Response of soil nematodes to elevated temperature in conventional and no-tillage cropland systems

Aims

Nematodes are sensitive to environmental changes and are strongly affected by tillage practices. However, it remains unclear whether an increase in soil temperature in conventional tillage (CT) and no-tillage (NT) cropland systems would have a significant effect on nematode communities. The response of soil nematodes to increases in temperature will provide valuable information about probable changes in soil ecology under global warming.

Methods

A field experiment using infrared heaters to simulate climate warming was performed in North China. The impacts of predicted warming on the nematode community in CT and NT systems were measured during the growing season of maize.

Results

The results showed that the diversity of nematodes responded positively to warming in both tillage systems early in the maize growing season, though the diversity in NT declined due to warming late in the growing season. However, no significant warming effects were found on the total nematode density, individual feeding group density or functional indices. Compared to CT, NT presented a rather different nematode community that was characterized by a large nematode diversity, low fungal feeder density due to a strong decrease in Aphelenchoides, and high maturity indices.

Conclusions

Tillage is an important factor that influences the soil properties and nematode community. It is proposed that future global warming with soil temperature increasing approximately 1 °C will have only small effects on soil nematodes in the two tillage systems.     

Assessment of carbon sustainability under different tillage systems in a double rice cropping system in Southern China

Purpose

Adoption of the carbon (C)-friendly and cleaner technology is an effective solution to offset some of the anthropogenic emissions. Conservation tillage is widely considered as an important sustainable technology and for the development of conservation agriculture (CA). Thus, the objective of this study was to assess the C sustainability of different tillage systems in a double rice (Oryza sativa L.) cropping system in southern China.

Methods

The experiment was established with no-till (NT), rotary tillage (RT), and conventional tillage (CT) treatments since 2005. Emission of greenhouse gasses (GHG), C footprint (CF), and ecosystem service through C sequestration in different tillage systems were compared.

Result and discussion

Emission of GHG from agricultural inputs (Mg CO₂-eq ha^?1 year^?1) ranged from 1.81 to 1.97 for the early rice, 1.82 to 1.98 for the late rice, and 3.63 to 3.95 for the whole growing season, respectively. The CF (kg CO₂-eq kg^?1 of rice year^?1) in the whole growing seasons were 1.27, 1.85, and 1.40 [excluding soil organic carbon (SOC) storage] and 0.54, 1.20, and 0.72 (including SOC storage) for NT, RT, and CT, respectively. The value of ecosystem services on C sequestration for the whole growing seasons ranged from ¥3,353 to 4,948 ha^?1 year^?1 and followed the order of NT > CT > RT. The C sustainability under NT was better than that under RT for the late, but reversed for the early rice. However, NT system had better C sustainability for the whole cropping system compared with CT.

Conclusions

Therefore, NT is a preferred technology to reduce GHG emissions, increase ecosystem service functions of C sequestration, and improve C sustainability in a double rice cropping region of Southern China.     

Effect of long-term tillage and mineral phosphorus fertilization on arbuscular mycorrhizal fungi in a humid continental zone of Eastern Canada

Aims

Evidence shows that tillage modifies soil properties, especially phosphorus (P) dynamics. Our objective was to disentangle long-term effects of P-fertilization and tillage on arbuscular mycorrhizal fungal (AMF) proliferation and community structure.

Methods

Changes in the community structure of AMF and in the density of their hyphae and spores induced by moldboard plow (MP) or no till (NT), and fertilization with 0, 17.5, or 35 kg?P?ha^?1 were sought in the 0–15 cm and 15–30 cm soil layers after soybean harvest, at a long-term (17 years) experimental site in a humid continental zone of eastern Canada. The relationships among AMF, soil and plant attributes were examined.

