[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.

[Regional differences of inputs of organic matter and chemical fertilizer in South Central China].

This article analyzed the inputs of organic matter and chemical fertilizer in the cropland of South Central China, i.e., Hunan, Hubei, Guangdong and Guangxi, and then calculated the budgets of nitrogen (N), phosphorus (P) and potassium (K), based on the data from field investigations and peasant household surveys in the four provinces. The results showed that total amounts of organic matter inputs in the four provinces was ranked as follow: 8993 kg · hm(-2) in Guangxi, 6390 kg · hm(-2) in Hunan, 5012 kg · hm(-2) in Hubei, 4630 kg · hm(-2) in Guangdong, and average NPK inputs in the four provinces were ranked as follow: 777.5 kg · hm(-2) in Guangxi, 501.6 kg · hm(-2) in Hunan, 486.4 kg · hm(-2) in Hubei, 340.4 kg · hm(-2) in Guangdong. The N and P input surpluses were greatest in Guangxi (67.2% and 99.0% as for N and P, respectively) , followed by Hunan (33.2% and 50.8%), Hubei (11.8% and 11.0%), and Guangdong (7.8% and 30.0%). However, K input was deficient in Hunan, Hubei, and Guangdong (6.6%, 18.7% and 12.4%), but surplus in Guangxi (19.5%).

[Effects of applying pig manure on lettuce yield and nitrate content and soil nutrients].

A pot experiment with two representative soils (purple soil and yellow soil) in Southwest China was conducted to study the effects of applying pig manure on the lettuce (Lactuca sativa L. var. capitata L. ) yield and nitrate content and the soil nutrients. Applying pig manure increased the lettuce yield significantly, and the increment was higher for yellow soil than for purple soil. The nitrate and total nitrogen, phosphorus, and potassium contents in lettuce plants were closely related to soil type and pig manure application rate. According to the evaluation standards of the nitrate pollution level of vegetables formulated by the Chinese Academy of Agricultural Sciences, the nitrate content in lettuce plants growing on purple soil was lower than the grade I (< or = 432 mg x kg(-1), slight pollution) in treatments CK (no pig manure application) and M1 ( applying 200 kg N x hm(-2) of pig manure), but generally higher than the grade II (< or = 758 mg x kg(-1), moderate pollution) while not exceeded the grade III (< or = 1440 mg x kg(-1), heavy pollution) in other treatments. The nitrate content in lettuce plants growing on yellow soil was lower than the grade I, except that in the treatments of chemical fertilizations and of M8 (applying 1600 kg N x hm(-2) of pig manure) where the plant nitrate content was exceeded the grade II. The critical value of Olsen-P characterizing the apparent leaching risk level of phosphorous in yellow soil and purple soil was 96.3 and 107.7 mg x kg(-1), respectively. The environmental safety capacity of pig manure was higher for yellow soil than for purple soil. Applying pig manure increased the organic carbon and total nitrogen contents of the two soils significantly.

[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.

[Effects of selective microbial inhibitors on the microbial transformation of phosphorous in aggregates of highly weathered red soil with rice straw amendment].

In order to further understand the mechanisms of microbial immobilization of phosphorous (P) in highly weathered red soil with organic amendment, an incubation test was conducted to investigate the roles of microbial functional groups in the transformation of P in 0.2-2 mm soil aggregates. Throughout the 90-day incubation period, amendment with rice straw induced a substantial increase in the amounts of microbial biomass C and P, Olsen-P, and organic P in the aggregates. Comparing with rice straw amendment alone, the amendment with rice straw plus fungal inhibitor actidione decreased the amount of microbial biomass C in the aggregates by 10.5%-31.8% in the first 30 days. Such a decrement was significantly larger than that (6.8%-11.6%) in the treatment amended with rice straw plus bacterial inhibitors tetracycline and streptomycin sulphate (P<0.01). After the first 30 days, the microbial biomass C remained constant. In the first 20 days, the amount of microbial biomass P in the aggregates was 10.0%-28.8% higher in the treatment amended with bacterial inhibitors than in the treatment amended with fungal inhibitor (P<0.01). All the results suggested that that both the fungal and the bacterial groups were involved in the microbial immobilization of P in the soil aggregates, and the fungal group played a relatively larger role.

[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 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.

[Advances in extraction and purification of soil microorganism].

For the studies on soil microbial ecology and bio-diversity as well as on the dynamics of C, N, S and P in soil, it is necessary to quantitatively extract microorganism from soil. The recent advances on extraction and purification of microorganism from soil were reviewed in this paper. The procedure for extracting microorganism from soil was comprised of three steps, i.e., soil dispersion, separation and purification. Most bacteria in soil could be successively extracted from soil particles by present techniques such as filtration, centrifugation and elutriation. However, filamentous fungi seem more difficult to be extracted than bacteria, because it is tangled with soil particles. Rotating wire frame, aqueous extraction and membrane filter technique, and low-speed centrifugation were used for extracting filamentous fungi from soil, but the extraction efficiency was rather low and needed to be raised. Aqueous two-phase partitioning system was successfully adopted as a technique for bacteria purification, and it would possibly become a technique suitable for fungi purification. The potential area of research on soil microbial ecology as well as on soil and plant nutrition by using microbial extraction technique was also discussed.