Soil carbon pools in different pasture systems

The aim of this study was to assess the carbon pools of a tropical soil where the native forest was replaced with different pasture systems. We studied five pasture production systems, including four monoculture systems with forage grasses such as Andropogon, Brachiaria, Panicum, and Cynodon, and an agroforestry system as well as a native vegetation plot. Greater availability of fulvic acid was detected in the agroforestry system as compared with that in the other systems. Higher lability of C was detected in the Andropogon system during the dry and rainy seasons and during the dry season in Cynodon. During the dry season, all pastures systems showed deficits in the net removal of atmospheric CO2. The structure and practices of the agroforestry system enables more carbon to be sequestered in the soil as compared with the monoculture pasture, suggesting that it is an important practice to mitigate climatic change and to improve soil quality. Additional key words: humic substances; carbon management; agroforestry system Abbreviations used: AE (alkaline extract); AFS (agroforestry system); AND (Andropogon); BRA (Brachiaria); C-FAF (C-fulvic acid fraction); C-HAF (C-humic acid fraction); C-HF (C-humin fraction); CMI (carbon management index); CPI (carbon pool index); CYN (Cynodon); HI (humification index); LC (labile organic carbon); L (lability); LI (lability index); NLC (non-labile organic carbon); NV (native vegetation); PAN (Panicum); SOM (soil organic matter); TOC (total organic carbon) Citation: Cardozo, F. M. Jr.; Carneiro, R. F. V.; Leite, L. F. C.; Araujo, A. S. F. (2016). Short communication: Soil carbon pools in different pasture systems. Spanish Journal of Agricultural Research, Volume 14, Issue 1, e11SC01. http://dx.doi.org/10.5424/ sjar/2016141-7939. Received: 29 Apr 2015. Accepted: 08 Feb 2016 Copyright © 2016 INIA. This is an open access article distributed under the terms of the Creative Commons Attribution-Non Commercial (by-nc) Spain 3.0 Licence, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Funding: The authors received no specific funding for this work Competing interests: The authors have declared that no competing interests exist. Correspondence should be addressed to Ademir Araujo: asfaruaj@yahoo.com.br


Introduction
The pasture ecosystem is characterized by interactions between plants, animals, soil, climate, and management practices implemented by the farmer. Meat and milk productions are important economic activities in Brazil, and the sustainable practices of these production systems contribute to efficient carbon cycling, thereby improving soil quality (Carvalho et al., 2014;Soussana & Lemaire, 2014). Soils are a carbon pool for terrestrial ecosystems; thus, they are relevant with regard to environmental problems associated with global warming and deforestation (Bao et al., 2015).
Conversion from native forest to livestock grazing has decreased soil C storage (Carvalho et al., 2014) and has been the most common land use change in Brazil (Sousanna & Lemaire, 2014). It is estimated that Brazil releases more than 1.090 Mt CO 2 /year into the 2 ing to different pasture system, we hypothetized that the management method applied for different pasture systems could influence the soil carbon pool. In this context, the objective of this study was to assess the C pools of a tropical soil where the native forest was replaced with different pasture systems.
Soil sampling was carried out in March (rainy season) and September (dry season) 2014. Soil samples were obtained from three transects from each plot (three points per transect) at a depth of 0-20 cm. The soil samples were ground and passed through a 0.21mm sieve to determine TOC by wet combustion using a mixture of potassium dichromate and sulfuric acid under heating (Yeomans & Bremmer, 1988). Labile organic carbon (LC) was quantified by wet oxidation with 0.33 M KMnO 4 , as described by Blair et al. (1995). Non-labile carbon (NLC), that is equivalent to non-oxidized carbon by KMnO 4 , was calculated as a difference (NLC = TOC -LC). Based on the difference between TOC-forest (reference) and TOC systems, a carbon pool index was created (CPI) and calculated as CPI = TOC-system/TOC-forest.
According to changes in the proportion of LC (i.e., L = LC/NLC) in the soil, a lability index (LI) was calculated as LI = L system/L reference. These two indices were used to calculate the carbon management index (CMI) using the following expression: CMI = CPI × LI × 100 (Blair et al., 1995). C-CO 2 emission or sequestration rate was estimated for 0-20 cm depth, using native vegetation as a reference (TOC stocks native vegetation -TOC stocks management systems/number of years). A conversion factor of C to CO 2 of 3.67 (molar mass of CO 2 /molar mass of C) was used.
Soil humic substances (humic acids, fulvic acids, and humin) were extracted and fractionated using the method recommended by the International Humic Substances Society, as described by Swift (1996). The carbon content of the fulvic acid (C-FAF), humic acid (C-HAF), and humin (C-HF) fractions was measured using the dichromate oxidation method (Yeomans & Bremmer, 1988). The ratios of C-HAF by C-FAF and the alkali soluble fractions (C-FAF + C-HAF = AE) by H (AE/C-HF) were calculated to characterize the humified fraction of soil organic matter (SOM). Additionally, the humification index (HI) was calculated using the following formula to estimate the proportion of humified organic matter in relation to TOC content: HI = (C-FAF + C-HAF + C-HF) / TOC × 100.
Data were analyzed using one-way analysis of variance, and means were compared using the least significant difference (LSD) values calculated at the 5% level of significance. All analyses were performed using STATISTICA 7.1 (StatSoft).