Results

The 0–15 cm and 15–30 cm soil layers had different properties under NT, but were similar under MP, after 17 years, and MP increased soil available P levels. Phosphorus fertilization increased P levels in soil and in soybean. Treatment effects on AMF spore and hyphal density at 0–15 cm were greater than that at 15–30 cm, whereas effects on AMF community structure did not change with soil depths. At 0–15 cm, P-fertilization increased AMF spore density and reduced AMF hyphal density, and MP reduced AMF spore density. A total of eight AMF phylotypes were detected. Phosphorus fertilization reduced AMF phylotype richness and Shannon diversity index. Soil P availability increased under MP and hence the influence of P-fertilization treatments on the frequency of AMF phylotype detection varied with tillage system; it declined with P-fertilization under MP, but increased under NT.

Conclusions

Phosphorus fertilization shifts resource partitioning in AMF propagules rather than in their hyphae, and degrades the genetic diversity of AMF in soil; tillage increases soil P availability and hence aggravates the impact of P-fertilization.     

Soil carbon dioxide and methane fluxes as affected by tillage and N fertilization in dryland conditions

Background and aims

The effects of tillage and N fertilization on CO₂ and CH₄ emissions are a cause for concern worldwide. This paper quantifies these effects in a Mediterranean dryland area.

Methods

CO₂ and CH₄ fluxes were measured in two field experiments. A long-term experiment compared two types of tillage (NT, no-tillage, and CT, conventional intensive tillage) and three N fertilization rates (0, 60 and 120 kg N ha^?1). A short-term experiment compared NT and CT, three N fertilization doses (0, 75 and 150 kg N ha^?1) and two types of fertilizer (mineral N and organic N with pig slurry). Aboveground and root biomass C inputs, soil organic carbon stocks and grain yield were also quantified.

Results

The NT treatment showed a greater mean CO₂ flux than the CT treatment in both experiments. In the long-term experiment CH₄ oxidation was greater under NT, whereas in the short-term experiment it was greater under CT. The fertilization treatments also affected CO₂ emissions in the short-term experiment, with the greatest fluxes when 75 and 150 kg organic N ha^?1 was applied. Overall, the amount of CO₂ emitted ranged between 0.47 and 6.0 kg CO₂?equivalent kg grain^?1. NT lowered yield-scaled emissions in both experiments, but these treatment effects were largely driven by an increase in grain yield.

Conclusions

In dryland Mediterranean agroecosystems the combination of NT and medium rates of either mineral or organic N fertilization can be an appropriate strategy for optimizing CO₂ and CH₄ emissions and grain yield.     

Root growth response of rainfed lowland rice to aerobic conditions in northeastern Thailand

Background and aims

Rice plants alternately experience anaerobic and aerobic conditions during their life cycle in rainfed lowlands. Each condition affects root growth differently. Our objective was to clarify the specific rice root response to aerobic conditions in rainfed lowlands.

Methods

At the Ubon Ratchathani Rice Research Center in northeastern Thailand, we obtained root samples from 17 ‘Surin1’ (Thai variety) BC₃-derived lines and 7 CT9993-5-10-1-M × IR62266-42-6-2 doubled-haploid lines from flooded and non-flooded paddy fields at the reproductive stage in 2010 and 2011.

Results

In the non-flooded trial, rice was grown aerobically by draining the perched water; soil moisture at a depth of 20 cm fluctuated between ?10 and ?30 kPa. Deep rooting was likely promoted under aerobic conditions, but slightly drier soils under longer dry spells seemed to restrict root penetration, as the topsoil rapidly hardened during dry spells of only a few days. Fine-root development in the topsoil was inhibited under aerobic conditions.

Conclusions

Even without drought stress, rice roots respond significantly to the disappearance of standing water in rainfed lowlands via deep rooting and root branching. We identified one promising ‘Surin1’ BC₃-derived line showing an adaptive response of deep rooting under aerobic conditions, which can be used as a breeding material for rainfed lowland rice in Thailand.     

Winter cereal root growth and aboveground–belowground biomass ratios as affected by site and tillage system in dryland Mediterranean conditions

Background and aims

Understanding the interaction between crop roots and management and environmental factors can improve crop management and agricultural carbon sequestration. The objectives of this study were to determine the response of winter cereal root growth and aboveground–belowground biomass ratios to tillage and environmental factors in the Mediterranean region and to test an alternative approach to determine root surface area.

