The return of an experimentally N-saturated boreal forest to an N-limited state: observations on the soil microbial community structure, biotic N retention capacity and gross N mineralisation

Abstract

Background and aims To find out how N-saturated forests can return to an N-limited state, we examined the recovery of biotic N sinks under decreasing N supply. Methods . We studied a 40-year-old experiment in Pinus sylvestris forest, with control plots, N0, three N treatments, N1-N3, of which N3 was stopped after 20 years, allowing observation of recovery. Results In N3, the N concentration in foliage was still slightly elevated, but the N uptake capacity of ectomycorrhizal (ECM) roots in N3 was no longer lower than in N0. Per area the amount of a biomarker for fungi, here mainly attributed ECM, was higher in N3 and N0 than in N1 and N2. Retention of labeled 15 NH 4 + by the soil was greater in the control (99 %) and N3 (86 %), than in N1 (45 %) and N2 (29 %); we ascribe these differences to biotic retention because cation exchange capacity did not vary. Gross N mineralisation and retention of N correlated, negatively and positively, respectively, with abundance of ECM fungal biomarker. Conclusions . The results suggest a key role for ECM fungi in regulating the N cycle. We propose, in accordance with plant C allocation theory, that recovery is driven by increased tree below-ground C allocation to ECM roots and fungi.

Diversity and Enzyme Activity of Ectomycorrhizal Fungal Communities Following Nitrogen Fertilization in an Urban-Adjacent Pine Plantation

“…ECM fungal richness, diversity, and evenness were not significantly different across N fertilization treatments. These results reconcile with earlier studies in oak [30] and spruce forests [3,[47][48][49], but seemingly contradict the majority of studies that show a rapid and substantial decline in ECM fungal diversity following N fertilization [e.g., 6,[9][10][11]30] and increasing soil N levels along natural gradients of productivity [50]. This result may reflect the short period (three years) over which the plots were fertilized [6,51] or (more likely) that high levels of ambient N deposition may have pre-empted any effects of the experimental N additions.…”

“…As a result, mycorrhizal diversity may underpin many forest ecosystem services including nutrient cycling and water use efficiency. Although studies have widely demonstrated declines in ECM diversity and changes in ECM community composition following N-enrichment [3,[6][7][8][9][10][11][12], the extent to which these changes influence the functional capacity of ECM is less well understood [13][14][15][16]. In this study, we examined the diversity and functioning in ECM communities under ambient N deposition and following N fertilization in an urban-adjacent Pinus elliottii plantation.…”

Forest Soil Bacteria: Diversity, Involvement in Ecosystem Processes, and Response to Global Change

“…In contrast to global warming and increased levels of CO 2 , however, their effects are largely local. Since N is a limiting nutrient in most terrestrial ecosystems, including boreal and temperate forests (255), anthropogenic N deposition may strongly influence NPP in these environments by reducing this limitation (256). N deposition also has a range of additional consequences that range from shifts in the soil C/N ratio, soil acidification, and root exudation to changes in the vegetation and microbiota (257).…”

“…There appears to be a general consensus that N deposition increases soil C sequestration due to the decline in SOM decomposition via the reduction of biomass and activity of soil microbial populations in many different soil environments, including temperate and boreal forests (258,(260)(261)(262)(263)(264). The most probable reason for C sequestration is the observed reduction of tree root exudation (256), although direct effects of a pH decrease induced by N deposition were proposed as an alternative explanation (265). Although the reduction of biomass following N input was mostly observed in fungi (256,266), it was recently noted that the biomass of soil bacteria can be reduced as well, by up to 50% (258,263,267).…”

“…The most probable reason for C sequestration is the observed reduction of tree root exudation (256), although direct effects of a pH decrease induced by N deposition were proposed as an alternative explanation (265). Although the reduction of biomass following N input was mostly observed in fungi (256,266), it was recently noted that the biomass of soil bacteria can be reduced as well, by up to 50% (258,263,267). Reduction of the bacterial biomass, however, depends on the level of N input.…”