Nitrogen mineralization in irrigated plantations of shisham and mulberry

Ammonification and nitrification rates and nitrogen uptake were measured using the buried-bag technique in irrigated mixed plantations of shisham (Dalbergia sissoo Roxb. ex DC.) and mulberry (Morus alba L.). Nitrogen transformations were rapid in these stands, particularly following thinning to reduce stand density. In young stands, net N mineralization was 26.72 mg N kg⁻¹ soil month⁻¹ (approximately 480 kg ha⁻¹ month⁻¹), but, as the end of the 22-year rotation approached, nitrification slowed to 13.41 mg N kg⁻¹ soil month⁻¹ (approximately 241 kg ha⁻¹ month⁻¹). N₂-fixing shisham appeared to respond after thinning only to the increased space and temporarily reduced competition for light and moisture, but mulberry appeared to benefit greatly from the nitrogen released through mineralization following thinning.     

Nitrogen deposition in Casuarina equisetifolia (Forst.) plantation stands in the dry tropics of Sonbhadra, India

Periodic variations in the concentration, deposition and canopy impact of different forms of N on annual N deposition through rainfall, throughfall and stemflow in 5 and 8 year old stands of Casuarina equisetifolia were studied. Throughfall and stemflow ranged from 70 to 76% and 5–6% of annual precipitation respectively. The total N deposition by rainfall was 11.1 kg ha⁻¹ year⁻¹, and by throughfall was 13.6 kg ha⁻¹ year⁻¹ and 16.5 kg ha⁻¹ year⁻¹ in 5-year-old and 8-year old plantations, respectively. The quantities of N deposited through stemflow in the two plantations were nearly identical, accounting for 1.6 kg ha⁻¹ year⁻¹. Observations of the monthly deposition of NH₄,N, NO₃-N, Kjeldahl-N and organic-N revealed that maximum deposition occurred in July and the minimum in September. Organic-N deposition was 17% less (5-year) than the rainwater content. Net deposition of N, as an effect of canopy, was 7–8.7 kg ha⁻¹ year⁻¹, which was added directly to the available nutrient pool of soil.     

Variation in the amount and quality of litterfall in a Pinus sylvestris L. stand growing on a bog

The amount and nutrient content of the above-ground litterfall was followed for 9 years in an unfertilized, PKMgB and NPKMgB fertilized Scots pine stand growing on a drained ombrotrophic bog in eastern Finland. The annual litterfall on unfertilized plots was 1995 kg ha⁻¹, of which needles accounted for 74%. The effective temperature sum (threshold value + 5°C) explained 99% of the annual variation in the amount of needle litterfall when the data from one atypical year were excluded from the analysis. Nutrient concentrations were, except for Fe, higher in needle litter than in the other litterfall fractions. Nitrogen, P and K concentrations were low in autumn, and those of Ca and Mn high, possibly owing to variation in the mobility of elements during senescence. The annual litterfall input of N to the soil was 12.4 kg ha⁻¹, and the corresponding values for P and K were 0.08 kg ha⁻¹ and 1.81 kg ha⁻¹, respectively. Fertilization reduced needle litterfall in the first year after treatment, but had no effect thereafter. The amount of other litterfall fractions was not affected by fertilization in any of the 9 years of the study. Nitrogen, P, K and B concentrations increased in the needle litter after both fertilization treatments. The results indicate long-term cycling of fertilizer nutrients on the site.     

