Surface Paddy Soil Research Articles

Denitrification contributes to N2O emission in paddy soils.

Denitrification is vital to nitrogen removal and N2O release in ecosystems; in this regard, paddy soils exhibit strong denitrifying ability. However, the underlying mechanism of N2O emission from denitrification in paddy soils is yet to be elucidated. In this study, the potential N2O emission rate, enzymatic activity for N2O production and reduction, gene abundance, and community composition during denitrification were investigated using the 15N isotope tracer technique combined with slurry incubation, enzymatic activity detection, quantitative polymerase chain reaction (qPCR), and metagenomic sequencing. Results of incubation experiments showed that the average potential N2O emission rates were 0.51 ± 0.20 μmol⋅N⋅kg-1⋅h-1, which constituted 2.16 ± 0.85% of the denitrification end-products. The enzymatic activity for N2O production was 2.77-8.94 times than that for N2O reduction, indicating an imbalance between N2O production and reduction. The gene abundance ratio of nir to nosZ from qPCR results further supported the imbalance. Results of metagenomic analysis showed that, although Proteobacteria was the common phylum for denitrification genes, other dominant community compositions varied for different denitrification genes. Gammaproteobacteria and other phyla containing the norB gene without nosZ genes, including Actinobacteria, Planctomycetes, Desulfobacterota, Cyanobacteria, Acidobacteria, Bacteroidetes, and Myxococcus, may contribute to N2O emission from paddy soils. Our results suggest that denitrification is highly modular, with different microbial communities collaborating to complete the denitrification process, thus resulting in an emission estimation of 13.67 ± 5.44 g N2O⋅m-2⋅yr-1 in surface paddy soils.

Type-specific quantification of particulate methane monooxygenase gene of methane-oxidizing bacteria at the oxic-anoxic interface of a surface paddy soil by digital PCR.

Aerobic methane-oxidizing bacteria (MOB) play an important role in mitigating methane emissions from paddy fields. In this study, we developed a differential quantification method for the copy number of pmoA genes of type Ia, Ib, and IIa MOB in paddy field soil using chip-based digital PCR. Three probes specific to the pmoA of type Ia, Ib, and IIa MOB worked well in digital PCR quantification when genomic DNA of MOB isolates and PCR-amplified DNA fragments of pmoA were examined as templates. When pmoA genes in the surface soil layer of a flooded paddy were quantified by digital PCR, the copy numbers of type Ia, Ib, and IIa MOB were 105 -106 , 105 -106 , and 107 copies g-1 dry soil, respectively, with the highest values in the top 0-2-mm soil layer. Especially, the copy numbers of type Ia and Ib MOB increased by 240% and 380% at the top layer after soil flooding, suggesting that the soil circumstances at the oxic-anoxic interfaces were more preferential for growth of type I MOB than type II MOB. Thus, type I MOB likely play an important role in the methane consumption at the surface paddy soil.

Spatial Variations in Organic Carbon Pools and Their Responses to Different Annual Straw Return Rates in Surface Paddy Soils in South China

To identify the effects of straw return on different organic carbon pools in surface paddy soils (0–20 cm), a total of 33 soil samples under different annual straw return rates (SRr) was collected, and then the samples were analyzed based on a 100-day incubation. The data from acid hydrolysis-incubation experiments were fitted to a three-pool first-order kinetics model that divided soil organic carbon (SOC) into active (Ca), slow (Cs) and resistant (Cr) pools. The results showed that the mean pool sizes of Ca, Cs, and Cr were 0.27, 10.26, and 13.46 g·kg−1, representing a mean of 1.35%, 41.91%, and 56.74% of the total SOC (TOC), respectively. The SOC pools in the surface paddy soils in Dongxiang had a small Ca pool but had longer mean residence times of the Ca and Cs pools than those in other regions in China. The three carbon pools were less affected by the paddy soil type but showed obvious spatial variations. The SRr contributed a strong positive effect on the variability of Cs and Cr, especially on Cs variability, while it had very little effect on Ca variability. Soil available nitrogen dominated the variability in TOC and Cr compared to the other soil properties. Therefore, the Cs pool is more sensitive than the other carbon pools to long-term straw return.

