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Microbial nitrogen (N) immobilization, which typically results in soil N retention but based on the balance of gross N immobilization over gross N production, affects the fate of the anthropogenic reactive N. However, global patterns and drivers of soil gross immobilization of ammonium (I

Acidic soils cover about 30% of the world's land. Liming is a management practice applied worldwide to reduce the negative effects of acidification on soil fertility and plant growth. Liming also affects the biotic and abiotic soil properties controlling the production and consumption of the greenhouse gases (GHGs) carbon dioxide (CO2), nitrous oxide (N2O) and methane (CH4). Although our understanding of how liming regulates net GHG emissions is increasing, the impact of liming on soil biological drivers of GHG emissions has not been quantitatively synthesized. Here we conducted a global meta-analysis using 1474 paired observations from 124 studies to explore the responses of GHG emissions to liming, with a focus on soil biological factors. We show that the N2O mitigation capacity of liming could be linked to (i) increases in bacterial abundance of N2O reductase genes (NosZ) and decreases in fungi:bacteria ratio, both contributing to a lower N2O:N2 product ratio of denitrification; and (ii) reductions in soil mineral nitrogen (N) via stimulation of plant N uptake. The limited evidence available indicates that liming reduced CH4 emissions and the abundance of methanogens, but it had no effect on CH4 uptake and abundance of methanotrophs. Liming-induced increases in soil CO2 emissions can be explained by higher heterotrophic and/or autotrophic respiration. The strong coupling between liming effects on GHG emissions and on soil microbial communities involved in GHG production and consumption can be used to identify strategies to reduce GHGs in response to liming, and to improve process-based models for better predictions of soil GHG emissions.

The response of soil gross nitrogen (N) cycling to elevated carbon dioxide (CO

G78 nitrogen deposition collector was used to investigate the background value of local nitrogen deposition flux in Daiyun Mountain National Nature Reserve of Fujian province. The results showed that dry and wet nitrogen deposition of Daiyun Mountain National Nature Reserve was 2.30 kg·hm

Excessive fertilization easily leads to the degradation of greenhouse vegetable fields, therefore rational fertilizations are important to maintain the production and sustainable development of vegetable. In this study, two fertilization treatments (optimized fertilization and conventional fertilization, noted as OF and CF, respectively) under continuous tomato-pepper cropping were arranged to investigate soil physicochemical properties, abundance and trophic groups of nematode and vegetable yield. The results showed that OF could maintain soil pH at the relatively higher level and increase the yield of tomato and pepper by 9.0% and 6.9% compared to CF treatment. In contrast to CF, OF increased nematode quantity and the relative abundance of bacterivores, but decreased the relative abundance of fungivores and plant-parasites, more obviously in the growth season of tomato. No obvious differences in plant parasite index, diversity, and richness were observed between CF and OF treatments across all sampling stages of tomato and pepper. Nematode channel ratio ranged from 0.39 to 0.64 in CF treatment, which was significantly lower than that in OF treatment (0.67-0.84), suggesting that the decomposition of food network was dominated by fungi in CF treatment but by bacteria in OF treatment. Based on soil physicochemical properties, nematode groups and vegetable yield, we concluded that optimized fertilization could not only increase vegetable growth but also improve soil ecological environment.高强度利用下设施蔬菜地的施肥过量问题导致了土壤质量的严重退化,合理施肥是维持蔬菜地生产力和可持续发展的重要措施.本研究比较了常规施肥和优化施肥两种施肥方式下连续种植番茄和辣椒后土壤理化性质、线虫群落及蔬菜产量的差异.结果表明: 优化施肥处理土壤pH显著高于常规施肥处理,且番茄和辣椒产量分别提高了9.0%和6.9%.与常规施肥相比,优化施肥提高了土壤线虫数量和食细菌线虫的相对多度,但降低了食真菌线虫和植食性线虫的相对多度.两季蔬菜种植过程中,不同施肥处理土壤寄生线虫成熟指数、多样性指数和丰富度指数无显著差异.优化施肥土壤线虫通路比值(0.67～0.84)显著高于常规施肥(0.39～0.64),前者土壤食物网的分解途径以细菌为主,而后者则为真菌控制.综合土壤理化性质、线虫数量和群落及蔬菜产量指标,优化施肥措施能够在促进蔬菜生长的同时,显著改善土壤生态环境.

