Nutrient and carbon cycling along nitrogen deposition gradients in broadleaf and conifer forest stands in the east of England

Southern California deserts and coastal sage scrub (CSS) are undergoing vegetation-type conversion to exotic annual grassland, especially in regions downwind of urban areas that receive high nitrogen (N), primarily as dry deposition. To determine critical loads (CLs) of N that cause negative impacts, we measured plant and soil responses along N deposition gradients, fertilized vegetation at different N levels, and used biomass production output from the DayCent model. Nitrogen deposition gradients were identified from the CMAQ model and compared with measured N deposition values. Coastal sage scrub receives N deposition as high as 30 kg ha− 1 year− 1, while the desert has levels up to 16 kg ha− 1 year− 1. These ecosystems are subject to increases in exotic species production, loss of native species diversity, and increased fire risk at relatively low CLs. For instance, a gradient survey in CSS showed that exotic grass cover increased and native plant species richness declined by almost 50 % above 10 kg N ha − 1 year− 1. Fertilization studies in desert creosote bush scrub showed a significant increase in exotic species biomass with 5 kg N ha− 1 year− 1 in a wet year, and biomass output from DayCent modelling indicated an increased fire risk from exotic grasses with 1 t per ha production during years with moderate to high precipitation at 2.2–8.8 kg N ha− 1 year− 1. The difference in CL between desert and CSS is related to the different criteria used (diversity loss in CSS, productivity and fire risk in desert), as well as responsiveness of native vs. exotic plant species to N and the degree to which precipitation and soil N limits plant growth in the two vegetation types.

Impacts of increased nitrogen deposition and altered precipitation regimes on soil fertility and functioning in semiarid Mediterranean shrublands

Deciduous forests may respond differently from coniferous forests to the anthropogenic deposition of nitrogen (N). Since fungi, especially ectomycorrhizal (EM) fungi, are known to be negatively affected by N deposition, the effects of N deposition on the soil microbial community, total fungal biomass and mycelial growth of EM fungi were studied in oak-dominated deciduous forests along a nitrogen deposition gradient in southern Sweden. In-growth mesh bags were used to estimate the production of mycelia by EM fungi in 19 oak stands in the N deposition gradient, and the results were compared with nitrate leaching data obtained previously. Soil samples from 154 oak forest sites were analysed regarding the content of phospholipid fatty acids (PLFAs). Thirty PLFAs associated with microbes were analysed and the PLFA 18:2omega6,9 was used as an indicator to estimate the total fungal biomass. Higher N deposition (20 kg N ha(-1)y(-1) compared with 10 kg N ha(-1)y(-1)) tended to reduce EM mycelial growth. The total soil fungal biomass was not affected by N deposition or soil pH, while the PLFA 16:1omega5, a biomarker for arbuscular mycorrhizal (AM) fungi, was negatively affected by N deposition, but also positively correlated to soil pH. Other PLFAs positively affected by soil pH were, e.g., i14:0, a15:0, 16:1omega9, a17:0 and 18:1omega7, while some were negatively affected by pH, such as i15:0, 16:1omega7t, 10Me17:0 and cy19:0. In addition, N deposition had an effect on the PLFAs 16:1omega7c and 16:1omega9 (negatively) and cy19:0 (positively). The production of EM mycelia is probably more sensitive to N deposition than total fungal biomass according to the fungal biomarker PLFA 18:2omega6,9. Low amounts of EM mycelia covaried with increased nitrate leaching, suggesting that EM mycelia possibly play an important role in forest soil N retention at increased N input.

Nutrient stoichiometry in Sphagnum along a nitrogen deposition gradient in highly polluted region of Central-East Europe

Fomes root rot caused by the pathogenic wood-rotting basidiomycete, Heterobasidion annosum, has caused much mortality in the pine plantations of Thetford Forest, East Anglia, which were first established in the period between the two World Wars. This paper presents an evaluation of over 50 years of research into the disease, and includes much previously unpublished data. Special emphasis is given to the position of Corsican pine, Pinus nigra ssp. laricio, as this is now the principal tree used in the plantations. Freshly cut stumps of both Corsican and Scots pine (P. sylvestris) are equally susceptible to colonization by air-borne basidiospores of H. annosum and, in both species, this colonization can be effectively prevented by treatment with spores of Phlebiopsis gigantea. It is well known that on first-rotation sites, a high soil pH favours the development of disease from thinning stumps. Data from one experiment show that on a site with a pH of 7.5-8.0, 1095 trees per hectare had been killed by the time the crop of Scots pine was 36 years old; representing 25 per cent of the original planting. Losses tend to fall as the crop reaches the end of the rotation. There are some data to show that pure Corsican pine is less vulnerable than Scots pine to disease developing after thinning. When a new crop of pine is planted on a site carrying H. annosum-colonized stumps, losses averaging 30 per cent can occur during the first 10 years on soils with a pH >6.0. Even on soils with a pH <6.0, losses average nearly 20 per cent. At this early age, Corsican pine is as susceptible to the disease as Scots pine. A series of experiments has shown that it is only through stump removal that adequate control can be achieved, and for 30 years, this process has been standard practice as the first-rotation crops have been progressively felled. Experiments with a wide range of trees on different soil types have shown that very few species are resistant both to killing and to buttrot caused by H. annosum. Those that have performed best - most notably Picea omorika, Abies grandis and Fagus sylvatica - have major limitations for use in the forest. Nevertheless it is concluded that, while the justification for using Corsican pine as the principal species is well founded, there are reasons for making some continued use of other species, most notably Scots pine, on certain sites. A continued commitment to good quality stump treatment is required in order to minimize H. annosum problems in future rotations.