Rice-crayfish co-culture increases microbial necromass' contribution to the soil nitrogen pool

The rice-crayfish co-culture (RC) is a putative sustainable agricultural system. However, studies on the ecological effects of long-term RC systems were still lacking. Here, we compare enzymatic stoichiometry, microbial necromass, and microbial community between the RC and rice monoculture systems (RM). Soil enzymatic stoichiometry analysis showed that after transformation from RM to RC for about three years, ammonium nitrogen (NH4+-N) availability increased to depress relative N-acquiring enzyme production, especially for leucine aminopeptidase. The contents of microbial necromass increased approximately onefold in the RC system, making microbial necromass' contribution to the soil nitrogen (N) reach up to 46.72%. Elevation in NH4+ decreased N-acquiring enzyme, and a relatively more effective C acquisition likely benefited microbial necromass retention and production in the RC system. This study highlights that the rice-crayfish co-culture could modify the N pool of the surface paddy soil.

A Comprehensive Exploration on Pollution Characteristics and Ecological Risks of Heavy Metals in Surface Paddy Soils around a Large Copper Smelter, Southeast China

To assess heavy metal pollution and ecological risk, a total of 28 surface paddy soil samples were collected and analyzed around a famous copper smelter in Guixi, China. The results showed that all sites were heavily contaminated by both Cu and Cd, compared with soil background values, whose average concentrations exceeded the standard by 5.7 and 12.3 times, respectively, posing a slight ecological risk related to Cu (potential ecological risk index <40) and an extremely serious ecological risk related to Cd (potential ecological risk index >320). The risks were also demonstrated through the speciation analyses of Cu (CaCl2-Cu 2.63%, acid-soluble Cu 8.67%, and residual Cu 74.17%, on average) and Cd (CaCl2-Cd 47.30%, acid-soluble Cd 45.02%, and residual Cd 28.87%, on average) in the surface paddy soil, including the use of a CaCl2 extraction procedure and the BCR (Community Bureau of Reference) sequential extraction scheme. Several soil properties (residual carbon, cation exchange capacity, and soil texture) were significantly correlated with soil Cd but made a small contribution to their variability with a poor linear fit because of external Cd input to the soil, while soil total potassium largely influenced the soil Cu species except for residual Cu. Therefore, an effective Cu pollution regulation strategy through soil potassium control is suggested for this smelter soil.

Distribution and determinants of organic carbon and available nutrients in tropical paddy soils revealed by high–resolution sampling

Rice is one of the most important crops in China; however, the loss of paddy area and imbalanced soil nutrient composition threaten food security in China owing to the increasing food demand. Soil nutrients significantly influence soil fertility, quality, and productivity, thereby controlling rice production and agroecosystem sustainability. However, a systematic assessment of the nutrient distribution and its driving factors is lacking in the low–yield paddy soils of southern China. In this study, 3239 surface paddy soil samples covering an area of 1278 km2 were collected and analyzed for soil organic carbon (SOC), alkaline hydrolyzable nitrogen (AN), available phosphorus (AP), and available potassium (AK). The dominant factors influencing SOC, AN, AP, and AK were identified using a Random Forest (RF) method. We identified deficient levels of SOC (8.94 g kg−1), SOC density (SOCD, 21.1 t ha−1), AN (55.3 mg kg−1), and AK (37.2 mg kg−1) in tropical paddy soils, but high levels of AP (34.7 mg kg−1). According to the RF, SOCD was significantly correlated with soil AN and AK, implying their coupled cycling in tropical paddy soils; however, the negative correlation between SOC and AP signified decoupling. Compared with waterloggogenic paddy soils, gleyed paddy soils had significantly higher SOC concentrations and densities but significantly lower AP and AK concentrations and pH. Analyses of cropping system effects indicated that double– and triple–cropping systems had significantly higher SOC, bulk density (BD), SOCD, AP, AK, and pH than those of single–cropping systems. These results suggested that tropical paddy soils can potentially sequester OC and available nutrients via efficient soil management. Considering the deficient AN and AK levels of single–cropping system and their high importance on SOCD and soil productivity, double– and triple–cropping systems (generally with high N and K fertilization and residue return) are recommended as an efficient approach to improve the poor soil nutrient status and enhance the OC pool and rice production in tropical paddy soils.

