An article in Nature analysed 439 studies and found that soils around the globe have increased their carbon dioxide emissions over the last few decades. Researchers found that, over the last decades, soil respiration increased by approximately 0.1% per year between 1989 and 2008, a global total of 98 billion tons (10x more than human induced atmospheric CO2). The additional flux could result from two sources - microbes and plants. Increasing soil temperature causes microbes to increase their rates of decomposition, likely tapping into long term stored carbon sources. Living plants could both emit and consume carbon dioxide - emit through roots, consume through photosynthesis - resulting in a 0 net increase. This is a huge, complex study that sheds light on how soil respiration is important when looking at the results of climate change.
The study: http://www.nature.com/nature/journal/v464/n7288/full/nature08930.html
The story: http://www.nature.com/news/2010/100324/full/news.2010.147.html