[Distribution and Dynamics of Cropland Soil Organic Carbon in Jianghan Plain: A Case Study of Qianjiang City].

Taking an example of Qianjiang City in Jianghan Plain, the distribution and dynamics of soil organic carbon (SOC) in croplands was studied in present study. The cropland included both paddy field and dry land. SOC contents were analyzed by taking soil samples of topsoil (0-20 cm) in 2011 according to land uses and soil types, and then compared with the initial SOC conducted in the period of the second soil survey (1983). The results showed that SOC density and storage in 2011 was 30.50 t . hm-2and 452. 82 x 10(4) t, respectively. During the past 28 years, the cropland SOC density was decreased at a rate of 0. 10 t . (hm2.a)-1, and SOC storage was reduced by 9% with the decreasing rate of 1. 53 t.a-1. SOC density and storage in paddy field was about 1. 6 and 1. 3 times over that in dry land in the two selected periods. However, the dynamics of SOC in paddy field and dry land were quite the opposite. In paddy field, SOC was lost by 16% (52. 83 x 10(4) t), with a decreasing rate of 0. 23 t . (hm2.a)-1; whereas in dry land, SOC was increased by 5% (8. 57 x 10(4) t), with an increasing rate of 0. 05 t . (hm2.a)-1. The loss of SOC in paddy field was mainly resulted from gleyed paddy soil, which suffered a fast decrease of SOC density and accounted for 80% of SOC lost in paddy field. In addition, Hydromorphic paddy soil, accounting for 50% of the area of paddy field, tended to loss another 15% of SOC in paddy field. While in dry land, the minor SOC storage increased was dominantly attributed to grey fluvo aquic soil, which accounted for 96% of the area of dry land. Thus, the dynamics of cropland SOC in Jianghan Plain was dominantly controlled by SOC changes in paddy field. Our findings suggest that effective management should be considered to enhance the capacity of SOC accumulation and sequestration in the low-yield paddy field and the types of soils that are large in area.

[Dynamics of the mineralization and transformation of rice photosynthesized carbon in paddy soils--a batch incubation experiment].

Photosynthesized carbon is an important part in C cycling of "atmosphere-plant-soil" and is the source of soil organic carbon (SOC), but its mineralization and transformation dynamics in paddy soils remains still unclear. Therefore, a batch incubation experiment was conducted to investigate the mineralization and transformation of rice photosynthesized carbon in paddy soils after rice harvest. The results showed that the mineralization rate of native SOC ranged from 4.44 to 17.8 microg x (g x d)(-1), while that of photosynthesized carbon (new carbon) was 0.15- 1.51 micro x (gx d)(-1) during the course of 100-day-incubation span. Rice photosynthesized carbon input significantly influenced the soil active carbon (DOC, MBC) transformation. During the incubation period (100 d), the amount of 14C-DOC transformation ranged from 1.89 to 5.32 mg x 8 kg(-1), and that of native DOC varied from 61.13 to 90.65 mg x kg(-2), with the transformation rates ranged from 0.18 to 0.34 mg x (kg x d)(-1) and from 4.10 to 5.48 mg. (kg x d)(-1), respectively. However, the 14C-MBC and native original MBC were 10.92-44.11 mg x kg(-1) and 463.31-1153.46 mg x kg(-1), respectively, and their transformation rates were 0.80-2.87, 41.60-74.46 mg x (kg-d)(-1), respectively. It suggested that the turnover of MBC was greater than that of DOC. Furthermore, "new carbon" was easier to be mineralized and decomposed than native SOC. The mineralized portion in "new carbon" was 13.5%-20.2%, whereas that in native SOC was only 2.2%-3.7%. Therefore, we concluded that the incorporation of rice photosynthesized carbon was vital to maintain the soil carbon sink for paddy soils.

[Carbon dioxide assimilation potential, functional gene amount and RubisCO activity of autotrophic microorganisms in agricultural soils].