Spatial Distribution Characteristic of Antimony in Typical Paddy Soil of Eastern Hunan Province, China

Considering the eastern part of Hunan Province as the research area, 34 sampling sites were set up, 198 samples were collected from representative paddy soil, the distribution characteristics of antimony (Sb) were studied. The results showed that: (1) The content of Sb on the surface of paddy soil ranging from 0.07 to 11.00 mg/kg and the geometric mean was 1.56 mg/kg. (2) The distribution of contents of Sb in paddy soil in different areas was shown as Yueyang > Changsha > Zhuzhou > Xiangtan. (3) Sb showed a strong migration in paddy soil in the research area and its content increased initially and then decreased or gradually decreased with the increase of profile depth. (4) The content of Sb in the substratum was significantly affected by parent materials.

Photic Biofilms Mediated Distant Nitrate Reduction at the Soil–Water Interface of Paddy Fields

Photic biofilms widely exist on the surface of paddy soil and play a key role in nutrient transformation and accumulation in the soil–water interface. However, whether and how photic biofilms affec...

Mean residence times of active and slow soil organic carbon pools in croplands across China

Using active (Ca) and slow (Cs) soil organic carbon (SOC) pools to simulate changes in SOC stocks has been determined to be a major improvement over treating SOC as a single, homogeneous pool. However, regional patterns and primary controls of the mean residence times of the Ca (MRTa) and Cs (MRTs) pools in China’s croplands are largely unknown. In this study, a total of 373 samples from six regions were analysed based on a 100-d incubation experiment. The data from acid hydrolysis-incubation experiments were fitted to a three-pool first-order kinetics model that estimated MRTa = 1/Ka and MRTs = 1/Ks (Ka and Ks represent the decomposition rate constants of the Ca and Cs pools, respectively). The results showed that significant regional differences were found in the MRTa and MRTs in the surface upland soils (0–20 cm) across China, in which the longest MRTa and MRTs values were 72 d in the north China and 24.0 yr in the northeast China, the shortest MRTa and MRTs values were 12 d and 4.7 yr, respectively, both in the south-central China, while relatively small regional differences were observed in the values of the surface paddy soils (0–20 cm). Both the MRTa and the MRTs in upland soils (average: 30 d and 13.5 yr) were longer than the corresponding values in the paddy soils (average: 11 d and 5.4 yr), especially in the southwest China. Thus, the carbon sequestration potential of paddy soils needs to be revalued. The climate could partly explain the regional distribution of the MRTa and MRTs in the surface upland and paddy soils in China; however, as Chinese agricuture intensified from 1980 to 2010, the regional variabilities of MRTa and MRTs in the surface upland and paddy soils were primarily dominated by the soil texture. Therefore, more attention should be paid to the destruction of mechanical composition caused by land-use change or conventional tillage methods when faced with future carbon sequestration in croplands.

Source Identification of Heavy Metals in Surface Paddy Soils Using Accumulated Elemental Ratios Coupled with MLR.

Source identification of heavy metals in agricultural soils using small sample sizes, simple experimental procedures, and convenient analysis is urgently required. This study employed a simple source identification model using a visual comparison via radar plots, cluster analysis, principal component analysis, and a multiple linear regression model to determine the source of heavy metal pollution in soil samples from the Chang-Zhu-Tan urban agglomeration area of China. The elemental compositions of major pollution sources (atmospheric deposition, organic fertilizer, irrigation water, and tailings) were compared with soil samples from 11 study locations and the model was used to determine the relative contribution of different pollution sources at each sample site. The results showed that the model successfully calculated the contribution of different pollution sources at each site based on the pollution characteristics and contaminant transport rules of the region. The proposed method overcomes the requirement for extensive data and complex experimental procedures. Furthermore, the model can determine the source of heavy metal contamination in single or small plots, which is important for the prevention and control of heavy metal soil pollution and remediation at the plot scale.