Methods

Winter cereal root growth and biomass ratios were studied in three sites with different yield potential according to their water deficit (high yield potential, HYP; medium yield potential, MYP; low yield potential, LYP) in the Ebro Valley (NE Spain). At all sites, three tillage systems were compared (conventional tillage, minimum tillage, no-tillage (NT)). Root surface density (RSD), soil water content, yield components, and grain yield were quantified and shoot-to-root and grain-to-root ratios were calculated. RSD was measured with the use of image analysis software comparing its performance to a more common intersection method.

Results

Significant differences on RSD between sites with different yield potential were found being the greatest at the HYP site and the lowest at the LYP one. Shoot-to-root ratio was 2.7 and 4.6 times greater at the HYP site than at the MYP and LYP sites, respectively. Moreover, the grain-to-root ratio was significantly affected by site, with a ratio that increased with yield potential. Tillage had no significant effects on RSD at any of the sites studied; however, tillage did affect grain yield, with NT having the greatest yields.

Conclusions

This study shows that in the Mediterranean dryland agroecosystems, winter cereals relative above- and belowground biomass growth is strongly affected by the yield potential of each area. NT in the Mediterranean areas does not limit cereal root growth and leads to greater grain yields. A highly significant linear relationship (P?<?0.001; r ² 0.77) was observed between the root surface values obtained with the free-software image analysis method and the most common intersection method, showing it to be a reliable method for quantifying root density.     

Modelling facilitation or competition within a root system: importance of the overlap of root depletion and accumulation zones

Background and aims

Relevant soil properties and nutrient distributions influencing crop root growth might be different under no-till (NT) and mouldboard plough (MP) management. The possible different root systems within different managements might have key impact on crop nutrient uptake and consequently crop production. Our objective was to assess the long-term combined effects of tillage and phosphorus (P) fertilization on corn (Zea mays L.) root distribution and morphology.

Methods

Corn root and soil samples were collected during the silking stage at five depths (0–5, 5–10, 10–20, 20–30 and 30–40 cm) and three horizontal distances perpendicular to the corn row (5, 15 and 25 cm) under MP and NT with three P fertilizations (0, 17.5, and 35 kg P ha^?1) for a long-term (22 years) experiment in eastern Canada. Root morphology and soil properties were determined.

Results

NT practice decreased corn root biomass by ?26 % compared to MP, mainly by decreasing the primary and secondary roots. Additionally, corn roots in NT tend to be more expansive on the surface layer with higher root length and surface densities for the depth of 0–5 cm at two sampling distances of 15 and 25 cm. The 35 kg P ha^?1 rate increased the root biomass by 26 and 41 % compared to the 0 and 17.5 kg P ha^?1 rates.

Conclusions

No-tillage practice and low rates of P fertilization reduce corn roots. This is probably caused by the weed competition in NT and the continued downward P status with low P rates over 22 years.

     

Conservation agriculture, increased organic carbon in the top-soil macro-aggregates and reduced soil CO2 emissions

Background and aims

Conservation agriculture, the combination of minimal soil movement (zero or reduced tillage), crop residue retention and crop rotation, might have the potential to increase soil organic C content and reduce emissions of CO₂.

Methods

Three management factors were analyzed: (1) tillage (zero tillage (ZT) or conventional tillage (CT)), (2) crop rotation (wheat monoculture (W), maize monoculture (M) and maize-wheat rotation (R)), and (3) residue management (with (+r), or without (?r) crop residues). Samples were taken from the 0–5 and 5–10?cm soil layers and separated in micro-aggregates (< 0.25?mm), small macro-aggregates (0.25 to 1?mm) and large macro-aggregates (1 to 8?mm). The carbon content of each aggregate fraction was determined.