Results and discussion
The C-HAF was lower than the C-FAF during the wet and dry seasons in all systems (Table 1). The lower C-HAF found in all systems may have occurred because this humic fraction can easily migrate in soil with high porosity (Martins et al., 2009). Therefore, in the evaluated areas the soil porosity may facilitate the movement of C-HAF through the soil horizons. The C-HAF values in plots under Brachiaria, Panicum and Cynodon were similar to those in native vegetation during the dry season. Greater C-FAF availability was detected in the agroforestry system during the rainy and dry seasons because this plot showed higher TOC and Ca +2 content, which is a favorable condition for the complexation of this C pool in the soil (Barros et al.,3 Short communication: Soil carbon pools in different pasture systems trees in the agroforestry system may indicate a high potential for nutrient cycling and increased fertility. No significant difference in the HA:FA ratio was observed between areas. The ratio (HAF + FAF)/HF in the native vegetation (rainy season) and agroforestry system (dry season) was higher than in the pasture systems, indicating that these systems contain more chemically stable organic C, for which the turnover time is approximately 2,000 years (Chan et al., 2001). The humification index did not differ between systems, which was likely because of the low proportion of labile C found in all plots. These results are indicative of soils with low organic matter input; however, with potential to stimulate the microbial growth in these ecosystems because of the complexity of their organic molecules (Bausenwein et al., 2008).
Higher lability of C was detected in the soil with Andropogon during the dry and rainy seasons, whereas soil with Cynodon showed higher lability of C during the dry season (Table 2). All plots showed a 2012). The high carbon content from FAF in this system may have stimulated microbial diversity because that fraction is more easily used as an energy source by soil microorganisms, generating more negative charges and improving nutrient cycling and ecosystem productivity (Moraes et al., 2011).
The C-HF was higher than the C-HAF and C-FAF in all evaluated plots during both seasons, suggesting a greater stability of humin to mineralization than of humic and fulvic acids (Barros et al., 2012). The C-HF was higher in the native vegetation (dry season) and agroforestry system (rainy season), indicating the presence of more stable humus, low degradation, and strong stimulus to soil microbial activity (Barros et al., 2012). The presence of lignin derived from plant residues increased also the humin in soil (Carvalho et al., 2014). This increase in humin in soil is resulting from the loss of oxidative C and, at the same time, an increase in the C stable (Moraes et al., 2011). Therefore, the permanent inputs of organic C from herbaceous plants and Table 1. Carbon content from humic acid (HAF), fulvic acid (FAF), and humin (HF) fractions, HAF:FAF ratio (HAF/FAF), alkaline extract and HF ratio (HAF+FAF/HF), and humification index (HI) under different pastures and during two seasons (rainy and dry). AND -Andropogon gayanus; BRA -Brachiaria brizantha; PAN -Panicum maximum; CYN -Cynodon dactilon; AFS -Agroforestry system; NV -native vegetation. Means followed by the same letter in each row and season do not differ statistically from each other at p < 0.05 (LSD test).  Means followed by the same letter in each row and season do not differ statistically from each other at p < 0.05 (LSD test). 4 CMI < 100 (Table 2), indicating high plant input and minimal soil disturbance (Leite et al., 2014). According to Blair et al. (1995), a CMI < 100 indicates a strong negative impact of management practices on a soil ecosystem. Specifically, the agroforestry system showed a higher CMI than in the Brachiaria, Panicum, and Cynodon. The absence of trees and different pasture management practices reduce the C management index over time, reflecting a decrease in the potential to restore pre-existing carbon stocks (Leite et al., 2014).

Rainy season
The agroforestry system showed highest carbon stock values and high C sequestration rate for the rainy and dry seasons, whereas Brachiaria, Panicum, and Cynodon showed the lowest C stock and a high C loss (Fig. 1). It means that in multicropping systems, such as agroforestry system, the presence of pioneer tree species and the lesser removal of plant residues contribute significantly to mitigate the loss of C and decrease greenhouse gas emissions. Also, agroforestry systems have higher potential to build up and sequester C in soils because of the increased rates of organic matter addition and retention (Lenka et al., 2012).
In conclusion, conversion of native vegetation to pasture system causes changes in C pools, increasing CO 2 emissions into the atmosphere. The agroforestry system has the potential to sequester more carbon in the soil than the pasture system, and it may be an alternative to produce forage for animal production.