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Soil organic phosphorus has broad agronomic and ecological significance, but remains a neglected topic of research. This opinion paper reflects a collaborative discussion between three generations of scientists who have collectively studied soil organic phosphorus for almost 50 years. We discuss personal reflections on our involvement in the field, opinions about progress and promising opportunities for future research. We debate an apparent overemphasis on analytical methodology at the expense of broader questions, and whether this has stifled progress in recent decades. We reiterate the urgent need to understand organic phosphorus cycling in the environment to address fundamental questions about phosphate supply, crop nutrition, water quality and ecosystem ecology. We also contend that we must encourage and integrate the study of organic phosphorus across all scales, from molecular chemistry to global cycling. Our discussion among three generations of researchers shows the value of a long-term perspective, emphasizes the changing nature of this field of research, and reinforces the importance of continuing to be curious about the dynamics of organic phosphorus in the environment.

The article “Organic phosphorus in the terrestrial environment: a perspective on the state of the art and future priorities”, written by Timothy S George et al., was originally published with incorrect affiliation information for one of the co-authors, E. Klumpp.

Made available in DSpace on 2018-12-11T16:49:52Z (GMT). No. of bitstreams: 0 Previous issue date: 2018-06-01 Biotechnology and Biological Sciences Research Council Background: The dynamics of phosphorus (P) in the environment is important for regulating nutrient cycles in natural and managed ecosystems and an integral part in assessing biological resilience against environmental change. Organic P (Po) compounds play key roles in biological and ecosystems function in the terrestrial environment being critical to cell function, growth and reproduction. Scope: We asked a group of experts to consider the global issues associated with Po in the terrestrial environment, methodological strengths and weaknesses, benefits to be gained from understanding the Po cycle, and to set priorities for Po research. Conclusions: We identified seven key opportunities for Po research including: the need for integrated, quality controlled and functionally based methodologies; assessment of stoichiometry with other elements in organic matter; understanding the dynamics of Po in natural and managed systems; the role of microorganisms in controlling Po cycles; the implications of nanoparticles in the environment and the need for better modelling and communication of the research. Each priority is discussed and a statement of intent for the Po research community is made that highlights there are key contributions to be made toward understanding biogeochemical cycles, dynamics and function of natural ecosystems and the management of agricultural systems. The James Hutton Institute Lancaster Environment Centre Lancaster University Lincoln University, Lincoln Universidade Estadual do Norte Fluminense Darcy Ribeiro (UENF - Laboratório de Solos), Av. Alberto Lamego 2000 Plant and Soil Sciences University of Delaware, 160 Townsend Hall Faculty of Environment and Natural Resources Chair of Soil Ecology University of Freiburg, Bertoldstraße 17 Rothamsted Research West Common Harpenden Rothamsted Research West Common Harpenden, North Wyke College of Agricultural Sciences Department of Crop Science Sao Paulo State University (UNESP), 1780, Jose Barbosa de Barros st Institute of Bio- and Geosciences IBG-3: Agrosphere Forschungszentrum Jülich GmbH Oak Ridge National Laboratory, P.O.Box 2008 DISAFA Soil Biogeochemistry University of Turin, largo Braccini 2 University of Minnesota-, Cities, 1479 Gortner Ave. Saint Paul China Agricultural University Le Laboratoire des Sciences du Climat et de l’Environnement IPSL-LSCE CEA/CNRS/UVSQ Saclay Institute of Soil Science University of Hohenheim, Emil Wolff Str. 27 INRA UMR ECO&SOLS Evolutionary Biology Centre EBC, Kåbovägen 4, house 7 Centre for Agri-environmental Research School of Agriculture Policy and Development University of Reading, Whiteknights, PO Box 237 Climate Impacts Research Centre Dep. of Ecology and Environmental Science Umeå University CSIRO Agriculture & Food D-USYS ETH Zurich, Tannenstrasse 1 Department of Environmental Sciences Norwegian University of Life Sciences, Post Box 5003 University of Guelph, 50 Stone Road East National Institute of Agricultural Research of Uruguay Department of Chemical & Biological Engineering The University of Sheffield, Mappin Street, S1 3JD School of Biological Sciences University of East Anglia, Norwich Research Park, Norwich International Plant Nutrition Institute, 18 Maplewood Drive Centro Interdipartimentale di Ricerca sulla Risonanza Magnetica Nucleare per l’Ambiente l’Agro-Alimentare ed i Nuovi Materiali (CERMANU) Università di Napoli Federico II, Via Università 100 Universidad de La Frontera School of Environment and Life Sciences University of Salford School of the Environment Natural Resources and Geography Bangor University School of Biological Sciences and Institute for Global Food Security The Queen’s University of Belfast, Medical Biology Centre, 97 Lisburn Road CENA University of Sao Paulo, Avenida Centenario, 303 Teagasc Environmental Research Centre, Johnstown Castle Centre for Ecology & Hydrology, Library Avenue, Bailrigg Department of Soil Biogeochemistry Bayreuth Center of Ecology and Environmental Research (BayCEER) University Bayreuth, Dr.-Hans-Frisch-Str. 1-3 Guangdong Provincial Key Laboratory of Applied Botany South China Botanical Garden Chinese Academy of Sciences, 723 Xingke Road, Tianhe District School of Geographical Sciences & School of Chemistry University of Bristol, University Road Soil Science and Soil Protection Martin Luther University Halle-Wittenberg, von-Seckendorff-Platz 3 Centre for Ice and Climate Niels Bohr Institute University of Copenhagen NERC Isotope Geosciences Facility British Geological Survey Department of Microbiology and Ecosystem Science University of Vienna, Althanstrasse 14 University of Northern British Columbia, 3333 University Way Brigham Young University Centre for Environmental Sciences Hasselt University Building D, Agoralaan Yamagata University Department of Biology Lund University, Biology Building Sölvegatan 35 Assessment of Microbial Environment Graduate School of Biosphere Science Hiroshima University College of Agricultural Sciences Department of Crop Science Sao Paulo State University (UNESP), 1780, Jose Barbosa de Barros st Biotechnology and Biological Sciences Research Council: BB/K018167/1 Biotechnology and Biological Sciences Research Council: BB/L025671/2

