Effects of Dry-Season N Input on the Productivity and N Storage of Mediterranean-Type Shrublands

Anthropogenic nitrogen (N) deposition is a globally important source of N that is expected to increase with population growth. In southern California, N input from dry deposition accumulates on vegetation and soil surfaces of chaparral and coastal sage scrub (CSS) ecosystems during the summer and fall and becomes available as a pulse following winter rainfall. Presumably, N input will act to stimulate the productivity and N storage of these Mediterranean-type, semi-arid shrublands because these ecosystems are thought to be N limited. To assess whether dry-season N inputs alter ecosystem productivity and N storage, a field experiment was conducted over a 4-year period where plots were exposed to either ambient N deposition (control) or ambient + 50 kgN ha⁻¹ y⁻¹ (added N) that was added as NH₄NO₃ during the fall dry-season of each year. Plots exposed to added N had significantly higher accumulation of NH₄ and NO₃ on ion exchange resins that was due in part to direct fertilization and N mineralization, and the increase in N availability lead to a significant increase in NO₃ leaching in chaparral but not CSS. Nitrogen addition also lead to an increase in litter and tissue N concentration and a decline in the C:N ratio, but failed to alter the ecosystem productivity and N storage of the chaparral and CSS shrublands over the 4-year study period. The reasons for the lack of a treatment response are unknown; however, it is possible that these semi-arid shrublands are not N limited, cannot respond rapidly enough to capture the ephemeral N pulse, are limited by other nutrients, or the N response is dependent on the amount and/or distribution of rainfall. These results have important implications for understanding the potential effects of anthropogenic N deposition on the C and N cycling and storage of Mediterranean-type, semi-arid shrublands. Author Contributions  GLV conceived or designed study, performed research, analyzed data, contributed new methods or models, and wrote the article. SCP performed research, analyzed data, and contributed to the writing of the article. RM performed research, analyzed data, and contributed to the writing of the article.     

Factors controlling the concentration of particulate carbohydrates and amino acids in the Paraná River

Particulate carbohydrates and particulate amino acids were determined in a set of samples from the Paraná River, Argentina, covering a stretch of over 1300 km. To gain an understanding of the factors which control the concentrations of these organic compounds, a multivariate statistical approach was used (R-mode varimax factor analysis). This technique separated the influence of various sources of organic matter. The floodplain of the middle reach (about 900 km long and 20–30 km wide) was the most significant source controlling the concentrations of particulate carbohydrates (PCH), providing angiosperm debris, phytoplankton and grasses. Particulate amino acids (PAA) concentrations were determined markedly both by contributions from the Paraguay basin, and the lotic and lentic environments of the floodplain. Sugars single out forested tributary basins as sources of bacteria while amino acids point toward the Paraguay River as a major source of biodegraded material; heterotrophic processes, however, appear more clearly depicted with PAA than with PCH. A downstream increase of the PCH/PAA ratio suggests a larger consumption of PAA in the lowermost reaches.     

Forest Soil Chemistry and Mesofauna 20 Years After an Amelioration Fertilization

Abstract Soil chemical properties and soil mesofauna composition were assessed at a forest site in northern Austria, where 20 years earlier an amelioration treatment had been performed. The site had been treated with limestone, a high P slag, and ammonium nitrate to replace the poorly growing pine (Pinus sylvestris) forest with a Norway spruce (Picea abies) stand. This treatment was at that time a common means for the amelioration of nutrient‐poor forest soils with recalcitrant forest floor layers. After treatment, a dense cover of a nitrophilic stinging nettle (Urtica dioica) developed. Most likely, the site had been over‐fertilized, and inadvertently, an experiment with extreme N enrichment had been conducted. The abundance of collembolans increased, and dominance structure shifted from Isotomiella minor, Lipothrix lubbocki, and Isotoma notabilis at fertilizer treatment to Friesea mirabilis, Isotomiella minor, and Sphaeridia pumilis in the control, but the abundance of soil mesofauna generally decreased in the fertilizer treatment. Fertilization reduced the mass of the litter layer from 7.6 to 2.4 kg/m². The total carbon pool in the soil was reduced due to reduction of the litter layer. However, the content of soil organic matter in the upper mineral soil was significantly increased. A part of the applied and mineralized nitrogen had been lost from the soil, but N retention in the upper mineral soil was still considerable. Soil pH and the base saturation were sustainably increased. Carbon losses upon mineralization of the litter layer were not offset by the increase in C content of the mineral soil. Presently, the C pool in the soil of the fertilized treatment is lower than in the control. However, the overall nutrient enrichment of the soil may facilitate C sequestration in the fertilized site in the future.     

