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It is well established that mycorrhizal symbiosis can affect plant-plant interactions. On the other hand, how intraspecific plant interactions influence communities of arbuscular mycorrhizal fungi (AMF) is still very poorly understood. We applied 454 pyrosequencing to determine the responses of AMF communities to intraspecific shoot and root interactions using Medicago sativa L. as host plant. We found that intraspecific plant interaction caused significant variation in the structure of the AMF community assessed from roots but not from soil. Intraspecific root system interaction resulted in significantly elevated AMF species richness, greater total mycorrhizal and arbuscular colonization, and a larger root:shoot ratio compared to plants experiencing intraspecific shoot interaction or no interaction. Intraspecific shoot interaction resulted in significantly depressed AMF species richness. Our results support two hypotheses. The first is that increased AMF species richness in plants experiencing intraspecific root interaction originates from competitive release provided by high root density. The second is that shoot interaction leads to reduced allocation to root systems, increasing the stringency with which host plants selectively reward the most cooperative fungi. Because intraspecific interactions among host plants can influence the structure of their AMF community, interactions among species in complex plant communities may influence AMF community structure in addition to a plant diversity effect.

Intensive attentions have been paid to the positive effects on nitrous oxide (N

Our previous studies indicated that ditch-buried straw return (DB-SR) can improve soil processes in the short term, i.e. increasing microbial metabolic capability, reducing nitrogen leaching loss and promoting soil aggregation. However, it remains unclear how long-term implementation of DB-SR affects soil carbon (C) and nitrogen (N) processes and crop yields. Here, the effects of DB-SR on soil C pool and N availability as well as grain yields were investigated after consecutive application of 6 (rice season) and 6.5 years (wheat season). We found that long-term DB-SR significantly increased rice yields, total organic C, NH4+ and NO3– in the rice soils, as well as enhanced wheat yields, microbial biomass C, microbial biomass N, microbial biomass C/total organic C ratio and microbial biomass C/N ratio, but reduced NH4+ and NO3– in the wheat soils when compared with rotary tillage straw return (RT-SR) and no tillage with straw removal (NT-NS). These findings suggest that long-term DB-SR application has positive effects on grain production, but possibly through different mechanisms in improving soil processes. The yield-increasing effects on rice might result from improvements in soil fertility, whereas increased wheat yields can be ascribed to stimulated soil microbial activity.

Ditch‐buried straw return (DB‐SR) is a novel soil tillage and fertility building practice that is effective in regulating soil carbon and nitrogen dynamics and hydrothermal processes in rice–wheat rotation systems. However, the effects of DB‐SR on soil bacterial community are still largely unclear. We deciphered soil bacterial community with high‐throughput sequencing under various returning approaches, burial depths, and straw amounts after 6.5 years of DB‐SR application. Our results showed that DB‐SR structured distinctive soil bacterial community with rotary tillage straw return (RT‐SR; one‐way analysis of similarities [ANOSIM]: P .05). These variations were mainly caused by water content‐driven changes in soil organic carbon. Also, bacterial community composition was distinctive among burial depth treatment (one‐way ANOSIM: P .05), but bacterial community composition was more dissimilar with increasing straw amount (one‐way ANOSIM: P < .01). Our results suggest that long‐term DB‐SR can maintain the bacterial community structure in the surface soil layers when compared with conventional RT‐SR, but taking the current production level into consideration, the burial depth should not be greater than 20 cm for incorporating the full amounts of straws.

Improving nitrogen (N) use efficiency (NUE) to reduce the application of N fertilisers in a way that benefits the environment and reduces farmers' costs is an ongoing objective for sustainable wheat production. However, whether and how arbuscular mycorrhizal fungi (AMF) affect NUE in wheat is still not well explored. Three independent but complementary experiments were conducted to decipher the contribution of roots and AMF to the N uptake and utilisation efficiency in wheat. We show a temporal complementarity pattern between roots and AMF in shaping NUE of wheat. Pre-anthesis N uptake efficiency mainly depends on root functional traits, but the efficiency to utilise the N taken up during pre-anthesis for producing grains (EsubN,g/sub) is strongly affected by AMF, which might increase the uptake of phosphorus and thereby improve photosynthetic carbon assimilation. Root association with AMF reduced the N remobilisation efficiency in varieties with high EsubN,g/sub; whilst the overall grain N concentration increased, due to a large improvement in post-anthesis N uptake supported by AMF and/or other microbes. The findings provide evidence for the importance of managing AMF in agroecosystems, and an opportunity to tackle the contradiction between maximising grain yield and protein concentration in wheat breeding.

