Long-term rice-rice-green manure rotation changing the microbial communities in typical red paddy soil in South China

On the basis of a long-term（30 years） field experiment that involved four rotation systems, rice-rice-winter fallow（RRF）, rice-rice-ryegrass（RRG）, rice-rice-rape（RRP）, and rice-rice-milk vetch（RRV）, this study described the effects of green manure on the microbial communities in the red paddy soils using 454 pyrosequencing for the 16 S r RNA gene. The Chao1 richness and non-parametric Shannon＇s index increased in all soil samples that received green manure treatments. The communities＇ structures with the green manure applications were significantly dissimilar from that under the winter fallow. Using Metastats tests, many genera in the RRG, RRP and RRV soils were significantly different from those in the RRF soil, including a number of genera that functioned in the nitrogen and sulfur cycles. Analyses of the genera with these functions revealed the shifts in microbial ecosystem functions after long-term green manuring. Changes in the microbial communities increased the ammonium supply and decreased the soil acidification in green-manure-amended soils. Together, these data suggested powerful effects of green manure on both the microbial communities and the biogeochemical cycle driven by the shifts in bacterial functional groups.

Size-asymmetric root competition in deep,nutrient-poor soil

Aims There is much evidence that plant competition below ground is size symmetric,i.e.that competing plants share contested resources in proportion to their sizes.Several researchers have hypothesized that a patchy distribution of soil nutrients could result in size-asymmetric root competition.We tested this hypothesis.Methods In a greenhouse experiment,Triticum aestivum(wheat)individ-uals of different initial sizes were grown alone or with below-ground competition from one neighbour,in 1 m tall,narrow containers in a nitrogen-poor field soil with(i)no added nitro-gen,(ii)nitrogen fertilizer mixed into the upper 50 cm,and(iii)the same amount of fertilizer mixed into a 20-30 cm deep layer.We measured total leaf length throughout the experiment,and above-ground biomass and nitrogen concentration at harvest.We also measured root depth and frequency over time in a subset of containers.Important Findings Competing plants were half the size of non-competing plants,meaning that root competition was very strong.Root competition was size-asym-metric to some degree in all soil treatments.Neighbours larger than the target plant showed a greater per-unit-size effect on target growth than neighbours smaller than the target.Size variation increased over time for competing individuals,but decreased for non-competing pairs.Contrary to expectations,the presence of a high-nutrient patch reduced the strength and size asymmetry of competition temporarily.Size asym-metry in poor,deep soils may result from directionality in resource interception as roots compete for limited nutrients by growing deeper into soil layers that have not yet been exploited.Root competition can be size asymmetric,but not to the same degree as competition for light.

Intercropping effect on root growth and nitrogen uptake at different nitrogen levels

Aims Intercropping legumes and non-legumes may affect the root growth of both components in the mixture,and the non-legume is known to be strongly favored by increasing nitrogen(N)supply.the knowledge of how root systems affect the growth of the individual species is useful for understanding the interactions in intercrops as well as for planning cover cropping strategies.the aim of this work was(i)to determine if different levels of N in the topsoil influence root depth(rD)and intensity of barley and vetch as sole crops or as an inter-cropped mixture and(ii)to test if the choice of a mixture or the N availability in the topsoil will influence the N uptake by deep roots.Methods In this study,we combined rhizotron studies with root extraction and species identification by microscopy with studies of growth,N uptake and 15N uptake from deeper soil layers,for studying the root interactions of root growth and N foraging for barley(Hordeum vul-gare l.)and vetch(Vicia sativa l.),frequently grown in mixtures as cover crops.N was added at 0(N0),50(N1)and 150(N2)kg N ha−1.the roots discrimination relying on the anatomical and morphologi-cal differences observed between dicots and monocots proved to be a reliable method providing valuable data for the analysis.Important Findingsthe intercrop and the barley attained slightly higher root intensity(rI)and rD than the vetch,with values around 150 crosses m−1 and 1.4 m,respectively,compared to 50 crosses m−1 and 0.9 m for the vetch.at deep soil layers,intercropping showed slightly larger rI values compared to the sole-cropped barley.the barley and the intercropping had larger root length density(r D)values(200-600 m m−3)than the vetch(25-130)at 0.8-1.2 m depth.the topsoil N supply did not show a clear effect on the rI,rD or r D;however,increasing topsoil N favored the proliferation of vetch roots in the intercropping at deep soil layers,with the barley:vetch root ratio ranging from 25 at N0 to 5 at N2.the N uptake of the barley was enhanced in the intercropping at the expense of the vetch(from~100 mg plant−1 to 200).the intercropped barley roots took up more labeled nitrogen(0.6 mg 15N plant−1)than the sole-cropped barley roots(0.3 mg 15N plant−1)from deep layers.

Using coloured roots to study root interaction and competition in intercropped legumes and non-legumes

Aims Root interactions between neighbour plants represent a fundamental aspect of the competitive dynamics in pure stand and mixed cropping systems.The comprehension of such phenomena places big methodological challenges,and still needs clarification.The objectives of this work were(i)to test if a species with coloured roots can be used to examine the interaction in a legume-non-legume intercropping system;(ii)to verify the importance of initial root growth on the successive root development ofmixture component plants;(iii)to test if the root interaction in the shallowlayers has consequences for deep root growth and(iv)to compare the effect of intraspecific and interspecific competition on root development and biomass growth.Methods A detailed study on root growth and interaction was carried out using rhizotron tubes where two legume species were grown in pure stands or were intercropped with red beet,a variety of Beta vulgaris L.with clear red roots.Within the rhizotrons,the three species were grown either without competitors,with two plants of the same species to measure intraspecific competition or with one legume and one red beet plant to study interspecific competition.The use of mixtures where one component has clearly coloured roots,together with several scalar measurements of root depth and proliferation,allowed the measurement of the root system of each species when grown in the mixtures.Important findings The use of rhizotron tubes coupled with species with coloured roots represented a valuable method to study the belowground interaction in mixed cropping systems.The initial root growth was a very important feature for the subsequent dominance of a species and it was not related to seed dimension.Initial root growth was also important because the root interactions in the shallower soil layers were found to influence the root growth in deeper soil.The root system of the red beet showed much faster and deeper growth than that of the legumes,and made red beet the dominant component in the mixtures while the legume root system was confined to the shallower soil layer.Intraspecific competition was well tolerated by the legumes,but it was limiting for the highly competitive red beet.The outcome of root interaction between neighbour plants was confirmed to be speciesspecific as it changed according to the intensity of the competitive effect/response of each species of the mixture:both legumes were slightly affected by the intraspecific and highly affected by interspecific competition while red beet was more affected by intraspecific competition but strongly dominant when intercropped with legumes.