Landscape Controls on Organic and Inorganic Nitrogen Leaching across an Alpine/Subalpine Ecotone,Green Lakes Valley,Colorado Front Range

Here we report measurements of organic and inorganic nitrogen (N) fluxes from the high-elevation Green Lakes Valley catchment in the Colorado Front Range for two snowmelt seasons (1998 and 1999). Surface water and soil samples were collected along an elevational gradient extending from the lightly vegetated alpine to the forested subalpine to assess how changes in land cover and basin area affect yields and concentrations of ammonium-N (NH₄-N), nitrate-N (NO₃-N), dissolved organic N (DON), and particulate organic N (PON). Streamwater yields of NO₃-N decreased downstream from 4.3 kg ha⁻¹ in the alpine to 0.75 kg ha⁻¹ at treeline, while yields of DON were much less variable (0.40–0.34 kg ha⁻¹). Yields of NH₄-N and PON were low and showed little variation with basin area. NO₃-N accounted for 40%–90% of total N along the sample transect and was the dominant form of N at all but the lowest elevation site. Concentrations of DON ranged from approximately 10% of total N in the alpine to 45% in the subalpine. For all sites, volume-weighted mean concentrations of total dissolved nitrogen (TDN) were significantly related to the DIN:DON ratio (R ² = 0.81, P < 0.001) Concentrations of NO₃-N were significantly higher at forested sites that received streamflow from the lightly vegetated alpine reaches of the catchment than in a control catchment that was entirely subalpine forest, suggesting that the alpine may subsidize downstream forested systems with inorganic N. KCl-extractable inorganic N and microbial biomass N showed no relationship to changes in soil properties and vegetative cover moving downstream in catchment. In contrast, soil carbon–nitrogen (C:N) ratios increased with increasing vegetative cover in catchment and were significantly higher in the subalpine compared to the alpine (P < 0.0001) Soil C:N ratios along the sample transect explained 78% of the variation in dissolved organic carbon (DOC) concentrations and 70% of the variation in DON concentrations. These findings suggest that DON is an important vector for N loss in high-elevation ecosystems and that streamwater losses of DON are at least partially dependent on catchment soil organic matter stoichiometry. Received 26 July 2001; accepted 6 May 2002.     

The Imprint of Land-use History: Patterns of Carbon and Nitrogen in Downed Woody Debris at the Harvard Forest

Few data sets have characterized carbon (C) and nitrogen (N) pools in woody debris at sites where other aspects of C and N cycling are studied and histories of land use and disturbance are well documented. We quantified pools of mass, C, and N in fine and coarse woody debris (CWD) in two contrasting stands: a 73-year-old red pine plantation on abandoned agricultural land and a naturally regenerated deciduous forest that has experienced several disturbances in the past 150 years. Masses of downed woody debris amounted to 40.0 Mg ha⁻¹ in the coniferous stand and 26.9 Mg ha⁻¹ in the deciduous forest (20.4 and 13.8 Mg C ha⁻¹, respectively). Concentrations of N were higher and C:N ratios were lower in the deciduous forest compared to the coniferous. Pools of N amounted to 146 kg N ha⁻¹ in the coniferous stand and 155 kg N ha⁻¹ in the deciduous forest; both are larger than previously published pools of N in woody debris of temperate forests. Woody detritus buried in O horizons was minimal in these forests, contrary to previous findings in forests of New England. Differences in the patterns of mass, C, and N in size and decay classes of woody debris were related to stand histories. In the naturally regenerated deciduous forest, detritus was distributed across all size categories, and most CWD mass and N was present in the most advanced decay stages. In the coniferous plantation, nearly all of the CWD mass was present in the smallest size class (less than 25 cm diameter), and a recognizable cohort of decayed stems was evident from the stem-exclusion phase of this even-aged stand. These results indicate that heterogeneities in site histories should be explicitly included when biogeochemical process models are used to scale C and N stocks in woody debris to landscapes and regions. Received 27 April 2001; accepted 4 January 2002.     

