Vegetation Nitrogen Research Articles

The Content of Amino Acids in Pasture Vegetation and Their Apparent Digestibility in 2-year-old Horses

Ten samples of plant vegetation and 10 samples of fresh excrement were taken from the same pasture area. The excrement were collected from 10 2-year-old Old Kladruber horses that received the pasture vegetation daily. The apparent digestibility of nitrogen and amino acids in pasture vegetation was determined by using the acid-insoluble ash marker method. In comparison with excrement, the pasture vegetation contained higher levels of Ser, Ala, Leu, and His and higher levels of Pro (P ≤ .01), Met (P ≤ .01), and Arg (P ≤ .05). The mean level of Ile in pasture vegetation was lower than in excrement (P ≤ .05). The apparent digestibility of amino acids from pasture vegetation was high for Pro and Met (86.75 and 89.39%), moderate for Ser, Ala, Leu, His, and Arg (68.61%-76%), and low for Asp, Thr, Glu, Gly, Val, Leu, Tyr, Phe, and Lys (56.15%-66.03%). The digestibility of lysine and Ile was relatively low (56.39% and 56.15%, respectively). The total content of nitrogen per dry matter was 10.98 ± 2.46 g/kg for pasture vegetation and 12.12 ± 2.38 g/kg for excrement, whereas the content of protein nitrogen was 7.20 ± 0.25 g/kg and 6.89 ± 0.21 g/kg of dry matter in pasture vegetation and excrement, respectively. This means that only 65.55% and 56.90% of N is bound in proteins in pasture vegetation and excrement, respectively. Non-protein nitrogen accounts for 34.45% in pasture vegetation and for 43.10% in excrement.

Carbon sequestration and biogeochemical cycling in a saltmarsh subject to coastal managed realignment

Globally, wetlands provide the largest terrestrial carbon (C) store, and restoration of degraded wetlands provides a potentially important mechanism for climate change mitigation. We examined the potential for restored saltmarshes to sequester carbon, and found that they can provide a modest, but sustained, sink for atmospheric CO2. Rates of C and nutrient cycling were measured and compared between a natural saltmarsh (high- and low-shore locations), claimed arable land on former high-shore saltmarsh and a managed realignment restoration site (high- and low-shore) in transition from agricultural land to saltmarsh 15 years after realignment, at Tollesbury, Essex, UK. We measured pools and turnover of C and nitrogen (N) in soil and vegetation at each site using a range of methods, including gas flux measurement and isotopic labelling. The natural high-shore site had the highest soil organic matter concentrations, topsoil C stock and below-ground biomass, whereas the agricultural site had the highest total extractable N concentration and lowest soil C/N ratio. Ecosystem respiration rates were similar across all three high-shore sites, but much higher in both low-shore sites, which receive regular inputs of organic matter and nutrients from the estuary. Total evolution of 14C-isotopically labelled substrate as CO2 was highest at the agricultural site, suggesting that low observed respiration rates here were due to low substrate supply (following a recent harvest) rather than to inherently low microbial activity. The results suggest that, after 15 years, the managed realignment site is not fully equivalent to the natural saltmarsh in terms of biological and chemical function. While above ground biomass, extractable N and substrate mineralisation rates in the high-shore site were all quite similar to the natural site, less dynamic ecosystem properties including soil C stock, C/N ratio and below-ground biomass all remained more similar to the agricultural site. These results suggest that reversion to natural biogeochemical functioning will occur following restoration, but is likely to be slow; we estimate that it will take approximately 100 years for the restored site to accumulate the amount of C currently stored in the natural site, at a rate of 0.92 t C ha−1 yr−1.

