David Johnson’s research while affiliated with Lancaster University and other places

Most plants extend their zone of interaction with surrounding soils and plants via mycorrhizal hyphae, which in some cases can form common mycorrhizal networks with hyphal continuity to other radial plants. These interactions can impact plant health and ecosystem function, yet the role of these radial plants in mycorrhizal interactions and subsequent plant performance remains underexplored. Here we investigated the influence of hyphal exploration and interaction with neighbouring mycorrhizal and non-mycorrhizal plants on the performance of Plantago lanceolata , a mycotrophic perennial herb common to many European grasslands, using mesh cores and the manipulation of neighbouring plant communities. Allowing growth of hyphae beyond the mesh core increased carbon capture above-ground and release below-ground as root exudates and resulted in the greater accumulation of elements relevant to plant health in P. lanceolata . However, contrary to expectations, the presence of mycorrhizal or non-mycorrhizal neighbours did not significantly alter the benefits of hyphal networks to P. lanceolata . Our findings demonstrate that enabling the development of a fungal network beyond the immediate host rhizosphere significantly influences plant leaf elemental stoichiometry, enhances plant carbon capture, and increases the amount of carbon they release via their roots as exudates.

Plant–soil feedbacks (PSFs) regulate plant growth, plant community dynamics and ecosystem functioning and are important for global biogeochemical cycles. However, human activities and their associated impacts on the environment can alter the strength and direction of PSFs, but these effects, and especially the interactions among human impacts, are poorly understood. In urbanised and other human‐modified landscapes, anthropogenic sources of change are more varied and pronounced, resulting in a myriad of biotic and abiotic human‐caused drivers simultaneously affecting ecological processes across multiple scales. These anthropogenic environmental drivers can have severe consequences for the delivery of ecosystem services in urbanised areas and beyond. Here, we systematically review the literature on the impacts of environmental drivers on PSFs to address the question: how do multiple anthropogenic drivers impact PSFs? Further, we will determine the dominant and interactive drivers of changes to PSFs across 21 potential anthropogenically influenced environmental drivers and assess the relative importance of biotic and abiotic drivers. We will assess how these drivers shape the plant and soil microbial communities involved in PSFs to determine their scale and directionality. We will also outline research gaps to guide future studies on PSFs in anthropogenically impacted ecosystems and especially urban environments. Besides extracting key variables, such as the range of values of the driver and impacts on plant growth or microbial diversity from reviewed articles, we will also determine how attributes of the studies themselves, such as location or duration of studies, influence the strength of findings. Practical implication: This work will be crucial to understand not only human impacts on ecosystems, but also developing mitigation and management solutions to reduce the negative consequences of altered PSF, and so can be instrumental for managing ecosystem services in human‐dominated landscapes.

Many applications of network theory to plant-mycorrhizal associations have used a bipartite description, in which one set of nodes is the plants, and the other set is the fungi. Most applications have relied on null models from algorithms that randomly rewire the observed connections to test for non-random patterns in the network. We used existing plant-arbuscular mycorrhizal (AM) fungal datasets to apply a new, well validated generation of network models relaxing the very limiting assumptions of traditional null models. We focused on nestedness and modularity, which have been related to the functioning and stability of communities. Given the existent literature, we expected nestedness and modularity to be prevalent. We modelled plant-AM fungal associations using maximum entropy networks with a degree sequence, soft constraint to generate null distributions for nestedness and modularity. Most plant-AM fungal associations were anti-nested and modular. This pattern was consistent across habitat types and multiple spatial scales. Anti-nestedness can easily emerge from modularity when network patterns are determined by the identity of the plant and AM fungal nodes. Future studies will have to test how the observed patterns determine the ability of the associations to adapt to environmental changes.

Tree-planting is increasingly presented as a cost-effective strategy to maximise ecosystem carbon (C) storage and thus mitigate climate change. Its success largely depends on the associated response of soil C stocks, where most terrestrial C is stored. Yet, we lack a precise understanding of how soil C stocks develop following tree planting, and particularly how it affects the form in which soil C is stored and its associated stability and resistance to climate change. Here, we present changes in C and nitrogen (N) stored as mineral-associated organic matter (OM), occluded particulate OM, free particulate OM and dissolved OM, from four regional chronosequences of Scots pine (Pinus sylvestris L.) forests planted on former grasslands across Scotland. We found that c. 58-68 years after the plantation, bulk soil C and N stocks in the organic layer and the top 20 cm of mineral soil decreased by half relative to unforested grasslands-a decrease roughly equivalent to a third of the simultaneous C gain in the tree biomass. This pattern was driven predominantly by a decrease in the amount of C and N stored as mineral-associated OM, an OM fraction considered as relatively long-lived. Our findings demonstrate the need to estimate C storage in response to tree planting based both on soil C stocks and tree biomass, as the use of the latter alone may significantly overestimate net C benefits of tree planting on permanent grasslands.

