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Scientia Agricola

On-line version ISSN 1678-992X

Sci. agric. (Piracicaba, Braz.) vol.77 no.3 Piracicaba  2020  Epub Sep 05, 2019

http://dx.doi.org/10.1590/1678-992x-2018-0164 

Review

Soils and Plant Nutrition

Processes that influence dissolved organic matter in the soil: a review

Maria Regina Gmach1 
http://orcid.org/0000-0001-7193-0828

Maurício Roberto Cherubin1 
http://orcid.org/0000-0001-7920-8362

Klaus Kaiser2 
http://orcid.org/0000-0001-7376-443X

Carlos Eduardo Pellegrino Cerri1  * 
http://orcid.org/0000-0002-4374-4056

1Universidade de São Paulo/ESALQ, Depto. de Ciência do Solo, Av. Pádua Dias, 11 – 13418-900 – Piracicaba, SP – Brazil

2Martin Luther University Halle-Wittenberg, Universitätsplatz 10 – 06120 – Halle (Saale) – Germany


ABSTRACT:

In tropical regions, climate conditions favor fast decomposition of soil organic matter (SOM), releasing into the soil organic composts in solid, liquid, and gaseous forms with variable compositions. Dissolved organic matter (DOM), a complex mixture of thousands of organic compounds, is only a small fraction of the decomposition products; however, it is highly mobile and reactive to the soil. Therefore, DOM play a key role in soil aggregation (formation of organometallic complexes), energy source for microorganisms, as well as C storage, cycling, and provision of plant-available nutrients. DOM multifunctionality to sustain soil functions and important ecosystem services have raised global scientific interest in studies on DOM fractions. However, previous studies were conducted predominantly under temperate soil conditions in natural ecosystems. Therefore, there is paucity of information on tropical soil conditions under agricultural systems, where DOM turnover is intensified by management practices. This review synthesized information in the literature to identify and discuss the main sources, transformations, and future of DOM in soils. We also discussed the importance of this fraction in C cycling and other soil properties and processes, emphasizing agricultural systems in tropical soils. Gaps and opportunities were identified to guide future studies on DOM in tropical soils.

Keywords: Brazil; dissolved organic carbon; agricultural soils; tropical soils

Introduction

Dissolved organic matter (DOM) is one of the most active and mobile C pools and has an important role in global C cycling (Kalbitz et al., 2000). In addition, dissolved organic carbon (DOC) affects the soil negative electrical charges denitrification process, acid-basic reactions in the soil solution, retention and translocation of nutrients (cations), and immobilization of heavy metals and xenobiotics (Zech et al., 1997). Soil DOM can be derived from different sources (inputs), such as atmospheric C dissolved in rainfall, litter and crop residues, manure, root exudates, and decomposition of soil organic matter (SOM) (Figure 1). In the soil, DOM availability depends on its interactions with mineral components (e.g., clays, Fe and Al oxides) modulated by adsorption and desorption processes (Saidy et al., 2015). It also depends on SOM fractions (e.g., stabilized organic molecules and microbial biomass) by mineralization and immobilization processes (Figure 1). In addition, the intensity of these interactions changes according to soil inherent properties (Kaiser and Guggenberger, 2007), land use, and crop management (Saidy et al., 2015).

Figure 1 Schematic representation of main inputs, transformation processes, and DOM losses in the soil system. 

During the decomposition of organic material, most C is lost as CO2 to the atmosphere by microbial oxidation. Soil type and landscape slope, leaching, and runoff (Figure 1) are also important processes associated to DOM losses in the soil (Veum et al., 2009). In well-drained soils, leached DOC can reach the water table and release nutrients and pollutants that can contaminate groundwater (Thayalakumaran et al., 2015; Sparling et al., 2016), whereas runoff transports DOM and xenobiotics to other areas, rivers, and lakes.