Three field manipulation experiments were carried out during 1993–1995 on the Northern Pennines to investigate the influences of temperature, solar radiation and rainfall on the release of dissolved organic carbon (DOC) from vegetated soil cores using zero-tension lysimeters. The cores were manipulated by being translocated to four sites down a climatic gradient, by artificial soil heating or being exposed to double normal rainfall. In each experiment three soil types, a brown earth, a micropodzol and a peaty gley, with differing organic matter content and distribution within the profile, were studied. DOC data, expressed as mg C m−2 day−1, were analysed following log10 transformation, by a repeated measures analysis of variance procedure, using climatic variables measured concurrently with sampling, and 1 and 2 months before sampling. DOC release was dominated by rainfall but was also associated with solar radiation and temperature. With each of the three climatic variables, rainfall, solar radiation and temperature, both positive and negative effects on DOC release have been found significant, indicating that the concurrent and delayed effects of the same variable may be different. DOC release was positively related with all three soils to concurrent rainfall, indicating rainfall's primary leaching action. DOC release was also negatively related to rainfall of the previous month indicating that its action depletes the leachable pool of DOC in the soil. DOC release was positively associated with solar radiation 2 months earlier, indicating that DOC's main source is that of primary production; DOC peaks closely followed peaks of annual primary production. DOC release was linked with temperature, the strongest association being with temperature 2 months earlier, indicating that temperature effects both primary production and DOC regeneration via organic matter decomposition. A conceptual model, relating our findings to those processes known to govern DOC release from soils, has been presented.

We analysed the 14C signal in soil solution DOC from the transition zone between the Ap and B horizon at 12 sites representing an afforestation chronosequence ranging from agricultural land to 89-year-old first generation Norway spruce forests. Although DOC concentrations had high spatial and temporal variation, the 14C signal in the DOC was consistent over two separate sampling periods for the individual plots. This indicates that the origin of DOC in the lower part of the A horizon is the same for each individual plot despite variations in DOC concentration in the soil solution. The DOC from the agricultural land had a 14C signal close to contemporary atmospheric values; mean 14C in DOC for 1999 and 2001 was 108 and 107% abs. modern, respectively. After establishment of forest, the 14C values in DOC fall to around 100% abs. modern. This provides evidence that a large part of the DOC in the younger stands is derived from older, pre-bomb carbon. The 14C enrichment gradually increased with stand age due to an increasing influence of decaying litter containing younger carbon, coming from that assimilated by the forest during the period with elevated 14C concentrations in the atmosphere. The average 14C concentration in DOC from the oldest stand was 111% abs. modern. Even if the younger carbon gradually influenced the proportion of 14C in DOC when stand age increased, a substantial part of the DOC must still originate from SOM formed before afforestation. It is hypothesised that 14C enrichment changes along the chronosequence is explained by two simultaneous processes, changes in input of litter and an increased mobilisation of older SOM from the A horizon due to the change in land-use.