Changes in the soil, litter, and vegetation nitrogen and carbon concentrations of semiarid shrublands in response to chronic dry season nitrogen input

Southern Californian chaparral and coastal sage scrub (CSS) shrublands are exposed to high-levels of dry-atmospheric nitrogen (N) deposition. A field experiment was conducted over a 6-year period to assess the effects of cumulative, dry-season N inputs on soil and vegetation carbon (C) and nitrogen (N). We hypothesized that plots exposed to exogenous N should exhibit signs of N enrichment once a cumulative N exposure threshold was reached and the N immobilization capacity was exceeded. Using mass-balance we found that chaparral and CSS have the capacity to immobilize 6.2 and 11.9gNm−2y−1, respectively. Soil extractable N increased significantly after 7–10g/m2 of cumulative N exposure, resulting in a simultaneous increase in the N concentration and a decline in the C:N ratio of shrub tissue. Similar results were observed for the surface litter pool and litter production but at a higher cumulative N exposure. While we cannot tease out the potential for lagged responses to affect our estimate of the cumulative N threshold, these results indicate that the C and N cycling of semi-arid shrublands is significantly altered once the ecosystem N uptake capacity is exceeded.

Seasonal dynamics in vegetation and rumen microbial nitrogen production and nutritional status of grazing sheep in a semiarid rangeland in eastern Rajasthan, India

Seasonal dynamics in the vegetation and rumen microbial nitrogen (MBN) production and nutritional status of grazing sheep were investigated in a semiarid rangeland in eastern Rajasthan, India, to obtain some information for conserving the grazing land. Total number of plant species was 47, 71 and 47 in the summer, monsoon and winter seasons, respectively. The frequency of plant occurrence, density of plants and aboveground biomass were ranked monsoon > winter > summer, showing considerably low values in the summer season (10–32% of the monsoon values). Nutrient intake by animals was highest in the winter, followed by the monsoon and summer, with considerable summer depressions (32–64% of the winter values). Rumen MBN production per kg organic matter (OM) digested in the rumen was higher in the monsoon than in the summer and winter (66 and 60% of the monsoon value, respectively). Compared with the maintenance requirement, the animals were able to ingest only 58% of dry matter in the summer, 89% of digestible crude protein in the monsoon and winter, and 82 and 62% of metabolizable energy in the summer and monsoon, respectively. The removal of nutrients by animals via herbivory always exceeded the deposition via excreta, leading to the annual nutrient deficits of 187 kg OM, 1.7 kg N and 8.2 kg inorganic ash per sheep. The results highlight the need for developing means of nutrient replenishment to sustain the grazing resource, while accurate evaluation of the nutrient balance is warranted taking account of other pathways of inputs and outputs as well.

Feedback of carbon and nitrogen cycles enhances carbon sequestration in the terrestrial biosphere

AbstractThe efforts to explain the ‘missing sink’ for anthropogenic carbon dioxide (CO2) have included in recent years the role of nitrogen as an important constraint for biospheric carbon fluxes. We used the Nitrogen Carbon Interaction Model (NCIM) to investigate patterns of carbon and nitrogen storage in different compartments of the terrestrial biosphere as a consequence of a rising atmospheric CO2 concentration, in combination with varying levels of nitrogen availability. This model has separate but closely coupled carbon and nitrogen cycles with a focus on soil processes and soil–plant interactions, including an active compartment of soil microorganisms decomposing litter residues and competing with plants for available nitrogen. Biological nitrogen fixation is represented as a function of vegetation nitrogen demand. The model was validated against several global datasets of soil and vegetation carbon and nitrogen pools. Five model experiments were carried out for the modeling periods 1860–2002 and 2002–2100. In these experiments we varied the nitrogen availability using different combinations of biological nitrogen fixation, denitrification, leaching of soluble nitrogen compounds with constant or rising atmospheric CO2 concentrations. Oversupply with nitrogen, in an experiment with nitrogen fixation, but no nitrogen losses, together with constant atmospheric CO2, led to some carbon sequestration in organismic pools, which was nearly compensated by losses of C from soil organic carbon pools. Rising atmospheric CO2 always led to carbon sequestration in the biosphere. Considering an open nitrogen cycle including dynamic nitrogen fixation, and nitrogen losses from denitrification and leaching, the carbon sequestration in the biosphere is of a magnitude comparable to current observation based estimates of the ‘missing sink.’ A fertilization feedback between the carbon and nitrogen cycles occurred in this experiment, which was much stronger than the sum of separate influences of high nitrogen supply and rising atmospheric CO2. The demand‐driven biological nitrogen fixation was mainly responsible for this result. For the modeling period 2002–2100, NCIM predicts continued carbon sequestration in the low range of previously published estimates, combined with a plausible rate of CO2‐driven biological nitrogen fixation and substantial redistribution of nitrogen from soil to plant pools.

