Is zakat obligatory on ten kilograms of pure gold after the passing of a hawl (lunar year) if its weight decreased due to loss?

Zakat is obligatory on whoever owns the equivalent of the nisab in gold (85 grams) every year. If the gold falls below the nisab, zakat is waived for the amount that fell short, and zakat becomes obligatory for the remainder, if it reaches the nisab. The gold you possess has reached the nisab, and if any of it decreases, you are obliged to pay zakat on theograms that have completed a full year.