N. van Breemen

Rock-eating fungi

Weatherable minerals under many European coniferous forests contain a network of numerous tubular pores, formed by organic acids exuded by fungi. We believe that symbiotic mycorrhizal hyphae translocate dissolved minerals from the isolated micropores directly to their host plants, bypassing competition for nutrient uptake by other organisms. The discovery of this pathway challenges current ideas about nutrient uptake from the bulk soil solution and criteria for critical loads of acidic deposition on forests.

The podzolization process. A review.

Abstract This paper reviews the major mechanisms proposed to explain podzolization. These include the production of organic acids that form soluble complexes with aluminium/iron thereby enhancing weathering, followed by illuviation by precipitation/adsorption processes occurring at greater depth. Precipitation of aluminium and iron is explained by decreasing solubility of increasingly metal-rich complexes, or by microbial degradation of the organic ligand. We also discuss proposed role of inorganic hydroxy-aluminium–silicate sols in podzolization. The paper is introductory to a multidisciplinary study of podzolization performed in the Nordic Countries presented in this volume.

Where did all the nitrogen go? Fate of nitrogen inputs to large watersheds in the northeastern U.S.A.

To assess the fate of the large amounts of nitrogen (N) brought into the environment by human activities, we constructed N budgets for sixteen large watersheds (475 to 70,189 km2) in the northeastern U.S.A. These watersheds are mainly forested (48–87%), but vary widely with respect to land use and population density. We combined published data and empirical and process models to set up a complete N budget for these sixteen watersheds. Atmospheric deposition, fertilizer application, net feed and food inputs, biological fixation, river discharge, wood accumulation and export, changes in soil N, and denitrification losses in the landscape and in rivers were considered for the period 1988 to 1992. For the whole area, on average 3420 kg of N is imported annually per km2 of land. Atmospheric N deposition, N2 fixation by plants, and N imported in commercial products (fertilizers, food and feed) contributed to the input in roughly equal contributions. We quantified the fate of these inputs by independent estimates of storage and loss terms, except for denitrification from land, which was estimated from the difference between all inputs and all other storage and loss terms. Of the total storage and losses in the watersheds, about half of the N is lost in gaseous form (51%, largely by denitrification). Additional N is lost in riverine export (20%), in food exports (6%), and in wood exports (5%). Change in storage of N in the watersheds in soil organic matter (9%) and wood (9%) accounts for the remainder of the sinks. The presence of appreciable changes in total N storage on land, which we probably under-rather than overestimated, shows that the N budget is not in steady state, so that drainage and denitrification exports of N may well increase further in the future.

Ecosystem effects of atmospheric deposition of nitrogen in The Netherlands.

Atmospheric deposition of inorganic N, mainly ammonium volatilized from manure produced in intensive stockbreeding, on sensitive terrestrial and aquatic ecosystems in The Netherlands is in the order of 40 to 80 kg ha(-1) year(-1). Proven effects of this deposition are (i) eutrophication with N, leading to floristic changes (ii) acidification of base-poor sandy soils and of moorland pools, leading to higher concentrations of dissolved, potentially toxic metals such as Al3+, and (iii) increased levels of nitrate in groundwater below woodlands. In acid forest soils, but not in soils under heathland, nitrification and leaching of nitrate is common. However, in very poor sandy forest soils and at very high ammonium inputs, nitrification may be too slow to prevent the development of high concentrations of ammonium. Both excessive acidification and excessive levels of ammonium probably play an important role in the general forest decline, which is most severe in the southern and central parts of the country, where ammonium inputs are highest.

Plant-soil interactions: ecological aspects and evolutionary implications.

Building on the concept of plants as ecosystem engineers, and on published information on effects of particular plant species on soils, we review the evidence that such effects can provide a positive feedback to such plants. Based on case studies involving dune formation by Marram grass, N supply by N2-fixing plants, depression of N availability by ericaceous plants, ‘islands of fertility’ in deserts, mull- and mor-forming temperate forest trees, and formation of peatbogs, as well as similar other cases, we conclude that there is strong evidence for plant-soil feedbacks in a variety of ecosystems. We argue, moreover, that these feedbacks could have played a role in the evolution of the plant species in question. These ideas are based mainly on correlative observations, and need further testing.

Advances in understanding the podzolization process resulting from a multidisciplinary study of three coniferous forest soils in the Nordic Countries

Advances in understanding the podzolisation process resulting from a multidisciplinary study at three coniferous forest soils in the Nordic countries

Soils as biotic constructs favouring net primary productivity

Abstract Many, if not most, physical and chemical properties of soils required for plant growth are affected strongly by biotic processes. Feedback processes involving primary producers and decomposers may be involved in the development of properties that favour net primary productivity in terrestrial ecosystems. However, both constructive and destructive effects of biota on soils can be observed. Apparently, effects favouring net primary production have accumulated and presently prevail in the various terrestrial ecosystems of the world. In some ecosystems, however, the dominant vegetation gains competitive advantage by making soils un “favourable” for most other plants. Ombrotrophic peat bogs and heathlands are cases in point. On the global scale, biotic processes can be seen as responsible for the persistence of water on the earth, through control of the earths surface temperature under the influence of greenhouse gases. As a result, therefore, the large-scale geochemical and hydrological cycles, which are essential for chemical rejuvenation of the earths surface, also depend on life processes. This is an aspect of Lovelocks Gaia hypothesis, which states that the earth has evolved over geologic time by feedback processes keeping the earth in a state comfortable for life by the action of living organisms. While the development of soils with “favourable” properties may be explained in evolutionary terms, such an explanation apparently does not suffice for the development of Gaia as a whole.

