[Effect of Water Management on Rice Growth and Rhizosphere Priming Effect in Paddy Soils].

The rhizosphere priming effect (RPE) caused by carbon inputs from crop rhizodeposits plays a key role in regulating the carbon emission flux and carbon balance of farmland soils. Due to frequent alternations between dry and wet conditions, CO2 and CH4 emissions and the RPE in paddy field ecosystems are significantly different to those of other ecosystems. Therefore, it is of great significance to determine the direction and intensity of the rice RPE under alternations of dry and wet to limit greenhouse gas emissions. In this study, using a 13C-CO2 continuous labeling method combined with a pot-based experiment, the response of rice growth and the RPE under alternating dry and wet and continuous flooding conditions was examined. The results showed that, compared with the continuous flooding treatment, the alternating dry and wet treatments significantly increased aboveground and root biomass and the root-to-root ratio, and also increased soil microbial biomass. Under continuous flooding conditions, fluxes of 13CO2 and 13CH4 increased with rice growth from 10.2 µg·(kg·h)-1 and 2.8 µg·(kg·h)-1 (63 d) to 16.0 µg·(kg·h)-1 and 3.2 µg·(kg·h)-1 (75 d), respectively. During the 12-day drying process, the emissions of 13CO2 and 13CH4 derived from rhizosphere deposited C decreased by 57.5% and 88.1%. Under continuous flooding conditions, the RPE for CO2 and CH4 were positive and increased with the growth of rice. Under the alternating dry and wet treatment, after 12 days of drying, the RPE for CO2 and CH4 was reduced from 0.29 mg·(kg·h)-1 and 12.3 µg·(kg·h)-1 (63 d) to -0.39 mg·(kg·h)-1 and 0.07 µg·(kg·h)-1 (75 d). Thus, alternating wet and dry treatment can effectively promote rice growth and reduce the cumulative emissions of CH4. Therefore, adopting appropriate field water management is of great significance for increasing rice yields and mitigating greenhouse gas emissions.

Effectiveness of simultaneous applications of lime and zinc/iron foliar sprays to minimize cadmium accumulation in rice.

Due to the large area of Cd-contaminated paddy soils worldwide, low-cost measures to reduce the accumulation of Cd in rice plant are necessary. A field experiment was therefore conducted to investigate the reducing effect of lime combined with foliar applications of Zn (ZnSO4) or Fe (EDTA·Na2Fe) on Cd concentrations in brown rice on a Cd-contaminated paddy soil. The results indicated that liming alone or in combination with foliar sprays of Zn or Fe increased the soil pH by 0.27-0.63 units. However, limited effects of lime or lime combined with foliar applications of Zn/Fe on soil DTPA-extractable Cd, rice grain and rice straw biomass were observed. Liming alone significantly reduced the Cd concentration in brown rice and rice straw by 31.8% and 42.3%, respectively. The Cd concentrations in brown rice decreased by 25.5% and 65.4% and in rice straw by 53.0% and 68.1% after liming combined with foliar applications of Fe and Zn, respectively. In contrast, liming combined with foliar spraying of Fe significantly increased the transfer ratio of Cd from the rice straw to the grain. As a low-cost technique, lime application combined with foliar application of ZnSO4 could be recommended for the remediation of Cd-contaminated paddy soils.

[Effect of Phosphorus Addition on the Abundance of Autotrophic CO2-Fixation Microorganisms in Rhizospheric Soil from a Phosphorus-Limited Paddy Field].

A rice pot experiment was conducted to investigate the effect of phosphorus addition on the abundance of autotrophic CO2-fixation microorganisms using phosphorus-limited paddy soil from the Changsha Observation and Research Station for the Agricultural Environment. Rice seedlings were transplanted in the paddy soil with or without phosphorus addition, corresponding to P-treated-pot (P) or control pot (CK), respectively. Rhizosphere soils were collected from the P and CK treatments during the tillering and shooting stages. The physical and chemical soil properties were measured and the abundance of autotrophic CO2-fixation microorganisms was quantified with a real-time PCR technique based on four functional genes (cbbL, cbbM, accA, and aclB) involved in three CO2-fixation pathways (CBB cycle, rTCA cycle, and 3-hydroxypropionate/4-hydroxybutyrate cycle). The results show that phosphorus addition improves the concentrations of DOC and Olsen-P and the pH value, whereas negative effects on the MBC and NH4+-N concentrations are revealed during the tillering stage. The effect of phosphorus addition on the NO3--N concentration in the tillering and shooting stages differs. Phosphorus addition significantly increases the abundances of the cbbL, cbbM, accA, and aclB genes, which are 156%, 99%, 110%, and 193% higher than those of the CK treatment in the tillering stage. However, this positive effect is not notable for the cbbL, accA, and aclB genes during the shooting stage. Redundancy analysis (RDA) shows that Olsen-P is the environmental factor that most significantly affects the abundance of autotrophic CO2-fixation microorganisms.

