If a negative nitrate reduction test does not change color after the addition of powdered zinc, how is it reported?

In the context of a nitrate reduction test, a negative result with no color change after adding powdered zinc indicates that nitrate was not reduced to nitrite and that either there was no nitrate present to begin with or the nitrate was further reduced to nitrogen gas or other nitrogenous compounds that do not produce a color change.

When zinc powder is added, it acts as a catalyst to reduce any unreduced nitrate to nitrite, which would then react with the reagents added to produce a red color. The absence of a color change after the addition of zinc suggests that neither nitrite nor unreduced nitrate is present, confirming the organism has either not reduced the nitrate at all or has reduced it completely past nitrite to nitrogen gas.

In this situation, the test is reported as positive for nitrate reduction, indicating that the organism has indeed the ability to reduce nitrate fully, which is why the answer is reported as positive. This outcome emphasizes the organism's capacity to perform denitrification if it has completely reduced the nitrate to nitrogen gas. The other options do not align with this interpretation of the test results.