What happens to trees and soils during five decades of experimental nitrogen loading?

High deposition of nitrogen was postulated to drive losses of NO3 - and nutrient base cations, causing soil acidification, nutrient deficiencies reducing tree growth and ultimately tree mortality. We tested these predictions in a uniquely long-term study involving three NH4NO3 addition treatments (N1-N3) in a boreal Pinus sylvestris forest. The lowest level (N1), 30 kg N ha− 1 yr− 1 was applied during 50 years. Twice this rate (N2) was added 38 years, followed by 12 years of recovery, while thrice this rate (N3) was added 20 years followed by 30 years of recovery. We compared tree growth, changes in foliar and soil chemistry among treatments including control plots without N additions. As predicted, the N treatments lowered soil pH and reduced soil base saturation by around 50 %. They also lowered foliar levels of Ca, Mg, K, P and B initially, but after 50 years only Ca and Mg remained lower than in the control. Lack of B motivated a single addition of 2.5 kg ha− 1 after ten years of N treatment. Tree stem growth became and then remained higher in N1 than in the other treatments through the 50 years of treatments. In N2 and N3, foliar δ15N increased during the N-loading phase, but declined during the recovery phase, indicating a return of ectomycorrhizal fungi and their role in tightening the N cycle in N-limited forests. In the terminated, initially highest N treatments, N2 and N3, the trees even show signs of returning to Nlimitation. In these treatments, the soil base saturation remains lower, while the pH was only lower at 0–10 depth in the mineral soil, but not in the 10–20 cm depth horizon or in the superficial organic mor-layer. Accurately documenting the effect of N additions on forest growth required a long-term approach, where reasonable rates of application could be compared with extreme rates. Such long-term experiments are necessary to support forest management in achieving goals for developing future forests as they shift in response to major, global-scale changes

Shifts in soil microbial community structure, nitrogen cycling and the concomitant declining N availability in ageing primary boreal forest ecosystems

AbstractPlant growth in boreal forests is commonly limited by a low supply of nitrogen, a condition that may be aggravated by high tree below-ground allocation of carbon to ectomycorrhizal (ECM) fungi and associated microorganisms. These in turn immobilise N and reduce its availability to plants as boreal ecosystems develop. Here, we studied a boreal forest ecosystem chronosequence created by new land rising out of the sea due to iso-static rebound along the coast of northern Sweden. We used height over the ocean to estimate ecosystem age and examined its relationship to soil microbial community structure and the gross turnover of N. The youngest soils develop with meadows by the coast, followed by a zone of N2-fixing alder trees, and primary boreal conifer forest on ground up to 560 years old. The young soils in meadows contained little organic matter and microbial biomass per unit area. Nitrogen was turned over at low rates when expressed per area (m−2), but specific rates (per gram soil carbon (C)) were the highest found along the transect. In the zone with alder, the amounts of soil C and microbial biomass were much higher (bacterial biomass had doubled and fungal biomass quadrupled). Rates of gross N mineralisation (expressed on an area basis) were highest, but the retention of added labelled NH4+ was lowest in this soil as compared to other ages. The alder zone also had the largest extractable pools of inorganic N in soil and highest N % in plant foliage. In the older conifer forest ecosystems the amounts of soil C and N, as well as biomass of both bacteria and fungi increased. Data on organic matter 14C suggested that the largest input of recently fixed plant C occurred in the younger coniferous forest ecosystems. With increasing ecosystem age, the ratio of microbial C to total soil C was constant, whereas the ratio of microbial N to total soil N increased and gross N mineralization declined. Simultaneously, plant foliar N % decreased and the natural abundance of 15N in the soil increased. More specifically, the difference in δ15N between plant foliage and soil increased, which is related to relatively greater retention of 15N relative to 14N by ECM fungi as N is taken up from the soil and some N is transferred to the plant host. In the conifer forest, where these changes were greatest, we found increased fungal biomass in the F- and H-horizons of the mor-layer, in which ECM fungi are known to dominate (the uppermost horizon with litter and moss is dominated by saprotrophic fungi). Hence, we propose that the decreasing availability of N to the plants and the subsequent decline in plant production in ageing boreal forests is linked to high tree belowground C allocation to ECM fungi, a strong microbial sink for available soil N

Obstetric fistulae in southern Mozambique: incidence, obstetric characteristics and treatment.

