4.4 Mangrove SOC

4.4.1 Amundson, 2001

“The carbon budget in soils” (Amundson 2001)

Key contribution:

Key notes: This article provides a review of the interface between soils and the global carbon budget. Discussion of its significance, different methods for estimating global SOC stocks, and the impacts of human activities on it.

Soil organic matter:

Soil organic matter is defined as organic remains of plants and animals that is either recognizable or has decomposed to the point in which it is no longer recognizable (humus). SOM may be living or non-living, and microbial communities form a key component that is primarily responsible for the cycling of OM.

Global soil C reservoir

Two primary approaches to estimating it:

1. Soil taxonomy - mean SOC values by soil taxon are extrapolated to their areal coverage globally via soil maps
2. Ecosystem-based approaches - based on observation that SOC and plant communities are both largely a function of temperature and precipitation, and thus use the Holdridge ecological life biomes. Mean SOC values are estimated by biome, and extrapolated to the total areal coverage

Estimates as of 2001 estimated total SOC within the top meter of soil to be approximately 1,500 Gt C

Controls of SOC

State factors of soil formation + human activity (Climate, Organisms, Parent material, Topography, Time, Humans).

Different pools of SOC exist, which have varying degrees of susceptibility to being cycled:

1. Fast (10⁰ years) - e.g., fresh plants and roots
2. Slow (10^1-2 years)
3. Passive (10^3-4 years) - physically or chemically protected OM

Change in SOC with time can be expressed as:

\[ \frac{dC}{dt} = I -kC\]

Where I is the SOC input, k is the fraction of C lost as CO₂ each year, and C is the mass of C per soil volume.

Variation of SOC with time

Two general approaches exist for determining rates of SOC accumulation and cycling over time:

1. Chronosequences - which monitor SOC in soils of different ages but with similar environments and bedrock
2. Mass balance approach - C cycling rates are inferred for soils near or at steady state

In using chronosequences, need to constrain time period in which one looks because of the importance of climate on SOC cycling. Most chronosequences of SOC are constrained to the Holocene period.

Understanding of SOC dynamics from using chronosequences includes:

1. C accumulates in soils for thousands of years (perhaps as long as 10,000 years)
2. Accumulation rates decrease with time, eventually approaching 0 as steady state is reached
3. Steady-state C is related to climate and other environmental factors
4. Rates of C accumulation in young soils are likely controlled by N availability in ecosystem

Understanding of SOC dynamics from mass balance approach:

At steady state, inputs can be assumed to equate to the decay of C present, thus: \(I = kC\). For soils at or near steady state, total C and either losses or inputs can be measured to derive the decay constant.

Soils in global CO₂ cycle

Several key questions exist per understanding the role of SOC in the global CO₂ cycle:

1. Are undisturbed soils sequestering atmospheric CO₂?
   • They may be. Particularly for young soils that are not at steady state and may now be newly exposed to increasing temperatures and OM inputs (i.e., northern Canadian soils that were priorly glaciated).
2. What is the effect of cultivation on SOC storage and atmospheric CO₂? Several key points are provided:
   • Soils initially low in C often will gain C upon cultivation.
   • At global scales, cultivation reduces SOC content by approximately 30%.
   • Rate of C loss is a function of climate
   • Agricultural soils approach new C steady states in approximately 100 years.
   • Fractional loss of C is controlled by climate.
3. How quickly will soils recover C if reverted to original flora?
   • Initial rates are quite high, but drop off exponentially with time. Inputs of OM to the soil system are often limited by N availability.
   • Production and transport of N fertilizers can be greenhouse gas intensive and may reduce the GHG reduction potential of SOC inputs from both reverting ecosystems to native flora as well as enhanced agricultural management practices.
4. How will global SOC change with increasing temperatures?
   • Only a fraction of SOC is cycled on time scales relevant for climate change, however it is expected that SOC emissions will increase with increasing temperature, and may be a significant positive feedback response for climate change.

4.4.2 Bouillon, 2003

“Sources of organic carbon in mangrove sediments: Variability and possible ecological implications” (Bouillon et al. 2003)

Key contribution: This study examines soil organic carbon at three different sites (one in India, two in Sri Lanka) and investigates various soil related parameters that may shed light on sourcing of organic carbon. The authors find high variability across the sites, and conclude that while at some sites mangrove contributions to SOC are very high, others may be dominated primarily by organic matter of marine origin.

This is one of the first studies to examine sources of organic matter in mangrove sediments.

Key notes:

Concentrations of organic carbon, total nitrogen, elemental ratios (C/N) and carbon stable isotopes were taken from soil samples to 5 cm depth, as well as from leaf litter in each of the sites. The geomorphology and species compositions of each of the sites were different.

C/N ratios of suspended particulate organic matter are typically much lower that those of mangrove litter and thus may act as an important source of N in sediment food webs.

Study design

They sampled soil and leaf litter as well as compose mixing models to investigate the relative contributions of mangrove derived organic matter versus marine derived organic matter.

Mixing models are a way of modeling relative contributions (often two end-members) to a system. In this case, it is the contribution of mangrove derived versus marine derived organic matter to total sediment organic matter.