Stable isotope systematics of C, N and S were studied in soils of 5 European forest ecosystems. The sites were located along a North–South transect from Sweden to Italy (mean annual temperatures from +1.0 to +8.5 °C, atmospheric deposition from 2 to 19 kg N ha −1 a −1 , and from 6 to 42 kg S ha −1 a −1 ). In Picea stands, the behavior of C, N and S isotopes was similar in 3 aspects: (1) assimilation favored the lighter isotopes 12 C, 14 N and 32 S; (2) mineralization in the soil profile left in situ residues enriched in the heavier isotopes 13 C, 15 N and 34 S; and (3) NO 3 –N as well as SO 4 –S in soil solution was isotopically lighter compared to the same species in the atmospheric input. In this study, emphasis was placed on S isotope profiles which so far have been investigated to a much lesser extent than those of C and N. Sulfate in monthly samples of atmospheric input had systematically higher δ 34 S ratios than total soil S at the 0–5 cm depth, on average by 4.0‰. Sulfate in the atmospheric input had higher δ 34 S ratios than in deep (>50 cm) lysimeter water, on average by 3.2‰. Organic S constituted more than 50% of total soil S throughout most of the profiles (0–20 cm below surface). There was a tendency to isotopically heavier organic S and lighter inorganic SO 4 –S, with ester SO 4 –S heavier than C-bonded S at 3 of the 5 sites. With an increasing depth (0 to 20 cm below surface), δ 13 C, δ 15 N and δ 34 S ratios of bulk soil increased on average by 0.9, 4.2 and 1.6‰, respectively, reflecting an increasing degree of mineralization of organic matter. The isotope effects of C, N and S mineralization were robust enough to exist at a variety of climate conditions and pollution levels. In the case of S, the difference between isotope composition of the upper organic-rich soil horizon (lower δ 34 S) and the deeper sesquioxide-rich soil horizons (higher δ 34 S) can be used to determine the source of SO 4 in streams draining forests. This application of δ 34 S as a tracer of S origin was developed in the Jezeři catchment, Czech Republic, a highly polluted site suffering from spruce die-back. In 1996–1997, the magnitude and δ 34 S of atmospheric input (20 kg S ha −1 a −1 , 5.8‰) and stream discharge (56 kg S ha −1 a −1 , 3.5‰) was monitored. Export of S from the catchment was 3 times higher than contemporary atmospheric input. More than 50% of S in the discharge was represented by release of previously stored pollutant S from the soil. Stable isotope systematics of Jezeři soil S (mean of 2.5‰ in the O+A horizon, 4.8‰ in the B horizon, and 5.8‰ in the bedrock) suggests that most of the soil-derived S in discharge must come from the isotopically light organic S present in the upper soil horizon, and that mineralized organically-cycled S is mainly flushed out during the spring snowmelt. The fact that a considerable proportion of incoming S is organically cycled should be considered when predicting the time-scale of acidification reversal in spruce die-back affected areas.

The indirect effects of acid-misting applied to tree canopies, on soil P availability, phosphatase activity and P mineralisation rates in a Sitka spruce (Picea sitchensis (Bong.) Carr) stand have been investigated. The treated trees, grouped according to five height classes, had for four years previously received acid mist consisting of a mixture of H2SO4 and NH4NO3 (pH 2.5) at concentrations of 3.2 mM H 1 and 1.6 mM each of NH 1 4 ; NO 2 3 and SO 22 4 : These simulate cloudwater composition in uplands affected by acidic deposition. Another group of mixed heightclass trees, received a double dose of acid mist. Control — no acid-misted-trees received only the usual precipitation inputs. The acid mist treatments had previously been found to induce a significant reduction in stem diameter growth, but no canopy visible injury symptoms were observed. Root bioassays had demonstrated the induction of P nutritional stress in acid-misted versus no acid-misted trees. Soil chemical analysis showed that acidifying inputs induced changes in the inorganic P subcycle by: (i) increasing acidity and P sorption capacity of the soils; and (ii) decreasing the concentration of labile inorganic P. We report here on changes in the organic P subcycle (phosphatase activity and gross mineralisation rates). We used a method which applies independent treatments to soil samples (control, HgCl 2-addition, HgCl2addition 1 autoclaving) to separate mineral solubilisation, organic mineralisation and immobilisation of solution P. Soil phosphatase activity was significantly lower under acid-misted trees (263.9mg pNP h 21 g 21 ) than under control trees (382.0 mg pNP h 21 g 21 ). Soils beneath double-dose acid-misted trees showed a 42% reduction in the labile inorganic P pool-size, and a 33% increase in the labile organic P poolsize, compared to soils under control trees. A pattern of decreasing net P solubilisation and gross P mineralisation rates with increasing acidmist dose-level was also found. Thus, the combined effects of acid-misting on the inorganic and organic P subcycles, in addition to the suggested effect of N saturation enhancing demand for P by the trees, may have induced P deficiency and reduced tree growth. The effects described are considered to be important factors in the forest decline syndrome. q 2000 Elsevier Science Ltd. All rights reserved.

Lysimeters located outdoors have been used to evaluate the decomposition of buried oily beach sand waste (OBS) prepared using Forties light crude oil and sand from different locations around the British coast. The OBS (5% oil by weight) was buried as a 12-cm layer over dune pasture sub-sand and overlain by 20 cm of dune pasture topsoil. Decomposition rates of oil residues averaged 2300 kg ha(-1) in the first year and the pattern of oil decomposition may be represented by a power curve. Oil decomposition was strongly related to the temperature in the OBS layer, but was also significantly affected by rainfall in the previous 12 h. The CO(2) flux at the surface of the treatment lysimeters followed the relationship [log(10) CO(2) (mg C m(-2) h(-1))=0.93+0.058x OBS temp. (degrees C)-0.042x12 h rain (mm)]. There was considerable variation in the rate of oil decomposition in sands collected from different sites. Sand from Askernish supported most microbial activity whilst sand from Tain was relatively inactive. The decomposition process appeared to cease when the sand became saturated with water, i.e. temporarily anaerobic. However, decomposition recommenced when the soil dried out. The fastest rate of decomposition occurred in sand from one of the two sites predicted to have high populations of hydrocarbon-degrading bacteria. Larger particle size and higher Ca content may also be significant factors governing the rate of decomposition.

