Peter Millard

Nitrogen storage and remobilization by trees : ecophysiological relevance in a changing world

The role of carbon (C) and nitrogen (N) storage by trees will be discussed in terms of uncoupling their growth from resource acquisition. There are profound differences between the physiology of C and N storage. C storage acts as a short-term, temporary buffer when photosynthesis cannot meet current sink demand and remobilization is sink driven. However, the majority of C allocated to non-structural carbohydrates such as starch is not reused so is in fact sequestered, not stored. In contrast, N storage is seasonally programmed, closely linked to tree phenology and operates at temporal scales of months to years, with remobilization being source driven. We examine the ecological significance of N storage and remobilization in terms of regulating plant N use efficiency, allowing trees to uncouple seasonal growth from N uptake by roots and allowing recovery from disturbances such as browsing damage. We also briefly consider the importance of N storage and remobilization in regulating how trees will likely respond to rising atmospheric carbon dioxide concentrations. Most studies of N storage and remobilization have been restricted to small trees growing in a controlled environment where (15)N can be used easily as a tracer for mineral N. We highlight the need to describe and quantify these processes for adult trees in situ where most root N uptake occurs via ectomycorrhizal partners, an approach that now appears feasible for deciduous trees through quantification of the flux of remobilized N in their xylem. This opens new possibilities for studying interactions between N and C allocation in trees and associated mycorrhizal partners, which are likely to be crucial in regulating the response of trees to many aspects of global environmental change.

Through the eye of the needle: a review of isotope approaches to quantify microbial processes mediating soil carbon balance

For soils in carbon balance, losses of soil carbon from biological activity are balanced by organic inputs from vegetation. Perturbations, such as climate or land use change, have the potential to disrupt this balance and alter soil-atmosphere carbon exchanges. As the quantification of soil organic matter stocks is an insensitive means of detecting changes, certainly over short timescales, there is a need to apply methods that facilitate a quantitative understanding of the biological processes underlying soil carbon balance. We outline the processes by which plant carbon enters the soil and critically evaluate isotopic methods to quantify them. Then, we consider the balancing CO(2) flux from soil and detail the importance of partitioning the sources of this flux into those from recent plant assimilate and those from native soil organic matter. Finally, we consider the interactions between the inputs of carbon to soil and the losses from soil mediated by biological activity. We emphasize the key functional role of the microbiota in the concurrent processing of carbon from recent plant inputs and native soil organic matter. We conclude that quantitative isotope labelling and partitioning methods, coupled to those for the quantification of microbial community substrate use, offer the potential to resolve the functioning of the microbial control point of soil carbon balance in unprecedented detail.

Does grassland vegetation drive soil microbial diversity

Does plant diversity drive soil microbial diversity in temperate, upland grasslands? Plants influence microbial activity around their roots by release of carbon and pot studies have shown an impact of different grass species on soil microbial community structure. Therefore it is tempting to answer yes. However, evidence from field studies is more complex. This evidence is reviewed at three different scales. First, studies from the plant community scale are considered that have compared soil microbial community structure in pastures of different vegetation composition, as a consequence of pasture improvement. These show fungi dominating the biomass in unimproved pastures and bacteria when lime and fertilizers have been applied. Secondly, evidence for interactions between individual grass species and soil microbes is discussed at the level of the rhizosphere, by considering both pot experiments and field studies. These have produced contrasting and inconclusive results, often due to spatial heterogeneity of soil properties and microbial communities. In particular, increased soil pH and fertility in urine patches and other nutrient cycling processes interact to increase the spatially complexity of soil microbial communities. Finally three studies which have measured microbial community structure in the rhizoplane are considered. These show that bacterial diversity is not directly related to plant diversity, although fungal diversity is. In addition, the soil fungal community has been demonstrated to have an effect upon the composition of the bacterial community. We suggest that while current vegetation influences fungal communities (particularly mycorrhizae) and litter inputs fungal saprotrophs, bacterial community structure is influenced more by the quality or composition of soil organic matter, thereby reflecting carbon inputs to the soil over decades.

