Methane and nitrous oxide emissions from organic and conventional rice cropping systems in Southeast China

To evaluate the impacts of organic cropping system on global warming potentials (GWPs), field measurements of CH₄ and N₂O were taken in conventional and organic rice (Oryza sativa L.) cropping systems in southeast China. Rice paddies were under various water regimes, including continuous flooding (F), flooding–midseason drainage–reflooding (F-D-F), and flooding–midseason drainage–reflooding and moisture but without waterlogging (F-D-F-M). Nitrogen was applied at the rate of 100 kg N ha^?1, as urea-N or pelletized, dehydrated manure product in conventional or organic rice paddies, respectively. Seasonal fluxes of CH₄ averaged 4.44, 2.14, and 1.75 mg m^?2 h^?1 for the organic paddy plots under the water regimes of F, F-D-F and F-D-F-M, respectively. Relative to conventional rice paddies, organic cropping systems increased seasonal CH₄ emissions by 20%, 23%, and 35% for the plots under the water regimes of F, F-D-F, and F-D-F-M, respectively. Under the water regimes of F-D-F and F-D-F-M, seasonal N₂O-N emissions averaged 10.85 and 13.66 μg m^?2 h^?1 in organic rice paddies, respectively, which were significantly lower than those in conventional rice paddies. The net global warming potentials (GWPs) of CH₄ and N₂O emissions from organic rice paddies relative to conventional rice paddies were significantly higher or comparable under various water regimes. The greenhouse gas intensities were greater, while carbon efficiency ratios were lower in organic relative to conventional rice paddies. The results of this study suggest that organic cropping system might not be an effective option for mitigating the combined climatic impacts from CH₄ and N₂O in paddy rice production.     

Assessment of Eco-Physiological Performance of Quercus ilex L. Leaves in Urban Area by an Integrated Approach

Bioretention cells, also known as raingardens, are increasingly being constructed as a means to collect, infiltrate, and treat stormwater runoff. There are concerns, however, about how stormwater management practices might function in terms of infiltration and pollutant removal as they age. Saturated hydraulic conductivity (K _sat) values were obtained for eight cells in 2006 and again for three of those cells in 2010 using an infiltrometer. A strong positive correlation of mean K _sat with service time was observed (slope = 10.2?±?2.4 cm/h per year, R ² = 0.67). Results from metals analyses of bioretention media cores collected from six bioretention cells showed the expected trend of Cu and Zn enrichment at the surface while Cd was detected only in one out of 72 media samples analyzed. Sorption isotherms from batch testing of field media samples (T = 22.5 °C, pH = 7.2) were used to estimate metal sorption capacities based on representative stormwater Cd and Zn concentrations. Cu was not considered, as very little of the metal is dissolved under these conditions (22.8?±?7.1 %). The mean equilibrium sorption capacities for Cd (10.2?±?3.1 mg/kg) and Zn (294.9?±?14.9 mg/kg) far exceeded observed levels in the bioretention media such that the remaining sorption capacity was ≥83 % for Zn and ≥90 % for Cd for the cells. Overall, the results of this investigation suggest that bioretention cells can provide many years of effective infiltration (>6 years) and metals removal performance (>25 years).     

Diazinon Mitigation in Constructed Wetlands: Influence of Vegetation

In intensively cultivated areas, agriculture is a significant source of pesticides associated with storm runoff. When these pollutants enter aquatic receiving waters, they have potential to damage nearby aquatic ecosystems. Constructed wetlands are a best management practice (BMP) designed to help alleviate this potential problem. A constructed wetland system (180?×?30 m) comprised of a sediment retention basin and two treatment cells was used to determine fate and transport of a simulated storm runoff event containing the insecticide diazinon and suspended sediment. Wetland water, sediment, and plant samples were collected spatially and temporally over 55 d. Results indicated that 43% of the study’s measured diazinon mass was associated with plant material, while 23 and 34% were measured in sediment and water, respectively. Mean diazinon concentrations in water, sediment, and plants for the 55-d study were 18.1?±?4.5 μg/l, 26.0?±?8.0 μg/kg, and 97.8?±?10.7 μg/kg, respectively. Aqueous concentrations fluctuated in the wetlands between 51–86 μg/l for the first 4 h of the experiment; however, by 9 h, aqueous concentrations were approximately 16 μg/l. During the 55 d experiment, 0.3 m of rainfall contributed to fluctuations in diazinon concentrations. Results of this experiment can be used to model future design specifications for mitigation of diazinon and other pesticides.     

