Urban belowground food-web responses to plant community manipulation - Impacts on nutrient dynamics

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Decomposition of labile and recalcitrant litter types under different plant communities in urban soils

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Urbanisation differently affects decomposition rates of recalcitrant woody material and labile leaf litter

Litter decomposition is a fundamental ecosystem process and service that supplies nutrients to the soil. Although decomposition rate is influenced by litter quality, climatic conditions, the decomposer community and vegetation type in non-urban ecosystems, little is known about the degradation of different organic matter types in urban settings. We investigated the decomposition rates of recalcitrant (wood sticks for 4 years) and labile litter (green tea leaves in pyramid-shaped teabags for 3 years) in urban habitats that differed in level of management and disturbance. We found that recalcitrant woody material decomposed slower in urban habitat types (ca. 60-75% mass loss after 4 years in remnant spruce forests, park lawns, ruderal habitats) than in natural to semi-natural spruce forest soils (84% mass loss) outside the city. Labile tea litter, however, decomposed faster in typical open urban habitats (70% mass loss after 3 years in park lawns, ruderal habitats) than in forested habitats (60% mass loss in semi-natural and remnant spruce forests), with a remarkable dichotomy in decomposition rate between open and forested habitats. We suggest that the slower rate of wood decomposition in the city relates to its depauperate saprotrophic fungal community. The faster rate of labile litter decomposition in open habitats is difficult to explain, but is potentially a consequence of environmental factors that support the activity of bacteria over fungi in open habitats. We propose that the reintroduction of decaying woody material into the urban greenspace milieu could increase biodiversity and also improve the ability of urban soils to decompose an array of organic material entering the system. This reintroduction of decaying woody material could either occur by leaving cut logs - due to management - in urban remnant forests, which has been shown to be accepted as natural features by residents in Fennoscandian cities, and by placing logs in urban parks in ways that communicate their intentional use as part of urban landscape design and management.Peer reviewe

Ecosystem services and biodiversity in Europe

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Soil sealing causes substantial losses in C and N storage in urban soils under cool climate

Urban soil can store large amounts of carbon (C) and nitrogen (N). To accurately estimate C and N storage in urban soils, C and N contents underneath impervious surfaces - the most prevalent land cover type in cities - should be taken into account. To date, however, only few studies have reported urban soil C and N content underneath impervious surfaces, and no data exist for cities under cold/cool climates, such as the Boreal zone. Here, we studied, for the first time, the effects of sealing on soil C and N storage in a Boreal city. Sealed soils were sampled for physico-chemical and biological parameters from 13 sites in the city of Lahti, Finland, at three depths (0–10 and 45–55 cm, representing the construction layer composed of gravel, other moraine material and crushed rock, and the native soil layer beneath the ca. 1 m thick construction layer). Our results show that urban soils underneath impervious surfaces in Finland contain 11 and 31 times less C and N content, respectively, compared with warmer regions. This is due to a deep C and N deficient construction layer below sealed surfaces. Even though impervious surfaces cover ca. twice the area of pervious surfaces in the centre of Lahti, we estimate that only 6% and 4% of urban soil C and N, respectively, are stored underneath them. Furthermore, we found very little C and N accumulation underneath the sealed surfaces via root growth and/or leakage through ageing asphalt. Our results show that soil sealing, in concert with a massive top soil removal typical to cold climates, induces a considerable loss of C and N in Boreal urban areas.Peer reviewe

Plant functional type affects nitrogen dynamics in urban park soils similarly to boreal forest soils

Purpose Although plant functional type can modulate soils and their processes in natural, nitrogen (N)-limited ecosystems, little is known about their ability to influence soil N dynamics in urban ecosystems that have high excess N input. We investigated whether i) plant functional type effects on soil N dynamics in urban parks follow the same pattern as those in undisturbed natural/semi-natural forests, and ii) park age influences plant functional type effects on soil N dynamics under boreal climate. Methods We selected 13 urban parks of varying ages (young: 10 to 15, old: > 70 years), and 5 undisturbed natural/semi-natural forests (> 80 years) in southern Finland. In these parks and forests, we measured soil total N concentration, availability of inorganic N, nitrous oxide (N2O) flux and earthworm biomass under three plant functional types (evergreen tree, deciduous tree, lawn). Results Our results showed that plant functional type influenced N dynamics also in urban greenspace soils, which may relate to the clear effect of plant functional type on earthworm biomass. Evergreen trees tended to have the highest ability to foster N accumulation and reduce N2O emissions in urban parks. Moreover, with increasing park age, N accumulation increased under trees but decreased under lawns, further emphasising the role of vegetation in affecting soil N dynamics in urban greenspaces. Conclusions Our results show that, similar to natural/semi-natural forests, plant functional type, irrespective of park age, can influence soil N dynamics in urban parks.Peer reviewe