Effects of environmental parameters, leaf physiological properties and leaf water relations on leaf water delta18O enrichment in different Eucalyptus species

Stable oxygen isotope ratios (delta18O) have become a valuable tool in the plant and ecosystem sciences. The interpretation of delta18O values in plant material is, however, still complicated owing to the complex interactions among factors that influence leaf water enrichment. This study investigated the interplay among environmental parameters, leaf physiological properties and leaf water relations as drivers of the isotopic enrichment of leaf water across 17 Eucalyptus species growing in a common garden. We observed large differences in maximum daily leaf water delta18O across the 17 species. By fitting different leaf water models to these empirical data, we determined that differences in leaf water delta18O across species are largely explained by variation in the Péclet effect across species. Our analyses also revealed that species-specific differences in transpiration do not explain the observed differences in delta18O while the unconstrained fitting parameter 'effective path length' (L) was highly correlated with delta18O. None of the leaf morphological or leaf water related parameters we quantified in this study correlated with the L values we determined even though L was typically interpreted as a leaf morphological/anatomical property. A sensitivity analysis supported the importance of L for explaining the variability in leaf water delta18O across different species. Our investigation highlighted the importance of future studies to quantify the leaf properties that influence L. Obtaining such information will significantly improve our understanding of what ultimately determines the delta18O values of leaf water across different plant species.     

Coupled biogeochemical and hydrological responses of streams and rivers to drought

• 1 Severe or extreme droughts occurred about 10% of the time over a 105‐year record from central New Mexico, U.S.A., based on the Palmer Drought Severity Index.
• 2 Drought lowers water tables, creating extensive areas of groundwater recharge and fragmenting reaches of streams and rivers. Deeper groundwater inputs predominate as sources of surface flows during drought. Nutrient inputs to streams and rivers reflect the biogeochemistry of regional ground waters with longer subsurface residence times.
• 3 Inputs of bioavailable dissolved organic carbon to surface waters decrease during drought, with labile carbon limitation of microbial metabolism a byproduct of drought conditions.
• 4 Decreased inputs of organic forms of carbon, nitrogen and phosphorus and a decrease in the organic : inorganic ratio of nutrient inputs favours autotrophs over heterotrophs during drought.
• 5 The fate of autotrophic production during drought will be strongly influenced by the structure of the aquatic food web within impacted sites.

     

Factors controlling long-term changes in soil pools of exchangeable basic cations and stream acid neutralizing capacity in a northern hardwood forest ecosystem

Understanding the factors regulating the concentrations of basic cations in soils and surface waters is critical if rates of recovery are to be predicted in response to decreases in acidic deposition. Using a dynamic simulation model (PnET-BGC), we evaluated the extent to which atmospheric deposition of strong acids and associated leaching by strong anions, atmospheric deposition of basic cations through changes in emissions of particulate matter, and historical forest cutting have influenced soil pools of exchangeable basic cations and the acid-base status of stream water at the Hubbard Brook Experimental Forest (HBEF) in New Hampshire. Historical deposition of basic cations was reconstructed from regression relationships with particulate matter emissions. Simulation results indicate that the combination of these factors has resulted in changes in the percent soil base saturation, and stream pH and acid neutralizing capacity (ANC) from pre-industrial estimates of 20%, 6.3 and 45 eq L^–1, respectively, to current values of 10%, 5.0 and –5 eq L^–1, respectively. These current values fall within the critical thresholds at which forest vegetation and aquatic biotic are at risk from soil and surface water acidification due to acidic deposition. While the deposition of strong acid anions had the largest impact on the acid-base status of soil and stream water, the reduction in deposition of basic cations associated with reductions in particulate emissions was estimated to have contributed about 27% of the depletion in soil Ca²⁺ exchange pool and 15% of the decreases in stream water concentrations of basic cations. Decline in stream water concentrations of basic cation occurred under both increasing and decreasing exchangeable pools, depending on the process controlling the acid base status of the ecosystem. Model calculations suggest that historical forest cutting has resulted in only slight decreases in soil pools of exchangeable basic cations, and has had a limited effect on stream ANC over the long-term.