Most studies have focused on understanding the soil DOM dynamics and its potential implications in water contamination in temperate forests and wetland areas; however, results from agricultural sites remain scarce in the literature (Van Gaelen et al., 2014), especially in tropical conditions.

Therefore, this literature review investigated discussions in previous studies on DOM and determined the current interest in this research topic in Brazil. For that purpose, we analyzed information in the literature to describe importance, source and production, transformation processes, and future of DOM in the soil-atmosphere system, emphasizing agricultural soils in tropical conditions. Finally, gaps and opportunities were delineated to guide further research for a better understanding of the importance and implications of DOM changes in tropical soils.

Increase in scientific interest in DOC/DOM

Since studies on soil DOC/DOM were introduced in the early 1980s, interest in this topic in aquatic and terrestrial systems has increased linearly. However, DOM studies in the soil systems, especially in agricultural soils are uncommon in Brazil. To illustrate this contrast between the number of publications on DOM/DOC in Brazil and abroad, we performed a simplified bibliometric study in the Web of Science (WoS) database.

Initially, searching the terms “dissolved organic carbon” or “dissolved organic matter” as a “topic” from 1990 to 2017 provided 14,168 and 13,054 publications, respectively. For comparison purposes, only the topic “DOC” was used to avoid an overlap of results. When the word “soil” was added in the searches, the total number of publications decreased to 4,347 (Figure 2A) during the same period. When the searches were restricted to studies conducted in Brazil, the terms “DOC” and “Brazil” showed only 134 publications (Figure 2B), while searching for “DOC” and “USA”, 593 publications were found for the same period. In addition, searching for “DOC” and “Europe” showed 265 publications and for “DOC” and “Germany”, 217 publications were found, which is a large number for a relatively small country in territorial terms (23 times smaller than Brazil).

Figure 2 Evolution in the number of publications of research on DOC in the Web of Science database. a) globally, and; b and c) in Brazil. DOC+Soil: “dissolved organic carbon” AND “soil”; DOC+BR: “dissolved organic carbon” AND “Brazil”; DOC+BR+Soil: “dissolved organic carbon” AND “Brazil” AND “soil”. 

The number of publications decreased further when the word “soil” was added (i.e., “DOC” and “Brazil” and “soil”), resulting in 38 publications until 2017 (Figure 2C), but only 14 publications actually showed results from soil experiments (Table 1), and only a few had DOC fluxes as the main variables of study or evaluated DOM dynamics in the soil profile. Moreover, only one article evaluated DOC in the soil in Brazil recently (2018) (Table 1).

Table 1 List of studies* that evaluated soil dissolved organic carbon in Brazil. 