A call to investigate drivers of soil organic matter retention vs. mineralization in a high CO 2 world

Understanding how elevated atmospheric CO 2 alters the formation and decomposition of soil organic carbon (SOC) is important but challenging. If elevated CO 2 induces even small changes in rates of formation or decay of SOC, there could be substantial feedbacks on the atmosphere's concentration of CO 2. However, the long turnover times of many SOC pools – decades to centuries – make the detection of changes in the soil's pool size difficult. Long-term CO 2 enrichment experiments have offered unprecedented opportunities to explore these issues in intact ecosystems for more than a decade. Increased NPP with elevated CO 2 has prompted the hypothesis that SOC may increase at the same time that increased vegetation nitrogen (N) uptake and accumulation indicates probable declines in SON. Varying investigators thus have hypothesized that SOC will increase and SON will decline to explain increased NPP with elevated CO 2; researchers also invoke biogeochemical theory and stoichiometric constraints to argue for strong limitations on the co-occurrence of these phenomena. We call for researchers to investigate two broad research questions to elucidate the drivers of these processes. First, we ask how elevated CO 2 influences compound structure and stoichiometry of that proportion of NPP retained by soil profiles for relatively long time periods. We also call for investigations of the mechanisms underlying the decomposition of mineralizable organic matter with elevated CO 2. Specifically, we need to understand how elevated CO 2 influences microbial priming (driven by enhanced microbial energy needs associated with increases in biomass or activity) and microbial mining of N (driven by enhanced microbial N demand associated with greater vegetative N uptake), two processes that necessarily will be constrained by the stoichiometry of both substrates and microbial demands. Applying technologies such as nuclear magnetic resonance and the detection of biomarkers that reveal organic matter structure and origins, and studying microbial stoichiometric constraints, will dramatically improve our ability to predict future patterns of ecosystem C and N cycling.

Nutrient concentration ratios and co‐limitation in South African grasslands

*Assessing plant nutrient limitation is a fundamental part of understanding grassland dynamics. The ratio of concentrations of nitrogen (N) and phosphorus (P) in vegetation has been proposed as an index of the relative limitation of biomass production by N and P, but its utility has not been tested well in grasslands. *At five sites in Kruger National Park, South Africa, across soil and precipitation contrasts, N and P were added in a factorial design to grass-dominated plots. *Although the N:P ratio of unfertilized vegetation across all sites (5.8) would have indicated that production was N-limited, aboveground production was consistently co-limited by N and P. Aboveground production was still greater in plots fertilized with N and P than in those fertilized with just N, but the N:P ratio did not exceed standard thresholds for P limitation in N-fertilized vegetation. Comparisons among sites showed little pattern between site N:P ratio and relative responses to N and P. *When combined with results from other grassland fertilization studies, these data suggest that the N:P ratio of grasses has little ability to predict limitation in upland grasslands. Co-limitation between N and P appears to be much more widespread than would be predicted from simple assumptions of vegetative N:P ratios.