Periphyton enhances arsenic release and methylation at the soil-water interface of paddy soils

Periphyton is ubiquitous in rice paddy fields, however its role in paddy soil arsenic (As) biogeochemistry remains unexplored. In this study, microcosm incubations and extensive field sampling were used to better understand the roles of periphyton on As mobility and transformation at the soil-water interface. Microcosm incubations revealed that periphyton on the paddy soil surface enhanced As release to water and increased methylated As contents at the soil-water interface. Experimental additions of dissolved phosphate did not significantly affect these processes. The presence of periphyton increased the dissolved organic carbon (DOC) content of the surface soil which may have played a role in the increased As mobility. However, the increase in methylated As species at the soil-water interface is indicative of detoxification processes of As by periphyton. The results from the field study revealed a high abundance and diversity of As biotransformation and detoxification genes in periphyton. Genera of Kineosporia, Limisphaera, Ornatilinea, Ktedonosporobacter and Anaerolinea played key roles in shaping arsM harboring microbe communities in field periphyton. These results highlight the importance of periphyton in the behavior of As in paddy soils and can potentially facilitate improved management of As contamination in paddy soils.

Effect of standing water levels on methane gas emission and yield performance of transplanted Aman rice (Oryza sativa L. cv. BRRI dhan51)

An experiment was conducted to investigate the effect of standing water levels on methane gas emission and yield of transplanted Aman rice (cv. BRRI dhan51) during July to December 2015 at medium low-lying area of Mohangonj upazila under Netrokona district. The experiment comprised five standing water levels on surface paddy soil viz., 5 cm, 10 cm, 15 cm, 20 cm and 25 cm. Methane (CH4) gas emission, yield components and yield of transplanted Aman rice were significantly affected by standing water levels on surface paddy soil. CH4 emission was gradually increased with rising standing water levels and remained static condition at 20-25 cm water level. The highest CH4 emission was observed at 20 cm standing water level and the lowest CH4 emission was recorded at 5 cm water level. The highest CH4 peak recorded at 85 days after transplanting (DAT). The highest CH4 flux (36.59 mg/m2/h) was observed in treatment 20 cm water level whereas the lowest CH4flux (21.17 mg/m2/h) was observed in 5 cm water level. Finally, the CH4 emission dropped at 108 DAT. On and average, the CH4 emission rate during rice cultivation followed 20 cm > 25 cm> 15 cm > 10 cm > 5 cm water level. Soil Eh gradually decreased with progress of time and plant growth and at 85 DAT highly reduced condition developed in all treatments. The maximum reduced condition was observed (-238.67 mV) in treatment 20 cm water level and minimum one (- 214.667 mV) was found in 5 cm water level. The highest grain (5260 kg ha-1) and straw (6725.0 kg ha-1) yields were obtained at 10 cm standing water level while the lowest grain (4191.6 kg ha-1) and straw (5050.0 kg ha-1) yields were recorded in 5 cm and 15 cm water level, respectively. It may be concluded that 10 cm standing water level is beneficial for transplant Aman rice (cv. BRRI dhan51) cultivation in low lying area in respect of grain yield and environmental issues.

The influence of periphyton on the migration and transformation of arsenic in the paddy soil: Rules and mechanisms

Periphyton, composed of algae, bacteria, protozoa, epiphytes, and detritus, is widely distributed on the surfaces of paddy soils. Little is known about the interactions between the periphyton and arsenic (As) in the paddy soil. In the present study, model paddy ecosystems with and without periphyton were set up to explore the effects of periphyton on As migration and transformation in soil. According to the results, periphyton played dual roles in the mobility of As in soil. Periphyton on the surface of paddy soil could significantly increase the mobility and bioavailability of As in soils in the rice tillering stage because of the increased pH and the decreased Eh. The As uptake by rice also increased in the presence of periphyton. However, a significant fraction of the released As was further entrapped by the periphyton, significantly decreasing As concentration in pore water. As biotransformation genes, including aioA, arrA, arsC, and arsM, were identified in periphyton, with arsM being the most abundant in periphyton and soil. Periphyton significantly decreased the abundance of aioA, but increased the abundance of arsC in soils. Cupriavidus and Afipia, which are involved in As(V) cytoplasmic reduction, significantly increased in the presence of periphyton. Periphyton exerted minor effects on the highly abundant and predominant bacteria but had major effects on the less abundant bacteria in the paddy soil. The results of the present study could facilitate the regulation of As contamination in paddy soil, and enhance our understanding of the role of periphyton in the As biogeochemical cycle.

