Carbon, nitrogen and natural abundance of δ13C e δ15N of light-fraction organic matter under no-tillage and crop-livestock integration systems

Carbono, nitrogênio e abundância natural de δ15N e δ13C da matéria orgânica leve sob sistemas plantio direto e integração lavoura-pecuária

SOIL

Carbon, nitrogen and natural abundance of δ¹³C e δ¹⁵N of light-fraction organic matter under no-tillage and crop-livestock integration systems

Carbono, nitrogênio e abundância natural de δ ¹⁵N e δ ¹³C da matéria orgânica leve sob sistemas plantio direto e integração lavoura-pecuária

Arcângelo LossI,^*; Marcos Gervasio Pereira^II; Adriano Perin^III; Sidinei Julio Beutler^I; Lúcia Helena Cunha dos Anjos^II

^IPrograma de Pós-graduação em Agronomia, Ciência do Solo, Departamento de Solos, Instituto de Agronomia, Universidade Federal Rural do Rio de Janeiro, BR-465, km 7, 2389000, Seropédica, Rio de Janeiro, Brazil

^IIDepartamento de Solos, Instituto de Agronomia, Universidade Federal Rural do Rio de Janeiro, Seropédica, Rio de Janeiro, Brazil

^IIIInstituto Federal de Educação Ciência e Tecnologia Goiano, Campus Rio Verde, Rio Verde, Goiás, Brazil

ABSTRACT

This study aimed to quantify the light organic matter mass (LOM) in water, free light fraction (FLF) of soil organic matter (SOM) and carbon (C), nitrogen (N) contents and natural abundance of δ¹³C e δ¹⁵N contained in the mass of LOM, C and N in the FLF in land use systems in the Cerrado region of Goiás State, Brazil. Were evaluated a crop-livestock integration system - CLIS (corn+brachiaria/bean/cotton/soybean) and no-tillage system - NTS (sunflower/millet/soybean/corn). The vegetal coverage of natural Cerrado, adjacent to NTS and CLIS, was considered as the original soil condition. Soil samples were collected at depths of 0.0-5.0 and 5.0-10.0 cm in full random experimental design. The CLIS showed the greatest mass and levels of C-LOM (5-10 cm), and too greatest mass and levels of C and N of FLF (0-10 cm) when compared with the NTS. Regarding the Cerrado, the CLIS showed higher values for N-LOM and 1δ15N-LOM (0-5 cm), C and N of FLF (0-5 cm). Through physical indicators LOM and FLF, it was possible to infer that the CLIS increased the SOM fractions when compared to the NTS.

Keywords: Cerrado, brachiaria, plant residues, isotopic composition, physical indicators.

RESUMO

Este trabalho teve como objetivo quantificar a massa da matéria orgânica leve (MOL) em água e da fração leve livre (FLL) da matéria orgânica do solo (MOS) e os respectivos teores de carbono (C), nitrogênio (N) e abundância natural de δ¹⁵N e δ¹³C contidos na massa da MOL, C e N na FLL em sistemas de uso do solo no Cerrado goiano (Cerradão). Foram avaliados um sistema de integração lavoura-pecuária - SILP (milho+braquiária/feijão/algodão/soja) e sistema plantio direto - SPD (girassol/milheto/soja/milho). A cobertura vegetal de Cerrado natural, adjacente ao SPD e SILP, foi considerada como condição original do solo. Coletaram-se amostras de solo nas profundidades de 0-5 e 5-10 cm, em delineamento inteiramente casualizado. O SILP apresentou maior massa e teores de C na MOL (5-10 cm) e, também maior massa e teores de C e N na FLL (0-10 cm) quando comparada com o SPD. Em relação ao Cerrado, o SILP apresentou maiores valores para N-MOL e δ¹⁵N-MOL (0-5 cm), C e N da FLL (0-5 cm). Por meio dos indicadores físicos, MOL e FLL, foi possível inferir que o SILP aumentou essas frações da MOS quando comparado ao SPD.