Key results

Sediments underneath Rhizophora were found to have higher OC concentrations than Avicennia and Excoecaria species at two of the sites.

Lower OC concentrations coincided with lower C/N ratios and less negative C¹³ isotope values, whereas soils with rich OC concentrations had higher C/N ratios and C¹³ values closer to those of the mangrove leaf litter.

Appears there is an inverse relationship between OC content and C¹³ values.

Although they note that organic C in mangrove sediment may originate entirely from mangrove-derived organic matter, they note that this is not likely to be the norm.

From the Coringa site, they find that suspended OM in the water column is significant and likely derived from phytoplankton-derived matter.

They describe the differences as “flow-through” systems versus “accumulation” systems, which may largely be a result of tidal amplitude (flushing) of system.

Mangrove litter has relatively high C/N ratio, and thus the influx of marine organic matter may be significant for provisioning of N rich material to microbial communities.

Notes on stable isotopes

Stable isotopes can be used for sourcing of organic matter and are commonly used in marine ecosystems.

Stable isotope compositions are typically expressed in “parts per thousand differences from a standard.” I.e.:

\[\delta X = (\frac{R_{sample}}{R_{standard}}-1)*1,000 \] Where:

• X represents the isotope of interest (i.e., ¹³C)
• R represents ratio of isotope of interest and its standard form (¹³C to ¹²C).

Higher values (less negative) represent increases in concentrations of the isotope of interest relative to the standard, whereas lower values represent decreases in isotope of interest relative to standard.

Carbon isotopes allow for understanding of primary production sources in an ecosystem. Transfer of ¹³C through trophic levels is relatively the same, except for slight increase (<1 ppt) in concentration.

4.4.3 Krauss, 2003

“Differential rates of vertical accretion and elevation change among aerial root types in Micronesian mangrove forests” (Krauss, Allen, and Cahoon 2003)

Key contribution: This study examines the differential effects of different root morphologies on sedimentation in mangroves. The study is summarized in Section 4.3.5.

4.4.4 Lovelock, 2008

“Soil respiration and belowground carbon allocation in mangrove forests” (Lovelock 2008)

Key contribution: Lovelock sought to examine variation in mangrove soil respiration as a result of forest structure and temperature at seven different sites, from 27 deg N to 37 deg S.

Key findings were that soil respiration approximated that of terrestrial forests, and correlated with leaf area index and aboveground net primary production.

Key notes:

Lovelock examines soil respiration across a range of stands that represent a wide variety of forest structure, ANPP and climate. In doing so, she seeks to primarily answer:

1. Whether soil respiration is higher in taller forests compared to dwarf forests, reflecting differences in aboveground productivity
2. Whether total belowground carbon allocation (TBCA) in mangrove forests is related to ANPP with a similar relationship as to that which exists in terrestrial forests
3. Whether dwarf mangroves, which are nutrient limited, have proportionally higher TBCA than taller forests

Study design

Soil respiration was measured in conjunction with soil temperature at 2 cm depth, leaf area index, and tree height.

In addition, TBCA was calculated using a mass balance approach:

\[ TBCA = F_{soil\ efflux} - F_{AG\ litter\ production} + F_{export} + F_{stored}\]

Assuming that export and storage are low relative to inputs and respiration, the mass balance is simply:

\[ TBCA = F_{soil\ efflux} - F_{AG\ litter\ production} \]

\(F_{soil\ efflux}\) was estimated via extrapolation of soil respiration measurements.

Key results

Soil respiration was similar for both dwarf forests and taller forests, and was approximately equivalent to terrestrial forests (though slightly on the lower end).

Soil respiration also varied with forest LAI and soil temperature. Soil respiration increased to soil temperatures of approximately 25-27 deg C, and then decreased with further increases in temperature.

Key findings

Lovelock notes that the direct links between above- and below-ground processes facilitates scaling CO₂ exchange measurements.

The correlation between LAI and soil respiration could be a result of variation in allocation of biomass to fine root components, or due to variation in heterotrophic component of respiration across sites.

LAI may be useful for estimating soil respiration over large spatial scales as it can be remotely sensed.

High rates of soil respiration in dwarf forests relative to taller forests are unlikely to be due to respiration of live roots as live root densities, root growth rates and root respiration rates are low in dwarf forests. Rather, may be due to high rates of C exudation and high levels of heterotrophic respiration.

4.4.5 Lunstrum, 2014

“Soil carbon stocks and accumulation in young mangrove forests” (Lunstrum and Chen 2014)

Key contribution: This study examines contributions of organic carbon to soil systems following mangrove afforestation of two species: Kandelia obovata and Sonneratia apetala. In addition, they examine a chronosequence of forest at different ages, and a literature review of mangrove inputs to soil organic carbon to investigate the influence of mangrove growth and species composition in soil organic carbon.

Key notes:

The authors hypothesize that given the relatively higher root to shoot biomass ratio in mangroves, soil C stocks will increase soon after afforestation.

They use primarily two key datasets:

1. Data consisting of repeated measures of surface soil C concentration in two young forests (K. obovata and S. apetala) from 0 to 6 years old
2. Chronosequence of 1 m soil cores from a mudflat, a 6 year, a 20 year, and a 70 year old forest.