BACKGROUND: Obstetric fistula is one of the most devastating consequences of unmet needs in obstetric services. Systematic reviews suggest that the pooled incidence of fistulae in community-based studies is 0.09 per 1000 recently pregnant women; however, as facility delivery is increasing, for the most part, in Africa, incidence of fistula should decrease. Few population-based studies on fistulae have been undertaken in Sub-Saharan Africa, including Mozambique. This study aimed to estimate the incidence of obstetric fistulae in recently delivered mothers, and to describe the clinical characteristics and care, as well as the outcome, after surgical repair. METHODS: We selected women who had delivered up to 12 months before the start of the study (June, 1st 2016). They were part of a cohort of women of reproductive age (12-49 years), recruited from selected clusters in rural areas of Maputo and Gaza provinces, Southern Mozambique, who were participating in an intervention trial (the Community Level Interventions for Pre-eclampsia trial or CLIP trial). Case identification was completed by self-reported constant urine leakage and was confirmed by clinical assessment. Women who had confirmed obstetric fistulae were referred for surgical repair. Data were entered into a REDCap database and analysed using R software. RESULTS: Five women with obstetric fistulae were detected among 4358 interviewed, giving an incidence of 1.1 per 1000 recently pregnant women (95% CI 2.16-0.14). All but one had Caesarean section and all of the babies died. Four were stillborn, and one died very soon after birth. All of the patients identified and reached the primary health facility in reasonable time. Delays occurred in the care: in diagnosis of obstructed labour, and in the decision to refer to the secondary or third-level hospital. All but one of the women were referred to surgical repair and the fistulae successfully closed. CONCLUSION: This population-based study reports a high incidence of obstetric fistulae in an area with high numbers of facility births. Few first and second delays in reaching care, but many third delays in receiving care, were identified. This raises concerns for quality of care

Tamm Review: On the nature of the nitrogen limitation to plant growth in Fennoscandian boreal forests

The supply of nitrogen commonly limits plant production in boreal forests and also affects species composition and ecosystem functions other than plant growth. These interrelations vary across the landscapes, with the highest N availability, plant growth and plant species richness in ground-water discharge areas (GDAs), typically in toe-slope positions, which receive solutes leaching from the much larger groundwater recharge areas (GRAs) uphill. Plant N sources include not only inorganic N, but, as heightened more recently, also organic N species. In general, also the ratio inorganic N over organic N sources increase down hillslopes. Here, we review recent evidence about the nature of the N limitation and its variations in Fennoscandian boreal forests and discuss its implications for forest ecology and management. The rate of litter decomposition has traditionally been seen as the determinant of the rate of N supply. However, while N-rich litter decomposes faster than N-poor litter initially, N-rich litter then decomposes more slowly, which means that the relation between N % of litter and its decomposability is complex. Moreover, in the lower part of the mor-layer, where the most superficial mycorrhizal roots first appear, and N availability matters for plants, the ratio of microbial N over total soil N is remarkably constant over the wide range in litter and soil C/N ratios of between 15 and 40 for N-rich and N-poor sites, respectively. Nitrogen-rich and -poor sites thus differ in the sizes of the total N pool and the microbial N pool, but not in the ratio between them. A more important difference is that the soil microbial N pool turns over faster in N-rich systems because the microbes are more limited by C, while microbes in N-poor systems are a stronger sink for available N. Furthermore, litter decomposition in the most superficial soil horizon (as studied by the so-called litter-bag method) is associated with a dominance of saprotrophic fungi, and absence of mycorrhizal fungi. The focal zone in the context of plant N supply in N-limited forests is further down the soil profile, where ectomycorrhizal (ECM) roots become abundant. Molecular evidence and stable isotope data indicate that in the typical N-poor boreal forests, nitrogen is retained in saprotrophic fungi, likely until they run out of energy (available C-compounds). Then, as heightened by recent research, ECM fungi, which are supplied by photosynthate from the trees, become the superior competitors for N. In N-poor boreal soils strong N retention by microorganisms keeps levels of available N very low. This is exacerbated by an increase in tree C allocation to mycorrhizal fungi (TCAM) relative to net primary production (NPP) with decreasing soil N supply, which causes ECM fungi to retain much of the available soil N for their own growth and transfer little to their tree hosts. The transfer of N through the ECM fungi, and not the rate of litter decomposition, is likely limiting the rate of tree N supply under such conditions. All but a few stress-tolerant less N-demanding plant species, like the ECM trees themselves and ericaceous dwarf shrubs, are excluded. With increasing N supply, a weakening of ECM symbiosis caused by the relative decline in TCAM contributes to shifts in soil microbial community composition from fungal dominance to bacterial dominance. Thus, bacteria, which are less C-demanding, but more likely to release N than fungi, take over. This, and the relatively high pH in GDA, allow autotrophic nitrifying bacteria to compete successfully for the NH4+ released by C-limited organisms and causes the N cycle to open up with leaching of nitrate (NO3−) and gaseous N losses through denitrification. These N-rich conditions allow species-rich communities of N-demanding plant species. Meanwhile, ECM fungi have a smaller biomass, are supplied with N in excess of their demand and will export more N to their host trees. Hence, the gradient from low to high N supply is characterized by profound variations in plant and soil microbial physiologies, especially their relations to the C-to-N supply ratio. We propose how interactions among functional groups can be understood and modelled (the plant-microbe carbon-nitrogen model). With regard to forest management these perspectives explain why the creation of larger tree-free gaps favors the regeneration of tree seedlings under N-limited conditions through reduced belowground competition for N, and why such gaps are less important under high N supply (but when light might be limiting). We also discuss perspectives on the relations between N supply, biodiversity, and eutrophication of boreal forests from N deposition or forest fertilization