Palavras-chave: Cerrado, braquiária, resíduos vegetais, composição isotópica, indicadores físicos.

Introduction

The impact of soil management systems on the dynamics of the soil organic matter (SOM), or on the carbon (C) cycle in agroecosystems deserves special attention. Regarding the Cerrado biome, this attention must be double, as this is a region of the agricultural frontier, which stands out as producer of food and energy to meet the domestic needs and also other countries. The soils of this biome are characterized by being highly weathered, with low natural fertility. In addition, the seasonal climate is also a factor, with hot and rainy summer and cold and dry winter, resulting in high rates of plant residues decomposition (RESCK et al., 2008).

Therefore, management systems able to maintain and/or even increase the organic C contents may contribute to maintaining the productive capacity of soils, reducing CO₂ emissions to the atmosphere (CERRI et al., 2010; SIQUEIRA NETO et al., 2011). Thus, efficient and practical methods to assess C dynamics are required in order to create soil management strategies that reduce the impact of agriculture on the environment (FRAZÃO et al., 2010; LOSS et al., 2009a and b; SOUZA et al., 2006).

Studies such as those of Pinheiro et al. (2004), Conceição et al. (2005), Xavier et al. (2006), Maia et al. (2007), Rangel et al. (2008), Loss et al. (2009a and b, 2010, 2011), Pereira et al. (2010), Pillon et al. (2011) and Fontana et al. (2011) have shown that some SOM compartments can more rapidly detect changes in soil C content associated with management. Reductions in these compartments are generally higher than those observed when considering only the total soil organic C content (JANZEN et al., 1992; LOSS et al., 2009b).

Light organic matter (LOM) is an active fraction in soil that consists of partially humified organic residues at various decomposition stages, which has a residence time in soil ranging from 1 to 5 years (JANZEN et al., 1992). LOM is a fraction with size between 0.25 and 2.0 mm and can be quantified by means of flotation of the light material in liquid of density ranging from 1.6 to 2.0 kg L^-1 (SOHI et al., 2001) or in water (ANDERSON; INGRAM, 1989). The LOM in soils is mainly composed of parts of plants and can also contain residues of animals and microorganisms in various decomposition stages. One of the most important activities of soil microorganisms is the organic matter decomposition, releasing nutrients to the soil solution and its absorption by plants (RUIVO et al., 2005). The maintenance of this compartment (LOM), is essential for the sustainability of farming systems, since it represents, in short and medium term, potential for nutrient cycling (COMPTON; BOONE, 2002).

The crop rotation in no-tillage system (NTS) associated with pasture create an integrated production system, which is commonly referred to as crop-livestock integration system (CLIS). These transfer large amounts of plant biomass to the soil, especially when cover plants, such as brachiaria, are used to increase the fodder production, also serving as food source for cattle. Thus, the CLIS is associated to the NTS, creating a more diverse environment compared to the NTS alone, without the use of brachiaria (LOSS et al., 2011).

The CLIS can lead to greater levels of labile fractions (LOM in water and free light fraction - FLF of SOM), when compared to NTS, without the use of brachiaria and pasture. The aim of this study was to quantify the mass of LOM and FLF of SOM and C, N content and natural abundance of δ¹³C e ¹δ¹⁵N in the LOM and C e N of FLF in land use systems in the Cerrado region of Montividiu, Goiás State, Brazil.

Material and methods

The study was conducted at "Vargem Grande" Farm, belonging to the Peeters Agriculture and Livestock, located at Montividiu, Goiás State, Brazil (17º21'S and 51º28'W). The climate is characterized by two well-defined seasons: a dry season (from May to September) and a rainy season (from October to April). For the 2008 crop year (period of soil sample collection), and the mean annual rainfall and temperature are shown in Figure 1.

