Biogeochemistry of paddy soils

Potential adsorption of dissolved organic matter in poorly podzolised, high-latitude soils

Changes in concentrations and properties of dissolved organic matter (DOM) caused by oxygen deficiency are poorly understood. We estimated the influence of redox conditions on DOM dynamics in the field, sampling soil solutions at different depths of three soils (Humic and Histic Gleysol, Chromic Cambisol) along a soil catena in the cool-humid Black Forest (Germany) over a period of 2 years. We measured dissolved organic carbon (DOC) and determined the specific absorbance at 280 nm and two humification indices derived from fluorescence spectra to describe the aromaticity and complexity of DOM. Redox potential (Eh) was monitored continuously in situ. In the forest floor, DOC concentrations ranged independent of soil organic matter content and redox regime between 40 and 60 mg C L−1. DOC concentrations in all soils decreased with depth, accompanied by a decrease in DOM aromaticity and complexity. In the mineral subsoil, DOC concentrations, aromaticity, and DOM complexity were smallest in the aerobic soil (Chromic Cambisol; Eh > 500 mV) and largest in the most anaerobic soil (Histic Gleysol; Eh < 100 mV). Large DOM retention in the aerobic soil could be related to high contents of Fe oxides, highlighting their importance for DOM adsorption. Despite significantly reduced DOM retention under anaerobic conditions, it remains relatively large because the main DOM adsorbents changed from Fe oxides under oxic conditions to clay minerals, which were about 100 times more abundant under anaerobic conditions than Fe oxides. We found indications that biodegradation of DOM contributes more to DOM retention under anaerobic conditions, and we conclude that large DOM fluxes from anaerobic forest soils are the result of limited DOM adsorption in the subsoil rather than large DOM release from the topsoil.

Dissolved organic matter (DOM) in soils plays an important role in the biogeochemistry of carbon, nitrogen, and phosphorus, in pedogenesis, and in the transport of pollutants in soils. The aim of this review is to summarize the recent literature about controls on DOM concentrations and fluxes in soils. We focus on comparing results between laboratory and field investigations and on the differences between the dynamics of dissolved organic carbon (DOC), nitrogen (DON), and phosphorus (DOP). Both laboratory and field studies show that litter and humus are the most important DOM sources in soils. However, it is impossible to quantify the individual contributions of each of these sources to DOM release. In addition, it is not clear how changes in the pool sizes of litter or humus may affect DOM release. High microbial activity, high fungal abundance, and any conditions that enhance mineralization all promote high DOM concentrations. However, under field conditions, hydrologic variability in soil horizons with high carbon contents may be more important than biotic controls. In subsoil horizons with low carbon contents, DOM may be adsorbed strongly to mineral surfaces, resulting in low DOM concentrations in the soil solution. There are strong indications that microbial degradation of DOM also controls the fate of DOM in the soil. Laboratory experiments on controls of DOM dynamics have often contradicted field observations, primarily because hydrology has not been taken into account. For example, laboratory findings on the effects of plant species (conifer vs. deciduous) on DOM release from forest floors and on the effects of substrate quality (e.g.: C/N ratio) or pH on DOC concentrations were often not confirmed in field studies. The high adsorption capacity of soil clay minerals and oxides for DOM shown in laboratory studies may not control the transport of DOM in soils in the field if macropore fluxes dominate under field conditions. Laboratory findings about the biodegradability of DOM also await verification under field conditions. Studies that include DON and DOP dynamics in addition to DOC are few. The rate of release and the fate of DOC, DON, and DOP in soils may differ to a far greater extent than previously assumed. Controls established for DOC might thus be not valid for DON and DOP. Despite intensive research in the last decade, our knowledge of the formation and fate of DOM in soils and its response to changing environmental conditions is still fragmented and often inconsistent. Predictions at the field scale are still very uncertain, and most of the information available today is the result of studies on temperate soils and forest ecosystems. Thus, future research on controls of DOM dynamics should be extended to soils under different land uses and in other climate zones. Emphasis should also be given to: (i) the effects of soil organic matter properties on the release of DOM (ii) environmental factors controlling DOM quantity and quality (iii) the assessment of biological versus physico-chemical controls on the release and retention of DOM in soils, and (iv) the differences between DOC, DON, and DOP. Finally, if our goal is to predict DOM concentrations and fluxes in soils, future research on the controls of DOM dynamics should have a strong focus on field studies.

