Nutrient Cycling Model Research Articles

A forest nutrient cycling and biomass model (ForNBM) based on year-round, monthly weather conditions, part I: assumption, structure and processing

A forest nutrient cycling and biomass growth model was developed to simulate nutrient cycling and NPP based on site and monthly mean weather conditions. A modular design was used to partition northern forest ecosystems into separate modules that address the following ecological variables: (1) hydrologic processes and temperatures to estimate moisture, percolation and temperature in forest floor, soil and subsoil; (2) soil acidity to estimate the H-ion balance in the soil, in the context of atmospheric deposition, nutrient uptake, weathering, and soil-ion retention; (3) cycling of N, S, Ca, Mg, and K to estimate nutrient uptake, mineralization, nitrification, immobilization, mineral soil weathering, nutrient exchange between soil exchange sites and solution, and nutrient leaching associated with atmospheric deposition; and (4) biomass to estimate NPP and its allocation to foliage, wood, and root, as well as litterfall and decomposition. After the model calibration, verification and validation, the model can be applied (1) to predict the rate of sustainable nutrient harvesting based on nutrient geochemical balance; (2) to determine limiting nutrients for forest growth; (3) to evaluate the effects of atmospheric acidic deposition on soil chemistry and growth; and (4) to evaluate the effects of forest harvesting on environmental issues, such as stream water quality.

A Forest Nutrient Cycling and Biomass Model (ForNBM) based on year-round, monthly weather conditions: Part II: Calibration, verification, and application

The ForNBM was applied to the Nashwaak Experimental Watershed Project in central New Brunswick, Canada. The data represented a mixed hardwood site and included information about nutrient leaching, foliage and wood biomass, leaf fall, and ancillary information required for model initialization. Ancillary information included forest cover type, stand density, forest floor depth, soil rooting depth, soil texture, soil substrate type, initial amounts of biomass and N, S, Ca, Mg, and K content in foliage, wood, and roots, and mineral soil nutrient contents in soil solution, on ion-exchange sites, and in the soil. The authors were able to calibrate the model with existing data. The model simulated observed monthly leaching reasonably well. The r 2-values of model simulations compared with field observations of monthly leaching of NO 3 −_N, NH 4 +_N, Ca, Mg, and K were 0.78, 0.7, 0.78, 0.84, and 0.75, respectively. Modeled multi-year cumulative leaching of NO 3 −_N, NH 4 +_N, Ca, Mg, and K compared with actual values gave r 2-values close to one for all cases considered. The results also showed that soil nutrient leaching had increased approximately five-fold for NO 3 −_N, 80% for NH 4 +_N, 71% for K, 20% for Ca, and 14% for Mg during the 3 years following a stem-only harvest operation applied watershed-wide. However, the model simulation showed that increased nutrient leaching during the 11 years following the stem-only harvest was small compared with the amounts removed in the biomass during the harvest. Increased nutrient leaching following harvesting did not appear to impact site productivity over the long term.

Defining nutritional constraints on carbon cycling in boreal forests – towards a less `phytocentric' perspective

Growing interest in possible global climate change has underlined the need for better information concerning the way in which carbon partitioning between ecosystem components is influenced by constraints on nutrient availability. Micro-organisms play a fundamental role in the cycling of carbon and nutrients in all ecosystems but the role of fungi in particular is pivotal in boreal forest ecosystems. Traditional models of nutrient cycling are based on methods and concepts developed in agricultural systems where microorganisms are considered primarily as nutrient processors providing plants with inorganic nutrients. The filamentous nature of fungi, their ability to translocate carbon and nutrients between different substrates and the capacity of ectomycorrhizal fungi to utilise organic nutrients have all been largely ignored. In this article, a new model is suggested which emphasises competition for organic nutrients between decomposer organisms and plants, with the plants depending on their associated mycorrhizal fungi for nutrient acquisition. Antagonistic interactions involving nutrient transfer between decomposer and mycorrhizal fungi are proposed as important pathways in nutrient cycling. Due to the nutrient conservative features of decomposer fungi, inorganic nutrients are considered less important for plant nutrition. The implications of the new nutrient cycling model on the carbon balance of boreal forests are discussed.

Nitrogen loss from unpolluted South American forests mainly via dissolved organic compounds.