Banana Fusarium wilt caused by Fusarium oxysporum f. sp. cubense (FOC) is a worldwide soil-borne disease that causes serious economic losses every year. Effective ways of preventing the disease are still not available. In this study, biological soil disinfestation (BSD) with incorporation of various organic matters under flooding conditions combined with plastic mulching was applied to suppress FOC and prevent banana Fusarium wilt disease. The biological properties of the soil during the BSD process were investigated using quantitative real-time PCR and denaturing gradient gel electrophoresis. The results showed that the values of soil redox potential significantly decreased by >800 mV in the treatments that incorporated rice straw and bagasse under flooding conditions compared with untreated soil. The lowest soil pH (5.36) and a significant decline in the FOC population to 2.79 % of untreated soil were obtained in flooded soil with the highest amount of rice straw. Incorporation of bagasse, pig manure, and lime also significantly decreased the populations of FOC, but were less effective than rice straw. Application of rice straw under flooding conditions significantly increased soil bacterial diversity, and the increased bacteria were considered to have an antagonistic effect on FOC. In 2013 field experiment, a wilt disease control efficiency of 82.3 % was obtained in flooded soil incorporated with 0.5 % (w/w) rice straw compared with untreated soil, and the control efficiency of BSD was further confirmed in 2014 field experiment. These results indicated that BSD can reduce FOC populations in soil, ameliorate soil microbial communities, and reduce the occurrence of banana Fusarium wilt disease.

To understand the effects of long-term amendment of organic manure and N fertilizer on N2O emission in the North China Plain, a laboratory incubation at different temperatures and soil moistures were carried out using soils treated with organic manure (OM), half organic manure plus half fertilizer N (HOM), fertilizer NPK (NPK), fertilizer NP (NP), fertilizer NK (NK), fertilizer PK (NK) and control (CK) since 1989. Cumulative N2O emission in OM soil during the 17 d incubation period was slightly higher than in NPK soil under optimum nitrification conditions (25 degrees C and 60% water-filled pore space, WFPS), but more than twice under the optimum denitrification conditions (35 degrees C and 90% WFPS). N2O produced by denitrification was 2.1-2.3 times greater than that by nitrification in OM and HOM soils, but only 1.5 times greater in NPK and NP soils. These results implied that the long-term amendment of organic manure could significantly increase the N2O emission via denitrification in OM soil as compared to NPK soil. This is quite different from field measurement between OM soil and NPK soil. Substantial inhibition of the formation of anaerobic environment for denitrification in field might result in no marked difference in N2O emission between OM and NPK soils. This is due in part to more rapid oxygen diffusion in coarse textured soils than consumption by aerobic microbes until WFPS was 75% and to low easily decomposed organic C of organic manure. This finding suggested that addition of organic manure in the tested sandy loam might be a good management option since it seldom caused a burst of N2O emission but sequestered atmospheric C and maintained efficiently applied N in soil.

High application rate of chemical fertilizers and unreasonable rotation in facility vegetable cultivation can easily induce the occurrence of soil acidification, salinization, and serious soil-borne diseases, while to quickly and effectively remediate the degraded facility vegetable soil can considerably increase vegetable yield and farmers' income. In this paper, a degraded facility vegetable soil was amended with 0, 3.75, 7.50, and 11.3 t C x hm(-2) of air-dried alfalfa and flooded for 31 days to establish a strong reductive environment, with the variations of soil physical and chemical properties and the cucumber yield studied. Under the reductive condition, soil Eh dropped quickly below 0 mV, accumulated soil NO3(-) was effectively eliminated, soil pH was significantly raised, and soil EC was lowered, being more evident in higher alfalfa input treatments. After treated with the strong reductive approach, the cucumber yield in the facility vegetable field reached 53.3-57.9 t x hm(-2), being significantly higher than that in un-treated facility vegetable field in last growth season (10.8 t x hm(-2)). It was suggested that strong reductive approach could effectively remediate the degraded facility vegetable soil in a short term.