Behavior of runoff-derived metals in a detention pond system

The removal and fate of runoff-derived heavy metals in a 1-yr old detention pond system were investigated during climatic conditions typical of the U.S. maritime Northwest. The catchment was a medium-sized, suburban parking lot near Portland, Oregon. Water samples from runoff, the detention pond system, and pond effluent were collected and analyzed for dissolved and particulate Cu. Copper was the dominant toxic metal for the study site, while analysis of selected samples for Pb and Cd showed these metals to be minor pollutants. Total Cu in runoff varied among different storm events over a wide range of concentrations (< 2 to 33 µg L^?1), while total Cu levels in pond effluent remained within a fairly narrow range (5 to 12 µg L^?1), Sediment samples collected from the detention pond system were analyzed for Cu in two size fractions (< 63 µm and < 125 µm). Copper was found to be deposited in the pond sediments in a small but highly concentrated plume (up to 130 mg kg^?1), extending axially from the runoff inlet pipe. Overall, results from this study showed that low-cost, small-scale detention ponds can be a useful management practice for runoff from parking lot areas and can be of value in preserving the integrity of receiving waters.     

Sediment denitrification in waterways in a rice-paddy-dominated watershed in eastern China

Purpose

Rice-paddy-dominated watersheds in eastern China are intensively cultivated, and lands with two crops receive as much as 550–600 kg?ha^–1?year^–1 of nitrogen (N), mainly through the addition of N-based fertilizers. However, stream N concentrations have been found to be relatively low. Waterways in the watersheds are assumed to be effective “sinks” for N, minimizing its downstream movement. We directly measured net sediment denitrification rates in three types of waterways (ponds, streams/rivers, and a reservoir) and determined the key factors that control net sediment denitrification. Such information is essential for evaluating the impact of the agricultural N cycle on the quality of surface water.

Materials and methods

The pond–stream–reservoir continuum was sampled every 2 months at nine sites in an agricultural watershed between November 2010 and December 2011. Net sediment N₂ fluxes/net sediment denitrification rates were determined by membrane inlet mass spectrometry and the N₂/Ar technique. A suite of parameters known to influence denitrification were also measured.

Results and discussion

Net denitrification rates ranged between 28.2?±?18.2 and 674.3?±?314.5 μmol N₂–N?m^–2?h^–1 for the streams, 23.7?±?23.9 and 121.2?±?38.7 μmol N₂–N?m^–2?h^–1 for the ponds, and 41.8?±?17.7 and 239.3?±?49.8 μmol N₂–N?m^–2?h^–1 for the reservoir. The mean net denitrification rate of the stream sites (173.2?±?248.4 μmol N₂–N?m^–2?h^–1) was significantly higher (p?<?0.001) than that of the pond sites (48.3?±?44.5 μmol N₂–N?m^–2?h^–1), and the three types of waterways all had significantly higher (p?<?0.01) mean net denitrification rates in summer than in other seasons. Linear regression and linear mixed effect model analysis showed that nitrate (NO₃ ^?–N) concentration in surface water was the primary controlling factor for net sediment denitrification, followed by water temperature. Using monitoring data on NO₃ ^?–N concentrations and temperature of the surface water of waterways and an established linear mixed effect model, total N removed through net sediment denitrification in the pond–stream–reservoir continuum was estimated at 46.8?±?24.0 t?year^–1 from July 2007 to June 2009, which was comparable with earlier estimates based on the mass balance method (34.3?±?12.7 t?year^–1), and accounted for 83.4 % of the total aquatic N. However, the total aquatic N was only 4.4 % of the total N input to the watershed, and thus most of the surplus N in the watershed was likely to be either denitrified or stored in soil.