Conceptual and numerical models of nitrogen cycling in temperate forests assume that nitrogen is lost from these ecosystems predominantly by way of inorganic forms, such as nitrate and ammonium ions. Of these, nitrate is thought to be particularly mobile, being responsible for nitrogen loss to deep soil and stream waters. But human activities-such as fossil fuel combustion, fertilizer production and land-use change-have substantially altered the nitrogen cycle over large regions, making it difficult to separate natural aspects of nitrogen cycling from those induced by human perturbations. Here we report stream chemistry data from 100 unpolluted primary forests in temperate South America. Although the sites exhibit a broad range of environmental factors that influence ecosystem nutrient cycles (such as climate, parent material, time of ecosystem development, topography and biotic diversity), we observed a remarkably consistent pattern of nitrogen loss across all forests. In contrast to findings from forests in polluted regions, streamwater nitrate concentrations are exceedingly low, such that nitrate to ammonium ratios were less than unity, and dissolved organic nitrogen is responsible for the majority of nitrogen losses from these forests. We therefore suggest that organic nitrogen losses should be considered in models of forest nutrient cycling, which could help to explain observations of nutrient limitation in temperate forest ecosystems.

A spreadsheet-based biogeochemical model to simulate nutrient cycling processes in forest ecosystems

A Nutrient Cycling Spreadsheet (NuCSS) model was developed to simulate nutrient cycling in forest ecosystems. This model is intended to provide a tool for nutrient management and is intermediate in complexity between simple budget calculations and complex, process-based ecosystem models. NuCSS simulates biomass production, organic matter decomposition, N mineralization, cation adsorption, weathering and leaching (hydrology not included) using empirical relationships and simple mechanistic process descriptions. The model calculates a target biomass production and associated nutrient uptake. By reviewing soil nutrient pools the user can assess whether available soil nutrients can sustain the targeted biomass production. NuCSS was calibrated to a Norway spruce site in the Solling Mountains, Germany. The model performance was evaluated using long-term data on deposition, vegetation, water quality and soil nutrient inventories. Long-term predictions were made using a ‘business as usual’ and two reduced deposition scenarios. Results from NuCSS were compared with simulations from other biogeochemical models. The element distribution over soil, forest floor and vegetation closely matched measurements both after one and 15 years. Forest floor nutrients were underestimated but data on forest floor accumulation proved to be unreliable. NuCSS overestimated N uptake, N mineralization and leaching but not enough data on these fluxes were available to determine whether NuCSS was wrong. Simulated base cation leaching agreed well with the measurements but K leaching was overestimated most likely due to an underestimation of the selectivity coefficient for K adsorption. The reduced deposition scenarios caused N and base cation leaching to decrease with time. When N deposition was reduced by 95%, a N deficit occurred and N mineralization and deposition could not sustain the (fixed) uptake. For most simulated parameters NuCSS agreed well with more detailed, process-based, biogeochemical models. In general, NuCSS simulated a higher N leaching and N mineralization than most other models. Trends in base saturation and base cation leaching simulated by NuCSS agreed well with other models. The model proved to be helpful in addressing some of the uncertainties in nutrient cycling regarding vegetation uptake, weathering rates and N mineralization. Given the uncertainties in description of certain processes, a quantitative use of NuCSS may not be justified. These uncertainties also apply to more complex nutrient cycling models, however.

A comparative study of nitrogen and phosphorus cycling in tidal and non-tidal riverine wetlands

This paper describes a study of nutrient dynamics in 12 tidal and non-tidal freshwater riverine wetlands in The Netherlands, Belgium, and Maryland (USA). The purpose of the study was to investigate the relationships between nutrient cycling processes in riverine wetlands that were geographically separated, that were dominated by different types of vegetation, and that had different hydrodynamics. We also compared restored and natural riverine wetlands. The results showed distinct differences in interstitial water chemistry between the sites in Maryland and Europe. No such regional differences were found in the soil variables, except for soil phosphorus, which was higher in The Netherlands. Soil organic matter, total nitrogen and phosphorus content, and bulk density were higher in tidal freshwater wetland soils. Forested wetland soils had higher organic matter and total nitrogen and lower bulk density and total phosphorus than soils from wetlands dominated by herbaceous species. Restored wetlands had lower soil organic matter and total soil nitrogen and phosphorus than similar types of natural riverine wetlands. There were no differences in nutrient-related process rates nor plant nutrient concentrations in tidal versus non-tidal riverine wetlands. Lower nitrogen and phosphorus concentrations in plants at the restored sites suggest that nutrient uptake by vegetation may be poorly coupled to rates of nutrient cycling during early stages of vegetation development. A principal components analysis of the data identified groupings of soil and water variables that were similar to those that had been previously identified when we applied the same methods to peatlands that were also geographically widely separated. Results of the study demonstrate that the techniques that we have been using are robust and repeatable. They are especially useful for making general comparisons of nitrogen and phosphorus cycling, when there are limitations on the number of wetland that can be sampled. The approach that we have developed may also be used to calibrate and refine nutrient cycling models that are incorporated into wetland assessment procedures.

