Carbonenitrogen interactions during afforestation in central China

We conducted a field study in Danjiangkou Reservoir region of central China to evaluate soil C and N dynamics following afforestation by comparing soil organic C and N (SOC and SON), soil net N mineralization and nitrification, and inorganic N concentrations in the plant rhizosphere and open areas in the forest, shrubland and adjacent cropland. Afforestation increased SOC but did not significantly affect SON in the plant rhizosphere. Due to large quantity of low-quality litter (with high C:N ratios) inputs, afforestation enhanced soil C recalcitrant indexes (RIC) but decreased soil N recalcitrance indexes (RIN) in the plant rhizosphere. Both SON and RIN significantly decreased following afforestation in the open areas. Afforestation decreased inorganic N concentrations and net N mineralization. Soil net N mineralization were negatively correlated with soil C:N ratios across land use types. These results suggest that afforestation could increase SOC stocks resulting from large low-quality litter input, but over the longterm, this increase was likely limited due to decreased soil N availability. 2013 Elsevier Ltd. All rights reserved. Afforestation has been proposed as an effective method for reducing the atmospheric CO2 concentration because of the ability to sequester C in vegetation and soil (IPCC, 2007). Despite numerous studies have been conducted to investigate the contribution of afforestation to C sequestration on both regional and global scales (Richter et al.,1999; Paul et al., 2002; Powers et al., 2011; D.J. Li et al., 2012;M. Li et al., 2012), effect of afforestation on soil C accumulation remains a widely debated topic (Smal and Olszewska, 2008; Mao and Zeng, 2010; Hernandez-Ramirez et al., 2011). For instance, Hernandez-Ramirez et al. (2011) demonstrated that afforestation caused increases in soil C due primarily to large litter production. In contrast, Smal and Olszewska (2008) reported that soil C decreased during afforestation when litter input is low with uneven spatial distribution. Vesterdal et al. (2002) inferred that soil C would increase following subsequent rooting and canopy closure during afforestation. Among those inconsistent controversies, increased nitrogen (N) availability and soil C recalcitrance index (RIC, the ratio of unhydrolyzable C to total soil organic C) due to large litter input have been considered to be twomajor mechanisms for determining C accumulation in afforestation soil over the long-term (Luo et al., 2004, 2006; Rovira and Vallejo, 2007). Particularly, rooting and canopy closure are largely dependent of soil N availability. Whether