Wetlands are the largest natural source of terrestrial CH

The importance of assembly processes in shaping biological communities is poorly understood, especially for microbes. Here, we report on the forces that structure soil bacterial communities along a 2000 m elevational gradient. We characterized the relative importance of habitat filtering and competition on phylogenetic structure and turnover in bacterial communities. Bacterial communities exhibited a phylogenetically clustered pattern and were more clustered with increasing elevation. Biotic factors (i.e., relative abundance of dominant bacterial lineages) appeared to be most important to the degree of clustering, evidencing the role of the competitive ability of entire clades in shaping the communities. Phylogenetic turnover showed the greatest correlation to elevation. After controlling the elevation, biotic factors showed greater correlation to phylogenetic turnover than all the habitat variables (i.e., climate, soil and vegetation). Structural equation modelling also identified that elevation and soil organic matter exerted indirect effects on phylogenetic diversity and turnover by determining the dominance of microbial competitors. Our results suggest that competition among bacterial taxa induced by soil carbon contributes to the phylogenetic pattern across elevational gradient in the Tibetan Plateau. This highlights the importance of considering not only abiotic filtering but also biotic interactions in soil bacterial communities across stressful elevational gradients., This work was funded by Special Fund for Forest Scientific Research in the Public Welfare of China (No. 201504401) and the National Natural Science Foundation of China (Nos. 30870231 and 31300105). MG thanks the Spanish Ministry of Economy and Competitiveness (MINECO) for funding through the Ramón y Cajal programme. We thank the editor and referees for insightful comments that helped the improvement of this article.

Arbuscular mycorrhizal fungi (AMF) can form symbiotic relationships with roots of plants and potentially affect nitrogen (N) cycling in cropping systems. However, it is unclear how AMF influence N2O emissions and crop N assimilation, two key N cycling steps which are closely related to climate change and food production, under different soil management regimes. A microcosm experiment was conducted to test the effects of AMF on N2O emission and plant N assimilation, with wheat as a model crop, under organic (straw) amendment in soils with different textures. We found that AMF suppressed N2O emissions, promoted mycorrhizal N uptake and remobilization to grains and increase grain biomass under organic amendment in the sandy soil. However, AMF did not affect N2O emission, although increased plant N uptake, but inhibited mycorrhizal N remobilization to grains under organic amendment in the clay soil. These findings highlight that AMF have different effects on N2O emission and N translocation between sandy and clay soils. Appropriate soil management for mycorrhizal symbiosis in the cropping system can combat climate change and ensure food production.

Increasing field experiments have been conducted in forests to better understand the response of plant growth and photosynthesis to climatic warming. However, it is still unknown whether there is a general pattern in relation to how and to what extent warming impacts woody plants in forests. In this study, a meta-analysis was conducted to investigate the warming effects. When temperatures increased between 0.3 and 10 °C, specific leaf area (SLA) was significantly increased by 5.9%, plant height by 7.8%, biomass by 21.9%, foliar calcium (Ca) and manganese (Mn) concentrations by 20.7% and 39.6% and net photosynthetic rate (Pn) by 9.9%. Enhanced growth and Pn may have a relationship with changing SLA, efficiency of PSII (photosystem II), photosynthetic pigment concentrations and foliar nutrients. The results will be useful to understand the underlying mechanisms of forests responding to global warming.

The structure and dynamics of microbial communities may be influenced by environmental selection, spatial processes and competition. However, the relative strengths of these processes need to be further elucidated. Here, with a large scale sampling along an elevational gradient of 2000 m in the Qinghai-Tibetan Plateau, we examined the interplay among the three processes, as well as their relative importance to the biogeography of bacterial communities. Variation partitioning showed that environmental selection, competition and spatial variables jointly explained 37% of the total variation among the bacterial communities. The non-spatially structured environmental variables explained more variation than spatial distance or competition. Two environmental factors, elevation and soil pH, were shown to be the main cues determining soil bacterial communities. Environmental variables and spatial distance also exerted indirect effects on community variation via competition. The main conclusion of this study is consistent with the idea that “the environment selects”. Competition and spatial processes were less important in shaping bacterial communities of Qinghai-Tibetan Plateau.

Waterlogging stress and rice residue management are two major problems for wheat production in rice-wheat rotation systems. Ditch-buried rice straw return (DB-SR) is designed to incorporate all rice straw residues and has exhibited positive effects on soil carbon sequestration and nitrogen retention. However, little is known about its effects on waterlogging and wheat yield after long-term application. We therefore determined whether DB-SR could reduce waterlogging stress and increase wheat yields through a 6.5-year field experiment. We found that the straw ditches served as hidden drainage channels and could significantly reduce soil moisture content and water potential after rainfall in the wheat fields. However, the drainage effect was dependent on DB-SR burial depths. DB-SR promoted more waterlogging reduction at the depth of 40 cm than 20 cm. Wheat yields began to increase only after DB-SR application for four years. Our results suggest that DB-SR has the potential to solve the problems of waterlogging stress and incorporation of total rice straws, as well as simultaneously maintain or increase wheat grain yield in the rice-wheat rotation system.