Long-term nutrient cycling patterns in Douglas-fir and red alder stands: a simulation study

Long-term patterns in nutrient cycling in regrowing Douglas-fir ( Pseudosuga menziesii Mirb. Franco) and red alder ( Alnus rubra Bong.) on native soils plus soils previously occupied by other species were simulated using the nutrient cycling model. Simulations of regrowing stands were also compared with observations of nutrient cycling in mature Douglas-fir and red alder. We hypothesized that (1) prolonged presence of red alder will cause a depletion in soil base cations due to increased nitrification and NO 3 − leaching; (2) lower base cation availability under red alder will ultimately cause biomass production to decline; (3) high N availability in red alder soils will favor regrowth of Douglas-fir; (4) higher base cation and P status of the Douglas-fir soils will favor growth of red alder both in the short- and long-term, since N is not limiting to red alder; and (5) in regrowing red alder, NO 3 − leaching will increase with time as a result of increased N fixation. All hypotheses were confirmed, but the effect of soil type on biomass production was minimal both for red alder and Douglas-fir. The higher soil organic matter content in the mature red alder stand most likely reflected previous occupation by old-growth Douglas-fir and also a large litter input from the understory vegetation. In general, the nutrient cycling model simulated differences in nutrient cycling patterns at least qualitatively between Douglas-fir and red alder and was helpful in identifying potential gaps in the understanding of biogeochemical cycling as well as uncertainties in the data. The nutrient cycling model did not fully elucidate differences in P cycling between Douglas-fir and red alder and overestimated weathering rates under Douglas-fir. Uncertainties in the data included: (1) temporal patterns in N fixation in the regrowing stands; (2) understory litterfall; and (3) site history and, consequently, presence of pre-existing differences in site conditions.

MARKED DIFFERENCES IN SURVIVORSHIP AMONG APPLE ROOTS OF DIFFERENT DIAMETERS

Fine roots are responsible for a substantial fraction of terrestrial net primary productivity, and a better understanding of fine root production and turnover is crucial to improving global carbon and nutrient cycling models. In most studies, roots less than 1 or 2 mm in diameter (“fine roots”) have been treated as structurally and physiologically identical individuals. We used minirhizotron data from 16-yr-old apple (Malus domestica) trees to investigate differences in life span and life history among fine roots whose diameters differed by tenths of a millimeter. We also introduced the use of Cox proportional hazards regression models to assess the effects of multiple covariates on root mortality. Overwinter survivorship differed markedly among diameter classes in both years: 3–12% for roots 0.5 mm in diameter and brown, and these roots gave rise to new white laterals during the spring root flush. Based on the considerable differences in morphology and life history within the class of roots <1 mm in diameter, we advocate the use of a functional definition for the fine root whenever possible.