Using the two datasets, they test the hypotheses that:

1. Soil C increases with forest age following mangrove forestation
2. Soil C accumulates more quickly in forested S. apetala than K. obovata plots

Chronosequence results

The growth of S. apetala and K. obovata differed greatly. S. apetala reached 7.6 m whereas K. obovata reached just 1.4 m. The stem density between the 20 year and 70 year old forests were markedly different, with the 70 year old forest (dominated by K. obovata) stocked more densely.

Soil C generally decreased with depth in all forests. Younger forests converged towards the mudflat values below 30 cm.

The 70 year forest had the highest SOC concentration at 10-20 cm, whereas the lowest value was found at the 70-100 cm level in the 6 year forests.

N content was also found to be higher in the 70 year old forest, though the difference was not as pronounced.

SOC differences between the three young forests and the mud flat was not significantly different, whereas the 70 year old forest was significantly different from the mud flat. This might indicate that soil C accumulation accelerates in older forests.

Field study results

Soil C concentration increased with age in the young forests (increase of approximately 0.06% per yr for K. obovata and 0.08% per yr for S. apetala).

Mangroves are often cited as having high average POC at 8.5% (Duarte et al., 2005), but Alongi, 2014 revised this estimate closer to 2-3% C.

Literature results

The literature results also showed a significant increase in SOC with age; however, the mean estimates of soil carbon percentage were lower than commonly cited values.

Discussion

Terrestrial forests are commonly cited as losing SOC upon afforestation, occasionally taking 20-30 years to recover initial values. Given the evidence here and the literature review, the results indicate that mangroves may behave differently and provide immediate SOC sequestration benefits.

The hypothesis that S. apetala and K. obovata would differ in SOC accumulation was not supported by the study.

Relative C:N concentrations in the biomass of different species could influence differences in SOC in forests of different species compositions, as the nutrient quality would affect the rate at which biomass is decomposed.

4.4.6 Atwood, 2017

“Global patterns in mangrove soil carbon stocks and losses” (Atwood et al. 2017)

Key contribution: The study provides an updated estimate of SOC patterns globally, through the lens of national scales.

Key notes: They describe the data quality (relatively poor) for SOC, although their database is likely less extensive than that of the ERL paper. They note the widespread prevalence of pedotransfer functions to estimate bulk density, as well as a common lack of SOC estimates deeper than 50 cm.

National and global trends:

Found a pattern in mean SOC per ha across latitudinal bands, but did not find total SOC to correlate with latitudinal change.

Found mixed species stands to have higher SOC than monospecific stands; stands with 5 genera had 70-90% higher soil C stocks than other richness levels. There are implications here, particularly for restoration efforts which typically focus on Rhizophora or Avicennia species.

Almost all variation at national level was explained by extent of mangroves, which points to the need for improved methods for estimating and mapping mangrove area.

Data did not exist for 44 countries in which mangroves occur.

They did not examine variation in mean SOC density across nations or global scales.

Indonesia, Malaysia, USA, and Brazil have the highest potential C emissions from deforestation, accounting for a total of approximately 86% of stocks.

Other countries with very high density (higher than these four) currently have low rates of deforestation, and thus may constrain total C emissions from mangroves. Prioritizing conservation of mangroves in these countries may be critical.

Myanmar may be a highly significant country in terms of C emissions and deforestation.

References

Amundson, Ronald. 2001. “The Carbon Budget in Soils.” Annual Review of Earth and Planetary Sciences 29 (1): 535–62. doi:10.1146/annurev.earth.29.1.535.

Bouillon, Steven, Farid Dahdouh-Guebas, AVVS Rao, Nico Koedam, and Frank Dehairs. 2003. “Sources of Organic Carbon in Mangrove Sediments: Variability and Possible Ecological Implications.” Hydrobiologia 495 (1): 33–39. doi:10.1023/A:1025411506526.

Krauss, KW, JA Allen, and DR Cahoon. 2003. “Differential Rates of Vertical Accretion and Elevation Change Among Aerial Root Types in Micronesian Mangrove Forests.” Estuarine, Coastal and Shelf Science 56 (2): 251–59. doi:10.1016/S0272-7714(02)00184-1.

Lovelock, Catherine E. 2008. “Soil Respiration and Belowground Carbon Allocation in Mangrove Forests.” Ecosystems 11 (2): 342–54. doi:10.1007/s10021-008-9125-4.

Lunstrum, Abby, and Luzhen Chen. 2014. “Soil Carbon Stocks and Accumulation in Young Mangrove Forests.” Soil Biology and Biochemistry 75: 223–32. doi:10.1016/j.soilbio.2014.04.008.

Atwood, Trisha B, Rod M Connolly, Hanan Almahasheer, Paul E Carnell, Carlos M Duarte, Carolyn J Ewers Lewis, Xabier Irigoien, et al. 2017. “Global Patterns in Mangrove Soil Carbon Stocks and Losses.” Nature Climate Change 7 (7): nclimate3326. doi:10.1038/NCLIMATE3326.