More evidence from the phytolith research group at the Chinese Institute of Geology in Beijing indicates that rice dispersal rapidly northwards from its presumed Yangzte origins into the temperate Yellow River Basin. Zhang et al (2010) report in a recent BOREAS article. phytolith sequences collected from scraped archaeological sections at Quanhu, Yangguanzhai and Anban, all Yangshao sites in the Wei river tributary to the Yellow River. All three sites start from the Middle Yangshao on cultural grounds, and sediment AMS dates support the start of these sequences from 3700-3500 BC. They continue through the Longshan and sometimes later. Rice bulliforms and double peak cells occur throughout the sequences, although it should be noted that broomcorn and foxtail millet husks occur too (applying the enhanced identification criteria developed in the same lab, famously applied at Cishan), and usually millet husks far outnumber rice husks indicating that millet cropping (dry farming) dominated over presumably wet rice.
These data need to be taken alongside other, even earlier indications, that rice spread northwards already in the Early Yangshao, by ca. 4000 BC. My colleague Arlene Rosen has explored the geoarchaeological evidence for rice cultivation from the Early Yangshao onwards in the Yiluo Valley in a recent Geomorphology article (2008). In addition, towards the end of last year we published the archaeobotanical evidence from Nanjiaokou (by Qin Ling & Dorian Fuller, in Chinese in the Nanjiaokou monograph, 2009). This includes some rice, alongside the millets, from early Yangshao levels dated between 4500 and 3800 BC, although the earliest direct AMS date on a rice grain was from end of this range, 3900-3800 BC. Still all of this indicates that rice diffused rapidly from the South (in the Daxi/ Later Majiabang horizon) as it came to be intensively cultivated (and was still undergoing population-wise morphological evolution of domestication syndrome traits: see Tianluoshan links), and was adopted into the expanding economies of Yangshao millet farmers.