Marked Differences in Survivorship among Apple Roots of Different Diameters

Fine roots are responsible for a substantial fraction of terrestrial net primary productivity, and a better understanding of fine root production and turnover is crucial to improving global carbon and nutrient cycling models. In most studies, roots less than 1 or 2 mm in diameter (“fine roots”) have been treated as structurally and physiologically identical individuals. We used minirhizotron data from 16-yr-old apple (Malus domestica) trees to investigate differences in life span and life history among fine roots whose diameters differed by tenths of a millimeter. We also introduced the use of Cox proportional hazards regression models to assess the effects of multiple covariates on root mortality. Overwinter survivorship differed markedly among diameter classes in both years: 3–12% for roots <0.3 mm in diameter, 30% for roots 0.3–0.5 mm in diameter, and 55–60% for roots 0.5–1.1 mm in diameter. Diameter was also shown to be negatively related to the risk of root mortality using a proportional hazards regression (P < 0.0001 in 1994–1995, P < 0.030 in 1995–1996). Smaller diameter fine roots were more likely to be found in densely proliferated patches, while the majority of larger diameter fine roots were found alone or with a single neighboring root. Number of neighbors was shown to be positively correlated (P < 0.0001) with the risk of root mortality in 1994–1995. Pigmentation had a marginally significant (P < 0.053 in 1994–1995, P < 0.078 in 1995–1996) negative effect on the risk of root mortality. Tetrazolium staining of brown fine roots harvested in March of 1996 indicated that 90% were viable. The majority of roots which survived from October to May in both years were >0.5 mm in diameter and brown, and these roots gave rise to new white laterals during the spring root flush. Based on the considerable differences in morphology and life history within the class of roots <1 mm in diameter, we advocate the use of a functional definition for the fine root whenever possible.

The nutrient cycling model: lessons learned

The nutrient cycling model (NuCM) is a stand level model that depicts the cycling of N, P, K, Ca, Mg, and S on daily, weekly or monthly time scales. NuCM has been applied to several forest ecosystems (ponderosa pine, red spruce, beech, eastern deciduous, loblolly pine, slash pine, Scots pine, and Norway spruce) to simulate the effects of changing atmospheric deposition, harvesting, species change, precipitation quantity, increased temperature, elevated CO 2, and liming. In some cases (e.g., harvesting, liming), the model output has matched field data quite well; however, it cannot be known whether the model does so because it accurately portrays nutrient cycling processes or simply because of chance. In other cases, NuCM simulations have either failed to match field data (as in the case of the observed chromatographic response of soil solution cations to a nitrate pulse in a beech forest) or produced results that are counterintuitive but as yet untested (as in the case where increased N translocation caused increased leaching). In that the primary purpose of these simulations has been heuristic rather than predictive, the simulation outputs that are either inconsistent with field data or counter-intuitive are of greatest interest. This review of NuCM applications led to the conclusion that the model has been more successful in matching decadal-scale changes in nutrient pools and soils and less successful in capturing intra-annual variations in soil solution chemistry. The NuCM model, like all models, can use improvements and these have been suggested; however, the model as it is has provided valuable insights into nutrient cycling in forest ecosystems, including the potential for short-term soil change and the great importance of nutrient translocation in N cycling.

Simulated effects of temperature and precipitation change in several forest ecosystems

The Nutrient Cycling Model (NuCM) was used to investigate the effects of increased temperature (+4°C) and changing precipitation (increased and decreased) on biogeochemical cycling at six forest sites in the United States: a Picea rubens forest at Nolan Divide in the Great Smoky Mountains, North Carolina; mixed deciduous forests at Walker Branch, Tennessee and Coweeta, North Carolina; a Pinus taeda forest at Duke, North Carolina; a P. eliottii forest at Bradford, Florida; and a P. contorta/P. jeffreyii forest at Little Valley, Nevada. Simulations of increased temperature indicated increased evapotranspiration and reduced water flux. Simulations of changes in precipitation indicated disproportionately large variations in soil water flux because of the relative stability of evapotranspiration with changes in precipitation. Increased temperature caused N release from forest floors at all sites. At the N-saturated Nolan Divide site, this resulted in no change in N uptake or growth but increased soil solution Al and NO 3 − and increased N leaching losses. At the N-limited sites, the release of N from the forest floor caused increased growth, and, in some cases, increased NO 3 − leaching as well, indicating that N released from the forest floor was not efficiently taken up by the vegetation. Increased precipitation caused increased growth, and decreased precipitation caused reduced growth in the N-limited sites because of changes in wet N deposition. Changes in precipitation had no effect on growth in the N-saturated Nolan Divide site, but did cause large changes in soil solution mineral acid anion and Al concentrations. Increased precipitation caused long-term decreases in soil exchangeable base cations in most cases because of the disproportionately large effects on soil water flux; however, increased precipitation caused decreases in exchangeable base cations in cases where atmospheric deposition was a major source of base cations for the system. The simulation results illustrate the extreme complexity of the possible responses of nutrient cycling processes to climate change. By virtue of the fact that the NuCM model does not contain physiological algorithms, these simulations demonstrate that changes in temperature and precipitation can produce widely varying ecosystem-level responses through their effects on biogeochemical cycling processes alone and that generalizations about the relative importance of temperature versus precipitation changes are hazardous.