We sampled 42 permanent vegetation plots in the Adirondack Mountains, NY to characterize the spatial patterns of, and controls on soil C and N contents. Average C (24 kg m −2 ) and N (1.1 kg m −2 ) contents of the combined organic and mineral horizons were high for northeastern U.S. forests. Contrary to our expectations, whole‐profile C and N amounts were not different among northern hardwood (NH), spruce‐fir (SF), and pine‐dominated (PW) plots. Most of the C and N were stored in organic horizons in the high‐elevation SF plots, and in the mineral horizons in NH and PW plots. Regression analyses of the pooled set of sites revealed that the factors that explained the most variability in soil C and N contents were different for organic and mineral horizons and differed between forest types. Overall, growing season degree‐days (GSDD) was the variable most closely correlated with C and N contents in both organic and mineral horizons, and varying combinations of N deposition, conifer importance, and soil texture were the principal secondary influences. Spruce‐fir plots received the most atmospheric N deposition, and multivariate regression tree (MRT) analysis indicated that N deposition rate was the environmental variable that explained the most variation in organic horizon C (sums of squares [SS] explained = 54%) and N (79% SS explained) amounts in these high‐elevation stands. However, even in the unlikely case of 100% retention of the atmospheric inputs of N deposited over the past 50 yr, this source only accounted for a small portion of soil N, and the differences in N among the plots along the deposition gradient. It is likely that differences in GSDD accounted for most of the differences in C and N amounts.

Regional patterns in foliar 15N across a gradient of nitrogen deposition in the northeastern US

Atmospheric deposition of nitrogen along the Connecticut coastline of Long Island Sound: a decade of measurements

To gain insight into fine roots decomposition in subtropical China, the litter bag method was used to examine the decomposition dynamics of dry mass, N, P, K, and organic fractions in six natural forests and a Chinese fir plantation over a 2-year period in the Wanmulin Nature Reserve, Fujian. The seven tree species examined, representative of this area, differed significantly in their initial chemical quality and were used to determine the best substrate quality parameters to predict decomposition dynamics. Dry mass loss varied significantly among the different roots, which showed fast decomposition in the first year, with mass loss regulated by extractive and acid-soluble fraction, followed by a low rate in the second year, with mass loss dominated by acid-insoluble fraction. Net N release was constantly slower than the mass loss of acid-insoluble fractions, while K release was the other way around. Release of P seemed to be independent of disappearance of acid-insoluble fraction. Not all the very fine roots (0–1 mm) decomposed faster than the fine ones (1–2 mm), and decomposition rates of coniferous roots were not always lower than broadleaved species. Correlation analysis demonstrated that dry mass loss and net N and P release rates were not correlated with initial N concentration, but with acid-insoluble organic fraction and P related parameters at the end of a 2-year decomposition period. Our results suggest that N is a limiting factor of fine root decomposition. Additionally, P could also be an important driver of fine root decomposition and N and P dynamics in this low soil P availability area.

Abstract. The Thetford Project (1968–1976) was a keystone project for the newly established Institute of Hydrology. Its primary objective was to elucidate the processes underlying evaporation of transpired water and intercepted rainfall from plantation forest, so as to explain hydrological observations that more water was apparently returned to the atmosphere from plantations than from grassland and heathland. The primary approach was to determine the fluxes of water vapour from a stand of Scots pine, situated within a larger area of plantations of Scots and Corsican pine, in Thetford Forest, East Anglia, UK, using the Bowen ratio approach. In 1976, advantage was taken of the methodology developed to add measurement of profiles of carbon dioxide concentration so as to enable the fluxes of CO2 also to be calculated. A team from Aberdeen and Edinburgh Universities collected 914 hours of 8-point CO2 concentration profiles, largely between dawn and dusk, on days from March to October, and the data from an "elite" data set of 710 hours have been analysed. In conditions of moderate temperature (&lt;25°C) and specific humidity deficit (&lt;15 g kg−1 with high solar irradiance (&gt;500 W m−2), CO2 uptake reached relatively high rates for pine of up to 20 µmol m−2 s−1 in the middle of the day. This rate of CO2 uptake is higher than has been recently found for four Scots pine forests in continental Europe during July 1997. However, the year of 1976 was exceptionally hot and dry, with air temperatures reaching 30°C and the water deficit in the top 3 m of soil at the site of 152 mm by August. Air temperatures of over 25°C led to large specific humidity deficits, approaching 20 g kg−1, and associated severe reductions in CO2 uptake, as well as in evaporation. However, when specific humidity deficits dropped below c. 15 g kg−1 on succeeding days, generally as a result of lower air temperatures rather than lower solar irradiance, there was rapid recovery in both uptake and evaporation, thus indicating that the large soil water deficit was not the main cause of the reductions in the CO2 and water fluxes. Based on earlier analysis of evaporation data on completely dry days, the concurrent reductions in CO2 flux and evaporation are largely attributable to decrease in canopy stomatal conductance. The air temperature, specific humidity deficit, and soil water deficit in 1976 were exceptional and similar conditions have most likely not been experienced again until 2003. We conclude that the information gained at Thetford in 1976 on the response of the pine forest ecosystem to such weather, may provide a good guide to the response of English pine forests to the projected climate change over the next 25 years.

