Aboveground Plant Material Research Articles

Comparing tree‐ring and permanent plot estimates of aboveground net primary production in three eastern U.S. forests

AbstractForests account for a large portion of sequestered carbon, much of which is stored as wood in trees. The rate of carbon accumulation in aboveground plant material, or aboveground net primary productivity (aNPP), quantifies annual to decadal variations in forest carbon sequestration. Permanent plots are often used to estimate aNPP but are usually not annually resolved and take many years to develop a long data set. Tree rings are a unique and infrequently used source for measuring aNPP, and benefit from fine spatial (individual trees) and temporal (annual) resolution. Because of this precision, tree rings are complementary to permanent plots and the suite of tools used to study forest productivity. Here we evaluate whether annual estimates of aNPP developed from tree rings approximate estimates derived from colocated permanent plots. We studied a lowland evergreen (Howland, Maine), mixed deciduous (Harvard Forest, Massachusetts), and mixed mesophytic (Fernow, West Virginia) forest in the eastern United States. Permanent plots at the sites cover an area of 2–3 ha, and we use these areas as benchmarks indicative of the forest stand. We simulate random draws of permanent plot subsets to describe the distribution of aNPP estimates given a sampling area size equivalent to the tree‐ring plots. Though mean tree‐ring aNPP underestimates permanent plot aNPP slightly at Howland and Fernow and overestimates at Harvard Forest when compared with the entire permanent plot, it is within the 95% confidence interval of the random draws of equal‐sized sampling area at all sites. To investigate whether tree‐ring aNPP can be upscaled to the stand, we conducted a second random draw of permanent plot subsets simulating a twofold increase in sampling area. aNPP estimates from this distribution were not significantly different from results of the initial sampling area, though variance decreased as sampling area approaches stand area. Despite several concerns to consider when using tree rings to reconstruct aNPP (e.g., upscaling, allometric, and sampling uncertainties), the benefits are apparent, and we call for the continued application of tree rings in carbon cycle studies across a broader range of species diversity, productivity, and disturbance histories to fully develop this potential.

Organic input quality is more important than its quantity: C turnover coefficients in different cropping systems

Abstract Annual C input to soil is a major factor affecting soil organic carbon (SOC) dynamics. However different types of C-sources can have different behaviour, in relation to their chemical characteristics and how they interact with soil. Root-derived C, in particular, should be more efficient than other organic materials as a result of the physicochemical and biological characteristics of the surrounding environment, leading to a reduction in the C decomposition rate. To test this hypothesis, we considered a long-term experiment underway in Northern Italy since 1962, comparing permanent meadow and 6 different crop rotations over a wide range of nutrient inputs, in both organic and inorganic forms. C inputs from amendments were measured and those from crops were calculated using allometric functions and crop and residues yields. The time evolution of SOC was studied through a single-pool, first-order kinetic model, allowing the estimation of humification coefficients for residues, roots, farmyard manure and cattle slurries. The highest value of the humification coefficient was estimated for farmyard manure, which confirmed its high efficiency in stabilising SOC content. Root C presented a humification coefficient 1.9 times higher than above-ground plant materials while slurries were intermediate, with a humification coefficient roughly half that of farmyard manure and even lower that of roots. The quality of C input thus seems of fundamental importance for evaluating the sustainability of different cropping systems in terms of SOC dynamics.

Impact of grassland management regimes on bacterial endophyte diversity differs with grass species.

Most plant species are colonized by endophytic bacteria. Despite their importance for plant health and growth, the response of these bacteria to grassland management regimes is still not understood. Hence, we investigated the bacterial community structure in three agricultural important grass species Dactylis glomerata L., Festuca rubra L. and Lolium perenne L. with regard to fertilizer application and different mowing frequencies. For this purpose, above-ground plant material was collected from the Grassland Management Experiment (GrassMan) in Germany in September 2010 and 2011. DNA was extracted from surface-sterilized plant tissue and subjected to 16S rRNA gene PCRs. Endophytic community structures were assessed by denaturing gradient gel electrophoresis (DGGE)-based analysis of obtained PCR products. DGGE fingerprints revealed that fertilizer application significantly altered the endophytic communities in L.perenne and F.rubra but not in D.glomerata. Although no direct effect of mowing was observed, mowing frequencies in combination with fertilizer application had a significant impact on endophyte bacterial community structures. However, this effect was not observed for all three grass species in both years. Therefore, our results showed that management regimes changed the bacterial endophyte communities, but this effect was plant-specific and varied over time. Endophytic bacteria play an important role in plant health and growth. However, studies addressing the influence of grassland management regimes on these bacteria in above-ground plant parts are still missing. In this study, we present first evidence that fertilizer application significantly impacted bacterial community structures in three agricultural important grass species, whereas mowing had only a minor effect. Moreover, this effect was plant-specific and thus not visible for all grass species in each year. Consequently, this study sheds new light into the complex interaction of microbes and plants.