Conclusions

High doses of N in a rice-paddy-dominated watershed did not lead to high stream N concentrations due to limited input of N into waterways and the high efficiency of waterways in removing N through denitrification.     

Temporal Trends of Trace Metals in Sediment and Invertebrates from Stormwater Management Ponds

Stormwater ponds are an increasingly common feature in urban landscapes. Because these ponds retain runoff and particulate-bound contaminants from impervious surfaces, organisms inhabiting stormwater ponds may be exposed to elevated metal levels in sediments. This study evaluated temporal changes in sediment and macroinvertebrate Cu, Pb and Zn over an eleven-year period with specific attention to land use in pond watersheds. Sediment and invertebrate metal levels were quantified using atomic absorption spectrophotometry (1993 samples) or inductively coupled plasma mass spectrometry (2003–2004 samples). Sediment trace element levels did not significantly change from 1993 to 2003-2004 with the exception of Zn in ponds receiving runoff from highways, which increased from a mean of 32 mg kg^?1 in 1993 to 344 mg kg^?1 in 2003–2004. Sediment Pb and Cu generally remained below published threshold effects concentrations (TEC) except for two instances of elevated Cu in 2003–2004. Zn remained below the TEC in 1993 but exceeded the TEC in six ponds in 2003–2004. Trace metal body burdens varied among invertebrate groups, and to a lesser extent among land uses, but in both cases this variation was a function of year. In general, trace element body burdens were more similar among invertebrate groups or land use or both during 2003–2004 when compared to levels in 1993. Our results suggest sediment and invertebrate trace metal levels are at steady state in these stormwater management ponds and that risk to organisms inhabiting these ponds does not vary as a function of pond age.     

Nitrogen dynamics of anaerobically digested slurry used to fertilize paddy fields

To determine nitrogen (N) fate and environmental impact of applying anaerobic digestion slurry (ADS) to rice paddy (Oryza sativa L.), a field experiment was established using three treatments based on contrasting N application rate. The ADS (with ammonium-N accounting for >80 % of total N) treatment at a conventional application rate of 270 kg N?ha^?1 was compared to a negative control (no N fertilizer) and a positive control of urea applied at 270 kg N?ha^?1. The N budget showed the following distribution of applied N from ADS and urea: 41.3?±?5.1 % for ADS and 36.6?±?4.4 % for urea recovered by the rice plant (including straw, grain, and root), 16.4?±?3.7 % for ADS and 7.4?±?1.8 % for urea lost via ammonia volatilization, 0.26?±?0.15 % for ADS and 0.15?±?0.12 % for urea lost by direct N₂O emission, 1.9?±?0.5 % for ADS and 2.3?±?0.8 % for urea leached downward, 0.70?±?0.15 % for ADS and 0.67?±?0.12 % for urea discharged with floodwater drainage, and 39.4?±?8.4 % for ADS and 53.0?±?9.1 % for urea retained by soil or lost by N₂ emission. Compared to urea application, ADS application impacts the environment mainly through gaseous N losses rather than water N losses. ADS application had a positive impact on rice grain yield and reduced chemical fertilizer use. Considering the wide distribution of paddy fields and the ever-increasing quantities of ADS, ADS may serve as a valuable N source for rice cultivation, although mitigating ammonia and N₂O losses should be further investigated.     

Detection of Water and Sediments Pollution of An Arid Saltern (Sfax, Tunisia) by Coupling the Distribution of Microorganisms With Hydrocarbons