Carbon dioxide (CO2) assimilation by autotrophic microorganisms plays a significant role in carbon sequestration in terrestrial ecosystems. Here, experiments were carried out to determine the contribution of autotrophic microorganisms to atmospheric CO2 fixation in 6 representative agricultural soils. Soils were incubated continuously in an atmosphere of 14CO2 and the distribution of labeled C into soil organic carbon (14C-SOC) was determined after 110 d. Meanwhile, the amounts of the cbbL genes were determined by Quantitative PCR and the RubisCO activity was measured in different soils. The results showed that substantial amounts of 14CO2 were fixed into 14C-SOC (ranged 10.63-133.81 mg x kg(-1) after 110 d of continuous labeling, with an annual, global rate of about 0.57-7.3 Pg. The microbially fixed C was also incorporated into the active carbon pool [the dissolved organic C (14C-DOC) and in the microbial biomass C (14C-MBC)], and ranged from 0.96 to 8.10 mg x kg(-1) and 1.70 to 49.16 mg x kg(-1), respectively. The proportion of 14C-SOC in SOC was 0.09%-0.64%. The 14C-DOC /DOC and 14C-MBC /MBC were 5.07%- 4.3% and 2.51%-13.12%, respectively. Thus, the distribution and transformation of microbially fixed C had a larger influence on the dynamics of DOC and MBC than on the total SOC dynamics. Moreover, the abundance of soil bacteria cbbL gene and RubisCO activity were in the range of 2.40 x 10(7) - 1.9 x 10(8) copies x g(-1) and 34.06-71.86 nmol x (g x min)(-1), respectively. The 14C-SOC content was significantly correlated with both the 14C-MBC content (P < 0.01) and the RubisCO activity (P < 0.01) in all tested soils. We concluded that autotrophic CO2 assimilation by soil microbes is significant to the global C cycle.

[Study on mechanism of SOM stabilization of paddy soils under long-term fertilizations].

Fourier transform infrared spectroscopy (FTIR) was applied to study the structure of soil organic matter (SOM) of paddy soils under long-term different fertilization treatments. The aim was to clarify the different distribution of SOM between different fertilization methods and between topsoil and subsoil, and to explore the stability mechanism of SOM under different fertilization treatments. The results showed that the content of topsoil organic carbon (SOC) was the highest under organic-inorganic fertilizations, with the increment of SOC by 18.5%, 12.9% and 18.4% under high organic manure (HOM), low organic manure (LOM) and straw returning (STW) respectively compared with no fertilization treatment (CK). The long-term fertilizations also changed the chemical structure of SOM. As compared with CK, different fertilization treatments increased the functional group absorbing intensity of chemical resistance compounds (aliphatic, aromaticity), carbohydrate and organo-silicon compounds, which was the most distinctive under treatments of HOM, LOM and STW. For example, the absorbing intensity of alkyl was 0.30, 0.25 and 0.29 under HOM, LOM and STW, respectively. These values were increased by 87% , 56% and 81% as compared with that under CK treatment. The functional group absorbing intensity of SOM in the topsoil was stronger than that in the subsoil, with the most distinctive difference under HOM, LOM and STW treatments. The present research indicated that the enhanced chemical resistance of functional group of SOM may contribute to the high contents of SOC in the paddy soils under long-term organic-inorganic fertilizations, which also suggested a chemical stabilization mechanism of SOM in the paddy soils.

[Quantifying soil autotrophic microbes-assimilated carbon input into soil organic carbon pools following continuous 14C labeling].

Soil autotrophic microbe has been found numerous and widespread. However, roles of microbial autotrophic processes and the mechanisms of that in the soil carbon sequestration remain poorly understood. Here, we used soils incubated for 110 days in a closed, continuously labeled 14C-CO2 atmosphere to measure the amount of labeled C incorporated into the microbial biomass. The allocation of 14C-labeled assimilated carbon in variable soil C pools such as dissolved organic C (DOC) and microbial biomass C (MBC) were also examined over the 14C labeling span. The results showed that significant amounts of 14C-SOC were measured in paddy soils, which ranged from 69.06-133.81 mg x kg(-1), accounting for 0.58% to 0.92% of the total soil organic carbon (SOC). The amounts of 14C in the dissolved organic C (14C-DOC) and in the microbial biomass C (14C-MBC) were dependent on the soils, ranged from 2.54 to 8.10 mg x kg(-1), 19.50 to 49.16 mg x kg(-1), respectively. There was a significantly positive linear relationship between concentrations of 14C-SOC and 14C-MBC (R2 = 0.957**, P < 0.01). The 14C-DOC and 14C-MBC as proportions of total DOC, MBC, were 5.65%-24.91% and 4.23%-20.02%, respectively. Moreover, the distribution and transformation of microbes-assimilated-derived C had a greater influence on the dynamics of DOC and MBC than that on the dynamics of SOC. These data provide new insights into the importance of microorganisms in the fixation of atmospheric CO2 and of the potentially significant contributions made by microbial autotrophy to terrestrial C cycling.

