The plant-available pools of calcium, magnesium and potassium are assumed to be stored in the soil as exchangeable cations adsorbed on the cation exchange complex. In numerous forest ecosystems, ...despite very low plant-available pools, elevated forest productivities are sustained. We hypothesize that trees access nutrient sources in the soil that are currently unaccounted by conventional soil analysis methods. We carried out an isotopic dilution assay to quantify the plant-available pools of calcium, magnesium and potassium and trace the soil phases that support these pools in 143 individual soil samples covering 3 climatic zones and 5 different soil types. For 81%, 87% and 90% of the soil samples (respectively for Ca, Mg and K), the plant-available pools measured by isotopic dilution were greater than the conventional exchangeable pool. This additional pool is most likely supported by secondary non-crystalline mineral phases in interaction with soil organic matter and represents in many cases (respectively 43%, 27% and 47% of the soil samples) a substantial amount of plant-available nutrient cations (50% greater than the conventional exchangeable pools) that is likely to play an essential role in the biogeochemical functioning of forest ecosystems, in particular when the resources of Ca, Mg and K are low.
Accurate nutrient budgets in forest ecosystems are needed in order to plan sustainable forest management on poor soils. Such budgets require precise measurements of water and nutrient leaching ...through the soil. This study aims to characterize the hydrological processes and compute hydrological budgets occurring in a very poor and acidic soil under a 30-year-old beech stand in the Morvan Mountains (France). A forest plot was set up with rain collectors, lysimeters and TDR probes, and we used a deuterated water tracing experiment and two hydrological models (BILJOU and HYDRUS-1D) to estimate the proportion of preferential and slow convective water flow, and to compute the influence of preferential flow paths on nutrient leaching.
Preferential flow paths were evidenced by the deuterium tracing experiment. Tracer dynamic through the soil and soil water content variations were successively modeled. This approach enabled us to define the main condition leading to preferential flow generation (rainfall above 3.5mm/h) and quantify the proportion of preferential flow (54%). Finally, the computed nutrient leaching fluxes of major elements Ca, Mg, NO3 and Al were strongly increased when considering preferential flow paths. The experimental and modeling approach proved to be complementary and we recommend the use of tracing experiments for better model calibration, especially when their outputs are used to compute nutrient leaching fluxes.
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► Experimental and modeling approaches were used to water and solute transport. ► Deuterium tracing experiment evidenced preferential flow in a forest soil. ► Preferential flow was quantified by the modeling approach: 30% of rainfall. ► Preferential flow increased nutrient leaching fluxes up to 2.5 fold.
Many forest stands grow on acid and nutrient poor soils. To better understand how they cope with very low mineral resources, we investigated (1) Mg and Ca uptake in relation to depth, and (2) the ...allocation of these elements from the roots to the canopy, using a multi-isotopic (²⁶Mg,⁴⁴Ca) tracing experiment in a beech stand on a very poor soil. The distribution of the tracers in the soil was taken from van der Heijden et al. (Plant Soil 369:33–45, 2013a, Geoderma 195–196:12–22, 2013b, For Ecol Manag 293:65–78, 2013c). A model simulating Mg, Ca,²⁶Mg and⁴⁴Ca uptake was developed and applied to estimate the vertical distribution of Mg and Ca uptake in the soil profile. The vertical distribution of tracers in aboveground biomass was measured from four felled trees 2 years after the application of the tracers. The modeled distribution of root uptake in relation to depth shows differences between Mg and Ca: the main source of Mg uptake is the litter layer (circa. 43 % of total uptake) and the top mineral soil (0–5 cm) for Ca (circa. 42 %). The deeper soil layers (15–60 cm) also contribute to uptake. The study does not show clear evidence that uptake occurs in the very deep soil layers (>70 cm). The distribution of tracers in the aboveground biomass shows a vertical gradient from the stump to the canopy with no or very small amounts of tracers being observed in the foliage during the 2 years after the application of tracers. This suggests that Mg and Ca transport from roots to leaves along the xylem sap flow is quite slow. As Ca and Mg supply to the trees from deep soil horizons is not evidenced, and tracer transport from roots to the leaves is slow, we suggest that the tree internal pool of Ca and Mg may be more active than previously thought. This pool may act as a buffer when nutrient availability is in shortage.
