Mark Poth

Nitrogen excess in North American ecosystems: predisposing factors, ecosystem responses, and management strategies

Most forests in North America remain nitrogen limited, although recent studies have identified forested areas that exhibit symptoms of N excess, analogous to overfertilization of arable land. Nitrogen excess in watersheds is detrimental because of disruptions in plant/soil nutrient relations, increased soil acidification and aluminum mobility, increased emissions of nitrogenous greenhouse gases from soil, reduced methane consumption in soil, decreased water quality, toxic effects on freshwater biota, and eutrophication of coastal marine waters. Elevated nitrate (NO3−) loss to groundwater or surface waters is the primary symptom of N excess. Additional symptoms include increasing N concentrations and higher N:nutrient ratios in foliage (i.e., N:Mg, N:P), foliar accumulation of amino acids or NO3−, and low soil C:N ratios. Recent nitrogen-fertilization studies in New England and Europe provide preliminary evidence that some forests receiving chronic N inputs may decline in productivity and experience greate...

Evidence for nitrogen saturation in the San Bernardino Mountains in southern California

Abstract Elevated N deposition has occurred in the Los Angeles Basin in southern California for at least the last 40 years. Elevated streamwater NO3− fluxes and high nitric oxide (NO) fluxes from soil, indicators of N saturation, have recently been reported for chaparral watersheds exposed to chronic N deposition in the San Gabriel Mountains north/northeast of Los Angeles. A number of nutritional and edaphic parameters across a deposition gradient in the San Bernardino Mountains (SBM) support the hypothesis that the mixed conifer forest in the western end of the range is also N saturated. Concentrations of NO3− in the soil solution or in soil extracts during the summer months were 14 to 44 times higher at Camp Paivika (CP), a western high N deposition site, than at Camp Osceola (CAO) or Barton Flats (BF), eastern low-pollution sites. Accumulation of NO3− in foliage of bracken fern (Pteridium aquilinum var. pubescens Underw.) and overstory species was also much greater at CP than at CAO and a site near BF. Nitric oxide fluxes in mid-August from relatively dry soil at CP were ca. 20 times higher than for typical forests in North America. Nitrous oxide (N2O) emissions, on the other hand, were low in the SBM sites. However, emissions of NO and N2O were several-fold higher at CP than at BF, a relatively low-pollution site. High NO emissions from otherwise undisturbed and well-drained forest soils of the western US may prove useful as a diagnostic indicator of N saturation. Nitrogen mineralization was greater at CP and Dogwood (high-pollution sites) than at CAO and Heartbar (low-pollution sites). Additional indicators of N enrichment at CP compared with the low N deposition sites include: low C:N ratios in soil and foliage, high foliar N:P ratios, higher nitrification rates and high soil acidity. Lower pH and base saturation were observed in soil from two high-pollution sites compared with two low-pollution sites. In summary, high NO emissions and elevated NO3− concentrations in the soil solution and in foliage, and high foliar N:P ratios at CP, indicate N in excess of biotic demand, with potential above-normal loss of N from the ecosystem - and thus, a N-saturated condition. Model outputs from the nutrient cycling model (NuCM) agreed well with field data from the SBM on elevated soil solution NO3− concentrations, reduced soil base saturation, and lack of a growth response to increasing N deposition.

Soil moisture affects survival of microorganisms in heated chaparral soil

Abstract Prescribed burning, the planned application of fire to reduce the hazards of wildland fuels, is coming into wider use in southern California chaparral. Soil was heated to various temperatures in factorial combination with various soil moistures to determine the direct effects of fire and soil moisture on the survival of microorganisms in chaparral soil. For fungi, mild heating increased germination of dormant forms yielding significantly higher counts than those in unheated soil. With increasing temperatures, microbe populations showed an exponential decrease. For heterotrophic soil bacteria, this decrease was summarized as a function of soil moisture and temperature. Microbial groups differed significantly in sensitivity to temperature: fungi > nitrite oxidizers > heterotrophic bacteria. Physiologically-active populations in moist soil were significantly more sensitive than were dormant populations in dry soil. The mathematical model presented shows qualitatively that more of the microbial biomass will be killed when the soil is moderately moist—as during prescribed burning—than when it is dry. Mineralization of killed microbial biomass in soil and release of plant nutrients may partially explain the increased plant growth and reduced response to fertilizer at burned sites.