Beneficial Use of Dewatered and Composted Sewage Sludge as Soil Amendments: Behaviour of Metals in Soils and Their Uptake by Plants

In Portugal, where soils have a very low topsoil organic matter content, the use of sewage sludge (SS) as organic soil improvers seems an attractive option, because it would enable organic matter, N, P, K and other nutrients to be recycled. However, the risk of this practice must be properly assessed. The aim of this study was to evaluate the potential risk of the use of dewatered and composted SS as soil amendments. For this purpose, SS from two different wastewater treatment plants (SS1 and SS2), and a compost produced from SS and agricultural wastes (AWSSC), were characterized for their total metal concentrations, organic contaminants and indicator pathogens, and used in a pot experiment with three application rates, 6, 12 and 24 ton dry matter/ha, cultivated with a hybrid variety of sorghum and Sudan grass (Sorghum bicolor × Sorghum sudanense var. Rocket). SS1 and AWSSC met the legal requirements to be applied to soils, but SS2 had a high content of pathogens, which compromised its use. Both dewatered SS had a marked beneficial effect on plant production and on soil nutritional characteristics, more pronounced than in the case of AWSSC application, without a significant increase in total and in mobile metals concentration in soils. Bioaccumulation factors for metals in plants were low, and their concentrations in the aboveground plant material were lower than the maximum tolerable level for cattle, used as a risk indicator of metal entry into the human food chain. However, it will be necessary, in future studies, to evaluate the potential risk of the observed increase in the mobilisable content of Cu and Zn in soil, as a consequence of the application of these organic materials.

Additional nitrogen in arctic‐alpine soils and plants—a pilot study with 15 NO$ _3^ - $ and 15 NH$ _4^+ $ fertilization along an elevation gradient

Nitrogen (N) deposition has been increasing in alpine ecosystems, but its fate in soils and plants remains unclear. We assumed that the increased N load will be efficiently retained in alpine ecosystems but that the degree of N use efficiency changes with elevation. Thus, we performed a 3-year 15N tracer experiment, in which we added 1 g m−2 of either NH415NO3 or 15NH4NO3 fertilizer to a plot of 1 m2 in size at three elevations. Composite soil samples and aboveground plant material from lichens, dwarf shrubs, and graminoids were collected annually for three years and analyzed for their 15N accrual. We found a cumulative and plateauing rise in 15N concentration in soils and plants at all sites. However, overall recovery of the tracer decreased with time, amounting to 71% of fertilizer recovered in the soils in the first year, 69% recovered in soils and plants in the second year, and 37% in soils and plants in the third year. Moreover, the fertilizer use efficiency varied among fertilizer types and plant functional types. This utilization pattern appears to be modulated by elevation.

Effects of Biochars and Compost Mixtures and Inorganic Additives on Immobilisation of Heavy Metals in Contaminated Soils