Deposition of atmospheric constituents and its impact on nutrient budgets of oak forests (Quercus petraea and Quercus robur) in Lower Austria

Geochemical processes in central European oak ecosystems (Quercus petraea and Quercus robur) suffering stand decline were studied in two oak stands of the Weinviertel, Lower Austria, about 30 km north of Vienna. Stores of chemical elements were determined by soil and biomass inventories. Deposition input was monitored over a 2 year period by bulk sampling of throughfall. Soil solution chemistry was studied by tension lysimetry over a 1 year period. Mineral nutrition of oak was judged by foliar analysis. Bulk deposition rates were 10–12 kg N ha⁻¹ year⁻¹, and 15–20 kg S ha⁻¹ year⁻¹. Total annual nitrogen gain is high. Both systems lose calcium and magnesium. Foliar nutrient levels indicate sufficient nutrition with main mineral nutrients, except for magnesium, which is in moderately low supply. Based on these findings, the hypothesis that pollutant deposition has been the cause of a sudden and severe appearance of decline symptoms in the second half of the 1980s must be dismissed. The data on deposition rates and ecosystem nutrient status, however, indicate that the soil of both systems is acidifying, nitrogen stores are increasing, and magnesium pools are depleted. If deposition of pollutants continues at current rates, a slow but steady degradation of many oak ecosystems in the Austrian Weinviertel is inevitable.     

Biomass production and allocation, including fine-root turnover, and annual N uptake in lysimeter-grown basket willows

Annual net primary production (NPP) and N uptake were estimated for lysimeter-grown basket willows (Salix viminalis L.) during 3 years after planting. The willows were grown in a stand structure and continuously supplied with water and liquid fertilizer through drip tubes. The lysimeters contained either clay from the site or washed quartz sand. Shoot growth and leaf litter were measured and fine-root dynamics observed in minirhizotrons. Destructive samples were taken annually in late autumn and entire root systems were washed out. Dry mass and N content of all plant parts were determined. Fine-root production was estimated by two methods, based on destructive samplings and observations in minirhizotrons.

The proportion of biomass allocated below ground increased considerably when estimates based on accumulated NPP were compared with those based on standing dry mass. In the first year, 49 and 58% of annual NPP in willows grown in clay and sand, respectively, was belowground. In subsequent years the proportions were 36–38% and 33–40%. Most belowground production was fine roots. Relatively more N was used belowground in the first year than subsequently, but no substrate-induced differences were observed in the allocation pattern. Both annual NPP and N uptake was always higher in plants in clay than in those in sand: in the final 2 years, 21–22 tonnes DM ha⁻¹ year⁻¹ and 190 kg N ha⁻¹ year⁻¹ in clay, and 9–10 tonnes DM ha⁻¹ year⁻¹ and 100 kg N ha⁻¹ year⁻¹ in sand.     

Interaction between moisture, nutrients and growth of white spruce in interior Alaska

Two thinning and fertilization studies, the first in 1969 and the second in 1971, were established to evaluate the question of nutrient limitation to tree growth and the consequences of stand manipulation of soil moisture supply. Fertilizer was applied yearly for the first 5 years in both studies; growth response has been measured through 1987. Results indicate that thinning is necessary to obtain a growth response to fertilizer applied at the rate of 111 kg nitrogen ha⁻¹. The response to fertilization after fertilization ended lasted for 4 years in plots thinned to 800 stems ha⁻¹, while a significant response continued for only 2 years in plots thinned to 1600 stems ha⁻¹.

A soil water-balance model was calibrated for the control and treatment plots of these two studies. Soil water-deficits were estimated and correlated with yearly average basal-area growth per tree. Results indicated that there is a correlation between seasonal soil-moisture deficit and growth during the years when soil moisture was measured for the unthinned control plots (r² = −0.787, P = 0.002) but not for the thinned and fertilized plots (r² = −0.652, P = 0.057).     