Results

Zero tillage combined with crop rotation and crop residues retention resulted in a higher proportion of macro-aggregates. In the 0–5?cm layer, plots with a crop rotation and monoculture of maize and wheat in ZT+r had the greatest proportion of large stable macro-aggregates (40%) and highest mean weighted diameter (MWD) (1.7?mm). The plots with CT had the largest proportion of micro-aggregates (27%). In the 5–10?cm layer, plots with residue retention in both CT and ZT (maize 1?mm and wheat 1.5?mm) or with monoculture of wheat in plots under ZT without residues (1.4?mm) had the greatest MWD. The 0–10?cm soil layer had a greater proportion of small macroaggregates compared to large macro-aggregates and micro-aggregates. In the 0–10?cm layer of soil with residues retention and maize or wheat, the greatest C content was found in the small and large macro-aggregates. The small macro-aggregates contributed most C to the organic C of the sample. For soil cultivated with maize, the CT treatments had significantly higher CO₂ emissions than the ZT treatments. For soil cultivated with wheat, CTR-r had significantly higher CO₂ emissions than all other treatments.

Conclusion

Reduction in soil disturbance combined with residue retention increased the C retained in the small and large macro-aggregates of the top soil due to greater aggregate stability and reduced the emissions of CO₂ compared with conventional tillage without residues retention and maize monoculture (a cultivation system normally used in the central highlands of Mexico).     

Seasonal variation in nitrogen fixation and effects of climate change in a subarctic heath

Background and aims

Nitrogen fixation associated with cryptogams is potentially very important in arctic and subarctic terrestrial ecosystems, as it is a source of new nitrogen (N) into these highly N limited systems. Moss-, lichen- and legume-associated N₂ fixation was studied with high frequency (every second week) during spring, summer, autumn and early winter to uncover the seasonal variation in input of atmospheric N₂ to a subarctic heath with an altered climate.

Methods

We estimated N₂ fixation from ethylene production by acetylene reduction assay in situ in a field experiment with the treatments: long- vs. short-term summer warming using plastic tents and litter addition (simulating expansion of the birch forest).

Results

N₂ fixation activity was measured from late April to mid November and 33 % of all N₂ was fixed outside the vascular plant growing season (Jun–Aug). This substantial amount underlines the importance of N₂ fixation in the cold period. Warming increased N₂ fixation two- to fivefold during late spring. However, long-term summer warming tended to decrease N₂ fixation outside the treatment (tents present) period. Litter alone did not alter N₂ fixation but in combination with warming N₂ fixation increased, probably because N₂ fixation became phosphorus limited under higher temperatures, which was alleviated by the P supply from the litter.

Conclusion

In subarctic heath, the current N₂ fixation period extends far beyond the vascular plant growing season. Climate warming and indirect effects such as vegetation changes affect the process of N₂ fixation in different directions and thereby complicate predictions of future N cycling.     

Root hairs explain P uptake efficiency of soybean genotypes grown in a P-deficient Ferralsol

Background and aims

Incorporating soybean (Glycine max) genotypes with a high nitrogen fixation potential into cropping systems can sustainably improve the livelihoods of smallholder farmers in Western Kenya. Nitrogen fixation is, however, often constrained by low phosphorus (P) availability. The selection of soybean genotypes for increased P efficiency could help to overcome this problem. This study investigated the contribution of different root traits to variation in P efficiency among soybean genotypes.

Methods

Eight genotypes were grown in a Ferralsol amended with suboptimal (low P) and optimal (high P) amounts of soluble P. Root hair growth was visualized by growing plants in a novel agar system where P intensity was buffered by Al₂O₃ nanoparticles.

Results

In the pot trial, P uptake was unaffected among the genotypes at high P but differed about 2-fold at low P. The genotypes differed in P uptake efficiency but not in P utilization efficiency. Regression analysis and mechanistic modeling indicated that P uptake efficiencies were to a large extent related to root hair development (length and density) and, to a lower extent, to colonization by mycorrhizal fungi.

Conclusion

Breeding for improved root hair development is a promising way to increase P uptake efficiency in soybean.     