Biochar (BC) as soil amendment has the potential to support the remediation of polluted soils. The aim of our study was to test metal extractability in a contaminated soil and metal accumulation in plants in response to the application of BC and compost-based and inorganic soil amendments. The soil used in our study was taken from a long-term polluted site in Austria (Arnoldstein). The treatments included three BC/compost mixtures (50/50 w/w; using three different BC types), two inorganic amendments (gravel sludge (GS) with siderite-bearing (Fe) material (GSFe, 50/50 w/w) and lime (L)) and an unamended control; each treatment consisted of 1.5 % in 1.2 kg soil. Poplar biochar (P-BC), GSFe and L significantly decreased the NH4NO3-extractable fraction of Pb, Zn and Cd, but not of Cu. The highest immobilisation effect within the organic amendments was found after P-BC, reducing NH4NO3-extractable Zn by 63 %. In contrast to the NH4NO3 extractions, heavy metal concentrations in the aboveground plant material (shoots of Lolium multiflorum) did not generally decrease. Still, Cd, Ni and Zn concentrations in L. multiflorum grown on the amended soil were lower after the P-BC treatment. Cu and Pb showed slightly higher plant concentrations upon amendment addition, indicating that NH4NO3 extractions did not entirely reflect plant availability. Our data show that the type of BC and the addition of compost are crucial when using sorbent additives as a remediation strategy for metal-polluted soils and that the desired immobilisation effects cannot be achieved for all heavy metals. Depending on the type of metal, specific biochars and compost mixtures can either cause immobilisation or mobilisation.

Effects of drought and elevated temperature on biochemical composition of forage plants and their impact on carbon storage in grassland soil

Aims The objective of this study was to investigate the effects of future warming and drought on (1) the biochemical composition of above-ground biomass of forage plants (Festuca arundinacea and Dactylis glomerata), (2) the potential mineralization of this material in soil, and (3) its priming effect on native soil organic matter. Methods We sampled above-ground plant material from spring regrowth and summer regrowth of a climate change experiment. While in spring, the plants were well watered, the summer regrowth was exposed to drought and elevated temperature (+3 °C) by infrared heating of the canopy during 3w eeks. We assessed the elemental and isotopic composition, lignin and non-cellulosic carbohydrate content and composition of plant material grown under all three conditions. Its mineralization potential in soil and priming effects were evaluated during laboratory incubation. Results Warming had no significant effect on elemental and stable isotope composition of both plant materials. In contrast, it resulted in reduction of lignin content for both plant species and decrease of the lignin-to-N ratio for F. arundinacea and increased non-cellulosic carbohydrate content for D. glomerata. Summer regrowth was characterised by increase of δ 13 C values, which is consistent with variations in stomatal conductance due to water shortage. Moreover, summer drought induced an increase in N content leading to decrease of the C/N ratio and increase of lignin-to-N ratio of summer regrowth compared to spring regrowth. Differences in decomposition were small, while priming effects were more strongly altered by the different exposure to enviromental. Conclusion Our results provide direct experimental evidence that extreme climatic events (high temperature and precipitation deficit) have an influence on soil carbon storage particularly through their effect on priming of native soil organic matter induced by altered plant litter. These effects seem to be governed by alterations of stoichiometry and to a smaller extent by alterations of plant chemical composition.

Estimating nutrient uptake requirements for wheat in China

Estimating balanced nutrient requirement for wheat (Triticum aestivum L.) in China is essential to manage nutrient application more effectively for increasing crop yields and reducing risk of negative environmental impact. Datasets from 2000 to 2011 dealing with nitrogen (N), phosphorus (P) and potassium (K) treatments across the winter and spring wheat growing regions in China were collected to assess the relationship between grain yield and nutrient uptake, and to estimate N, P and K optimal nutrient requirements for a target yield using the QUEFTS (Quantitative Evaluation of the Fertility of Tropical Soils) model. In the QUEFTS model, two boundary lines described the minimum and maximum internal efficiencies (IEs, kg grain per kg nutrient in above-ground plant dry matter) of N, P and K. The minimum and maximum IEs for wheat were 28.8 and 62.6kg grain per kg N, 98.9 and 487.4kg grain per kg P, and 23.0 and 112.9kg grain per kg K. The QUEFTS model predicted a linear–parabolic–plateau curve for balanced nutrient uptake with target yield increasing. The linear part continued until the yield was approximately at 60–70% of the potential yield, and 22.8kg N, 4.4kg P and 19.0kg K were required to produce 1000kg grain. The corresponding N:P:K ratio was 5.18:1:4.32, and the corresponding IEs were 43.9, 227.0 and 52.7kg grain per kg N, P and K, respectively. The QUEFTS model simulated balanced N, P and K removal by 1000kg grain were 18.3, 3.6 and 3.5kg, respectively, with a N:P:K ratio of 5.08:1:0.97. Approximately 80%, 82% and 18% of N, P and K in total above-ground plant material were presented in the grain and removed from the field. The relationship between grain yield and nutrient uptake was also estimated to suggest fertilizer application avoiding excess or deficient nutrient supply. Field experiment validation confirmed that the QUEFTS model could be used as a practical tool for the Nutrient Expert decision support system to make fertilizer recommendation.

