R split and get the sum of each group

Question

I am a beginner to R and I am stuck with a probably simple problem. I have a data frame NEI_small like that:

Now I want to calculate the sum of the emissions per year. So It should look like that:

 year  Emission
 1999  29920139
 2002  41757550

How can I get the sum of a group ?

I tried this but it didn`t work :

   sapply(split(NEI_small,NEI_small$year),sum(NEI_small$Emissions))

But I got this error :

Error in match.fun(FUN) : 'sum(NEI_small$Emissions)' ist nicht Funktion, Zeichen oder Symbol

Can anybody help me please?