Mini-Review: Nutrient Cycling in Lakes and Streams: Insights from a Comparative Analysis

Understanding of general ecosystem principles may be improved by comparing disparate ecosystems. We compared nutrient cycling in lakes and streams to evaluate whether contrasts in hydrologic properties lead to different controls and different rates of internal nutrient cycling. Our primary focus was nutrient cycling that results in increased productivity, so we quantified nutrient cycling by defining the recycling ratio (ρ) as the number of times a nutrient molecule is sequestered by producers before export. An analytic model of nutrient cycling predicted that in lakes ρ is governed by the processes that promote the mineralization and retard the sedimentation of particulate-bound nutrients, whereas in streams, ρ is governed by processes that promote the uptake and retard the export of dissolved nutrients. These differences were the consequence of contrast between lakes and streams in the mass-specific export rates (mass exported · standing stock-1· time-1) of dissolved and particulate nutrients. Although ρ is calculated from readily measured ecosystem variables, we found very few published data sets that provided the necessary data for a given ecosystem. We calculated and compared ρ in two well-studied P-limited ecosystems, Peter Lake and West Fork Walker Branch (WFWB). When ecosystems were scaled so that water residence time was equal between these two ecosystems, ρ was three orders of magnitude greater in WFWB. However, when we scaled by P residence time, ρ was nearly equal between these two ecosystems. This suggests broad similarities in ρ across ecosystem types when ecosystem boundaries are defined so that turnover times of limiting nutrients are the same.

Simulated Effects of Reduced Sulfur, Nitrogen, and Base Cation Deposition on Soils and Solutions in Southern Appalachian Forests

AbstractEffects of reduced deposition of N, S, and CB on nutrient pools, fluxes, soil, and soil solution chemistry were simulated for two Appalachian forest ecosystems using the nutrient cycling model. In the extremely acidic, N‐ and S‐saturated red spruce [Picea rubens (Sarg.)] forest (Nolan Divide), reducing CB deposition by 50% reduced CB leaching by ∼40% during the 24‐yr simulation period. This was due solely to the effects of CB deposition on the soil exchanger rather than effects on soil solution. Reducing S and N by 50% caused immediate reductions in total anion and cation leaching at Nolan Divide, but the effects on soil solution CB diminished and CB leaching was reduced by only 17% over the simulation period. Reducing S and N deposition had a greater effect on soil solution aluminum (Al) and molar Ca/Al ratio than reducing base cation deposition at Nolan Divide. In the moderately acidic, N‐ and S‐accumulating mixed deciduous forest at Coweeta, reduced CB deposition by 50% caused a very slight (&lt;4%) reduction in CB leaching as a result of slightly reduced base saturation and increased soil sulfate adsorption. The effects on reducing S and N deposition by 50% on CB leaching (16% over the simulation period) were greater than those of reduced CB deposition. The system continued to accumulate both S and N even at reduced deposition at Coweeta, although growth and vegetation uptake were slightly reduced (−5%) because of increased N deficiency. Base saturation remained well above the Al buffering range at all times at Coweeta and Al was an unimportant component of soil solutions in all scenarios.

Simulated nitrogen cycling response to elevated CO(2) in Pinus taeda and mixed deciduous forests.