Year Authors Article Local Soil type Soil Texture Situation Main evaluated variables
2018 Sousa Jr. et al. Three-year soil carbon and nitrogen responses to sugarcane straw management Piracicaba, Brazil Oxisol Sandy-clay-loam Sugarcane straw management DOC, Total C and N stocks in the soil
2017 Costa et al. Influence of hydrological pathway on dissolved organic carbon fluxes in tropical streams Northeastern, Brazil Ultisol / Oxisol Sandy / Medium clayey Cacao agroforestry / Natural forest DOC fluxes
2016 De Conti et al. Soil solution concentration and chemical species of copper and zinc in a soil with a history of pig slurry application and plant cultivation Rio Grande do Sul state, Brazil Typic Hapludalf Sandy clay loam Agricultural soil Soluble Cu and Zn concentrations, and complexation with DOC
2015 Zanchi et al. Water balance, nutrient and carbon export from a heath forest catchment in central Amazonia, Brazil Amazonia, Brazil - Sandy soil Forest soil Water balance, DOC and nutrients export
2014 Bayer et al. Yield-scaled greenhouse gas emissions from flood irrigated rice under long-term conventional tillage and no-till systems in a Humid Subtropical climate Southern, Brazil Flooded soil - Agricultural flooded soil CH4, N2O, DOC, microbial biomass
2014 Ding et al. Environmental dynamics of dissolved black carbon in wetlands Pantanal biome, Brazil Wetland Wetland water and soil Dissolved black carbon dynamics
2012 Bayer et al. Methane emission from soil under long-term no-till cropping systems Southern, Brazil Aluminic Acrisol 22 % of clay Agricultural soil CH4 emission, DOC, ammonium, and others.
2012 Marques et al. Variations of dissolved organic carbon and soil physical properties under different land uses in central Amazonia Amazonia, Brazil Oxisol, Ultisol, and Spodosol Clayey Land use systems (forest, grassland and agroforestry) DOC and physical attributes
2009 Rueckamp et al. Carbon and nutrient leaching from termite mounds inhabited by primary and secondary termites Tocantins state, Brazil Umbric Acrisol Sandy clay loam Soil from mounds C, N, and P leaching from different mounds of termites
2008 Zambrosi et al. Liming and ionic speciation of an Oxisol under no-till system Paraná state, Brazil Rhodic Hapludox Clayey Agricultural soil Soil chemical properties and DOC
2007 Mariot et al. Dissolved organic matter fluorescence as a water-flow tracer in the tropical wetland of Pantanal of Nhecolandia, Brazil Pantanal biome, Brazil Wetland - Water and soil from lakes DOM as a tracer of water flows in contrasting range of salinity of the lakes
2007 Zambrosi et al. Nutrient concentration in soil water extracts and soybean nutrition in response to lime and gypsum application to an acid Oxisol under no-till system Paraná state, Brazil Rhodic Hapludox Clayey Agricultural soil Application of lime and gypsum on soil water extracts composition (nutrients and DOC)
2003 Rosa et al. Carbon forms of a Typic Eutroferric Red Latossol under no-tillage in a savanna biogeographic system Goiás state, Brazil Typic Eutroferric Oxisol Clayey Land use systems Chemical and physical soil characterization
2001 Martinez et al. Thermally induced changes in metal solubility of contaminated soils is linked to mineral recrystallization and organic matter transformations Cerrado biome (Savanna), Brazil Oxisol, Typic Acrustox Clayey Soil Heavy metals and DOC complexation
1999 Benke et al. Retention of dissolved organic carbon from vinasse by a tropical soil, kaolinite, and Fe-oxides Pernambuco state, Brazil Ultisol Agricultural soil Retention of DOC from vinasse by soil minerals

*Articles found in Web of Science database using the terms “Dissolved organic carbon” AND “Brazil” AND “Soil” (Accessed September 21st, 2018).

Complementary to this search in Web of Science database, the same search was performed in the Scopus and Scielo databases (i.e., databases that comprise scientific papers published in Brazilian and some Latino-American journals). The results found in the Scopus database are very similar to those found in the Web of Science. In the Scielo database, the aim was also to find publications in Portuguese; however, the addition of the terms “dissolved organic carbon” and “soil” showed only six publications. In addition, terms such as “soluble carbon,” “crop,” and “carbon leaching” were also searched jointly with DOC or DOM and Brazil, but there were few results.

These searches in the main scientific databases showed the lack of the studies in Brazil on this important C fraction. While the international scientific community is concerned with understanding DOM implications on the functioning of natural and anthropic ecosystems, in Brazil, there is much to advance to understand DOM dynamics, especially in agricultural systems with diversified management practices (e.g., no-till, cover crop, crop-livestock-forest integration, and green sugarcane harvesting).