Many forest soils are acidic and have very low plant-available pools of magnesium. Past and present sylvicultural, nutritional and/or climatic pressures endured by forest ecosystems can result in net ...losses of nutrients and ecosystem function losses. Liming with a carbonate product is an alternative to counteract these degradations but the effects of liming on the biogeochemical cycling of nutrients over time and the dynamics of Mg released from liming products are still unclear. We studied the Mg isotopes composition in four paired-treatment experimental beech forest ecosystems in northern France. At the sites where dolomitic lime was applied, the variation in exchangeable and foliar δ
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Mg demonstrated the direct contribution of dolomite-derived Mg to the replenishment of topsoil exchangeable pools and to tree nutrition improvement: dolomite-derived Mg was incorporated into the biological cycling which allows its retention on the mid to long term in the soil–plant system. At the sites limed with calcium carbonate, the changes in exchangeable and foliar Mg contents and δ
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Mg observed on the long term suggest that the applied product contained a small amount of Mg and/or that Mg cycling changed after liming, to cope in particular with the low Mg availability. Lastly, our results highlight the high δ
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Mg of the organic layer (humus): fractionation processes occurring within this layer (mineralization/ageing of organic matter, preferential retention of
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Mg) could explain these singular signatures that could greatly influence the topsoil Mg exchangeable pools.
► Mg was depleted from the soil over the 1974–2001 period. ► S and Ca atmospheric deposition has decreased since 1990 in the Morvan Mountains. ► Mg and Ca depletion occurs still due to an important ...sulphate desorption flux. ► Mg, Ca and K budgets were negative during the 2003–2008 period. ► Tree nutrition did not seem affected by small and decreasing nutrient pools.
Compared to conifers, broadleaf forests are less prone to soil and water acidification, because (1) they often have soils with larger exchangeable base cation pools, (2) the pollution scavenging capacity of deciduous trees is lower, and (3) they grow more slowly and are often less intensively managed. Since the 1980s, atmospheric deposition acidity has strongly decreased in forest ecosystems, so that the present acidification status of broadleaf forests should be improving. We used a 35year-old beech plot in the Morvan Mountains (Burgundy, France) to question past and present acidification processes in broadleaf ecosystems.
Soil exchangeable Mg, Ca and K pools measured from samples collected in 1974 and 2001 were compared and input–output budgets were computed over the 2003–2008 period. The objectives were (i) to assess Mg and Ca pool size changes over 1974–2008, (ii) to discuss the potential causes of these changes and, (iii) discuss the limits of conventional methods to study nutrient pool size changes (soil data comparison and nutrient budgets).
Soil exchangeable Mg pools decreased during the 1974–2001 period while Ca and K pools remained constant, and very small. Soil solution monitoring and input–output budgets over the 2003–2008 period suggested an ongoing loss of exchangeable Ca and Mg, partly due to the desorption of sulphate from the soil which induced Mg, Ca and K depletion. Given the very low concentration in exchangeable base cations, and assuming no change in soil spatial variability, we computed that resampling soils at 10years intervals may not unequivocally demonstrate a gain/loss of exchangeable base cations. Foliar Mg concentrations were continuously below the deficiency level, K and N concentrations decreased but Ca did not.
From this, we discuss the validity/limits of the different approaches used, how the ecosystem can cope with such low levels of nutrients, and the fluxes and processes within the ecosystem that should be investigated in this perspective.
•A typology of nutritional functioning of forest ecosystems emerges from our study.•Forest productivity can be high even when pools of nutrients in soil are low.•Chemical fertility of forest ...ecosystems needs to encompass biogeochemical cycling.•Biological cycling becomes predominant when the soil nutrient reservoir is small.