The magnitude and persistence of soil NO, N2O, CH4, and CO2 fluxes from burned tropical savanna in Brazil

Among all global ecosystems, tropical savannas are the most severely and extensively affected by anthropogenic burning. Frequency of fire in cerrado, a type of tropical savanna covering 25% of Brazil, is 2 to 4 years. In 1992 we measured soil fluxes of NO, N2O, CH4, and CO2 from cerrado sites that had been burned within the previous 2 days, 30 days, 1 year, and from a control site last burned in 1976. NO and N2O fluxes responded dramatically to fire with the highest fluxes observed from newly burned soils after addition of water. Emissions of N-trace gases after burning were of similar magnitude to estimated emissions during combustion. NO fluxes immediately after burning are among the highest observed for any ecosystem studied to date. These rates declined with time after burning and had returned to control levels 1 year after the burn. An assessment of our data suggested that tropical savanna, burned or unburned, is a major source of NO to the troposphere. Cerrado appeared to be a minor source of N2O and a sink for atmospheric CH4. Burning also elevated CO2 fluxes, which remained detectably elevated 1 year later.

Prescribed fire, soils, and stream water chemistry in a watershed in the Lake Tahoe Basin, California

Before Euro-American settlement fire was a common process in the forests of the Lake Tahoe Basin. The combination of drought, fire suppression, and past harvesting has produced ecosystems that are susceptible to high-severity wildfires. Consequently, a program of prescribed fire has been recommended but there is incomplete understanding of the ecological effects of fuels treatments, especially with regard to how treatments will affect the flow of nutrients to Lake Tahoe. Nitrogen and phosphorus are the most important nutrients affecting algal growth, and thus lake clarity. Existing data demonstrate a long-term shift from a co-limitation by both nitrogen and phosphorus to phosphorus limitation. Two high-consumption, moderate-intensity prescribed fires were conducted to determine their effects on soil and stream water chemistry. Stream water calcium concentrations increased in burned watersheds whereas soluble reactive phosphorus concentrations were not significantly different. Prescribed fires released calcium and raised soil pH and this may have resulted in the incorporation of phosphorus into insoluble forms. Stream monitoring data indicates water quality effects last for ~3 months. Prescribed fires did not significantly increase the amount of soluble reactive phosphorus in stream waters. However, additional research is needed to determine if prescribed fire increases erosion or movement of particulate P, particularly in areas with steep slopes.

Microbial N and biomass, respiration and N mineralization in soils beneath two chaparral species along a fire-induced age gradient

Abstract A decline in available nutrients may contribute to the loss of vigor observed in older chaparral stands. We examined N mineralization and the storage of C and N in the microbial biomass of soil along a fire-induced chaparral chronosequence in San Diego County, Calif. Soil was collected under chamise (Adenostoma fasciculatum H. & A.) and ceanothus [Ceanothus greggii Gray var. perplexans (Trel.) Jeps] shrubs in stands burned 0, 2, 4, 11, 20, 54 and 80 yr prior to the study. Soil collected from the top 5 cm beneath chamise and ceanothus had similar microbial biomass patterns with stand aging. The amount of microbial C in soil remained relatively constant across the age gradient. Concentrations of microbial N and the amount of N mineralized fluctuated, with no significant trend across the stand-age gradient. For both chaparral species, NH4+ concentrations in soil were significantly higher in the recently burned stand (age = 0) than in stands burned from 2 to 80 yr earlier. Ammonium and nitrate concentrations in soil were not significantly different in most instances among soils from 2 to 80 yr-old stands. Soil collected under A. fasciculatum had significantly higher respiration rates, and higher concentrations of NH4+ and NO3− compared to C. greggii, suggesting that these chaparral species have differential influences on microbial processes in the soil around them.

L-tryptophan transaminase of a bacterium isolated from the rhizosphere of Festuca octoflora (graminae)

Abstract Cell-free extracts of a rhizobacterial isolate from Festuca octoflora Walt. contained a soluble α-ketoglutarate-dependent L-tryptophan (L-TRP) transaminase which catalyzes the conversion of L-TRP to indole-3-pyruvic acid (IPyA). The enzyme was purified 35-fold with protamine sulfate and ammonium sulfate (45–65%) precipitation. The IPyA released in the reaction mixture was trapped and partially stabilized as an enol-borate-arsenate complex and measured spectrophotometrically. IPyA production was directly proportional with time (up to 120min) and protein concentration in the ranges employed. The optimum pH and temperature for the catalytic conversion was pH 8.0 and 40°C, respectively. Using L-TRP as the substrate in the presence of α-ketoglutarate (as an amino-group acceptor) and pyridoxal phosphate (as a coenzyme), the average Km was 1.22 mM and the activation energy 21.0 kJ mol−1. The role of this enzyme in the microbial synthesis of auxins and its possible influence on plant growth and development are discussed.