Summary To test the hypothesis that canopy thinning might result from phosphorus limitations induced by acidic pollutant deposition, paired stands with ‘thin’ and ‘good’ canopies were compared by examining (1) the phosphorus nutrition of the trees using a root bioassay and (2) availability of phosphorus in surface soil using a resin-extraction procedure. Three pairs of stands of each of Sitka spruce, Scots pine and beech on a range of comparable soils were investigated. A higher level of phosphorus stress in stands with the thinner canopy compared with their corresponding ‘partners’ with the better canopy was demonstrated for eight out of the nine pairs; only the pair of beech stands on gley soils did not comply with this finding. A similar pattern was demonstrated with the soil available phosphorus measurements, namely that surface soils had lower amounts of resin-extractable phosphorus under trees with the thinner canopy compared with their respective ‘partner’ stand with the better canopy; again the beech stands on gley soils did not comply. Analysis of variance indicated that there were significant and complex interactions between tree canopy condition, tree species and soils, in phosphorus stress and surface soil available phosphorus. These results derived from stands distributed over a wide area of northern Britain suggest that there may be a general link between canopy condition and phosphorus stress in trees. However, no link was found between phosphorus stress and estimates of net acidic deposition inputs derived from broad-scale modelling. The critical load for acidic deposition appeared, from these estimates, to be exceeded for all the stands, irrespective of canopy condition. The demonstration of a possible link between tree canopy condition and phosphorus stress suggests that there is a need for a wider investigation, possibly as an integrated part of the UK Tree Health monitoring programme. The hypothesized induction by acidic deposition of phosphorus stress in trees, if true, will require actual monitoring, rather than prediction, of that deposition at each study site.

The impact of climate change on N leaching from hill land plant/soil systems was investigated using a transplant technique involving the movement of intact lysimeter cores of three contrasting soil types down an altitudinal gradient at Great Dun Fell, Cumbria. Air and soil temperatures and precipitation were monitored at four elevations down an altitudinal transect using automatic weather stations for a period of two years. The altitudinal sequence of air temperature followed the anticipated pattern, providing mean annual temperatures at the four locations of 3.4, 5.0, 6.3 and 8.1 °C. Lapse rates of both mean air and soil temperatures over the altitudinal range 171–845 m were 6.6 (1993) and 7.0 °C km–1 (1994). Soil monthly temperature gradients for a particular soil type for each of the two years showed a seasonal range of 6.0 and 7.4 °C km–1, respectively, and for air temperature of 4.3 and 3.1 °C km–1. Precipitation gradients showed the expected general increase with altitude, but were less predictable. Inorganic nitrogen leaching was studied in lysimeter leachates with climatic amelioration resulting in dramatic reductions in leachate nitrate concentrations and associated total concentrations of inorganic nitrogen. Decreases in leachate nitrate concentrations were observed for all three soil types studied. Soils receiving supplemented rainfall also showed decreased N concentrations, suggesting that temperature was the main controlling factor responsible for the observed reductions. Increased N uptake by the vegetation, in response to the increases in temperature, is considered to be critical in controlling soil solution chemistry at these sites.

The effects of acid-misting applied to tree canopies, on soil P availability, pH, effective cation exchange capacity and Ca + Mg:Al ratio, plus tree P nutrition in a Sitka spruce (Picea sitchensis (Bong.) Carr) stand at Glencorse, Scotland have been investigated. The treated trees, grouped according to five height classes, had for four years previously received acid mist consisting of a mixture of H2SO4 and NH4NO3 (pH 2.5) at concentrations of 3.2 mM H+ and 1.6 mM each of NH4+ NO3− and SO42−, with and without soda-glass ballotini, thus simulating cloudwater composition. The treatments were applied approximately twice a week at the equivalent of 2 mm precipitation per treatment from July to December 1990, and throughout the growing season (May–November) in 1991–1992–1993. Another group of mixed height class trees, received a double dose of acid mist at each application. Control trees received only the usual precipitation inputs. The acid mist treatments had been found, reported elsewhere, to induce a significant (p < 0.05) reduction in stem diameter growth compared to control trees. Acid-mist treatments significantly (p < 0.01) reduced the water-soluble P content in forest litter as well as the labile inorganic P (resin-extractable) content (p < 0.05) and phosphate concentrations in equilibrium soil solution (p < 0.01) in the surface soil, compared to soils under control trees. The labile organic P content (p < 0.01) and P sorption capacity (p < 0.01) of the surface soil however were increased compared to soils under control trees. Using a sensitive root bioassay technique, based on the metabolic uptake of 32P, the acid treatment was shown to have significantly (p < 0.001) increased P stress within the trees, relative to control trees. Acid-mist applied to the tree canopies caused significant (p < 0.01) decreases in the soil pH, and marked changes in the cation exchange complex composition, though total effective cation exchange capacity of the soil showed little variation. However, base saturation was 90% in soils under control trees but decreased significantly under acid-misted trees and very significantly to less than 30% under trees given the double acid dose treatment. Consequently, the (Ca + Mg):Al ratio was reduced markedly from 9.5 ± 2.9 to 0.4 ± 0.3. The results from this study therefore support the working hypothesis that acidifying inputs induce P deficiency by slowing the cycling of phosphorus and decreasing P availability within the forest ecosystem. The effects are related to the general effects of acidifying inputs on soil pH and base status. The effects described are considered to be important factors in the forest decline syndrome.