Many forest ecosystems are developed on acidic and nutrient-poor soils and it is not yet clearly understood how forests sustain their growth with low nutrient resources. In forestry, the soil chemical fertility is commonly defined, following concepts inherited from agronomy, as the pool of plant-available nutrients in the soil at a given time compared to the nutritional requirement of the tree species. In this two-part study, Part 1 (Hansson et al., 2020) showed, through the compiled dataset of 49 forest ecosystems in France, Brazil and Republic of Congo, the limits of this definition of soil chemical fertility in forest ecosystem contexts. In this study (Part 2), we investigated the nutrient pools and fluxes between the different ecosystem compartments at 11 of the 49 sites in order to better characterize the role of the biogeochemical cycling of nutrients in the chemical fertility of forest ecosystems, and in particular the roles of the biological and geochemical components of biogeochemical cycling.
The analysis of our dataset shows different types of biogeochemical functioning. When the geochemical component (inputs through mineral weathering and/or atmospheric inputs, capillary rise) is predominant, sufficient nutrients are provided to the plant-soil system to ensure tree nutrition and growth. Conversely, when the geochemical component of the cycle brings too few nutrients to the plant-soil system, the biological component (litterfall, plant internal cycling) becomes predominant in tree nutrition and growth. In the latter case, forest production may be high even when pools of nutrients in the soil reservoir are low because small but active nutrient fluxes may continuously replenish the soil reservoir or may directly ensure tree nutrition by bypassing the soil reservoir.
This study highlights the necessity to include biogeochemical cycling and recycling fluxes in the definition and diagnosis methods of soil chemical fertility in forest ecosystems. We show that the chemical fertility is not only supported by the soil in forest ecosystem but by the sum of all the ecosystem’s compartments and fluxes between these pools.
Accurately quantifying soil base cation pool sizes is
essential to interpreting the sustainability of forest harvests from element
mass-balance studies. The soil-exchangeable pool is classically ...viewed as
the bank of “available” base cations in the soil, withdrawn upon by plant
uptake and leaching and refilled by litter decomposition, atmospheric
deposition and mineral weathering. The operational definition of this soil
bank as the exchangeable (salt-extractable) pools ignores the potential role
of “other” soil nutrient pools, including microbial biomass, clay
interlayer absorbed elements, and calcium oxalate. These pools can be large
relative to “exchangeable” pools. Thus neglecting these other pools in
studies examining the sustainability of biomass extractions, or need for
nutrient return, limits our ability to gauge the threat or risk of
unsustainable biomass removals. We examine a set of chemical extraction data
from a mature Norway spruce forest in central Sweden and compare this
dataset to ecosystem flux data gathered from the site in previous research.
The 0.2 M HCl extraction released large pools of Ca, K, Mg, and Na,
considerably larger than the exchangeable pools. Where net losses of base
cations are predicted from biomass harvest, exchangeable pools may not be
sufficient to support more than a single 65-year forest rotation, but
acid-extractable pools are sufficient to support many rotations of
net-ecosystem losses. We examine elemental ratios, soil clay and carbon
contents, and pool depth trends to identify the likely origin
of the HCl-extractable pool. No single candidate compound class emerges, as very strongly supported by the data, as being the major constituent of the HCl-extractable fraction. A combination of microbial biomass, fine
grain, potentially shielded, easily weatherable minerals, and non-structural
clay interlayer bound potassium may explain the size and distribution of the
acid-extractable base cation pool. Sequential extraction techniques and
isotope-exchange measurements should be further developed and, if possible,
complemented with spectroscopic techniques to illuminate the identity of and
flux rates through these important, and commonly overlooked, nutrient pools.
•The water balance is an important factor for stand productivity in forests.•We assessed the impact of soil type on the water balance and beech growth.•Deep loamy and shallow stony soils were ...compared.•The water cycle is driven by the soil water holding capacity and root distribution.•Trees transpiration was the primary driver of stand biomass production.
