Role of Crop Rotations in the Dynamic of Soil Organic Matter Pools

Soil organic matter is considered a key attribute for a sustainable agricultural production and is influenced by the quantity and quality of the crop residue deposited on the soil surface. Therefore, different crop rotations could change the soil organic matter pools. The objectives of this study were to evaluate the soil carbon pools obtained by chemical and physical fractionation methods and the humification index under different crop rotations in a no-till system. We test the following hypothesis: a) the distribution of C and N among the soil organic matter fractions depends on plant species rotation schemes and; b) labile fractions are more sensitive to the input of crop residues and therefore, more suitable for evaluating the impact of different crop rotations in the soil organic matter quality. We evaluated four crop sequences (corn/corn/corn; corn/wheat/corn; soybean/wheat/corn and soybean/corn/corn) in a no-till system. A five-year reforested area was used as reference. We determined the total C and N contents, the mineral-associated C and N, the light fraction of C and N, the labile carbon extracted with KMnO4 and the soil organic matter humification index. We found narrow differences between the crop rotation systems in the total C and N levels, the mineral-associated C and N fractions and the labile C extracted with KMnO4. The diversification of the agricultural system with soybean in crop rotation favored the accumulation of light fraction C and N in the soil that were more efficient to provide information about the changes in the soil organic matter quality.


Introduction
Soil organic matter (SOM) is a complex mix of plant and animal residues in various stages of decomposition and synthesis that plays an essential role in the chemical, physical and biological processes relate to soil ecosystem functions.For this reason, maintaining the SOM status is particularly important to preserve the capacity of soils sustain plant production (Favoretto et al., 2008).
Due to its heterogeneous nature, a separation of SOM into pools that differ in stability may be useful to understanding the environment functionality of the soil organic carbon (SOC) (Strosser, 2010).Changes in the crop management system altered the quantity and quality of organic residue input in an agricultural soil (Mujuru et al., 2013).Therefore, different crop rotations associated with a no-tillage system may affect the size, cycling and distribution of carbon and nitrogen among the SOM fractions.
Soil is one of the main carbon compartments of the planet.Thus, changes in the total SOC contents due to the agricultural management occur slowly due to the high background C in soil (Blair et al., 1995).However, labile fractions tend to respond faster to the crop management changes and by driving crucial soil processes, such as nutrient cycling and biological activity, these fractions can significantly influence the quality of an agricultural land (Chen et al., 2012;Duval et al., 2013).
Labile carbon is the fraction of SOM that is easily decomposed and could be estimate by the chemical oxidation with KMnO 4 (Blair et al., 1995) and by physical fractionation methods (Cambardella & Elliott, 1992).The light fraction is a labile pool of C, which consists of organic materials in various stages of decay, but no having undergone a complete transformation and still retains characteristics of the source material (Ramnarine et al., 2014).In comparison, the mineral-associated fraction is composed of a highly decomposed organic material that is chemically stabilized, therefore, is more resistant to changes (Junior et al., 2014).
Another way to characterize SOM is through the humification index (Milori et al., 2006) that can provide information about the stability and its availability of C to the soil microorganisms.High rates of straw deposition on the soil surface in no-tillage can increase the labile forms of carbon and decrease the humification index of the organic matter in the superficial layers (Segnini et al., 2013;Martins et al., 2011).Although no-tillage is a conservationist agricultural system widely adopted, many farmers do not use an adequate crop rotation, which may compromise the ability of this system in improving and conserve the soil quality.
Hence, the objectives of this study were to evaluate the soil carbon pools isolated by chemical and physical methods and the humification index of SOM in different crop rotations.We test the following hypothesis: a) the distribution of C and N among the SOM fractions depends on plant species rotation schemes and; b) labile fractions are more sensitive to the input of crop residues and therefore more suitable for evaluating the impact of different crop rotations in the soil organic matter quality.We hope that the distribution of carbon and nitrogen among the SOM pools can provide information about how different management systems influence the quality of agricultural soils.

Study Site
The field trial was established at the research station of the Sao Paulo Agency of Agribusiness Technology, city of Capao Bonito, Sao Paulo State, Brazil (aprox.coordinates 24°00′14″S and 48°20′21″W) from September 2010 to September 2015.The soil was classified as a clayey Ferralsol (FAO, 1988).The mean annual temperature is 18.6 °C and the mean annual rainfall is 1241 mm.

Processing of Yield Data
Grain yield of corn (Zea mays L.), soybean (Glycine max L.) and wheat (Triticum aestivum L.) were collected at each plot for 5 years (10 harvests).Productivity of every plot was expressed by standardization of yield data as follows: Where, RCY is the relative cumulative yield of plot p (a value between 0 and 1), Y p is the yield of parcel p (t ha -1 ) and Y max is the maximum yield, considering the crop of the plot p, on the total research site over all plots (t ha -1 ).