We investigated the coupling of abundance of bacteria, phytoplankton and ciliates with hydrocarbons in the surface water and sediments of five interconnected ponds in the arid Sfax solar salterns. This study aimed at determining the potential sources of hydrocarbons and the effects of salinity gradients on microorganism metabolism. Hydrocarbon analysis was performed by gas chromatography (GC-FID) and gas chromatography coupled with mass spectrometry (GC-MS). The GC-FID allowed the detection of aliphatic hydrocarbons and n-alkanes ranging from n-C₁₃ to n-C₃₀. Total aliphatic hydrocarbon concentrations varied from 92.5 mg. l^?1 in the first pond (having marine characteristics) to 661.1 mg. l^?1 in the last pond (crystallizer) (316.8?±?120.1 mg. l^?1) for water samples and from 26.7 to 127.8 μg. g^?1 dry weight for sediment samples. The GC-MS enabled us to detect halogenated hydrocarbons (bromoalkanes and chloroalkanes) and n-alkenes. The distribution of n-alkanes indices coupled to several environmental factors suggests that a major fraction of hydrocarbons resulted from both prokaryotic (bacteria) and eukaryotic (protists) developments. A low hydrocarbon fraction might be petrogenic.     

Mineral and Anthropogenic Indicator Inorganics in Urban Stormwater and Snowmelt Runoff: Sources and Mobility Patterns

Inorganic chemicals in urban stormwater and snowmelt runoff originate from catchment geology and anthropogenic activities. The occurrence, partitioning and mobility of six minerals and six trace metal (TM) indicators of anthropogenic activities were studied in stormwater, snowmelt and baseflow in four urban catchments, and the sampling of inorganics was supplemented by measurements of electrical conductivity (EC), pH and total suspended solids (TSSs). Minerals occurred at concentrations several orders of magnitude higher (1–10² mg/L) than those of TMs (10^?2–10² μg/L) and reflected the composition of local groundwater seeping into sewers. Concentrations of Ca, K, Mg and Na were enhanced by baseflow contributions and followed closely the electrical conductivity. Al and Fe minerals occurred in insoluble forms, and their pollutographs were similar to those of TMs, whose concentrations mimicked, to some extent, the flux of TSS. The TMs with the highest and lowest particulate fractions were Cr&Pb and Cu&Zn, respectively. The concentrations of total TMs in snowmelt were two to four times higher than those in stormwater, and both sources likely exceeded some of the stormwater effluent limits (for Cd, Cu and Zn) proposed in Sweden. Where such concentrations depended on water hardness, the risk of toxicity might be reduced by elevated hardness of the monitored snowmelt and stormwater. Recognizing the good ecological status of the study area receiving water, Lake Storsjön, some protection against polluted runoff and snowmelt may be needed and could be achieved by implementing stormwater management measures controlling TSS and TMs.     

Influence of Three Aquatic Macrophytes on Mitigation of Nitrogen Species from Agricultural Runoff

Agricultural runoff containing nitrogen fertilizer is a major contributor to eutrophication in aquatic systems. One method of decreasing amounts of nitrogen entering rivers or lakes is the transport of runoff through vegetated drainage ditches. Vegetated drainage ditches can enhance the mitigation of nutrients from runoff; however, the efficiency of nitrogen removal can vary between plant species. The efficiency of three aquatic macrophytes, cutgrass (Leersia oryzoides), cattail (Typha latifolia), and bur-reed (Sparganium americanum), to mitigate dissolved and total nitrogen from water was investigated. Replicate mesocosms of each plant species were exposed to flowing water enriched with ammonium and nitrate for 6?h, allowed to remain stagnant for 42?h, and then flushed with non-enriched water for an additional 6?h to simulate a second storm event. After termination of the final simulated runoff, all vegetated treatments lowered total nitrogen loads exiting mesocosms by greater than 50%, significantly more than unvegetated controls, which only decreased concentrations by 26.9% (p????0.0023). L. oryzoides and T. latifolia were more efficient at lowering dissolved nitrogen, decreasing ammonium by 42?±?9% and 59?±?4% and nitrate by 67?±?6% and 64?±?7%, respectively. All treatments decreased ammonium and nitrate concentrations within mesocosms by more than 86% after 1?week. However, T. latifolia and L. oryzoides absorbed nitrogen more rapidly, lowering concentrations by greater than 98% within 48?h. By determining the nitrogen mitigation efficiency of different vegetative species, plant communities in agricultural drainage ditches can be managed to significantly increase their remediation potential.     