Ditch-buried straw return (DB-SR) is a novel soil fertility building method by combing straw incorporation and tillage rotation in rice–wheat annual double cropping system. Several benefits of DB-SR have been shown, e.g. improving carbon sequestration, nitrogen retention and soil aggregation. However, how long-term DB-SR affecting the metabolic activity and diversity of soil microbial communities is still unclear. In this study, we investigated the effects of DB-SR application after 5 years (rice season) and 5.5 years (wheat season) on functional patterns, including the overall functional activity, reflected by fluorescein diacetate hydrolase (FDA) activity, the overall growth activity, reflected by soil respiration rate, extracellular element-cycling enzyme activity, glomalin content and metabolic diversity in different soil layers. Our results first showed that DB-SR significantly increased the overall functional activity (FDA hydrolase) and growth activity (respiration rate) in 10–20 cm and 20–30 cm soils. Second, β-glucosidase, lipase, acid phosphatase and arylsulphatase activities were significantly increased by DB-SR in different soil layers. However, urease activity was only increased by DB-SR in wheat soils, but reduced in the 0–10 cm and 20–30 cm soils after growing rice. In addition, DB-SR significantly increased the content of total glomalin and easily extracted glomalin. Finally, DB-SR significantly increased microbial metabolic diversity of carbon substrates in different soil layers. Overall, the metabolic activity and diversity of microbial communities were shown with a larger increase in 10–20 cm (above the straw layer) than 0–10 cm and 20–30 cm soil layers; and tended to increase with straw burial duration. These findings suggest that the long-term application of DB-SR is helpful to drive material transformation and nutrient supply, which can further benefit crop production in rice–wheat rotation systems.

Uncertainties remain as to the potential for tree plantations to affect soil microbial biomass. Our aim was to determine the factors accountable for the maintenance and the increase of soil microbial biomass following tree plantation. Based on mixed effect models, we conducted a meta-analysis with three fixed and two random factors to test the impact of tree plantation on soil microbial biomass. Previous land use was more important than climate or plant species in its effect on soil microbial biomass after tree plantation. There was a positive impact on soil microbial biomass for tree plantations on bare land but a negative impact for which on previously forested land. Climate and plant species were found to be not as important in their effects on soil microbial biomass. Our meta-analysis gives a general pattern that previous land use type is the major controlling factor of soil microbial biomass following tree plantations and promotes our understanding of the effects of rehabilitation of degraded sites on vegetation recruitment.

As a novel soil tillage practice, ditch-buried straw return (DB-SR) has exhibited positive effects on soil carbon sequestration, nitrogen retention and rice yield in previous studies. However, little is known about how long-term DB-SR affects soil hydrothermal and microbial processes. Our objective is to test whether DB-SR will alter the soil water potential, temperature and microbial community in a wheat field following rice cultivation. In this study, we found significant alterations in soil water potential, temperature, and microbial communities driven by DB-SR. On average, soil water potential was significantly reduced by 37.33% and 17.56% under DB-SR to a depth of 20 cm (DB-SR-20) and 40 cm (DB-SR-40), respectively. DB-SR-20 increased soil mean daily temperature and daily range of temperature more than DB-SR-40, possibly caused by decreased water content, especially at soil depths of 10 and 15 cm. Both DB-SR-20 and DB-SR-40 led to distinct shifts in soil bacterial and fungal community composition. DB-SR-20 significantly increased the activities of peroxidase, cellobiohydrolase, urease, and acid phosphatase by 3.5%, 75.0%, 81.4% and 41.7%, respectively, but had no effects on β- d -glucosidase activity. DB-SR-40, in contrast, significantly increased the activities of peroxidase and cellobiohydrolase by 2.4% and 36.0%, respectively, but showed no effects on urease and acid phosphatase. It did, however, reduce β- d -glucosidase activity by 15.0%. Overall functional diversity was increased by 29.9% under DB-SR-20 but was not affected by DB-SR-40. Our results suggest that these improvements in soil ecological processes driven by DB-SR will promote wheat yield in a rice-wheat rotation system.