Effect of grazing intensity on carbon and nitrogen in soil and vegetation in a meadow steppe in Inner Mongolia

The effects of grazing on grassland ecosystems must be understood to develop effective soil conservation measures and sustain livestock production. The objective of this research was to characterize the impact of dairy cow grazing intensity on C and N in soil and vegetation on a meadow steppe. The meadow steppe site, located near Hailaer, Inner Mongolia, China, has been grazed by dairy cattle for more than 30 years, with the last 23 years managed by the same herders. The study site never received any form of mineral fertilizer. Grazing intensity was classified as light (LG), moderate (MG) and heavy (HG) according to the vegetation utilization rate. Grazing affected biomass, total N (TN) and 15N levels, but not 13C, of above-ground plants. The above-ground biomass and δ 15N decreased but TN content increased with the grazing intensity for all nine non-legume species while five legume species were not affected. Coarse root biomass also decreased with grazing intensity while TN or δ 15N responses to grazing showed no consistent patterns. Soil OC and total N content decreased significantly with grazing intensity for all three sampling depths (0–10, 10–20 and 20–30 cm) while nitrate, ammonium and 13C levels were not affected by grazing intensity in all sampling depths. The decreases in soil OC suggest carbon storage in the meadow steppe declined as grazing intensity increased. Combined with the decreases in soil TN content, this also means heavy grazing could lead to decreases in soil quality and fertility. Uneven grazing intensity is an inherent feature of a season-long grazing system and there are risks to soil quality and environmental health associated with HG. We need to find the proper balance between light to heavy grazing intensity through proper grazing management and in some cases using judicious herding to distribute livestock. Some combination of HG, MG and LG could be desirable in sustainable rangeland grazing production.

Effects of nutrient loading and extreme rainfall events on coastal tallgrass prairies: invasion intensity, vegetation responses, and carbon and nitrogen distribution

AbstractSoil fertility and precipitation are major factors regulating transitions from grasslands to forests. Biotic regulation may influence the effects of these abiotic drivers. In this study, we examined the effects of extreme rainfall events, anthropogenic nutrient loading and insect herbivory on the ability of Chinese tallow tree (Sapium sebiferum) to invade coastal prairie to determine how these factors may influence woody invasion of a grassland. We manipulated soil fertility (NPK addition) and simulated variation in frequency of extreme rainfall events in a three growing season, full factorial field experiment. Adding water to or pumping water out of plots simulated increased and decreased rainfall frequencies. We added Sapium seeds and seedlings to each plot and manipulated insect herbivory on transplanted Sapium seedlings with insecticide. We measured soil moisture, Sapium performance, vegetation mass, and carbon and nitrogen in vegetation and soils (0–10 cm deep, 10–20 cm deep). Fertilization increased Sapium invasion intensity by increasing seedling survival, height growth and biomass. Insect damage was low and insect suppression had little effect in all conditions. Recruitment of Sapium from seed was very low and independent of treatments. Vegetation mass was increased by fertilization in both rainfall treatments but not in the ambient moisture treatment. The amount of carbon and nitrogen in plants was increased by fertilization, especially in modified moisture plots. Soil carbon and nitrogen were independent of all treatments. These results suggest that coastal tallgrass prairies are more likely to be impacted by nutrient loading, in terms of invasion severity and nutrient cycling, than by changes in the frequency of extreme rainfall events.

Nitrogen Fixation in the High Arctic: Role of Vegetation and Environmental Conditions

The course of nitrogen fixation by moss-associated cyanobacteria in Svalbard (78°N, 16°E), Norway, was studied using the acetylene reduction assay. In situ field measurements of nitrogen fixation activity were conducted in six different types of moss-dominated arctic vegetation from the beginning of the snowmelt in early June to the end of July 1998. Concurrently, the water content of the soil/vegetation layer was determined and correlated with the nitrogen fixation rates. At all sites with diminishing water content during the summer season, nitrogen fixation activity was positively correlated with the amount of available water in the vegetation. At two sites, where water content of the vegetation was constantly higher than 80% (w/w) throughout the season, nitrogen fixation activity was correlated with temperature. Depending on the type of vegetation, nitrogen fixation became limited when the water status fell below a minimum threshold level. The most desiccation-tolerant vegetation for nitrogen fixation activity was the cryptobiotic crust, where nitrogen fixation decreased only after the water content of the soil/vegetation was less than 50% of the its fresh weight, while in the other types of vegetation nitrogen fixation stopped when water content was around 60%. The results from the present study confirm that, in arctic regions with low precipitation during the growing season, nitrogen fixation in different types of vegetation is mostly limited either to the period of snowmelt when water is sufficiently available, or to habitats that stay wet during summer.