The soil in the systems evaluated was classified as LatossoloVermelho Distrófico (Oxisol) with clayey texture (EMBRAPA, 2006). In the mineralogical composition of the clay fraction, gibbsite, kaolinite and hematite stand out.

The original vegetal coverage of the soil is the Cerrado field (Cerradão), taken in 1975 to establish pastures (Urochloa decumbens), with continuous use for 10 years, until 1985 (Figure 2).

Then, the systems were managed by means of plowing and harrowing (CTS) until 1991 (7 years) for the cultivation of grains (corn, bean, soybean and sunflower). Later, the NTS was implemented with crop rotation - corn, soybean, cotton, bean - (1991-2008) and, from 1999, part of the NTS was transformed into CLIS (1999-2008). Therefore, the systems evaluated had been conducted in NTS with the same crop rotation, with only NTS (1991-2008) and CLIS (1999-2008) (Figure 2).

The systems evaluated are cultivated in NTS for 17 years: one with crop rotation only (sunflower-millet-soybean-corn), at coordinates 17º21,120'S, 51º29,461'W and altitude of 958 m, and another with brachiaria (U. ruziziensis) intercropped with corn to increase the fodder production in the dry season (corn-brachiaria-bean-cotton-soybean), at coordinates 17º21,854'S, 51º28,599'W; and altitude of 859 m. The vegetal coverage of natural Cerrado, adjacent to cultivated systems (17º26,642'S, 51º22,522' W and altitude of 951 m) was considered as the original soil condition.

In the CLIS (corn-brachiaria-bean-cotton-soybean) was used corn and brachiaria being planted simultaneously (brachiaria between corn rows). After the corn harvest, the introduction of cattle in the system was made (2.0 ha per AU; AU - animal unit) for 90 days (July-September). After removal of the cattle, only brachiaria clumps remained. Then, after the first rains, brachiaria was fertilized with 200 kg ha^-1 of N P K formulation (20-00-20) in the first half of September. After regrowth, when the soil was completely covered by the grass, desiccation was made and beans were grown.

Liming was made in the CLIS in July 2005 using a dose of 3.60 Mg ha^-1 of dolomitic limestone, with PRNT = 70%, to raise the base saturation to 70%. The same procedure was performed for the NTS, also in July 2005, using a dose of 2.90 Mg ha^-1 of dolomitic limestone, with PRNT = 70%, to raise the base saturation to 60%. The sequence of crops and fertilizers used in both systems from 2002 to 2008 is described in Table 1.

The sytems were planted with sunflower in NTS and corn + brachiaria in CLIS at the time of soil sample collection for evaluation. These collections were held in March 2008, being bounded an area around 600 m², where four trenches were opened across the plant rows in each system. Layers of 0.0 to 5.0 and 5.0 to 10.0 cm were sampled, with three single samples collected in each trench to form a composite sample. Totaling four replications in each land use system. After collection, the samples were identified, placed in plastic bags and transported to the laboratory, air dried and passed through 2 mm sieve to obtain the air-dried fine soil (ADFS), which was used for the quantification of LOM and FLF of SOM.

For the quantification of LOM, 50 g of ADFS were weighed and placed in a 250 mL beaker added of 100 mL of NaOH 0.1 mol L^-1, leaving to rest overnight. After this period, the suspension was stirred with glass rod and the material was passed through 0.25 mm sieve, discarding the silt and clay fraction (ANDERSON; INGRAM, 1989).