Climate change and particularly increasing frequency of drought events during the vegetation period may threaten tree vitality and forest biomass productivity in many temperate regions in the future. In that context, the identification of critical environmental factors and a better understanding of their impact on forests are decisive. The water balance is recognized as one of the most important soil factors for stand productivity in temperate forests. Hence, the consequences of short or long term climate change might vary considerably spatially in function of soil type within a given forest. Our study objective was to assess the impact of contrasting soil types on the water balance and stand growth of a beech (Fagus sylvatica) forest ecosystem of similar age and management during four climatically contrasting years. The experimental forest site of Montiers presents different soils with contrasting physicochemical properties (Dystric Cambisol, Eutric Cambisol and Rendzic Leptosol) monitored to quantify water fluxes and stand biomass increment. Using data collected over the period 2012–2015, including a particularly dry year (−24% precipitations in 2015), we also quantified the impact of water shortage on stand productivity at the annual scale as a function of soil type. We evidenced important differences in soil water holding capacities (SWHC) along the studied soil sequence, ranging between 57mm for the Rendzic Leptosol downhill over limestone and 205mm for the Dystric Cambisol uphill over detrital sediments. The results show that the canopy intercepted the same amount of incident rainfall in the three plots and that there were no significant differences in annual soil moisture dynamics among the studied soils. We evidenced different rooting patterns depending on soil type. Under a same climate and with stand, site exposition and solar radiation equivalency, trees transpiration was the evident primary driver of the stand potential to produce aboveground biomass. Soil water holding capacity, annual trees transpiration and aboveground biomass production increased in that order: Rendzic Leptosol<Eutric Cambisol<Dystric Cambisol. During the drier year 2015, the decrease in aboveground biomass productivity was of similar amplitude on the three soil types.
Intensive silvicultural practices and the planting of monospecific forests of coniferous, that are more productive compared to hardwoods may threaten over the mid to long-term the sustainability of ...soil chemical fertility of forest ecosystems, and are a major concern for forest managers and policy.
We investigated the tree species effect (Quercus sessiliflora Smith, Fagus sylvatica L., Picea abies Karst., Pseudotsuga menziesii Mirb. Franco., Abies nordmanniana Spach. and Pinus nigra Arn. ssp. laricio Poiret var. corsicana) on the change over time of soil chemical properties and nutrient pool sizes in the mineral and organic layers of the soil during the 45 years after the plantation of the Breuil-Chenue common garden experiment (Burgundy, France). The organic and mineral soil layers down to 70-cm depth were sampled in the different monospecific plots in 1974, 2001 and 2019.
Exchangeable Ca and Mg pools and soil pH increased on average over time in the 0–70 cm soil profile in most stands. However, in the topsoil layers (0–15 cm), the decrease of pH, the increase of exchangeable acidity over time under the coniferous stands and the decrease of exchangeable K pools in most stands highlighted that soil acidification is still on-going at Breuil-Chenue site but the intensity of this process depends on the tree species. Indeed, three groups of species could be distinguished: i) Nordmann fir (Abies nordmanniana Spach.)/Norway spruce (Picea abies Karst.) where acidolysis and chelation occurred, resulting in the most pronounced pH decrease in the topsoil, ii) Douglas fir (Pseudotsuga menziesii Mirb. Franco.)/Laricio pine (Pinus nigra Arn. ssp. laricio Poiret var. corsicana) where acidification caused by elevated nitrification rates is probably currently compensated by larger weathering and/or atmospheric depositions fluxes, and iii) oak (Quercus sessiliflora Smith)/beech (Fagus sylvatica L.) where soil acidification was less intense. Counterintuitively, soil acidification at Breuil-Chenue site resulted in an increase in soil CEC which limited the loss of nutrient cations. This change in soil CEC was most likely explained by the precipitation/dissolution dynamics of aluminium (Al) (hydr)oxides in the interfoliar space of phyllosilicates and/or the increase in soil carbon (C) content in the topsoil layers.
After 45 years, tree species continue to exert influence on the chemical fertility of the soil and the pedogenetic processes which in turn may impact forest ecosystem functions and services.
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