Seedlings of Eucalyptus grandis were grown at five different rates of nitrogen supply. Once steady-state growth rates were established, a detailed set of CO 2 and water vapour exchange measurements were made to investigate the effects of leaf nitrogen content ( N ), as determined by nitrogen supply rate, on leaf structural, photosynthetic, respiratory and stomatal properties. Gas exchange data were used to parametrize the Farquhar-von Caemmerer photosynthesis model. Leaf mass per area (LMA) was negatively correlated to N . A positive correlation was observed between both day ( R d ) and night respiration ( R n ) and N when they were expressed on a leaf mass basis, but no correlation was found on a leaf area basis. An R d / R n ratio of 0·59 indicated a significant inhibition of dark respiration by light. The maximum net CO 2 assimilation rate at ambient CO 2 concentration ( A max ), the maximum rate of potential electron transport ( J max ) and the maximum rate of carboxylation ( V cmax ) significantly increased with N , particularly when expressed on a mass basis. Although the maximum stomatal conductance to CO 2 ( g scmax ) was positively correlated with A max , there was no relationship between g scmax and N . Leaf N content influenced the allocation of nitrogen to photosynthetic processes, resulting in a decrease of the J max / V cmax ratio with increasing N . It was concluded that leaf nitrogen concentration is a major determinant of photosynthetic capacity in Eucalyptus grandis seedlings and, to a lesser extent, of leaf respiration and nitrogen partitioning among photosynthetic processes, but not of stomatal conductance.

In Australia, irrigating tree plantations with domestic sewerage effluent is becoming an increasingly common and necessary alternative to disposal of effluent in rivers. This study estimated field rates of soil N mineralization and concentrations of N in soil solution under a newly established stand of radiata pine (Pinus radiata D. Don) to which secondary‐treated sewerage effluent was applied by spray‐irrigation at various rates. These rates nominally were: medium treatment (M) irrigated at the rate at which water was used by the plantation (allowing for inputs in rain); high treatment (H) irrigated at twice the rate M; low treatment (L) irrigated at half the rate M; and a bore‐water control (W, groundwater containing no N or P) irrigated at the rate of water use (less rainfall). Annual rates of in situ N mineralization were large in all treatments, ranging from 290 kg ha−1 in the L treatment to 410 kg ha−1 in the H treatment. The amount of soil N decreased, on average, by about 190 kg ha−1 during the first two irrigation seasons in the H, M, and W treatments. A total of 374 kg of N was added in effluent during the first three irrigation seasons in the H treatment. Despite this addition and the large amount of N mineralized, most of which was nitrified, leaching of NO3‐N (and of total N) was limited. During a 2‐yr sampling period, for example, the concentrations of NO3‐N leaching at 100 cm averaged only 1.1 mg L−1 across all treatments. Weed growth and weed management, in combination with irrigation and its effect on soil moisture availability can greatly influence soil N turnover and leaching in young tree plantations.

Eucalyptus grandis seedlings were grown in growth units in which plant roots were suspended in air while continuously being sprayed with nutrient solution (aeroponic system). Phosphorus was added to nutrient solutions in exponentially increasing amounts which determined plant growth rates. Plants were grown at five relative phosphorus addition rates, and photosynthetic performance of leaves was compared across treatments. Carbon assimilation rates ranged from 11.7 pmol m-2 s-' for plants with lowest phosphorus status to 23.1 pmol m-2 s-' for plants with highest phosphorus status. Intercellular partial pressures of COz concomitantly decreased from 260 pbar for plants with lowest to 220 pbar for plants with highest phosphorus status. Leaves in all treatments showed a decrease in assimilation rate at intercellular partial pressures of C 0 2 above c. 600 pbar. There was no consistent correlation between the extent of that decrease and the phosphorus status of leaves.Assimilation rates were correlated with leaf phosphorus content. This relationship was apparent on either a unit leaf area or unit leaf weight basis. Assimilation rates and leaf nitrogen content per unit leaf weight were also correlated. In contrast, there was no correlation between leaf assimilation rate per unit leaf area and nitrogen content per unit leaf area, as nitrogen content per unit area was similar for all phosphorus treatments. The differences between correlations on a weight and area basis were due to differences in specific leaf area in different treatments, with plants with lower phosphorus status having less leaf area per unit leaf weight. The photosynthetic measurements showed that C02 assimilation rate, together with relative leaf growth rate, was one of the processes most sensitive to phosphorus nutrition.

Fifteen‐year old Monterey pine (Pinus radiata) was irrigated for 3 yr with wastewater derived from industrial and municipal sources. The wastewater contained high concentrations of Na2+ and HCO3− and was quite alkaline. Irrigation thus caused substantial increases in exchangeable Na2+, extractable P, exchangeable K+, pH, and the electrical conductivity of the soil solution. Highly colored organic compounds derived from pulp‐mill effluent apparently combined with inorganic N from municipal effluent to form organic N compounds. Nitrogen remained largely in the organic form and became concentrated in the groundwater with colored humic compounds. The greatest environmental hazard in the use of such blended wastewater for irrigation is the contamination of drainage water with colored, saline water containing high concentrations of organic N. The organic compounds appeared to inhibit nitrification in the groundwater.

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