Subsequently, the material retained on the sieve (LOM and sand) was quantitatively transferred to the beaker and filled up with water. The floated material was passed through 0.25 mm sieve, taking care to separate the LOM from the sand fraction. Then, water was added to the beaker, manually stirring to resuspend the remaining LOM and slowly pour the material through a 0.25 mm sieve. This operation was repeated until all the material that floated with the stirring in water was removed. The material retained in the sieve (LOM) was transferred to aluminum containers (previously weighed), led to an oven at 65ºC until constant weight was reached (72h), the whole material being weighted. Then, the LOM mass was obtained to determine the C and N contents in the LOM mass through the dry combustion method in a C and N autoanalyzer at 900ºC (CHN-600 Carlo Erba EA-1110, Italy). The δ¹⁵N and δ¹³C values were evaluated using a mass spectrometer (Finnigan Mat Delta Plus, Germany) in the CENA Laboratory of Isotopic Ecology (Centre for Nuclear Energy in Agriculture, Piracicaba, São Paulo State).

The FLF of SOM was obtained according to recommendations described in Sohi et al. (2001) and Machado (2002). Centrifuge bottles of 50 mL were added of 5 g ADFS and 35 mL of sodium iodide solution (NaI) with density of 1.8 (± 0.1 g cm^-3). The bottles containing the mixture were manually and slowly shaken for 30 seconds in order to disperse unstable aggregates and allow the FLF flotation in NaI solution and, being then centrifuged at 8000 x g for 30 min. After centrifugation, the FLF on the surface of the NaI solution was aspirated and filtered through a vacuum system using a glass fiber filter with 47 mm in diameter and 2 µm of retention. The retained FLF was carefully washed with distilled water in order to remove excess NaI, being then taken to an oven with forced hot air circulation at 65ºC for 72h.

To determine the FLF mass, only the filter was weighed, and after complete drying, the sample was weighed along with the filter. Thus, by subtracting the weight of the filter {(FLF mass = (Mass of filter plus sample) - weight of the filter)}, obtaining the FLF mass, in grams. Subsequently, the C and N contents in the FLF mass were determined through the dry combustion method in a C and N autoanalyzer at 900ºC (CHN-600 Carlo Erba EA-1110, Italy).

The results were analyzed for normality and homogeneity of data by means of the Lilliefors and Cochran and Barttlet tests, respectively. Later, it was analyzed as a completely randomized design with three land use systems (NTS, CLIS and Cerrado) with four replications. The land use systems were evaluated under the same soil climatic and topographic conditions, differing only in the land use system. The results were submitted to analysis of variance with application of the F test and the mean values, when significant, were compared by LSD-student test at 5%.

Results and discussion

Light-fraction organic matter in water

The highest LOM mass values were found in the Cerrado at depth from 0.0 to 5.0 cm, with no differences between NTS and CLIS. For the depth from 5.0 to 10.0 cm, the CLIS showed values equal to those of the Cerrado, with lower LOM value for the NTS (Table 2).

These results (Table 2) demonstrate that there is a greater amount of plant material (litter) in the Cerrado when compared with the cultivated systems (0.0-5.0 cm). For the depth from 5.0 to 10.0 cm, between the cultivated systems, CLIS showed higher LOM mass values when compared with the NTS. These differences are due to the higher amount of plant residues in the subsurface through rizo-deposition and renewal of the root system of brachiaria in the CLIS. This contribution when compared with the amount of litter in the Cerrado was more efficient than in the NTS (with millet) in increasing the soil LOM levels. These results are corroborated by those of Pereira et al. (2010), who found higher LOM levels in NTS (0.0-2.5 cm) cultivated with corn and soybean on brachiaria residues when compared with the cultivation of these crops on millet and crotalaria residues in the Cerrado region of Uberaba, Minas Gerais State, Brazil. For other depths studied (2.5-5.0 and 5.0-10.0 cm), no differences were observed between the systems evaluated (PEREIRA et al., 2010).

In relation to the C-LOM levels, higher values were observed in the Cerrado at the depth of 0.0-5.0 cm, and values equal to the CLIS at the depth of 5.0-10.0 cm. Among the cultivated, only the depth of 5.0-10.0 cm showed higher C-LOM levels for the CLIS. These results showed the same pattern for the LOM mass contents, indicating that for the CLIS, which has higher LOM mass amounts, there is a greater supply of substrate used as source of energy for microbial growth. This fact will lead to greater release of nutrients through cycling of microbial biomass, resulting in increased levels of C-LOM, total soil C and macronutrients (N, P, K and Ca), which are significantly higher in the CLIS (LOSS et al., 2011).

