Low plant-available phosphorus (P) in degraded arid steppes greatly limits plant yields. However, whether exterior P addition will improve the soil P availability and thus increase plant yield in these degraded arid steppes is still not certain. In the current study, a severely degraded arid steppe in Inner Mongolia, China, with soil-available P <5 mg/kg, was fertilised annually with chemical or manure P for two years (2014, dry year; 2015, wet year). There were six fertilisation treatments: 0, 30 kg P/ha, 60 kg P/ha, 90 kg P/ha, 4000 kg sheep manure/ha (equalling 16.4 kg P/ha) and 8000 kg sheep manure/ha (32.8 kg P/ha). A pot experiment with Stipa krylovii (the dominant plant species in the tested steppe) and five P application rates (0, 30, 60, 90 and 120 kg P/ha) was also conducted, under well-watered and nitrogen-fertilised conditions, using surface soils from unfertilised plots in the field. Results indicated that the tested soils had strong P adsorption capacity and weaker desorption capacity, and that the labile P fractions were quickly transformed into less labile fractions, reducing P availabilities. Overall, chemical P fertiliser resulted in the accumulation of Ca₁₀-P and occluded P, whereas sheep manure resulted in the accumulation of moderately resistant organic P and highly resistant organic P. Phosphorus fertilisation was associated with an increase in plant P concentrations in both 2014 and 2015, and a low P rate (30 kg P/ha in the current study) was able to improve the aboveground biomass in both the field experiment in the wet year and the pot experiment under well-watered conditions. Thus, in degraded arid steppes, P fertilisation may be unnecessary in dry years. A low rate of P fertilisation is recommended in wet years to improve soil P status and steppe plant productivity.