[Distribution characteristics of rice photosynthesized carbon in soil aggregates of different size and density].

Rice growth affects the distribution of organic matter in soils and soil fractions, and is thus an important factor to control the storage of soil organic matter. The aims of our study were to quantify the photosynthesized C in soil fraction pools of different size and density during the rice growth, and also to offer data evidence not only in the mechanisms of SOC accumulation, but also in C sequestration potential in paddy soils. Therefore, the microcosm experiment was carried out to quantify the input and distribution of photo-assimilated carbon (C) in soils size and density aggregates pools by using continuous 14C labeling technique. Destructive samplings of rice (Oryza sativa) were conducted after labeling for 80 days. The allocation of 14C-labeled photosynthates in soil C pools was examined in rice-planted soil over the 14C labeling span using the size (250-2 000 microm, 20-250 microm, < 20 microm) and density (light and heavy) fractionation procedure. The amount of 14C in the soil organic C (SOC14) in the 250-2 000 microm particle size was dependent on the soils, ranged from 118.23 mg x kg(-1) to 309.94 mg x kg(-1), accounting for 0.52%-1.55% of its SOC, respectively, which was much larger than those of aggregates with the other two sizes (20-250 microm, < 20 microm). Moreover, the amounts of SOC14 in light fractions of 250-2 000 microm and 20-250 microm particle size aggregate were significantly greater than those in their heavy fractions (P < 0.05). The data suggested that rice photosynthesize C mainly entered into the light fraction of 250-2 000 microm particle size aggregate by rhizodeposition, which enhanced the contents of SOC. There was a significant positive correlation between the light and heavy fraction and 250-2000 microm particle size aggregate, 20-250 microm and < 20 microm particle size aggregate of SOC14, although significant negative correlation between light fractions in < 20 microm, 20-250 microm aggregates was observed.

[Microbial activity and community structure analysis under the different land use patterns in farmland soils: based on the methods PLFA and MicroResp].

Soil microbe plays an essential role in terrestrial ecosystem through its role in cycling mineral compounds and decomposing organic matter. The objective of this paper is to determine the influences of different land use patterns on soil microbial activity and community structure, which were analyzed by phospholipids fatty acid (PLFA) and MicroResp method, based on a long-term fertilization experiment in Taoyuan County, Hunan Province. There were three land use patterns included, i. e. paddy fields (PS), paddy-upland rotation (PU) and upland land (US) soils. The results showed that the amounts of bacteria PLFA, fungi PLFA and the total PLFA were generally following the order PS > PU > US. The ratio of bacteria PLFA/fungi PLFA followed the order PU > US > PS, however, the ratio of Gram-positive bacteria PLFA (G+ PLFA) to Gram-negative bacteria PLFA (G(-) PLFA) in PU was the highest, and there's no significant difference in PU and US. In addition, principal components analysis (PCA) and the average concentration (mol x mol(-1)) of biomarker-PLFAs also revealed that the relative content of fungi and G(-) in PS was much higher than those of in PU and US (P < 0.05). However, the relative content of G(+) in PU was higher than those of other two land use patters. Therefore, Microbial community structure was influenced significantly by land use patterns in our study. Soil microbial biomass had significant relationships (P < 0.05) with SOC, TN and MBC, but had no significant correlation with CEC. Meanwhile, MicroResp analysis indicated that most of the carbon substrate addition enhanced the microbial respiration rates, although different substrate had different use efficiency. The average of carbon substrate use efficiency could be also ranked in the order: PS > PU > US. We conclude that microbial activity and community structure were influenced significantly by land use patterns in farmlands.

Spatial patterns and climate drivers of carbon fluxes in terrestrial ecosystems of China.

