Matter In Paddy Soils Research Articles

Warming promotes the decomposition of oligotrophic bacterial-driven organic matter in paddy soil

The ongoing global warming is causing paddy soil to come under threat by hitherto unseen levels of carbon and nitrogen loss. The mechanism of soil organic matter (SOM) components and microbial metabolism responding to increased temperature is complicated; for example, the temperature sensitivity (Q10) of SOM decomposition in rice topsoil and subsoil remains poorly understood. In this study, 0–30 cm columns of undisturbed paddy soil were collected and incubated at 5, 15, or 25 °C for 117 days to investigate the effect of temperature on SOM decomposition and microbial characteristics in different soil layers. Results showed that Q10 of subsoil (15–30 cm) was more than twice that of topsoil (0–15 cm), indicating that SOM mineralisation in subsoil was facilitated more by temperature than that in topsoil. Warming promoted the decomposition of recalcitrant SOM and reduced the fluorescent intensity of dissolved organic matter. Increased temperature exerted no significant effect on microbial biomass, but it did enhance the relative abundance of oligotrophic bacteria and promote anabolism. Therefore, recalcitrant SOM decomposition driven by oligotrophic bacteria was more sensitive to warming than labile organic matter consumption mediated by copiotrophic bacteria. Our findings revealed the underlying mechanisms by which elevating temperature promoted SOM mineralisation, thereby emphasising the need to remain vigilant regarding the threat to carbon dynamics in deep soil poised by global climate changes.

Distinct response of arsenic speciation and bioavailability to different exogenous organic matter in paddy soil

Land application of organic waste has been increasingly encouraged since it could sequester carbon to mitigate climate change. Considering the susceptibility of arsenic (As) bioavailability in soils to organic matter, understanding the influence of different exogenous organic matter on As biogeochemical behavior in rice-soil system is crucial to reasonably recycle organic waste on soils and ensure the food safety. In this study, impacts of two typical organic matter amendments, rice straw and humic substance, on the As speciation and bioavailability in paddy soil were investigated. Results showed that addition of both rice straw and humic substance could increase the dissolved organic carbon (DOC) content in soil solution by 16.4%–34.4% and 21.7%–53.2%, respectively, but the response of As speciation and bioavailability was quite different, showing the decoupling between As release and DOC. Rice straw addition increased As release to porewater by 28.0%–28.4%, particularly at the initial 0–18 days after the soil was flooded, but humic substance presented the opposite effect, decreasing As release by 27.4%–43.1% which was mainly attributable to the AsIII immobilization. This study suggests that the organic matter with high contents of labile heteroaliphatic/aliphatic carbon, being easily to be biodegraded, should not be applied on As contaminated soils.

Roles of soil active constituents in the degradation of sulfamethoxazole by biochar/persulfate: Contrasting effects of iron minerals and organic matter

The biochar/persulfate (BC/PS) has been extensively applied in the degradation of organic contaminants in the aqueous solutions. However, much less work has been done on the degradation of organic contaminants in soil by BC/PS, especially on the unclear roles of soil active constituents in the degradation. This study was conducted to investigate the degradation of sulfamethoxazole (SMX) in two soils through PS oxidation activated by biochar. Biochar was produced via the pyrolysis of peanut shell at 400 °C and 700 °C, which was denoted as BC400 and BC700, respectively. Two soils used were red soil and paddy soil, mainly differing in iron minerals and organic matter. Both biochar promoted SMX degradation (42.6 %–90.7 %) in two soils, compared to PS alone (20.9 %–41.7 %). In BC400/PS system, the free radicals were the dominant reactive species for SMX degradation, while the electron transfer pathway played a vital role in the SMX degradation by BC700/PS. Higher SMX degradation was observed in red soil (41.7 %–97.8 %) than that in paddy soil (20.3 %–94.8 %), which was ascribed to the promotion of iron minerals in red soil yet the inhibition of organic matter in paddy soil. Specifically, the reaction between ≡Fe(III)/≡Fe(II) and PS on the surface of iron minerals in red soil generated more SO4•− and •OH, resulting in the enhanced SMX degradation. However, the consumption of free radicals and suppression of electron transfer pathway by organic matter in paddy soil inhibited SMX degradation. As the comparative carbonaceous materials to biochar, graphite exerted no obvious degradation effect, whereas activated carbon exhibited the comparable promoting efficacy to BC700. Both biochar, especially BC700, significantly (p < 0.05) alleviated the adverse effects of PS treatment on wheat (Triticum aestivum L.) growth. Overall, this study demonstrates that biochar/persulfate was effective in SMX degradation in soil and the degradation was affected by soil iron minerals and organic matter, which should be paid more attention in the persulfate remediation of organic contaminated soils at a specific site.

