The vertical distribution of N and K uptake in relation to root distribution and root uptake capacity in mature Quercus robur , Fagus sylvatica and Picea abies stands

We have measured the uptake capacity of nitrogen (N) and potassium (K) from different soil depths by injecting 15N and caesium (Cs; as an analogue to K) at 5 and 50cm soil depth and analysing the recovery of these markers in foliage and buds. The study was performed in monocultures of 40-year-old pedunculate oak (Quercus robur), European beech (Fagus sylvatica) and Norway spruce (Picea abies (L.) Karst.) located at an experimental site in Palsgård, Denmark. The markers were injected as a solution through plastic tubes around 20 trees of each species at either 5 or 50cm soil depth in June 2003. After 65days foliage and buds were harvested and the concentrations of 15N and Cs analysed. The recovery of 15N in the foliage and buds tended to be higher from 5 than 50cm soil depth in oak whereas they where similar in spruce and beech after compensation for differences in immobilization of 15N in the soil. In oak more Cs was recovered from 5 than from 50cm soil depth whereas in beech and spruce no difference could be detected. Out of the three investigated tree species, oak was found to have the lowest capacity to take up Cs at 50cm soil depth compared to 5cm soil depth also after compensating for differences in discrimination against Cs by the roots. The uptake capacity from 50cm soil depth compared with 5cm was higher than expected from the root distribution except for K in oak, which can probably be explained by a considerable overlap of the uptake zones around the roots and mycorrhizal hyphae in the topsoil. The study also shows that fine roots at different soil depths with different physiological properties can influence the nutrient uptake of trees. Estimates of fine root distribution alone may thus not reflect the nutrient uptake capacity of trees with sufficient accuracy. Our study shows that deep-rooted trees such as oak may have lower nutrient uptake capacity at deeper soil layers than more shallow-rooted trees such as spruce, as we found no evidence that deep-rooted trees obtained proportionally more nutrients from deeper soil layers. This has implications for models of nutrient cycling in forest ecosystems that use the distribution of roots as the sole criterion for predicting uptake of nutrients from different soil depth

Bacterial, Archaeal and Fungal Succession in the Forefield of a Receding Glacier

Glacier forefield chronosequences, initially composed of barren substrate after glacier retreat, are ideal locations to study primary microbial colonization and succession in a natural environment. We characterized the structure and composition of bacterial, archaeal and fungal communities in exposed rock substrates along the Damma glacier forefield in central Switzerland. Soil samples were taken along the forefield from sites ranging from fine granite sand devoid of vegetation near the glacier terminus to well-developed soils covered with vegetation. The microbial communities were studied with genetic profiling (T-RFLP) and sequencing of clone libraries. According to the T-RFLP profiles, bacteria showed a high Shannon diversity index (H) (ranging from 2.3 to 3.4) with no trend along the forefield. The major bacterial lineages were Proteobacteria, Actinobacteria, Acidobacteria, Firmicutes and Cyanobacteria. An interesting finding was that Euryarchaeota were predominantly colonizing young soils and Crenarchaeota mainly mature soils. Fungi shifted from an Ascomycota-dominated community in young soils to a more Basidiomycota-dominated community in old soils. Redundancy analysis indicated that base saturation, pH, soil C and N contents and plant coverage, all related to soil age, correlated with the microbial succession along the forefiel

Rocks create nitrogen hotspots and N:P heterogeneity by funnelling rain

We postulated that soil nutrient heterogeneity arises not only through physical and biological processes in the soil, but also through emergent rocks diverting precipitation containing nutrients to the surrounding soil. To test this idea—which we call the ‘funnelling effect' of such rocks—we placed ion-exchange resin in small boxes beside rocks and in open soil on a pristine glacial forefield site in Switzerland, and measured the amounts of NH4 +, NO3 −, NO2 − and PO4 3− that were adsorbed. We also placed resin bags beneath PVC funnels of different sizes so that we could calibrate the natural funnelling effect of rocks. We obtained strong linear relationships between nitrogen (N) adsorbed and rain-collecting area of both rocks and funnels. Although the mean rain-collecting area of rocks was only 0.02m2, mean N adsorption was around 10 times higher within 1cm of rocks than further away. In contrast, phosphorus (P) was not concentrated beside rocks, so that N:P stoichiometry varied spatially. Rumex scutatus and Agrostis gigantea plants that rooted beside rocks had significantly higher foliar N concentrations than those growing further away, in line with the resin data. However, the two species showed differing responses in foliar P and N:P. We propose that R. scutatus benefits from the increased N supply by increasing its uptake of soil P, while A. gigantea is unable to do so. This study clearly demonstrates that aboveground rain-funnelling structures can produce spatial heterogeneity in N supply, thereby creating a diversity of nutritional niches for plants

The Vertical Distribution of Roots, Mycorrhizal Mycelia and Nutrient Acquisition in Mature Forest Trees

The vertical distribution of the nutrient uptake of Norway spruce (Picea abies (L.) Karst.), European beech (Fagus sylvatica L.) and pedunculate oak (Quercus robur L.) has been investigated in southern Scandinavia. Two approaches were employed. The first involved estimation of the nutrient uptake capacity of the trees at different soil depths by determining the distributions of roots, external ectomycorrhizal mycelia (EEM) and the nutrient uptake capacity of the roots located at different soil depths. The fine root biomass and length (Ø<1 mm) were determined down to a soil depth of 55 cm. The amount of EEM was estimated by measurements of the PLFA 18.2w6,9 using a new incubation technique. It was thus possible to separate the ectomycorrhizal and saprophytic mycelia. The uptake capacity of fine roots was determined by root bioassays using labelled rubidium, 86Rb+ (an analogue to potassium (K)), and ammonium 15NH4+. In the second method, direct measurements of the relative uptake capacity of the trees from different soil depths were made by injection of 15NH4+, labelled phosphorus (H232PO4- and H233PO4-) and caesium (another analogue to K) into the soil, and after 21-339 days the tracers were recovered in the foliage. Generally, the amount of EEM seemed to follow the root distribution. The uptake capacity of 86Rb+ by fine roots decreased with soil depth for oak, but was similar in beech and Norway spruce irrespective of soil depth. The nutrient uptake capacity of the tree was estimated by multiplying the root weight by the uptake capacity of the roots at the different soil horizons, as EEM followed the root distribution. In oak, the estimated uptake of K from 50 cm soil depth relative to 5 cm was lower than in beech and Norway spruce due to the low uptake capacity of the oak's fine roots in deep soil layers. Direct measurements of the K uptake capacity of trees confirmed that the oaks had a lower uptake capacity at greater soil depth than beech and Norway spruce. The relative uptake of K and N from 50 cm soil depth was higher using direct measurements than estimates using the first method. This was probably due to extensive overlapping of the soil volumes around the roots and hyphae from which nutrients can diffuse in the top layer, which decreases the uptake per unit root length. The results show that the nutrient uptake dose not always follow the root and EEM distribution in forest soils. This may be due to an overcapacity of nutrient uptake for mobile ions in the top layer and/or differences in the nutrient uptake capacity of roots at different soil depths