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The Loess Plateau in China constitutes an area short of soil nitrogen and organic carbon due to local land degradation induced by various factors (i.e. long term changes of land use, climate conditions, and soil properties). The present study aimed to examine the effects of species and land management by afforestation on tree N acquisition capacity and soil N and C availability in degraded soils of the Loess Plateau area. We quantified root N uptake of inorganic and organic N sources as well as soil N and C availability, both at the tree species (i.e. Robinia pseudoacacia L., Juglans regia L. and Pinus tabulaeformis Carr.) and the land management (i.e. arable vs. monoculture vs. mixed afforested stands) levels. Our results indicated that afforestation improved soil N and organic C availabilities compared to abandoned arable land ( p < 0.05). In particular, the presence of N 2 -fixing R. pseudoacacia enhanced root N concentrations (ca. 3.0 times) and soil NO 3 - (ca. 5.4 times), soil total N (ca. 1.9 times) and organic C (ca. 3.4 times) availabilities, but decreased soil NH 4 + (ca. −33%), microbial biomass carbon (ca. −74%) and nitrogen (ca. −54%) in the mixed stand compared to J. regia monoculture. Under the experimental conditions applied, the afforested trees preferred organic over inorganic N compounds as well as NO 3 − over NH 4 + ; J. regia in monoculture had a highest root amino acids N uptake capacities (i.e. 76.6 ± 7.7 nmol N (g fw) − 1 h −1 for glutamine, 90.3 ± 8.9 nmol N (g fw) − 1 h −1 for arginine) compared to other tree species whereas such high uptake capacities were largely repressed in the mixed stand with R. pseudoacacia . Thus, in the Loess Plateau area, the inter-planting system of J. regia with N 2 -fixing R. pseudoacacia could improve the total soil N and organic C pools as well as plant N cycling compared to traditional arable land use and J. regia monoculture system. This study shows that inter-planting R. pseudoacacia with economic fruit trees can be considered a successful strategy for soil regeneration by afforestation in future land management projects. [ABSTRACT FROM AUTHOR]

European beech forests growing on marginal calcareous soils have been proposed to be vulnerable to decreased soil water availability. This could result in a large-scale loss of ecological services and economical value in a changing climate. In order to evaluate the potential consequences of this drought-sensitivity, we investigated potential species range shifts for European beech forests on calcareous soil in the 21st century by statistical species range distribution modelling for present day and projected future climate conditions. We found a dramatic decline by 78% until 2080. Still the physiological or biogeochemical mechanisms underlying the drought sensitivity of European beech are largely unknown. Drought sensitivity of beech is commonly attributed to plant physiological constraints. Furthermore, it has also been proposed that reduced soil water availability could promote nitrogen (N) limitation of European beech due to impaired microbial N cycling in soil, but this hypothesis has not yet been tested. Hence we investigated the influence of simulated climate change (increased temperatures, reduced soil water availability) on soil gross microbial N turnover and plant N uptake in the beech-soil interface of a typical mountainous beech forest stocking on calcareous soil in SW Germany. For this purpose, triple 15N isotope labelling of intact beech seedling-soil-microbe systems was combined with a space-for-time climate change experiment. We found that nitrate was the dominant N source for beech natural regeneration. Reduced soil water content caused a persistent decline of ammonia oxidizing bacteria and therefore, a massive attenuation of gross nitrification rates and nitrate availability in the soil. Consequently, nitrate and total N uptake of beech seedlings were strongly reduced so that impaired growth of beech seedlings was observed already after one year of exposure to simulated climatic change. We conclude that the N cycle in this ecosystem and here specifically nitrification is vulnerable to reduced water availability, which can directly lead to nutritional limitations of beech seedlings. This tight link between reduced water availability, drought stress for nitrifiers, decreased gross nitrification rates and nitrate availability and finally nitrate uptake by beech seedlings could represent the Achilles' heel for beech under climate change stresses.