Phosphorus Release and Equilibrium Dynamics of Canal Sediments within the Everglades Agricultural Area, Florida

High phosphorus (P) in surface drainage water from agricultural and urban runoff is the main cause of eutrophication within aquatic systems in South Florida, including the Everglades. While primary sources of P in drainage canals in the Everglades Agricultural Area (EAA) are from land use application of agricultural chemicals and oxidation of the organic soils, internal sources from canal sediments can also affect overall P status in the water column. In this paper, we evaluate P release and equilibrium dynamics from three conveyance canals within the EAA. Incubation and flux experiments were conducted on intact sediment cores collected from four locations within the Miami, West Palm Beach (WPB), and Ocean canal. After three continuous exchanges, Miami canal sediments reported the highest P release (66?±?37 mg m^?2) compared to WPB (13?±?10 mg m^?2) and Ocean (17?±?11 mg m^?2) canal over 84 days. Overall, the P flux from all three canal sediments was highest during the first exchange. Miami canal sediments showed the highest P flux (2.4?±?1.3 mg m^?2 day^?1) compared to WPB (0.83?±?0.39 mg m^?2?d^?1) and Ocean canal sediments (0.98?±?0.38 mg m^?2 day^?1). Low P release from WPB canal sediments despite having high TP content could be due to carbonate layers distributed throughout the sediment column inhibiting P release. Equilibrium P concentrations estimated from the sediment core experiment corresponded to 0.12?±?0.04 mg L^?1, 0.06?±?0.03 mg L^?1, and 0.08?±?0.03 mg L^?1 for Miami, WPB, and Ocean canal sediments, respectively, indicating Miami canal sediments behave as a source of P, while Ocean and WPB canal sediments are in equilibrium with the water column. Overall, the sediments showed a significant positive correlation between P release and total P (r?=?0.42), Fe_ox (r?=?0.65), and Al_ox (r?=?0.64) content of sediments. The contribution of P from the three main canals sediments within the EAA boundary corresponded to a very small portion of the total P load exiting the EAA. These estimates, however, only take into consideration diffusive fluxes from sediments and no other factors such as canal flow, bioturbation, resuspension, and anaerobic conditions.     

Fecal Sterol and Bile Acid Biomarkers: Runoff Concentrations in Animal Waste-Amended Pastures

Nonpoint source pollution is the leading remaining cause of water quality problems. The extent of NPS pollution is often more difficult or expensive to monitor at the point(s) of origin, as compared to monitoring of point sources. This study evaluated the hypothesis that animal manure (chicken, cow, horse, and pig) applied to pasture contribute fecal sterols and bile acids to runoff. The study also assessed the potential benefit of fecal sterols and bile acids as biomarkers in distinguishing fecal pollution and its sources. Fecal sterol and bile acid concentrations were determined in flow-weighted composite runoff samples collected from 2.4?×?6.1 m plots (n?=?3) amended with manure. Runoff was generated from simulated rainfall (152 mm.h^?1). Runoff samples from manure-amended plots showed high concentrations of fecal sterol (ranged from 13?±?1 to 1,287?±?183) and bile acid (ranged from 24?±?1 to 2,251?±?248) biomarkers. The profiles of fecal sterols and bile acids in runoff samples were similar to those of fresh manure for all selected animals. For runoff and fresh manure, chenodeoxycholic acid, deoxycholic acid, epicoprostanol, and hyodeoxycholic acid were consistent biomarkers for chicken, cow, horse, and pig, respectively, suggesting that sterols and bile acids can be used to identify sources and occurrence of fecal matter in water and sediments.     

