Andrea Cavalieri

Total biomass production of plant monocultures growing over a range of densities and harvested after a period of growth increases monotonically with density and then levels out at higher densities. This pattern is called constant final yield (CFY) and is considered one of the most general phenomena in plant ecology. If CFY applies to plant communities, it would be a key to understanding and predicting many community‐level phenomena. We tested two primary hypotheses experimentally: (1) Mixtures of several species show CFY. (2) If so, the proportion of biomass production by the component species in a mixture does not change at densities above the density that reaches CFY. We performed a series of glasshouse experiments over 3 years using a “community density series,” in which the overall density of five species was varied while their proportions remained unchanged. In the first experiment, we grew a mixture of annual and perennial herbaceous species in mesocosms, and all species were also grown in monocultures at the corresponding densities. A similar experiment was performed in the second and third years, but only with annuals. A third experiment with mixtures only was performed in pots over 2 years. In all cases, aboveground biomass was harvested, separated by species, dried, and weighed. Perennials with underground storage organs produced maximum aboveground biomass at low or intermediate densities. In the second experiment, two of the species produced maximum biomass at the second‐highest density in monoculture, while mixtures of all five species showed classical CFY behavior, and the contribution of the species to the mixture changed very little above the lowest density producing CFY. The results of the third experiment were also consistent with the hypotheses. In conclusion, CFY in aboveground biomass production was observed in communities of annual species, and the contribution of the individual species was relatively insensitive to an increase in density above that reaching CFY, i.e., competitive performance of the species changed with density until CFY was reached. Evidence for CFY was stronger in mixture than in monoculture. Coexistence theory must include density as well as frequency dependence if densities are below CFY.

Rice‐fish co‐culture system has a history of more than 2,000 years in Asia and has been recognized as one of FAO's Globally Important Agricultural Heritage Systems (GIAHS). To date, few studies have explored rice‐fish co‐culture effects on the relationship between predator–prey aggregations and associated tri‐trophic cascades of predatory spiders, insect herbivores and rice. Indeed, predators have been shown to increase prey spatial aggregation but it is still not clear how fish (tertiary consumers) affect aggregations and interactions of their prey on multiple trophic levels (i.e. predatory spiders and rice insect herbivores). Here, we conducted a 2‐year experiment to measure the abundance and distribution of herbivores (leafrollers, stemborers and planthoppers) and their predators (spiders) along with rice yield in rice‐fish co‐culture versus monoculture‐rice (fish free) systems in eastern China. We analysed spatial aggregations of herbivores and predators using geostatistics and used correlations to examine tri‐trophic interactions. Results showed that co‐culture enhanced predatory spider and herbivore spatial aggregations, which in turn increased biocontrol of herbivores by predators, decreased herbivore damage and increased rice productivity. Synthesis and applications. Fish presence increased spatial aggregation of two different trophic levels, which enhanced a tri‐trophic cascade of predatory spiders, insect herbivores and rice in co‐culture systems. Our approach linking spatial aggregations of herbivores and predators with trophic cascades can be applied to study trophic interactions in other ecosystems. Incorporating fish into monoculture paddy fields can increase both natural pest control by predatory spiders and crop productivity via aggregation, and thus increase multiple ecosystem services.

The use of ground cover vegetation is becoming a prominent way of promoting biodiversity and associated ecosystem services in Chinese orchards. Despite the large number of studies that have addressed the effects of ground cover vegetation on promoting natural enemy populations and related pest control, it is still unclear whether enhanced natural pest control can increase yield and reduce the use of pesticide. We performed an experiment comparing three cover vegetation practices (ryegrass, clover and hairy vetch) versus a bare ground control in commercial pear orchards in the Yangtze River Delta of East China (YRDEC), China. Natural enemy density (predator and parasitoid abundance), invertebrate herbivore performance (piercing‐sucking herbivore abundance and branch‐boring and fruit‐boring percentage), pesticide input, and pear fruit yield were recorded. The results indicated that cover vegetation decreased herbivore abundance and boring percentage by 50% and 64%, respectively, thus decreasing pesticide use by 26%. We also found that cover vegetation increased the abundance of natural enemies by 621%, and increased pear fruit yield by 7%. Piecewise structural equation modelling confirmed that increased natural enemy densities, decreased herbivore performance by 102%, pesticide use by 51% and increased fruit yield by 10%. This study suggests that the use of ground cover vegetations, especially with clover and hairy vetch, is an effective way of promoting biocontrol services and associated ecosystem services in pear orchards.