Fate of 14C-labeled dissolved organic matter in paddy and upland soils in responding to moisture

Response of Vertisols, Andosols, and Alisols to paddy management

Dissolved organic matter (DOM) affects several processes in soil and water including nutrient cycling, soil and water pollution and CO2 flux between the soil and atmosphere. The aim of this review is to collate and synthesize the literature on the transport processes of DOM in soil. The DOM normally comprises of only a small fraction of soil organic matter (SOM) and originates mainly from the decomposition and solubilization of SOM, which is accumulated on soil surface or soil profile from plant residues and additions of organic amendments such as animal and poultry manures and other biosolids. The DOM is one of the most reactive and mobile SOM fractions and has a major influence on biogeochemical processes in both terrestrial and aquatic environments. Terrestrially borne DOM is subjected to microbial decomposition, photodegradation and adsorption on soil mineral surfaces. It is sorbed on mineral surfaces and high adsorption capacities of clay minerals and oxides for DOM sorption are demonstrated in laboratory studies. However, these high sorption values are not reproduced in limited field studies. Similarly, a few data available on the transport of DOM through macropores also demonstrate the limited control of sorption on DOM retention in soil profile. Thus, there is a need to further investigate the physical and chemical protection mechanisms, as well as the biodegradability of DOM shown in laboratory studies. There is an increasing need to clearly understand the formation, fate and transport of DOM at field scales. The environmental factors such as precipitation and temperature, land use change, land management, and biological factors have profound and discrete influences on DOM dynamics in soil profile. Future research efforts must focus on the assessment of the influences of these factors by conducting field studies in different climatic zones, soils, and land use and management systems.

Dissolved organic matter (DOM) plays an important role in transporting carbon and nitrogen from forest floor to mineral soils in temperate forest ecosystems. Thus, the retention of DOM via sorption or microbial assimilation is one of the critical steps for soil organic matter formation in mineral soils. The chemical properties of DOM are assumed to control these processes, yet we lack fundamental information that links litter quality, DOM chemistry, and DOM retention. Here, we studied whether differences in litter quality affect solution chemistry and whether changes in litter inputs affect DOM quality and removal in the field. The effects of litter quality on solution chemistry were evaluated using chemical fractionation methods for laboratory extracts and for soil water collected from a temperate coniferous forest where litter inputs had been altered. In a laboratory extraction, litter type (needle, wood, root) and the degree of decomposition strongly influenced solution chemistry. Root litter produced more than 10 times more water-extractable dissolved organic N (DON) than any other litter type, suggesting that root litter may be most responsible for DON production in this forest ecosystem. The chemical composition of the O-horizon leachate was similar under all field treatments (doubled needle, doubled wood, and normal litter inputs). O-horizon leachate most resembled laboratory extracts of well-decomposed litter (that is, a high proportion of hydrophobic acids), in spite of the significant amount of litter C added to the forest floor and a tendency toward higher mean DOM under doubled-Litter treatments. A lag in DOM production from added litter or microbial modification might have obscured chemical differences in DOM under the different treatments. Net DOM removal in this forest soil was strong; DOM concentration in the water deep in the mineral soil was always low regardless of concentrations in water that entered the mineral soil and of litter input manipulation. High net removal of DOM from O-horizon leachate, in spite of extremely low initial hydrophilic neutral content (labile DOM), coupled with the lack of influence by season or soil depth, suggests that DOM retention in the soil was mostly by abiotic sorption.

The role of DOM sorption to mineral surfaces in the preservation of organic matter in soils

Selective sorption and desorption of DOM in podzol horizons — FTIR and Py-GC/MS of leachates from a column experiment

Tracking litter-derived dissolved organic matter along a soil chronosequence using 14C imaging: Biodegradation, physico-chemical retention or preferential flow?