Interactions between elevated CO(2) and N cycling were explored with a nutrient cycling model (NuCM, Johnson et al. 1993, 1995) for a Pinus taeda L. site at Duke University, North Carolina, and a mixed deciduous site at Walker Branch, Tennessee. The simulations tested whether N limitation would prevent growth increases in response to elevated CO(2), and whether growth responses to CO(2) in N-limited systems could be facilitated by increasing the biomass/N ratio (reducing N concentration) or increasing litter N mineralization, or both. Nitrogen limitation precluded additional growth when target growth rates and litterfall were increased (simulating potential response to elevated CO(2)) at the Duke University site. At the Walker Branch site, increasing target growth and litterfall caused a 7% increase in growth. Reducing foliar N concentrations reduced growth because of N limitation created by reduced litter quality (C:N ratio), reduced decomposition and increased N accumulation on the forest floor. These effects were most pronounced at the Duke University site, because the forest floor N turnover rate was lower than at the Walker Branch site. Reducing wood N concentration allowed prolonged increases in growth because of greater biomass/N; however, N uptake was reduced, allowing greater N immobilization on the forest floor and in soil. Increased N mineralization caused increased growth at the Duke University site, but not at the Walker Branch site. These simulations pose the counterintuitive hypothesis that increased biogeochemical cycling of N (as a result of increased litterfall N) causes reduced growth in an N-limited system because of increased accumulations of N on the forest floor and in soil. Translocation of N from senescing leaves before litterfall mitigates this response by allowing the trees to retain a greater proportion of N taken up rather than recycle it back to the forest floor and soil where it can be immobilized. Eliminating N translocation at Walker Branch changed the direction as well as the magnitude of the responses in three of the four scenarios simulated. Because the NuCM model does not currently allow translocation in coniferous species, the effects of translocation on N cycling in the Duke University simulations are not known.

Simulation of the long-term carbon and nitrogen dynamics in Dutch forest soils under Scots pine

Abstract. Dynamics of C and N in forest soils in the Nutrient Cycling and Soil Acidification Model (NUCSAM) are described by the transformation and decomposition of three organic matter compartments, litter, fermented material and humic material. These three compartments are allocated to the morphological distinguishable L, F and H horizons of the organic layer. Changes in the pools of these organic compartments are described with first order equations for decomposition and transformation. Rate constants for decomposition and transformation were derived by calibrating the model to measured organic matter pools in organic layers of a chrono-sequence of five first succession Scots pine stands between 15 and 120 years old. Simulated pools of organic matter in the organic layers were in agreement with measured pools in the five pine stands, except for the first thirty years of the H-horizon. During this period, an increase in organic matter in the H horizon was simulated while no H horizons were observed in the field. The simulated total pool of organic matter in the organic layer agreed well with values from a field inventory in 20 other Scots pine stands, but the simulated distribution over the three horizons differed from the field measurements which varied among sites. For the Scots pine stands the model was able to simulate the organic matter accumulation in the top 40-cm of the mineral soil; derived almost completely from fine root turnover. The accumulated pool of nitrogen in the organic layer was in agreement with measured pools for the oldest Scots pine stand but was too high for the younger stands. Especially, the accumulation of N in the F-horizon was too fast, presumably due to an overestimated retention of nitrogen.

Soil Erosion and Crop Production: A Modeling Approach

Since soil erosion can be relatively slow process, it is required that the model be capable of simulating on a daily time step using readily available inputs. In order to meet this requirement., a mathematical model EPIC (Williams et. al. 1989) has been used and efforts are being made to develop a new raster/pixel based approach to simulate crop yield and erosion-sedimentation incorporating hydrology, weather and nutrient cycling models. The paper describes an application of EPIC in the northeastern part of India namely Bihar based on climate/land conditions, fertilizer use and management practices. The result suggests that the model can be used for simulating the effects of agricultural practices in response to the environmental changes such as climatechange or soil erosion.

Simulating effects of S and N deposition on soil water chemistry by the nutrient cycling model NuCM

The nutrient cycling model (NuCM), developed during the Integrated Forest Study in the USA, is one of the most detailed models simulating processes in forest ecosystems. The present study is an approach to validate NuCM by testing the model against data from a highly controlled model lysimeter experiment. NuCM was calibrated against data from lysimeters receiving no S and N treatment. The calibrated model was then used to simulate observed changes in soil water and leachate chemistry due to the application of artificial acid rain of pH 3 and/or N-fertiliser (90 kg N/ha∗year). By adjusting key parameters like the soil's ability to adsorb SO 4 2− and nitrification/denitrification rates, NuCM was able to simulate the observed changes in mean ion concentration for the experimental period fairly well. The conformity between observed and simulated seasonal/year-to-year variations was improved at elevated N-deposition. This study illustrates, however, the uncertainty which appears when complex models are applied to systems with less extensive parameter collection. Conditions for proper model testing are difficult to obtain when working with environmental systems.