Definition and main sources of DOM

Dissolved organic matter (DOM) is considered a complex mixture of thousands of organic compounds with diversified chemical compositions and properties (Catalá et al., 2015; Flerus et al., 2012; Thurman, 1985). However, a small proportion of DOM can be chemically identified, mostly as low molecular weight substances, such as organic acids, sugars, and amino acids (Herbert and Bertsch, 1995), hindering a thorough chemical definition of DOM (Silveira, 2005). DOM is a source of energy and organic nutrient forms, such as nitrogen (N) and phosphorus (P) readily accessible for soil microbiota (Burford and Bremner, 1975; McDowell et al., 2006). The origin, function, and future of these compounds in terrestrial ecosystems are only partially understood (Wang et al., 2016), as well as the factors that control soil DOM in the soil profile (Zhou et al., 2015). The DOC is a minor fraction of soil organic carbon (SOC), although DOC is one of the most mobile and bioavailable portions (Ghani et al., 2013; Marschner and Kalbitz, 2003). The decomposition of DOM can indicate processes that control SOM accumulation and stabilization (Kaiser and Kalbitz, 2012).

The main inputs of DOM into the soil are the rainfall, plant residues, root exudates, SOM, and microbial biomass (Kalbitz et al., 2000; Yano et al., 2005). DOM can be produced mainly by recent plant residues/litter and from relatively stable SOM decomposition (McDowell and Linkes, 1988; Michalzik et al., 2003). Some studies suggest that fresh C substrates are some of the most important DOM sources, such as plant residues, roots, and exudates, and their secretions including organic acids, phenols, sugars, and amino acids (Högberg and Högberg, 2002), (Wang et al., 2016). DOC originating from fresh leaf litter may contribute to the formation of an A horizon, whereas DOC originating from root litter may explain the presence of SOC at soil depths (Uselman et al., 2007).

In contrast, studies have shown that decomposition of stable SOM is the most important DOM source since wetter compounds predominate in DOM, suggesting that it originates from the large stock of native SOM than from recently added litter (Fröberg et al., 2003; Zsolnay, 1996) depending on the organic material. Thus, part of DOM is derived from old SOM, indicating that the release of C from the plant into the soil solution is at a steady state with its decomposition or that litter and young SOM can be degraded by the microbiota without solubilizing first (De Troyer et al., 2011). Therefore, wet SOM and exchanges with aqueous phase may determine DOM chemical composition (Sanderman et al., 2008).

In general, recent litter and wet SOM constitute the most important DOM sources in soils (Kalbitz et al., 2000), varying in DOM concentration according to soil characteristics, soil use and tillage, and local climate. Thus, some compounds are specific to different functional soil or plant types, improving the capacity to use DOM as a soil quality indicator (Jones et al., 2014).

Furthermore, rainfall contributes to DOC content in the soil. A global study showed that 80 % of C in rainfall is in the organic form (DOC), corresponding to 430 × 1012 g C yr–1, and 20 % in the inorganic form (DIC), corresponding to 80 × 1012 g C yr–1, totaling 510 × 1012 g C yr–1, from which 70 % is deposited over land (Willey et al., 2000). These results show the importance of including rainfall into the global C balance. Besides containing C, rainfall also contributes to the DOC movement and flux in the soil, and an increase in soil water flux may cause an increase in DOC content in soil solution (Chantigny, 2003).

Factors associated with production and inputs of DOM in the soil

The concentration of DOM in soil solution is controlled by several factors and processes, namely climate conditions, quantity and quality of the organic inputs, microbial activity (consumption and immobilization), soil texture, mineral adsorption, and leaching (Chantigny, 2003; Filep and Rékási, 2011; McDowell, 2003).

Climate and soil type

Climate characteristics can modify DOM production and release. Warm and humid weather conditions, such as the tropical climate, increase the microbial activity and the release of DOM from decomposing materials (Kalbitz and Knappe, 1997). Rainfall coming after dry periods may release a higher concentration of DOM into the soil solution than in normal rainy periods probably because of reduced decomposition rates in dry soils accumulate microbial products (Kalbitz et al., 2000). Rainfall intensity also may influence DOM sorption or leaching (Fröberg et al., 2007; Herbrich et al., 2017).

In general, high soil temperature and soil moisture were positively correlated to plant material decomposition rates, affecting directly DOM concentration in the surface soil layers (Zhou et al., 2015). Thus, DOC inputs and fluxes may be higher in tropical regions than in temperate regions.

