Soil inorganic N leaching in edges of different forest types subject to high N deposition loads

We report on soil leaching of dissolved inorganic nitrogen (DIN) along transects across exposed edges of four coniferous and four deciduous forest stands. In a 64-m edge zone, DIN leaching below the main rooting zone was enhanced relative to the interior (at 128 m from the edge) by 21 and 14 kg N ha-1 y-1 in the coniferous and deciduous forest stands, respectively. However, the patterns of DIN leaching did not univocally reflect those of DIN throughfall deposition. DIN leaching in the first 20 m of the edges was lower than at 32–64 m from the edge (17 vs. 36 kg N ha-1 y-1 and 15 vs. 24 kg N ha-1 y-1 in the coniferous and deciduous forests, respectively). Nitrogen stocks in the mineral topsoil (0–30 cm) were, on average, 943 kg N ha-1 higher at the outer edges than in the interior, indicating that N retention in the soil is probably one of the processes involved in the relatively low DIN leaching in the outer edges. We suggest that a complex of edge effects on biogeochemical processes occurs at the forest edges as a result of the interaction between microclimate, tree dynamics (growth and litterfall), and atmospheric deposition of N and base cations

Utility of proximal plant sensors to support nitrogen fertilization in Chrysanthemum

Chrysanthemum morifolium Ramat. is a commonly grown ornamental worldwide. A proper timing of nitrogen (N) supply is essential for a qualitative crop and the return on investment for growers. Sub-optimal nitrogen nutrition negatively influences the commercial plant quality, while supra-optimal N has an environmental impact due to nitrate leaching. Therefore, (a) reliable indicator(s) of plant nitrogen status is/are needed. Two field-grown potted Chrysanthemum cultivars, 'Maya' and 'Orlando' were studied for three consecutive years (2016-2018). Three different N treatments were applied in order to obtain a variation in N content. Plant quality measurements consisted of plant height, diameter, leaf mass per area (2017 and 2018 only), biomass and foliar and plant N content analysis. Optical measurements were performed with a SPAD sensor (2016 and 2017) and a Dualex Scientific sensor (2017 and 2018) on leaf level and with a GreenSeeker NDVI meter on canopy level. Biomass, height and diameter tended to be smaller in the minimal fertilizer treatments. Leaf mass per area did influence the relation between N and chlorophyll measured with SPAD and Dualex. Epidermal polyphenolics measured with Dualex correlated better with foliar nitrogen than non-destructive chlorophyll measurements and the nitrogen balance index. Since abaxial epidermal polyphenolics were highly correlated with foliar nitrogen and convenient to measure in-field, we propose this measurement for decision support in Chrysanthemum fertilization. Because of cultivar and sometimes year-to-year variability, reference plots can be of help for growers and advisors. NDVI was found to be more susceptible for yearly variation, but very high correlation with several quality parameters and convenience in use make this vegetation index useful for detecting the extent of spatial quality variability and thus support site dependent N requirements to reach the desired plant diameter at the end of the growing season

Application of proximal optical sensors to fine-tune nitrogen fertilization : opportunities for woody ornamentals

Today, high amounts of residual nitrogen are regularly being reported in the open field production of hardy nursery stock. In some cases, excessive fertilizers or side-dressings are applied when circumstances are not favorable for uptake. Aquatic as well as terrestrial ecosystems are sensitive to enrichment with nutrients, but growers also benefit when losses are avoided. In this study, the potential of proximal optical sensors to optimize nitrogen fertilization was investigated in four woody species: Acer pseudoplatanus L., Ligustrum ovalifolium Hassk., Prunus laurocerasus 'Rotundifolia' L. and Tilia cordata Mill. For three consecutive growing seasons, plants were grown under three different fertilization levels to generate different nitrogen contents. Plant growth and nitrogen uptake were monitored regularly and combined with sensor measurements including Soil Plant Analysis Development (SPAD), Dualex and GreenSeeker. Here, we show that optical sensors at the leaf level have good potential for assisting growers in the sustainable management of their nursery fields, especially if leaf mass per area is included. Nevertheless, care should be taken when plants with different leaf characteristics (e.g., wax-layer, color, and leaf thickness) are measured. When all measuring years were considered, high correlations (R-2 >= 0.80) were found between area-based foliar nitrogen content and its non-destructive proxy (i.e., chlorophyll)measured by Dualex or SPAD. Based on our results, we recommend a relative rather than absolute approach at the nursery level, as the number of species and cultivars produced is very diverse. Hence, knowledge of absolute threshold values is scarce. In this relative approach, a saturation index was calculated based on the sensor measurements of plants grown in a reference plot with an ample nitrogen supply