Molecular linkages between chemodiversity and MCPA complexation behavior of dissolved organic matter in paddy soil: Effects of land conversion

Complexation of dissolved organic matter (DOM) plays a crucial role in regulating the fate and risk of agrochemicals. Here, taking a toxic herbicide MCPA (4-chloro-2- methylphenoxyacetic acid) as the target, the effect of land conversion on complexation behavior of DOM to agrochemicals was investigated in paddy soil. Furthermore, the mechanisms were explored in a new perspective of DOM chemodiversity. Soil DOMs were selected from four long-term cropping systems, including paddy field (PF), vegetable field (VF), rice-vegetable rotation (RV) and abandoned land (AL). The results showed that the DOMs in PF and AL were rich in hydrophilic substances (e.g., carbohydrates or protein-like molecules) with low aromaticity. However, after converting PF to VF and RV, abundant aromatic macromolecules and aliphatic alkanes were observed in DOM. Due to those changes in DOM chemodiversity, the binding site and capability of DOM were highest in VF and RV, and were positively correlated with DOM aromaticity, MW, humus and polar groups (e.g., amino). This was because the complexation of “DOM-MCPA” was static binding via ligand exchange and H-bonding among polar groups and hydrophobic interaction among aromatic skeletons. The EEM-PARAFAC confirmed that microbial humic-like substances dominated the complexation of DOM rather than terrestrial humic-like and tryptophan-like matters. The 2D-COS analysis further revealed that the complexation of DOM preferentially occurred in amino, polysaccharide C–O and aliphatic C–H for PF and AL, but in aromatic C=C, amide C=N for RV and VF. In summary, these findings provide molecular insight into the effect of land conversion on DOM complexation activity, which highlight the importance of DOM chemodiversity. These results will contribute to the risk assessments of agrochemicals in paddy soil.

Effects of Agricultural Organic Waste Incorporation on the Metabolic Capacity of Microbial Carbon Sources of Dissolved Organic Matter in Paddy Soil

A long-term fertilization experiment with a system of rice-wheat rotation was conducted in Chengdu Plain. Three fertilization treatments including conventional fertilization (T1), pig manure substituting for 50% nitrogen fertilizer (T2), and T2 plus straw (T3) were set up to study the characteristics of microbial carbon source utilization of soil and dissolved organic matter (DOM). The results showed that T3 improved the soil microbial carbon source metabolism in comparison with those of the T1 and T2 treatments; the average color change rate (AWCD) increased by 16% and 48%, respectively. Meanwhile, T3 improved the soil DOM microbial carbon source metabolism, and the AWCD value was 0.43. The highest Shannon, Simpson, and McIntosh indexes of soil and DOM were all found in the T3 treatment, and the Shannon, Simpson, and McIntosh indexes of DOM were 2.73, 0.91, and 3.75, respectively. The results of principal component analysis and enrichment analysis showed that the main carbon sources used by microorganisms of soil and DOM were different under different fertilization treatments. For DOM, the main carbon source used by microorganisms in the T1 and T2 treatments was sugar, whereas T3 increased the utilization of amino acids, carboxylic acids, polymers, and amines. The changes in soil pH and texture were the main factors that caused the difference in soil DOM microbial carbon source metabolism. In conclusion, the application of organic fertilizer (pig manure plus straw) significantly increased the microbial community diversity and carbon source metabolic capacity of soil and DOM and promoted the diversification of microbial carbon source preference.