Cadmium accumulation in Crassocephalum crepidioides (Benth.) S. Moore (Compositae) in heavy-metal polluted soils and Cd-added conditions in hydroponic and pot cultures

Abstract

We have identified Crassocephalum crepidioides (Benth.) S. Moore (Compositae) as a cadmium (Cd)-accumulator plant in a heavy-metal polluted environment. In soil polluted with Cd, 5.7–17.5 mg kg^?1 Cd, concentrations in the above-ground plant tissues were measured as 14.6–78.6 mg kg^?1 with transfer factors in the above-ground plant tissues (concentration in above-ground tissues/soil concentration) of 1.5–6.0. No other toxic heavy metals or plant micronutrients were found to have accumulated into the above-ground plant tissues. In a hydroponic culture with 1 µmol L^?1 Cd added to Hoagland's nutrient solution, Cd concentration in the above-ground plant tissues was 121.0 mg kg^?1, with a transfer factor of more than 1000. In a pot culture carried out for 9 weeks in a greenhouse, the highest Cd concentration in the above-ground plant tissues, 121.2 mg kg^?1, was found in a treatment with 5 mg kg^?1 Cd, whereas the highest Cd content in an above-ground plant tissue, 106.1 µg, was found in a treatment with 2 mg kg^?1 Cd. These results clearly showed that C. crepidioides is a Cd accumulator. In all samples, the Cd concentration in the above-ground plant tissues was higher than that in the roots. The results obtained in the present study show that this plant has a strong potential for use in phytoremediation in farm fields contaminated with Cd.     

Effects of Arsenic‐Contaminated Irrigation Water on Growth,Yield, and Nutrient Concentration in Rice

Abstract

A study was undertaken to determine the effects of different concentrations of arsenic (As) in irrigation water on Boro (dry‐season) rice (Oryza sativa) and their residual effects on the following Aman (wet‐season) rice. There were six treatments, with 0, 0.1, 0.25, 0.5, 1, and 2 mg As L^?1 applied as disodium hydrogen arsenate. All the growth and yield parameters of Boro rice responded positively at lower concentrations of up to 0.25 mg As L^?1 in irrigation water but decreased sharply at concentrations more than 0.5 mg As L^?1. Arsenic concentrations in grain and straw of Boro rice increased significantly with increasing concentration of As in irrigation water. The grain As concentration was in the range of 0.25 to 0.97 µg g^?1 and its concentration in rice straw varied from 2.4 to 9.6 µg g^?1 over the treatments. Residual As from previous Boro rice showed a very similar pattern in the following Aman rice, although As concentration in Aman rice grain and straw over the treatments was almost half of the As levels in Boro rice grain. Arsenic concentrations in both grain and straw of Boro and Aman rice were found to correlate with iron and be antagonistic with phosphorus.     

Tubificid role in soil mineralization and recovery of algal nitrogen by lowland rice

Uptake of blue-green algal nitrogen (N) and total N uptake by lowland rice (Oryza sativa) was affected by tubificid (Oligochaeta) presence in submerged soils. Recovery of algal ¹⁵N by the first crop was 24–43% but only 4–7% for the second, and recovery was determined by both the method of algal application (surface vs buried) and the presence of tubificids. Tubificid activities reduced recovery of algal N by rice, increased its total N content and doubled losses of ¹⁵N to the atmosphere. Soil N uptake by rice was increased by tubificid presence. Soil N mineralization, measured as NH₄⁺ production, was doubled over 7 days by their activities and algal mineralization was also apparently enhanced. The NH₄⁺ release rate of Limnodrilus sp. was 4.11 ± 0.06 ng NH₄⁺-N mg ash free dry wt^?1 h^?1. Effects of tubificids on rice nutrition are discussed.     

Application of membrane inlet mass spectrometry to directly quantify denitrification in flooded rice paddy soil

Denitrification has long been considered a major mechanism of N loss when N fertilizer is applied to flooded rice paddies. However, the direct determination of denitrification in soils is almost impossible because of the high atmospheric background of dinitrogen (N₂). Dissolved N₂ in a small water sample can be rapidly and precisely measured through membrane inlet mass spectrometry (MIMS). This study is the first to directly measure N₂ flux through MIMS in flooded rice paddy plots that received different amounts of urea. Ammonia (NH₃) volatilization was measured simultaneously to verify whether NH₃ volatilization and denitrification are complementary loss mechanisms. The average cumulative N₂–N loss measured by MIMS 21 days after fertilization was 4.7?±?1.7 % of the applied N, which was within the range of the reported values obtained by cumulative recovery of (N₂ + N₂O)–¹⁵N and ¹⁵N-balance technique. Underestimation or overestimation of denitrification can be prevented in MIMS given that N₂ can be measured directly without ¹⁵N-labeled fertilizer. A good positive correlation was found between the dissolved in situ N₂ concentrations of floodwater and the denitrification rates of intact soil cores. Urea incorporation reduced NH₃ volatilization unlike surface broadcasting. However, urea incorporation significantly increased cumulative N₂–N loss during the 21 days after fertilization. Correlation analysis showed that nitrate (NO₃ ^?–N) concentration in floodwater could be the primary restricting factor for soil denitrification in the experimental field. Results suggest that MIMS is a promising technique for the measurement of denitrification in a flooded rice paddy.     