Impacts of grazing on montane heath vegetation in Wales and implications for the restoration of montane areas

The impact of long-term heavy grazing pressure on montane vegetation and soils was investigated on the Carneddau plateau in North Wales. At this site, historical information suggests that formerly extensive Racomitrium- and Vaccinium-dominated heaths have become degraded and reduced in extent over the last 40–50 years. This coincides with a period when both sheep numbers and atmospheric deposition of nitrogen have greatly increased. Vegetation composition, plant tissue chemistry, soil chemistry and sheep presence were compared between areas of ‘good’ condition Racomitrium and Vaccinium heath and degraded areas. Comparison of species composition data with sheep presence and environmental data showed no significant link between current grazing distribution and species composition in the areas investigated, with all habitats having sheep presence above the recommended maximum for this habitat. Soil chemistry showed several differences between degraded and ‘good’ condition habitats, suggesting that degraded areas had experienced loss of soil organic matter and upper organic horizons. Comparison of soil and vegetation nitrogen content with data from other UK sites suggests a build up of nitrogen has occurred in Carneddau vegetation and soils, possibly related to high levels of nitrogen deposition. It is concluded that restoration of severely degraded areas of montane vegetation may be retarded by deleterious changes in soil properties which could slow recolonisation by montane species. Reductions in stocking density aiming to restore montane communities might have more immediate impact on less degraded areas.

Off-season uptake of nitrogen in temperate heath vegetation

In this field study we show that temperate coastal heath vegetation has a significant off-season uptake potential for nitrogen, both in the form of ammonium and as glycine, throughout winter. We injected 15N-ammonium and 15N 2x(13C)-glycine into the soil twice during winter and once at spring. The winter temperatures were similar to those of an average winter in the northern temperate region of Europe, with only few days of soil temperatures below zero or above 5 degrees C. The vegetation, consisting of the evergreen dwarf shrub Calluna vulgaris, the deciduous dwarf shrub Salix arenaria, and the graminoids Carex arenaria and Deschampsia flexuosa, showed high root uptake of both forms of nitrogen, both 1 day after labelling and after a month, in species specific temporal patterns. Plant uptake of 13C was not significant, providing no further evidence of intact uptake of glycine. Translocation of the labelled nitrogen to shoots was generally evident after 1 month and increased as spring approached, with different translocation strategies in the three plant functional types. Furthermore, only the graminoids showed shoot growth during winter. Increasing plant nitrogen concentration from fall to spring at temperate heaths may, hence, be due to nitrogen uptake. Our results suggest that the potential for nitrogen uptake in plants at winter is of the same order of magnitude as at summer. Hence, winter nitrogen uptake in ecosystems in the temperate/boreal region should be considered when making annual nitrogen budgets of heath ecosystems, and the view of plant nutrient uptake as low in this climatic region during winter should be revised.

