Evergreen trees stimulate carbon accumulation in urban soils via high root production and slow litter decomposition.

Urban soils can, when not sealed, store a considerable amount of carbon (C) especially under cool climates. Soil C sequestration is controlled by plant functional type, but the mechanisms by which plant types affect C accumulation in urbanised settings is poorly known. We selected 27 urban parks of varying ages (young: 5-15, old: >70 years) and 10 reference forests (>80 years) in southern Finland to study whether the ability of soils to store C relates to (i) the decomposition rate of different litter types (recalcitrant vs. labile), and/or (ii) organic matter (OM) input via root production among three common plant functional types (deciduous trees, evergreen trees, grass/lawn). Our results suggest that the high soil C accumulation under evergreen trees can result from low needle litter decomposability, accompanied by a low soil CO2 efflux. Furthermore, high root production by evergreen trees compared to deciduous trees and lawns, likely reflects the high % OM under evergreen trees. We showed that plant effects on C inputs and outputs are modulated, either directly or indirectly, by park age so that these effects are accentuated in old parks. Our results suggest that despite the capacity of evergreen trees to accumulate C in soils in urban parks, this capacity is far less compared to soils in forests of the same age. OM content under deciduous trees did not differ between old parks and reference forests, suggesting that the raking of leaves in the fall has a surprisingly small impact on OM and C accumulation in urban parks. Soil OM content is an important measure that controls various ecosystem services in cities and elsewhere. Therefore, increasing the proportion of evergreen trees in urban parks in cool cities is a good option to boost the ecosystem services capacity in the often strongly disturbed urban soils.

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.

Legacy effects of drought on plant growth and the soil food web.

Soils deliver important ecosystem services, such as nutrient provision for plants and the storage of carbon (C) and nitrogen (N), which are greatly impacted by drought. Both plants and soil biota affect soil C and N availability, which might in turn affect their response to drought, offering the potential to feed back on each other's performance. In a greenhouse experiment, we compared legacy effects of repeated drought on plant growth and the soil food web in two contrasting land-use systems: extensively managed grassland, rich in C and with a fungal-based food web, and intensively managed wheat lower in C and with a bacterial-based food web. Moreover, we assessed the effect of plant presence on the recovery of the soil food web after drought. Drought legacy effects increased plant growth in both systems, and a plant strongly reduced N leaching. Fungi, bacteria, and their predators were more resilient after drought in the grassland soil than in the wheat soil. The presence of a plant strongly affected the composition of the soil food web, and alleviated the effects of drought for most trophic groups, regardless of the system. This effect was stronger for the bottom trophic levels, whose resilience was positively correlated to soil available C. Our results show that plant belowground inputs have the potential to affect the recovery of belowground communities after drought, with implications for the functions they perform, such as C and N cycling.