Forest Ecology and Management 199 (2004) 273–282
Effects of subsistence farming system on soil surface CO2–C flux on Cofre de Perote volcano slopes, Veracruz (Mexico) Adolfo Campos C.* Instituto de Ecologia, A.C. Apartado Postal 63, C.P. 91000 Xalapa, Veracruz, Mexico Received 24 September 2003; received in revised form 2 December 2003; accepted 16 May 2004
Abstract Mountainous parts of the eastern slope of the Mexican Cofre de Perote volcano have suffered great ecological disturbance due to the conversion of forest to subsistence corn (Zea mays) cropland. This research was conducted in order to evaluate the response of surface soil CO2–C flux to land use modification, specifically the adoption of subsistence farming in this mountainous forest–cropland setting. The treatments consisted of a corn plot (CP), recently abandoned cropland (RA), old abandoned cropland (OA), and coniferous (Pinus species) forestland (CF). For an 8-month period (August 1999–June 2000), CO2–C flux concentration was measured monthly using the static chamber method. CO2–C flux ranged from 1.80 to 5.22 g C m2/day in the CF treatment, 2.76–8.45 g C m2/day in the CP treatment, 3.24–7.48 g C m2/day in the RA treatment, to 2.99–8.84 g C m2/day in the OA treatment. Results showed that the conversion of coniferous forest to corn cropland leads to a significant increase in CO2–C flux concentration from the soil surface to the atmosphere. There were few differences between current and former agricultural plots, demonstrating that crop abandonment did not lower CO2–C flux from the soil’s surface. Soil temperature and moisture content were poorly correlated with CO2–C flux; however, soil moisture data did show greater scattering. This indicates that soil temperature had a major effect on CO2–C flux concentration. To offset some of the organic carbon depletion from soil caused by land use change, it is recommended that the subsistence farming system subject corn crop residue to a composting process. This can improve short-term soil fertility and increase corn crop yield, playing a central role in converting abandoned cropland into permanent cultures that facilitate forest conservation. # 2004 Elsevier B.V. All rights reserved. Keywords: CO2–C flux; Corn; Forest; Subsistence farming; Abandoned cropland; Mountain landscape
1. Introduction Soil processes are an important part of the global carbon cycle, not only because of atmospheric CO2–C sequestration but also their contribution to annual greenhouse effect gas flux (Bajracharya et al., 2000; Torbert et al., 2000). CO2–C flux from the soil surface to the atmosphere is the result of root respiration and * Tel.: þ52 228 842 1832; fax: þ52 228 818 7809. E-mail address:
[email protected] (A. Campos C.).
the decomposition of organic products by microorganisms and invertebrates. CO2–C flux is direct evidence of carbon loss and nutrient recycling (Mielnick and Dugas, 2000; Rustad et al., 2000; Marschner and Kalbitz, 2003). It is estimated that the CO2–C flux produced by soil respiration processes varies from 75 to 80 Gt/year and is equivalent to 10% of the atmosphere’s CO2–C (Raich and Schlesinger, 1992; Raich and Potter, 1995; Raich and Tufekcioglu, 2000; Raich et al., 2002; Schlesinger and Andrews, 2000). Agroforestry activities that affect CO2–C release to the
0378-1127/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.foreco.2004.05.045
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atmosphere include deforestation, biomass burning, and agricultural practices (Potter et al., 1997; FAO, 2000; Post and Kwon, 2000; Lal, 2003). Therefore, the monitoring of CO2–C flux through land use modification allows us to determine the soil’s C sink capacity, which in turn permits the implementation of restorative land use (Conant et al., 2003; Lal, 2003). The Cofre de Perote volcano in the Mexican state of Veracruz reaches an altitude of 4200 m above sea level, and mountainous areas show severe ecological disturbance due to the conversion of forest to cropland. On the volcano’s eastern slope, at approximately 2600 m above sea level, lies the upper border between forest and corn crop. At this altitude, subsistence agriculture is the norm, and the farmers involved have very limited economic resources. In the area, forests are often cleared and converted to corn crops. The tillage system that is practiced is lowintensity, the only tool used being the hoe. Plot size is generally less than 0.5 ha, with many plots scattered throughout the forestland. It is common to observe agricultural plots that have been abandoned for varying periods of time; grass grows at first and is followed by the establishment of secondary shrub vegetation locally known as ‘‘escobo’’ (Baccharis conferta). These lands are often deforested and used as cropland again but are sometimes abandoned indefinitively, benefiting natural restoration in some cases and sheep and goat grazing in others. The reason that such land is abandoned seems to be diminished corn yield due to a degradation in soil quality that is directly related to the change from forest by cropland. The objectives of this study were three-fold: (1) to evaluate the impact of the subsistence farming system on soil surface CO2 flux to the atmosphere (2) to examine how CO2 flux responds to the change from forest to cropland, and (3) to quantify the variation in CO2 flux as it relates to the moisture and temperature of mountain soils.