Distribution Characteristics of Soil Heavy Metals, their Source Identification and their Changes Influenced by Anthropogenic Cultivation Activities in Purple Hilly Regions of Sichuan Basin, China

To investigate the distribution and, possible sources of soil heavy metals and the effects of anthropogenic cultivation activities on the contents of these metals in purple hilly regions of China, samples of soils and parent materials in purple hilly regions of China were collected before and after land consolidation and after 7 and 19 months of intensive agricultural production, and contents of Pb, Cd, Cu, Zn, Cr, Ni, Hg, and As were analyzed. The results showed that, in the soil samples compared with the parent materials, the greatest accumulation of Cd, As, and Hg occurred in surface layer and a few other horizons. Furthermore, the Cd and Hg contents showed the largest differences among different soil horizons. Parent materials played an important role and anthropogenic cultivation activities greatly affected the soil heavy metals. Under traditional agriculture, Pb, Cd, and Hg had higher contents in surface paddy soils than in surface dry land soils. Land consolidation induced the homogenization of heavy metal elements and dramatically reduced Cd and Hg in the surface soils. The As and Hg contents in the surface soils increased after 19 months of intensive agricultural production. Multivariate statistical analysis (correlation analysis and principal component analysis (PCA)) showed that all eight metals were well represented by the first two principal components (PCs). PC1, which showed relatively high loadings for Cr, Zn, Cu, Ni, and As, was associated with parent materials and therefore represented natural sources. PC2 showed relatively high loadings for Pb, Cd, and Hg, reflecting the effect of air pollution under traditional agriculture, and showed relatively high loadings for Cd, Hg, As, and Pb, reflecting the effect of fertilization under intensive agriculture. This study provides guidelines for the prevention of soil heavy metal pollution and the remediation of polluted soils in purple hilly regions of Sichuan Basin, China.

Farm-Scale Estimation of Total Nitrogen Content in Surface Paddy Soils by Extraction with Commercially Available Hydrogen Peroxide

We recently proposed a simple method for estimating total nitrogen content in paddy soil. In this method, soil is extracted with a commercial 3% hydrogen peroxide (H2O2) solution at 25 °C for 40 h, and electrical conductivity (EC (H2O2)) of the extract is measured. This study aimed to further evaluate the method’s applicability to soil samples collected at the farm scale by using the original and six additional H2O2 solutions that are locally and commercially available. The results obtained with the original solution indicated that the determination coefficients between EC (H2O2) and total N were statistically significant at all farms examined: Moka, 0.78 (n = 13); Kyoto, 0.50 (n = 16); Kizu, 0.43 (n = 89); and Kawatabi, 0.25 (n = 18). The EC of the tested H2O2 solutions varied from less than 0.05 to 1.4 mS cm−1 because of the addition of different stabilizers. EC (H2O2) values obtained with the less stabilized H2O2 solutions (one from Japan, one from USA, and the analytical grade 6% solution) agreed well with those obtained with the original solution. Thus, the proposed method can be useful for estimating the farm-scale variation in soil total N, provided a H2O2 solution with a low EC (<0.2 mS cm−1) is used for the extraction.

Evolution of soil surface charge in a chronosequence of paddy soil derived from Alfisol