Clear-cutting reduces nitrate leaching in a pine plantation of high natural N status

Following the tree harvest, the biogeochemistry of a catchment is modified by changes in soil temperature and moisture, and nutrient cycling. We monitored soil-solution and stream-water chemistry, and soil properties in a Pinus radiata D. Don plantation in New Zealand before and after clear-cutting and replanting in 1997. The annual rainfall during the study was 1440–1860 mm. The soil was a 1800-year-old pumice soil of high natural N status; the catchment had received large inputs of volcanic N in rain, probably over the 1800 years since the pumice had been deposited. The leaching loss of nitrate-N was 28 kg ha⁻¹ yr⁻¹ in 1996, and then decreased sharply after clear-cutting to 3 kg ha⁻¹ yr⁻¹ in 1998 and <1 kg ha⁻¹ yr⁻¹ in 1999. Weed growth and soil microbial biomass increased during this time, and would have removed much of the N from soil solution in the upper soil layers. Although the catchment was small (8.7 ha), there was a 2-year lag until N decreased in stream-water; the losses of dissolved organic N to stream-water were low. There was no change in soil pH over the 4 years, but spring-water pH appeared to increase, which was consistent with the increase in bicarbonate that accompanied grass/weed growth. The export of cations (mmol_c l⁻¹) in the spring-water was Na>Ca>Mg=K as expected for rhyolitic pumice, and the total concentration was probably controlled by the accompanying anions. The export of anions was NO₃=Cl>SO₄=HCO₃ before harvest and HCO₃=Cl>SO₄=NO₃ after harvest.     

Some nutrient dynamics associated with litterfall and litter decomposition in hoop pine plantations of southeast Queensland, Australia

Litterfall was collected over a 12-month period with littertraps in hoop pine (Araucaria cunninghamii) plantations aged 10, 14 and 62 years in southeast Queensland, Australia. The bulk of litterfall occurred during spring, mainly as hoop pine foliage with the annual litterfall ranging between 6.0 and 10.9 t ha⁻¹, respectively, for the younger stands (10 and 14 years) and the mature 62-year old stand. The amount of nitrogen (N) and phosphorous (P) recycled annually through litterfall was lower in the younger stands (28–37 kg N ha⁻¹ and 4.4–5.3 kg P ha⁻¹) compared with that of the mature stand (85 N ha⁻¹ and 6.2 kg P ha⁻¹). The N and P retranslocated during senescence varied across the three stands studied with a trend for N and P retranslocation to increase as availability of soil mineral-N decreased.

Decomposition of the hoop pine foliage component of litter was also studied in the same stands using a litterbag technique and mass-balance analysis. The estimated half-life of hoop pine foliage mass ranged between 1.5 and 1.8 years. Litter-mass loss was strongly correlated with litter substrate quality indicators of N, C, P, C/N ratio, lignin, lignin/N ratio and polyphenols. During the course of the study, there was no difference in litter-mass loss between the stands of different ages. During the 15-month period, the order of element release from the hoop pine litter was K>Na>C>Mg>P, with N, Ca and Mn generally demonstrating varying degrees of net accumulation. During the course of the study, the lignin/C ratio of the hoop pine litter increased from 0.61 to 0.96. This suggested that the litter-C was predominantly in a recalcitrant form and, therefore, the associated N was unlikely to be rapidly released in the hoop pine litter layer.     

Effects of intensive harvesting on nutrient capitals of three forest types in New England

Effects of whole-tree clearcutting are being studied in three major forest types in the northeastern United States: a spruce-fir forest in central Maine, a northern hardwood forest in New Hampshire, and a central hardwood forest in Connecticut. At each site we sampled total and extractable nutrient capitals, inputs and outputs of nutrient ions in precipitation and streamflow, nutrient removals in harvested products, and nutrient accumulation in regrowth. Depending upon location, combined losses of nutrients in harvested products and increased leaching to streams were in the ranges of 374–558 kg ha⁻¹ for Ca, 135–253 kg ha⁻¹ for K, 50–65 kg ha⁻¹ for Mg, 248–379 kg ha⁻¹ for N, and 19–54 kg ha⁻¹ for P. Opportunities for replacing these losses over the next rotation are best for N. Data on inputs in precipitation versus outputs in streamflow indicate that, once effects of harvest subside, most N in precipitation will stay within the forest. By contrast, Ca shows a net output of 8–15 kg ha⁻¹ year⁻¹ from uncut watersheds, and the added leaching losses due to harvest may have a serious impact on Ca capital. This is especially the case for the Connecticut site, where total site capital for Ca is only about 4000 kg ha⁻¹.     