Dating Recent Peat Accumulation in European Ombrotrophic Bogs

This study compares age estimates of recent peat deposits in 10 European ombrotrophic (precipitation-fed) bogs produced using the 14C bomb peak, 210Pb, 137Cs, spheroidal carbonaceous particles (SCPs), and pollen. At 3 sites, the results of the different dating methods agree well. In 5 cores, there is a clear discrepancy between the 14C bomb peak and 210Pb age estimates. In the upper layers of the profiles, the age estimates of 14C and 210Pb are in agreement. However, with increasing depth, the difference between the age estimates appears to become progressively greater. The evidence from the sites featured in the study suggests that, provided aboveground plant material (seeds, leaves) is selected for dating, the 14C bomb peak is a reliable dating method, and is not significantly affected by the incorporation of old carbon with low 14C content originating from sources including air pollution deposition or methane produced by peat decomposition. 210Pb age estimates that are too old may be explained by the enrichment of 210Pb activity in the surface layers of peat resulting from a hypothesized mechanism where rapidly infilling hollows, rich in binding sites, may scavenge 210Pb associated with dissolved organic matter passing through the hollow, as part of the surface drainage network. Until further research identifies and resolves the cause of the inaccuracy in 210Pb dating, age estimates of peat samples based only on 210Pb should be used with caution.

Net ammonification as influenced by plant diversity in experimental grasslands

Previous plant diversity experiments have mainly reported positive correlations between diversity and N mineralization. We tested whether this relationship can be explained by plant diversity-induced changes i) in the quantity or quality of organic matter or ii) in microclimatic conditions of central European grassland mixtures.We measured ex-situ net ammonification in a laboratory incubation of aboveground plant material and soil sampled in differently diverse plant mixtures. Secondly, in-situ net ammonification was assessed in a field incubation with mineralization cores containing standardized material in four treatments: soil only (control), and soil mixed with field-fresh plant tissue (grass, legume, or tall herb). We used 82 plots with varying species numbers (1, 2, 4, 8, 16, and 60) and numbers of functional groups (1–4; grasses, short herbs, tall herbs, and legumes). We determined the soil water content, total N concentrations of plant and soil, and NH4–N release rates.In the ex-situ incubation under constant climatic conditions, functional group or plant species richness did not influence net ammonification rate constants (k) or the proportion of the organic N pool involved in ammonification (NH4–N0). The presence of legumes in plant mixtures significantly increased NH4–N0 and decreased k indicating elevated N leaching risks in legume-containing grassland mixtures. Mean in-situ net ammonification rates in the mineralization cores decreased in the following order: mixtures of soil with grasses (0.30 ± standard error 0.01 mg NH4–N (g Ninitial)−1 d−1) > tall herbs (0.25 ± 0.01) > legumes (0.22 ± 0.01) > control (0.07 ± 0.00). The type of incubated plant tissue also influenced the soil water content in the mineralization cores at the end of field incubation, likely because of different water retention capacities of the different plant tissue/soil mixtures. Significant plant functional group and species richness effects explained up to 13% of the variance of in-situ net ammonification rates. Because the effect of plant species richness disappeared if the type of incubated plant tissue and the soil water content were accounted for in a sequential ANCOVA, we infer that the soil water content was the main driver underlying the plant species richness effect.

