THE EFFECT OF TROPICAL LAND USE ON SOIL CARBON DYNAMICS

Acknowledgements:Funding for this Research Experiences for Undergraduate program is provided by the National Science Foundation’s Division of Earth Sciences (EAR-1659848).

Tropical land use change accounts for the second largest source of human-caused greenhouse gas emissions (about 12-30%). 0.2 Gt of Carbon emissions from tropical soils are attributed to land use change each year (Don et al. 2010). Soils are the largest terrestrial carbon (C) sink, storing two to three times as much C as the atmosphere and above ground vegetation (Gougoulias, et al. 2014). The majority of soil C is associated with soil aggregates of different size classes that have distinct potentials for C storage. Here we investigated the effect of different tropical land uses on soil C dynamics. We measured soil respiration and bioavailable C and nitrogen from three soil aggregate classes across three common tropical land uses; agricultural cropping system, a tree plantation (a man-made forest purposely for C sequestration), and a transitional premontane rainforest.

Premontane

Rainforest

Tree PlantationTexas A&M

Soltis Center

Land use change has a significant effect on CO2 flux and soil aggregate-CCO2 Flux

• CO2 flux is highest in the cropping system possibly due to higher soil temperatures (assuming flux and temperature have a linear relationship)

• Premontane rainforest have low flux rates due to it’s age and had more time to stabilize carbon in aggregates.

Soil Aggregate-C• High DOC in the premontane rainforest due to the forest having less soil disturbance.• More microaggregates in the premontane rainforest.

The tree plantation and premontane rainforest were composed of ~70% macroaggregates, while the cropping system had a more equal distribution of macroaggregates and microaggregates. Literature supports that greatest amount of biological activity is associated with macroaggregates (Strong et al. 2004) - possibly resulting in higher respiration rates. Moreover, since the cropping system soils are more homogenized, it would explain why the site has a smaller percentage of macroaggregates. Nitrogen differed by site, with the premontane rainforest having the highest amount and the tree plantation having the least amount. This could be due to the premontane rainforest having more leaf litter input and root exudates compared to the tree plantation and the cropping system. Soil carbon was variable between and within sites – limiting our ability to detect significant differences by land use. More replicates are necessary to understand DOC relationships within a single land use (n>3).

Conclusions

Funding for this Research Experiences for Undergraduate program is provided by the National Science Foundation’s Division of Earth Sciences. Thank you to the Texas A&M Soltis Center Director, Eugenio Gonzalez, and the Soltis Center staff who made this research possible. Thank you to Juan Carlos Rojas for allowing me to use his farm, and to Johan Rodriguez for providing transportation. Thank you to the owner of the tree plantation, Kurt Clemson, for allowing me to work there. Thank you to my lab partner, Abigail Keebler for helping me.

Methods:

Results:

• Main Result I. Land use has a significant effect on average CO2 flux. The tree plantation had the highest flux, whereas the cropping system had the lowest flux (Fig 1).

• Main Result II. Land use has a significant effect on average soil temperature. The cropping system was the highest, whereas the premontane rainforest was the lowest (Fig 2).

• Main Result III. There is no statistically significant effect of land use on daily fluxes (Fig 3), and there was no relationship between flux and temperature (Fig 4).

Carbon Fluxes

• We used an Li-8100A to measure carbon dioxide flux from three land uses (n = 10).

• Measured soil temperature at time of CO2measurements.

• Fluxes calculated aswhere FCO2 is the soil CO2 efflux rate (μmol m-2 s-1), V is volume, P0 is the initial pressure, W0 is the initial water vapor mole fraction, S is soil surface area, T0 is initial air temperature in the chamber, and δC’/δt is the initial rate of change in water corrected CO2 mole fraction.

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Cropping System Tree Plantation Premontane Rainforest

Cel

cius

Land Use (r2 = 0.3, p < 0.0001)

Average Soil Temperature per Site

Figure 1: Mean flux of ten collars per site, measured over six days.

Figure 2: The cropping plantation had the highest temperature, whereas the premontane cloud forest had the lowest.

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F CO

2 (µ

mol

m-2

s-1)

Dates

Dates Flux Averages were Taken

Cropping System Tree Plantation Premontane Rainforest

Figure 3: Average fluxes varying per date they were taken.

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F CO

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mol

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Temperature

Flux vs. Temperature

Cropping System Tree Plantation Premontane Rainforest

Figure 4: Relationship between flux and soil temperature.

