Even though the main nucleosynthetic products of type Ia supernovae belong to the iron-group, intermediate-mass alpha-nuclei (silicon, sulfur, argon, and calcium) stand out in their spectra up to several weeks past maximum brightness. Recent measurements of the abundances of calcium, argon, and sulfur in type Ia supernova remnants have been interpreted in terms of metallicity-dependent oxygen burning, in accordance with previous theoretical predictions. It is known that α-rich oxygen burning results from ¹⁶O→¹²C followed by efficient ¹²C+¹²C fusion reaction, as compared to oxygen consumption by ¹⁶O fusion reactions, but the precise mechanism of dependence on the progenitor metallicity has remained unidentified so far. I show that the chain ¹⁶O(p,α)¹³N(γ,p)¹²C boosts α-rich oxygen burning when the proton abundance is large, increasing the synthesis of argon and calcium with respect to sulfur and silicon. For high-metallicity progenitors, the presence of free neutrons leads to a drop in the proton abundance and the above chain is not efficient. Although the rate of ¹⁶O(p,α)¹³N can be found in astrophysical reaction rate libraries, its uncertainty is unconstrained. Assuming that all reaction rates other than ¹⁶O(p,α)¹³N retain their standard values, an increase by a factor of approximately seven of the ¹⁶O(p,α)¹³N rate at temperatures in the order 3−5 × 10⁹ K is enough to explain the whole range of calcium-to-sulfur mass ratios measured in Milky Way and LMC supernova remnants. These same measurements provide a lower limit to the ¹⁶O(p,α)¹³N rate in the mentioned temperature range, on the order of a factor of 0.5 with respect to the rate reported in widely used literature tabulations.