Development of the Bio-Geographical and GeoChemical (BGGC) Model for Assessments of Climate Change Impacts on the Terrestrial Ecosystem in Japan

This study developed the Bio-Geographical and GeoChemical model (BGGC model) to assess the future impacts of global climate change, as prescribed by the IPCC-SRES, on the distribution and functioning of terrestrial ecosystems in Japan. The BGGC model simulates the cycling of carbon, nitrogen, and water in vegetation, soil ecosystems and the atmosphere. Some modules in the model borrow most of their basic structures from the sub-model of CENTURY4 (Parton et al., 1993) and the modified BIOME3 (Ishigami et al., 2002). The BGGC model is broadly divided into two sub-models, the vegetation competition sub-model and the soil organic matter sub-model. The vegetation competition sub-model consists of the photosynthesis model and the canopy model. The photosynthesis model estimates the optimized net primary productivity (NPP) and leaf area index (LAI) to satisfy the soil nitrogen, and annual moisture. The canopy model considers the canopy structure for forest vegetation types. On the other hand, the soil organic matter sub-model, which is linked to the vegetation competition sub-model, simulates carbon, nitrogen, and water dynamics in the soil ecosystem. The distribution of potential natural vegetation types was determined by the estimated NPP and LAI. In order to assess the impact of climate change on the terrestrial ecosystem, this study estimated the distribution of potential natural vegetation in Japan using NPP. The GCMs experimental data used were the CSIRO-Mk2 and ECHAM4/OPYC3 for each of A2 and B2 scenarios in the SRES. Comparison of the averages of simulated NPP under each scenario with the average NPP under current climate conditions showed that the average NPP could increase about 19 to 33 percent by the year 2050 and 25 to 53 percent by the year 2080.

Forest fragmentation affects early successional patterns on shifting cultivation fields near Indian Church, Belize

Abstract This study examines whether vegetation and soils sampled in slash-and-burn fields in an agricultural landscape near Indian Church, Belize varied as a function of cropping status (in use, 1–2 year fallow, 3–10 year fallow) and distance to older forest (adjacent to or >150 m from forest). Multi-response permutation procedures (MRPPs) indicated that species composition (woody species density, herbaceous species cover) differed significantly among all treatments (woody: P=0.006; herbaceous: P=0.0001) and between distance classes (woody: P=0.03; herbaceous: P=0.002). The frequency of herbaceous life forms also differed between distance classes (MANOVA: P=0.04), with lianas (P=0.03) and legumes (P=0.008) being more common at greater distances from forest. Soil macronutrients (P, K) were significantly lower in long-term fallows than in use fields due to sequestration by the regenerating vegetation but ammonium nitrogen was significantly lower far from older forests (two-way ANOVA: P 0.05), most likely due to seed inputs from surrounding early successional habitats. This study indicates that changes in landscape structure may influence ecological processes such as succession in fragmented tropical landscapes and underscores the need to reject a conceptualization of the landscape matrix as a featureless, inhospitable habitat in favor of one that recognizes and incorporates the influence of matrix quality and heterogeneity.

Canopy cover effects on nitrogen and phosphorus status in understory vegetation in oak and pine stands

The rate of change of leaf mass, N and P levels in understory vegetation at various levels of canopy cover were measured for 2 years following canopy cover manipulations in northern red oak (Quercus rubra L.) and red pine (Pinus resinosa Ait.) stands in northern Lower Michigan, USA. Canopy cover treatments consisted of clearcut, 25% (50% during first sampling year), 75%, and uncut. Leaf mass, and N and P contents were significantly higher in the clearcut treatment than in other canopy cover treatments, except for the 25% treatment in red pine stands. Leaf N concentrations in understory vegetation were also significantly higher in the clearcut (1991, 20.8mgg−1; 1992, 22.4mgg−1) than in the uncut treatment (1991, 16.5mgg−1; 1992, 16.9mgg−1). Canopy type (northern red oak and red pine) had little influence on understory nutrient status and leaf mass. In addition, fronds of bracken ferns in all canopy cover treatments in both northern red oak and pine stands were a major sink of nutrients in the understory. The results of this study showed that partial canopy removal generally had only a minor impact on understory leaf production and nutrient status compared with clearcuts during the 2-year period following canopy removal.