Characteristics of Fluorescent Dissolved Organic Matter in Paddy Soil Amended With Crop Residues After Column (0–40 cm) Leaching

Dissolved organic matter (DOM) has a critical impact on various processes in the ecological environment, especially in soil and surface water due to its mobility and reactivity. In the present study, a column experiment simulating a paddy field was conducted to evaluate the feasible application of inorganic fertilizer incorporated with three types of organic materials (wheat straw, rapeseed shell, and astragalus smicus). Researchers investigated the vertical distinctions of soil (combined with different organic materials in 0–10 cm) DOM in 0–40 cm profiles after long-term leaching. Excitation–emission matrices coupled with parallel factor analyses (EEM-PARAFAC) and UV-visible techniques were employed to diagnose the spectral characteristics of soil DOM at four depths in different treatments, qualitatively and quantitatively. The results showed that the DOM content of CF (conventional fertilizer) (0.14 g kg−1) was the highest at the 10–20 cm layer and the CK (no fertilizer) (0.05 g kg−1) at 30–40 cm was the lowest, and the mean DOC concentration decreased with the increase in soil depth. Four fluorescent components, protein and tryptophan-like, humic acid-like, fulvic acid-like, and solubility-like microbial metabolite product materials, were identified by the PARAFAC model. Relative distributions of calculated components suggested that DOM mainly contained more aromatic humic materials and litter fulvic acid with the increase in soil depth. However, the SUVA254 showed a maximum in 20–30 cm layers, indicating strong aromatic and abundant hydrophobic fractions. Fluorescent index (FI) values of soil DOM from various treatments have a similar terrestrial and allochthonous source in topsoil and microbial source in deeper soil. In short, the findings obtained in our work could commendably provide some valuable optical information of DOM released from soil column and offer technical guidance when applying organic or green fertilizer to paddy soil during rice production.

The mass water content of paddy soil after harvest is strongly associated with the accumulation of organic matter as the source of available nitrogen

ABSTRACT Paddy soils are the dominant source of nitrogen for rice via mineralization, and knowledge about the available nitrogen (AN) in paddy soil is important for rice cultivation. Although the relationship between AN and soil physicochemical parameters has been studied, no systematic studies of the relationships between AN and the soil moisture content of various soil types in Japan have been studied. Therefore, we examined the impacts of soil types and treatment of organic matter application or paddy–upland rotation on these relationships. We sampled topsoils from 30 paddy fields throughout Japan. We compared the relationships between AN and soil physicochemical properties, such as total nitrogen (TN), cation exchange capacity, soil water-holding capacity (WHC), mass water content after harvest (MWH), and water content after harvest (WCH). The scatter diagram revealed that AN and either MWH or WCH (p < 0.01) had the highest positive linear correlation coefficient; the pattern became clearer after separating Andosols and non-Andosols. We analyzed the relationship between AN and either TN or MWH in 100 topsoils, including soils from treatment plots with successive organic matter application or paddy-upland rotation. A significantly positive correlation between AN and TN was observed, although the regression slopes for volcanic or non-volcanic soils differed. The decrease in AN due to frequent paddy–upland rotation was greater than that for TN. On the other hand, a highly positive correlation between AN and MWH was observed, and the coefficient of determination was higher than that for AN and TN. TN and the WHC increased when the accumulation of organic matter in paddy soil was greater. As there was a positive correlation between WHC and MWH, a positive correlation was also observed between AN and MWH. Additionally, we investigated year-to-year variation in MWH using topsoil samples collected before autumn tillage, for 3–5 consecutive years in 24 plots, for various soil types throughout Japan. In order to eliminate the effects of precipitation and to minimize the fluctuation of MWH in paddy topsoil, it was desirable to collect topsoil samples within 24–96 hours of the last precipitation of 1 mm h−1 or more, for the measurement of moisture content.

Distinct Response of Arsenic Speciation and Bioavailability to Different Exogenous Organic Matter in Paddy Soil

Distinct Response of Arsenic Speciation and Bioavailability to Different Exogenous Organic Matter in Paddy Soil

Long‐term biochar application governs the molecular compositions and decomposition of organic matter in paddy soil