SummaryThe relationship between the proportion of the stump surface covered by the biological stump treatment agent PG Suspension, containing Phlebiopsis gigantea and its efficacy against the pathogen Heterobasidion annosum sensu stricto was studied during a first thinning of Corsican pine (Pinus nigra ssp. laricio) in Thetford Forest, UK. PG Suspension was manually applied to 100%, 75%, 50% or 0% of the surface of 150 stumps. Spores of H. annosum were inoculated onto 75 of the stumps, and the remaining stumps exposed to natural airborne spore deposition. The relationship between coverage and efficacy was found to be quantitative. Covering all the stump surface with PG Suspension completely excluded the pathogen, whereas stumps not treated with PG Suspension (the 0% treatment) became infected with H. annosum. Partial (75%) PG Suspension coverage resulted in the pathogen colonizing 40% of stumps following artificial inoculation with H. annosum, and just 7% of stumps exposed to ambient H. annosum spore infection. Decreasing levels of coverage allowed increasing areas of the stump surface to be colonized by H. annosum. Some small gaps in coverage were closed by lateral growth of P. gigantea, but it is recommended that operators aim for full stump coverage to give complete protection against H. annosum.

Soil microorganisms of a native shrub and exotic grasses along a nitrogen deposition gradient in southern California

Chronic atmospheric N deposition has modified relative N availability, altering the biogeochemical cycles of forests and the stoichiometry of nutrients in trees, inducing P limitation, and modifying the N:P ratios of plant biomass. This study examines how the variation in the foliar stoichiometry of Abies pinsapo across an N deposition gradient affects foliar traits and photosynthetic rate. We measured the maximum net assimilation rates (Amax) and the foliar nitrogen (N) and phosphorus (P) concentrations in A. pinsapo needles of five age classes. The leaf mass per area and photosynthetic N and P use efficiencies (PNUE and PPUE, respectively) were also estimated. The results from the N-saturated stand (Sierra Bermeja, B) differed from the comparable N-limited stands under investigation (Yunquera, Y, and Sierra Real, SR). The trees from Y and SR exhibited a reduction in the N content in older needles, whereas the foliar N concentration at the B site increased with needle age. N and P were positively correlated at Y and SR, but not at B, suggesting that the overload of N in the trees at site B has exceeded the homeostatic regulation capacity of the N-saturated stand in terms of foliar stoichiometry. A max and PNUE were correlated positively with P and negatively with the N/P ratio at the three study sites. The foliar N concentration was positively correlated with A max at Y and SR. However, this relationship was negative for the B site. These findings suggest that the nutritional imbalance caused by increased chronic deposition of N and an insufficient supply of P counteracts the potential increase in net photosynthesis induced by the accumulation of foliar N.

The declining health of high-elevation red spruce (Picea rubens Sarg.) and Fraser fir (Abies fraseri (Pursh) Poir.) in the southern Appalachian region has long been linked to nitrogen (N) deposition. Recently, N deposition has also been proposed as a source of negative health impacts in lower elevation deciduous forests. In 1998 we established 46 plots on six sites in North Carolina and Virginia dominated by American beech (Fagus grandifolia Ehrh.), sugar maple (Acer saccharum Marsh.), and yellow birch (Betula alleghaniensis Britt). We evaluated several response variables across an N deposition gradient, including annual basal area growth; foliage percent N, Al, P, K, Mg, and Ca; and forest floor percent N, Mg, and C, pH, and potential net nitrification and N mineralization rates. We found a significant linear relationship between N deposition and basal area growth in sugar maple, but not in American beech or yellow birch. In addition, we found a significant relationship between N deposition and foliar chemistry (foliar %N in all species, foliar Mg/N and %K in sugar maple, and %P in sugar maple and yellow birch). Foliar %N of the three studied species was high relative to values reported in other studies in the United States and Canada. Several forest floor response variables (%N, C/N, pH, Mg/N, and potential net nitrification and N mineralization rates and nitrification/mineralization fractions) were also correlated with N deposition. The correlations between the above response variables and N deposition are consistent with the influence of chronic N deposition on forested ecosystems measured in other regions and suggest that chronic N deposition may be influencing forest structure and chemistry within the southern region.

Precipitation and pollution interaction effect on the abundance of Collembola in hardwood forests in the lower Midwestern United States