Understanding the dynamics and underlying mechanism of carbon exchange between terrestrial ecosystems and the atmosphere is one of the key issues in global change research. In this study, we quantified the carbon fluxes in different terrestrial ecosystems in China, and analyzed their spatial variation and environmental drivers based on the long-term observation data of ChinaFLUX sites and the published data from other flux sites in China. The results indicate that gross ecosystem productivity (GEP), ecosystem respiration (ER), and net ecosystem productivity (NEP) of terrestrial ecosystems in China showed a significantly latitudinal pattern, declining linearly with the increase of latitude. However, GEP, ER, and NEP did not present a clear longitudinal pattern. The carbon sink functional areas of terrestrial ecosystems in China were mainly located in the subtropical and temperate forests, coastal wetlands in eastern China, the temperate meadow steppe in the northeast China, and the alpine meadow in eastern edge of Qinghai-Tibetan Plateau. The forest ecosystems had stronger carbon sink than grassland ecosystems. The spatial patterns of GEP and ER in China were mainly determined by mean annual precipitation (MAP) and mean annual temperature (MAT), whereas the spatial variation in NEP was largely explained by MAT. The combined effects of MAT and MAP explained 79%, 62%, and 66% of the spatial variations in GEP, ER, and NEP, respectively. The GEP, ER, and NEP in different ecosystems in China exhibited 'positive coupling correlation' in their spatial patterns. Both ER and NEP were significantly correlated with GEP, with 68% of the per-unit GEP contributed to ER and 29% to NEP. MAT and MAP affected the spatial patterns of ER and NEP mainly by their direct effects on the spatial pattern of GEP.

[Quantifying rice (Oryza sativa L.) photo-assimilated carbon input into soil organic carbon pools following continuous 14C labeling].

The microcosm experiment was carried out to quantify the input and distribution of photo-assimilated C into soil C pools by using a 14C continuous labeling technique. Destructive samplings of rice (Oryza sativa) were conducted after labeling for 80 days. The allocation of 14C-labeled photosynthates in plants and soil C pools such as dissolved organic C (DOC) and microbial biomass C (MBC) in rice-planted soil were examined over the 14C labeling span. The amounts of rice shoot and root biomass C was ranged from 1.86 to 5.60 g x pot(-1), 0.46 to 0.78 g x pot(-1) in different tested paddy soils after labeling for 80 days, respectively. The amount of 14C in the soil organic C (14C-SOC) was also dependent on the soils, ranged from 114.3 to 348.2 mg x kg(-1), accounting for 5.09% to 6.62% of the rice biomass 14C, respectively. The amounts of 14C in the dissolved organic C (14C-DOC) and in the microbial biomass C(14C-MBC), as proportions of 14C-SOC, were 2.21%-3.54% and 9.72% -17.2%, respectively. The 14C-DOC, 14C-MBC, and 14C-SOC as proportions of total DOC, MBC, and SOC, respectively, were 6.72% -14.64%, 1.70% -7.67%, and 0.73% -1.99%, respectively. Moreover, the distribution and transformation of root-derived C had a greater influence on the dynamics of DOC and MBC than on the dynamics of SOC. Further studies are required to ascertain the functional significance of soil microorganisms (such as C-sequestering bacteria and photosynthetic bacteria) in the paddy system.

[Effect of fertilization on the absorption, partition and accumulation of carbon and nitrogen of rice under the equal N conditions].

In this study, the assimilation, partition and accumulation of carbon (C) and nitrogen (N), as well as the relationship between C and N accumulation of rice, were studied from typical paddy ecosystems under long-term fertilizer applications with equal N inputs in subtropical China. The results showed that chemical fertilizer plus low organic manure (LOM) could promote effectively the distribution of C in the rice plant. The N content in the stem-leaf and grain of rice under organic-inorganic fertilization was 8.9-10.2 g x kg(-1) and 11.9-14.8 g x kg(-1) respectively. It was much higher than under other treatments, with about 13% - 53% and 9% - 19% higher than under the chemical fertilization (NPK), separately and 12% - 77% and 23% - 32% higher than under the control treatment (CK), respectively. The C and N storages of rice were mainly accumulated in the aboveground part. Organic-inorganic fertilization treatment possessed higher storages of C (3467.8-4 323.9 kg x hm(-2)) and N (120.3-135.2 kg x hm(-2)) in the rice grain,which was about 13% - 23% of C and 26% - 45% of N higher than under NPK treatment. It indicated that rice grain was the main sink of C and N. The organic-inorganic fertilization was in favor of C accumulation and N absorption in the rice plant and it still possesses an obvious potential in C and N sequestration and absorption in subtropical paddy field.

[Effects of different nitrogen, phosphorous, and potassium fertilization modes on carbon- and nitrogen accumulation and allocation in rice plant].