AbstractBiochar addition can enhance soil quality and sequester carbon. However, changes in soil organic matter (SOM) molecular compositions in response to long‐term biochar addition have rarely been studied. Therefore, we quantified soil organic carbon fractions, carbon‐cycling enzyme activities, and a range of organic compounds and lignin‐derived phenols in the rhizosphere and bulk soils in different rice growth stages in the 8‐year field trial with biochar application rates of 0 (BC‐0), 4.5 (BC‐L), and 13.5 t ha−1 year−1 (BC‐H). We found that higher amounts of biochar addition (BC‐H) increased labile organic carbon (LOC), dissolved organic carbon (DOC) and particulate organic carbon (POC), and promoted activities of α‐1,4‐glucosidase, β‐D‐cellobiohydrolase and β‐1,4‐xylosidase; in contrast, BC‐L treatment reduced activities of these enzymes. The concentrations of dichloromethane/methanol‐extractable plant‐ and microbial‐derived organic compounds in the rhizosphere and bulk soils at tillering decreased significantly in the treatment with low amount of biochar addition. BC‐L also significantly altered the concentrations of extracted compounds in the rhizosphere soil at tillering and harvest. Concentration of lignin in the bulk soil was significantly reduced in BC‐L at tillering (by 19%) and harvest (by 28%). The concentrations of extracted compounds (e.g., n‐alkanols, n‐alkanoic acids, steroids, and carbohydrates) and lignin were generally significantly higher in the bulk than the rhizosphere soil at tillering and harvest. Long‐term biochar application (BC‐H) promoted lignin decomposition in the bulk soil (at tillering) and the rhizosphere soil (at harvest). Hence, biochar decreased stability of lignin in paddy soil. Our study provided evidence that long‐term biochar application changed the molecular composition and dynamics of degradation of SOM. These results deepen our understanding of the mechanisms governing SOM stability in agricultural ecosystems.

Influence mechanisms of iron, aluminum and manganese oxides on the mineralization of organic matter in paddy soil

The mineralization of soil organic matter (SOM) is closely related to the emission of greenhouse gas into atmosphere and the stability of organic carbon in soil. The influence of minerals on SOM mineralization in the specific soil received very few attentions. The influence characteristics and potential mechanisms of oxides on the mineralization of SOM in the paddy soil were observed in this study by incubating soil with the addition (dosage: 10 g kg−1) of prepared gibbsite, goethite, ferrihydrite or birnessite for 60 days. A sequence control treatment (753 mg CO2–C kg−1) > goethite treatment (656 mg CO2–C kg−1) ≈ gibbsite treatment (649 mg CO2–C kg−1) > birnessite treatment (529 mg CO2–C kg−1) > ferrihydrite treatment (441 mg CO2–C kg −1) was found in the cumulative amount of released CO2 in 60 days of incubation. Oxides especially ferrihydrite significantly decreased the content of dissolved organic matter (DOM) but tended to increase the content of microbial biomass carbon (MBC). The molecular structure of DOM in the paddy soil was simplified by gibbsite, ferrihydrite and birnessite after the incubation. Oxides especially birnessite and ferrihydrite reduced soil pH and the content of soil available N but increased soil redox potential (Eh). All examined oxides especially Fe oxides enhanced soil bacterial abundance but only birnessite significantly affected bacterial composition at phyla level. The stimulation on the immobilization and/or microbial assimilation of labile organic carbon, the modulation on soil basic properties (available N, pH, Eh), and the decrease of the relative abundance of some decomposing bacteria phyla such as Actinobacteria were the potential pathways of oxides in decreasing SOM mineralization.

Electron shuttles facilitate anaerobic methane oxidation coupled to nitrous oxide reduction in paddy soil

Anaerobic oxidation of methane (AOM) coupled with simultaneous consumption of nitrous oxide (N2O) has been recently reported, but the underlying coupling mechanism remains unclear. Here, we investigated N2O-mediated AOM in paddy soil under amendment of electron shuttles (anthraquinone-2,6-disulphonate and methylviologen). Sixty-day anaerobic incubation showed that N2O-mediated AOM occurred in paddy soil, and that electron shuttles stimulated AOM coupled to N2O reduction. Approximate 60% of 13CH4–C was converted into soil organic matter in paddy soil during anaerobic oxidation. Our results suggest that electron shuttles may play important roles in mitigating the emission of greenhouse gases (CH4 and N2O) and increasing organic matter in paddy soils.