Soil Aggregates Carbon & NitrogenMethods:

Results:

• Main Result I. Tree plantation had greatest amounts of large macroaggregates (~ 72% of soil structure classified as macroaggregates). The cropping system had a more equal distribution of macroaggregates and microaggregates.

• Main Result II. DOC did not differ between sites or among aggregate classes possibly due to low replication, and high variation in forested sites.

• Main Result III. Land use (site) had a significant effect on TDN. The premontane rainforest had highest N, whereas the tree plantation had lowest N.

1San Jacinto Community College, 2 Texas A&M University (TAMU); 3 Dept. of Soil and Crop Sciences, TAMU; 4Dept. of Civil Engineering, TAMU; 5Dept. of Ecosystem Science and Management, TAMU

M. Rivera1,2, P. Smith3, G. Miller4, G. Moore5

Figure 9: DOC and TDN for each aggregate class.

Figure 9a: TDN average per site. This includes all aggregate classes.

Figure 5: Soil aggregate separation.

• We used wet aggregate stability to separate soil into difference aggregate sizes: large macroaggregate, small macroaggregate, and microaggregates (Fig 5) across the three different land uses (Six et al. 1998).

• Measured bioavailable carbon, i.e. dissolved organic carbon (DOC), and nitrogen (TDN) from each aggregate size (n = 3).

Introduction:

Hypotheses

Site Description

MapCroppin

g System

ab

a

b

0123456789

10

Cropping System Tree Plantation Premontane Rainforest

F CO

2 (µ

mol

m-2

s-1)*

Land Use (r2 = 0.84, p = 0.04)Statistical test performed* on log transformed data

Average Flux per Site

Land use has a significant effect on soil respiration. Although we hypothesized that the cropping system would have higher flux rates due to having higher temperatures, it had lower flux rate. There was no statistical relationship between soil temperature and flux. Traditionally, higher temperatures mean higher fluxes. Perhaps respiration depended more on soil moisture or another soil property, than temperature at our sites. In contrast, the tree plantation had the largest flux in spite of its purpose to sequester carbon. This suggests that the tree plantation acts more like a carbon source than a carbon sink. Further research quantifying the carbon fluxes of these tree plantation sites is necessary to better support this argument.

Discussion CO2 Flux Discussion Aggregate C & N

• Land use influences soil respiration and soil nutrients, but not as expected.• The tree plantation was planted under Costa Rica’s Payment for Ecosystems Services Programme in order to offset greenhouse gas emissions through increased carbon sequestration in soils and

in above ground biomass. However, our results show that the tree plantation may be a carbon source instead of a sink. More respiration surveys from different tree plantations are necessary to further assess whether or not these sites acts as a carbon source.

• Furthermore, the terrestrial carbon cycle is dominated by the balance between photosynthesis and respiration (Gougoulias et al. 2014); therefore, photosynthetic studies are needed to better understand the balance of the tree plantation.

abb

a

00.10.20.30.40.50.60.70.80.9

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Cropping System Tree Plantation Premontane Rainforest

TDN

mg/

L*

Land Use (r2 = 0.74, p < 0.0001)*Statistical test performed on log transformed data

Total Dissolved Nitrogen Average per Site

Acknowledgments:

Texas A&M Soltis Center in the PeñasBlancas region of Costa Rica

ReferencesAxel Don, Jens Schumacher, Annette Freibauer. Impact of tropical land use change on soil organic carbon stocks - a meta-analysis.

Global Change Biology, Wiley, 2010, 17 (4), pp.1658Gougoulias, Christos et al. “The role of soil microbes in the global carbon cycle: tracking the below-ground microbial processing of

plant-derived carbon for manipulating carbon dynamics in agricultural systems.” Journal of the science of food and agriculture (2014).

Six, J, et al. "Aggregation and Soil Organic Matter Accumulation In Cultivated and Native Grassland Soils." Soil Science Society of America journal 62.5 (1998): 1367-1377.

Strong, D. T., Wever, H. D., Merckx, R. and Recous, S. (2004), Spatial location of carbon decomposition in the soil pore system. European Journal of Soil Science, 55: 739-750.

Figure 8: Aggregate structure within each site.

Cropping System• Dominant vegetation

is Yuca, Manihot esculenta.

• Planted 3 months prior to sampling.

• Cleared for agriculture 34 years ago.

Tree Plantation• Dominant tree is the

Sebo tree, Vochysiaguatemalenis.

• Planted in 2005• Was previously a

pasture land• Unmanaged

Premontane Rainforest• Dominant tree is the

Crabwood tree,Carapa guianensis,

• Secondary forest approximately 50 years old.

• Unmanaged