Genetic structure and seed-mediated dispersal rates of an endangered shrub in a fragmented landscape: a case study for Juniperus communis in northwestern Europe

<p>Abstract</p> <p>Background</p> <p>Population extinction risk in a fragmented landscape is related to the differential ability of the species to spread its genes across the landscape. The impact of landscape fragmentation on plant population dynamics will therefore vary across different spatial scales. We quantified successful seed-mediated dispersal of the dioecious shrub <it>Juniperus communis </it>in a fragmented landscape across northwestern Europe by using amplified fragment length polymorphism (AFLP) markers. Furthermore we investigated the genetic diversity and structure on two spatial scales: across northwestern Europe and across Flanders (northern Belgium). We also studied whether seed viability and populations size were correlated with genetic diversity.</p> <p>Results</p> <p>Unexpectedly, estimated seed-mediated dispersal rates were quite high and ranged between 3% and 14%. No population differentiation and no spatial genetic structure were detected on the local, Flemish scale. A significant low to moderate genetic differentiation between populations was detected at the regional, northwest European scale (PhiPT = 0.10). In general, geographically nearby populations were also genetically related. High levels of within-population genetic diversity were detected but no correlation was found between any genetic diversity parameter and population size or seed viability.</p> <p>Conclusions</p> <p>In northwestern Europe, landscape fragmentation has lead to a weak isolation-by-distance pattern but not to genetic impoverishment of common juniper. Substantial rates of successful migration by seed-mediated gene flow indicate a high dispersal ability which could enable <it>Juniperus communis </it>to naturally colonize suitable habitats. However, it is not clear whether the observed levels of migration will suffice to counterbalance the effects of genetic drift in small populations on the long run.</p

Unravelling the functions of biogenic volatiles in boreal and temperate forest ecosystems

Living trees are the main source of biogenic volatile organic compounds (BVOCs) in forest ecosystems, but substantial emissions originate from leaf and wood litter, the rhizosphere and from microorganisms. This review focuses on temperate and boreal forest ecosystems and the roles of BVOCs in ecosystem function, from the leaf to the forest canopy and from the forest soil to the atmosphere level. Moreover, emphasis is given to the question of how BVOCs will help forests adapt to environmental stress, particularly biotic stress related to climate change. Trees use their vascular system and emissions of BVOCs in internal communication, but emitted BVOCs have extended the communication to tree population and whole community levels and beyond. Future forestry practices should consider the importance of BVOCs in attraction and repulsion of attacking bark beetles, but also take an advantage of herbivore-induced BVOCs to improve the efficiency of natural enemies of herbivores. BVOCs are extensively involved in ecosystem services provided by forests including the positive effects on human health. BVOCs have a key role in ozone formation but also in ozone quenching. Oxidation products form secondary organic aerosols that disperse sunlight deeper into the forest canopy, and affect cloud formation and ultimately the climate. We also discuss the technical side of reliable BVOC sampling of forest trees for future interdisciplinary studies that should bridge the gaps between the forest sciences, health sciences, chemical ecology, conservation biology, tree physiology and atmospheric science

Dry deposition and canopy exchange for temperate tree species under high nitrogen deposition