Dynamics and potentials of carbon sequestration in managed stands and wood products in Finland under changing climatic conditions

Carbon (C) sequestration was studied in managed boreal forest stands and in wood products under current and changing climate in Finland. The C flows were simulated with a gap-type forest model interfaced with a wood product model. Sites in the simulations represented medium fertile southern and northern Finland sites, and stands were pure Scots pine and Norway spruce stands or mixtures of silver and pubescent birch.

Changing climate increased C sequestration clearly in northern Finland, but in southern Finland sequestration even decreased. Temperature is currently the major factor limiting tree growth in northern Finland. In southern Finland, the total average C balance over the 150 year period increased slightly in Scots pine stands and wood products, from 0.78 Mg C ha⁻¹ per year to 0.84 Mg C ha⁻¹ per year, while in birch stands and wood products the increase was larger, from 0.64 Mg C ha⁻¹ per year to 0.92 Mg C ha⁻¹ per year. In Norway spruce stands and wood products, the total average balance decreased substantially, from 0.96 Mg C ha⁻¹ per year to 0.32 Mg C ha⁻¹ per year. In northern Finland, the total average C balance of the 150 year period increased under changing climate, regardless of tree species: in Scots pine stands and wood products from 1.10 Mg C ha⁻¹ per year to 1.42 Mg C ha⁻¹ per year, in Norway spruce stands and wood products from 0.69 Mg C ha⁻¹ per year to 0.99 Mg C ha⁻¹ per year, and in birch stands and wood products from 0.43 Mg C ha⁻¹ per year to 0.60 Mg C ha⁻¹ per year.

C sequestration in unmanaged stands was larger than in managed systems, regardless of climate. However, wood products should be included in C sequestration assessments since 12–55% of the total 45–214 Mg C ha⁻¹ after 150 years' simulation was in products, depending on tree species, climate and location. The largest C flow from managed system back into the atmosphere was from litter, 36–47% of the total flow, from vegetation 22–32%, from soil organic matter 25–30%. Emissions from the production process and burning of discarded products were 1–6% of the total flow, and emissions from landfills less than 1%.     

Use of a gradient of N-deposition to calculate effect-related soil and vegetation measures in deciduous forests

Deposition of N and S has increased since the 1950s in most European countries and N accumulates in ecosystems that are not N saturated. This study shows long-term effects of a (modelled) N deposition of 7–17 kg N ha⁻¹ per year on biological and chemical processes in soil, vegetation composition, and functional types of field-layer plant species in deciduous forests. Soil pH largely determined the response of the soil processes, emphasising the importance to compare soils of similar acidity regarding the effects of N deposition. The most pronounced effects were demonstrated for the most acid study plots. When we compared regions with a deposition of 7 and 17 kg N ha⁻¹ per year we found a 40–80% higher soil N mineralisation rate, 2–90% higher nitrification rate and 10–25% lower C:N ratio in the region with the highest deposition. Similar but smaller differences were indicated when regions with a deposition of 7 and 10 kg N ha⁻¹ per year were compared. Number of species was lower in the regions with the highest deposition. Literature data for plants on N concentration, nitrate reductase activity (NRA), growth rates, morphology and height were calculated on a site basis. They varied to different extent between the regions. The N concentration was 7–24% higher in the regions with the highest N deposition. We argue that the effect-related critical load based on our results should be set to a N deposition of 7–10 kg N ha⁻¹ per year. Critical loads for a subdivision of deciduous forests would give lower critical loads for the most acid soils compared to less acid soil.     