and 198260 3200 N, 978070 2500 W), between 2550 and 2600 m above sea level; the landscape consists of steep slopes and deep gullies The soil type is Hydric Pachic Melanudand, the principal characteristics of which appear in Table 1 (Campos et al., 2001). Mean soil bulk density is 0.50 g cm3. The climate is moist, cold temperate with frequent fog, and the mean temperature is 9.4 8C. January is the coldest month (6.9 8C) and May the hottest (11.6 8C). Total annual rainfall is 1670 mm. Average monthly air temperature and cumulative rainfall during the experiment (1999– 2000) appear in Fig. 1. Native vegetation is coniferous forest (Pinus species). In this part of the Cofre de Perote volcano are the highest concentrations of soil organic carbon, ranging from 426 mg C ha1 at a depth of 100 cm to 743.5 mg C ha1 at 150 cm. The high soil carbon concentration is attributable to the formation of complexes from organic matter and noncrystalline materials that are favored by environmental factors (Shoji et al., 1993). Furthermore, the large proportion of micropores present in these soils may play a crucial role in organic matter conservation by restricting interaction with microorganisms. The study plots are close together, lie in a similar topographic position, and have the same parent material. Treatments were a corn (Zea mays) plot (CP), recently (2 years previous) abandoned cropland (RA), old (10 years previous) abandoned cropland (OA), and coniferous (Pinus species) forestland (CF). For corn cultivation, soil preparation usually begins in mid-February and consists of using a hoe to till topsoil from 0 to 12 cm. Corn seeding begins in mid-March and the harvest is reaped in December. Approximately 3.0 t ha1 of sheep and goat manure are commonly applied during seeding. In mid-May, when the corn plants have reached an approximate height of 80 cm, farmers till the land to eliminate weeds that compete with the corn; this consists of using a hoe to till the topsoil (0–5 cm). Around this time, they apply inorganic fertilizer, 155 kg N ha1 and 20.5 kg P ha1.
2. Materials and methods
2.2. Field CO2–C measurements
2.1. Description of sites
Soil respiration was measured using the static chamber method (Freijer and Bouten, 1991; Raich and Schlesinger, 1992; Aslam et al., 2000; Kabwe et al., 2002). This method is based on CO2 absorption
The study area is located on the eastern slope of the Cofre de Perote Volcano (198260 2400 N, 978070 1700 W
Values are expressed on an air-dried basis. Subscripts d, o and p indicate that the element was extracted with dithionite-citrate-bicarbonate, oxalate, and pyrophosphate.
0.16 0.06 0.10 0.21 0.14 0.65 0.35 0.21 0.37 0.10 2.30 0.74 1.00 1.40 0.17 8.0 5.0 4.4 4.3 4.8 0.30 1.00 1.75 1.55 2.57 0.57 0.75 1.01 1.43 0.31 0.78 1.02 1.87 1.68 1.84 1.13 1.81 2.94 2.36 2.73 1.81 1.72 2.13 2.98 1.27 1.82 2.63 4.62 4.53 5.88 2.45 2.16 2.76 3.45 2.45 3.5 2.3 1.4 3.8 0.9 11.0 11.1 11.2 11.3 11.2 0–10 10–50 50–90 90–180 180–200 A11 A12 A13 A14h Bw
5.5 5.7 5.7 5.6 5.3
12.9 7.0 7.9 12.7 3.4
Fep (%) Feo (%) Fed (%) Alp (%) Alo (%) Ald (%) Cp (%) C (%) pH NaF pH H2 O Depth cm Horizon
Table 1 Selected properties of soil ( CP > OA > CF. In the region’s agricultural subsistence systems, there is no efficient corn crop residue management. When the
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