Soil acidification induced by nitrogen (N) deposition has important effects on species diversity and ecosystem function. The potential role of phosphorous (P) limitation on these effects was overlooked and observed only after decades of atmospheric N deposition. Few studies have addressed the long-term effect of P addition on soil acidification resulting from N deposition. Changes in precipitation patterns could also have important effects on soil acidification in semi-arid regions. To investigate the effects of long-term N, P, and water addition on soil acidification, a nine-year experiment was conducted on the semi-arid steppe in north-eastern China, in which N, P and water were added in a factorial design. N addition decreased soil pH significantly, P addition decreased soil pH significantly under ambient precipitation, while there was no effect on pH when water was added. Under the ambient precipitation, N and P addition decreased soil acid buffering capacity (ABC) by 86% and 42%, respectively, and NP addition decreased ABC by 63%. Under water addition, N and NP addition decreased ABC by 74% and 58%, respectively, P addition increased ABC by 83%. Addition of water changed the way soil N and P affect soil ABC. Without additional water, there was a direct negative influence of inorganic N on soil ABC, whereas with additional water there was an indirect effect on soil pH. Without addition of water P had a positive effect on soil ABC via soil effective cation exchange capacity, whereas with addition of water, the effect of P occurred via its effects on soil organic matter. Our results suggest that long-term P addition could alleviate damage to the soil acid buffering system due to N deposition to some extent by increasing ABC. We conclude that water availability mediates the effects of N and P addition on soil ABC.

There have been very few studies on the effects of plant competition on the rhizosphere bacterial community. To investigate the impacts of intra- and interspecific plant competition, we analyzed the responses of rhizosphere bacterial communities to plant density as determined by 16S rRNA gene targeted sequencing. We included five weedy plant species growing in field soil in monocultures and mixed cultures at three densities in a greenhouse experiment. The rhizosphere bacterial community of each species changed more with density in a mixture of all five plant species than in monocultures, so intra- and interspecific plant competition had different effects on the bacterial community. For the dominant plant competitor, Centaurea cyanus, neither intra- nor interspecific competition had major effects on the composition of its rhizosphere bacterial communities. In contrast, the bacterial communities of the weakest competitor, Trifolium repens, were affected differently by intra- and interspecific competition. During increasing intraspecific competition T. repens maintained a highly specialized bacterial community dominated by Rhizobium; while during interspecific competition, the relative abundance of Rhizobium declined while other nitrogen fixing and potentially plant growth promoting taxa became more abundant. Contrary to previous observations made for soil microbial communities, the bacterial rhizosphere community of the weakest competitor did not become more similar to that of the dominant species. Thus, the process of competition, as well as the plant species themselves, determined the rhizosphere bacterial community. Our results emphasize the role of plant-plant interactions for rhizosphere bacterial communities. These effects may feedback to affect plant-plant interactions, and this is an important hypothesis for future research.

Distinctive strains of Pantoea are used as soil inoculants for their ability to promote plant growth. Pantoea agglomerans strain C1, previously isolated from the phyllosphere of lettuce, can produce indole-3-acetic acid (IAA), solubilize phosphate, and inhibit plant pathogens, such as Erwinia amylovora. In this paper, the complete genome sequence of strain C1 is reported. In addition, experimental evidence is provided on how the strain tolerates arsenate As (V) up to 100 mM, and on how secreted metabolites like IAA and siderophores act as biostimulants in tomato cuttings. The strain has a circular chromosome and two prophages for a total genome of 4,846,925-bp, with a DNA G+C content of 55.2%. Genes related to plant growth promotion and biocontrol activity, such as those associated with IAA and spermidine synthesis, solubilization of inorganic phosphate, acquisition of ferrous iron, and production of volatile organic compounds, siderophores and GABA, were found in the genome of strain C1. Genome analysis also provided better understanding of the mechanisms underlying strain resistance to multiple toxic heavy metals and transmission of these genes by horizontal gene transfer. Findings suggested that strain C1 exhibits high biotechnological potential as plant growth-promoting bacterium in heavy metal polluted soils.

Summary: Potato is very susceptible to weed interference during the early growth stages due to slow emergence, and again at the end of the growing cycle when branches collapse and the canopy opens. Weed control usually is performed through a combination of physical and chemical methods. A growing concern for the environment and human health has encouraged the development of non‐chemical weed control. We evaluated the effects of an integrated weed management strategy consisting of physical and cultural methods on naturally emerging weeds over two field seasons in central Italy. One harrowing plus one hilling operation were conducted during the early crop stages, and the competitive abilities against late emerging weeds of six different cultivars of potato, characterised by differences in developmental timing and growth habit, were evaluated. Two measures of competition were evaluated, the competitive balance index (Cb) and the relative total biomass of crop and weed. Higher competitive ability (Cb) was associated with traits such as fast early above‐ground biomass production, height and final above‐ground biomass. Medium late maturity cultivars showed higher Cb than earlier ones, but this was associated with lower yield, providing evidence for a trade‐off between competitive ability and yield. The trade‐off was in part biased by the lack of adaptation of the medium late cultivars to hot weather conditions, so we concluded that cultivars characterised by different developmental time need to be screened and tested for local systems.