Effects of high concentrations of sulfate on dissolved organic matter in paddy soils revealed by excitation-emission matrix analyzing

The problem of sulfate pollution is becoming increasingly serious in freshwater and wetlands. Since paddy fields are the largest constructed wetland in Earth’s surface, the increased sulfate input may have great effect on dissolved organic matter (DOM) in paddy soils. To understand these effects, a 24-day anaerobic incubation experiment was conducted with four Chinese paddy soils amended with high concentrations (0, 10, 25, 50, and 100 mmol L−1) of Na2SO4. Dissolved organic carbon (DOC) and chlorophyll a (Chl a) concentrations were determined after incubation. Parallel factor analysis (PARAFAC) of the excitation–emission matrix (EEM) spectra was used to analyze the DOM composition. In all four soils, DOC concentrations generally increased with increasing sulfate concentration, while the Chl a concentrations decreased. The EEM spectra of DOM were resolved into four components by PARAFAC. With increasing sulfate concentration, the proportion of the ultraviolet C humic acid-like compound decreased and the tyrosine-like compound increased in two algae-rich soils (Sichuan and Tianjin). No obvious variation was observed in the humification index (HIX) or the ratio of peak β to peak α (β:α) in any soils with added sulfate. Specific ultra-violet absorbance at 254 nm (SUVA254) decreased with increasing sulfate concentration in Jilin, Tianjin, and Ningxia soils, and the fluorescence index (FI) decreased in two algae-rich soils. In conclusion, although sulfate addition increased the DOC concentration, the DOM composition depended more strongly on soil type and physicochemical properties than sulfate. Sulfate addition only affected soil DOM origin and composition by inhibiting algal growth in algae-rich paddy soils.

The effects of fertilization treatments and cropping systems on long-term dynamics and spectroscopic characteristics of dissolved organic matter in paddy soil

ABSTRACT We investigated a series of long-term soil samples (1976–2015) collected from a paddy field that underwent a conversion of cropping systems from double cropping to single cropping in 1991, with consistently different fertilization treatments. The fertilization treatments include (i) chemical fertilizer, (ii) chemical fertilizer + rice straw, (iii) chemical fertilizer + cattle manure compost, and (iv) chemical fertilizer + rice straw + fused phosphate + calcium silicate. We extracted dissolved organic matter (DOM) from the soil samples and analyzed it for the concentration of humic substances as well as for optical properties using ultraviolet-visible and excitation-emission matrix (EEM) spectroscopy. Different fertilization treatments did not significantly (P > 0.05) affect DOM and humic substance concentrations, while the soil total carbon (TC) content and DOM concentration consistently increased in the double cropping system and decreased in the single cropping system. The DOM/TC ratio, which is an indicator of the proportion of the labile fraction in TC pool, also showed a similar pattern. These results indicate that cropping systems had a larger effect on DOM concentration than the fertilization treatments. Five fluorescent components identified by parallel factor analysis of EEMs (three humic-like and two protein-like components) showed significant differences in the relative abundances under the different fertilization treatments. The manure compost application resulted in a higher tyrosine-like component relative abundance than the other chemical fertilizer applications. Overall, fertilization affected the DOM composition, while the cropping systems impacted both the concentration and spectroscopic property of paddy soil DOM.

Molecular Chemodiversity of Dissolved Organic Matter in Paddy Soils.

Organic matter (OM), and dissolved organic matter (DOM), have a major influence upon biogeochemical processes; most significantly, the carbon cycle. To date, very few studies have examined the spatial heterogeneity of DOM in paddy soils. Thus, very little is known about the DOM molecular profiles and the key environmental factors that underpin DOM molecular chemodiversity in paddy soils. Here, Fourier-Transform Ion Cyclotron Resonance Mass Spectrometry was applied to unambiguously resolve 11 361 molecular formulas in 16 paddy soils; thereby elucidating the molecular characteristics of paddy soil DOM. Soil pH, iron complexing index (Fep/FeR) and C/N ratio were established to be key factors controlling DOM profiles. Polycyclic aromatics (derived from combustion) and polyphenols (derived from plants) increased with increasing pH, while polyphenols molecules, pyrogenic aromatics, and carboxylic compounds decreased with increasing iron complexing index. Patterns in molecular profiles indicated DOM in paddy soils to become more recalcitrant at higher soil C/N ratio and higher pH. Furthermore, plant-derived polyphenols and pyrogenic DOM were retained favorably by iron and the chemodiversity of DOM in paddy soil increased with increasing soil C/N ratios. This study provides critical information about DOM characteristics at a molecular level and will inform better global management of soil carbon in paddy soil ecosystems.

