Carbon (C) stored in forest ecosystems represents a substantial part of the global C budget. It is estimated that forest standing biomass accounts for 82–86% of above-ground C content and forest soils contain 70–73% of the global soil organic C (soil C hereafter) (Six et al., 2002). Following cultivation of previously forested lands (deforestation), soil C can be rapidly lost as a result of enhanced C decomposition and erosion caused by soil disturbance (Lal, 2005). It was reported that up to 50% of soil C was lost within the first 20 yr (Lal, 2005). Forest plantation, either afforestation or reforestation, on the other hand, has been suggested to reverse the process of deforestation and may bring about accumulation of C (Metz et al., 2007). For this reason, afforestation and reforestation were proposed as an effective method of C sequestration in Article 3.3 of the Kyoto Protocol.
However, the effects of afforestation on soil C accumulation and the related influencing factors are not well understood. According to the results of individual studies, afforestation either increased (Resh et al., 2002; Lemma et al., 2006; Hernandez-Ramirez et al., 2011) or decreased (Farley et al., 2004; Zhao et al., 2007; Mao & Zeng, 2010) soil C accumulation. Other studies found a negligible change in soil C pool after afforestation. For example, Richter et al. (1999) reported that, despite high C inputs to the mineral soil, C sequestration was quite limited (< 1% of total ecosystem C accumulation) relative to trees (c. 80%) and the forest floor (c. 20%). The inconsistent conclusions from the individual studies likely arise because the magnitude and direction of soil C dynamics are affected by multiple factors, including climate, soil type, tree species planted and nutrient management (Paul et al., 2002; Lal, 2004; Laganière et al., 2010). It will undoubtedly be helpful to determine the general patterns and the major controlling factors of soil C accumulation in order to provide support for policy-making in relation to C sequestration via afforestation.
Several quantitative reviews have been published that are relevant to C stock dynamics following afforestation (Post & Kwon, 2000; Guo & Gifford, 2002; Paul et al., 2002; Berthrong et al., 2009; Laganière et al., 2010). However, the results of these studies are inconsistent, possibly because of the methods of handle sampling depths. It is well documented that soil C stock changes after afforestation vary with depth (Paul et al., 2002; Don et al., 2011), but most reviews did not consider the sampling depths well in their analysis (Guo & Gifford, 2002; Berthrong et al., 2009; Laganière et al., 2010; Don et al., 2011). Another reason for the inconsistency could be the method of handling the organic layer of the soil profile; this was included in some reviews but was combined with the mineral layer for data analysis, although data from the organic layer were quite limited (Laganière et al., 2010).
Furthermore, the existing quantitative reviews only focused on soil C accumulation, neglecting soil nitrogen (N) dynamics and C–N interactions, which are very important in determining whether the C sink in land ecosystems could be sustained over the long term (Luo et al., 2004, 2006a; Finzi et al., 2006). It has been suggested that N dynamics is a key parameter in the regulation of long-term terrestrial C sequestration (Rastetter et al., 1997; Luo et al., 2004). The modeling studies demonstrated that N capital in an ecosystem determines the long-term trend of the terrestrial C sink, which will be sustained only when there is increased N input into an ecosystem (Rastetter et al., 1997). Similarly, a conceptual framework of progressive N limitation also predicted that N would increasingly constrain terrestrial C dynamics only if ecosystem N capital does not change over time (Luo et al., 2004). If additional C inputs stimulate the capital gain of N through biological fixation and atmospheric deposition, increased uptake for soil N availability or decreased N losses, progressive N limitation will not occur (Luo et al., 2006a). Therefore, it is imperative to quantify N capital changes and to analyze relationships between C and N stock dynamics following afforestation.
In this paper, a meta-analysis was carried out to study the dynamics of both soil C and N stocks. Data from organic and mineral layers were extracted from the published papers and standardized to improve the comparison. The major questions we aimed to answer were as follows: how do C and N stocks change following afforestation; how do prior land use, climate and tree species planted affect C and N dynamics following afforestation; and how does the change in N stock change correlate with the change in C stock?