Utilização de Lolium Perenne L. na fitoestabilização controlada de solos degradados por actividades mineiras

We studied the application of Lolium perenne L. and of different organic residues in the assisted phytostabilization of mine-contaminated soils (Aljustrel Mine Area, Iberian Pyrite Belt). Although the application of the organic residues led to immobilization of Cu, Pb, and Zn in the soil, decreasing their effective available fraction, their concentrations in the above-ground plant material did not change. The accumulation factors (AF) obtained were AF(Cu) and AF(Pb) <<1, and AF(Zn) <1, allowing its election as a possible candidate for phytostabilization purposes. However, Pb and Cu concentrations in ryegrass shoots were considered toxic, which suggests that there is a risk of contamination of the human food chain. Multivariate statistical analysis emphasized sewage sludge as the organic residue with the greatest capacity to correct soil acidity, and to improve soil characteristics, followed by the compost obtained from municipal solid waste residues. However, an impaired growth was obtained when using the highest application rate (100 Mg ha-1) of both residues. All garden waste compost rates were unable to correct general soil constraints to plant growth.

Variation in Yield, Phosphorus Uptake, and Physiological Efficiency of Wheat Genotypes at Adequate and Stress Phosphorus Levels in Soil

Genetic differences among crop genotypes can be exploited for identification of genotypes more suited to a low‐input agricultural system. Twenty wheat (Triticum aestivum L.) genotypes were evaluated for their differential yield response, phosphorus (P) uptake in grain and straw, and P‐use efficiency at the zero‐P control and 52 kg P ha−1 rates. Substantial and significant differences were obvious among genotypes for both grain and straw yields at stress (8 mg P kg−1 soil, native soil P, no P addition) and adequate (52 kg P ha−1) P levels. Genotype 5039 produced maximum grain yield at both P levels. Relative reduction in grain yield due to P‐deficiency stress [i.e., P stress factor (PSF)] ranged between none and 32.4%, indicating differential P requirement of these genotypes. Pasban 90, Pitic 62, Rohtas 90, Punjab 85, and line 4943 did not respond to P application and exhibited high relative yield compared to those at adequate P level. FSD 83 exhibited the best response to P with maximum value for PSF (32.4%). Genotypes were distributed into nine groups on the basis of relationship between grain yield and total P uptake. Rohtas 90 and lines 4072 and 5039 exhibited high grain yield and medium P uptake (HGY‐MP). However, line 5039 with high total index score utilized less P (12.2 kg P ha−1) than line 4072 and Rohtas 90 (13.5 and 13.6 kg P ha−1, respectively). Moreover, this genotype also had greater P harvest index (PHI, %) and P physiological efficiency index (PPEI) at stress P level. Pasban 90, Pitic 62, and Pak 81 had the greatest total index score (21), mainly due to high total P uptake, but yielded less grain than lines 5039 and 4072 under low available P conditions. Line 6142 had minimum total index score (15) and also produced minimum grain yield. A wide range of significant differences in PPEI (211 to 365 kg grain kg−1 P absorbed at stress and 206 to 325 kg grain kg−1 P absorbed by aboveground plant material at adequate P) indicated differential utilization of absorbed P by these genotypes for grain production at both P levels. It is concluded from the results that wheat genotypes differed considerably in terms of their P requirements for growth and response to P application. The findings suggest that PSF, PHI, and PPEI parameters could be useful to determine P‐deficiency stress tolerance in wheat.

Carbon Sequestration Potential of Extensive Green Roofs

Two studies were conducted with the objective of quantifying the carbon storage potential of extensive green roofs. The first was performed on eight roofs in Michigan and four roofs in Maryland, ranging from 1 to 6 years in age. All 12 green roofs were composed primarily of Sedum species, and substrate depths ranged from 2.5 to 12.7 cm. Aboveground plant material was harvested in the fall of 2006. On average, these roofs stored 162 g C x m(-2) in aboveground biomass. The second study was conducted on a roof in East Lansing, MI. Twenty plots were established on 21 April 2007 with a substrate depth of 6.0 cm. In addition to a substrate only control, the other plots were sown with a single species of Sedum (S. acre, S. album, S. kamtshaticum, or S. spurium). Species and substrate depth represent typical extensive green roofs in the United States. Plant material and substrate were harvested seven times across two growing seasons. Results at the end of the second year showed that aboveground plant material storage varied by species, ranging from 64 g C x m(-2) (S. acre) to 239 g C x m(-2) (S. album), with an average of 168 g C x m(-2). Belowground biomass ranged from 37 g C x m(-2) (S. acre) to 185 g C x m(-2) (S. kamtschaticum) and averaged 107 g C x m(-2). Substrate carbon content averaged 913 g C x m(-2), with no species effect, which represents a sequestration rate of 100 g C x m(-2) over the 2 years of this study. The entire extensive green roof system sequestered 375 g C x m(-2) in above- and belowground biomass and substrate organic matter.