Limited information is available on the changes in surface electro–chemical properties of paddy soils during pedogenesis. This study evaluated the category, evolution and origin of soil surface charge along an Alfisol–derived paddy soil chronosequence. Ammonium acetate adsorption, as well as cesium (Cs+) adsorption and sequential desorption methods were applied to reveal the surface charge evolution along a paddy soil chronosequence. Results indicated that organic matter increased by 52.79%, 84.90%, 144.35%, and 126.52% in the surface paddy soils after 3, 8, 30, and 70 years of rice cultivation, respectively. The main phyllosilicates in Alfisol and Alfisol–derived paddy soils were montmorillonite and illite, which increased with paddy cultivation over time. Soil negative charge, as determined by ammonia (NH4+) adsorption, was strongly correlated with data from Cs+ adsorption methods. The negative charge on the surface paddy soil, as estimated by NH4+ adsorption, increased by 16.41%, 33.21%, 56.49%, and 31.68% after 3, 8, 30, and 70 years of rice cultivation, respectively. Further investigations revealed that an increasing variable surface charge is mainly attributed to soil organic matter accumulation. Also, hydroponics cultivation was shown to prompt the destruction and weathering of primary minerals and formation of soil secondary phyllosilicate minerals, which are mainly responsible for increasing permanent surface charge along the paddy soil chronosequence. Thus, the nutrients holding capacities of paddy soils are maintained or even improved, thereby increasing agricultural productivity, when compared with upland soils.

Distributions and environmental drivers of archaea and bacteria in paddy soils

The aim of this study is to investigate the abundance, diversity, and distribution of archaea and bacteria as affected by environment parameters in paddy soils, with focus on putative functional microbial groups related to redox processes. Because there is generally a high iron content in the soil, we also want to test a hypothesis that soil iron concentration significantly affects microbial diversity and distribution. Quantitative PCR and barcoded pyrosequencing of 16S ribosomal RNA genes were employed to investigate the abundance and community composition of archaeal and bacterial communities in 27 surface paddy soil samples. Pearson’s correlation, analysis of variance, partial least squares regression, principal coordinates analysis, and structural equation models were performed for the analyses of gene copy numbers, α-diversity, β-diversity, and relative abundances of archaea and bacteria and their relationships with environmental factors. Archaeal abundance was correlated greatest with temperature, but bacterial abundance was affected mainly by soil organic matter and total nitrogen content. Soil pH and concentrations of different ions were associated with archaeal and bacterial β-diversity. The relative abundances of Euryarchaeota and Thaumarchaeota were 61.3 and 13.1% of archaea and correlated with soil pH, which may affect the availability of substrates to methanogens and ammonia oxidizers. Dominant bacterial phyla were Proteobacteria (32.4%), Acidobacteria (17.8%), Bacteroidetes (9.3%), and Verrucomicrobia (6.0%). The relative abundances of putative bacterial reducers of nitrate, Fe(III), sulfate, and sulfur, and oxidizers of ammonia, nitrite, reduced sulfur, and C1 compounds had positive, negative, or non-significant correlations with the concentrations of their substrates. Soil iron concentration was correlated only with the distributions of some putative iron-reducing bacteria. In paddy soils characterized by dynamic redox processes, archaea and bacteria differ in relationships of abundance, diversity, and distribution with environmental factors. Especially, the concentrations of electron donors or acceptors can explain the distributions of some but not all the putative functional microbial groups related to redox processes. Depending on pH range, soil pH has a strong impact on microbial ecology in paddy soils.

The biogeography of fungal communities in paddy soils is mainly driven by geographic distance

The aim of this study is to answer how the biogeographic patterns of fungi are affected by spatial and environmental factors in paddy soils characterized by unique field management. Given the generally low C/N ratios of paddy soils, we also want to test a hypothesis that the dominant fungi in paddy soils are Ascomycota, which reportedly prefer habitats with low soil C/N ratios. Using quantitative PCR and barcoded pyrosequencing, we investigated the abundance, diversity, and community composition of fungal communities in 30 surface paddy soil samples collected from 10 rice cultivation regions of China. Pearson’s correlation, analysis of variance, partial least squares regression, principal coordinates analysis, and variation partition were performed for analyses of gene copy numbers, α-diversity, β-diversity, and relative abundances of fungal taxa and their relationships with environmental factors. The abundance of fungal 18S rRNA gene varied from 106.4 to 108.6 copies g−1 soil, and was positively correlated with soil sand, organic matter, and total nitrogen content, and negatively correlated with soil chloride concentration. Ascomycota comprised 88% of total fungal sequences and increased in relative abundance with increasing soil pH and decreasing mean annual temperature (MAT) and precipitation (MAP). The predominance of Ascomycota in fungal communities is probably due to the low soil C/N ratios (9–15) in the paddy soils studied. The α-diversity increased with MAT, MAP, and soil nitrate-N and total nitrogen content but decreased with soil pH, clay content, chloride concentration, and C/N ratio. Variation partition revealed that fungal β-diversity was mainly driven by geographic distance. In paddy soils which are characterized by intensive rice cropping practices, fungal abundance is mainly influenced by soil properties, fungal α-diversity is constrained by both climatic factors and soil properties, while fungal community compositions are mainly structured by geographic distance.