Nutrient cycling in a clonal stand of Eucalyptus and an adjacent savanna ecosystem in Congo: 3. Input–output budgets and consequences for the sustainability of the plantations

Clonal plantations of Eucalyptus have been introduced since 1978 on savanna soils of the coastal plains of Congo. Atmospheric deposition, canopy exchange and transfer through the soil were estimated on the whole rooting depth (6 m) over 3 years, in an experimental design installed in a native savanna and an adjacent 6-year-old Eucalyptus plantation. Complementary measurements after planting the experimental savanna made it possible to establish input–output budgets of nutrients for the whole Eucalyptus rotation and to compare them with the native savanna ecosystem.

In this highly-weathered soil, atmospheric deposits and symbiotic N fixation by a legume species balanced the nutrient budgets in savanna, despite large losses during annual burnings. After afforestation, weeding in the Eucalyptus stands eliminated the leguminous species responsible for a N input by symbiotic fixation of about 20 kg ha⁻¹ year⁻¹. Whereas the budgets of P, K, Ca and Mg were roughly balanced, the current silviculture led to a deficit of about 140 kg N ha⁻¹ in the soil, throughout a 7-year rotation. This deficit was large relative to the pool of total N in the upper soil layer (0–50 cm), which was about 2 t ha⁻¹. Therefore, the sustainability of Congolese plantations will require an increase in N fertilizer inputs over successive rotations to balance the N budget. These results were consistent with field trials of fertilization. Practical consequences of these budgets were identified, in order to: (i) direct field trials of fertilization, (ii) select appropriate methods of soil preparation, weed control and harvest, (iii) highlight the importance of fire prevention in this area, and (iv) support the implementation of field trials aiming at introducing a biological nitrogen fixing understorey in Eucalyptus stands.     

Long-term soil potassium availability from a Kanhapludult to an aggrading loblolly pine ecosystem

A long-term (1962 to 1990) forest biogeochemistry study in the southeastern Piedmont of the USA provided estimates of soil K release in response to forest regrowth. We investigated the sources of soil K that buffered the exchangeable K pools during forest growth and we estimated soil K release rates through greenhouse and acid extraction studies for comparison to our field estimate.

In these acid Kanhapludults, derived from granitic-gneiss, the disparity between measured depletions of soil exchangeable K and estimated forest removals indicated a buffering of exchangeable K on the order of 0.31 kmol_c ha⁻¹ per year. Non-exchangeable K extracted by boiling with 1 M HNO₃ exceeded exchangeable K by up to 40-fold. Non-exchangeable K was not depleted during the three decades of stand growth, however, thus was not the long-term source of exchangeable K buffering. Total K in these soils ranged from 0.4 to 3.8% by weight. Mineralogical data indicated a presence of hydroxy-interlayered vermiculite throughout the upper 4 m of soil for <2 μm clay fraction and a presence of micaceous minerals in the 2 to 45 μm silt fraction. XRD analysis of micaceous flakes extracted from 4 to 8 m in the soil indicated a presence of muscovite mica.

Estimated K releases in the greenhouse and extraction studies were generally consistent with long-term results. The accumulation of K during two rotations of pine seedling growth in the greenhouse exceeded the measured depletions in exchangeable and non-exchangeable K over all soil depths tested by 0.007 to 0.026 cmol_c kg⁻¹. Potassium removal by sequential extraction/incubations with 1 mM HCl and 1 mM oxalic acid continued through 24 extractions and K recovered in extract solutions exceeded the sum of depletions in exchangeable and non-exchangeable K pools by 0.001 to 0.028 cmol_c kg⁻¹. These excess removals in plant uptake or solution recovery indicate a release of mineral K. Thirty-day extractions with H⁺-resins in both 1 mM HCl and 1 mM oxalic acid were well fit by the Elovich equation but were not well correlated with plant K uptake in the greenhouse study. The release rate coefficients ranged from 0.012 to 0.025(cmol_c kg⁻¹) h⁻¹.