Nitrous oxide (N2O) fluxes from rice paddy fields were measured in Nanjing, Yingtan and Fengqiu, using closed chamber method in 1994. The results showed that N2O fluxes varied temporally, spatially and geographic regionally, with the total amounts of N2O emissions during the period of rice growth ranged from 13.66 to 98.11mg/m2 in Nanjing, 1.73 to 3.65mg/m2 in Yingtan and 178.04 to 472.26mg/m2 in Fengqiu, respectively. Soil water regime and soil texture had significant effects on N2O production and emission from rice paddy fields. The mean N2O fluxes from sandy, loamy and clayey rice paddy fields were 182.2,82.8 and 68.7 µg N2O-N/m2/h, respectively. High N2O fluxes occurred when rice paddy fields were imposed by alternation of irrigation and drainage and almost no N2O emitted when the fields were submerged continuously. The rice paddy field applied with ammonium sulphate emitted more N2O than with urea and N2O-N losses of applied ammonium sulphate and urea ranged from 0.038 to 0.28% and 0.033 to 0.16%, respectively.

Methane oxidation in paddy soils was investigated under laboratory conditions. Paddy soils collected before early rice transplanting could not oxidize atmospheric CH4 but could oxidize CH4 when the concentration exceeded 10 µl l-1. Initial CH4 oxidation rate increased with the increase of initial CH4 concentration. Soil with the maximum potential to produce CH4, also had the maximum CH4 oxidation activity and the maximum emission flux from paddy soil. High CH4 concentration stimulated the oxidation of CH4. After 10 days' incubation under atmosphere containing 1000 µl-1 or 104 µl l-1 CH4, the soil which could not oxidize atmospheric CH4 was able to oxidize it.

Applying large amount of nitrogen fertilizer into vegetable field can induce soil NO(3-)-N accumulation, while rapidly removing the accumulated NO(3-)-N can improve vegetable soil quality and extend its service duration. In this study, a vegetable soil containing 360 mg N x kg(-1) was amended with 0, 2500, 5000, and 7500 kg C x hm(-2) of ryegrass (noted as CK, C2500, C5000, and C7500), respectively, and incubated in a thermostat at 30 degrees C for 240 h under flooding condition, aimed to investigate the effects of organic material amendment on vegetable soil nitrate concentration and nitrogenous gases emission. By the end of the incubation, the soil NO(3-)-N concentration in CK was still up to 310 mg N x kg(-1). Ryegrass amendment could remove the accumulated NO(3-)-N effectively. In treatments C2500, C5000, and C7500, the duration for the soil NO(3-)-N concentration dropped below 10 mg N x kg(-1) was 240 h, 48 h, and 24 h, respectively. After the amendment of ryegrass, soil pH increased significantly, and soil EC decreased, with the increment and decrement increased with increasing amendment amount of ryegrass. The cumulative emissions of soil N2O and N2 in ryegrass amendment treatments amounted to 270-378 mg N x kg(-1), and the N2O/N2 ratio ranged from 0.6 to 1.5. Incorporating with ryegrass under flooding condition could rapidly remove the accumulated NO(3-)-N in vegetable soil, but the high N2O emission during this process should be attached importance to.

By the method of static chamber, a field experiment was conducted to study the effects of applying controlled-release fertilizer (CRF) and its combination with urea on the N2O emission during rice growth period. Four treatments, i.e., no fertilization (CK), urea (U), urea and CRF with a ratio of 3:7 (U+C), and CRF (C) were installed, and the N application rate in treatments U, U+C, and C was the same. Compared with treatment U, treatments U+C and C decreased the N2O emission during rice growth season by 40.4% and 59.6%, and decreased the emission at midseason aeration stage by 65.1% and 83.9%, respectively (P0.05). Compared with that in treatment C, the N2O emission in treatment U+C had a slight decrease, and decreased by 53.9% at midseason aeration stage. Applying CRF increased rice yield, and the increment in treatments C and U+C was 7.8% and 9.8%, respectively, as compared to treatment U. Applying CRF delayed the peak time of soil inorganic nitrogen concentration, resulting in the reduction of N2O emission at midseason aeration stage. During rice growth season, no significant correlation was observed between N2O flux and soil Eh or soil temperature.

