1. EFFECTS OF CHANGES IN FALLOW LENGTH ON SOIL ORGANIC C DUE TO CLIMATE CHANGE AND SOCIOECONOMIC FACTORS IN POTATO-BASED CROPPING SYSTEMS IN THE BOLIVIAN HIGHLANDS P.P. Motavalli 1, J. Aguilera 2, B. Jintaridth 1, C. Valdivia 1, M. Gonzales 2 and C. Chambilla 2 1 College of Agriculture, Food and Natural Resources, University of Missouri, Columbia, MO 65211 USA 2 Fundación PROINPA, La Paz, Bolivia Figure 1 A-D. (A-D) location and landscape example of the study communities and villages in the Altiplano of Bolivia. Introduction The Bolivian highland plateau region (Altiplano) is a semi-arid region in the Andes Mountains that occupies approximately 27% of the area of Bolivia and has a range in elevation of between 3600 and 4300 m above sea level. The region’s climate is characterized by high diurnal temperature variations, frost risks, low and irregular precipitation and high risks of drought during the growing season (Garcia et al., 2007).Potato-based cropping systems and livestock rearing of cows, sheep and camelids (e.g., alpaca and lama) are the primary agricultural activities (Valdivia et al., 2001). Crop rotations are initiated with potato followed by two to three years of cereal crops (e.g., quinoa, barley) and then an extended uncultivated fallow period which can last from 1 to over 40 years. This long fallow period allows for restoration of soil fertility, control of crop disease and pests, grazing of natural vegetation (e.g., “thola” (Parastrephia lepidophylla)) and fuel for cooking (Bottner et al., 2006; see Fig. 2 A-D)). The Bolivian highland plateau region (Altiplano) is a semi-arid region in the Andes Mountains that occupies approximately 27% of the area of Bolivia and has a range in elevation of between 3600 and 4300 m above sea level. The region’s climate is characterized by high diurnal temperature variations, frost risks, low and irregular precipitation and high risks of drought during the growing season (Garcia et al., 2007). Potato-based cropping systems and livestock rearing of cows, sheep and camelids (e.g., alpaca and lama) are the primary agricultural activities (Valdivia et al., 2001). Crop rotations are initiated with potato followed by two to three years of cereal crops (e.g., quinoa, barley) and then an extended uncultivated fallow period which can last from 1 to over 40 years. This long fallow period allows for restoration of soil fertility, control of crop disease and pests, grazing of natural vegetation (e.g., “thola” (Parastrephia lepidophylla)) and fuel for cooking (Bottner et al., 2006; see Fig. 2 A-D)). Increasing human population in the Altiplano and competing land uses have caused a growing reduction in the use and length of fallow (Coûteaux et al., 2008). Moreover, practices, such as use of mechanized disc plowing and cutting native vegetation for fuel, have reduced the regrowth of natural vegetation during the fallow period, possibly diminishing the amount of organic inputs and the rate of soil fertility restoration. The impact of fallowing in this environment on soil properties is unclear. For example, Hervé (1994) in a comparison of soils with up to 20 years of fallow from the Bolivian Altiplano did not observe any clear improvement in soil physical (e.g., bulk density) and chemical properties (e.g., soil total organic C) with increasing fallow. Cabaneiro et al. (2008) in the Andes in Venezuela determined that fallowing only increased labile organic C in soils collected from slopes with a northeast compared to a southwest aspect, possibly due to differences in microclimate. Objectives 1.To determine the effects of cropping, fallow periods and landscape position on soil total organic C and soil organic C fractions. 2.To compare different analytical techniques to evaluate changes in soil organic C fractions. Conclusions The decreasing length of the fallow period and reduction in native vegetation caused by competing uses and mechanized tillage, may be removing an important mechanism by which total and active soil organic C is restored in potato-based cropping systems in this region.The decreasing length of the fallow period and reduction in native vegetation caused by competing uses and mechanized tillage, may be removing an important mechanism by which total and active soil organic C is restored in potato-based cropping systems in this region. The impact of fallowing appears to be greater in soils collected at higher elevations which may reflect differences in climate and management practices with elevation in this region.The impact of fallowing appears to be greater in soils collected at higher elevations which may reflect differences in climate and management practices with elevation in this region. Several analytical methods for detecting changes in soil active organic C fractions were used in this study and they were sensitive to the effects of fallowing. Additional