Belowground nitrogen partitioning in experimental grassland plant communities of varying species richness

Partitioning of soil nitrogen (N) by niche separation among species may be an important mechanism explaining species coexistence and positive biodiversity-productivity relationships in terrestrial plant communities. However, there is little experimental evidence for such partitioning, in particular, as assessed across a gradient of species richness. In experimental communities of one, three, and six temperate grassland species in the field, we tested whether increasing species richness (1) decreases niche breadths of individual species, (2) decreases niche overlap among species, and (3) increases niche breadth of whole communities. Six N sources consisting of three different chemical forms of 15N-labeled N (15NO3(-), 15NH4+, 13C2-15N-glycine) injected at two soil depths (3 and 12 cm) were applied to each community. The chemical form and the soil depth of N characterize the niches for which niche breadth (Levins' B) and overlap (proportional similarity) were measured. After 48 hours, aboveground plant material was harvested to measure 15N enrichment. As expected, niche breadth of single species and niche overlap among species decreased with increased species richness, but community niche breadth did not increase. The decrease in niche breadth and niche overlap mostly occurred among subordinate species or pairs of subordinate and dominant species, rather than among dominant species. Species in the six-species mixtures mostly preferred NO3(-) from shallow soil. This may be partly explained by the presence of legumes in all six-species mixtures which allowed "N sparing" (i.e., increased availability of soil N since legumes rely more on atmospheric N2 than on soil N). Niche separation with respect to N uptake from different chemical forms and soil depths did not contribute much to facilitating the coexistence of dominant species, nor do our results suggest it as a major driver of positive diversity-ecosystem functioning relationships. However, partitioning of N may be important for the persistence of subordinate species.

Determination of the soil organic carbon, nitrogen, available phosphorus and the combined aboveground plant materials in the semi-arid Mbulu District, Tanzania

Soil of the semi-arid Mbulu District is part of the tropical soils, covered with sparse trees, shrubs or grasses in which domestic grazing animals have prevented the wide spread of vegetation cover. The study aimed at determining soil organic carbon (OC), total nitrogen (N), available phosphorus (P) and the combined aboveground plant materials. Six study sites were established in which soil samples were collected at the depths of 0–5, 6–10 and 11–20 cm. Soil samples were analysed for OC, N and P as well as the levels of N and P in the combined aboveground materials of Panicum coloratum and Hyparrhenia filipendula. The percentage concentrations of OC, N and P were high in the top soil than in the deeper soil horizons. However, analysis of variance showed significant differences of OC in some sites whereas no difference for N and P between soil depth classes. OC was highly related with N and P along soil depth classes. It was concluded that the availability of N and P was because of the decomposition of organic matter in the soil. Soil N and P were highly related with the same in the combined aboveground plant materials. It was concluded that the increased concentration of N and P in the soil resulted into availability of the same in P. coloratum and H. filipendula. There was a very high variation in N and P among sites with different levels of intensity of grazing. It was concluded that grazing animals contributes to the redistribution of soil elements in the rangelands because they graze upon plant parts but the excreta are dropped away from the grazed spot. Le sol du district semi-aride de Mbulu fait partie de ces sols tropicaux couverts d’arbres, de buissons et d’herbes rares où le pâturage des animaux domestiques a empêché une large dispersion du couvert végétal. L’étude visait à déterminer le carbone organique (CO) du sol, l’azote (N) total, le phosphore (P) disponible et l’ensemble combiné de la matière végétale aérienne. Six sites d’étude furent établis, où l’on a récolté des échantillons de sol à des profondeurs de 0 à 5 cm, 6 à 10 cm, et 11 à 20 cm. Les échantillons de sols ont été analysés pour le CO, le N et le P ainsi que les niveaux de N et de P dans la matière végétale aérienne composée de Panicum coloratum et Hyparrhenia filipendula. Le pourcentage des concentrations de CO, N et P était plus élevé dans la couche supérieure du sol que dans les couches plus profondes. Cependant, une analyse de variance a révélé des différences significatives du CO dans certains sites alors qu’il n’y avait pas de différences pour N ni P aux différentes profondeurs de sol. Le CO était fortement liéà N et à P selon les classes de profondeur. On en a conclu que la disponibilité de N et de P était due à la décomposition de la matière organique dans le sol. Le N et le P du sol étaient fortement liés aux mêmes éléments présents dans la matière végétale aérienne combinée. On a conclu que la concentration accrue de N et de P dans le sol résultait de la concentration de ces mêmes éléments dans P. coloratum et H. filipendula. Il y a avait une très grande variation de N et de P entre des sites où l’intensité de pâturage était différente. On a conclu que les animaux qui pâturent contribuent à la redistribution des éléments du sol dans tous les endroits fréquentés étant donné qu’ils mangent des plantes à certains endroits mais que leurs excréments peuvent être rejetés ailleurs que les zones pâturées.