Microbial Differentiation on Chemical Properties of Paddy Soils in Reclaimed Tidal Lands at Western-coast Area of Korea

The scientific information on the microbial differentiation according to the changes in chemical properties of paddy soil in reclaimed tidal lands is not enough to understand the reclamation processes. The changes in microflora based on the chemical properties of paddy soils at the same sites of reclaimed tidal lands (21 samples from Nampo, Ewon, Sukmoon and Shihwa sites) were investigated in 2013 and 2015. In general, organic matter in paddy soils increased whereas pH decreased with the reclamation time. The electrical conductivities (EC) of soil samples were closely related to the exchangeable Na+. With an increases in EC of paddy soils from 0.39 to 48.9 dS m -1 , the ratios (%) of halotolerant and halophilic bacteria to mesophilic bacteria proportionally increased from 0.2% to 102,000%. The population of halotolerant and halophilic bacteria in total microflora was also differentiated with the changes in EC of the same sites from reclaimed tidal soils within 2 years. The population of mesophilic bacteria decreased with an increase in EC above 5 dS m -1 . The microflora including halotolerant and halophilic bacteria could be a candidate as a biological parameter in evaluating the reclamation processes in addition to the chemical index of EC.

Biochar decreased microbial metabolic quotient and shifted community composition four years after a single incorporation in a slightly acid rice paddy from southwest China

While numerous studies both in laboratory and field have showed short term impacts of biochar on soil microbial community, there have been comparatively few reports addressing its long term impacts particular in field condition. This study investigated the changes of microbial community activity and composition in a rice paddy four years after a single incorporation of biochar at 20 and 40t/ha. The results indicated that biochar amendment after four years increased soil pH, soil organic C (SOC), total N and C/N ratio and decreased bulk density, particularly for the 40t/ha treatment compared to the control (0t/ha). Though no significant difference was observed in soil basal respiration, biochar amendment increased soil microbial biomass C and resulted in a significantly lower metabolic quotient. Besides, dehydrogenase and β-glucosidase activities were significantly decreased under biochar amendment relative to the control. The results of Illumina Miseq sequencing showed that biochar increased α-diversity of bacteria but decreased that of fungi and changed both bacterial and fungal community structures significantly. Biochar did not change the relative abundances of majority of bacteria at phylum level with the exception of a significant reduction of Actinobacteria, but significantly changed most of bacterial groups at genus level, particularly at 40t/ha. In contrast, biochar significantly decreased the relative abundances of Ascomycota and Basidiomycota by 11% and 66% and increased the relative abundances of Zygomycota by 147% at 40t/ha compared to the non-amended soil. Redundancy analysis (RDA) indicated that biochar induced changes in soil chemical properties, such as pH, SOC and C/N, were important factors driving community composition shifts. This study suggested that biochar amendment may increase microbial C use efficiency and reduce some microorganisms that are capable of decomposing more recalcitrant soil C, which may help stabilization of soil organic matter in paddy soil in long term.

Influence of pH on the redox chemistry of metal (hydr)oxides and organic matter in paddy soils

The primary purpose of this study was to determine how flooding and draining cycles affect the redox chemistry of metal (hydr)oxides and organic matter in paddy soils and how the pH influences these processes. Our secondary purpose was to determine to what extent a geochemical thermodynamic equilibrium model can be used to predict the solubility of Mn and Fe during flooding and draining cycles in paddy soils.