A study of Sitka spruce plantations of varying age on stagnopodsols in Wales has shown that relatively little nitrate leaches from forests of under 30 years, but it does so to a statistically significant and increasing extent from older forests. It has been hypothesised that P and K deficiencies develop in later stages of the forest rotation which effectively limit N demand by the trees resulting in nitrate leaching. To test this hypothesis, bioassays to determine tree nutrient demands relative to supply, based on rates of uptake of isotopically labelled ions (32P, 86Rb and 15N) by live roots were applied to four stands on stagnopodsol soils in each of five Welsh forests (Beddgelert, Dyfi, Dyfnant, Hafren and Tywi). Older forest sites where stream nitrate concentrations were generally over 0.16 mg N 1−1 tended to have a higher demand for P and K, but a low demand for N, indicating that nitrate leaching may be associated with deficiencies in supply of P and K. Multivariate analysis of root bioassay data showed that sites leaching nitrate could be distinguished at P < 0.01 from those not leaching nitrate. The critical load for pollutant N inputs to the Welsh forests will therefore need to be calculated using the mass balance equation modified to take account of P and K deficiencies on certain soil types.

In 1992 and 1994, sites where oiled beach material, resulting from tanker spills at sea, had been buried in sand dunes in 1978 were located. Analysis of samples from one site (Hemsby, Norfolk) was inconclusive but samples from Pendine in south Wales contained small quantities of hydrocarbon residues and bacterial populations capable of metabolizing catechol. Gas sampling also showed elevated concentrations of volatile hydrocarbons and carbon dioxide within the deposit compared with controls in adjacent sand dunes. Following the beaching of heavy fuel oil at Pendine in January 1994, oiled sand from the beach was deposited in the nearby dune system and a monitoring experiment was set up. Evidence of hydrocarbon degradation was found soon after deposition and was especially rapid close to the previous deposit. Progress of the oil decomposition is being monitored by collection and analysis (chemical and microbiological) of core samples and in-situ measurements of gaseous emissions. Because of problems of sand being moved by the wind and consequent concentration of oil residues on the surface, alternative methods of surface stabilization by vegetation are also being tested.

Mature Sitka spruce (Picea sitchensis (Bong.) Carr.) plantations in the Beddgelert area of North Wales have comparatively large concentrations and leaching rates of soil water nitrate. To determine whether elimination of any phosphorus or potassium deficiency in the trees can result in increased demand for N and reduce nitrate leaching rates, P and K fertiliser was applied at two rates (P1K1 and P2K2) in a randomized block experiment. Soil water nitrate concentrations dropped significantly in the P2K2 treatment, compared with the Control, for two growing seasons following application of the fertiliser. There was no significant effect of the P1K1 treatment. Demand for N, P and K by the trees was estimated using root bioassay techniques. Three months after application of the fertiliser, uptake of all three nutrients from solution by excised roots was significantly lower in the P1K1 treatment, and very significantly lower in the P2K2 treatment, compared with the Control. These results imply that at least part of the demand for P and K in the trees had been satisfied by increased uptake of the applied fertiliser, resulting in an increased uptake of N. Foliar analysis confirmed that additional N was present in the trees in the P2K2 treatment. Fertiliser application is a technique which could reduce nitrate leaching rates. Only at sites where nitrate is a major component of the total inorganic anion concentration will reduction in nitrate concentration reduce acidification rates significantly.

A study of sulphur movement through a Scots pine forest involved the analysis of S and 35S in rainfall, throughfall and stem flow solutions, and aqueous extracts of needles and soil, together with measurement of total levels in soil and needles. Solution preparation procedures were developed so that the solutions could be used for liquid scintillation counting, ion chromatography and ICP/OES instrumental methods, ie., it was possible to analyse stable and radioactive sulphur fractions on the same sample solutions. Counting efficiency of the beta radiation from 35S in the study solutions ranged from 10 to 80%, with the greatest quenching occurring in soil nitric acid digestion solutions. A single non‐linear mathematical relationship was applied to correct the beta counts for energy quenching in all solutions with minimal bias when compared to the values derived from using separate factors for each solution type.

Mature plants of Calluna vulgaris. Trifolium repens and Agrostis capillaris were dug up from field sites. Plants were artificially labelled with 137Cs by uptake through the roots from a liquid medium. Shoot material was allowed to dry to give plant litters adequately labelled for leaching experiments, using microcosms subjected to a number of applied treatments. Measurements were made of 137Cs activity, potassium content and pH on the leaches from each micrososm obtained every 2 weeks over a 3-month period. The respiration rate of the litter was also measured to give an index of microbial activity. At the end of the experiment. 137Cs and K contents of the residual litter were measured. Generally, the rate of release of 137Cs from Calluna litter was slower than that from Trifolium or Agrostis. After 3 months, the loss of 137Cs from Calluna litter had stabilised at about 30–40% of the original (i.e. 60–70% remained in the litter). Trifolium had lost 60% of the initial 137Cs over the same period, whilst Agrostis had lost about 70%. Repeated drying and rewetting treatment, compared to a continuous moistened state, resulted in the stabilisation of 137Cs within Trifolium and Agrostis litters. In Agrostis and Trifolium, the release of potassium was greater and more rapid than the release of 137Cs, with only 10% of the original K remaining in the litter after 2 months. In Calluna, the release of K resembled the release of 137Cs more closely.

