Response of Water Requirement and Microbial Biomass Carbon, Nitrogen, and Phosphorus Stoichiometric Ratios in Next Generation Dryland Rice to Organic Fertilizer Application

Purpose This study aimed to investigate the effects of organic fertilizer on water requirements, water use efficiency (WUE), soil physicochemical properties, and microbial biomass carbon (MBC), nitrogen (MBN), and phosphorus (MBP) contents at different growth stages of next generation dryland rice (NGDR), and to clarify the coupling relationships among these factors, determining the optimal organic fertilizer application rate.

Methods Based on field experiment, five treatments were established, including four organic fertilizer application rates, T₁ (6000 kg/hm²), T₂ (12000 kg/hm²), T₃ (18000 kg/hm²), and T₄ (24000 kg/hm²), and a control (CK, no fertilizer). Using water balance equations, ecological stoichiometry, partial least squares structural equation modeling (PLS-SEM), and the entropy-TOPSIS method, this study investigated the effects of organic fertilizer application on water requirements during the seedling, tillering, heading, and maturity stages of NGDR, as well as the influence of soil depth on soil MBC, MBN, and MBP contents and nutrient distribution.

Results Water requirements for NGDR increased with higher organic fertilizer application rates, and initially increased and then decreased as the growth period progressed (P<0.05). Compared with CK, T₃ and T₄ treatments saved 2.24% and 4.96% of water over the entire growth period, respectively; meanwhile, WUE initially increased and then decreased with increasing fertilizer application rates. Nutrient contents in the 0-20 cm soil layer were significantly higher than those in the 20-40 cm layer. At the heading stage, MBC reached its peak in T₃ treatment at 746.20 mg/kg, while T₄ treatment showed a significant decrease to 713.45 mg/kg. MBN showed an increasing trend with the progression of growth stages, reaching 158.60 mg/kg at the heading stage for T₃ treatment. The most significant variations in MBN content were observed between the tillering and heading stages of T₃ treatment and the seedling and maturity stages of T₄ treatment (P<0.05). MBP content reached its maximum in T₄ treatment during the tillering stage at 56.51 mg/kg, with T₄ treatment maintaining high levels during both seedling and tillering stages. The stoichiometric ratios of soil microbial biomass showed stage-specific differences in response to fertilizer application across growth stages. The PLS-SEM model showed the best fit for yield with a coefficient of determination (R²) of 0.989. Organic fertilizer positively promoted water use, yield, and microbial stoichiometric ratios in NGDR. The entropy-TOPSIS analysis indicated that T₃ and T₄ treatments had the highest comprehensive rankings for yield and soil factors.

Conclusion The water requirement of NGDR increases with higher fertilizer application rates, while WUE first rises and then declines, with T₃ treatment showing the optimal results. Organic fertilizer enhances soil organic matter, strengthens the surface accumulation effects of carbon, nitrogen, and phosphorus, and boosts microbial activity, thereby improving soil water retention capacity. The microbial stoichiometric ratios and WUE are significantly regulated by organic fertilizer, and these three factors collectively determine the crop yield. In conclusion, an organic fertilizer application rate of 18000 kg/hm² is the optimal amount for maximizing the yield of NGDR.