Extrapolations to annual releases of K in the greenhouse and sequential extraction studies were a similar order of magnitude as long-term releases estimated at the long-term Calhoun plots. Surface soil (0 to 15 cm) releases ranged from 0.15 to 0.65 kmol_c ha⁻¹per year while deeper soils ranged up to 1.54 kmol_c ha⁻¹per year. Results indicate that soils similar to those at Calhoun that contain a similar micaceous and HIV component will be able to supply K at rates adequate to keep pace with demands of forest regrowth even under intensive forest management.     

Modelling the yield of Pinus radiata on a site limited by water and nitrogen

A process-based model is described and applied to a range of Pinus radiata D. Don stands, aged 9–12 years, growing on stabilised sand dunes in a stocking × fertiliser experiment in Woodhill State Forest, New Zealand. The model requires inputs of daily weather data (maximum and minimum air temperatures and rainfall), physical characteristics of the site (longitude, latitude, rootzone depth and relationship between root-zone soil matric potential and volumetric water-content) and crop (stocking, crown dimensions and leaf-area index) and crop physiological parameters (e.g., maximum stomatal conductance). The model was used to simulate components of the forest water-balance and annual net photosynthesis for a defined crop canopy architecture. Simulated daily root-zone water storage in both open and closed canopy stands generally agreed with monthly measurements made over a complete year. Simulated net annual photosynthesis ranged from 23 to 33 t C ha⁻¹ year⁻¹ and comparison with measured stem-volume increments of 12–38 m³ ha⁻¹ year⁻¹ over the same time periods resulted in a strong positive correlation. Ratios of stem-volume increment to net photosynthesis suggested that fertilised and unfertilised stands had a 26 and 14%, respetively, allocation of C to stem growth. Simulations using weather data for a dry year with 941 mm year⁻¹ rainfall indicated that annual net photosynthesis and transpiration of fully stocked stands were reduced by 41 and 45%, respectively, compared to those in a wet year with 153 mm year⁻¹ rainfall. Operational applications of the model to forest management in quantifying environmental requirements for stand growth and examining silvicultural alternatives are discussed.     

Litter and fine root dynamics of a relict sacred grove forest at Cherrapunji in north-eastern India

In a sacred grove climax forest (protected by the local Khasi tribe for religious reasons) at Cherrapunji in north-eastern India (mean annual rainfall, 10 372 mm), litter dynamics and related fine root dynamics were studied. Litterfall and its decomposition were very pronounced during the monsoon season, unlike in other rain forests in the region. High levels of nitrogen (2.13–3.58%) phosphorus (0.62–0.91%) and potassium (1.45–1.98%) in the leaf litter of four dominant tree species—Englehardtia spicata Bl., Echinocarpus dasycarpus Benth, Sysygium cuminii (L.) Skeels and Drimycarpus racemosus Hk.f.—suggest that these species help in conserving nutrient elements, thus ensuring their rapid recycling. Different species have different nutrient release patterns which are related to litter quality and seasonal environmental factors.

The highly developed fine root system (14 000 kg ha⁻¹) of which about 48% is located in the 0–10 cm soil depth, is important for nutrient storage and rapid recycling of nutrients. With a productivity of 3200 kg ha⁻¹ per year, the fine root component of the climax sacred grove ecosystem has a key role to play in tight nutrient cycling. It is concluded that these results are important in understanding the fragility of rainforest ecosystems and their management.     

Water/nutrient interactions affecting the productivity of stands of Pinus radiata

The accumulation of above-ground biomass and the seasonal patterns of leaf-area development, foliar nutrient concentrations and tree and soil water-status have been measured for fertilised, irrigated, and control stands of Pinus radiata D. Don growing on a low-productivity site, average annual precipitation of 790 mm, near Canberra in southeastern Australia. In the second growing-season after treatments commenced, projected leaf-area index reached peak values of 7 on the irrigated/fertilised stands compared with approximately 5 on the other stands. Average canopy nitrogen concentration (dry-weight basis) varied across the treatments from 9 to 17 mg g⁻¹. Measurements of soil and tree water-status over a 2-year period indicated that stands which were not irrigated experienced summer droughts of up to 4 months duration.