Simulation modeling of nitrogen trace gas emissions along an age gradient of tropical forest soils

Applications of a process simulation model for ecosystem production and soil nutrient cycling were carried out to test hypotheses concerning the controls on nitrogen trace gas fluxes in tropical forests. Assuming that emissions of trace gases (N 2O and NO) can help characterize patterns in N cycling among forests of different age and nutrient status, we applied a new daily time step version of the CASA (Carnegie-Ames-Stanford approach) model in an attempt to reproduce soil biophysical conditions previously measured in three forest stands that comprise a soil age gradient (ranging from 200 to 185 000 years old) on the island of Hawaii. We compared model-predicted soil moisture, temperature, N mineralization, and N trace gas emission rates to measurements at forest sites made during 1990 and 1991. Results showed that predicted water filled pore space (WFPS) in the soil is consistently lower than measured WFPS, possibly due to incomplete understanding of moisture holding capacity, drainage properties, and small scale variability of these volcanic soils. Simulations correctly predicted the observed difference of an order of magnitude between N 2O emission fluxes in the youngest and the oldest forest soils. Nevertheless, specific day-by-day comparison of predicted and measured N 2O fluxes reveal only occasional agreement within an order of magnitude tolerance level. Although soil moisture conditions appear favorable for emission on NO at these sites, as predicted by the generalized model design, lack of observed NO emissions from the previous field and laboratory studies suggest that: (1) our hypothesized NO emission levels as a function of WFPS should be truncated at lower levels of soil moisture availability; and/or (2) nitrification potentials of natural forest soils should be considered in revised versions of the model.

Plant Response to Fish Farming Wastes in Volcanic Soils

AbstractAgricultural use of aquaculture wastes appears to be a sound ecological and economical means to improve soil fertility and to decrease the potential for adverse water quality impacts in South Argentina. A 3‐mo greenhouse experiment with ryegrass (Lolium perenne L.) was conducted to determine plant availability of N and P from aquaculture wastes and compare results with previous laboratory incubations. Treatments included: (i) wastes taken from sediments under cages in a 3‐yr‐old fish farm (EA) applied at 40 and 80 Mg ha−1; (ii) wastes taken from sediments under cages in an 8‐yr‐old farm (LM), applied at 10 and 20 Mg ha−1; (iii) the lowest waste rates amended with 50 kg ha−1 synthetic N; (iv) four synthetic fertilizer treatments including a single N rate of 80 kg N ha−1 and four P levels of 30, 40, 60, and 80 kg P ha−1, and (v) a no‐fertilizer control. Although total N and P were higher in LM than in EA, and N mineralization rates in lab incubations were similar (13%), ryegrass yields and N uptake were much higher in EA than in LM. When mineralized N was estimated as the difference between added N and total N uptake, values were 50 to 63% for EA and 5 to 14% for LM. Ryegrass yields were approximately as follows: EA &gt; FI &gt; LM &gt; Control. Synthetic fertilizer treatments showed the lowest values of P utilization efficiency, residual Olsen‐P in soil and plant P uptake in biomass, and the highest P retention in soils. The lowest waste rates with addition of N produced comparable ryegrass yields with the highest rates. Results emphasize: (i) the need to include living plants in mineralization potential evaluations to avoid significant errors in nutrient cycling models, and (ii) the importance of organic amendments in ameliorating plant P availability in high P‐fixing volcanic soils.

Simulated Responses of Red Spruce Forest Soils to Reduced Sulfur and Nitrogen Deposition

Implications of reducing S and N deposition on red spruce forests of the southern Appalachians are explored using the Nutrient Cycling Model (NuCM). We hypothesized that reducing deposition would cause (i) large reductions in soil solution NO{sub 3}{sup {minus}}, SO{sub 4}{sup 2{minus}}, Al and Ca/Al ratios, but (II) small changes in exchangeable base cation reserves. Hypothesis (I) was supported in part: simulated reductions in atmospheric deposition had substantial and nearly immediate effects upon soil solution mineral acid anions, Ca{sup 2+}, and Al{sup 3+} concentrations. Ca/Al molar ratios were much less sensitive to changes in deposition and soil solution ionic strength than either Ca{sup 2+} or Al{sup 3+} separately. Hypothesis (II) was not supported: although the increases in base saturation and exchangeable cation pools were small relative to cation exchange, they were large relative to initial exchangeable base cation pools. These changes in base cation pools and base saturation were of sufficient magnitude to affect simulated soil solution composition during a very short time. 29 refs., 11 figs., 2 tabs.