Effect of Afforestation and Reforestation of Pastures on the Activity and Population Dynamics of Methanotrophic Bacteria

ABSTRACT We investigated the effect of afforestation and reforestation of pastures on methane oxidation and the methanotrophic communities in soils from three different New Zealand sites. Methane oxidation was measured in soils from two pine (Pinus radiata) forests and one shrubland (mainly Kunzea ericoides var. ericoides) and three adjacent permanent pastures. The methane oxidation rate was consistently higher in the pine forest or shrubland soils than in the adjacent pasture soils. A combination of phospholipid fatty acid (PLFA) and stable isotope probing (SIP) analyses of these soils revealed that different methanotrophic communities were active in soils under the different vegetations. The C18 PLFAs (signature of type II methanotrophs) predominated under pine and shrublands, and C16 PLFAs (type I methanotrophs) predominated under pastures. Analysis of the methanotrophs by molecular methods revealed further differences in methanotrophic community structure under the different vegetation types. Cloning and sequencing and terminal-restriction fragment length polymorphism analysis of the particulate methane oxygenase gene (pmoA) from different samples confirmed the PLFA-SIP results that methanotrophic bacteria related to type II methanotrophs were dominant in pine forest and shrubland, and type I methanotrophs (related to Methylococcus capsulatus) were dominant in all pasture soils. We report that afforestation and reforestation of pastures caused changes in methane oxidation by altering the community structure of methanotrophic bacteria in these soils.

Remobilised nitrogen and root uptake of nitrate for spring leaf growth, flowers and developing fruits of pear (Pyrus communis L.) trees

Both uptake of fertiliser N and remobilisation of stored N were quantified for the early growth of spur and shoot leaves, flowers and fruit development of pear trees. One-year old Abbé F. trees grafted on quince C rootstocks were fertilised with a generous N supply for one year and while dormant during the winter, transferred to sand cultures. Each tree received 3 g of labelled nitrate-N at the end of winter and in early spring. Leaves, flowers and fruit were sampled on 5 separate occasions and the recovery of labelled N used to distinguish the remobilisation of N and the root uptake of nitrate. Remobilisation of stored N accounted for most of the N present in leaves and flowers during blossoming. Remobilisation of nitrogen stopped between petal fall and the beginning of fruit development. Root uptake of nitrate linearly increased over time and at the last sampling, 55 days after bud burst, fertiliser N contributed approximately half of the total N recovered in both spur and shoot leaves, the remainder coming from remobilisation. Flowers and fruits based their N metabolism more on remobilisation as compared to the leaves. This pattern of internal cycling of N is discussed in relation to fertilisation strategies for pear trees.

Effects of severity of defoliation on root functioning in grasses.

Grass shoots after defoliation can be supplied with the nitrogen required for regrowth by either root uptake or remobilization of stores. Whilst it is accepted that after a single defoliation inhibition of root uptake and remobilization from root occurs, it has not been established how the capability of roots to supply nitrogen by uptake and from storage is affected with differing severities of regular defoliation, as experienced by grazed swards. The objective was to examine this question using Agrostis castellana Boiss et Reut., Festuca rubra L., Lolium perenne L. and Poa trivialis L., grasses associated with sites of differing fertility, grown in sand culture and defoliated weekly at a height of either 4 or 8 cm. Nitrogen was supplied as NH4NO3 in a complete nutrient solution. The use of 15N as a tracer allowed the nitrogen supplied to the shoot by root uptake and remobilization to be discriminated over a 35 day period. An increased severity of defoliation resulted in decreased root mass, and increased nitrogen uptake per unit root weight for all species. Increased severity of defoliation did not affect uptake on a per plant basis for A. castellana, 0.54 mg N (plant)-1 (week)-1 and P. rubra, 0.40 mg N (plant)-1 (week)-1, whilst mg N (plant)-1(week)-1 decreased from 0.54 to 0.14, and 0.54 to 0.34 for L perenne and P. trivialis respectively. For plants clipped at 4 or 8 cm, over 88% and 77% respectively of uptake appeared in the shoot. Nitrogen was remobilized from roots to the shoot for A. castellana and F. rubra when clipped at 4 cm, and for A. castellana, L. perenne and P. trivialis when clipped at 8 cm. Uptake by roots was more important than remobilization from roots in supplying nitrogen to the shoot. The ability to maintain the supply of nitrogen by uptake and remobilization to the shoot with increased severity of defoliation was species dependent.

