Pitching for the first time in a week, and working in a steady drizzle, Bell (0-3) couldn't find the plate. The All-Star closer walked four and threw a whopping 46 pitches.
The Mets trailed 2-1 when David Wright drew a leadoff walk in the ninth. One-out passes to Ike Davis and Josh Thole loaded the bases for Turner, a pinch hitter.
Turner fell behind in the count 0-2 and later fouled off seven pitches before his walk made it 2-all. After a forceout at the plate, Nieuwenhuis hit a one-hopper off the right-field wall.
Before Nieuwenhuis' hit the Mets were 0 for 16 with two walks when batting with the bases loaded this season.
The Mets finished off a three-game sweep on Bell's third blown save in five chances.
Ramon Ramirez (2-1) pitched two scoreless innings for the win.
GIANTS 6, REDS 5
CINCINNATI (AP) — Angel Pagan hit a three-run homer in the ninth inning, lifting San Francisco to the win over Cincinnati.
Joaquin Arias led off the ninth against Cincinnati closer Sean Marshall (0-2) with a walk, and Ryan Theriot followed with a single. After pinch-hitter Brett Pill struck out, Pagan lofted a 1-2 pitch into the left field seats to snap the Giants' losing streak in Cincinnati at seven.
Javier Lopez (2-0) allowed two hits but no runs in the eighth to earn the win. Santiago Casilla struck out the side in the ninth for his second save in two opportunities.
Jay Bruce hit a two-run homer in the sixth for the Reds and Scott Rolen added a solo shot.
PADRES 2, NATIONALS 1
SAN DIEGO (AP) — Pinch-hitter Mark Kotsay's two-run double off reliever Tyler Clippard in the eighth inning lifted San Diego over Washington.
With the Padres trailing 1-0, Orlando Hudson drew a one-out walk and Cameron Maybin dropped down a bunt single against Clippard (1-2). Kotsay followed with a double to right-centre.
Andrew Cashner (1-1) pitched a perfect eighth and Huston Street earned his second save. Three pitchers combined on a four-hitter for San Diego, which improved to 6-14 and averted a three-game sweep.
The NL East-leading Nationals (14-5) had won four straight and eight of 10.
Jayson Werth's home run in the seventh off Padres starter Edinson Volquez snapped a scoreless tie.
Edwin Jackson pitched 6 2-3 shutout innings for the Nationals, who have eight scoreless starts in 19 games this season.