A test for nitrogen deficiency in forest trees was developed, in which the uptake of 15N from a solution of labelled ammonium chloride by excised roots was measured. Laboratory development of the method involved the growth of Picea sitchensis (Bong.) Carr and Pinus sylvestris L. seedlings in sand culture supplied with nutrient solution containing varying concentrations of nitrogen as ammonium nitrate. When the roots of these seedlings were assayed, a negative curvilinear relationship was demonstrated between 15N uptake and the concentration of nitrogen previously supplied in sand culture. No differences were found in the total corrected rate of nitrogen uptake from 20 or 98% 15N-enriched solutions. In field trials of the bioassay, roots were excised from a Picea abies (L.) Karst. mixture experiment and from P. sitchensis nitrogen fertilization experiments. The pattern of 15N up-take by these roots indicated that the method is potentially useful as a rapid test for nitrogen deficiency in forest trees.

Two idealised Se budget scenarios have been generated to assess the role of atmospheric cycling on the Se status of UK agricultural grasslands. Based on a thorough literature review, the budgets demonstrate that varying the factors for losses by volatilisation from plants and soils or inputs by deposition may significantly affect the net balance of a grassland. The contribution of wet, dry, vapour and particulate deposition inputs to the soil-herbage system are assessed and are thought to be dominated by a soluble form in wet deposition. The total input is calculated to be typically in the range 0.2–0.7 mg m −2 year −1 . Volatile losses from the soil are difficult to derive accurately, but are estimated to range between 0.1 and 10 mg m −2 year −1 . The budget suggests there is a net input of Se to herbage from the atmosphere, even in remote/rural locations, where inputs are generally low. The implications for the availability of Se to plants and grazing livestock are briefly considered.

Agrostis capillaris (Agrostis) and Calluna vulgaris (Calluna), two species with differing phenologies and widespread presence in upland areas of Britain where high Chernobyl fallout occurred, were grown in pot culture with varying concentrations of potassium in the rooting medium. Tissue content of potassium increased with increasing supply in both species. Roots, excised from these plants, were placed in a solution of 137-Cs-labelled caesium chloride for 15 min to determine uptake potential. There were clear negative relationships between the rate of uptake of 137Cs by both species and (a) the concentration of potassium supplied and (b) plant tissue potassium concentrations. With Agrostis, there was an approximately ten-fold difference in 137Cs uptake between potassium-deficient and optimum plants; with Calluna, it was approximately eight-fold. These results demonstrate the suppression of 137Cs uptake into plants by potassium supply.

Using a root bioassay, a peak of phosphorus demand of a developing series of Sitka spruce (Picea sitchensis (Bong.) Carr) plantations, growing on a peaty-gley soil in upland Great Britain, has been shown to occur at canopy closure. Demonstration of the increased demand by the forest at this stage of development agrees well with theoretical considerations of nutrient immobilization but does not correlate to changes in foliar P concentration nor to soil chemical analysis. It does, however, correlate with litter depth, indicating a relationship between peak demand by the crop due to maximal growth rate, and reduced soil supply due to nutrient accumulation in recalcitrant organic matter prior to the development of efficient retranslocation in the canopy. The bioassay detected true nutrient demand; this was alleviated by phosphorus supply at 100 kg ha−1 which resulted in significant increase in girth. The potential of the bioassay as a management tool, allowing detection and subsequent alleviation of phosphate deficiency by fertilization at a critical stage in the development of the crop, is discussed.

The NIPHYS/CANIF project of the EEC has provided the unique opportunity to examine forest ecosystem processes and diversity along a transect through Europe ranging from north Sweden to central Italy. The main objectives of this study were (1) to identify and quantify effects of N deposition on ecosystem processes, particularly the C cycle, by extending the range of observations across a deposition maximum in central Europe, and (2) to study feedback effects between ecosystem processes and biodiversity. The study resulted in a very comprehensive and consistent set of data on ecosystem processes over a 5-year period. However, in contrast to earlier large-scale ecosystem studies (IBP: Reichle 1981; Acid Rain Programme: Last and Watling 1991), the samples were collected and data generated by the same scientists at all sites. This assured comparisons of results on a broad geographic scale. In addition, key parameters were assessed by different methods, and integrating parameters were collected for different processes, in order to test and verify predictions made at higher and lower scales, ranging from physiological responses to ecosystem level processes.

Research on effects of air pollutants on forests concentrated initially on quantifying wet deposition into ecosystems, because of its significance in acidifying soils (Ulrich 1987; Last and Watling 1991) and because it can be easily monitored. By contrast, the deposition of pollutant gases has not received an equivalent amount of attention, even though the ecological importance of this process has long been recognised (Nilgard 1985; Roelofs et al. 1985). Understanding of gas interactions with canopies has led to a general explanation of the processes leading to forest decline (Schulze 1989). The processes involved in deposition and canopy uptake of pollutant N have remained difficult to quantify due to their complexity (e.g. Duyzer et al. 1992; Hanson and Lindberg 1991; Joslin et al. 1990) and a lack of adequate techniques to measure uptake fluxes directly under field conditions. Thus, estimates of the amounts of nitrogen entering into the ecosystem directly via the canopy, bypassing soils and roots, and the induced physiological responses in the trees and ground flora, have been assessed only by indirect methods (Pearson and Stewart 1993; Sutton et al. 1993).