Summary Arbuscular mycorrhizal fungi (AMF) are key regulators of ecosystem processes, yet how their biodiversity works in ecosystems remains poorly understood. We documented the extent to which taxonomic resolution influenced the effect of biodiversity of AMF taxa on plant performance (growth, nutrient uptake and stress tolerance) in a meta-analysis of 902 articles. We found that the effect of biodiversity of AMF taxa depended on taxonomic resolution. Plant performance was positively promoted by AMF family richness, while no effect was found for fungal species richness. In addition, negative effect was found between AMF phylogenetic diversity and plant growth. This pattern can be explained by functional conservatism within AMF families and functional differentiation among AMF families. Synthesis. Conservation of AMF communities to maintain a full complement of ecosystem functions requires the presence of diverse families and not simply diverse species within a family. This finding may be of key importance for the function of ecosystems under various environmental perturbations to which AMF families may respond differently.

The arbuscular mycorrhiza (AM) is among the most ubiquitous symbiosis in the world. A meta-analysis of 759 articles (1978–2012) was conducted to test whether ecologically important host plant traits (N-fixation and C-fixation pathway) affect the response of the plant to mycorrhizal colonization. We found that the effect of N-fixation on mycorrhizal growth response (MGR) depended on whether the plant was woody or a forb. N-fixing forbs had a higher MGR than non-N-fixing forbs, but the reverse was true for woody plants. Moreover, C 4 -grasses had significantly higher MGR than C 3 -grasses, but no significant difference was found between C 3 and C 4 forbs, or between C 3 and C 4 woody species. Overall, woody species had higher MGR than any other functional group. These results demonstrate that MGR does depend on host functional characteristics, but neither N-fixation capacity nor C-fixation pathway are apparently fundamental controllers of MGR. Instead, it would appear possible that these traits influence MGR only insofar as they influence more fundamental functions such as P demand and P supply.

As a novel soil tillage practice to build fertility, ditch-buried straw return (DBSR) can affect soil carbon sequestration, nitrogen availability and hydrothermal processes in rice-wheat rotation systems. However, it is still not well-known how DBSR influences soil fungal community. A 6.5-year's field experiment was conducted to test the effects of DBSR on soil fungal community via comparing with rotary tillage straw return (RTSR) and no tillage with no straw (NTNS). Our results showed that soil fungal species composition under DBSR was significantly different from NTNS and RTSR. Also, DBSR showed distinctive impacts on soil fungal composition at the order level. DBSR significantly increased Shannon diversity index and evenness, decreased dominance, but did not affect species richness. Moreover, DBSR significantly increased the abundance of symbiotrophic fungi, but reduced the abundance of pathotrophic fungi. Especially, DBSR largely increased the abundance ratio of arbuscular mycorrhizal fungi to plant fungal pathogens. DBSR significantly reduced β-D-glucosidase activity but increased peroxidase activity while RTSR increased both cellobiohydrolase and peroxidase activity. Our results suggest that long term ditch-buried straw return can structure a unique soil fungal community from that of RTSR and NTNS, increase soil fungal diversity and promote the dominance of beneficial fungi over soil-borne plant fungal pathogens. These findings can provide solid evidence for future large-scale extension of ditch-buried straw return in practical agriculture.

A large part of crop nutrient uptake occurs through the interaction of roots with symbiotic arbuscular mycorrhizal fungi (AMF). However, it is still an open question how straw management and earthworm activity affect AMF community structure and their nitrogen-transferring function in wheat. A split-plot field experiment was conducted to address this question. Three straw management regimes including different tillage treatments (no-till with no straw, NTNS; rotary tillage with straw return, RTSR and ditch-buried straw return, DBSR), and two earthworm treatments (no earthworm, −E; and earthworm addition, +E) were conducted. The AMF community structure in the wheat roots was characterized with high-throughput sequencing, and its function in terms of N acquisition was measured with 15N isotope tracing through hyphal in-growth cores. Our results showed that both the DBSR and RTSR treatments significantly changed AMF community composition and enhanced the mycorrhiza-mediated plant N uptake when compared to NTNS. The effect of earthworm activity on AMF community composition and mycorrhiza-mediated N uptake strongly depended on the straw management regimes. While earthworm presence increased AMF dominance (+32.9%) and mycorrhizal N uptake (+2.05-fold) under straw removal, they decreased AMF dominance (−30.4% and −41.9% respectively) and mycorrhizal N uptake (−37.3% and −34.3% respectively) under both DBSR and RTSR treatments in comparison with the absence of earthworms. It is concluded that straw addition shifts the AMF community structure and increases N uptake by the host plants; and that the effect of earthworms on AMF community structure and functioning depends on the straw management regime. The results suggest that straw management and its interaction with earthworms can affect mycorrhiza-mediated plant N uptake, possibly through altering some dominant AMF taxa.