For the N-LOM at the depth of 0.0-5.0 cm, no differences were found (F test, p < 0.05) and at the depth of 5.0-10.0 cm, the cultivated systems showed higher N-LOM contents compared with the Cerrado. The increased N-LOM levels of cultivated systems may be due to the deposition of plant material with higher N concentrations from the decomposition of brachiaria (CLIS) and millet (NTS) root system, which can extract soil nutrients at greater depths, being subsequently released on the depth 5.0–10.0 cm through rizo-deposition.

For the C/N ratio, the Cerrado showed the highest values, followed by the CLIS at the depth of 5.0-10.0 cm. The highest C/N ratios in the CLIS reflect the quality of the residue, in this case, brachiaria. The highest C/N ratios in the Cerrado may be due to the lower decomposition of plant residues, which may have higher proportions of phenolic compounds and cellulose, with lower N concentrations (PILLON et al., 2011).

The δ¹³C-LOM values reflect the origin vegetation, being well characterized in the Cerrado, with lower isotopic values, with the sign from -29.01 to -28.27‰, and in the NTS, with intermediate values from -24.06 to -24.40‰, being these values were corresponding to the 0.0–5.0 and 5.0-10.0 cm, respectively. Where for C₃ plants, the isotopic composition (δ¹³C) ranges from -24 to -34‰, whereas for C₄ plants, the composition in δ¹³C ranges from -6 to -19‰ (Smith and Epstein, 1971). Therefore, in the Cerrado, the plant material is only from C₃ plants, while in the NTS and CLIS, there is a mixture of plants, where the contribution of C₄ plants is clearly observed, especially millet and corn in the NTS and corn + brachiaria in the CLIS. In relation to δ¹³C-LOM, the CLIS showed the highest δ¹³C-LOM values, with sign ranging from -21.98 to -22.64, which allows inferences about greater contribution of C₄ plants in the CLIS (corn + brachiaria) compared to the NTS.

The natural abundance of δ¹⁵N-LOM showed small values in all systems and at both depths, which is a pattern resulting from the lower SOM decomposition (PICCOLO et al., 1996). In general, as the SOM decomposition and soil depth increase, higher δ¹⁵N values are observed due to the N fractionation through nitrification and denitrification processes (BUSTAMANTE et al., 2004). However, comparing the δ¹⁵N-LOM values between systems, it was found that the CLIS had the highest values, which may be related to N fertilization of cultures and brachiaria as well as the planting of legumes such as soybeans and beans (Table 1).

FLF of SOM

The Cerrado showed the highest FLF at the layer of 0.0 to -5.0 cm, and values similar to those observed in the CLIS at the layer of 5.0-10.0 cm (Table 3). Among the systems under cultivation, the CLIS showed higher FLF mass values when compared to the NTS, at both layers assessed. The higher FLF mass values in the Cerrado can be attributed to the absence of human activity and to the presence of larger amounts of plant residues (litter). While among the cultivated systems in the CLIS, the planting of brachiaria (phytomass and root system) resulted in higher FLF mass values when compared with the NTS, in the absence of fodder.

A similar study was carried out by Assis et al. (2006), who evaluated the organic compartments of a medium-textured Oxisol in Capinópolis, Minas Gerais State, Brazil, with different land uses and managements. The treatments consisted of NTS for 4 years in corn (silage)/soybean succession; NTS for 4 years with corn/corn/corn/soybean succession; NTS for 3 years followed by Tifton (hay) and soybean in the last year, about 30 years with conventional tillage (corn/soybean), with only soybean in the last four years, and native forest as reference. The depths evaluated were: 0.0-5.0; 5.0-10.0 and 10.0-20.0 cm. The authors observed that the native forest showed higher FLF mass values at all depths probably due to the greater amount of organic residues. The same result was found in this study, in which the Cerrado showed the highest FLF mass values at both depths studied, except for 5.0-10.0 cm, where the Cerrado had values equal to the CLIS.