Paddy soil management is generally thought to promote the accumulation of soil organic matter (SOM) and specifically lignin. Lignin is considered particularly susceptible to accumulation under these circumstances because of the recalcitrance of its aromatic structure to biodegradation under anaerobic conditions (i.e., during inundation of paddy fields). The present study investigates the effect of paddy soil management on SOM composition in comparison to nearby agricultural soils that are not used for rice production (non‐paddy soils). Soil types typically used for rice cultivation were selected, including Alisol, Andosol and Vertisol sites in Indonesia (humid tropical climate of Java) and an Alisol site in China (humid subtropical climate, Jiangxi province). These soil types represent a range of soil properties to be expected in Asian paddy fields. All upper‐most A horizons were analysed for their SOM composition by solid‐state 13C nuclear magnetic resonance (NMR) spectroscopy and for lignin‐derived phenols by the CuO oxidation method. The SOM composition was similar for all of the above named parent soil types (non‐paddy soils) and was also not affected by paddy soil management. A substantial proportion (up to 23%) of the total aryl‐carbon in some paddy and non‐paddy soils was found to originate from condensed aromatic‐carbon (e.g., charcoal). This may be attributed to the burning of crop residues. On average, the proportion of lignin was low and made up 20% of the total SOM, and showed no differences between straw, particulate organic matter (POM), and the bulk soil material. The results from CuO oxidation are consistent with the data obtained from solid‐state 13C NMR spectroscopy. The extraction of lignin‐derived phenols revealed low VSC (vanillyl, syringyl, cinnamyl) values for all investigated soils in a range (4 to 12 g kg−1 OC) that was typical for agricultural soils. In comparison to adjacent non‐paddy soils, the data do not provide evidence for a substantial accumulation of phenolic lignin‐derived structures in the paddy soils, even for those characterized by higher organic carbon (OC) contents (e.g., Andosol‐ and Alisol (China)‐derived paddy soils). We conclude that the properties of the parent soil types are more important for the lignin content of the soils than the effect of paddy management itself.

&amp;lt;p&amp;gt;Dissolved organic matter (DOM) is one of the most mobile components of the global carbon cycle. Corresponding transport processes in the environment have received plenty of attention in the context of carbon sequestration as well as the mobility of DOM-associated contaminants.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;However, most previous transport studies have been conducted exclusively under continuous flow conditions, which are not comparable to real water flow characteristics in soil. The present study aims to address that gap in knowledge by systematically assessing the effect of defined flow interruption phases on the retention of DOM.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;For that, the breakthrough behavior of DOM as affected by phases of flow interruption was investigated in an increasingly complex system of solid matrices rich in oxide mineral coatings: goethite coated quartz sand, disturbed Cambisol subsoil, and undisturbed Cambisol subsoil. The classic DLVO and extended DLVO (XDLVO) models including Lewis acid&amp;amp;#8212;base parameters were applied based on measurements of sessile drop contact angles and zeta potentials. &amp;amp;#160;&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;DOM retention was increasing with the duration of flow interruption, and retention was considerably higher in the soils than in goethite coated sand. After 112 hours of flow stagnation, DOM release from the soils was reduced to 16 to 22 % as compared to continuous flow conditions. The retention in the different solid matrix materials was well correlated with the respective amounts of oxalate and dithionite extractable oxide mineral phases. The DLVO model was capable of correctly predicting the mobility of DOM in goethite coated sand, but not in the soils, due to the fact that soil surface charge heterogeneities could not be measured. The XDVLO model predicted short-range hydrophilic repulsive interactions that may have contributed to the distinct tailing of the DOM breakthrough curves.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;We conclude that the significant DOM retention during phases of flow stagnation phases shows that more complex flow regimes need to be considered in order to assess the mobility of DOM in soils. In fact, many previous studies excluding phases of flow stagnation likely overestimated the mobility of DOM in the environment.&amp;lt;/p&amp;gt;

Review on the interactions of arsenic, iron (oxy)(hydr)oxides, and dissolved organic matter in soils, sediments, and groundwater in a ternary system

Retention of dissolved organic matter during podzolisation: Testing processes in laboratory experiments and at the submicron scale

Can soil properties of Fluvisols be influenced by river flow gradient?