Moreover, soil characteristics affect DOM inputs to the soil, such as clay content, water holding capacity, porosity and infiltration rates, and affect mainly the sorption force controlled by the concentration of clays and oxides in the soil (Saidy et al., 2013). The DOM concentration in the soil profile is a result of continuous sorption combined with microbial processing and subsequent desorption (Kaiser and Kalbitz, 2012). Aluminum (Al) and iron (Fe) oxides and hydroxides are some of the most important DOM adsorbents (Kaiser et al., 1996), especially in tropical soils.

Quantity and quality of organic material

The role and dynamics of DOM in soils are related to the quantity and quality of organic residues, which depend largely on their sources (Kalbitz et al., 2000; McDowell and Likens, 1988). The lignin content in residues regulates the litter decomposition rate and thus is important for DOM production (Guggenberger, 1994; Kalbitz et al., 2006). There is a strong relationship between DOC flux and soil C:N ratio (Aitkenhead and McDowell, 2000) in which decomposition of poor-N materials seems to result in the production of more soluble compounds, explaining the positive correlation between C:N ratio and DOC concentration (Kalbitz and Knappe, 1997). When the C:N ratio is lower than 10, most C associated to SOM is consumed or re-assimilated by the soil microbiota to ensure that only a small portion of C remains in the soil as DOC (Kindler et al., 2011). Furthermore, a reduction of the C:N ratio in the soil could lead to significant declines in DOC flux, especially in soils with lower initial mean soil C:N ratio, such as grasslands, the savanna and others (Aitkenhead and McDowell, 2000).

Root exudates can also release different organic compounds, leading to intensive changes in the physical, biological, and chemical nature of soils (Jones et al., 2009). The dominant organic C compounds in roots reflect the key compounds for cell metabolism, including sugars, amino acids, and organic acids (Kraffczyk et al., 1984).

Soil use and management

The labile DOC fraction is more sensitive to tillage disturbance than total SOC pool (Roper et al., 2010). Moreover, the use of DOM as an indicator for environmental changes and a tool for classifying ecosystems has been proposed in aquatic and marine sciences; consequently, using DOM in soil science seems desirable (Kaiser and Kalbitz, 2012). In the short-term, the relation between DOC and SOC concentration is not significant (Zhou et al., 2015), but the relationship is significant in the long-term perspective (Gregorich et al., 2000).

DOM production is sensitive to changes in land uses and management, such as the conversion of native forest into agriculture systems and the use of conventional tillage, that is, activities that can increase microbial activity (Van Gaelen et al., 2014). Higher microbial activity increases DOM release for a short period (Brye et al., 2001; Leinweber et al., 2008) and induces faster turnover of C fractions. In a study conducted in tropical soil from the Brazilian Amazon, DOC concentration was higher in an agroforestry system than in native forest and pasture (Marques et al., 2012). In addition, in the Brazilian savanna (Cerrado biome), Silva et al. (2007) found higher DOC flux in the soil under sugarcane crop than under eucalypt forest and native forest areas. Unfortunately, little has been done to quantify factors that affect DOM production in tropical conditions (Wang et al., 2016).

Residues amendment from the soil surface and the resulting release of easily biodegradable DOM by plant residues clearly induce microbial growth (De Troyer et al., 2011). Keeping crop residues on the soil surface is important to maintain C inputs and subsequently SOC (Cherubin et al., 2018). Thus, soils under no cover can suffer significant C losses as DOC forms (Baldock and Skjemstad, 2000; Sousa Jr. et al., 2018).

In summary, a defined chemical composition of DOM is difficult, and DOM origins are still little understood. To date, it is known that the main DOM sources are the plant residues/litter and stable SOM, which vary mainly according to the organic material. Thus, the production and release of DOM depend on a range of factors, such as soil characteristics (e.g., quantity of clays and oxides), climate conditions (e.g., temperature and humidity), characteristics of the plant residues (e.g., C:N ratio, lignin content, roots length), soil use, and management practices. This information shows that DOM production in the soil is higher in tropical conditions, under crop cultivation, and with plants with high C:N ratios and lignin content.