Based on a 20-year field site-specific fertilization experiment in Taoyuan Experimental Station of Agriculture Ecosystems under Chinese Ecosystem Research Network (CERN), this paper studied the effects of different fertilization modes of N, P, and K on the accumulation and allocation of C and N in rice plant. The fertilization mode N-only showed the highest C and N contents (433 g kg(-1) and 18.9 g kg(-1), respectively) in rice grain, whereas the modes balanced fertilization of chemical N, P and K (NPK) and its combination with organic mature recycling (NPKC) showed the highest storage of C and N in rice plant. In treatments NPK and NPKC, the C storage in rice grain and in stem and leaf was 1960 kg hm(-2) and 2015 kg hm(-2), and 2002 kg hm(-2) and 2048 kg hm(-2), and the N storage in rice grain was 80.5 kg hm(-2) and 80.6 kg hm(-2), respectively. Treatment NPK had the highest N storage (59.3 kg hm(-2)) in stem and leaf. Balanced fertilization of chemical N, P, and K combined with organic manure recycling increased the accumulation of C and N in rice plant significantly. Comparing with applying N only, balanced fertilization of chemical N, P, and K was more favorable to the accumulation and allocation of C and N in rice plant during its growth period.

[Effects of water content on redox potential and carbon mineralization of wetland sediments].

To better understand the effect of soil water contents on redox potential (Eh), and their impacts on C mineralization in natural wetland, sediment samples from 3 types of wetlands (fen, humus marsh and marshy meadow) in the San-jiang Plate region of North China were incubated (25 degrees C) for 155 d under a range of reducing and oxidizing conditions by controlling water contents (varied from 24% to 232% of water holding capacity) (WHC). CO2-C evolved during incubation was measured at different time intervals. Results showed that Eh of sediments decreased significantly as water content increased from 24% WHC (lighted moisturized) to about 100% WHC, then decreased slightly as water content increased further to a level of submersed (about 2 cm water-depths). The accumulative amount of CO2-C evolved from the sediments indicated that the optimum water contents for mineralization of organic C are 32%, 48% and 76%-100% WHC for sediments of fen, humus marsh, and marshy meadow, respectively. The relationship between mineralization rates and redox potentials (Eh) were well fitted with second order parabola equations (p < 0.05). Mineralization rates and accumulative amount of organic C displayed a positive correlation with Eh up to 300 mV. However, a significant negative correlation was observed when Eh increased above 300 mV. Results demonstrated that low redox potential is the controlling factor of carbon accumulation of wetland in San-jiang Plate region.

[Effects of soil texture and water content on the mineralization of soil organic carbon in paddy soils].

To understand how soil texture and water content affect the mineralization of organic C in paddy soil, 3 selected soils (sandy loam, clay loam, and silty clay) were incubated (25 degrees C) with 14 C-labelled rice straw (1.0 g x kg(-1)) at water content varied from 45% to 105% of water holding capacity (WHC). Data indicated that, in the sandy loam and clay loam, the mineralization rate of 14 C-labelled rice straw reached the maximum at 75% WHC, as 53% and 58% of the straw C mineralized in the incubation period of 160 d, whereas in the silty clay, it increased gradually (from 41.8% to 49.0%) as water content increased up to 105% WHC. For all of the three soils, the mineralization rate of soil native organic C reached the maximum at 75% WHC, with 5.8% of the organic C mineralized in the same period for the sandy loam, and 8.0% and 4.8% for the clay loam and silty clay, respectively. As water content increased further, the mineralization rate of native organic C in the three soils significantly declined. The mineralization rate of added rice straw and native organic C in all the three soils, was well fitted with a conic curve. These results suggest that water-logging can decrease the mineralization of organic C in paddy soils.

[Responses of CO2 fluxes to light intensity and temperature in rice paddy field].

CO2 fluxes in rice paddy ecosystem in subtropical hilly region were measured continuously using eddy covariance technique. The objectives were to investigate the responses of CO2 fluxes to light intensity and temperature in the paddy ecosystem. Results showed a rectangular hyperbolic light-response function could be used to describe the relationship of CO2 flux and photosynthetic photon flux density (PPFD). The absolute values of CO2 fluxes increased with the increment of PPFD. When PPFD was higher than 1000 micromol/(m2 x s), the maximum was observed. CO2 fluxes responded differently to light between early and late rice. Values of quantum yield of late rice (0.0465-0.0999 micromol/micromol) were general higher than that of early rice (0.0176-0.0541 micromol/micromol). Moreover, the quantum yield and the maximum rate of photosynthesis assimilation in the blooming stage were higher than that in tillering and ripening stages. In nighttime, respiration from soil and plants (ecosystem respiration, Reco) changed exponentially with the increase of soil temperature at the depth of 5 cm (T5), 10 cm (T10), and 20 cm (T20), respectively. Whereas, T5 was more feasible than others to be considered as the temperature parameter for Reco calculation. During early rice growing season, Reco was more sensitive to temperature change than that during late rice growing season.

