Mean soil carbon content and predicted amount of carbon (Mg C ha–1) in sampled
1. Introduction
Recently there has been an increasing interest of research related to improve the understanding of carbon (C) sequestration mainly under Article 3.4 of the Kyoto Protocol of the United Nations Framework Convention on Climate Change where countries can count this sequestration as a contribution to reduce greenhouse gas emission (IPCC, 2001). Data on C storage in forests, grasslands and shrublands are essential for understanding the importance of rapidly increasing level of CO2 in the atmosphere and its potential effect on global climate change. In South America, mean annual temperature is predicted to increase by 3-4 ºC in both summer and winter between 30° and 55° SL (Manabe & Wetherald, 1987). Such an increase would have significant effects on Patagonian ecosystems. In this context, secondary indigenous forests are considered efficient C sink ecosystems.
The steppe ecosystem, mainly characterised by the presence of tussock, short grasses and shrubs, covers 85% of the total area. Grazing has modified the structure of Patagonian ecosystems by reducing vegetation cover, increasing bare areas, and changing floristic composition. Erosion and degradation processes have occurred in several areas of Patagonia due to an overestimation of the carrying capacity of these rangelands, inadequate distribution of animals in very large and heterogeneous paddocks, and year-long continuous grazing (Golluscio et al., 1998). In Patagonia, most of the actual knowledge about the environmental factors that affect net primary production of grasslands at regional level derives from the importance of mean annual precipitation, radiation and temperature (Jobbágy & Sala, 2000). However, data on C accumulation in both above- and belowground components of plant functional types are essential for evaluating the impacts of grazing on C cycle and long -term effects on the C balance of grasslands. Global estimates of the relative amounts of C in different vegetation types suggest that grasslands approximately contribute more than 10% of the total biosphere store (Nosberger et al., 2000). Also, it has been demonstrated that most temperate grasslands under existing management conditions are considered to be C sink and sequester more C than arable crops (Connan et al., 2001).
Therefore, the aim of this manuscript was to describe the amount of C in both above- and below-ground components for the main cold temperate ecosystems in Southern Patagonia (Argentina). In particular, the aim was to quantify the C storage in an age sequence and among crown classes for individual trees grown at different site qualities of deciduous
2. Carbon storage in main Patagonian ecosystems
2.1. Nothofagus antarctica native forest
Above- and below-ground C pools were measured in pure even-aged stands of
Sigmoid functions were fitted for total C accumulation and C root/shoot ratio of individual trees against age for each site class. The parameters are given in
Peri et al. (2010
). Total C accumulation over time followed the order: dominant > codominant > intermediate > suppressed trees (Fig. 1a). For example, dominant trees growing on SC III had accumulated 228 kg C tree-1 after 180 years and suppressed trees only 46 kg C tree-1. Also, site quality of the stands had a strong impact on total C accumulation over time. For example, while the mean total C accumulated for dominant trees grown in SC V at 150 years was 109 kg C tree-1, dominant trees growing on SC III had 207 kg C tree-1. C accumulation was divided by tree age to establish the average annual rate at which C was accumulated by trees. The rate of C accumulation showed a parabolic relationship with tree age and increased to reach a maximum and then declined as tree age increased further (Fig. 1b). Crown class also affected the maximum value and shape of this response. For example, maximum accumulation rate for dominant trees growing on SC III was 1.44 kg C tree-1 year-1 at 116 years and then declined to 1.10 kg C tree-1 year-1 at 220 years (Fig. 1b). In contrast, maximum accumulation rate for suppressed trees was 0.26 kg C tree-1 year-1 at 139 years (Fig. 1b). Also, the site quality modified the maximum values and the shape of the rate of C accumulation. The greater C accumulation of dominant trees at any age compared to inferior crown classes was very closely related to the C accumulation rates. This is consistent with Rötzer et al. (2009) who estimated that the amount of C storage in both above- and belowground components over time for a mixed beech stand changed with variations in site conditions, especially when precipitation decreased. Dominant trees and trees growing in better site qualities had larger crowns with more biomass of photosynthetic green leaves, and consequently had faster growth rates. In contrast, the leaves of suppressed trees located in the inferior stratum receive less available light for photosynthesis and these less active leaves may accumulate less C. For
In primary
The equations for total C accumulation from individual trees were used to estimate the C storage at the stand level using forest inventory data. Total C storage in
