Samsung Electronics Co Ltd posted its first decline in quarterly profit in two years, hit by one-off special bonus as well as slowing smartphone sales as it braces for a year of tougher competition with rival Apple Inc. The worlds biggest smartphone maker kept its full-year investment plan conservative and warned it would be difficult to increase earnings in the first quarter as demand pulls back from the year-end holiday period. October-December operating profit fell 6 percent on year to 8.3 trillion won ($7.7 billion), in line with a previous estimate of 8.3 trillion won, Samsung said in a statement on Friday. Shares of Samsung Electronics, worth $204 billion, edged up 0.4 percent after the earnings release, compared with the benchmark index which fell 0.3 percent. The stock has fallen 8 percent in the past month on concern over slowing smartphone growth.