[Evolvement characteristics and coupling relationship of soil organic carbon and total nitrogen in subtropical paddy field ecosystem under different fertilization practices].

Soil samples were collected from long-term monitoring plots of paddy field ecosystem in Hunan Province to study the evolvement characteristics and coupling relationship of soil organic carbon and total nitrogen under different fertilization practices. The results showed that in 1986-2003, the contents of soil organic carbon and total nitrogen had a slight decrease under no fertilization (CK), basically remained stable under chemical fertilization (NPK), and increased under the combined application of organic manure and chemical fertilizers. Compared with those in CK, the contents of soil organic carbon and total nitrogen in treatments NPK, low application rate organic manure, and high application rate organic manure were increased by 13% and 18%, 54% and 45%, and 89% and 67% respectively. There was a significant positive correlation (P < 0.01) between soil organic carbon and total nitrogen. The soil C/N was around 10, with the majority ranged from 8.5 to 12.9. It was suggested that a combined application of organic manure and chemical fertilizers on paddy field could increase the sequestration and accumulation of soil carbon and nitrogen to a certain extent, and there existed a better coupling relationship between soil organic carbon and total nitrogen in paddy field ecosystem.

[Effects of long-term application of fertilizers on soil microbial biomass nitrogen and organic nitrogen components in subtropical paddy soils].

The effect of long-term fertilization on soil organic nitrogen components and microbial biomass nitrogen (B(N)) in paddy soils from two experiment sites in Hunan province were studied. Soil samples were collected from the plough layers of different fertilizer treatments. Soil B(N) was measured by the fumigation-extraction method, and soil organic N was fractionated by acid hydrolysis-distillation method according to the scheme of Bremner (1965). Results showed that the soil N increased 40 mg x kg(-1) every year at Ningxiang site (low N level) for 17 years under the application combined of fertilizers and manure, while that at Nanxian site (high N level) was 55 mg x kg(-1). Soil total nitrogen (T(N)), total hydrolysable nitrogen (THN) and microbial biomass nitrogen(B(N)) were increased by long-term combined application of chemical fertilizer and manure (NPKM). NPKM significantly increased the content of T(N), B(N), total hydrolysable nitrogen (THN), ammonia acid nitrogen (AAN), hydrolysable unidentified nitrogen (HUN) and the percentage of B(N) to T(N). Besides, NPKM increased the easily mineralizable B(N), AAN, and low decomposed HUN. There was positive correlated relationship between B(N) and THN and different THN components, and the effect of AAN and HUN on B(N) was biggest. It is obvious that NPKM increased soil fertility and enhanc the nitrogen-supplying capability of paddy soils. NPKM had the effect on increasing soil nitrogen capability of paddy soils, both easily decomposed fractions and difficultly decomposed ones.

[Simulating and predicting of carbon cycling in typical wetland ecosystems].

A model was developed based on the theories of physiological ecology and turnover dynamics of organic carbon in wetland ecosystem. It aimed to illustrate the process and characteristics of carbon cycling and its potential changes under climate change scenarios in wetland ecosystems. The key environmental parameters to determine the effects of temperature, water-logging, and freeze-thaw were gained from the results of incubation experiments. Effects of CO2 fertilizing on the carbon sequestration and plant cover on organic carbon mineralization were also taken into account in this model. It was verified by the conventional observed meteorologic data in temperate and subtropical wetland ecosystems. Sensitivity analysis and prediction under climate change scenarios by this model were also discussed. There were significant correlations between the simulated and observed values of sediment respiration in temperate wetlands in Northeast China. It was estimated that the annual net carbon sequestration rate was about 104 g x m(-2) in permanently water-logged wetland ecosystems and 76 g x m(-2) in the seasonally water logged ones in temperate northeast China. The simulated value of the accumulated organic carbon density was within the changing range of the investigated data. The model was sensitive to the change of parameters of CO2 concentration and temperature. The potential changes in carbon cycling characteristics were also predicted under assumed climate change scenarios of A1B and A1FI. It indicated that the exchange of carbon between the atmosphere and the wetland ecosystem became more active under climate change scenario of warming and increased CO2 concentration assuming no changing of hydrological condition. The net primary production (NPP) and the organic carbon density in temperate wetland ecosystems would increase under the scenario of doubled CO2 concentration and less than 2.5 degrees C increment of temperature. In this case wetland ecosystem would act as a net carbon sink of the atmosphere. However, it had no advantage to the carbon sequestration and accumulation in wetland ecosystems under the more warming scenarios. Under these more warming scenarios, the amount of increased NPP resulted from CO2 fertilizing and warming could not compensate the carbon loss from the accelerated respiration caused by warming. The organic carbon accumulated in seasonally water-logged wetland even would decrease under the assumed scenarios.