Comparison of common bean (Phaseolus vulgaris L.) genotypes for nitrogen fixation tolerance to soil drying

Aims

Common bean is a major source of protein for many people worldwide. However, the crop is often subjected to drought conditions and its advantage in undertaking symbiotic nitrogen fixation can be severely decreased. The primary objective of this study was to compare the resistance of nitrogen fixation of 12 selected genotypes to soil drying.

Methods

Twelve common bean genotypes of diverse genetic background were compared. Plants were grown in pots and subjected to soil drying over about 2 weeks. Nitrogen fixation was measured daily using a flow-through acetylene reduction technique. The plants were exposed to acetylene for only a short time period allowing repeated measures. The acetylene reduction rate of plants on drying soil was normalized against the rates measured for well-watered plants.

Results

Substantial variability was identified among genotypes in the threshold soil water content at which nitrogen fixation was observed to decrease. Genotypes SER 16, SXB 412, NCB 226, and Calima were found to have the greatest delay in their decrease in nitrogen fixation rates based on soil water content. These four genotypes expressed substantial tolerance of nitrogen fixation to soil drying. These experiments also resulted in data on the threshold soil water contents at which transpiration rates decreased. A decrease in transpiration rates at high soil water contents is potentially advantageous since it allows soil water conservation for use as the severity of the drought increases. There was a general trend of those genotypes with sustained nitrogen fixation rates to low soil water contents also expressing decreased transpiration rates at high soil water contents.

Conclusions

This study identified genetic variation among common bean genotypes in their response of nitrogen fixation and transpiration to soil drying. Five genotypes (SER 16, SXB 412, NCB 226, Calima, and SEA 5) expressed the desired traits for water-limited conditions, which might be exploited in breeding efforts.     

Field performance of STG06L-35-061, a new genetic resource developed from crosses between weed-suppressive indica rice and commercial southern U.S. long-grains

Aims

Weed control in rice is challenging, particularly in light of increased resistance to herbicides in weed populations including Echinochloa crus-galli (L.) Beauv. Indica rice cultivars can produce high yields and suppress barnyardgrass, but have not been commercially acceptable in the U.S. due to inferior agronomic traits and grain quality. Our objectives were to combine high yield and weed-suppressive characteristics from indica cultivars with commercially acceptable grain quality and plant types from long-grain cultivars grown in the southern U.S.

Methods

Crosses between indica and commercial tropical japonica (cv. Katy, and cv. Drew) rice were evaluated for weed suppression and agronomic traits in a breeding program.

Results

In some tests, the selection STG06L-35-061 was nearly as weed suppressive as PI 312777, the suppressive parent, and more suppressive than its tropical japonica parents. Its main crop yield is commercially acceptable, and intermediate between PI 312777 and Katy. Its milling quality and cooking quality are similar to long-grain commercial cultivars, and it has resistance to rice blast disease. Marker analyses identified introgressions from the indica parents on chromosomes 1 and 3 of STG06L-35-061 that require further analysis as possible sources of weed suppressive traits.

Conclusions

STG06L-35-061 might be suitable for organic rice or reduced input conventional systems.     

High surface area biochar negatively impacts herbicide efficacy

Background and Aims

Amendment of soil by biochar may reduce efficacy of soil-applied herbicides due to sorption.

Methods

Bioassays with Green Foxtail (Setaria viridis) tested the influence of two biochars on phytoavailability of S-metolachlor and sulfentrazone under biochar amendment of 0, 13, 26 and 52?Mg?ha^-1.

Results

Adsorption of both herbicides was an order of magnitude greater on a high specific surface area (SSA) biochar (EUC-800; SSA 242?m²?g^-1) than on a low SSA biochar (BC-1; SSA 3.6?m²?g^-1). Herbicide doses near the lowest recommended label rates controlled the weed at 13 and 26?Mg?ha^-1 of BC-1; sulfentrazone was also effective at 52?Mg BC-1?ha^-1. These same herbicide doses controlled weed germination and development only at 13?Mg?ha^-1 of EUC-800; at herbicide doses near the highest label rates, weed control was also achieved at 26?Mg EUC-800?ha^-1, but not at 52?Mg EUC-800?ha^-1.

