Languishing for many years in the shadow of plant inorganic nitrogen (N) nutrition research, studies of organic N uptake have attracted increased attention during the last decade. The capacity of plants to acquire organic N, demonstrated in laboratory and field settings, has thereby been well established. Even so, the ecological significance of organic N uptake for plant N nutrition is still a matter of discussion. Several lines of evidence suggest that plants growing in various ecosystems may access organic N species. Many soils display amino acid concentrations similar to, or higher than, those of inorganic N, mainly as a result of rapid hydrolysis of soil proteins. Transporters mediating amino acid uptake have been identified both in mycorrhizal fungi and in plant roots. Studies of endogenous metabolism of absorbed amino acids suggest that L- but not D-enantiomers are efficiently utilized. Dual labelled amino acids supplied to soil have provided strong evidence for plant uptake of organic N in the field but have failed to provide information on the quantitative importance of this process. Thus, direct evidence that organic N contributes significantly to plant N nutrition is still lacking. Recent progress in our understanding of the mechanisms underlying plant organic N uptake may open new avenues for the exploration of this subject.

https://nph.onlinelibrary.wiley.com/doi/epdf/10.1111/j.1469-8137.2008.02751.x

Uptake of organic nitrogen by plants.

The importance of mycorrhizas in heathland and boreal forest biomes, which together cover much of the landmass of the Northern Hemisphere and store most of the global stocks of carbon, is reviewed....

Mycorrhizal fungi as drivers of ecosystem processes in heathland and boreal forest biomes

The recently recognized importance of organic nitrogen (ON), particularly amino acids, to plant nutrition in many types of agricultural and natural ecosystems has raised questions about plant-microbe interactions, N availability in soils, and the ecological implications of ON use by plants in the light of climate change and N pollution. In this review we synthesize the recent work on availability and plant uptake of amino acids with classic work on ON in soils. We also discuss recent work on the use of natural abundance levels of 15N to infer N sources for plants. Reliance on ON is widespread among plants from many ecosystems. Authors have reached this conclusion based on laboratory studies of amino acid uptake by plants in pure culture, amino acid concentrations in soils, plant uptake of isotopically labeled amino acids in the field and in plant-soil microcosms, and from plant natural abundance values of 15N. The supply of amino acids to plants is determined mainly by the action of soil proteolytic enzymes, interactions between amino acids and the soil matrix, and competition between plants and microbes. Plants generally compete for a minor fraction of the total amino acid flux, but in some cases this forms a significant N resource, especially in ecosystems where microbial biomass undergoes large seasonal fluctuations and contributes labile ON to the soil. A quantitative understanding of ON use by plants is confounded by incomplete data on partitioning of ON between plants, mycorrhizal fungi, and competing soil microbes. Further research is needed to predict the ecological implications of ON use by plants given the influence of climatic change and N pollution.

The unexpected versatility of plants: organic nitrogen use and availability in terrestrial ecosystems.

There is a growing recognition that accurate predictions of plant and ecosystem responses to global change require a better understanding of the mechanisms that control acquisition of growth-limiting resources. One such key mechanism is root physiological capacity to acquire nutrients. Changes in kinetics of root nitrogen (N) uptake might influence the extent to which terrestrial ecosystems will be able to sequester excesses in carbon (C) and N loads. Despite its significant role in determining plant and ecosystem cycling of C and N, there is little information on whether, or how, root nutrient uptake responds to global change. In this review various components of global change, namely increased CO2 concentration, increased soil temperature and increased atmospheric N deposition and their effects on kinetics of root nutrient uptake are examined. The response of root nutrient uptake kinetics to high CO2 is highly variable. Most of this variability might be attributable to differences in experimental protocols, but more recent evidence suggests that kinetic responses to high CO2 are also species-specific. This raises the possibility that elevated CO2 might alter community composition by shifting the competitive interaction of co-occurring species. Uptake of NH4+ and NO3− seem to be differentially sensitive to high CO2, which could influence ecosystem trajectory toward N saturation. Increased soil temperature might increase N and P uptake capacity to a greater extent in species from warm and fluctuating soil habitats than in species from cold and stable soil environments. The few available data also indicate that increased soil temperature elicits a differential effect on uptake of NH4+ versus NO3−. Root uptake kinetics are generally down-regulated in response to long-term exposure to atmospheric N deposition. The extent of this down-regulation might, however, vary among species, stages of succession, land-use history and plant demand. Nonetheless, it is suggested that root N uptake kinetics might be an accurate biological indicator of the ecosystem capacity to retain N. The results reviewed here clearly highlight the scanty nature of the literature in the area of root nutrient absorption responses to global change. It is also clear that effects of one component of global change on root nutrient absorption capacity might be counterbalanced by another. Therefore, the generalizations offered here must be viewed with caution and more effort should be directed to rigorously test these initial observations in future research.