In forest ecosystems, physical, chemical and biological processes which regulate the uptake and flow of matter cause differences in the isotopic signatures of N, C and S compounds due to fractionation. Thus, tracing processes of isotopic fractionations in forest ecosystems can help identify transfer pathways and capacities. Atmospheric nitrogen pollution has been recognised as the cause for formerly N-limited forests to approach N saturation (Aber et al. 1989, 1998). The increase in N availability in forest ecosystems may have consequences for net ecosystem productivity on a local scale, but also for the carbon budget of terrestrial ecosystems on a global scale (Schulze 1994; Schimel 1995). Clearly, knowledge about the amount of N being deposited and cycling through forest ecosystems is of paramount importance for the carbon assimilation of the terrestrial biosphere (Lloyd and Farquhar 1996). In a similar manner, information on stable sulphur isotopes is of importance regarding pollution inputs and their effect on ecosystem health (Krouse 1989; Gebauer et al. 1994).

The NIPHYS/CANIF sites Fig. 2.1 were chosen to represent a latitudinal, climatic and deposition transect through Europe containing conifer and broadleaf stands. The main species are Norway spruce, Picea abies L. (Karst.) and European beech, Fagus sylvatica L. In southern France, Picea and Fagus are lacking at suitable sites and were replaced by maritime pine, Pinus pinaster Ait. (P. maritima Lam.), Aleppo pine, P. halepensis Mill. and white oak, Quercus pubescens Willd. for some of the studies. F. sylvatica has its most northern distribution in southern Scandinavia. Therefore, in northern Sweden, F. sylvatica was replaced with birch, Betula pubescens Ehrh.

The CANIF project experimentally investigates the carbon and nitrogen flows in Picea abies and Fagus sylvatica forest stands. The experimental subprojects encompass a large diversity of research subjects ranging from root studies for uptake of nutrients, root turnover, the diversity and the role of mycorrhizae, soil fauna and microorganisms, soil organic matter dynamics, tree growth and nutrient relations to measurements of leaching fluxes. Only by putting all these different aspects together is an overview of the functioning of the ecosystem possible. Process-based models that incorporate the carbon, nutrients and water flows of the ecosystem are very appropriate to use for this integrative function. Their great advantage is that such models with site-specific input data for climate and deposition levels can highlight the major differences between sites.

Radioactive sulphate (35SO4) was applied to the soil below a Scots pine forest on 23 June 1989, and its movement into the canopy and into throughfall and stemflow was measured over 4 months. The specific activity, Bq (mg S)(-1), of the canopy increased monotonically; uptake by current-year (1989) expanding needles was initially twice as fast as by older needles or live twigs. By 10 October the canopy average specific activity was 62 Bq (mg S)(-1). The specific activity of net throughfall (throughfall + stemflow - rain), deduced from measurements from six throughfall collectors, six stemflow collectors and two rain collectors, fell rapidly from 12.6 Bq (mg S)(-1) in late July to1 Bq (mg S)(-1) in mid-August. The results suggest (assuming rapid equilibration of 35S with sulphate in soil) that root-derived sulphate contributed c. 3% of sulphate in net throughfall and that dry deposition of SO2 and sulphate particles contributed c. 97% of the 0.56 g S m(-2) measured in net throughfall over the period. Simultaneous measurements of SO2 at canopy height and of NH3 above and within the canopy gave mean concentrations of 5.9 and 0.86 microg m(-3), respectively, sufficient to account for the sulphate measured in net throughfall only if codeposition of NH3 and SO2 occurred to canopy surfaces. The large values of specific activity observed in July, however, indicate that throughfall composition may be closely related to recent soil input of sulphate, and that equilibrium cannot be safely assumed. The possibility of a significant contribution of soil-derived sulphate to sulphate deposition in net throughfall cannot be ruled out on the basis of this experiment.

The role of nitrogen (N) in limiting herbage production was assessed in an altitudinal sequence of Agrostis-Festuca grasslands in the Moor House National Nature Reserve. For most of the growing season the concentration of ammonium-N found in the soils at the times of sampling greatly exceeded that of nitrate-N. Laboratory determinations of soil N mineralization rates at field growing season temperatures showed that mineral N accumulated almost entirely as nitrate in samples from lower altitudes and as ammonium in soil samples taken at the highest altitude. A linear relationship between the mean annual concentration of N in the herbage and primary production in the four grassland sites suggests that soil N supply was a limiting factor (...)

Retrospective analysis of archived soil and herbage samples from Rothamsted Experimental Station, southeast England, has determined the long-term changes in selenium deposition over the last century. Three out of four soils (those under permanent grassland, or growing wheat and barley) accumulated Se at a rate of circa 0.15% y(r)-1 (rate based on Se concentration, normalized to the earliest date circa 100 years earlier), with a net flux in the order 60-90 mug m-2 yr-1. The increase in soil growing root crops was smaller, with an increase of only 0.07% yr-1, possibly reflecting larger volatilization losses from this soil. Herbage samples were sensitive to changes in air composition. In the earlier half of the twentieth century there was an increase in the selenium content of herbage, probably from increased atmospheric deposition following increased use of fossil fuels. However, following the Clean Air Act (1956) the atmospheric loading of Se at this UK site appears to have declined, with contemporary Se concentrations in herbage considerably lower than they were in the 1970s, probably reflecting a change in fossil fuel usage from coal to oil and gas. The atmosphere has been a significant source of Se to plants and therefore grazing livestock. If the decline in the atmospheric input of selenium to herbage continues, selenium deficiency in livestock may become more prevalent in areas where soil concentrations are marginal.