Using biochar to purify runoff in road verges of urbanised watersheds: A large-scale field lysimeter study

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Dog Urine Has Acute Impacts on Soil Chemistry in Urban Greenspaces

Urban residents and their pets utilize urban greenspaces daily. As urban dog ownership rates increase globally, urban greenspaces are under mounting pressure even as the benefits and services they provide become more important. The urine of dogs is high in nitrogen (N) and may represent a significant portion of the annual urban N load. We examined the spatial distribution and impact of N deposition from dog urine on soils in three urban greenspace typologies in Finland: Parks, Tree Alleys, and Remnant Forests. We analyzed soil from around trees, lampposts and lawn areas near walking paths, and compared these to soils from lawn areas 8 m away from pathways. Soil nitrate, ammonium, total N concentrations, and electrical conductivity were significantly higher and soil pH significantly lower near path-side trees and poles relative to the 8 m lawn plots. Also, stable isotope analysis indicates that the primary source of path-side N are distinct from those of the 8 m lawn plots, supporting our hypothesis that dogs are a significant source of N in urban greenspaces, but that this deposition occurs in a restricted zone associated with walking paths. Additionally, we found that Remnant Forests were the least impacted of the three typologies analyzed. We recommend that landscape planners acknowledge this impact, and design parks to reduce or isolate this source of N from the wider environment.Peer reviewe

Quantifying carbon stocks in urban parks under cold climate conditions

Removing CO2 from the atmosphere and storing carbon in vegetation and soil are important ecosystem services provided by urban green space. However, knowledge on the capacity of trees and soils to store carbon in urban parks - especially in the northern latitudes - is scarce. We assessed the amount of organic carbon stored in trees and soil of constructed urban parks under cold climatic conditions in Finland. More specifically, we investigated the effects of management, vegetation type and time since construction on the amount of carbon stored in park trees and soil. We conducted two tree surveys and collected soil samples (0 to 90 cm) in constructed parks managed by the city of Helsinki. The estimated overall carbon density was approximately 130 t per park hectare, when the carbon stock of trees was 22 to 28 t ha-1 and that of soil 104 t ha-1 at the very least. The soil to tree carbon storage ratio varied from 7.1 to 7.5 for vegetated, pervious grounds and from 3.7 to 5.0 for entire park areas. The effects of park management and vegetation type could not be entirely separated in our data, but time was shown to have a distinct, positive effect on tree and soil carbon stocks. The results indicate that park soils can hold remarkable carbon stocks in a cold climate. It also seems that park soil carbon holding capacity largely exceeds that of forested soils in Finland. Preservation and augmentation of carbon stocks in urban parks implies avoidance of drastic tree and soil renovation measures.Peer reviewe

Impacts of urban roadside forest patches on NO2 concentrations

Although it is commonly believed that trees can improve air quality, recent studies have shown that such pollution mitigation can be negligible – or that tree canopies can even increase pollutant concentrations near their sources compared to adjacent treeless areas. We explored the impacts of urban roadside forest patches on the concentrations of nitrogen dioxide in summer and winter in the Helsinki Metropolitan Area, Finland, and especially investigated if canopy cover can result in increased concentrations of NO2 below the canopy. Our results, however, did not show significantly higher – or lower – NO2 concentrations underneath tree canopies compared to levels above canopies. Neither did NO2 levels at the below-canopy sampling height differ significantly between forest patches and adjacent open, treeless areas. The lack of a canopy effect may derive from the rather small size of the forest patches, and – compared to previous studies with similar design – divergent tree species composition forming a dense canopy structure. Our results corroborate previous studies that the potential ecosystem services offered by urban near-road forests are more likely due to benefits other than those related to the removal of air pollutants.Peer reviewe