https://nph.onlinelibrary.wiley.com/doi/pdf/10.1046/j.1469-8137.2000.00682.x

Kinetics of nutrient uptake by roots: responses to global change

Mycorthizae play a critical role in nutrient capture from soils. Arbuscular mycorrhizae (AM) and ectomycorrhizae (EM) are the most important mycorrhizae in agricultural and natural ecosystems. AM and EM fungi use inorganic NH4+ and NO3-, and most EM fungi are capable of using organic nitrogen. The heavier stable isotope N-15 is discriminated against during biogeochemical and biochemical processes. Differences in N-15 (atom%) or delta(15)N (parts per thousand) provide nitrogen movement information in an experimental system. A range of 20 to 50% of one-way N-transfer has been observed from legumes to nonlegumes. Mycorrhizal fungal mycelia can extend from one plant's roots to another plant's roots to form common mycorrhizal networks (CMNs). Individual species, genera, even families of plants can be interconnected by CMNs. They are capable of facilitating nutrient uptake and flux. Nutrients such as carbon, nitrogen and phosphorus and other elements may then move via either AM or EM networks from plant to plant. Both N-15 labeling and N-15 natural abundance techniques have been employed to trace N movement between plants interconnected by AM or EM networks. Fine mesh (25similar to45 mum) has been used to separate root systems and allow only hyphal penetration and linkages but no root contact between plants. In many studies, nitrogen from N-2-fixing mycorrhizal plants transferred to non-N-2-fixing mycorrhizal plants (one-way N-transfer). In a few studies, N is also transferred from non-N-2-fixing mycorrhizal plants to N-2-fixing mycorrhizal plants (two-way N-transfer). There is controversy about whether N-transfer is direct through CMNs, or indirect through the soil. The lack of convincing data underlines the need for creative, careful experimental manipulations. Nitrogen is crucial to productivity in most terrestrial ecosystems, and there are potential benefits of management in soil-plant systems to enhance N-transfer. Thus, two-way N-transfer warrants further investigation with many species and under field conditions.

Nitrogen transfer within and between plants through Common Mycorrhizal Networks (CMNs)

It is well established that ectomycorrhizal fungi can use amino acids as nitrogen and carbon sources, but data on the kinetic properties of amino acid uptake systems of ectomycorrhizal systems are scarce. Using 14C-labelled compounds we have determined the kinetics of uptake of amino acids by excised ectomycorrhizal roots for a range of distinct mycorrhizal types from three tree species, beech, spruce, and pine. All mycorrhizal types examined took up amino acids via high-affinity transport systems (KM values ranging from 19 to 233 mmol m–3). A comparative analysis of kinetic parameters for uptake of amino acids and the ammonium analogue methylammonium showed that ectomycorrhizal roots have similar or even higher affinities (lower KM values) for the amino acids, indicating that absorption of these organic forms of nitrogen (N) can contribute significantly to total N uptake by ectomycorrhizal plants. Analysis of amino acid uptake by ectomycorrhizal roots collected along a European north/south gradient of increasing mineral N pollution from northern Sweden to south Germany revealed no obvious trend in the uptake capabilities for amino acids by ectomycorrhizal roots in relation to the location of the sampling site on this gradient. Rather, the fungal species forming a particular morphotype was the factor determining uptake kinetics. It can therefore be deduced that the species composition of the fungal community will contribute significantly to the functional diversity of a population of mycorrhizal roots.

https://onlinelibrary.wiley.com/doi/pdf/10.1046/j.1365-3040.1999.00385.x

T. Wallenda

Papers

Kinetics of amino acid uptake by ectomycorrhizal roots