Soil DOM changes and their implications for the biogeochemical cycle

Adsorption/desorption of DOM in soil

Sorption processes of organic C on mineral surfaces contribute to accumulation and stabilization of SOC in the environment (Feng et al., 2005; Saidy et al., 2015). Free DOM movement is controlled mainly by its adsorption to soil clay surfaces (Ussiri and Johnson, 2004). The sorption of OC to mineral surfaces is strong and only partially reversible, with only a small portion extractable into fresh water, salt water, or organic solvents (Kahle et al., 2007; Kaiser and Guggenberger, 2007). Desorption varies according to the mineral and all DOC adsorbed by kaolinite is completely desorbed, while only 28 to 35 % of adsorbed DOC is desorbed by Fe-oxides. These findings highlight the importance of goethite and hematite in DOM adsorption in tropical soils (Benke et al., 1999). Moreover, there is high correlation between DOM adsorption and specific surface area (SSA) of the clay fraction (Singh et al., 2016).

The biological stability of SOC sorbed to clay-oxide associations is influenced by the balance between the negative charge of clays and the positive charge of Fe-oxides (Saidy et al., 2015). Fe-oxides tend to be positively charged, especially in acids soils, and kaolinitic clays tend to carry less negative charges than other clays do (Saidy et al., 2013). In this sense, oxides can interact with both clay minerals and organic compounds to form organic-mineral associations that may influence significantly the size of the organic matter fraction resistant to biodegradation (Schneider et al., 2010).

Polyvalent cations usually reduce DOM leaching and increase DOM adsorption due to cation bridging and precipitation. Comparing cation adsorption, Singh et al. (2016) found that DOM adsorption was higher with increasing concentration of Ca2+ than of Na+. In contrast, anions, such as phosphate and sulfate, compete with DOM for adsorption sites, increasing DOM leaching (Kalbitz et al., 2000).

In general, soil with predominance of clays with high SSA, higher CEC, and especially high content of Fe/Al oxides are more efficient to protect chemically and physically C of microbial mineralization and other loss processes (Kahle et al., 2003). Moreover, high concentration of oxides reduces DOM concentration in the soil solution, reducing losses by leaching. Thus, oxidic soils are expected to retain DOM more effectively.

DOC effects on C sequestration

Soil organic carbon is the largest terrestrial SOM pool, containing about 1550 Pg of C, three-fold the amount found in the atmosphere or terrestrial vegetation (Lal, 2004). Therefore, the soil plays a key role in C sequestration, mitigating global warming and climate changes. For its characteristics, DOM is important in soil biogeochemical and is a crucial component of the net ecosystem C balance (Kindler et al., 2011). The DOM fraction is a potential source of stabilized C, occurring in subsoil by C redistribution in deep layers (Fröberg et al., 2007; Kalbitz and Kaiser, 2008) leading to SOC accumulation (Schneider et al., 2010; Saidy et al., 2015), making it an important way to sequester C and decrease C loss in the CO2 form (Lal, 2004; Smith, 2004).

In long-term studies, De Troyer et al. (2011), Fröberg et al. (2003), and Hagedorn et al. (2004) found that OM from plant residues do not accumulate in the DOC pool; instead, it is mostly released as CO2. However, Uselman et al. (2007) found that during high rainfall and low temperatures, a larger fraction of 14C from plant litter is lost as DOC, translocated or leached, than released as CO2, probably favoring more leaching than microbial metabolism. These results indicate that the proportion of C released as CO2 or as DOC is closely related with local climate characteristics.

