Soil and vegetation play a vital role in the global C cycle because C exchange is affected by
both. Thus a change in land use may result in either a loss or gain of C in the soil-plant
system. This study was conducted in the Weatherley catchment in the northerly Eastern
Cape Province, a former grassland area. Approximately half of the 160 ha in the catchment
was afforested with three tree species, viz. P. elliottii, P. patula, and E. nitens in 2002.
Before afforestation, a baseline study (Le Roux et al., 2005) on soil organic matter was
conducted on the areas designated for the above mentioned tree species. Therefore, this
study was a continuation of the mentioned study with the aim to quantify the soil and
biomass C stocks (in some instances N stocks also) eight years after afforestation.
For comparable purposes the same 27 sites studied by Le Roux et al. (2005) were
investigated, viz. 25 afforested sites and two control sites. Soil samples were collected in
2010 at various depths from the 27 sites: 0-50, 0-100, 0-150, 0-200, 0-250, 0-300, 0-400, 0-
500, 0-600, 0-700, 0-800, 0-900, 0-1000, 0-1100, and 0-1200 mm and analysed for organic
C and total N as organic matter indices. At each site, three sub-samples were taken per
depth interval and mixed together to give a composite sample. The procedure was
replicated four times at each site.
At each of the 27 sites, fallen litter and undergrowth were collected simultaneously with the
soil sampling, also in four replicates. After being dried in a glasshouse, the litter was milled
and analysed for C and N contents. A year after soil and litter sampling, when trees were
eight years old, the height and diameter at breast height of 12 trees were measured at each
of the 25 afforested sites. The measured data were used to calculate the utilisable stem
volume, and hence the tree C stocks.
Afforestation of the former grassland areas influenced soil organic matter in the upper 300
mm layer, resulting in either increases or decreases in soil C stocks, N stocks and C:N
ratios. Soil C stocks decreased by 0.9 Mg ha-1 at site 235 (Katsptuit soil with grass) to 23.6
Mg ha-1 at site 232 (Katspruit soil with P. elliottii trees). The rate of decrease ranged
between 0.11 and 2.95 Mg C ha-1 yr-1. The soil C stocks increased by 0.9 Mg ha-1 at site 244
(Pinedene soil with P. patula trees) to 11.3 Mg ha-1 at site 242 (Longlands soil with P. patula
trees). The rate of increase ranged from 0.11 Mg C ha-1 yr-1 to 1.41 Mg C ha-1 yr-1. Soil C
stocks decreased significantly by 5.5 Mg ha-1, 10.0 Mg ha-1, and 12.4 Mg ha-1 for grass, E.
nitens, and P. elliottii areas, respectively. Soils under P. patula showed an increase in C
stocks of 1.9 Mg ha-1. When soils were grouped according to mapping units, drainage
classes or first subsoil (B1 or E1) horizons there was generally a significant decrease in soil C stocks due to afforestation. The soil N stocks to a large extent behaved like the soil C
stocks.
The aboveground biomass C stocks were obtained by adding the litter C stocks and the tree
C stocks together. These aboveground biomass C stocks varied from 3.71 Mg ha-1 at site
209 (Katspruit soil with grass) to 167.2 Mg ha-1 at site 246 (Pinedene soil with E. nitens
trees). On average the aboveground biomass C stocks for the 27 sites was 64.9 Mg ha-1.
However, aboveground biomass C stocks averaged 69.7 Mg ha-1 for the 25 afforested sites
and only 4.8 Mg ha-1 for the two control sites. The aboveground biomass C stocks varied
significantly from 4.8 Mg C ha-1 for the grass to 41.2 Mg ha-1 for the P. elliottii and 67.3 Mg
ha-1 for the P. patula and 113.2 Mg ha-1 for the E. nitens areas. Based on the soil mapping
units, aboveground biomass C stocks varied from 45.6 Mg ha-1 for the C soil group to 83.3
Mg ha-1 for the A soil group. In the drainage class soil group, the aboveground biomass C
stocks varied significantly from 44.1 Mg ha-1 for the poorly drained soils to 81.8 Mg ha-1 for
the moderately drained soils and 74.4 Mg ha-1 for the freely drained soils. The aboveground
biomass C stocks varied significantly from 44.7 Mg ha-1 for the G horizon soils to 86.2 Mg
haâ1 for the red apedal B horizon soils. In general, the tree C stocks contributed the greatest
portion to the aboveground biomass C stocks, which in turn contributed more to the total C
stocks in the catchment. The C (undifferentiated hydromorphic), poorly drained, and G
horizon soil groups had the lowest aboveground biomass C stocks because the conditions in
these soil groups limited tree growth and hence C sequestration.
Total C stocks in the catchment before afforestation were estimated to be 7 209 Mg. After
eight years of afforestation C stocks were estimated to be 11 912 Mg. Therefore the trees
added 4 702 Mg C to the catchment, at a rate of 588 Mg C yr-1 or 3.67 Mg C ha-1 yr-1. The
rate of C sequestration in the afforested areas was 7.74 Mg ha-1 yr-1.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ufs/oai:etd.uovs.ac.za:etd-05172013-115355 |
| Date | 17 May 2013 |
| Creators | Lebenya, Relebohile Mirriam |
| Contributors | Prof CC du Preez, Prof CW van Huyssteen |
| Publisher | University of the Free State |
| Source Sets | South African National ETD Portal |
| Language | en-uk |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | http://etd.uovs.ac.za//theses/available/etd-05172013-115355/restricted/ |
| Rights | unrestricted, I hereby certify that, if appropriate, I have obtained and attached hereto a written permission statement from the owner(s) of each third party copyrighted matter to be included in my thesis, dissertation, or project report, allowing distribution as specified below. I certify that the version I submitted is the same as that approved by my advisory committee. I hereby grant to University Free State or its agents the non-exclusive license to archive and make accessible, under the conditions specified below, my thesis, dissertation, or project report in whole or in part in all forms of media, now or hereafter known. I retain all other ownership rights to the copyright of the thesis, dissertation or project report. I also retain the right to use in future works (such as articles or books) all or part of this thesis, dissertation, or project report. |
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