Soil organic carbon dynamics: variability with depth in forested and deforested soils under pasture in Costa Rica.

Dynamics of soil organic carbon (SOC) inchronosequences of soils below forests that had beenreplaced by grazed pastures 3–25 years ago, wereinvestigated for two contrasting soil types (AndicHumitropept and Eutric Hapludand) in the Atlantic Zoneof Costa Rica. By forest clearing and subsequentestablishment of pastures, photosynthesis changes froma C-3 to a C-4 pathway. The accompanying changes inC-input and its δ13C and 14Csignals, were used to quantify SOC dynamics. C-input from rootturnover at a pasture site was measured by sequentialharvesting and 14C-pulse labelling. With aspatial resolution of 5 cm, data on total SOC,δ13C and δ14C of soil profileswere interpreted with a model that distinguishes threepools of SOC: ‘active’ C, ‘slow’ C and ‘passive’ C,each with a 1-st order decomposition rate(ka, ks and kp). The modelincludes carbon isotope fractionation and depth-dependentdecomposition rates. Transport of C between soillayers was described as a diffusion process, whichaccounts for physical and biotic mixing processes.Calibrated diffusion coefficients were 0.42 cm2yr-1 for the Humitropept and 3.97 cm2yr-1 for the Hapludand chronosequence.Diffusional transport alone was insufficient foroptimal simulation; it had to be augmented bydepth-dependent decomposition rates to explain thedynamics of SOC, δ13C andδ14C. Decomposition rates decreasedstrongly with depth. Upon increased diffusion,differences between calibrated decomposition rates ofSOC fractions between surface soils and subsoilsdiminished, but the concept of depth-dependentdecomposition had to be retained, to obtain smallresiduals between observed and simulated data. At areference depth of 15–20 cm ks was 90 yr-1in the Humitropept and 146 yr-1 in the Hapludand.Slow C contributed most to total organic C in surfacesoils, whereas passive C contributed most below 40 cmdepth. After 18–25 years of pasture, net loss of C was2180 g C m-2 for the Hapludand and 150 g m-2for the Humitropept soil.

Uncertainties in the fate of nitrogen I: an overview of sources of uncertainty illustrated with a Dutch case study.

This study focuses on the uncertainties in the ‘fate’ of nitrogen (N) in the Netherlands. Nitrogen inputs into the Netherlands in products, by rivers, and by atmospheric deposition, and microbial and industrial fixation of atmospheric N2 amount to about 4450 Gg N y−1. About 60% of this N is transported out of the Netherlands in products. The fate of the remaining 40%, however, is less clear. We discuss uncertainties in losses to the atmosphere (as ammonia or through denitrification), by leaching and runoff, and in N accumulation in biomass and soils. These processes may account for the fate of about 40% of the N in the Netherlands, and for the fate of about 60% of the N in Dutch agricultural soils. Reducing uncertainties in the estimates of these fluxes is necessary for reducing the impact of excess N in the environment. In particular, monitoring the environmental effects of ammonia emissions and nitrate leaching to groundwater and aquatic systems requires an increased understanding of the fate of N. Uncertainties arise because (1) some N fluxes cannot be measured directly and are usually quantified indirectly as the balance in N budgets, (2) direct measurements of N fluxes have inevitable inaccuracies, (3) lack of experimental data and other information (e.g. statistics) needed for upscaling, (4) large spatial and temporal variability of fluxes, and (5) poor understanding of the processes involved. These uncertainties can be reduced by additional experimental studies and by further development of process-based models and N budget studies. We prioritize these future research needs according to a range of different criteria.

NITREX: responses of coniferous forest ecosystems to experimentally changed deposition of nitrogen

Abstract In large regions of Europe and eastern North America atmospheric deposition of inorganic nitrogen compounds has greatly increased the natural external supply to forest ecosystems. This leads to nitrogen saturation, in which availability of inorganic nitrogen is in excess of biological demand and the ecosystem is unable to retain all incoming nitrogen. The large-scale experiments of the NITREX project (nitrogen saturation experiments) are designed to provide information regarding the patterns and rates of responses of coniferous forest ecosystems to increases in N deposition and the reversibility and recovery of impacted ecosystems following reductions in N deposition. The nitrogen input-output data from the NITREX sites are consistent with the general pattern of nitrogen fluxes from forest ecosystems in Europe. At annual inputs of less than about 10 kg ha−1 year−1, nearly all the nitrogen is retained and outputs are very small. At inputs above about 25 kg ha−1 year−1 outputs are substantial. In the range 10–25 kg ha−1 year−1 these forest ecosystems undergo a transition to nitrogen saturation. The 10 kg ha−1 year−1 apparently represents the minimum threshold for nitrogen saturation. The NITREX experiments indicate that nitrogen outputs respond markedly across the 10–25 kg ha−1 year−1 range of inputs. In contrast, the nutrient concentrations in foliage, a measure of tree response, is delayed by several years. Nitrogen saturation can apparently be induced or reversed within only a few years, at least with respect to the commonly used diagnostic of nitrogen saturation-nitrogen output in leachate or runoff.