Recently, Deng et al. (2017) showed that DOC leaching from the litter layer to topsoil in a subtropical forest was the major cause of rain-induced soil CO2 pulse; consequently, there is great concern with DOC contribution to increasing CO2 release in tropical soils, due to the increase in DOC fluxes by accelerated microbial activity. Nevertheless, correlations between DOC fluxes and CO2 release in tropical soils still need to be further investigated.

Effects of DOM on soil properties

The DOC is a sensitive fraction and can be considered an alternative tool to monitor adverse impacts on soil quality (Silveira, 2005). Due to its high mobility, the DOM movement is significant to the cycling and distribution of nutrients, such as N and P (Veum et al., 2009) and Fe and Al complexes (Fujii et al., 2009), in ecosystems.

Soluble organic acids that comprise DOM have functional groups, especially carboxylic and phenolic, which participate in many chemical reactions in the soil, such as organic metal complexation, increasing the ion adsorption rate and metal detoxification (Franchini et al., 2003; Roberts, 2006). These acids make exchangeable Al complex in the soil solution, making it nontoxic to plants (Amaral et al., 2004; Franchini et al., 1999). Therefore, in tropical soils, these organic acids can compete with other ions, such as phosphate ions, for adsorption sites, increasing P availability to plants (Andrade et al., 2003; Jones, 1998). The organic acids can also form stable organometallic complexes with Fe and Al in a wide pH range (Sposito, 1989). In addition, greater soil structural quality (e.g., higher aggregate stability, soil porosity, and water retention) is positively associated to DOM movement in the soil profile (Marques et al., 2012), since its movement and sorption are related to the water fluxes (Herbrich et al., 2017).

The metal detoxification activity depends on the DOM origin, since DOM originating from plant residues does not contribute significantly to the transport of organic pollutants and metals (Amery et al., 2007), because of this DOM is easily degradable and quickly decomposed rather than leached through deeper soil horizons. However, DOM derived from SOM can be used to predict the movement of both organic and inorganic pollutants in the soil (Amery et al., 2008).

The soil pH can affect DOM mobility; however, effects are still uncertain. Nonetheless, Tipping and Woof (1990) reported reduced adsorption capacity at high pH values with increase of DOM mobilization. Consequently, small increases in the soil pH lead to higher amounts of mobilized SOM.

In summary, DOM dynamics and processes are mainly affected by adsorption in the soil mineral phase and more strongly adsorbed by Fe and Al oxides, higher SSA clays, and polyvalent cations. Moreover, DOM is important in nutrient cycling and distribution in the profile, in phosphate availability, and in complexation of Al, heavy metals, and pollutants. In subsoil, DOC is an important source of stabilized SOC and a potential C reservoir in deep soils, playing an important role in C cycling and sequestration in the soil. Therefore, in tropical conditions, DOM is possibly strongly adsorbed by Fe and Al oxides; however, fast production and changes of DOC can boost CO2 emission. Considering the direct and indirect influence and benefits of DOM on multiple soil chemical, physical, and biological properties, as well as the lack of information in tropical soils, this topic needs to be further explored in those conditions.

DOM output and losses

Terrestrial hydrological pathways of C flow include rainfall, surface runoff, and drainage or leaching. The DOC fraction is more linked to leaching, while the particulate C fraction is more linked to superficial runoff (Edwards et al., 2008). Then, the process of DOM percolation from the soil surface transfers C and nutrients to deeper layers through soil solution (Fröberg et al., 2007). Thus, DOM can undergo sorption and be stored or transported to aquifers, moving from the terrestrial to the aquatic system (Sparling et al., 2016). Therefore, DOM leaching may be an important pathway of continuous soil C and nutrient losses (Kindler et al., 2011).

The main source of DOM leaching is SOM, because DOM from fresh plant residues is largely retained or consumed in topsoil, while only a small fraction is moved through the soil profiles (Fröberg et al., 2007, 2009). Some microorganisms can also contribute to DOM leaching, such as mycorrhizal symbionts that contribute to C flow, mainly through their structures, resulting in the release of exudates into the mycorrhizosphere (Jones et al., 2009).