Estimativa do Índice de Área Foliar (IAF) e Biomassa em pastagem no estado de Rondônia, Brasil

Medidas mensais da altura da pastagem, biomassa total, variações de biomassa viva e morta, a área específica foliar (SLA) e o Índice de Área de Folha (IAF) de fevereiro de 1999 a janeiro de 2005 na Fazenda Nossa Senhora (FNS) e em Rolim de Moura (RDM) entre Fevereiro a Março de 1999, Rondônia, Brasil. A pastagem predominante é Urochloa brizantha (Hochst. ex A. Rich) R. D. Webster (99% na FNS e 76% em RDM), com pequenas manchas de Urochloa humidicula (Rendle). A altura média anual da grama foi de ~0,16 m. Com o pastejo, o mínimo mensal foi de 0,09 m (estação seca) e máximo de 0,3 m sem pastejo (estação úmida). O IAF, biomassa total, material morto, vivo e SLA tiveram valores médios de 2,5 m² m-2 , 2202 kg ha-1, 2916 kg ha-1 e 19 m² kg-1 respectivamente. A média mensal da biomassa foi 4224 kg ha-1 em 2002 e 6667 kg ha-1 em 2003. Grande variação sazonal do material vivo e morto, sendo mais alto o vivo durante a estação úmida (3229 contra 2529 kg ha-1), sendo o morto maior durante a seca (2542 contra 1894 kg ha-1). O nível de água no solo variou de -3,1 a -6,5 m durante as estações. Em médias anuais os IAF foram de 1,4 em 2000 a 2,8 em 2003 e o SLA entre 16,3 m² kg-1 em 1999 e 20,4 m² kg-1 em 2001. As observações do Albedo variaram de 0,18 para 0,16 em relação aos altos valores de IAF.

Nutrient Status in Alpine Soils of the Colorado Front Range Using the Nitrogen/Phosphorus Ratio Index

The suitability of the foliar N/P ratio was evaluated as a predictor of nutrient limitation in an alpine ecosystem of the Colorado Front Range. We hypothesized that foliar N/P ratios are directly correlated with the alpine soil nutrient status. We used a long‐term fertilization experiment conducted in three alpine plant communities, where 48 plots were established consisting of four replicates of control, N, P, and N + P additions. We characterized four extractable P fractions, maximum P sorption capacity, and extractable NO3–N and NH4–N, measured N and P adsorption fluxes, determined the soil moisture content, and measured the total N and P concentrations in soils and below‐ and aboveground plant material. The fertilized plots exhibited significantly higher nutrient concentrations in above‐ and belowground plant materials and adsorption fluxes of N and P (P &lt; 0.001 for all additions) than the control plots. The N‐added plots were significantly acidified (&lt;3.8) vs. the control plots (&gt;4.14), which may have partially contributed to increased P flux. A correlation analysis showed that N and P accumulations in aboveground plant material could not be predicted by the traditional extractable N and P tests. Nutrient adsorption flux measurements and especially P showed better correlation with nutrient accumulation in below‐ and aboveground plant material (r = 0.53 and 0.43, respectively, P &lt; 0.001). This moderate correlation suggested, however, that the N/P ratio index is somewhat limited in providing a definitive answer for the nutrient limitation status of this alpine ecosystem.