development of low-cost and rapid field methods for detecting changes in soil C fractions would be useful for developing and monitoring sustainable agricultural practices.Several analytical methods for detecting changes in soil active organic C fractions were used in this study and they were sensitive to the effects of fallowing. Additional development of low-cost and rapid field methods for detecting changes in soil C fractions would be useful for developing and monitoring sustainable agricultural practices. Materials and Methods Four small communities in Umala in the central Altiplano of Bolivia (Fig. 1 A-D)were selected as study sites in 2006 to represent typical Aymara farming communities at different relative elevations (Table 1).Four small communities in Umala in the central Altiplano of Bolivia (Fig. 1 A-D) were selected as study sites in 2006 to represent typical Aymara farming communities at different relative elevations (Table 1). Replicate soil samples were collected to a 20 cm depth from agricultural fields of the same cropping system and sandy loam soil type (locally classified as “cha’lla” soil)to determine changes in soil C due to cropping and fallow length.Replicate soil samples were collected to a 20 cm depth from agricultural fields of the same cropping system and sandy loam soil type (locally classified as “cha’lla” soil) to determine changes in soil C due to cropping and fallow length. Results and Discussion Increases in soil total organic C due to fallowing were more rapid in soils collected from communities at higher elevations compared to the lower elevation soils and generally a maximum accumulation was reached at higher elevations after approximately 20 to 30 years of fallow (Fig. 3A).Increases in soil total organic C due to fallowing were more rapid in soils collected from communities at higher elevations compared to the lower elevation soils and generally a maximum accumulation was reached at higher elevations after approximately 20 to 30 years of fallow (Fig. 3A). Cumulative CO 2 -C mineralized during incubation provides a relative measure of active organic C and results from this analysis showed similar differences between the lower and upper elevation communities as was observed for total organic C except this form of organic C continued to accumulate in the upper communities over 40 years of fallow (Fig 3B). Others have observed in the Andes in Venezuala that the maximum level of active C occurred after 8 years of fallow (Cabaneiro et al., 2008).Cumulative CO 2 -C mineralized during incubation provides a relative measure of active organic C and results from this analysis showed similar differences between the lower and upper elevation communities as was observed for total organic C except this form of organic C continued to accumulate in the upper communities over 40 years of fallow (Fig 3B). Others have observed in the Andes in Venezuala that the maximum level of active C occurred after 8 years of fallow (Cabaneiro et al., 2008). Additional measurements of soil C fractions showed a general increase with fallowing in KMnO4 active C and POM C in all communities with fallowing except for San José de Llanga (Table 2).Additional measurements of soil C fractions showed a general increase with fallowing in KMnO4 active C and POM C in all communities with fallowing except for San José de Llanga (Table 2). Ongoing analysis using MIR-DRIFT has shown some sensitivity in detecting changes in the ratio of labile to recalcitrant C (O/R ratio) with fallowing (Fig. 4).Ongoing analysis using MIR-DRIFT has shown some sensitivity in detecting changes in the ratio of labile to recalcitrant C (O/R ratio) with fallowing (Fig. 4). Portable NIR units may allow for rapid determination of soil C fractions due to management practices, such as fallowing. Fig. 5 shows the results of developing a predictive model for KMnO 4 active C using NIR spectra and how it significantly compared to measured KMnO 4 active C. Development of a low-cost NIR field unit may facilitate these type of measurements in locations, such as the Altiplano, where laboratory facilities are difficult and costly to access.Portable NIR units may allow for rapid determination of soil C fractions due to management practices, such as fallowing. Fig. 5 shows the results of developing a predictive model for KMnO 4 active C using NIR spectra and how it significantly compared to measured KMnO 4 active C. Development of a low-cost NIR field unit may facilitate these type of measurements in locations, such as the Altiplano, where laboratory facilities are difficult and costly to access. Soils were air-dried and passed through a 2-mm sieve and analyzed for soil total organic C using a LECO C/N analyzer.Soils were air-dried and passed through a 2-mm sieve and analyzed for soil total organic C using a LECO C/N analyzer. Active