Carbon losses by superficial runoff can be avoided with management for soil conservation. Continuous vegetal cover can provide a significant reduction in runoff, preventing potential contamination of waters by DOM (Veum et al., 2009). The DOC mobilization in runoff water results from antecedent soil moisture, as more DOC is released from drier soils (Van Gaelen et al., 2014). Then, the monitoring of C losses by runoff and leaching to deeper layers is required in agricultural soils to estimate C balances (Nachimuthu and Hulugalle, 2016).

The DOM leaching is also controlled by the magnitude and direction of drainage water fluxes. During intensive and frequent rainfalls, elevated DOC concentration was found in groundwater from a sugarcane crop in Australia and was supplied via water flow (Thayalakumaran et al., 2015). Fast water movement, such as strong rains, might decrease DOM sorption in the soil, as well as microbial processing, resulting in fresh residues derived from DOM transported deeper into the soil (Fröberg et al., 2007). On the other hand, less time is available for SOM desorption, which may cause lower DOC concentration in the soil solution compared to a slower water percolation (Herbrich et al., 2017). Consequently, with more water volume in the soil, more DOM is probably derived from fresh residues than from SOM desorption.

The DOM leaching is considered a continuous form of C and nutrient losses from the soil and becomes a pollutant, as it reaches aquifers. In contrast, DOM may be a large reservoir of C in deep soils when it is adsorbed and stored in deep layers. Carbon losses by soil surface are generally linked to soil management system; however, C loss by leaching depends on many factors, such as soil characteristics, soil management, and rainfall intensity. In the case of tropical conditions, where most areas contain deep soil, the DOM fraction may be labeled as an important reservoir of C at depth. To verify this hypothesis, further studies on DOM production and leaching should be carried out in tropical soils to estimate a complete C balance.

Final remarks and perspectives

The DOM concentration in soil solution is highly variable and depends on site-specific soil, climate, and land management conditions (Sparling et al., 2016). Most studies on DOM have been performed in temperate soils, predominating shallow soils. In contrast, little is known about tropical soils, which are highly weathered, deeper, and contain large amounts of Al and Fe oxides and hydroxides, leading to large adsorption.

The DOM fraction is an important active and bioavailable C source for microbial biomass, besides sequestering and storing C in deep layers. Despite its benefits, DOM dynamics has been preferentially evaluated in forests and peat soil, whereas only few studies have been conducted in agricultural soils (Wang et al., 2016). While land use and management practices affect directly the C fractions in the soil, there is little experimental data involving DOM mechanisms and processes.

Future studies are essential to determine the potential of best management practices (e.g., no-till, cover crop, crop rotation) to increase soil DOM, such as the removal of crop waste to feed animals or produce bioenergy can affect DOM dynamics in soils, and avoidance of DOM leaching in agricultural soils. Our research shows that little importance has been given to this topic in Brazilian agricultural soils, revealing a gap of information on DOM, which should be addressed in future studies.

Acknowledgement

MRG thanks the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) for providing her PhD scholarship, and also thanks “Fundação Agrisus” for the financial support for the research visit in Germany. MRC thanks the “Fundação de Estudos Agrários Luiz de Queiroz” (Project # 67555) for providing his postdoctoral fellowship. We would also like to thank the Banco Nacional de Desenvolvimento Econômico e Social (BNDES) and Raízen Energia S/A for funding our research (Project #14.2.0773.1).

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Received: May 23, 2018; Accepted: November 25, 2018

*Corresponding author <cepcerri@usp.br>

Edited by: Paulo Cesar Sentelhas

Authors' Contributions

Conceptualization: Gmach, M.R.; Cherubin, M.R.; Cerri, C.E.P. Data acquisition: Gmach, M.R. Writing and Editing: Gmach, M.R.; Cherubin, M.R.; Cerri, C.E.P.; Kaiser, K.

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