Ditch-buried straw return (DB-SR) is a novel soil tillage and fertility-building practice, which may allow for the formation of a straw layer structure in rice–wheat rotation systems. However, it is presently unknown how the straw layer affects soil physicochemical and microbial processes in the interface between straw layer and bulk soils. Thus, we conducted a field experiment from 2008 to 2014 to determine whether there were changes in soil structure, temperature, available nitrogen, and microbial properties in the soil layer close to the straw layers (± 5 cm). Three treatments were included: control with no-till and straw removal (CK), ditch-buried straw return to a depth of 20 cm (DB-SR-20), and ditch-buried straw return to a depth of 40 cm (DB-SR-40). Our results showed that soil macroaggregate formation was increased with the duration of straw return in the soils 5 cm below the straw layers under DB-SR-20 and above the straw layers under DB-SR-40. The straw layers could significantly increase soil temperature below the straw layers, but a larger increase was observed for DB-SR-40 than DB-SR-20. We also found that DB-SR-20 largely increased the NH4+ concentration in both the soil layers above and below the straw layers, but DB-SR-40 only increased NO3− in the soils above the straw layers. In addition, the catabolic profile of soil microbial communities was largely affected by the straw layers, but the effects decreased with the duration of the treatment. These findings suggest that the straw layer largely alters the spatio-temporal processes of physicochemical and microbial properties in the interface soils after long-term implementation of ditch-buried straw return. Some of the positive alterations may cascade to facilitate crop growth and enhance the productivity of rice-wheat rotation systems.

Rice-wheat rotation is the main farming system in the Yangtze River regions, and plays a critical role in maintaining food safety in China. However, some serious issues are emerging during agricultural production. One is that long-term conservation tillage leads to shallow and stringent ploughing layers. The other is that large quantity of straws is difficult to effectively manage in the limited rotation gaps between both crops. Our group designed a novel soil tillage practice, ditch-buried straw return (DB-SR), to overcome such problems. In this chapter, we summarized our current advances from 10-year's research for DB-SR in the rice-wheat rotation systems. DB-SR showed excellent straw incorporation capability, improvements in soil ecological processes as well as positive effects on pest control, crop growth and final yield performance. Our results suggested that DB-DR is effective in sustaining crop production and has a great potential for extension in the rice-wheat rotation farming regions. However, we also need more agricultural engineering scientists to join us for developing an all-in-one machine which supports this novel soil tillage practice.

Our aim was to investigate the effects of short-term waterlogging in shaping communities of the obligately aerobic AMF in roots of Populus deltoides. AMF community in populus roots was deciphered with 454-pyrosequencing method from a field plot before and after 15 days waterlogging. We found that short-term waterlogging significantly altered the AMF community as confirmed with NMDS. Indicator species analysis identified 12 AMF OTUs that shown with different relative abundance between before and after waterlogging. In contrast to expectation, short-term waterlogging increased detectable AMF species richness: 4 OTUs were detected only before and 17 only after waterlogging. Variation in abundance of dominant OTUs was possibly due to short-term waterlogging disturbance and the role of competitive release in the increase in species richness following waterlogging.

Ditch-buried straw return (DBSR) is a novel farming system that not only efficiently eliminates the need to burn straw, but also shows positive effects on soil carbon sequestration and crop yields. Implementation of DBSR, however, may penetrate the tillage pan, increasing the risk of N leaching losses. We therefore determined whether N retention could be increased by DBSR in order to reduce the risk of N loss to the environment. A four-year field experiment and a complementary greenhouse experiment were conducted to test the effects of DBSR on N retention in a rice-wheat rotation system. We found that DBSR altered the spatial distribution of fertilizer N. N content was significantly increased above but reduced below the straw layer in the field experiment. The greenhouse experiment further confirmed the N retention effects by the straw layer. In theory, a maximum of 9.09 mg urea-N could be adsorbed by one gram dry wheat straw. Our results suggest that DBSR has the potential to increase N retention in the soil, thus increasing crop uptake and minimizing leaching N loss in the rice-wheat rotation system.

In microbial ecology, the “everything is everywhere” hypothesis has long been controversial. In the present study, we performed data-mining for 18S rDNA sequences of glomeromycotan fungi in order to test this hypothesis. 18S rDNA sequences targeted using AM1–NS31 fragments were retrieved from GenBank, with a total of 1768 sequences collected from 34 sites worldwide. In total, 229, 330 and 518 operational taxonomic units (OTUs) were defined based on 97, 98 and 99 % similarity, respectively. The 97 % OTUs showed a limited geographical range of glomeromycotan fungi. Among the OTUs, 58.1 % were endemic, and 17.9 % and 9.2 % were found in two and three sites, respectively. The most widespread OTU was shared by 17 sites. Phylogenetic structure analysis demonstrated that most local communities (26 of 34) were clustered. OTUs with larger host breadth had wider geographic ranges. A significant distance–decay relationship was revealed that was independent of habitat. Cluster analysis showed that fungal composition was not related to habitat, while Fast UniFrac analysis indicated that the distribution of Glomeromycota was affected by temperature. Taken together, these results suggest that glomeromycotan fungi were not randomly distributed under natural conditions; rather, they were affected by host plants, dispersal ability and temperature. Thus, the distribution of glomeromycotan fungi argues against the hypothesis that “everything is everywhere.”