Annual volume production measured over the 2-year period ranged from 17 to 45 m³ ha⁻¹. The extent to which this variation could be attributed to differences in leaf area, rats of photosynthesis, duration of the period of positive net photosynthesis, and hence growth, was analysed in terms of a process-based model of stand growth dependent on climate and soil water-balance.

Annual canopy net photosynthesis simulated by the model ranged from 18 t carbon ha⁻¹ for the control stand to 38 t ha⁻¹ for the irrigated/fertilised stands. Simulations indicated that 67% of this difference could be attributed to the role of irrigation in extending the period of active growth. The additional leaf area carried by the irrigated/fertilised stands contributed a further 23%, while differences in rates of photosynthesis, related to nitrogen nutrition, explained the remaining 10%.     

Growth potential of twelve Acacia species on acid soils in Hawaii

Reforestation of degraded tropical sites is often hampered by soils of high acidity, high aluminum saturation, and low fertility. To evaluate the possibility of cultivating Acacia species on such soils, a study was conducted at Waiawa, HI, to test growth under conditions of (1) high acidity (primarily aluminum) and nutrient stress, and (2) no acidity stress and high nutrient availability. Twelve Acacia species, including the important native Hawaiian species Acacia koa, were established on a Ustic Kanhaplohumult soil. The experimental design was a split plot with two fertility treatments as the main plots and the 12 Acacia species as subplots. The treatments were: low fertility (F₀; 143 kg ha⁻¹ 14-14-14 plus micronutrients) and high fertility (F₁; 8 Mg ha⁻¹ lime, 143 kg ha⁻¹ 14-14-14 plus micronutrients, 200 kg P ha⁻¹, and 77 kg K ha⁻¹). Acacia angustissima, Acacia aulacocarpa, Acacia auriculiformis, Acacia cincinnata, Acacia crassicarpa, Acacia implexa, Acacia koa, and Acacia mangium grew significantly faster under the high fertility treatment. Three species, A. cincinnata, A. crassicarpa, and A. mangium, are recommended for planting on infertile acid soils. The volume of A. koa was increased ten-fold by the high fertility treatment. Additional study on koa's nutritional requirements is suggested in order to identify the nutrients contributing to this increased growth.     

Effects of repeated urea doses on soil chemistry and nutrient pools in a Norway spruce stand

Pools of macro-nutrients in soil and vegetation were studied in an old fertilization experiment with a large previous input of N. Different doses of N, in the form of urea, had been added four times during a 20-year period. In total, between 480 and 2400 kg N ha⁻¹ had been given. The experiment was established in a relatively productive Norway spruce stand and the expectation was that the large N input would cause an accelerated leaching of N, especially nitrate, accompanied by soil acidification and losses of several nutrients. The aim was to test for possible residual effects. Thirteen years after the last N addition, samples from the aboveground part of trees, field layer, S-layer, humus layer and mineral soil (0–10 cm) were analyzed for concentrations of most major nutrients. Nutrient pools were calculated. In the humus layer, the concentration of N increased and the C/N-ratio decreased with increasing N dose. The calculated recovery of added N in soil including ground vegetation was complete for the lowest N dose, while it was 25–50% for higher doses. The amount of N retained was unaffected by the N dose. The amount of extractable P in the upper part of the mineral soil was negatively correlated with N dose, as was also the concentration of total P in the S-layer. Neither soil pH, nor concentrations or amounts of Ca, Mg and K were affected by the previous fertilization. The calculated total soil-plant pool was only influenced by N dose in the case of P, which was 20% lower at the highest N dose compared with unfertilized conditions. Despite the large extra N input, the nutritional changes in plants and soil of the actual study site seemed surprisingly small.     