Air-drying is always accompanied by soil moisture loss. The different influences of air-drying on soil nitrification might due to the different nitrification responses to moisture changing of acid subtropical soils. So, after applying 0 and 150 mg x kg(-1) of ammonium bicarbonate, a 35-day incubation study was conducted to determine the nitrification potential of four acid subtropical soils under 5 soil moisture levels, namely 30% water-holding capacity (WHC), 45% WHC, 60% WHC, 75% WHC and 90% WHC. Four soils, designated QR, QU, SR and SU, derived from Quaternary red earth and Tertiary red sandstone, were collected from rice and upland field. The results indicated that the soil nitrification was significantly influenced by the moisture content (p0.01), and the nitrification sensitivities to soil moisture content varied with land-use types. For the treatments without ammonium input, the ranges of nitrification ratio were 11% and 8% in upland soils, QU and SU respectively, which were obviously lower than those in paddy soils (35% for QR, and 20% for SR). The ammonium input would increase the ranges of nitrification ratio which were 56%, 26%, 31%, and 26% for soil QR, QU, SR, and SU, respectively. And the ammonium input would accelerate soil acidification under high moisture levels. In a word, the land-use type presents a significant influence on the nitrification response to moisture content, which might lead to the difference of air-drying effect.

In order to understand how nitrogen (N) fertilization affects soil respiration, a pot experiment with splitting-root compartment and by root-cutting was conducted in a greenhouse. The experiment had four treatments, i. e., unplanted and N-unfertilized (CKO), unplanted but fertilized with 150 mg N x kg(-1) CKN), planted maize (Zea mays L.) but N-unfertilized (MO), and planted maize and fertilized with 150 mg N x kg(-1) (MN). Soil respiration, soil basal respiration, root respiration, and rhizospheric microbial respiration were measured simultaneously. In unplanted soils (treatments CKO and CKN), soil respiration rate (soil basal respiration) ranged from 13.41 to 77.27 mg C x m(-2) x h(-1), and N fertilization had less effect; while in planted soils, the averaged soil respiration rate in treatment MN amounted to 138.54 mg C x m(-2) x h(-1), and was 17.7% higher (P0.05) than that in treatment MO. This increment mainly occurred at tasselling and flowering stages. During maize growth season, the contribution of soil basal respiration, root respiration, and rhizospheric microbial respiration to soil respiration in treatments MN and MO was 36.2%, 45.9%, and 17.9%, and 35.5%, 36.9%, and 37.6%, respectively.

Populations and activities of nitrifier can be significantly influenced by air-drying. A 35-day incubation study was conducted to determine the effects of air-drying on nitrification potential of four acid subtropical soils after applying 0 and 150 mg x kg(-1) of ammonium bicarbonate. Four soils, designated QR, QU, SR and SU, derived from Quaternary red earth and Tertiary red sandstone, were collected from rice and upland field. The results indicated that, without ammonium input, the responses of nitrification in upland soils to air-drying were insignificant (p0.05). The nitrification ratio in fresh and air-dried samples of soil QU were 48% and 54% respectively, and 76% and 78% for SU; however, air-drying did have a significant effect on the nitrification in paddy soils (p0.01). Nitrification ratios were 40% and 89% for fresh and air-dried samples of QR, 76% and 94% for SR, and there was a lag-phase of the nitrification in air-dried paddy soils. Ammonium input would accelerate nitrification and make the nitrification ratios of fresh samples significantly higher than those of air-dried ones. In generally, land-use type presents a significant impact on the response of nitrification to air-drying effect, and there was a significant interaction of land-use type and fertilization.

The effects of chlorothalonil and carbendazim on nitrification and denitrification in six soils in upland and rice paddy environments were investigated. Laboratory aerobic (60% water holding capacity) and anaerobic (flooded) conditions were studied at 25 degrees C and fungicide addition rates of 5.5 mg/kg A. I. (field rate, FR), 20 times (20FR) and 40 times (40FR) field rate, respectively. The results indicated that chlorothalonil at the field rate had a slight inhibitory effect on one soil only, and that soil did not nitrify much in the first place. But chlorothalonil at higher rates inhibited nitrification significantly in all soils. For soils JXP and JXU with a pH of less than 5.0, chlorothalonil almost completely stopped their nitrification at 20FR and 40FR during the whole 14 d incubation period. For soils HNP and HNU with a pH of greater than 8.0, chlorothalonil also significantly inhibit nitrification at 20FR and 40FR (p < 0.05). However, NH4+ that was added to the soil was also almost completely nitrified by the end of the incubation period in these two soils. The effects of chlorothalonil at 20FR and 40FR on the nitrification of JSP and JSU soils, with a pH of 5.4 and 7.2, respectively, were intermediate between the other soil types. Chlorothalonil had no effect on denitrification at the field rate and had little effect at the higher rates of application in some soils. Carbendazim had essentially no effect on nitrification and denitrification in soils assessed.

