Deforestation of tropical forests for the establishment of grass monoculture for livestock production is responsible for about 30 % of CO 2 emissions. This issue is particularly severe in degraded pastures because degraded soils favor CO 2 flow to the soil surface. Silvopastoral systems could reduce CO 2 emissions, notably by using live fences. Here, we hypothesized that live fences of Gliricidia sepium in livestock systems should reduce variations in environmental relative humidity and soil temperature and, in turn, reduce soil CO 2 emissions. Here, we studied two livestock systems: (1) grass monoculture of Brachiaria decumbens with live fences of G. sepium and (2) grass monoculture of B. decumbens without live fences. We measured soil CO 2 seasonal emissions at different times of the day, soil temperature, and environmental relative humidity. Nine 600-m 2 plots were established in each system. All variables were measured over four 6-h period during a 24-h period, twice a month from April to September. Our results show that soil CO 2 emissions showed less variability with G. septum live fences than without live fences. This lower variability is explained by the creation of a microclimate with a higher and more stable environmental relative humidity, provided by the shade of trees. Results also show, however, that global soil CO 2 emissions did not differ between the two systems, with and without live fence. Moreover, soil CO 2 emissions varied according to season, as shown by 1.082 g CO 2 m −2 h −1 in the wet season versus 0.871 g CO 2 m −2 h −1 in the dry season. Soil CO 2 emissions varied also according to sampling time, as shown by 1.116 g CO 2 m −2 h −1 in the night versus 0.960 CO 2 m −2 h −1 in the morning.

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G. Villanueva-López, P. Martínez-Zurimendi, L. Ramírez-Avilés, F. Casanova-Lugo, and A. Jarquín-Sánchez. 2014. Influence of livestock systems with live fences of Gliricidia sepium on several soil properties in Tabasco, Mexico. Cien Inv. Agr. 41(2): 175-186. The aim of the current study was to evaluate the effects of two livestock systems, a livestock system with live fences (LSLF) of Gliricidia sepium associated with signal grass (Brachiaria decumbens) and a livestock system based on a grass monoculture (LSPM), on specific physical and chemical soil characteristics at different depths and distances from the fence. In each system, we randomly selected 9 plots of 600 m 2. A completely randomized design was used with a 2 x 3 factorial arrangement in which we analyzed the influence of the livestock systems (LSLF and LSPM), soil strata (0-10, 10-20 and 20-30 cm) and the interaction of both factors using a multivariate analysis of variance. In addition, we performed analysis of variance to determine the effect of distance sampling in the LSLF (0-3, 3-6 and 6-9 m). The LSLFs were associated with higher (P≤0.05) soil organic matter (OM), carbon (C) and nitrogen (N) content as well as lower pH and bulk density (BD) when compared with the LSPM. In both livestock systems, the soil OM, C and N were higher (P≤0.05) in the upper (0-10 cm) strata and in the LSLF at a 3 to 6 m distance from the live fences. In the LSLF soil, the pH and BD were similar (P>0.05) at different depths and distances from the live fences. However, the soil pH varied between soil depths in the LSPM. Regarding the physical soil properties, only the sand and clay content varied (P≤0.05) at different depths in both systems but not at different distances from the LSLF. We concluded that the LSLF presents high potential to substantially improve the physical and chemical soil properties and provide an important option for reducing soil degradation in future in livestock production systems based on pasture monoculture.

Live fences have the potential to improve microclimatic conditions, moderate soil CO2 fluxes and function as carbon sinks. We quantified variation in soil CO2 fluxes from livestock silvopastoral systems under the canopies of live fences (LF), formed by Gliricidia sepium trees, or artificial fences (AF). We determined the responses of soil CO2 fluxes to environmental factors, including diurnal and seasonal variations in temperature and relative humidity in each fencing system. Measurements were made from April to June (dry season) and from July to September (rainy season), 2012. Fluxes were similar between the two livestock systems; LF emitted 1.00 μmol CO2/m2/s and AF 1.02 μmol CO2/m2/s. Soil temperatures at 5 cm depth were 3% warmer in AF than in LF, and relative humidity was 16% greater in LF than in AF. Seasonal variation in temperature greatly affected soil CO2 fluxes, which changed seasonally in parallel with temperature of the topsoil and relative humidity at 1 m height, peaking in late summer. Fluxes in LF and AF were greater in the rainy season (1.1 μmol CO2/m2/s, for both systems), when soil temperature was cooler and relative humidity was greatest, than during the dry season (0.9 μmol CO2/m2/s, for both systems). Soil fluxes were larger at night (00:00–06:00 h), when soil temperature was cooler and relative humidity greater, than during the morning (6:00–12:00 h), when soil temperature was warmer and relative humidity was less. The presence of G. sepium trees in LF did not influence soil CO2 fluxes.

Agroforestry systems (AFS) play a major role in the sequestration of carbon (C). The objectives of this study were to quantify the organic C stocks in the above-and below-ground tree biomass and in the soil in a cattle-farming system with live fences (CFSLF) of Gliricidia sepium and to compare the levels with those of a cattle-farming system based on a grass monoculture (CFSGM). The methodology included a forest inventory in nine randomly assigned plots and the destructive sampling of G. sepium 32 trees, measuring for each tree the diameter at breast height (DBH), stem height, total tree height, branch weight, leaf weight and coarse root weight. In addition, we measured grass biomass, collected litterfall and collected soil samples at depths of 0-10, 10-20 and 20-30 cm in the plots. A logarithmic model was developed to quantify the above-and below-ground tree biomass. The soil organic matter was determined by the dry combustion method. The total carbon stored in the CFSLF was 119.82 Mg C ha -1 , with the G. sepium trees contributing 5.7 % of the total C (6.48 Mg C ha -1 ). The CFSGM stored 113.34 Mg C ha -1 . The grass biomass stored 15.32 Mg C ha -1 year -1 in the CFSGM and 15.68 Mg C ha -1 year -1 in the CFSLF, and the litterfall in the CFSLF stored 0.205 Mg C ha -1 year -1 . Despite the modest contribution of G. sepium trees to the C storage, the total carbon accumulated in the CFSLF and CFSGM was similar.

Land use change from forests to grazing lands is one of the important sources of greenhouse gas emissions in many parts of the tropics. The objective of this study was to analyze the extent of soil organic carbon (SOC) loss from the conversion of native forests to pasturelands in Mexico. We analyzed 66 sets of published research data with simultaneous measurements of soil organic carbon stocks between native forests and pasturelands in Mexico. We used a generalized linear mixed effect model to evaluate the effect of land use change (forest versus pasture), soil depth, and original native forest types. The model showed that there was a significant reduction in SOC stocks due to the conversion of native forests to pasturelands. The median loss of SOC ranged from 31.6% to 52.0% depending upon the soil depth. The highest loss was observed in tropical mangrove forests followed by highland tropical forests and humid tropical forests. Higher loss was detected in upper soil horizon (0–30 cm) compared to deeper horizons. The emissions of CO2 from SOC loss ranged from 46.7 to 165.5 Mg CO2 eq. ha−1 depending upon the type of original native forests. In this paper, we also discuss the effect that agroforestry practices such as silvopastoral arrangements and other management practices like rotational grazing, soil erosion control, and soil nutrient management can have in enhancing SOC stocks in tropical grasslands. The results on the degree of carbon loss can have strong implications in adopting appropriate management decisions that recover or retain carbon stocks in biomass and soils of tropical livestock production systems.

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