Coupling Sap Flow Velocity and Amino Acid Concentrations as an Alternative Method to 15N Labeling for Quantifying Nitrogen Remobilization by Walnut Trees

The temporal dynamics of N remobilization was studied in walnut (Juglans nigra × regia) trees growing in sand culture. Trees were fed with labeled N (15N) during 1999 and unlabeled N in 2000. Total N and15N contents in different tree compartments were measured during 80 d after bud burst and were used to estimate N remobilization for spring growth. The seasonal (and occasionally diurnal) dynamics of the concentration and 15N enrichment of the major amino acids in xylem sap were determined concurrently. Sap flow velocity was also measured for sample trees. A new approach coupling amino acid concentrations to sap flow velocity for quantifying N remobilization was tested. A decrease of the labeled N contents of medium roots, tap roots, and trunk was observed concurrently to the increase in the labeled N content of new shoots. Remobilized N represented from previous year storage 54% of N recovered in new shoots. Arginine, citruline, γ-amino butyric acid, glutamic acid, and aspartic acid always represented around 80% of total amino acid and amide N in xylem sap and exhibited specific seasonal trends and significant diurnal trends. N translocation was mainly insured by arginine during the first 15 d after bud burst, and then by glutamic acid and citruline. The pattern of N remobilization estimated by the new approach was consistent with that measured by the classical labeling technique. Implications for quantifying N remobilization for large, field-growing trees are discussed.

Evidence of Microbial Regulation of Biogeochemical Cycles from a Study on Methane Flux and Land Use Change

ABSTRACT Microbes play an essential role in ecosystem functions, including carrying out biogeochemical cycles, but are currently considered a black box in predictive models and all global biodiversity debates. This is due to (i) perceived temporal and spatial variations in microbial communities and (ii) lack of ecological theory explaining how microbes regulate ecosystem functions. Providing evidence of the microbial regulation of biogeochemical cycles is key for predicting ecosystem functions, including greenhouse gas fluxes, under current and future climate scenarios. Using functional measures, stable-isotope probing, and molecular methods, we show that microbial (community diversity and function) response to land use change is stable over time. We investigated the change in net methane flux and associated microbial communities due to afforestation of bog, grassland, and moorland. Afforestation resulted in the stable and consistent enhancement in sink of atmospheric methane at all sites. This change in function was linked to a niche-specific separation of microbial communities (methanotrophs). The results suggest that ecological theories developed for macroecology may explain the microbial regulation of the methane cycle. Our findings provide support for the explicit consideration of microbial data in ecosystem/climate models to improve predictions of biogeochemical cycles.

Response of methanotrophic communities to afforestation and reforestation in New Zealand

Methanotrophs use methane (CH4) as a carbon source. They are particularly active in temperate forest soils. However, the rate of change of CH4 oxidation in soil with afforestation or reforestation is poorly understood. Here, soil CH4 oxidation was examined in New Zealand volcanic soils under regenerating native forests following burning, and in a mature native forest. Results were compared with data for pasture to pine land-use change at nearby sites. We show that following soil disturbance, as little as 47 years may be needed for development of a stable methanotrophic community similar to that in the undisturbed native forest soil. Corresponding soil CH4-oxidation rates in the regenerating forest soil have the potential to reach those of the mature forest, but climo-edaphic fators appear limiting. The observed changes in CH4-oxidation rate were directly linked to a prior shift in methanotrophic communities, which suggests microbial control of the terrestrial CH4 flux and identifies the need to account for this response to afforestation and reforestation in global prediction of CH4 emission.

Storage and remobilisation of nitrogen by pear (Pyrus communis L.) trees as affected by timing of N supply

Abstract Orchard nitrogen (N) management should aim at reconciling productivity, fruit quality and environmental concerns. Fertilisation strategies should, therefore, maximise the efficiency of N fertilisers, including the choice of the optimal timing of N supply. In the present study, with the aid of labelled [Ca( 15 NO 3 ) 2 ] fertiliser, we assessed: (1) the uptake; (2) the partitioning of nitrogen in 1 year; and (3) its remobilisation the following year, as affected by the timing of N supply. Two-year old Abbe Fetel trees on quince C were grown in pots filled with sand and divided into three groups: one group (A) received 3 g (in total) of labelled N from mid March to mid June, while trees of group B received 3 g of labelled N from late June to fruit harvest (August 20). Trees of group A and B received unlabelled N (3 g/tree) from late June to fruit harvest and from mid March to mid June, respectively. A third set of trees (C) received labelled N (6 g/tree) throughout the season. At fruit harvest, fruits and leaves contained similar amounts of N derived from remobilisation of stored N and from spring uptake (March–June, treatment A) and only a small fraction (around 10%) of N derived from N taken up after June (treatment B). Although abscised leaves contained 10 times higher amounts of N taken up early (A) than late (B) treatments, similar amounts of labelled N were recovered in the tree framework in winter in trees of groups A and B. Remobilisation of N in the following spring accounted for 23–24% of the labelled N in the tree, regardless the timing of N uptake. Trees remobilised with preference N taken up during the previous year than N absorbed earlier. Results indicate that a limited supply of N before fruit harvest does not increase the fruit N content significantly, while it increases N storage in roots during winter to be remobilised the following spring.