[Effects of Phosphorus and Straw Addition on CO₂ Emission in Black Soil and Its Driving Factors]

Straw return is a crucial agricultural practice for enhancing soil carbon accumulation and fertility， but it may induce nutrient limitation on soil microbes， potentially affecting CO₂ emissions. Phosphorus （P）， a key nutrient， plays an essential role in this context， yet how P availability and straw return regulate the CO₂ emissions of black soil remains poorly understood. We utilized soil samples from a long-term fertilization experiment in Gongzhuling black soil under a non-fertilized treatment. Nine gradients of P addition （0， 5， 10， 15， 30， 50， 80， 100， and 150 mg·kg^-1 Na₂HPO₄ solution， on a P basis）， combined with straw addition， were carried out in a 28-day incubation experiment， during which their dynamic CO₂ emissions were monitored to examine the effects of P and straw addition on CO₂ emissions from the soil to identify the driving factors. The results indicated that in the absence of straw addition， the cumulative CO₂ emissions from the tested black soil ranged from 311.0 to 386.5 mg·kg^-1 （on a carbon basis）. As P addition rates increased， the cumulative CO₂ emissions exhibited a nonlinear pattern， initially decreasing and then increasing， with the lowest value occurring at 15 mg·kg^-1 P addition rate， which was 18.1% lower than that in the no-phosphorus treatment. Under straw addition， the cumulative CO₂ emissions from the tested black soil ranged from 721.9 to 855.5 mg·kg^-1， which was on average 2.3 times higher than those in the absence of straw. As P addition increased， the CO₂ emissions showed a linear increase， peaking at 100 mg·kg^-1 P addition rate， which was 18.5% higher than that in the no-phosphorus treatment. Correlation analysis revealed that， in the absence of straw， cumulative CO₂ emissions were significantly positively correlated with dissolved organic carbon （DOC） and dissolved inorganic nitrogen （DIN）. Under straw addition， cumulative CO₂ emissions were significantly positively correlated with DOC， DIN， and microbial biomass carbon （MBC）. P addition significantly altered the contents of DOC， DIN， MBC， and metabolic quotient （qCO₂）， thereby regulating CO₂ emissions. In conclusion， in the absence of straw， moderate P addition can alleviate carbon decomposition loss in the tested black soil. However， with straw addition， P addition might promote carbon decomposition， increasing CO₂ emissions from the tested black soil. In practical agricultural production， applying phosphate fertilizer should be rationally regulated based on soil P availability to avoid excessive application， thereby achieving the dual objectives of carbon sequestration and CO₂ emission reduction.