Organic C and N mineralization as affected by dissolved organic matter in paddy soils of subtropical China

In this study, dynamics of dissolved organic matter (DOM) and mineralization of soil organic C and N in subtropical China were investigated. Paddy and upland soils were derived from Tertiary sandstone, Quaternary red clay and river alluvium with low, middle, and high levels of soil fertility. Dissolved organic carbon contents (DOC) varied from 3.8 to 68.7 mg/kg and dissolved organic nitrogen contents (DON) ranged from 2.9 to 18.3 mg/kg, respectively, during the experimental period. Both DOC and DON increased with increasing soil fertility. The content of DOM was characterized by strong seasonal fluctuation in soils, and seasonal patterns showed the maximum value in Nov and the minimum in July. The contribution of DOM to mineralization of C and N was significantly affected by soil types and seasons. DOM removal significantly decreased the cumulative organic C mineralization in soils derived from Tertiary sandstone and River alluvium (by 1.6%–20.3% with an average of 7.1%), while it did not significantly change those in soils derived from Quaternary red clay. Similarly, the cumulative mineralization of N in paddy soils after DOM removal decreased by 6.7%–27.3% over seasons. In this study, DOM played an important role in soil C and N mineralization in subtropical China. It appears that contribution of DOM to minerilization of C and N in paddy soils was possibly related to DOM content and composition.

Crop residue management effects on organic matter in paddy soils — The lignin component

Intensified rice cropping is not only increasing rice yields but also the amount of crop residue. In particular the lignin component of the residues may accumulate in submerged systems such as paddy soils due to incomplete decomposition under oxygen-limited conditions. Here, we examined the short and long-term effects of crop residue management on soil organic matter (SOM) in paddy soils of two experimental sites in China (Nanjing, Changsha) and one in the Philippines (Los Baños, Laguna). We determined organic C and total N, characterized the composition of alkaline-extractable SOM by 13C NMR spectroscopy, and the lignin component by CuO oxidation. The results revealed no significant changes in organic C, total N and total lignin-derived phenols upon incorporation of crop residue at the recently established experiments at Nanjing and Los Baños. In contrast, all tested variables increased at the long-term experiment at Changsha. Within 16 years (1990 to 2005), the organic C increased by 41% in the ‘residue incorporated’ plots while at the ‘residue removed’ plots the increase was 16%. Similar trends were found for total N. Lignin-derived phenols in ‘residue incorporation’ plots increased about 45% (12% for ‘residue removed’ plots). Parameters addressing alteration of lignin changed at the initial stages of the experiments. These changes are comparable to those observed in aerated cropping systems. No further changes in the lignin composition occurred at later stages of the experiments. The 13C NMR spectra revealed a largely unchanged SOM composition, thus supporting the results of the CuO oxidation. In summary, the additional input of crop residue in paddy soils seems to result in the mere accumulation of OM; the larger proportion of lignin-derived phenols reflects the input of crop residue. The comparable degree in oxidative alteration of lignin between ‘residue removed’ and ‘residue incorporated’ treatments indicates that the soil systems can cope with the increased residue-derived lignin input.

Effects of Temperature, Water Regime, Light, and Soil Properties on 15N2 Fixation Associated with Decomposition of Organic Matter in Paddy Soils

Effective utilization of the biological nitrogen fixation (BNF) activity is important for enhancing the N fertility of paddy soils and for developing sustainable rice cultivation systems. To analyze the soil factors that affect BNF in paddy soils, in this study, the effects of the temperature, water regime, and long-term soil management on 1 5 N 2 fixation were examined in relation to the decomposition of organic matter in incubation experiments. Within the range of 15-30°C, heterotrophic 1 5 N 2 fixation under dark conditions changed almost proportionally to the formation of CO 2 +CH 4 with glucose and straw as C sources by increasing the temperature, while the C use efficiency (N 2 fixed/(CO 2 +CH 4 )) was relatively higher at low temperatures in the presence of cellulose. The examination of the effect of the water regime on heterotrophic 1 5 N 2 fixation indicated that flooding after aerobic conditions promoted heterotrophic 1 5 N 2 fixation as well as the decomposition of cellulose. Among the soils with different types of management, the soils amended with manure and rice straw. showed the largest photodependent 1 5 N 2 fixation. On the other hand, the soils with a lower content of mineralizable-N tended to depend more on heterotrophic 1 5 N 2 fixation. Soils from paddy fields into upland fields (hereafter referred to as converted upland soils), particularly showed a high heterotrophic 1 5 N 2 fixation. Overall, it appeared that the management of organic matter application and the water regime may result in significant variations in BNF in paddy soils. The results obtained suggested that further studies should he conducted on the C and N metabolism involved in BNF during the decomposition processes of organic matter along with the changes in the soil redox status to identify methods for efficient soil management to promote BNF.