C was determined using a dilute 0.02 M KMnO 4 oxidation procedure described by Weil et al. (2003).Active C was determined using a dilute 0.02 M KMnO 4 oxidation procedure described by Weil et al. (2003). Particulate organic matter C (POM-C) was determined using the wet sieving procedure described by Cambardella and Elliot (1994).Particulate organic matter C (POM-C) was determined using the wet sieving procedure described by Cambardella and Elliot (1994). Near infrared (NIR) spectra of the soils were determined using a portable field spectrometer Fieldspec Pro FR (Analytical Spectral Devices, Inc, Boulder, CO) based on methods detailed in Sudduth and Hummel (1993).Near infrared (NIR) spectra of the soils were determined using a portable field spectrometer Fieldspec Pro FR (Analytical Spectral Devices, Inc, Boulder, CO) based on methods detailed in Sudduth and Hummel (1993). Ratios of labile (O containing) and recalcitrant (R) C functional groups were determined by MIR-DRIFT analysis of HF-treated humic acid fractions separated from the samples (Ding et al., 2002).Ratios of labile (O containing) and recalcitrant (R) C functional groups were determined by MIR-DRIFT analysis of HF-treated humic acid fractions separated from the samples (Ding et al., 2002). Soils were also incubated for 84 days in 150 mL Falcon filter units at -47 kPa soil moisture tension and a constant temperature of 25 ºC. Samples were periodically placed in sealed mason jars and the headspace swept with CO 2 - free air. Changes in head space CO 2 concentration in the head space due to soil CO 2 evolution was then determined using a gas chromatograph (GC) (Buck Scientific Inc., East Norwalk, CT, USA) fitted with a thermal conductivity detector (TCD).Soils were also incubated for 84 days in 150 mL Falcon filter units at -47 kPa soil moisture tension and a constant temperature of 25 ºC. Samples were periodically placed in sealed mason jars and the headspace swept with CO 2 - free air. Changes in head space CO 2 concentration in the head space due to soil CO 2 evolution was then determined using a gas chromatograph (GC) (Buck Scientific Inc., East Norwalk, CT, USA) fitted with a thermal conductivity detector (TCD). Altiplano A. B. C. Umala LakeTiticaca Umala Kellhuiri San José de Llanga San Juan Cerca Vinto Coopani La Paz Based on community surveys, fallow land represents between 68 to 82% of cultivated lands among both lower and upper elevation communities (Table 1). Average time of fallow varied between 3.5 to 7.4 years among the communities and community members perceived that the duration of fallow has generally decreased over the last 10 years due to increased human population and increased forage production.Based on community surveys, fallow land represents between 68 to 82% of cultivated lands among both lower and upper elevation communities (Table 1). Average time of fallow varied between 3.5 to 7.4 years among the communities and community members perceived that the duration of fallow has generally decreased over the last 10 years due to increased human population and increased forage production. Soil total organic C was significantly lower in soils collected from farm fields in the lower elevation communities compared to the upper elevation communities (Table 2). This difference was probably due to the generally higher sand content of the soils in the lower communities and possibly due to the more intensive mechanized tillage used in those communities compared to animal-based tillage in the upper communities.Soil total organic C was significantly lower in soils collected from farm fields in the lower elevation communities compared to the upper elevation communities (Table 2). This difference was probably due to the generally higher sand content of the soils in the lower communities and possibly due to the more intensive mechanized tillage used in those communities compared to animal-based tillage in the upper communities. Figure 2 A-D. Fallow and native vegetation growing during the fallow period has an important role in (A) grazing for sheep, (B & C)) a source of fuel for cooking and (D) soil fertility restoration. A. B. C. D. D. Umala landscape Table 1. Selected characteristics of the lower and upper communities in Umala in 2006/2007. Table 2. Effects of changes in length of fallow period on soil total organic C and soil C fractions in Umala. Figure 3. Changes in A) soil total organic C and B) cumulative CO 2 -C after 84 days of incubation of soils with different times of cropping and fallow. Regression lines show relationship for lower and upper elevation communities. Figure 4. MIR-DRIFT scans and O/R ratios of soils with A) 1 year of fallow and B) 40 years of fallow from Umala. A. B. O/R ratio = 0.69 O/R ratio = 0.79 Figure 5. Predicted KMnO 4 active C of soils from Umala using NIR spectra versus measured values. A. B.