A 14 d incubation test at 60% WHC and 25 degrees C was conducted to study the effects of fungicide chlorothalonil at its application rates of 0, 5.5 mg x kg(-1) (field application rate, FR), 110 mg x kg(-1) (20FR) and 220 mg x kg(-1) (40FR) on the nitrous oxide (N2O) and carbon dioxide (CO2) emissions from acidic, neutral, and alkaline soils. The results indicated that the effects of chlorothalonil on the two gases emissions depended on its application rate and soil type. Comparing with no chlorothalonil application, the chlorothalonil at 20FR and 40FR inhibited the N2O emission from acid soil significantly, while that at FR, 20FR and 40FR stimulated the N2O emission from neutral soil, with the strongest effect at FR. Higher application rates (20FR and 40FR) of chlorothalonil inhibited the N2O emission from alkaline soil at the early stage of incubation, but stimulated it at late incubation stage. Chlorothalonil at FR had no obvious effects on the CO2 emission from test soils, but that at 20FR and 40FR promoted the CO2 emission from acid soil while inhibited it from neutral and alkaline soils significantly.

Methane production and emission were investigated in 45 subtropical soil samples, collected from different land use and derived from different soil parent materials in Jiangxi province, by incubating flooded soil slurries in a closed system under N2 gas in the headspace after treatment with or without NO3(-)-N (200 mg x kg(-1)) for 28 days at 30 degrees C. The results indicated that the content and availability of soil organic C were the dominant factors influencing CH4 production and emission whether NO3(-)-N was added or not under this anaerobic incubation condition. Methane emission was higher in the soils derived from granite in the unamended soils and used for rice cultivation in the amended soils. During the anaerobic incubation, the NO3(-)-N added significantly inhibited the production and emission of CH4. The inhibitory effect of NO3(-)-N on CH4 emission might be stronger than that of N2O. The amount and rate of NO3(-)-N denitrified in the first 7 days of incubation determined the CH4 emission amount in the soils with NO3(-)-N. The Fe2+ content increased exponentially with the CH4 emission in 73% of the control soils without NO3(-)-N, which indicated that Fe3+ reduction processed simultaneously with CO2 reduction. Nitrate nitrogen inhibits not only the production and emission of CH4 but also the reduction of Fe3+.

0-20 cm soil samples were collected from an 18-year wheat-maize rotation field on the North China Plain to study the effects of long-term fertilization on the contents and storages of soil particulate organic carbon (POC), particulate organic nitrogen (PON), incorporated organic carbon (IOC), and incorporated organic nitrogen (ION). The long-term fertilization experiment was designed to include 7 treatments, i.e., chemical NPK (NPK), organic manure (OM), 1/2 organic manure plus 1/2 chemical NPK (1/20 MN), chemical NP (NP), chemical PK (PK), chemical NK (NK), and control (CK). After 18 years experiment, all the fertilization treatments showed higher contents of soil POC, PON, IOC and ION, higher proportions of soil POC to soil total organic carbon (TOC) and of soil PON to soil total nitrogen (TON), and higher C/N ratio of soil particulate organic matter. The storages of soil POC and PON under fertilization treatments were increased by 11.7%-196.8% and 13.0%-152.2%, respectively, compared with the control. The contribution of soil POC to the increased storage of soil TOC ranged from 31.5% to 67.3%, and that of soil PON to the increased storage of soil TON ranged from 14.3% to 100.0%. The storages of soil IOC and ION under fertilization treatments increased by 2.0%-75.0% and 0.0%-69.8%, respectively, compared with the control. Among the fertilization treatments, treatment OM had the highest storages of soil POC, PON, IOC and ION, followed by treatment 1/20 MN, and the treatments of applying chemical fertilizers alone. Balanced application of chemical fertilizers (treatment NPK) showed higher storages of soil POC, PON, IOC and ION than imbalanced application (treatments NP, PK, and NK). It was suggested that applying organic manure or its combination with chemical NPK and the balanced application of chemical NPK could be the keys for the increase of soil POC, PON, IOC and ION contents and storages.

