How does the U.S agriculture subsidy not constitute export subsidies under the WTO? The WTO explicitly prohibits subsidies on export goods to prevent dumping and market distortion but the United States spends $2.4 Billion annually on corn subsidies and has a trade surplus of $19.5 billion in agricultural goods. 

How does this subsidy not constitute dumping under the WTO guidelines?
Certain types of agricultural subsidies are permitted under the WTO's Green Box. These include direct income support for farmers, and environmental and regional development programmes. However the rules state that, to qualify, such subsidies must be no more than minimally trade-distorting.  Whether such subsidies (in the US and other countries) actually meet that requirement is a matter of controversy, as discussed in this ICTSD Information Note which also shows (in Figure 1) the very high Green Box proportion of US subsidies.