Conclusions

Increased doses of soil-applied herbicides cannot necessarily offset decreases in herbicide phytoavailability in biochar-amended soils, particularly if the biochar has a high SSA. Considering the long half-life of biochar in soil, pest control needs will be best served by low SSA biochars.     

Transfer of fixed-N from N2-fixing cyanobacteria associated with the moss Sphagnum riparium results in enhanced growth of the moss

Background

Despite the general assumption that nitrogen fixed by associated cyanobacteria will be readily utilised for growth by the Sphagnum, no empirical evidence is available in the literature. Therefore the effects of nitrogen transfer from cyanobacteria associated with S. riparium were investigated.

Methods

Cultivation of S. riparium with and without cyanobacteria was performed under laboratory conditions for 57 days.

Results

We show that nitrogen fixation by cyanobacteria associated with Sphagnum mosses, influences moss growth by transfer of fixed nitrogen to the moss. More than 35 % of the nitrogen fixed by cyanobacteria was transferred to the newly formed moss biomass and resulted in an increase in the growth of Sphagnum biomass compared to the controls. The variation in the increase of nitrogen content explained 76 % of the biomass increment.

Conclusion

Hence, nitrogen fixation will have immediate effect on the carbon fixation by Sphagnum. This shows that factors regulating nitrogen fixation will have a direct effect on the role of Sphagnum dominated ecosystems with respect to carbon cycling.     

Conventional tillage versus cover crops in relation to carbon fixation in Mediterranean olive cultivation

Background and Aims

For croplands, controversy persists concerning the adequacy of the soil use and the management of environmental problems such as soil erosion and fertility in a context of climate change. In this study, we used the RothC model to evaluate the capacity of carbon fixation by the soil in a Mediterranean olive grove for two different scenarios: the land-use change from native vegetation (NV) to conventional tillage (T) in the olive grove, and for the change in soil management from conventional tillage to cover crop (CC).

Methods

In three experimental olive groves in Andalusia (S Spain) two soil-management systems were sampled: T and CC. Areas of NV adjacent to the grove were also sampled as indicative of the initial state of the soil without olive trees. We measured the aboveground biomass production of the cover and the clay content, bulk density, and soil organic carbon (SOC) for 0–5, 5–15 and 15–30 cm depth.

Results

The removal of NV to implement T resulted in a significantly loss of SOC that depended mainly on the relief of the terrain. However, the use of CC increased the SOC because of greater inputs (above-and belowground plant inputs) to the soil. The final concentration at each location was related to the carbon inputs and the clay content. The CC resulted in carbon storage during the first year of 4.02?±?1.65 Mg C ha^?1, and a total carbon fixation by the soil of 5.91?±?2.06 Mg C ha^?1.

Conclusion

The use of cover crops in Mediterranean olive groves proved to be a suitable strategy to increase the carbon storage into the soil and then to decrease the CO₂ concentration in the atmosphere.     

Inoculation with indigenous <Emphasis Type="Italic">rhizobium</Emphasis> strains increases yields of common bean (<Emphasis Type="Italic">Phaseolus vulgaris</Emphasis> L.) in northern Spain,although its efficiency is affected by the tillage system