The ammonia volatilization on the Typic Gleyi-stagnic Anthrosol with application of common urea and controlled release urea (LP-S100) fertilizers in the rice seasons in paddy soil of Taihui region of China was modeled by Jayaweera-Mikkelsen model. Results showed great difference of ammonia volatilization from two type fertilizers was detected with lysimeter experiment in the rice season. Nitrogen loss via ammonia volatilization after common urea application with conventional ways was 29%-35%, while only 5% of controlled release urea-N was volatilized. The Jayaweera-Mikkelsen model was over estimated the total amount of ammonia volatilization in the whole season, and great deviation from the measured data was obvious for the higher volatilization from common urea fertilizer. The estimated data were 2.95-4.19 times of the measures one for common urea treatments, while they were 1.19-1.40 times of those measured for LP-S100 treatments. The order of magnitude quotient was one of the indicators to evaluate the model estimation. The value of it was 0.8, which indicated the estimation of the model need improvement. Though sensitive analysis for the five parameters in the model was tested and amended the parameter of the concentration of NH4+ -N, a limited term was inducted in the model operation. The amended model got better results as the ratio of estimation to measured data was decreased to 1.12-1.28. The alga activity in the paddy field influenced ammonia volatilization and might make the failure of the model estimation of the original model.

Soil samples were collected from the paddy fields at the Ecological Experimental Station of Red Soil, Chinese Academy of Sciences under different treatments of long-term fertilization, and their phospholipid fatty acids (PLFAs) and enzyme activities were determined. The results showed that soil enzyme activities, nutrient contents, microbial biomass, and PLFAs varied greatly with different fertilizations. Fertilization increased the kinds and amount of soil PLFAs. Compared with fertilized soil, unfertilized soil had more fungal PLFAs but less bacterial PLFAs, indicating that fungus was more adaptable to infertile soils than bacteria. Soils applied with NPK and organic fertilizer had higher amount of total PLFAs, which was 3.22 and 1.79 times higher than that under N fertilization and no fertilization. It was indicated that balanced fertilization with NPK or applying organic fertilizer was more beneficial to the growth of plants. Fertilization could also increase soil enzyme activities, and soil urease and phosphatase activities could be used as the indicators of soil fertility.

This paper reviewed the varieties and characteristics of aerobic denitrifiers, their action mechanisms, and the factors affecting aerobic denitrification. Aerobic denitrifiers mainly include Pseudomonas, Alcaligenes, Paracoccus and Bacillus, which are either aerobic or facultative aerobic, and heterotrophic. They can denitrify under aerobic conditions, with the main product being N2O. They can also convert NH4+ -N to gas product. The nitrate reductase which catalyzes the denitrification is periplasmic nitrate reductase rather than membrane-bound nitrate reductase. Dissolved oxygen concentration and C/N ratio are the main factors affecting aerobic denitrification. The main methods for screening aerobic denitrifiers, such as intermittent aeration and selected culture, were also introduced. The research advances in the application of aerobic denitrifiers in aquaculture, waste water processing, and bio-degradation of organic pollutants, as well as the contributions of aerobic denitrifiers to soil nitrogen emission were summarized.

With incubation experiment, this paper studied the decomposition rate of organic carbon in black soil, fluvo-aquic soil, and red soil. It was shown that these three soil types had significantly different decomposition rate constant of organic carbon (P0.05 ), with the corresponding value being 2.2 x 10(-4) x d(-1), 6.0 x 10(-4) x d(-1), and 3.4 x 10(-4) x d(-1), respectively. The decomposition rate constant had a significant correlation with soil pH, which was increased greatly when the soil pH was5.5 or8.0, and a significant negative correlation with the contents of soil clay and silt, indicating that soil clay and silt content was one of the dominant factors affecting the decomposition rate of organic carbon in these soil types.

