Soil carbon stocks and accumulation in young mangrove forests

Highlights

• We conducted a field study of soil carbon change following mangrove afforestation.

• We also conducted a literature review of soil carbon change with mangrove age.

• Soil carbon increased significantly with age in young forests after afforestation.

• Soil carbon accumulation rates did not differ between two species after 6 years.

• The rate of soil carbon accumulation was 155 g C m⁻² y⁻¹.

Abstract

Mangrove reforestation and afforestation programs have been initiated in many countries recently to compensate for historical losses. At the same time, awareness of the high carbon (C) sink potential of mangrove forests is growing, and C sequestration is beginning to be considered among forestation goals. To assess whether and at what rate C accumulates in the soil of young mangrove forests following afforestation, we conducted a field study at an afforestation project in southeast China, including repeated measures taken over six years at two young forests (consisting of Kandelia obovata and Sonneratia apetala, aged 0–6 years old), and also a chronosequence of forests aged 0 (mudflat), 6 (both species), 20 (S. apetala), and 70 (K. obovata) years old. In the repeated measures, surface (0–10 cm) soil C concentration (%C of dry soil mass) increased significantly over six years, from 1.14% to 1.52% (K. obovata) and 1.23% to 1.68% (S. apetala). The rates of increase did not differ significantly between the two species, despite much greater biomass of S. apetala. In the chronosequence, soil C also increased with age across sites, but only the 70-year-old forest was statistically different, suggesting that localized environmental differences may obscure age-related patterns in soil C. At all sites, soil C concentration for 1-m soil depth (0.62%–2.43%) was low compared to published global averages, yet the estimated soil C accumulation rate (155 g C m⁻² y⁻¹) was comparable to published averages for mature forests. We supported this field study with a literature review of similar studies containing soil C concentration data from young mangrove forests: data compiled from 15 studies, comprising 31 sites, showed consistent, positive changes in soil C concentration with forest age, even in the youngest (<5 years old) forests, supporting our field observation that soil C increases over time following mangrove afforestation.

Introduction

Recent studies have shown that coastal wetlands such as mangroves, salt marshes, and seagrass beds are among the most efficient carbon (C) sinks on the planet (Donato et al., 2011). Reviews of the available data estimate that carbon sequestration in these systems (popularly termed “blue carbon”) per unit area is on average more than an order of magnitude higher than terrestrial forests. These blue carbon systems are also among the most threatened ecosystems on the planet. For mangroves, the most recent areal estimates range from 13.8 million to 16.7 million ha (Valiela et al., 2001, Giri et al., 2011), which is a 20% reduction since 1980, and an estimated 35% loss compared to historical extents (Valiela et al., 2001, FAO, 2007).

To address this trend, mangrove reforestation and afforestation programs (collectively referred to as forestation in this study, unless otherwise specified) have been initiated in many countries over the past few decades (Field, 1998, Ellison, 2000). The earliest forestation efforts were essentially silviculture-oriented, while more recent projects have included ecological objectives, such as shoreline stabilization and environmental remediation (Ellison, 2000). With increasing concerns related to climate change, carbon sequestration is also beginning to be considered among conservation and forestation goals (Murray et al., 2011). An example of this trend is the recent release of guidelines for C sequestration accounting in mangrove restoration projects certified under the Cleaner Development Mechanism (CDM) program of the United Nations Framework Convention on Climate Change (UNFCCC, 2011).