Aims The allocation of recent plant photosynthates to soil via arbuscular mycorrhizal (AM) fungi is a critical process driving multiple ecosystem functions in grasslands. Yet, our understanding of how defoliation modifies below-ground allocation of recent plant photosynthate and its response to drought, which is becoming more intense and frequent, remains unresolved. Methods Here we undertook a ¹³C pulse-labelling experiment in a mesotrophic temperate grassland to evaluate in situ how defoliation intensity modifies the transfer of recently assimilated ¹³C from plant shoots to roots, extraradical AM fungal hyphae, soil, and ¹³C-CO2 efflux (soil respiration) in response to simulated drought. Results We found that, individually, both defoliation and drought reduced initial plant ¹³C uptake, but when defoliation and drought were combined, we detected a significant reduction in below-ground ¹³C allocation to soil. Furthermore, while defoliation stimulated ¹³C transfer to plant roots and soil, high intensity defoliation amplified ¹³C-CO2 efflux relative to the amount of ¹³C taken up by plants. Drought stimulated ¹³C transfer to fungal hyphae relative to initial plant uptake. High intensity defoliation, however, suppressed both ¹³C enrichment of extraradical AM fungal hyphae and ¹³C transfer to fungal hyphae relative to initial uptake. Conclusions Our findings suggest that defoliation can reduce the transfer of recent photosynthate below-ground under simulated drought and provide new insights into how defoliation may influence grassland C allocation dynamics and cycling between plants and AM fungi in grasslands facing drought.

Common mycorrhizal networks (CMNs) are an enigmatic feature of soil and mycorrhizal ecology. The current use of the term ‘common mycorrhizal network’ stipulates a direct, continuous physical link between plants formed by the mycelium of mycorrhizal fungal genets. This means that a specific case (involving hyphal continuity) is used to define a much broader phenomenon of hyphae interlinking among roots of different plants. We here embrace a more inclusive definition of the CMN as a network formed by mycorrhizal fungal genets among roots of different plants, irrespective of the type of connection or interaction, and not limited to direct hyphal linkages. Implicitly, this broader version of the term has been used by many researchers already. We propose using the term ‘common mycorrhizal networks with hyphal continuity’ (CMN‐HC) to capture the more specific case of a continuous link via hyphae between the roots of different plants, which is important to study for some (notable carbon and nutrient exchange), but not all functions of a CMN (e.g. transfer of infochemicals or microbes). In addition, and becoming more general than CMN, we introduce the term ‘common fungal network’ (CFN) to include networks of any type of connection formed between different plants by any type of fungus; this includes also non‐mycorrhizal fungi, and indeed a combination of non‐mycorrhizal and mycorrhizal networks. We assert that this new conceptual framework incorporating three hierarchical terms (CMN‐HC, CMN and CFN), ranging from the most specific to the very broad, can usher in a period of new research activity on fungal networks. Read the free Plain Language Summary for this article on the Journal blog.

The current use of the term ‘common mycorrhizal network’ (CMN) stipulates a direct link between plants formed by the mycelium of a mycorrhizal fungus. This means that a specific case (involving hyphal continuity) is used to define a much broader phenomenon of hyphae interlinking among plant roots. We here offer a more inclusive definition of the common mycorrhizal network as a network formed by a fungus among plant roots, irrespective of the type of connection or interaction, not limited to direct hyphal linkages. We propose the term ‘common mycorrhizal networks with hyphal continuity’ (CMN-HC) to capture the more specific case, which is important to study for some (notable carbon and nutrient exchange), but not all functions of a common mycorrhizal network. In addition, we introduce the term ‘common fungal network (CFN)’ to include networks of any type of connection formed by any type of fungus; this includes also non-mycorrhizal fungi, and indeed a combination of non-mycorrhizal and mycorrhizal networks. We feel this new set of three hierarchical terms (CMN-HC, CMN and CFN) can usher in a period of research activity unburdened by some of the difficulties (logistics, experimental design challenges) of studying CMN-HC and thus can help attract additional researchers to this fascinating topic of mycorrhizal ecology.

Modification of soil food webs by land management may alter the response of ecosystem processes to climate extremes, but empirical support is limited and the mechanisms involved remain unclear. Here we quantify how grassland management modifies the transfer of recent photosynthates and soil nitrogen through plants and soil food webs during a post-drought period in a controlled field experiment, using in situ ¹³C and ¹⁵N pulse-labelling in intensively and extensively managed fields. We show that intensive management decrease plant carbon (C) capture and its transfer through components of food webs and soil respiration compared to extensive management. We observe a legacy effect of drought on C transfer pathways mainly in intensively managed grasslands, by increasing plant C assimilation and ¹³C released as soil CO2 efflux but decreasing its transfer to roots, bacteria and Collembola. Our work provides insight into the interactive effects of grassland management and drought on C transfer pathways, and highlights that capture and rapid transfer of photosynthates through multi-trophic networks are key for maintaining grassland resistance to drought.