Variations in concentrations of N and P forms in leachates from dried soils rewetted at different rates

The rate at which dried soils are rewetted can affect the quantities and forms of nutrients in leachates. Both dried and moist replicated (n?=?3) samples of two contrasting grassland soil types (clayey vs brown earth) were irrigated during laboratory experiments with identical total amounts of water, but at different rates, ranging from 0 h, increasing by 30-min increments up to 4 h, and additionally a 24-h rewetting rate. Total P concentrations in leachates from dried samples of both soils generally decreased as rewetting rate increased, ranging from 2,923?±?589 μg P L^?1 (0.5 h rewetting rate) to 731?±?46.0 μg P L^?1 (24 h, clayey soil) and 1,588?±?45.1 μg P L^?1 (0.5 h) to 439?±?25.5 μg P L^?1 (24 h brown earth). Similar patterns in concentrations occurred for molybdate reactive P (MRP), although concentrations were generally an order of magnitude lower, indicating that the majority of the leached P was probably organic. The moist brown earth leached relatively high concentrations of MRP (maximum 232?±?10.6 μg P L^?1, 0.5 h), unlike the moist clayey soil (maximum 20.4?±?10.0 μg P L^?1, 0 h). The total oxidised N concentrations in leachates were less affected by rewetting rate, although longer rewetting rates resulted in decreased concentrations in leachates from the dried samples of both soils. The difference in responses to rewetting rates of the two soils is probably due to differences in the fate of the microbial biomass and adsorption properties in the soils. Results show that soil moisture could be an important factor in regulating nutrient losses and availability, especially under changing patterns of rainfall predicted by future climate change scenarios.     

Assessment of the Hyperaccumulating Lead Capacity of Salvinia minima Using Bioadsorption and Intracellular Accumulation Factors

Salvinia minima has been reported as a cadmium and lead hyperaccumulator being the adsorption and intracellular accumulation the main uptake mechanisms. However, its physicochemical properties, the effect of metal concentration and the presence of organic and inorganic compounds on its hyperaccumulating capacity are still unknown. Furthermore, the specific adsorption and accumulation mechanisms occurring in the plant are not clear yet. Thus, based on a compartmentalization analysis, a bioadsorption (BAF) and an intracellular accumulation factor (IAF) were calculated in order to differentiate and quantify these two mechanisms. The use of kinetic models allowed predicting the specific type of uptake mechanisms involved. Healthy plants were exposed to five lead concentrations ranging from 0.80?±?0.0 to 28.40?±?0.22 mg Pb²⁺l^?1 in batch systems. A synthetic wastewater, amended with propionic acid and magnesium sulfate, and deionized water were used as media. The BAF and IAF contributed to gain an in-depth insight into the hyperaccumulating lead capacity of S. minima. It is clear that such capacity is mainly due to adsorption (BAF 780–1980) most likely due to its exceptional physico-chemical characteristics such as a very high surface area (264 m² g^?1) and a high content of carboxylic groups (0.95 mmol H⁺g^?1 dw). Chemisorption was predicted as the responsible mechanism according to the pseudo-second order adsorption model. Surprisingly, the ability of S. minima to accumulate the metal into the cells (IAF 57–1007) was not inhibited at concentrations as high as 28.40±0.22 mg Pb²⁺l^?1.     