Effect of an invasive grass on ambient rates of decomposition and microbial community structure: a search for causality

In situ decomposition of above and belowground plant biomass of the native grass species Andropogon glomeratus (Walt.) B.S.P. and exotic Imperata cylindrica (L.) Beauv. (cogongrass) was investigated using litter bags over the course of a 12 month period. The above and belowground biomass of the invasive I. cylindrica always decomposed faster than that of the native A. glomeratus. Also, belowground biomass of both species decomposed at a consistently faster rate when placed within an invaded area consisting of a monotypic stand of I. cylindrica as opposed to within a native plant assemblage. However, there was no similar such trend observed in the aboveground plant material. The microbial communities associated with the invaded sites often differed from those found in the native vegetation and provide a possible causal mechanism by which to explain the observed differences in decomposition rates. The microbial communities differed not only compositionally, as indicated by ordination analyses, but also functionally with respect to enzymatic activity essential to the decomposition process. This study supports the growing consensus that invasive plant species alter normal ecological processes and highlights a possible mechanism (alteration of microbial assemblages) by which I. cylindrica may alter an ecosystem process (decomposition).

Salt Marsh Dieback: An overview of recent events in the US

This paper provides an overview of the marsh dieback events that have been observed along the east and gulf coasts of the U.S. over the past decade. It is likely that some of the recently reported changes in marsh vegetation were affected by physical or biotic disturbances that are known to generate bare areas, such as overgrazing or wrack smothering. Other areas may be experiencing a state change such as that caused by long-term changes in sea level. However, sites in many areas are not readily explained by these causes and are considered to have experienced “sudden dieback.” In such cases, there are observations that the above-ground plant material thinned or browned or, in some cases, failed to re-emerge in the spring; the dieback occurred over a period of months and usually affected multiple sites within the area; and there is evidence that these events are transient (through successful transplants or natural regrowth/recovery), although some areas take years to recover. We explored the potential linkage of dieback with drought (as characterized by the Palmer Severity Drought Index), and found that there is evidence for an association in the southeast (GA and SC) and the Gulf (LA), but not in the mid-Atlantic (DE, VA) or northeast (ME, RI, CT). We also review the evidence for potential causes of sudden dieback, including changes in soil chemistry, fungal pathogens, top–down consumer controls, and multiple stressors. There is currently no single explanation that can be applied to recent dieback. We highlight the need for the development of improved diagnostics that will allow us to better classify dieback areas and provide evidence for (or against) potential causes.

Chemical changes to nonaggregated particulate soil organic matter following grassland‐to‐woodland transition in a subtropical savanna

Encroachment of thorn woodlands into grasslands of southern Texas has resulted in greater aboveground and belowground biomass and greater soil organic carbon (SOC) stocks. Our previous studies showed that a large percentage of the SOC accrued under invading woody clusters was not stabilized within protective soil aggregates or on mineral‐surfaces. Here we evaluated lignin and cutin‐ and suberin‐derived substituted fatty acid (SFA) chemistry to determine if the accrual of nonaggregated particulate organic matter (POM) in woodlands was promoted by inherently greater recalcitrance of tissues from woody versus grass species, and if there was selective input of aboveground versus belowground plant carbon to POM. Woody clusters exhibited reduced concentrations of cutin‐derived SFA and cinnamyl phenols within surface litter compared to fresh aboveground plant material. However, root litter exhibited relatively minor changes in biopolymer chemistry compared to fresh root tissue, suggesting it was either more stable or was refreshed at a greater rate. Between 14 and 105 years of woody plant encroachment, SFA in free POM fractions appeared to be consistently derived from root material while SFA within intraaggregate POM were increasingly derived from cutin sources. In addition, the shift from herbaceous to woody input was accompanied by enrichment in the amount of cutin and suberin‐derived aliphatics with respect to lignin in both root and surface litter as well as nonaggregated POM. Woody plant encroachment at this site results in the rapid accrual of POM pools that are biochemically recalcitrant, providing a mechanism by which soil organic carbon can accumulate in this sandy soil system. Our results also lend further credence to the hypothesis that aliphatic biopolymers, particularly root‐derived suberin, are important components of long‐term soil organic carbon stabilization.