Although numerous studies about the nature and turnover of soil organic matter(SOM) in light and heavy fractions(LFOM and HFOM, respectively) have been made, little information is available in relation to the relationship between LFOM and HFOM, and no attempts have been made to quantify a general relationship between LFOM and HFOM for agricultural soils under field condition. Our hypothesis is there may be an inherent relationship between LFOM and HFOM for agricultural soils under certain unaltered management practices for a long period, to this end, we therefore studied typically soils taken from different parts in China by using a simple density fractionation procedure. The results indicated that LFOM was positively correlated with LFOM/HFOM ratio for three typical soils. This information will be of particular use not only in deepening our understanding of the dynamics of SOM fractions but also in evaluating the potential of agricultural soils to sequestrate C under different management practices in a long term.

The terrestrial ecosystem may be either a source or a sink of CH4 in rice paddies, depending, to a great extent, on the change of ecosystem types and land use patterns. CH4 emission fluxes from paddy fields under 4 cultivation patterns (conventional plain culture of rice (T1), no-tillage and ridge culture of rice (T2), no-tillage and ridge culture of rice and wheat (T3), and rice-wheat rotation (T4)) were measured with the closed chamber technique in 1996 and 1998 in Chongqing, China. The results showed that differences existed in CH4 emission from paddy fields under these land management practices. In 1996 and 1998, CH4 emission was 71.48% and 78.82% (T2), 65.93% and 57.18% (T3), and 61.53% and 34.22% (T4) of that in T1 during the rice growing season. During the non-rice growing season, CH4 emission from rice fields was 76.23% in T2 and 38.69% in TI. The accumulated annual CH4 emission in T2, T3 and T4 in 1996 decreased by 33.53%, 63.30% and 65.73%, respectively, as compared with that in T1. In 1998, the accumulated annual CH4 emission in T1, T2, T3 and T4 was 116.96 g/m2, 68.44 g/m2, 19.70 g/m2 and 11.80 g/m2, respectively. Changes in soil physical and chemical properties, in thermal and moisture conditions in the soil and in rice plant growth induced by different land use patterns were the dominant causes for the difference in CH4 emission observed. The relative contribution of various influencing factors to CH4 emission from paddy fields differed significantly under different land use patterns. However, the general trend was that chlorophyll content in rice leaves, air temperature and temperature at the 5 cm soil layer play a major role in CH4 emission from paddy fields and the effects of illumination, relative humidity and water layer depth in the paddy field and CH4 concentration in the crop canopy were relatively non-significant. Such conservative land use patterns as no-tillage and ridge culture of rice with or without rotation with wheat are thought to be beneficial to reducing CH4 emission from paddy fields and are, therefore, recommended as a significant solution to the problems of global (climatic) change.

A laboratory incubation experiment was conducted to investigate nitrous oxide (N2O) emission and reduction in a paddy soil (Stagnic Anthrosol) response to the pretreatment of water regime. The paddy soil was maintained under either air-dried (sample D) or submerged (sample F) conditions for 110 d before the soil was adjusted into soil moisture of 20% , 40% , 60% , 80% and 100% water holding capacity (WH(-2)) respectively, and then incubated with or without 10% (v/v) acetylene for 138 h at 25 degrees C. At lower soil water content (or = 60% WHC), N2O emission from the sample F was 2.29 times higher than that from the sample D (P0.01). While, N2O emission from the sample F was only 29 and 14 percent of that from the sample D at the soil moisture of 80% and 100% WHC, respectively (P0.01). The maximal N2O emissions observed at soil moisture of 80% WHC were about 24 and 186 times higher than the minima obtained at the soil moisture of 20% WHC for the sample F and D, respectively. But at the soil moisture of 80% and 100% WHC, N2O emission from the sample F with acetylene (F + ACE) was comparable to that of the sample D with acetylene (D + ACE). The results showed that the F sample produced N2O ability in denitrification was similar to the sample D, however, the sample F was in the better reduction of N2O to N, than the sample D even after the soil moisture was adjusted into the same level of 80% or 100% WHC. Therefore, the pretreatment of water regime influenced the strength and product composition of denitrification and N2O emission from the paddy soil.