|
2.2. Silvopastoral systems
Deciduous
System component | Mg C ha–1 |
|
|
Litter | 7.4 |
Organic horizon (0-03 m) | 13.6 |
Mineral horizon (0.03-0.10 m) | 17.6 |
Mineral horizon (0.10-0.30 m) | 81.3 |
Mineral horizon (0.30-0.60 m) | 98.6 |
|
|
|
|
Leaves | 0.5 |
Small branches | 1.0 |
Sapwood | 7.3 |
Heartwood | 8.2 |
Bark | 3.7 |
Rot | 0.5 |
Fine roots | 0.1 |
Coarse roots | 8.7 |
|
|
|
|
Green leaves | 0.5 |
Dead leaves | 0.1 |
Pseudostem | 0.4 |
Roots | 2.5 |
|
|
|
|
patterns compared with tree plantations or pasture monocultures. This study improved the understanding about the potential of C sequestration of
2.3. Shrubland
In Santa Cruz province there is a "matorral" thicket area dominated mainly by
Total plant C varied from 4.4 to 12.5 Mg C ha–1 for 20 and 60% shrub cover, respectively (Fig. 2). In this ecosystem, the C root/shoot ratio ranged between 0.17 and 0.30. The soil carbon store (0-30 cm) dominates the carbon budget at all
2.4. Grasslands
The Magellanic Patagonian steppe (southern Patagonia, Argentina) is a cold semiarid environment characterized by strong winds and high evaporation rates that cover 3 million hectares where grasses and shrubs are the dominated plant functional types with contrasting root systems. In this ecosystem
The effect of long-term livestock grazing on C content of the plant-soil grassland system (to 30 cm) of Dry Magellanic Grass Steppe and Sub-andean Grassland areas is presented in Figure 3. The vegetation of the steppe is dominated by grasses and sedges (
Pool |
|
Green leaves | 2.50 |
Senesced leaves | 2.84 |
Pseudostem | 1.48 |
Fine roots | 2.38 |
Coarse roots | 2.25 |
Soil (0-0.30 m) | 86.1 |
|
97.55 |
|
|
Green leaves | 2.92 |
Senesced leaves | 4.38 |
Pseudostem | 1.73 |
Fine roots | 5.17 |
Coarse roots | 3.49 |
Soil (0-0.30 m) | 205.2 |
|
222.89 |
|
|
Green leaves | 0.81 |
Senesced leaves | 0.76 |
Pseudostem | 0.98 |
Fine roots | 0.79 |
Coarse roots | 1.63 |
Soil (0-0.30 m) | 78.9 |
Total | 83.87 |
On these extensively managed grasslands, grazing intensity was the main management practices that affected ecosystem C levels. This varied from 50 Mg C ha-1 at a heavy stocking rate (0.70 ewe ha-1 yr-1) to 130 Mg C ha-1 under low grazing intensity (0.10 ewe ha-1 yr-1) (Fig. 3). A slightly higher total C content was detected in the low grazing intensity (0.10 ewe ha-1 yr-1) grassland compared with the non grazed areas (130 vs. 120 Mg C ha-1). This is consistent with Reeder & Schuman (2002) who reported that soil C content was highest in US mixed-grass and short-grass rangelands under grazing, while non-grazed enclosures caused immobilization of C in excessive above-ground plant litter. From a low grazing intensity of 0.10 ewe ha-1 yr-1, total grassland C declined as grazing intensity increased by reaching a value of 103 Mg C ha-1 at a medium stocking rate of 0.35 ewe ha-1 yr-1 (Fig. 3). Then, total grassland ecosystem C was followed by a further decline to the lowest estimated C value. For Dry Magellanic Grass Steppe grassland under sheep grazing, this response showed that 0.35 ewe ha-1 yr-1 stocking rate was a critical value below which ecosystem C was severely restricted. The effect of sheep on system C arose from the influence of grazing intensity on plant floristic composition.
Thus, long-term grazing at heavy stocking rates has tended to decrease plant species diversity and plant cover, and consequently by increasing bare areas. However, the biggest impact on grassland C ecosystem due to overgrazing was the C lost from soil (mainly the organic layer) as a consequence of soil erosion by strong winds.
3. Conclusions
Carbon storage has become an important issue in international negotiations on the management of greenhouse gas emissions, because increased carbon storage can be useful in offsetting emissions of carbon from fossil fuel burning and other sources. Estimates of native forest C storage under different management practices are required for estimating regional and national greenhouse gas balance. The use of functions provide a valuable tool for understanding and estimating C accumulation of primary forests of
C accumulation in grasslands and shrublands was strongly affected by plants composition, size and cover. In both ecosystems, it was critically important to quantify and understand belowground C allocation as well as soil C pools. Thus, quantification of roots C was important to improving our understanding of C cycles and storage in these ecosystems. In particular, long-term grazing intensity in grasslands was the main management practices that affected ecosystem C levels where overgrazing determined C lost mainly from soil as a consequence of soil erosion. Changes in grassland management that reverse the process of declining productivity can potentially lead to increased soil C. Therefore, proper grassland management is important to carbon sequestration.
Acknowledgments
I want to thank to Romina Lasagno, Verónica Gargaglione, Martín Viola and Juan Ruiz for their invaluable help during sampling in the field and contribution with data analysis.
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