[Effect of CO2 Doubling and Different Plant Growth Stages on Rice Carbon, Nitrogen, and Phosphorus and Their Stoichiometric Ratios].

The variation characteristics of ecological stoichiometric ratios can reflect the nature of plant adaptation to environmental changes. The C, N, and P contetns, and their stoichiometric ratios in different organs of rice were studied using a CO2 continuous labeling system, by simulating the increase of atmospheric CO2 concentration (800×10-6). The results showed that CO2 doubling promoted the growth of rice organs and increased the root/shoot ratio. CO2 doubling reduced the shoot TN content in different growth periods, increased the C/N ratio in the rice root, shoot, and grain, decreased the N use efficiency, and improved the P use efficiency. Multiple comparison and Venn diagram analyses showed that CO2 concentration only has a significant impact on the TN content in the rice shoot; it contributed little to the variation in rice nutrient content and their stoichiometric ratios, indicating that CO2 doubling had no effect on these. Under the condition of elevated atmospheric CO2 concentrations, the C, N, and P contents and their stoichiometirc ratios, in rice organs had good homeostasis, and the stoichiometric change during growth periods was consistent with "the Growth Rate Theory". In farmland management, appropriate nitrogen fertilizers can alleviate the nutrient balance pressure caused by the increase in CO2 concentration.

Speciation and phytoavailability of cadmium in soil treated with cadmium-contaminated rice straw.

When grown on Cd-contaminated soil, rice typically accumulates considerable Cd in straw, and which may return to the soil after harvest. This work was undertaken to assess the pollution risk of Cd associated to the Cd-contaminated rice straw after incorporating into an uncontaminated soil. With the Cd-contaminated rice straw added at 0, 1, 2, 3, 4 and 5 % (w/w), an incubation experiment (28 days) with non-planting and a followed pot experiment sequent with two planting (rice and Chinese cabbage, transplanted after 28-day incubation) were carried out to investigate the changes of soil Cd speciation and phytoavailability. The results indicated that the Cd-contaminated rice straw addition significantly increased soil pH and dissolved organic carbon during the 28-day incubation. For the high availability of Cd in contaminated rice straw, diethylenetriaminepentaacetic acid (DTPA) extractable Cd significantly increased, and the percentages of acetic acid extractable and reducible Cd in soil significantly enhanced after the addition of Cd-contaminated rice straw. However, the Cd-contaminated rice straw addition inhibited the rice growth and induced the decrease of Cd in rice grain and straw by 12.8 to 70.2 % and 39.3 to 57.3 %, respectively, whereas the Cd contents increased by 13.9 to 84.1 % in Chinese cabbage that planted after rice harvest. In conclusion, Cd associated with Cd-contaminated rice straw was highly available after incorporating into the soil, and thus the Cd pollution risk via the Cd-contaminated rice straw incorporation should be evaluated in the Cd-contaminated paddy region.

[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.

[Effects of ex situ rice straw incorporation on organic matter content and main physical properties of hilly red soil].

Two typical land-use types, i.e., newly cultivated slope land and mellow upland, were selected to investigate the effects of ex situ rice straw incorporation on the organic matter content, field water-holding capacity, bulk density, and porosity of hilly red soil, and to approach the correlations between these parameters. The results showed that ex situ incorporation of rice straw increased soil organic matter content, ameliorated soil physical properties, and improved soil water storage. Comparing with non-fertilization and applying chemical fertilizers, ex situ incorporation of rice straw increased the contents of organic matter (5.8%-28.9%) and > 0.25 mm water-stable aggregates in 0-20 cm soil layer, and increased the field water-holding capacity (6.8%-16.2%) and porosity (4.8%-7.7%) significantly (P < 0.05) while decreased the bulk density (4.5%-7.5%) in 10-15 cm soil layer. The organic matter content in 0-20 cm soil layer was significantly correlated to the bulk density, porosity, and field water-holding capacity in 10-15 cm soil layer (P < 0.01), and the field water-holding capacity in 0-20 cm and 10-15 cm soil layers was significantly correlated to the bulk density and porosity in these two layers (P < 0.05).