Common bean (Phaseolus vulgaris L.) crops hold the potential to obtain higher yields by enhancing their biological nitrogen fixation (BNF) with Rhizobium. However in contrast to other legumes, common bean has shown a lack of positive response to inoculation with Rhizobium in many cases. This has led to a limited use of rhizobial inoculants in this crop, especially in Europe. The adaptation of bacterial strains to the rhizosphere is a key factor in the success of any inoculant, especially in a promiscuous legume such as common bean. This research aimed at increasing common bean yields via inoculation with effective indigenous Rhizobium leguminosarum strains. Three highly effective strains (LCS0306, LBM1123 and ZBM1008) which were selected according to their effectiveness at BNF in hydroponic conditions were separately inoculated onto common bean in a field experiment. The experiment was carried out under three environments and three tillage systems: conventional-tillage (CONVT), no-tillage (NT) and a cover-crop (CC). The grain yield observed with seed inoculation was significantly higher than the yield obtained with uninoculated seed under CONVT and CC. However, under NT inoculation had no effect. Furthermore, under CONVT and CC, inoculation with R. leguminosarum LCS0306 produced even higher yields than those obtained in nitrogen-fertilised or control plots. This is the first attempt to explain the inoculation performance of common bean under different tillage systems in Europe. A gene–based hypothesis has been used to explain the effectiveness of indigenous common bean rhizobia as nitrogen fixers in this crop.     

保护性耕作对东北黑土微生物碳循环功能基因的影响

土壤微生物既参与土壤有机碳的分解也是土壤有机碳转化和固定的驱动者,是影响土壤碳循环和有机碳稳定性的关键因素。然而,保护性耕作（秸秆还田）如何通过调节土壤微生物碳循环功能基因组成来影响土壤CO₂释放的机理尚不明确。因此,依托中国科学院东北地理与农业生态研究所长春保护性耕作观测站,借助鸟枪法宏基因组测序技术,分析了玉米连作系统不同耕作方式（免耕（NT）、秋翻（MP）以及常规耕作（CT））对土壤CO₂释放速率、碳水化合物活性酶（CAZy）、碳循环功能基因（碳固定、甲烷代谢以及碳水化合物代谢）组成的影响。研究表明：基于生长季节土壤CO₂释放速率6年平均值分析发现,生长季前期免耕土壤的平均CO₂释放速率显著低于秋翻和常规耕作,分别比秋翻低28%（5月份）、11%（6月份）和23%（7月份）;比常规耕作低31%（5月份）、19%（6月份）和7%（7月份）。基于CAZy数据库注释结果,发现耕作处理显著影响一些糖苷水解酶（如GH102、GH5_38和GH13_17）、糖基转移酶（如GT39）和多糖裂解酶（如PL17和PL5_1）的基因丰度,与常规耕作相比,秸秆还田的免耕和秋翻处理的这些差异基因的相对丰度较高。基于京都基因与基因组百科全书（KEGG）数据库注释结果,发现耕作方式显著影响土壤碳循环功能基因组成（Adonis,多元方差分析,R²=0.45;P=0.006）,且免耕处理土壤的碳固定、甲烷代谢以及碳水化合物代谢功能基因组成不同于常规耕作和秋翻处理,单独聚为一类。免耕土壤上调的碳固定功能基因的相对丰度（所有上调功能基因相对丰度的平均值）分别比常规耕作和秋翻高17%和11%,而下调的2个功能基因（K01007和K00170）的丰度分别低19%（CT）、21%（MP）和14%（CT）、17%（MP）。免耕土壤上调的甲烷代谢基因相对丰度分别较常规耕作和秋翻高15%和10%;下调基因的丰度分别低13%（CT）和11%（MP）。免耕土壤上调的碳水化合物代谢功能基因丰度较常规耕作和秋翻高23%和14%;下调的基因丰度分别低25%（CT）和18%（MP）。冗余分析（db-RDA）表明土壤容重及土壤水溶性有机碳（DOC）是驱动土壤碳循环功能基因组成差异的主要因子（P<0.05）,且免耕土壤上调的碳固定功能基因（K00625、K01676、K09709、K00925和K14470等）、甲烷代谢基因（K03520、K00830、K10713、K15633和K00625等）和碳水化合物代谢功能基因（K00886、K00830、K01676、K00117和K00114等）与土壤DOC、容重或含水量呈显著正相关。此外,研究发现土壤CO₂释放速率与土壤碳循环功能基因组成显著相关（R²=0.80;P<0.01）,尤其是与一些碳水化合物代谢功能基因显著相关。这些结果说明免耕处理通过影响土壤理化性质改变土壤碳循环过程,且推断免耕秸秆还田和减少干扰的叠加效应通过调节碳循环功能基因组成来提高土壤固碳潜力。     

Nitrogen fertilizer increases spikelet number per panicle by enhancing cytokinin synthesis in rice

Key message

Nitrogen fertilizer enhances local cytokinin synthesis to increase flower numbers in the panicles of rice. Localized cytokinin biosynthesis is an important response to nitrogen.