Potential phosphorus (P) uptake from various depths by trees in a 30-year-old birch stand located at Brathens, Banchory, Scotland, on a brown podzolic soil, was studied using 32 P injected at 0, 10, 25 and 50 cm depth at 40 points within a 0.5 m radius of each tree. Two experiments were carrried out, the first using 8 trees, the second using 16 trees. The ground vegetation had been hand weeded within a 3 m radius of each tree to remove competition for 32 P uptake. (…)

Patterns of 14C enrichment in the superficial plant debris and mineral soil horizons of an established woodland have been monitored at regular intervals during the past 15 years. These data are compared with a model evaluation of carbon turnover based on the recorded changes in atmospheric 14C concentration since AD 1900.Leaf litter and decomposing plant debris are characterized by steady-state turnover values of ca 2 and ca 8 years, respectively. A two-component system of ‘fast’ (≤20 yr) and ‘slow’ (ca 350 yr) cycling carbon is indicated for the surface (0–5cm) soil humus; below 10cm, the “fast’ component is rare (Selective microbal humification of leaf litter, branch, and root debris is proposed to explain a delay of several years in the peak transfer of ‘bomb’ 14C to the soil carbon pool.

Summary A bioassay method has been developed to test for potassium deficiency in forest trees; it is based on the metabolic uptake by excised roots of 86Rb from 5 × 10−6 M rubidium chloride solution during a period of 15 min. Betula pubescens Ehrh. seedlings were grown in sand culture supplied with Hewitt's nutrient solution containing K at concentrations between 1 and 200 mg l−1. A negative curvilinear relationship was demonstrated between uptake by the seedlings of 86Rb from the rubidium chloride solution and the concentration of K previously supplied in the growth medium. When roots of intact plants were immersed in 86Rb-labelled solution for 15 min, less than 1% of 86Rb taken up was translocated to the shoots. A similar response curve, relating 86Rb uptake to K supplied to growing plants, was demonstrated when Picea sitchensis (Bong.) Carr seedlings were grown in peat culture with K added at 0 to 400 mg l−1. In three field trials of the bioassay, roots were excised from P. sitchensis and Pinus contorta Douglas ex Loud, growing on peat or peaty gley plots where different amounts of fertilizer had been added to give a range of degrees of K deficiency or from unfertilized, control plots. The patterns of 86Rb uptake by excised roots indicated that the method is potentially useful for the rapid determination of K deficiency in forest trees.

The distribution of the elements Cd, Cr, Cu, Mn, Ni, Pb, Sr, V and Zn has been examined in the horizons of soils under aged Sitka spruce ( Picea sitchensis (Bong.) Carr.) stands at a plantation in Northern England. The stands are under first-generation cultivation and are up to 33 years old. Cadmium, Mn, Pb and Zn concentrations were consistently higher in the organic layers than in the underlying mineral soil. This contrasted with the situation for Sr and V. Cadmium, Pb and Zn all showed an increase in concentration in the L + F horizons with stand age and a corresponding increase in the difference between L + F and H + mineral horizon concentrations. Soil pH declined with increasing stand age. Cadmium, Cr, Pb and Zn were all present at higher concentrations in the F horizon than in any other, while Cu and Ni were relatively constant through all the horizons studied. For all nine elements, the H horizon was the largest store of the three organic layers. Calculated rates of accumulation of Cd, Pb and Zn in the L + F horizons gave good agreement with estimated regional atmospheric deposition rates. In comparison to atmospheric deposition, biological mobilisation and deposition of Cd, Pb and Zn make a relatively minor contribution to the surface soil metal burden. Cadmium appeared to be the most readily leached of these three metals from the forest floor, although some transfer of atmospherically-derived Pb to the H + mineral soil horizons was indicated.

Transfer of selenium (Se) to pasture is important for prevention of Se deficiency in livestock, yet little is known about the relative importance of inputs to pasture from soil and the atmosphere. An isotope dilution method was used to assess quantitatively the importance of these inputs to ryegrass. Soil was labelled with 75Se and subjected to two field treatments that were untreated (pH 6.0) and limed (pH 7.0). After an initial period of equilibration, the specific activity of Se associated with unwashed leaves became lower than that of soil. This indicated that atmospheric Se had been deposited onto and possibly incorporated into the ryegrass. The percent contribution of 75Se in pasture leaves derived from the soil was 47% (pH 6.0) and 70% (pH 7.0), with, by inference, the remainder coming from the atmosphere.

(1) The precision of estimates of soil bulk density from organic matter content, expressed as % loss-on-ignition of dry weight, was examined for eight groups of data collected from surface and sub-surface soils on various sites by different workers. (2) The regression derived by pooling all data for surface soils differed significantly in elevation from its counterpart for sub-surface soils. (3) Regressions for individual groups of data for both surface and sub-surface soils, differed in slope and elevation from their respective overall equations and from each other. (4) For greatest precision in estimating soil bulk-densities, the authors recommend research workers to derive equations for their own research sites rather than to rely on general equations.