Equality testing for soil grid unit resolutions to polygon unit scales with DNDC modeling of regional SOC pools

Matching soil grid unit resolutions with polygon unit map scales is important to minimize the uncertainty of regional soil organic carbon (SOC) pool simulation due to their strong influences on the modeling. A series of soil grid units at varying cell sizes was derived from soil polygon units at six map scales, namely, 1:50 000 (C5), 1:200 000 (D2), 1:500 000 (P5), 1:1 000 000 (N1), 1: 4 000 000 (N4) and 1:14 000 000 (N14), in the Taihu Region of China. Both soil unit formats were used for regional SOC pool simulation with a DeNitrification-DeComposition (DNDC) process-based model, which spans the time period from 1982 to 2000 at the six map scales. Four indices, namely, soil type number (STN), area (AREA), average SOC density (ASOCD) and total SOC stocks (SOCS) of surface paddy soils that were simulated by the DNDC, were distinguished from all these soil polygon and grid units. Subjecting to the four index values (IV) from the parent polygon units, the variations in an index value (VIV, %) from the grid units were used to assess its dataset accuracy and redundancy, which reflects the uncertainty in the simulation of SOC pools. Optimal soil grid unit resolutions were generated and suggested for the DNDC simulation of regional SOC pools, matching their respective soil polygon unit map scales. With these optimal raster resolutions, the soil grid units datasets can have the same accuracy as their parent polygon units datasets without any redundancy, when VIV < 1% was assumed to be a criterion for all four indices. A quadratic curve regression model, namely, y =–0.80 × 10–6 x 2 + 0.0228x + 0.0211 (R 2 = 0.9994, P < 0.05), and a power function model R = 10.394Ŝ 0.2153 (R 2 = 0.9759, P < 0.05) were revealed, which describe the relationship between the optimal soil grid unit resolution (y, km) and soil polygon unit map scale (1:10 000x), the ratio (R, %) of the optimal soil grid size to average polygon patch size (Ŝ, km2) and the Ŝ, with the highest R 2 among different mathematical regressions, respectively. This knowledge may facilitate the grid partitioning of regions during the investigation and simulation of SOC pool dynamics at a certain map scale, and be referenced to other landscape polygon patches’ mesh partition.

N2O Consumption Ability of Submerged Paddy Soil and the Regulatory Mechanism

A large number of researches showed that the N2O negative emissions from flooding paddy fields, peatlands and other wetlands ecosystem were frequent and considerable, which is of great significance on alleviating the greenhouse gas effect. However, there are few reports about the transformation and microbial mechanism of N2O between atmosphere and paddy soil. The slurry of surface paddy soil (0-5 cm) was incubated in laboratory conditions, and the effect of enhanced N2O concentrations in headspace on the N2O consumption capacity of submerged paddy soil and the response of nosZ gene abundance were explored. The results showed that, paddy soil under flooding and anaerobic conditions harbored very strong potential of N2O reduction along with a relatively high nosZ gene abundance (108 copies·g-1 dry soil at DNA level). Regression analysis presented the N2O concentrations in headspace were positively correlated (r2=1, P<0.001) to the N2O consumption rates of paddy soil slurry, indicating the high N2O concentration could stimulate the N2O consumption power, to a very high rate of 4567.99 μg·(m2·h)-1. Meanwhile, there were no significant differences in the high abundance of nosZ gene among N2O treatments, demonstrating the nosZ gene abundance at DNA level might not be the main controller of N2O consumption ability in submerged paddy soil and further study on the key microbial factor is needed.