Wheat (Triticum aestivum L.) production by intercropping with faba bean (Vicia faba L.) has increased in popularity but is often associated with severe wheat powdery mildew (Blumeria graminis (DC.) Speer). Very little is known about the effects of below-and aboveground interspecific interactions on wheat nitrogen (N) nutrition and occurrence of wheat powdery mildew. A greenhouse pot experiment examined four N application rates and three belowground partition types (plastic film, nylon mesh partition or no partition) to study N nutrition and interactions between wheat and faba bean growing together. A field experiment investigated three N application rates and growth of wheat in monoculture and intercropped with faba bean with or without belowground plastic film partitions between wheat and faba bean. Disease incidence (DI) and disease severity index (DSI) were assessed at flowering stage and wheat leaves were sampled and analyzed for N. Foliar N was enhanced substantially by N addition in greenhouse and field conditions and also by belowground interactions (no partition compared with plastic film partition) in the pot experiment (all P < 0.001). There was a significant synergistic effect between N rate and belowground interactions on the enhancement of wheat N uptake (P < 0.01) in the pot experiment. DI and DSI of mildew increased markedly with increasing N rate in both experiments (all P < 0.001). In the pot experiment DI and DSI showed no marked differences among belowground partitions (both P > 0.05) but belowground interactions had soil but promoting disease with 0.05 g added N kg -1 soil. In the field experiment DI and DSI showed no significant differences between wheat monoculture and intercropping (both P > 0.05). However, the contributions of below-and aboveground interactions to disease control were different under different N rates, with interspecific root interactions increasing DI and DSI under different N rates and aboveground interactions increasing DI and DSI under zero-N application but decreasing DI and DSI at 150 and 300 kg N ha -1 . The data suggest that the microclimate in the field and biological control mechanisms due to belowground interactions in wheat-faba bean associations may influence the incidence and severity of wheat powdery mildew.

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Leucaena leucocephala was introduced into Panxi, Sichuan, China, in the 1980s and 1990s for afforestation and preventing water loss and soil erosion in this area. The co-introduction of rhizobial symbionts of introduced plants has drawn attention since they may influence local soil communities. We studied the phylogenetic position of the L. leucocephala isolates and assessed if the rhizobia were introduced together with the host to Panxi, Sichuan, China. The glnII and atpD genes of fifteen representative isolates were sequenced and analyzed, and applied multilocus sequence analyses in which the housekeeping genes recA, glnII and atpD were included. Furthermore, we estimated the within species diversity directly with 23S rDNA and IGS RFLP and indirectly through phenotypic analysis of forty L. leucocephala isolates. The isolates represented seven species and 38 diversified strains in the genera Ensifer, Mesorhizobium, Bradyrhizobium and Rhizobium. The within species diversity of the Ensifer isolates was large, proposing a potential to occupy novel niches. There was not conclusive evidence to show that any of the strains would have been co-introduced with L. leucocephala. On the contrary, we came to a conclusion that the possible introduction should not be inferred from sequence data alone.

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Wheat (Triticum aestivum L.)/maize (Zea mays L.)/soybean (Glycine max L.) relay strip intercropping (W/M/S) system is commonly used by the smallholders in the Southwest of China. However, little known is how to manage phosphorus (P) to enhance P use efficiency of the W/M/S system and to mitigate P leaching that is a major source of pollution. Field experiments were carried out in 2011, 2012, and 2013 to test the impact of five P application rates on yield and P use efficiency of the W/M/S system. The study measured grain yield, shoot P uptake, apparent P recovery efficiency (PRE) and soil P content. A linear-plateau model was used to determine the critical P rate that maximizes gains in the indexes of system productivity. The results show that increase in P application rates aggrandized shoot P uptake and crops yields at threshold rates of 70 and 71.5 kg P ha-1 respectively. With P application rates increasing, the W/M/S system decreased the PRE from 35.9% to 12.3% averaged over the three years. A rational P application rate, 72 kg P ha-1, or an appropriate soil Olsen-P level, 19.1 mg kg-1, drives the W/M/S system to maximize total grain yield while minimizing P surplus, as a result of the PRE up to 28.0%. We conclude that rational P application is an important approach for relay intercropping to produce high yield while mitigating P pollution and the rational P application-based integrated P fertilizer management is vital for sustainable intensification of agriculture in the Southwest of China.

IntroductionA better understanding of the regulatory role of microorganisms on soil phosphorous (P) mobilization is critical for developing sustainable fertilization practices and reducing P resource scarcity. The phoD genes regulate soil organic P (Po) mobilization.MethodsBased on the long-term P application experiments in acid purple soil of maize system in Southwest China (started in 2010), the experiment included five P levels: 0, 16, 33, 49, and 65.5 kg P hm–2 (P0, P16, P33, P49, and P65.5, respectively). The molecular speciation of organic P in soil was determined by 31P-nuclear magnetic resonance (NMR), high-throughput sequencing technology, and real-time qPCR were used to analyze the bacterial community and abundance of phoD-harboring bacterial genes, exploring the bacterial community and abundance characteristics of phoD gene and its relationship with the forms of Po and alkaline phosphatase (ALP) activity in the soil.ResultsThe results showed that the orthophosphate monoesters (OM) were the main Po speciation and varied by P fertilization in acid purple soil. ALP activity decreased as P fertilization increased. Co-occurrence network analysis identified the overall network under five P fertilizations. The keystone taxon base on the network showed that Collimonas, Roseateles, Mesorhizobium, and Cellulomonas positively correlated with both OM and Po. The random forest showed that Cellulomonas, Roseateles, and Rhodoplanes were the key predictors for ALP activity. The keystone taxon was a more important predictor than the dominant taxon for ALP, OM, and Po. The structural equation model (SEM) showed that soil organic matter (SOM), available P (AP), and OM were the main factors influencing the ALP by reshaping phoD-harboring bacteria alpha diversity, community composition, and phoD abundance.DiscussionThe phoD-harboring bacterial community composition especially the keystone taxon rather than alpha diversity and abundance dominated the ALP activity, which could promote P utilization over an intensive agroecosystem. These findings improve the understanding of how long-term gradient fertilization influences the community composition and function of P-solubilizing microorganisms in acid purple soil.

BACKGROUND: Effective nitrogen (N) management measures are required to control environmental problems caused by N fertilizer use in intensive maize production systems. Soil N losses associated with high precipitation and over-fertilization in maize production can cause substantial environmental problems, whereas there is a lack of quantitative data and effective study countermeasures. A 2-year field study was conducted in the subtropical maize production system in Southwest China to quantify N leaching under varying N application rates of 0, 90, 180, 270 and 360 kg N ha −1 yr −1 . RESULTS: The results indicated that N leaching accounted for 16-38% of N fertilizer input. For farmer practice treatment (360 kg N ha −1 yr −1 ), N leaching loss was high at 110 kg N ha −1 yr −1 and accounted for 31% of the N applied. As an indicator of the ambient water quality pollution, the grey water footprint across all treatments ranged from 376 to 1092 m 3 Mg −1 , with an average of 695 m 3 Mg −1 . Reducing N rate to agronomically optimized treatment (180 kg N ha −1 yr −1 ) significantly decreased N leaching by 77%, and maintained high grain yield of 8.1 Mg ha −1 . The grey water footprint was reduced by 52-63% with N rates from 270 or 360 kg N ha −1 yr −1 to 180 kg N ha −1 yr −1 . CONCLUSION: Nitrogen surplus (applied N rate minus N uptake by maize) resulted in higher soil residual nitrate concentration and consequently high N leaching. High precipitation and low soil pH were the main ecological factors leading to high N leaching.

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