Abstract

Flower number per panicle is one of the most important traits in rice productivity determination. The number of flowers is established in the early stages of panicle development. Nitrogen fertilizer application before panicle initiation is well known to increase flower number. Nitrogen increases cytokinin (CKs) biosynthesis in plants, and CKs have very similar effects as nitrogen fertilizer on panicle branching. The effects of nitrogen fertilizer on panicle branching may be mediated by CKs, in which accumulation in the inflorescence meristem can regulate panicle development, resulting in increased numbers of flowers and branches. Adenosine phosphate-isopentenyltransferase (IPT) catalyzes the rate-limiting step of CKs biosynthesis. We analyzed the effect of nitrogen fertilizer (urea) on the expression of OsIPT genes (OsIPTs). The results showed that OsIPTs were markedly increased, and CKs accumulated in panicle when nitrogen fertilizer was applied. CKs biosynthesis in the roots and leaves was not up-regulated by nitrogen. These results suggest that nitrogen fertilizer enhances local CKs synthesis to increase flower numbers in the panicles of rice. Localized CKs biosynthesis is an important response to nitrogen.     

Polyphenols, tannins and antioxidant activities of eight true mangrove plant species in South China

Background and aims

(i) compare the concentrations of total polyphenols (TP) and condensed tannins (CT), and CT profiles in different organs of mature trees and seedlings of eight true mangrove species in Hong Kong; (ii) examine the antioxidant activities of CT and (iii) relate the non-enzymatic antioxidative defence system with the vertical zonation pattern of mangrove species.

Methods

Mature trees and seedlings of eight species were collected from a Hong Kong mangrove swamp to determine TP and CT concentrations and the antioxidant activities of CT.

Results

According to TP concentrations, the true mangrove species could be broadly classified into three groups, (i) Lumnitzera racemosa and Aegiceras corniculatum > (ii) Heritiera littoralis, Excoecaria agallocha, Bruguiera gymnorrhiza and Kandelia obovata > (iii) Acanthus ilicifolius and Avicennia marina. The last two are pioneer species in the most foreshore location. They also had significantly lower antioxidant activities, CT concentrations and different CT profiles than the other six species in mid- and low-tides.

Conclusions

Classification of the eight true mangrove species into three groups based on polyphenols was similar to their vertical zonation from land to sea. The relationships between these antioxidants and zonation should be further verified by transplantation studies.     

Weeds influence soil bacterial and fungal communities

Background and aims

Vineyards harbour a variety of weeds, which are usually controlled since they compete with grapevines for water and nutrients. However, weed plants may host groups of fungi and bacteria exerting important functions.

Methods

We grew three different common vineyard weeds (Taraxacum officinalis, Trifolium repens and Poa trivialis) in four different soils to investigate the effects of weeds and soil type on bacterial and fungal communities colonising bulk soil, rhizosphere and root compartments. Measurements were made using the cultivation-independent technique Automated Ribosomal Intergenic Spacer Analysis (ARISA).

Results

Weeds have a substantial effect on roots but less impact on the rhizosphere and bulk soil, while soil type affects all three compartments, in particular the bulk soil community. The fungal, but not the bacterial, bulk soil community structure was affected by the plants at the late experimental stage. Root communities contained a smaller number of Operational Taxonomic Units (OTUs) and different bacterial and fungal structures compared with rhizosphere and bulk soil communities.

Conclusions

Weed effect is localised to the rhizosphere and does not extend to bulk soil in the case of bacteria, although the structure of fungal communities in the bulk soil may be influenced by some weed plants.