Contents
I. Introduction 000
II. Morphological responses  000
III. Root demography 000
IV. Physiological plasticity 000
V. Root plasticity in patches in competition and symbiosis with microorganisms 000
VI. Influence of patch attributes 000
VII. Control of root proliferation 000
VIII. Conclusions 000

Acknowledgements 000


References 000



Summary
When roots encounter a nutrient-rich zone or patch they often proliferate within it. Roots experiencing nutrient-rich patches can also enhance their physiological ion-uptake capacities compared with roots of the same plant outside the patch zone. These plastic responses by the root system have been proposed as the major mechanism by which plants cope with the naturally occurring heterogeneous supplies of nutrients in soil. Various attempts to predict how contrasting species will respond to patches have been made based on specific root length (SRL), root demography and biomass allocation within the patch zone. No one criterion has proved definitive. Actually demonstrating that root proliferation is beneficial to the plant, especially in terms of nitrogen capture from patches, has also proved troublesome. Yet by growing plants under more realistic conditions, such as in interspecific plant competition, and with a complex organic patch, a direct benefit can be demonstrated. Thus, as highlighted in this review, the environmental context in which the root response is expressed is as important as the magnitude of the response itself.

https://www.jstor.org/stable/info/1514473

The plastic plant: root responses to heterogeneous supplies of nutrients

Soil nutrients are distributed in a non-uniform or 'patchy' manner. It is well established that the modular nature of root systems allows them to show both morphological and/or physiological plasticity upon encountering nutrient-rich patches. These plastic responses are widely believed to be foraging mechanisms by the plant to enhance nutrient resource capture. Although morphological plasticity has traditionally been viewed as the more expensive option as it requires new root construction, more recent evidence suggests this may not necessarily be the case. Moreover, plants may be able to recapture most of the initial outlay involved in new root construction, again lowering the overall cost to the plant. Under natural conditions the roots of most plant species have an additional nutrient acquisition mechanism namely mycorrhizal symbiosis. However, the impact of these important symbiotic associations upon the host plant's response to nutrient patches has received relatively little attention. The mycorrhizal fungal symbiont should, in theory, be better able to compete directly with the rest of the microbial community for the nutrients in the patch. This could potentially be important to the host plant, as generally, root proliferation responses are more important for interspecific plant, than plant-microbial, competition.

Plastic plants and patchy soils

Response of the underground parts of a whole grassland community to heterogeneity in soil resources was studied under field conditions. The increased availability of nutrients (N, P) in patches was combined with suppression of arbuscular mycorrhizal symbiosis (by fungicide benomyl) and with modification of substrate structure (sieved soil, sieved soil mixed with coarse or fine sand). Three experimental runs were established in 1999, each with seven replicate blocks. Root and rhizome biomass was determined before the experimental treatment and after 53 days of the experiment. Plant community composition in neighbourhood of patches was recorded. Nutrient application had the most pronounced effect on the root biomass grown into the patches (the root biomass increased 1.64 times). The root biomass in patches with fungicide application was 1.17 times higher than in the untreated patches. The modification of substrate structure had no effect on the root biomass in patches but it did modify the effect of benomyl. The amount of root biomass proliferating into the patches was not related to species composition of vegetation near the patches. However, higher frequency of grass tillers in the patch neighbourhood and the mycorrhizal status of neighbouring species correlated significantly with the amount of root biomass in the experimental patches. In the pre-treatment community, amount of root biomass was significantly related to the abundance of grasses and mycorrhizal forbs surrounding the sampling points.

Plant root response to heterogeneity of soil resources: Effects of nutrient patches, AM symbiosis, and species composition

Arbuscular mycorrhizal fungi (AMF) can form a mutualistic symbiosis with over two-thirds of all land plants, providing phosphorus and/or nitrogen in exchange for carbon. They can have a significant effect on the surrounding soil, altering pH, water content, structure, and drainage. Important greenhouse gases (GHG) including carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) can be influenced by these factors, yet to date the interactions between AMF and soil GHG fluxes are surprisingly understudied.

A microcosm system was developed to study GHG fluxes in the presence and absence of AMF hyphae. A central compartment contained an AMF host plant (Zea mays L.), with two outer compartments, that either allowed (AMA) or prevented (NAMA) AMF hyphal access. Organic matter patches of dried, milled, Z. mays leaves mixed with soil were added to the outer compartments to encourage proliferation of AMF hyphae and GHG production. Soil-atmosphere fluxes of N2O, CO2 and CH4 from the outer compartments were quantified, and gas probes were developed to measure N2O concentrations within the organic matter patches.

Data from a series of microcosm experiments provide evidence for AMF interactions with soil fluxes of N2O and CO2, but not CH4. Soil CO2 fluxes were found to be a useful non-invasive method for determining the presence of AMF in hyphal compartments. The N2O concentrations in organic patches decreased in AMA treatments, and a subsequent experiment demonstrated that N2O production by nitrifiers may be limited in the presence of AMF hyphae. In contrast, following harvesting, N2O fluxes from organic matter patches were higher in the AMA treatment; possibly because carbon release from severed AMF hyphae fuelled denitrification. These interactions have important implications for N cycling and sustainable agriculture. The evidence presented in this thesis suggests that AMF may play a previously unappreciated role in reducing soil-atmosphere losses of N2O.

Interactions between arbuscular mycorrhizal fungi and soil greenhouse gas fluxes

The problem of how best to find and exploit essential resources, the quality and locations of which are unknown, is common throughout biology. For plants, the need to grow an efficient root system so as to acquire patchily distributed soil nutrients is typically complicated by competition between plants, and by the costs of maintaining the root system. Simple mechanistic models for root growth can help elucidate these complications, and here we argue that these models can be usefully informed by models initially developed for foraging fish larvae. Both plant and fish need to efficiently search a spatio-temporally variable environment using simple algorithms involving only local information, and both must perform this task against a backdrop of intra- and inter-specific competition and background mortality. Here we develop these parallels by using simple stochastic models describing the growth and efficiency of four contrasting idealized root growth strategies. We show that plants which grow identically in isolation in homogeneous substrates will typically perform very differently when grown in monocultures, in heterogeneous nutrient landscapes and in mixed-species competition. In particular, our simulations show a consistent result that plants which trade-off rapid growth in favour of a more efficient and durable root system perform better, both on average and in terms of the best performing individuals, than more rapidly growing ephemeral root systems. Moreover, when such slower growing but more efficient plants are grown in competition, the overall community productivity can exceed that of the constituent monocultures. These findings help to disentangle many of the context-dependent behaviours seen in the experimental literature, and may form a basis for future studies at the level of complex population dynamics and life history evolution.

/pdf/fishing-for-nutrients-in-heterogeneous-landscapes-modelling-gzubyiyq0l.pdf

Fishing for nutrients in heterogeneous landscapes: modelling plant growth trade-offs in monocultures and mixed communities

Phosphate uptake kinetics of Artemisia tridentata roots exposed to multiple soil enriched-nutrient patches

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Phosphate uptake kinetics of Artemisia tridentata roots exposed to multiple soil enriched-nutrient patches