Increased deposition of atmospheric nitrogen (N) and sulphur (S) on forest ecosystems has caused changes in biogeochemical processes, which may have adverse effects on forest structure and functioning. Therefore, an accurate quantification of total atmospheric deposition is required to establish cause-effect relationships and evaluate abatement measures and mitigation strategies. However, measurements of dry deposition, which is a major fraction of total deposition on forests, are still subject to several problems and are complicated by canopy exchange processes in tree canopies. This study aimed to address existing knowledge gaps on dry deposition and canopy exchange, in particular of N, and to gain a better understanding of the influence the forest canopy exerts on these processes. Next to this, we aimed to evaluate the often-used canopy budget model to calculate total atmospheric deposition from throughfall measurements. Dry deposition and canopy exchange processes were examined at different spatio-temporal scales for five temperate tree species widespread in Western Europe, a region with a high N deposition load. The effect of forest canopy characteristics, i.e. leaf characteristics and canopy structure, on the measured response variable was examined. Retention of inorganic N from wet (ammonium (NH4+), nitrate (NO3-)) and dry (ammonia (NH3)) deposition by foliage and twigs was quantified for saplings of European beech (Fagus sylvatica L.), pedunculate oak (Quercus robur L.), silver birch (Betula pendula Roth.) and Scots pine (Pinus sylvestris L.) by means of stable N isotope (15N) tracing. Retention patterns for dissolved inorganic N were mainly determined by foliar uptake, except for Scots pine. In general, retention rates were 3-10 times higher for 15NH4+ than for 15NO3-, 2-3 times higher for deciduous species than for Scots pine and lower for developing leaves, although this was tree species dependent. The observed effects of tree species and phenological stage in foliar retention could be related to differences in leaf wettability, and not to water storage capacity. The small 15NH4+ retention by twigs was mainly due to physicochemical adsorption to the woody plant surface. Retention of 15NH4+ calculated from throughfall water was, on average, 20 times higher than retention by the plant material, indicating that a large part of the applied 15NH4+ could not be accounted for. The 15NH3 retention by leaves and twigs was affected by tree species, treatment date, applied NH3 concentration and the interaction between these factors. Maximum 15NH3 retention occurred at 5, 20 or 50 ppb, depending on the treatment date, but never at the highest NH3 concentration level. Birch, beech and oak leaves showed the highest 15NH3 retention in August, while for pine needles this was in June, and was generally higher for the deciduous species than for pine. Both leaf characteristics as well as measured 13CO2 retention did not provide a strong explanation for the observed differences in 15NH3 retention. Throughfall deposition, dry deposition and canopy exchange of major ions calculated by the canopy budget model were studied along a vertical gradient within a beech and two Norway spruce (Picea abies Karst.) canopies, located in forests in Belgium and Denmark, respectively. Throughfall and net throughfall deposition of all ions other than H+ increased significantly with canopy depth in the middle and lower canopy of the beech tree and in the whole canopy of the spruce trees. Dry deposition of all ions and canopy uptake of inorganic N and H+ occurred mainly in the upper canopy, while canopy leaching of K+, Ca2+ and Mg2+ was observed at all canopy levels. Canopy exchange was always higher during the growing season compared to the dormant season. This observational study illustrated that biogeochemical deposition models would benefit from a multilayer approach for shade-tolerant tree species such as beech and spruce. For the beech canopy, a comparison of dry deposition calculated by the canopy budget model was made with dry deposition calculated from air concentration measurements and dry deposition onto multi- and single-layered artificial foliage. The multi-layered artificial foliage showed a good agreement with the canopy budget model for coarse aerosol deposition. For NHx, the canopy budget model and the air concentration measurements yielded similar results, however, for SOx the canopy budget model yielded higher dry deposition than estimated from air concentration measurements and for NOy the model resulted in lower estimates. Combining throughfall measurements with multi-layered artificial foliage could aid to quantify dry deposition of NOy. Net throughfall and dry deposition of all variables except throughfall volume, H+ and NO3-, were significantly correlated with the volume of canopy elements above the throughfall collectors, which was measured by terrestrial laser scanning. To determine the effect of different canopy budget model approaches on atmospheric deposition estimates, we calculated the range of atmospheric deposition obtained by combining canopy budget model approaches for three typical case studies: (i) total N and potentially acidifying deposition onto a forest canopy, (ii) the ratio of these deposition variables between a coniferous and a deciduous stand and (iii) the parameters of a deposition time trend analysis. The effect of each step in the canopy budget model on the obtained range was assessed. The time step, type of precipitation data and tracer ion used in the model had a significant effect on the derived deposition in the three case studies. In addition, including or excluding canopy leaching of weak acids and canopy uptake of NH4+ and NO3- during the leafless season largely affected the results, while including or excluding canopy uptake of NO3- generally showed no effect. Our results provided more insight into the role of the forest canopy on dry deposition and canopy exchange of major ions. Furthermore, canopy retention of N was accurately quantified and methodological recommendations with regard to the quantification of dry deposition and canopy exchange for N compounds from the canopy budget model could be formulated