BACKGROUND Crop residue management and nitrogen loss are two important environmental problems in the rice–wheat rotation system in China. This study investigated the effects of burial of straw on water percolation, nitrogen loss by leaching, crop growth and yield. Greenhouse mesocosm experiments were conducted over the course of three simulated cropping seasons in a rice1–wheat–rice2 rotation. RESULTS Greater amounts of straw resulted in more water percolation, irrespective of crop season. Burial at 20 and 35 cm significantly reduced, but burial at 50 cm increased nitrogen leaching. Straw at 500 kg ha−1 reduced, but at 1000 kg ha−1 and at 1500 kg ha−1 straw increased nitrogen leaching in three consecutive crop rotations. In addition, straw at 500 kg ha−1 buried at 35 cm significantly increased yield and its components for both crops. CONCLUSIONS This study suggests that N losses via leaching from the rice–wheat rotation may be reduced by the burial of the appropriate amount of straw at the appropriate depth. Greater amounts of buried straw, however, may promote nitrogen leaching and negatively affect crop growth and yields. Complementary field experiments must be performed to make specific agronomic recommendations. © 2015 Society of Chemical Industry

A novel straw return method (ditch-buried straw return, DB-SR) with a four-year field experiment was implemented to investigate the carbon sequestration effect under various burial depths and with different amounts in a rice–wheat rotation system. The DB-SR significantly increased the soil total organic carbon (TOC) and labile organic carbon (LOC) contents. In the two soil layers 10 cm above and below the straw layer, the TOC and LOC levels increased. The DB-SR treatment using the total amount of straw showed higher TOC, LOC and carbon pool management index (CPMI) values than when half and twice the amount of straw were used. The rice yield was significantly higher when the total amount was used with the DB-SR to depth of 20 cm than at other soil depth treatments. However, the DB-SR with twice the amount showed a negative, but not significant, effect on rice yields. These results suggest that DB-SR might be a better straw return method to increase soil organic carbon (SOC) and improve soil quality, particularly when returning the total amount of straw to a soil depth of 20 cm.

Nitrogen (N) and phosphorus (P) losses are a potential limitation for the direct application of biogas slurry as a substitute for chemical fertilizer in irrigated rice production systems. The hypothesis was tested that a rice-duck co-culture promotes the rice N and P use efficiencies, reducing the losses of these nutrient elements through run-offs and enabling the use of biogas slurry as a substitute for chemical fertilizers. A field split-plot experiment was carried out to test the hypothesis. Our results showed that the direct application of biogas slurry was harmful for rice production. Compared with rice monoculture under chemical fertilization, biogas slurry application reduced N and P accumulation in grains, P use efficiency, and grain yield by 3.6%, 7.8%, 12.7%, and 14.8%, respectively, but increased the total N and P concentrations in the surface water 1.4- and 2.7-fold, respectively, on average on the eleventh day after fertilization. However, rice-duck co-culture compensated for the negative effects of biogas slurry on rice production. Under the biogas slurry application and in line with our hypothesis, the rice-duck co-culture significantly increased N and P accumulation and use efficiencies, as well as grain yield to levels similar to those acquired with chemical fertilization treatments. Meanwhile, total N and P concentrations were significantly lower for rice-duck co-culture than those of rice monoculture under biogas slurry application. Our results suggest that rice-duck co-culture can maintain rice yield and reduce the risks of N and P loss to local environments when utilizing biogas slurry as a substitute for chemical fertilizers.

The roots of most land plants form mycorrhizal associations with soil fungi, in which plants trade carbon for increased nutrient acquisition (e.g., N and P) under nutrient deficiency conditions. However, how nutrient enrichment affects mycorrhiza is still not well understood, in particular under future global changing scenarios such as nitrogen deposition. In this chapter, we first review the major pathways of mycorrhizal-mediated nutrient acquisition and molecular mechanisms of sensing nutrient availability for mycorrhizal fungi and roots. Next, we propose two conceptual models that may control plant C allocation to mycorrhizal fungi in response to nutrient enrichment: reciprocal reward model and root-mycorrhiza trade-off model. We also describe a plant-centric model and fungal-centric model to explain responses of the mycorrhizal fungal community to nutrient enrichment as well as examine impacts of nutrient inputs on mycorrhizas functioning.