Variation of carbon and nitrogen cycling processes along a topographic gradient in a central Amazonian forest

AbstractIt is well recognized in the literature that topography can influence soil nutrient stocks and dynamics in temperate regions, but for tropical forests, this source of variation has sometimes been ignored. The nature of such variations may depend upon the soil type, which in turn, is closely linked to local or regional topography. This study characterizes the soil and describes the status of carbon and nitrogen in vegetation, litterfall, litter‐layer and soil upper layers along the main positions of a topographic gradient (plateau, slope and valley), 60 km north of Manaus, on Cuieiras Reserve watershed. Nitrogen concentrations in living leaves, fresh litterfall, litter‐layer and soil upper layers were lower in the valley than in both slope and plateau plots. Carbon concentrations in plant material were not significantly different among the three topographic positions, resulting in higher C : N ratios in valley plots. Local topography (plateau, slope and valley) clearly was an influential factor in the nutrient distribution along the study locations. Lower rates of N cycling processes in the valley are probably related to its sandy soil texture and seasonal flooding.

Urea fertilizations of a Norway spruce stand: effects on nitrogen in soil water and field-layer vegetation after final felling

Effects of previous fertilization with N (in total, 600 kg urea-N·ha–1 applied in 1976, 1980, and 1985) were studied after final felling in 1992 of a Norway spruce (Picea abies (L.) Karst.) stand in southern Sweden. The logging residues were removed from the site. In the clearcut, soil water at 50 cm depth was sampled 16 times with ceramic suction samplers (P80) in experimental plots during 1992–1995. The biomass and N content of the field layer was measured on seven occasions. The N storage of the field layer was significantly (p < 0.05) higher in the urea treatment than in the control. Significant interactions between treatment and time were found in soil water for nitrate-N and total N but not for ammonium-N, organic N, and pH. During the first year after final felling, nitrate-N tended to increase faster in the urea treatment than in the control. After a period with similar concentrations in both treatments, nitrate-N in the urea treatment declined while at the same time, a peak was observed in the control showing four to seven times higher concentrations than in the urea treatment. At the end of the study, the concentrations still appeared to be highest in the control. Thus, the study demonstrated the importance of using a sufficiently long study period when investigating environmental effects. Total leaching of nitrate-N from the urea treatment was roughly 40% ([Formula: see text]20 kg·ha–1) less than that from the control. The difference in leaching may be partly explained by the greater accumulation of N in the field-layer vegetation in the urea treatment.

Vernal nitrogen and phosphorus retention by forest understory vegetation and soil microbes

Northern hardwood forests experience annual maximal loss of nutrients during spring. The vernal dam hypothesis predicts that spring ephemeral herbs in northern hardwood forests serve as sinks for nutrients during this season and reduce the loss of nutrients from the terrestrial ecosystem. Soil microbes of northern hardwood forests also sequester nutrients during spring. We compared the vernal nutrient acquisition ability of a soil microbial community and an understory plant community with species of mixed leaf phenology. We monitored nitrogen and phosphorus pool sizes in understory vegetation and soil microbes during spring from 1999 through 2001 in a northern hardwood forest in the Catskill Mountains, New York. Vegetation nutrient content increased during two spring seasons by an average of 3.07 g N m−2 and 0.19 g P m−2 and decreased during one spring by 0.81 g N m−2 and 0.10 g P m−2. Evergreen, wintergreen, and deciduous plant species were able to sequester nutrients during spring. Soil microbial nutrient content decreased during one spring by 1.29 g N m−2 and remained constant during the other two springs. Streamwater nitrogen losses were not correlated with biotic nutrient uptake suggesting a temporal disconnect between the two processes. We conclude that understory vegetation is a larger potential sink for vernal nutrients than are soil microbes in this northern hardwood forest and understory and species representing multiple phenologies are capable of vernal nutrient uptake.