Soil Sampling and Analyses
Soil sampling was performance in October 2006 after harvest at 0-10, 10-20, 20-30 and 30-40 cm soil layers.Soil samples were air-drying, and visible crop residues were removed by handpicking before passing the soil through a 2-mm sieve to obtain the fine-earth fraction used for soil analyses.
The total soil organic carbon (TOC) and total nitrogen (TN) was determined by dry combustion, using an elemental analyzer LECO-CN.Labile carbon (C KMnO4 ) was determined by chemical oxidation with KMnO 4 according to Blair et al. (1995).For physical fractionation, 10 g of soil sample and 30 mL of Na hexametaphosphate solution (5 g L -1 ) were added to a 50 ml centrifuge tubes and horizontally shaken for 15 h (200 rpm).The soil suspension was passed through a 0.053 mm mesh was rinsed with a weak stream of distilled water, dried at 50 °C, ground and analyzed for mineral-associated fraction carbon (MFC) and mineral-associated Cultivation changed the amount and the quality of crop residues input to the soil and allowed a faster oxidation of the SOM compared to the natural vegetation sites (Mujuru et al., 2013;Junior et al., 2014).Depending on the agricultural management system carried out, the decrease of SOM can, therefore, reduce soil fertility, negatively impact soil structure, lowers water holding capacity and consequently lead to a decrease in crop yield (Islam et al., 2015;Lal, 2015;Vanhie et al., 2015).As observed by the high correlation (R 2 = 0.95) (Table 2) between total C and N contents, the both follows a similar pattern.Thus, the maintenance of SOM levels could increase the N supply in soil, which could enhance the crop productivity.Due to the importance of the organic matter, soil organic carbon is considered a key attribute to evaluate how management practices affect soil quality (Favoretto et al., 2008).However, total C and N contents changed slightly between the crop rotations.These results indicate that five years were not sufficient for the crop rotation systems to promote a significant increase in TOC content when compared with monoculture and grasses succession systems (CCC and CWC treatments).Therefore, the evaluation of different compartments of the SOM can provide an insight into how the management systems adopted affect the dynamics of soil carbon.
Regarding the carbon and nitrogen fractions analyzed in this study, we observed that the amounts of MFC and MFN in the soil surface (0-10 cm) were higher in the reference site compared to the cultivated area (Figure 2).For MFN in the 30-40 cm depth, the rotation CWC resulted in higher values than the reference area (Figure 3).Reference site also had a higher labile carbon (C KMnO4 ) content compared to the all rotations, except for the 30-40 cm soil layer (Figure 3).In relation to the light fraction of SOM, in general, the rotation SCC and the reference site showed the highest levels of LFC and LFN in all depths while in the rotation CWC the contents of C and N in this fraction were significantly lower (Figure 3).As shown in Table 2, all the C and N pools were positively correlated at p < 0.05.The percen pool in all 2014; Rya fractions t carbon.Th carbon ag mineral-as carbon lab Carbon an (Figure 3 Vieira et al. (2007) showed that the chemical oxidation might attack even some mineral-associated organic material, which is not readily available for soil microorganism.The results presented in this paper supported this assumption once the proportion of the C KMnO4 in TOC ranged between 9.5 and 14.1%, while the LFC in TOC ranged between 4.6 to 10.2% (Table 3).Thus, the chemical oxidation with KMnO 4 may overestimates the levels of the soil labile C and shows why C KMnO4 not present the same sensitivity that the LFC to changes in the crop rotation system.
Despite the no significant difference in total C and N contents between crop rotations, changes in labile carbon forms could be detected early compared to the TOC (Gong et al., 2009;Chen et al., 2009;Zhu et al., 2015).In this study, all the C and N pools analyzed were positively correlated each other (Table 2).These results indicate that more labile compartments of C and N can be used to provide early information about how different management practices may affect the SOM.Thus, it is possible that treatments with higher LFC levels could, in the future, lead to an increase in total SOM.However, to substantiate this assertion, a long-term study is required.The relative cumulative yield considering the five years of the study was higher in the treatments with legume-based annual rotation followed by the rotations CCC and CWC (Figure 4).Soil quality influence significantly the crop yield, therefore we expected that well-managed soils could support a sustainable production (Karami et al., 2012;Malhi & Lemke, 2007).Considering the five years of this study, the treatments with legume-based annual rotation showed a higher relative cumulative yield (Figure 4).Gan et al. (2015) and Islam et al. (2015) observed that the addition of legumes in crop rotation increased the productivity of the agricultural system.According to Fornara and Tilman (2008) the productivity of plants can be favored by the complementarity between grasses and legumes, since grass crops have a high efficiency in the use of nitrogen  The reference area presented the lowest humification index of SOM (Figure 5).This result can be explained by the thick layer of litter present in these areas providing a volume of organic material that overwhelms the capacity of microorganisms to metabolize them, reducing the humification index (González-Pérez, 2007).The H LIF was negatively correlated with LFC and C KMnO4 , showing that there is similarity between the techniques in estimating the lability of the soil organic matter.However, the LFC showed to be more sensitivity to discriminate the management systems and may be a way to evaluate the quality of a cropped soil.

Conclusion
The C and N labile fractions isolated by the physical fractionation are more efficient to provide information about the changes in soil organic matter quality in a no-till system compared to the labile C extracted with KMnO 4 .The diversification of the agricultural system with legume-based annual rotation changes the distribution of C and N among the SOM fractions confirming the hypothesis tested.

Note.
Different latters in the lines indicate significant difference among treatments at p < 0.05.MFC/TOC = proportion of mineral-associated fraction carbon in TOC; LFC = proportion of light fraction carbon in TOC; EOC = proportion of easy organic carbon in TOC; MFN = proportion of mineral-associated fraction nitrogen in TN; LFN = proportion of light fraction nitrogen in TN.

Table 1 .
Crop rotation arrangements used in the experiment

Table 3 .
Proportion of carbon and nitrogen fractions to total organic carbon and total nitrogen of different crop systems at four soil depths