[Seasonal characteristics of CO2 fluxes from the paddy ecosystem in subtropical region].

CO2 fluxes from paddy ecosystem in subtropical hilly region were measured continuously using eddy covariance technique. Based on data rejecting, correcting and filling, the daily and annual CO2 fluxes were calculated from the instantaneous values, respectively. The objectives were to investigate the variation of CO2 fluxes on seasonal temporal scale, analyze the relationship between CO2 fluxes and environmental factors, and to quantify the annual net ecosystem exchange (NEE) from the paddy ecosystem. Results show the values of GPP, R(eco) and NEE are higher from Jun. to Sep. and lower in the other months. The NEE from May to Sep. accounted for above 80% of the annual value and is crucial to the whole annual value. Photosynthetically active radiation (PAR) and mean daily air temperature (T(a)) were two main influential factors for controlling the seasonal trend of GPP and NEE and could be described by binary linear functions, respectively. The annual NEE in paddy ecosystem was 2 475.6 g/(m2 x a). This is showed that paddy ecosystem was a carbon sink for the atmosphere in subtropical region.

[Effects of temperature on organic carbon mineralization in paddy soils with different clay content].

An incubation test with three kinds of paddy soil (sandy loam, clay loam, and silty clay soils) in subtropical region was conducted at 10, 15, 20, 25 and 30 degrees C to examine the response of the mineralization of soil organic carbon (SOC) to temperature change. The results showed that during the period of 160 d incubation, the accumulative mineralized amount of SOC in sandy loam, clay loam, and silty clay soils at 30 degrees C was 3.5, 5.2 and 4.7 times as much as that at 10 degrees C, respectively. The mineralization rate was lower and relatively stable at lower temperatures (< or = 20 C), but was higher at the beginning of incubation and decreased and became stable as the time prolonged at higher temperatures (> or = 25 degrees C). During incubation, the temperature coefficient (Q10) of SOC mineralization in test soils fluctuated, with an average Q10 in sandy loam, clay loam, and silty clay soils being 1.92, 2.37 and 2.32, respectively. There was a positive exponential correlation between SOC mineralization constant k and temperature (P < 0.01), and the response of SOC mineralization to temperature change was in the order of clay loam soil > silty clay soil > sandy loam soil.

[Estimation of CO2 fluxes from rice paddies based on transparent chamber measurement].

Closed chamber provide a valuable tool for measuring CO2 exchange fluxes. In general, the change rate of CO2 concentration is assumed to be constant in the short measurement time and a linear regression method is used to estimate the CO2 fluxes. However, due to the physical and physiological effects, the change rate of CO2 is not always constant. A linear regression method and an exponential regression method are compared in this study. Results show that during the growing stages except for the ripening, nonlinear relationship of CO2 concentration versus time was estimated in the sunny daytime. Absolute values of CO2 fluxes calculated by linear regression were lower than that by exponential regression. Whereas, CO2 concentration changed linearly with time in cloudy days or in nighttime. And no significant difference was found between the results calculated with these two methods. Accumulative CO2-C fluxes with exponential and linear regressions were compared with the values of net ecosystem exchange of CO2-C (NEE), which were calculated by net primary production (NPP) minus soil respiration. The values with the exponential regression method were closer to NEE than those with linear regression. Therefore, the linear regression method could result in underestimation of carbon budget of ecosystem. Based on transparent chamber measurement, the exponential regression is more feasible to calculate CO2 fluxes than the linear regression.