The yield of direct-seeded rice has been shown to decrease after straw amendment. However, the reasons for the yield decrease, and any measures to alleviate it, are currently unknown. We hypothesised that straw return exerts negative effects on soil fertility and on root growth of direct-seeded rice, which subsequently reduces the remobilisation of reserves to grains under continuous flooding (CF); and that alternate wetting and drying (AWD) irrigation can alleviate these negative impacts. Field and greenhouse experiments were conducted to test the hypotheses, by comparing CF and AWD in combination with two wheat-straw treatments (incorporation and mulching). Under CF, wheat-straw incorporation decreased soil available phosphorus by 23–79%, root biomass by 10%, leaf biomass by 13%, and leaf area by 15% compared with the control with no straw incorporation; negative effects on these characteristics were lessened if the straw was mulched. The AWD treatment alleviated the negative effects of straw incorporation compared with CF, and straw mulching with AWD had no negative effects or resulted in positive effects. The results suggest that CF along with straw incorporation limits soil phosphorus availability, root growth and grain yield by affecting photosynthate accumulation and remobilisation. AWD irrigation mitigates these undesirable effects by decreasing soil total reductants, which subsequently increases soil pH and plant-available phosphorus. The proposed AWD treatment could be a promising strategy for the sustainable production of direct-seeded rice.

Our aim was to explore the way that root system type affects mycorrhizal growth response of plants. An extensive meta-analysis with 943 peer-review publications was conducted to test the difference in mycorrhizal responses between taproot plants and plants with a fibrous root system. We found that taproot plants showed greater growth response (biomass, P and N uptake) to colonization by arbuscular mycorrhizal fungi (AMF) than do plant species with fibrous root systems. This response pattern was dependent on stress types, AMF identity and species richness, and particularly the type of stress (abiotic vs. biotic). Taproot plants respond more to AMF than plants with a fibrous root system; but no difference was shown under biotic stress. The interaction effect seen for AMF and biotic stress was significantly higher for plants with fibrous root system, but was not significant between taproot plants and abiotic stress. Difference in biomass response was only found for Glomeraceae and Gigasporaceae between the two types of plants, while difference was found in P uptake response for Glomeraceae and Claroideoglomeraceae. However, plants with fibrous root system showed higher growth response than taproot plants under nematode stress. Taproot plants might be more dependent on mycorrhiza than plants with fibrous root system. This indicates that environmental conditions can modify the relative abundance of taproot plants and plants with fibrous root system through mycorrhizal functioning, which will regulate plant community dynamics and processes.

Arbuscular mycorrhizal fungi (AMF) are important components of soil microbial communities, and play important role in plant growth. However, the effects of AMF phylogenetic groups (Glomeraceae and non-Glomeraceae) on host plant under various heavy metal levels are not clear. Here we conducted a meta-analysis to compare symbiotic relationship between AMF phylogenetic groups (Glomeraceae and non-Glomeraceae) and host plant functional groups (herbs vs. trees, and non-legumes vs. legumes) at three heavy metal levels. In the meta-analysis, we calculate the effect size (ln(RR)) by taking the natural logarithm of the response ratio of inoculated to non-inoculated shoot biomass from each study. We found that the effect size of Glomeraceae increased, but the effect size of non-Glomeraceae decreased under high level of heavy metal compared to low level. According to the effect size, both Glomeraceae and non-Glomeraceae promoted host plant growth, but had different effects under various heavy metal levels. Glomeraceae provided more benefit to host plants than non-Glomeraceae did under heavy metal condition, while non-Glomeraceae provided more benefit to host plants than Glomeraceae did under no heavy metal. AMF phylogenetic groups also differed in promoting plant functional groups under various heavy metal levels. Interacting with Glomeraceae, herbs and legumes grew better than trees and non-legumes did under high heavy metal level, while trees and legumes grew better than herbs and non-legumes did under medium heavy metal level. Interacting with non-Glomeraceae, herbs and legumes grew better than trees and non-legumes did under no heavy metal. We suggested that the combination of legume with Glomeraceae could be a useful way in the remediation of heavy metal polluted environment.

It has been shown that plant biomass–density relationships are altered under extreme or stressed conditions. We do not know whether variation in biomass–density relationships is a direct result of stress tolerance or occurs via changes in plant–plant interactions. Here, we evaluated biomass–density relationships and neighbor effects in six plant species that differ in salt tolerance in a salt marsh, and conducted a literature review of biomass–density relationship under higher and lower stress levels. Our field study showed that both neighbor effects and the exponent of the biomass–density relationship (α) varied among plant species with different degrees of salt tolerance. There was a positive relationship between neighbor effects (measured as relative interaction index) and α-value among the tested species. The literature review showed that α and its variation increased under higher stress. Our results indicate that plant species with different salinity tolerance differ in the direction and strength of neighbor effects, resulting in variation in biomass–density relationships. Our results support the hypothesis that differences in biomass–density relationships among species are not due to differences in stress tolerance alone, they are mediated by changes in plant–plant interactions.