Remote sensing of nitrogen and lignin in Mediterranean vegetation from AVIRIS data: Decomposing biochemical from structural signals

Remote sensing estimates of vegetation nitrogen (N) and lignin concentration are central to assess ecosystem processes such as growth and decomposition. Although remote sensing techniques have been proven useful to assess N and lignin contents in continuous green canopies, more studies are needed to address their capabilities, particularly in low and sparsely vegetated ecosystems. We investigated the possibility of estimating canopy N and lignin concentrations in chaparral vegetation using Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) reflectance acquired over an area around Point Dume in the Santa Monica Mountains (Los Angeles, CA, USA). Two approaches were tested: multiple stepwise regression based on first difference reflectance (FDR) and reflectance ( R) indices. Multiple stepwise regressions (of three or fewer wavelengths) accounted for a large variance in canopy biochemical concentration ( r 2∼0.9, P<0.01). Log transformed R indices [log (1/ R)] formulated on the basis of previously known N and lignin absorption wavelengths also showed significant correlations ( P<0.01) with canopy biochemical concentration ( r 2 ranging from 0.39 to 0.48). In addition, the contribution of structural and biochemical signals and background effects on the performance of these indices was evaluated. These indices accounted for a increased variance when adding information on canopy structural attributes (e.g., relative contribution of each species and biomass amount) to foliar biochemical concentration. The relative contributions of foliar biochemical concentration and canopy structure (biomass amount) on the spectral signal were further evaluated by analyzing the residuals from linear regressions: foliar N concentration accounted for 42% of the variance for a normalized difference index based on the 1510-nm N absorption feature, while the foliar lignin concentration accounted for 44% of the variance for a normalized difference index based on the 1754 nm lignin absorption feature. These percentages increased to 58% when stands with senescing vegetation were disregarded. We propose the two indices, Normalized Difference Nitrogen Index (NDNI=[log (1/ R 1510)−log (1/ R 1680)]/[log (1/ R 1510)+log (1/ R 1680)]) and Normalized Difference Lignin Index (NDLI=[log (1/ R 1754)−log (1/ R 1680)]/[log (1/ R 1754)+log (1/ R 1680)]) as indices to assess N and lignin in native shrub vegetation.

Parameter estimation for a global model of terrestrial biogeochemical cycling by an iterative method

We have implemented a geographically distributed ecosystem model TERRA for the carbon, nitrogen, and water dynamics of the global terrestrial biosphere. The ecosystem model in each grid cell has state variables of soil water; vegetation carbon, soil carbon, vegetation nitrogen, soil organic nitrogen, soil inorganic nitrogen, and a variable for allocation. Eight parameters associated with eight carbon or nitrogen fluxes are determined during model calibration at specific sites for each of 17 vegetation types that cover the globe. Calibration is performed by an iterative method that brings calculated fluxes into agreement with observed fluxes over successive iterations. For the 17 vegetation types, calibration required a geometric mean number of 123 iterations for convergence with a geometric S.D. of 2.25. The minimum and maximum iterations required was 52 and 1060 for xeromorphic woodland and tropical evergreen forest, respectively. The parameter controlling gross primary productivity C max was found to be correlated with maximum projected leaf area index (plus cover of nonvascular plants) with a correlation coefficient of 0.90 ( r 2=0.81). Correlation of parameters with the input fluxes and average standing crops corresponding to the iteration equation was high in those cases in which temperature and precipitation were not explicit factors in the iteration equation. The parameters K fall, L nc, and N loss had correlation coefficients of 1.0 each with the appropriate ratio of the matching observed transfer flux and standing crop. In iteration equations with explicit dependence on temperature, soil moisture, and other factors, the correlation coefficients were less than 1.0 with these ratios; the parameters C max, K r, K d, N max, and N up had correlation coefficients of 0.83, 0.66, 0.60, 0.61, and 0.87, respectively. These correlation coefficients indicate the importance of environmental factors such as temperature and soil moisture in the calibration process and the robustness of the iteration method in determining parameters in systems with time-varying coefficients.