The larger amounts of FLF mass in the CLIS may be related to the root system of brachiaria (at the time of sample collection, by opening a narrow channel up to 40.0 cm, a good root system distribution of brachiaria was observed). It was found that brachiaria presented good root development, which could reach greater depths. After the death of these roots, all this material remains in the soil profile, thereby raising the SOM levels in various forms, in this case, in the form of FLF. A similar pattern was also observed in the study by Assis et al. (2006), in which the author found that the NTS with the presence of Tifton showed higher FLF values at depths (5.0-10.0 and 10.0-20.0 cm) when compared to other NTS. This result justifies the potential of intercropping and/or rotating forage grasses with Tifton and also brachiaria, for the management of agricultural systems, such as CLIS.

The highest C-FLF levels were found in the CLIS at the layer of 0.0-5.0 cm, and values equal to those of the Cerrado at layer from 5.0 to 10.0 cm. In the topsoil, the cultivated systems showed higher C-FLF levels when compared to Cerrado. The higher C-FLF levels found in cultivated systems in the surface layer are the result of the use of C₄ plants (grasses), while in the Cerrado, only C₃ plants were found. Among the systems under cultivation, the CLIS, for having brachiaria intercropped with corn (both C₄ at the time of sample collection), has higher C-FLF levels when compared to the NTS, which despite having millet and corn rotation, also C₄, differs from the CLIS at the time of sample collection. At this time (collection of samples for FLF analysis), only effect of sunflower cultivation was observed as dominant in the NTS. Among the cultivated systems, the highest C-FLF levels observed on the surface and subsurface of the CLIS reinforce the contribution of the root systems of forage species (grasses) in these systems on subsurface and/or SOM redistribution from surface layers to deeper layers (PILLON et al., 2011).

For the N-FLF contents, a dynamics similar to that of C-FLF was found, with lower N-FLF levels of in the Cerrado at the surface layer, and from 5.0-10.0 cm, the Cerrado did not differ from the NTS, with the highest N-FLF contents being found in the CLIS. The lower N-FLF contents found in the Cerrado may be due to the lower decomposition level of plant residues, which may show higher proportions of phenolic compounds and cellulose, with lower N concentrations. At deeper layers, the oxidation of organic compounds by microorganisms and by-products of microbial decomposition contribute to the higher N-FLF contents (PILLON et al., 2011), which are equal to those of the NTS.

The greater N-FLF contents in the CLIS can be explained by the great capacity of brachiaria of absorbing and accumulating this nutrient (CRUSCIOL; BORGHI, 2007), which is released into the soil from the decomposition of its fodder (plant residues that are quantified as FLF). The use of legumes and N fertilizer used in CLIS must be added to these results (Table 1).

Conclusion

The CLIS increased the mass and the C content of the LOM in water and the FLF of SOM compared to the NTS. Regarding the Cerrado, the CLIS showed equal mass and C and N values of LOM (5-10 cm) and higher C and N values of FLF (0-5 cm).

Acknowledgements

To the Agrisus Foundation for funding the research project, to CNPq and FAPERJ for granting the PhD scholarship to the first author, to the Post-graduation Program in Agronomy - Soil Science (CPGA-CS) for their assistance and to Dr. Adriano Perin and colleagues for their help in collecting soil samples for this study.

Received on September 26, 2011.

Accepted on January 21, 2012.

License information: This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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Publication Dates

• Publication in this collection
  01 Nov 2012
• Date of issue
  Dec 2012

History

• Received
  26 Sept 2011
• Accepted
  21 Jan 2012