Assessing the nutrition of juvenile hybrid poplar using a steady state technique and a mechanistic model

Fiber farming utilizing fast growing trees provides a means to satisfy the increasing demand for hardwood fiber. To maximize growth during the establishment phase, it is important to understand the relationship between maximal growth rate, plant nutrient requirements, and the ability of the soil to supply nutrients. The objectives of this study were: (i) to use a steady state nutrition technique to establish threshold N, P, and K ratios associated with maximal growth and (ii) to use the Barber–Cushman nutrient uptake model as a means of coupling information on the nutrient supplying capacity of the soil with the optimum N:P:K ratio. Growth chamber and greenhouse studies, using Populus nigra×maximowiczii (NM-6), were conducted to determine maximal growth rate, optimal N, P, and K content, uptake kinetics, and soil supply characteristics. Maximal growth rate was 0.12 g g⁻¹ per day at a whole plant ratio of 100N:11P:37K. The Barber–Cushman model was run using soil supply and root growth data from the greenhouse study. Nitrogen, P, and K uptake were simulated for a 105-day period in response to the addition of a slow release 17–6–12 fertilizer at rates equivalent to 0, 75, or 150 kg ha⁻¹ of N. Model predictions of uptake improved as the amount of fertilizer added increased. Uptake estimates at the 150 level were 96, 120, and 98% of observed uptake for N, P, and K, respectively. The model predicted that the supply of N was not adequate to support sustained plant uptake throughout the study period. Plant uptake and soil supply observations confirmed that N uptake occurred primarily in the first half of the growth period and that soil N supply was quickly depleted. Model simulations of P uptake support earlier observations that uptake is not solely a function of supply. A 10-fold increase in simulated supply increased uptake by a factor of 6. Much more needs to be done before a soil supply model like Barber–Cushman can be used as a nutrient management tool in forestry applications. However, establishing optimum levels, ratios, and rates of nutrient addition provides a good starting point for further evaluation.     

Growth, biomass production and nutrient accumulation by seven tree species irrigated with municipal effluent at Wodonga, Australia

In the Murray-Darling basin, irrigation of tree crops is being evaluated as an alternative method for the disposal of municipal effluent. A study was carried out at Wodonga in which seven tree species were irrigated with effluent for a period of 4 years. Irrigation was calculated weekly on the basis of pan evaporation and rainfall during the preceding week. Annual irrigation varied between 1190 mm and 1750 mm with a total input over the 4-year-period of 4940 mm.

Height and diameter growth varied significantly between species. At age 4, mean dominant height of Eucalyptus grandis, E. saligna and Populus deltoides × P. nigra ranged from 14.3 to 15.0 m compared with 6.6 to 9.8 m for Casuarina cunninghamiana, E. camaldulensis, P. deltoides and Pinus radiata. Wood production of the faster-growing species (E. grandis and E. saligna) was approximately 130 m³ ha⁻¹, or around 32 m³ ha⁻¹ year⁻¹ over a 4-year period. This was nearly three-fold the production of the other native species and twice that of Pi. radiata. Volume growth of P. deltoides × P. nigra (85 m³ ha⁻¹) was significantly greater than that of P. deltoides (42 m³ ha⁻¹).

Accumulation of nutrients in the above-ground biomass varied significantly between species and ranged from 24 to 41 g m⁻² for N, 2.6 to 5.9 g m⁻² for P, 0.5 to 9.2 g m⁻² for Na, 12 to 27 g m⁻² for K, 7 to 52 g m⁻² for Ca and 3.1 to 7.9 g m⁻² for Mg. Nutrient accumulation was generally greater in species with a comparatively large crown biomass relative to stem size such as C. cunninghamiana and E. camadulensis. Average nutrient accumulation by trees as a percentage of input from effluent was estimated at 19% for N, 9% for P, 1% for Na, 14% for K, 52% for Ca and 32% for Mg.

Results of this study indicate the importance of selecting species on the basis of not only growth but also nutrient accumulation to optimise renovation of wastewater by tree plantations.