Stoichiometric homeostasis of carbon (C), nitrogen (N), and phosphorus (P) in the plant-soil-microbial continuum is crucial for the maintenance of nutrient cycling stability in fragile ecosystems. To address the challenges of imbalanced C:N:P stoichiometry in eolian sandy soils and the uncertain effects of organic amendments in the Horqin Sandy Land, this study conducted a field experiment to compare the impacts of biochar and straw additions on the stoichiometric characteristics of C, N, and P within an oat cultivation system. The experimental design included a control group (CK), biochar amendment treatments (low B1: 3%, medium B2: 5%, and high B3: 10% by mass), and straw amendment treatments (low S1: 3%, medium S2: 5%, and high S3: 10% by mass). Grounded in ecological stoichiometry and homeostasis theory, this study systematically analyzed the C:N:P response characteristics of oat plants, soil, and microbial communities. The results showed that (1) Biochar amendments (B2 and B3) significantly enhanced the C-N-P cycling efficiency of the oat system and demonstrated superior regulation over elemental allocation compared with straw addition. While biochar had negligible effects on C, N, and P content in oat shoots, it considerably increased these nutrients in the roots. Under the B3 treatment, root C, N, and P content increased by 45.2%, 65.2%, and 71.4%, respectively, relative to the control (CK), outperforming straw S3 by 28.7%, 60.2%, and 14.5%. Soil C, N, and P pools exhibited even greater responses: biochar B3 elevated soil C (240.2% vs. CK; 118.4% vs. S3), N (157.8% vs. CK; 81.4% vs. S3), and P (81.2% vs. CK; 17.5% vs. S3) contents. Microbial biomass followed a similar trend, with C, N, and P increasing by 95.3%, 88.7%, and 134.7% over CK, and by 61.2%, 21.7%, and 43.5% over S3, respectively. Additionally, biochar significantly reduced the C:N and C:P ratios in the shoots, roots, and microbial biomass while increasing these ratios in the soil. This bidirectional regulatory effect was not observed with straw treatment. (2) The homeostasis of oat roots under biochar and straw addition exhibited weaker stability and higher sensitivity to exogenous nutrient inputs compared with stems, leaves, and microorganisms. (3) The C, N, and P concentrations in the plants and microbial biomass of different organs of jasmine followed the order of C>N>P, and the C:N, C:P, and N:P ratios followed the order of N:P>C:P>C:N. Collectively, biochar amendments significantly enhanced the C sequestration capacity and elemental stoichiometric stability of the plant-soil-microbial continuum by improving the oat’s N and P assimilation efficiency. The high-dose biochar (B3 treatment) demonstrated the most pronounced effects. This study establishes a mechanistic foundation for sustainable management of sandy agroecosystems and provides practical guidelines for using biochar as a carbon-negative soil amendment in arid regions.