Land use affects soil organic carbon of paddy soils: empirical evidence from 6280 years BP to present

Purpose

Submerged rice cultivation has been practiced in China for 7000 years. Empirical evidence on changes of soil organic carbon (SOC) contents in paddy soils over this historical time period is scarce. Therefore, a field study was conducted to investigate the effect of submerged rice cultivation on the accumulation and preservation of SOC in paddies.

Materials and methods

Two buried ancient paddy profiles (6280 years BP, named P-01 and P-03) in the Yangtze Delta of eastern China were excavated to illustrate the development of SOC contents in soils during the evolution of paddies under anthropogenic land use and environmental changes from the prehistoric period to the present time. Trends in SOC concentrations, total nitrogen concentrations, and stable carbon isotope ratio were identified for different points in time.

Results and discussion

Accumulation of organic carbon was found in the paddy soil layers of P-01 at 100–174 cm depth. This site was taken under submerged rice cultivation in about 6280 years BP. The average SOC concentration in the prehistoric paddy topsoil in 100–130 cm depth was 1.27 %, which is seven times higher than that in the adjacent uncultivated land at 103–130 cm depth of P-03. This implies that the paddy soil has experienced substantial CO₂ sequestration under submerged management during that time. By about 3320 years BP, organic carbon contents were halved, potentially due to marine inundation by sea level rise. Up to the year 2003, the SOC contents in all horizons in the present time paddy soil have increased, especially in the surface layer, indicative of continuous rice cultivation. However, due to rapid urbanization and industrialization, the cultivation of paddies in eastern China has gradually been discontinued leading to the loss of SOC stocks of approximately 10 % in a 6-year interval (from 2003 to 2009). A significant relationship between SOC and rice phytolith contents was found in the paddy soil horizons of P-01 (r?=?0.71, p?<?0.01) and P-03 (r?=?0.72, p?<?0.01), suggesting that phytolith-occluded organic carbon could be used as a biomarker to ascertain the development of SOC in the evolution of rice paddies over the past 6000 years.

Conclusions

Submerged rice cultivation led to a noticeable accumulation of SOC in paddies. Phytolith-occluded organic carbon could be used as a biomarker to monitor changes of OC contents in paddy soils.

     

Differences in soil organic carbon stocks and fraction distributions between rice paddies and upland cropping systems in China

Purpose

A large body of research suggests that rice (Oryza sativa L.) cropping facilitates soil organic carbon (SOC) storage, while the stability of the sequestered carbon is still not well understood. The objective of this study was to determine the differences in SOC stocks and fraction distributions between rice paddies and upland cropping fields and their variation in different rice cropping areas.

Materials and methods

Data from the national soil survey were analyzed to assess the differences in SOC contents between paddy and upland cropping fields at the regional scale. In addition, three pairs of rice and upland cropping systems were selected in Heilongjiang [single rice vs. single corn (Zea mays L.) cropping], Jiangsu [rice-wheat (Triticum aestivum L.) vs. corn-wheat cropping], and Jiangxi (double rice vs. double corn cropping) provinces, representing the major cropping patterns in China. Physical fractionation techniques were used to investigate the differences in SOC stocks and distribution among different pools between rice-based cropping systems and non-rice cropping systems in China.

Results and discussion

SOC concentrations were, on average, 74.9% higher at the regional scale and 56.8% higher at the field scale in paddy than in upland cropping fields. Carbon proportion of particulate organic matter within microaggregates increased from 14.4% in upland cropping soils to 25.3% in paddy soils at the Heilongjiang site and from 12.4 to 25.5% at the Jiangxi site. Meanwhile, the free silt and clay-associated carbon was significantly greater in paddy than in upland cropping soils at the both sites. Nevertheless, SOC distribution did not markedly differ between paddy and upland cropping fields at the Jiangsu site where rice was rotated with winter wheat annually.

Conclusions

As compared to upland cropping or rice-upland crop rotation, continuous rice cropping, such as single and double rice cropping, could favor SOC stabilization by occlusion within microaggregates and adsorption to the silt and clay outside microaggregates, which may promote the long-term storage of SOC in paddies.