Aims We investigated how rhizosphere factors (total rhizosphere, roots, arbuscular mycorrhizal fungal hyphae [AMF], and soil solution) and water availability affect interactions between neighboring Medicago sativa plants.

Abiotic and biotic factors can alter the nature and strength of plant–plant interactions and therefore self-thinning (density-dependent mortality), but few studies have looked at how such factors interact. We investigated how salt stress and arbuscular mycorrhizal fungi (AMF) influence plant neighbour effects and self-thinning in experimental populations of Medicago sativa. We obtained two mycorrhizal levels by applying the fungicide benomyl (low AMF) or not (high AMF) at three salinity levels (0.05%, 0.2% and 0.5%). In experiment 1, we investigated how salinity and AMF interact to influence plant interaction intensity using a neighbour removal treatment. In experiment 2, we investigated how self-thinning dynamics vary under salinity conditions and different AMF levels at two initial plant densities (6000 and 17,500 seeds m−2). Shoot biomass and plant density were measured 30, 60 and 90 days after sowing. Standardized major axis regression was used to estimate self-thinning parameters. In experiment 1, AMF increased competitive plant neighbour effects when there was no salinity stress, but this enhancement was not significant with increasing salinity. In experiment 2, there were effects of salinity and AMF on the self-thinning trajectory. The slope of the log (mean shoot biomass per unit area) vs. log density relationship was significantly steeper for the high AMF treatment than for the low AMF treatment without salinity, but the effect of AMF level on the self-thinning exponent was not significant under the two higher salinity levels. The effect of AMF treatments on the intercept of the self-thinning line was not significant at 0.2% salinity but was significant at 0.5% salinity, higher elevation for high AMF treatment. In self-thinning populations, AMF decreased the survival rate without salinity, but increased the survival rate at the highest salinity level. Our results support the hypothesis that salinity and AMF interact to influence plant neighbour effects and self-thinning. Under no-salinity conditions, AMF increased competition, steepened the self-thinning line and decreased survival rate, but these effects of AMF were not significant in the presence of salinity.

Changes in soil organic carbon, total nitrogen, pH, and the abundance of arbuscular mycorrhizal fungi are examined along a large-scale aridity gradient from southeast to northwest in China. Soil organic carbon and total nitrogen decreased but pH increased with increased aridity. Aboveground plant biomass, spore abundance, and colonization of roots by arbuscular mycorrhizal fungi also declined as the aridity increased. Soil organic carbon and total nitrogen were positively correlated with aboveground plant biomass, and arbuscular mycorrhizal fungal spore number and root colonization were positively correlated with soil organic carbon, total nitrogen, and aboveground plant biomass but were negatively correlated with soil pH. A structural equation model suggested that aridity affected soil organic carbon and total nitrogen by limiting aboveground plant biomass. Aridity exerted a large direct effect and smaller indirect effects (via changes in aboveground plant biomass) on the abundance of arbuscular mycorrhizal fungi. Soil pH also directly influenced arbuscular mycorrhizal fungal abundance. These results suggest that aboveground plant biomass could be a key factor driving the changes of soil organic carbon, total nitrogen, and arbuscular mycorrhizal fungal abundance along this aridity gradient in China.

Crop residue management and nitrogen loss are two important environmental problems in the rice-wheat rotation system in China. This study investigated the effects of burial of straw on water percolation, nitrogen loss by leaching, crop growth and yield. Greenhouse mesocosm experiments were conducted over the course of three simulated cropping seasons in a rice1-wheat-rice2 rotation.Greater amounts of straw resulted in more water percolation, irrespective of crop season. Burial at 20 and 35 cm significantly reduced, but burial at 50 cm increased nitrogen leaching. Straw at 500 kg ha(-1) reduced, but at 1000 kg ha(-1) and at 1500 kg ha(-1) straw increased nitrogen leaching in three consecutive crop rotations. In addition, straw at 500 kg ha(-1) buried at 35 cm significantly increased yield and its components for both crops.This study suggests that N losses via leaching from the rice-wheat rotation may be reduced by the burial of the appropriate amount of straw at the appropriate depth. Greater amounts of buried straw, however, may promote nitrogen leaching and negatively affect crop growth and yields. Complementary field experiments must be performed to make specific agronomic recommendations.