Palustrine wetlands (PWs) cover the majority of wetland types in the world (Lehner and Döll 2004). PWs refer to non-tidal marshes, swamps, bogs, and fens (Ramsar Convention Secretariat 2013). Only lakes, rivers and saline wetlands are not included as wetland types within the PW classification. PWs are important in the global carbon (C) cycle because of their large soil carbon pools held under largely anaerobic conditions, their high methane emissions, and their potential for considerable future carbon sequestration (Page et al 2011, Vicari et al 2011, Carlson et al 2012, Kleinen et al 2012, Sapart et al 2012, Charman et al 2013). Undisturbed PWs are usually observed to be small sinks of C annually, but large sinks over many years (Gorham 1991, Thormann et al 1999, Bellamy et al 2005, Huang et al 2010, Vicari et al 2011, van der Valk 2013, Maltchik et al 2014). However, if there is warming-induced acceleration of organic matter decomposition then net C release could occur. Alternatively, if climate change enhances plant productivity over decomposition then there may be an enhanced net C uptake. There is large uncertainty as to how different types of PWs will respond in different regions to climate change but it is clear that as a starting point there is an urgent need to map and quantify the existing extent of PWs and their C stock so that better estimates of potential feedbacks to climate change may be developed.

Human activities such as drainage and prescribed burning threaten PWs and may enhance C release. Protection of PWs has been considered as an important component of climate change mitigation (Holden 2005, Davidson and Janssens 2006, Song et al 2006, Smith et al 2007, Vicari et al 2011), but without good maps or reliable soil organic carbon (SOC) stock estimates, organizing such protection by national and international agencies becomes more difficult (Post et al 1982, Sigua et al 2009, Charman et al 2013, Lu and Xu 2014).

In China there are a number of soil maps and databases on SOC. However, the maps vary in spatial resolution and detailed estimates of SOC storage for PWs in China are only available for some wetland-rich regions (Tian et al 2003, Peng et al 2005a, 2005b, Kang et al 2009, Li 2009, Bai et al 2010). The lack of spatially explicit wetland area databases combined with a lack of representative in situ data on SOC is a major constraint for accurate estimation of SOC storage in China's wetlands. Thus previous estimates of wetland SOC storage in China are highly uncertain (Wang et al 2003, 2014, Mitra et al 2005, Wang 2009, Liu et al 2012, Zheng et al 2013). Many of the earlier SOC studies have predominantly focused on the Northeast of China (NEC), Zoige plateau and the middle and lower reaches of the Yangtze River, and do not provide a representative estimate of country-wide SOC stocks. In addition, the method of quantification of SOC stock for each region differs between studies, resulting in difficulties with comparisons between regions.

High-resolution remote sensing (RS) data, geographical information systems (GIS) and global positioning systems have made the mapping of PWs feasible. Maps of national and global PWs are now becoming available (Lehner and Döll 2004, Niu et al 2009, BFU 2011, Chen and Jessel 2011) but considerable discrepancies remain between these datasets, due to both different wetland classification systems and the methods used for mapping wetlands (table S1). Comparisons of these spatially explicit wetland area datasets with the Chinese first wetland census (National Bureau of Statistics of China 2007), showed large disagreements between the maps and national and provincial wetland census data. For example, the first Chinese wetland census suggested the total PW area was 137 002 km2 around the year 2000, while Niu et al (2009) reported the total PW area to be only 90 885 km2 in 2000. Based on the first Chinese wetland census database, PWs accounted for 35.6% of the total wetland area in China (National Bureau of Statistics of China 2007). However, there is no map focused on the spatial distribution of PWs in China. The SOC stock in the PWs of China is also not clear. In this study, we establish the first standardised map of PWs for China, evaluated against ground-based data. In a second step, we combined this dataset with in situ SOC measurements to provide a spatially explicit estimate of the SOC storage distribution across Chinese PWs.