* FTSEurofirst 300 down 1.7 percent, biggest slide in a month
* Major indexes break below their 50-day moving average
* Doubts arise on impact of rising commodities on earnings
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By Blaise Robinson
PARIS, April 12 (Reuters) - European stocks suffered their biggest one-day fall in a month on Tuesday, with the main indexes breaking below their 50-day moving averages, as Japan's worsening nuclear crisis sparked a bout of profit taking.
Alcoa's lower-than-expected revenue figures also fuelled concerns over the upcoming earning season and rattled investors, who were quick to book profits following an 8 percent rally over the past month.
Mining and metal stocks featured among the top losers, with Xstrata down 4.1 percent and ArcelorMittal down 3 percent.
The FTSEurofirst 300 index of top European shares dropped 1.7 percent at 1,127.28 points, the index's lowest close in nearly two weeks.
The benchmark index, as well as the broader STOXX Europe 600 , the euro zone's blue chip Euro STOXX 50, France's CAC 40 and Germany's DAX all broke below their 50-day moving average, sending a clear bearish signal.
"Strictly speaking, valuation levels remain very low, but there are mounting doubts over the impact of rising energy and commodity costs, as well as interest rates, on companies' results. This hasn't been completely priced in yet in earnings forecasts," said Pierre-Yves Gauthier, head of research at AlphaValue in Paris.
"Earnings growth will not be as strong as hoped earlier this year, probably 2-3 percent lower, and forecasts are being trimmed which is never good for the market. On the positive side, the long-awaited M&A revival has started. With current interest rates, making acquisitions seems a no brainer."
Energy shares dropped, with BP down 2.8 percent, Total down 2.5 percent and Repsol down 3 percent, as U.S. oil futures tumbled 3.6 percent to around $106 a barrel, trimming a small portion of their recent sharp rally, as Goldman Sachs warned again of a price reversal and the International Energy Agency said high prices could be eroding demand.
Around Europe, UK's FTSE 100 index fell 1.5 percent, Germany's DAX index shed 1.4 percent, and France's CAC 40 lost 1.5 percent.
Investors' risk aversion was on the rise on Tuesday, with the Euro STOXX 50 volatility index surging as much as 15 percent to 22.9 points, a two-week high.
The higher the volatility index, based on sell- and buy-options on the Euro STOXX 50 stocks, the lower investors' appetite for risky assets such as equities.
"I'm not expecting very positive surprises from the earnings season. All sorts of negative things should emerge. The dollar has weakened and we have to adjust (European corporate earnings) forecasts with that. The pressure will also come from higher commodity prices," said AXA IM fund manager Chrysoula Zervoudakis, who manages around 520 million euros ($752 million).
"On the positive side, stocks are very cheap compared with other asset classes. Companies have started to raise their dividend, and yields are significant," Zervoudakis said.
According to Thomson Reuters StarMine, STOXX 600 companies due to post first-quarter results in the upcoming reporting season are expected to post an average negative earnings surprise of 1 percent. (Reporting by Blaise Robinson; additional reporting by Dominic Lau in London and Alexandre Boksenbaum-Granier in Paris; Editing by Jon Loades-Carter)