserving scarce regional freshwater resources. Transitioning from the conventional practice of residue burning to conservation agriculture is strongly recommended to optimize water footprints and mitigate the impacts of climate change on agricultural ecosystems.