The San Dimas experimental forest: 50 years of research

The San Dimas Experimental Forest serves as a field laboratory for studies of chaparral and related ecosystems, and has been recognized by national and international organizations. It covers 6,945 ha (17,153 acres) in the foothills of the San Gabriel Mountains northeast of Los Angeles, and has a typical Mediterranean-type climate. The Forest encompasses the San Dimas and Big Dalton watersheds, which have vegetation typical of southern California, are separated by deep canyons from the rest of the San Gabriel Mountains, have small tributaries suitable for study, and are harnessed by flood control dams. Unique physical features and a broad database covering over 50 years of research make the Forest an irreplaceable resource. Over the years data has been collected on water (precipitation, streamflow, etc.), soils and slope stability, effects of fire, vegetation management, chaparral ecology and physiology, vegetation classification, litter decomposition, and community structure of fauna. On-going studies include these: investigations into erosion processes; sediment movement in streams; particle size shifts with burning; air pollution impacts on vegetation, soil and water; denitrification in streams and nitrogen fixation; regeneration of oaks, postfire vegetation composition changes, dynamics of seed populations in soil; long term changes in site quality including Ceanothus dieback; and wildlife interactions.

Forest decline, nutrient supply and diagnostic fertilization in southwestern Germany and in southern California

Abstract Since the mid-1970s new types of forest damage (of unknown origin) were observed in West Germany. Nonspecific foliage losses and foliar discolorations are the most common symptoms. This resulted in a comprehensive research program being initiated in the fall of 1983 to investigate the nutritional status of Norway spruce (Picea abies (L.) Karst.) stands in southwest Germany, growing on a variety of sites. Forests at these sites revealed light to moderate damage or acute site-and species-specific nutritional disturbances involving Mg, K, Ca, Mn, and Zn. Historical comparisons of needle-analysis data showed a dramatic change of the nutrient supply at many sites over the last one or two decades. As a working hypothesis, the adverse impacts of air pollutants (e.g. photo-oxidants) and/or acidic atmospheric precipitation are seen as inciting factors causing increased leaching of nutrient elements. The trees potential to compensate for the nutrient losses by increased uptake are limited by the often poor nutrient supply for forest soils. Accelerated soil acidification increase nutrient leaching from the soil, probably enhancing decline. Over a short period, fertilization led to a decrease or to the disappearance of the symptoms, and to the improvement in the nutrient composition of the foliage. Microscopy investigation of Norway-spruce needles indicated the regeneration potential on a histological level for moderate yellowed needles after fertilization. Furthermore, needles of nutrient-deficient-trees reveal typical tissue damages different to those caused by SO2 or O3. Acid mist increased the leaching of K, Ca, Mg, Mn, and Zn from the foliage. Even when nutrients are being leached from the foliage, nutrient uptake canbe increased by improving the nutrient supply in the soil. Preliminary studies on the effects of damages caused by ozone (O3) on ponderosa pine suggest imbalance in micronutrient concentrations of foliage.

A throughfall collection method using mixed bed ion exchange resin columns.

Measurement of ionic deposition in throughfall is a widely used method for measuring deposition inputs to the forest floor. Many studies have been published, providing a large database of throughfall deposition inputs to forests. However, throughfall collection and analysis is labor intensive and expensive because of the large number of replicate collectors needed and because sample collection and chemical analyses are required on a stochastic precipitation event-based schedule. Therefore we developed and tested a throughfall collector system using a mixed bed ion exchange resin column. We anticipate that this method will typically require only one to three samplings per year. With this method, bulk deposition and bulk throughfall are collected by a funnel or snow tube and ions are retained as the solution percolates through the resin column. Ions retained by the resin are then extracted in the same column with 2N KCl and analyzed for nitrate and ammonium. Deposition values in throughfall from conventional throughfall solution collectors and colocated ion exchange samplers were not significantly different during consecutive 3- and 4-month exposure periods at a high (Camp Paivika; >35 kg N ha-1 year-1) and a low deposition (Barton Flats; 5–9 kg N ha-1 year-1) site in the San Bernardino Mountains in southern California. N deposition in throughfall under mature pine trees at Camp Paivika after 7 months of exposure was extremely high (87 and 92 kg ha-1 based on the two collector types) compared to Barton Flats (11 and 13 kg ha-1). A large proportion of the N deposited in throughfall at Camp Paivika occurred as fog drip, demonstrating the importance of fog deposition as an input source of N at this site. By comparison, bulk deposition rates in open areas were 5.1 and 5.4 kg ha-1 at Camp Paivika based on the two collector types, and 1.9 and 3.0 kg ha-1 at Barton Flats.