In mangrove forests, a large proportion—from half to over 90%—of total ecosystem C is found in soil organic matter as opposed to biomass (Alongi et al., 2003, Khan et al., 2007, Donato et al., 2011). Direct measurements of soil C accumulation in young forestation projects are scarce, however. Several reviews of C sequestration in mangrove soil have been published recently (e.g. Bouillon et al., 2008, Mcleod et al., 2011, Alongi, 2012), but such reviews focus on mature forests and the existing data appear to be biased toward highly organic systems (Kristensen et al., 2008). Similarly, reviews of mangrove restoration projects (e.g. Bosire et al., 2008) provide data on post-restoration biomass accumulation, but not soil properties. Along these lines, CDM approval of restoration projects does not require soil C monitoring before or during the project lifetime. Of the published data that do exist, only a few studies provide direct measurements of change in soil C using repeated measures over time (Ren et al., 2010, Chen et al., 2012). Several more have used forestation chronosequences (a.k.a. space for time substitution) (e.g. Bosire et al., 2003, Alongi et al., 2004, Ren et al., 2008), and additional insight has been gained from similar studies of natural chronosequences, in which forest age varies in relation to naturally changing coastal conditions (Marchand et al., 2003, Lovelock et al., 2010).

In contrast, much more is known about soil C accumulation in terrestrial forestation projects, and several reviews have been written on this topic (Paul et al., 2002, Berthrong et al., 2009, Laganiere et al., 2010). Such studies have found that on average across all projects, soil C initially decreases following afforestation, and may maintain a deficit for up to 30 years (Paul et al., 2002). However, the results vary significantly depending on forestation species, previous land use, soil clay content, and climate. Mangroves allocate a large proportion of biomass belowground compared to many terrestrial tree species (Lovelock, 2008), so we hypothesize that soil C would increase relatively sooner following forestation. Given the high proportion of C stored in soil in mangrove ecosystems, accurately quantifying this change is especially important for assessing total ecosystem C during forest development.

China lies at the northern edge of global mangrove distribution, and similar to other countries with mangroves, has experienced drastic losses in forest cover in recent history. The most recent estimate of mangrove cover is 22,700 ha (Chen et al., 2009), less than a third of the historical extent (Li and Lee, 1997). To address this loss, the Chinese government began investing in mangrove reforestation in the early 1990s (Zheng et al., 2003), and by 2002 more than 2600 ha (more than 10% of the existing area) were replanted (Chen et al., 2009). This reforestation program presents an opportunity to study soil C accumulation in young forests following afforestation, including results across different species. Of particular interest is Sonneratia apetala, a non-native mangrove species from Bangladesh introduced to China in 1985, which has been used for approximately half the country's forestation projects so far (Chen et al., 2009). Kandelia obovata, a native mangrove species distributed along the southeast Chinese coast, has also been widely used in mangrove forestation (Chen et al., 2009). Previous research has shown that S. apetala has much higher rates of biomass C accumulation than most native species (Ren et al., 2008, Ren et al., 2010). Ren et al.'s (2008) study also found higher soil C values in S. apetala forests compared to neighboring native forests, suggesting that S. apetala may have higher rates of C accumulation in soil as well as biomass. According to the soil development model proposed by Chen and Twilley (1999), soil C is correlated with biomass inputs to soil, particularly root biomass. Thus, it is reasonable to expect that soil C will accumulate faster in S. apetala forests. However, comparative data of direct measurements is only available from a single study of 2.5-year-old forests (Chen et al., 2012).

In this study, we aimed to assess whether, and at what rate, soil C content increases over time in young, afforested mangrove forests. To achieve this, we collected two distinct data sets at four young forests in a mangrove forestation project in southeast China. The first data set consisted of repeated measures of surface soil C concentration taken in two young forests (K. obovata and S. apetala) from age 0 to 6 years old. The second data set consisted of a chronosequence (one-time measurements) of 1-m soil cores taken at the two 6-year-old forests, a 20-year-old S. apetala forest, a 70-year-old K. obovata forest, and a nearby mudflat. Using these data sets, we tested the hypotheses 1) that soil C increases with forest age following mangrove forestation, even in very young forests; and 2) that soil C accumulates more quickly in forested S. apetala, compared to K. obovata plots, due to higher rates of above- and belowground biomass growth in the former species. To support our first hypothesis, we also compiled all available data (published and unpublished) on soil C change in young mangrove forests, aiming to assess whether trends in the literature support or refute the results of this field study.