Effects of Various Straw Return Strategies on Selenium and Cadmium Accumulation in Rice (Oryza sativa L.) Under Field Conditions

The widespread co-occurrence of elevated selenium (Se) and cadmium (Cd) in paddy soils presents a dual challenge of maximizing agronomic Se biofortification while simultaneously mitigating Cd toxic risks. Although straw return is a prevalent agricultural management practice, its specific impacts on the bioavailability and subsequent grain accumulation of Se and Cd remain complex. The aim of this paper was to evaluate the effects of various straw return strategies—including straw applied to the soil surface (SS), deep soil incorporation (SI), and the application of a straw-decomposing microbial agent (SD)—on soil physicochemical properties, elemental dynamics, and rice growth performance, with particular emphasis on Se and Cd accumulation, in a typical subtropical paddy field ecosystem in Guangxi, China. The findings indicate that straw return effectively ameliorated soil conditions by enhancing macronutrient availability (nitrogen, phosphorus, and potassium concentrations), though improvements in soil pH and organic matter were treatment-specific: significant increases occurred under SI and SD for pH and only under SD for organic matter, with SS showing no significant change. These soil improvements culminated in a theoretical grain yield increase of 6.51–9.07% across all straw-return treatments. These practices primarily modulated Se and Cd bioavailable fractions through complex biogeochemical interactions. Consequently, Se biofortification in rice grains was substantially achieved, with increments ranging from 28.64% to 41.62%. However, this process was accompanied by a 4.99% to 19.69% increase in grain Cd accumulation. In contrast, incorporating straw deeply into the soil (below the 20 cm layer) emerged as a promising strategy under the conditions of this experiment. The synergistic combination of deep tillage and straw application significantly increased theoretical yield and achieved reliable grain Se biofortification (exceeding 0.1 mg kg−1) without inducing a significant increase in Cd accumulation. Ultimately, deep straw incorporation provides a practical management framework for achieving Se enrichment with stable grain Cd levels in paddy fields characterized by high Se and elevated Cd geological backgrounds, offering valuable implications for agricultural sustainability.