High temporal resolution tracing of photosynthate carbon from the tree canopy to forest soil microorganisms.

Half of the biological activity in forest soils is supported by recent tree photosynthate, but no study has traced in detail this flux of carbon from the canopy to soil microorganisms in the field. Using (13)CO(2), we pulse-labelled over 1.5 h a 50-m(2) patch of 4-m-tall boreal Pinus sylvestris forest in a 200-m(3) chamber. Tracer levels peaked after 24 h in soluble carbohydrates in the phloem at a height of 0.3 m, after 2-4 d in soil respiratory efflux, after 4-7 d in ectomycorrhizal roots, and after 2-4 d in soil microbial cytoplasm. Carbon in the active pool in needles, in soluble carbohydrates in phloem and in soil respiratory efflux had half-lives of 22, 17 and 35 h, respectively. Carbon in soil microbial cytoplasm had a half-life of 280 h, while the carbon in ectomycorrhizal root tips turned over much more slowly. Simultaneous labelling of the soil with (15)NH(+)(4) showed that the ectomycorrhizal roots, which were the strongest sinks for photosynthate, were also the most active sinks for soil nitrogen. These observations highlight the close temporal coupling between tree canopy photosynthesis and a significant fraction of soil activity in forests.

Growth of conifer seedlings on organic and inorganic nitrogen sources.

Effects of organic and inorganic nitrogen sources on growth and mineral nutrient concentrations of Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies (L.) Karst.) seedlings were compared in a 100-day experiment in a greenhouse. Seedlings were grown in pots containing peat. Nutrient solutions differing in ammonium, nitrate, arginine and glycine composition were supplied to the seedlings at three nitrogen (N) concentrations: 1, 3 and 10 mM. We used dual (13C, 15N) and single (15N) isotopic labeling to determine the uptake of organic and inorganic N at the end of the experiment. Seedling dry weights and mineral nutrient concentrations of the needles showed that both conifer species were able to grow well and maintain nutrient balance on all investigated N forms except for the ammonium-dominated nutrient mixtures at the 10 mM N concentration. In Scots pine, no significant differences in dry weights were found between seedlings grown on the amino acids and seedlings grown on a commercial fertilizer containing 61.5% NO(3-)-N and 38.5% NH(4+)-N. Isotopic labeling of seedlings indicated that uptake rates of arginine-N, glycine-N and NH(4+)-N were similar, and 7-8 times greater than uptake rates for NO(3-)-N in both species. In Scots pine seedlings, 100% of arginine-N, and at least 67% of glycine-N was derived from the uptake of intact amino acids through seedling roots or mycorrhizae. Corresponding figures for Norway spruce were 83% for arginine and 96% for glycine. The gas chromatography-mass spectrometry analyses confirmed the presence of intact labeled molecules of both arginine and glycine in seedlings. We conclude that arginine and glycine are comparable to inorganic N as N sources for growth of conifer seedlings.

Nitrogen forms in bark, wood and foliage of nitrogen-fertilized Pinus sylvestris.

Cycling of soluble non-protein N compounds is thought to be indicative of the N-nutritional status of trees. We determined the major N forms in bark, wood and foliage and estimated the dependence of prevalent N forms on N availability in Pinus sylvestris L. trees from northern Sweden. Trees subjected to severe N limitation and trees that had been fertilized with an average 64 kg N ha(-1) year(-1) for 25 years were analyzed. Bark and wood samples were collected by tangentially cryo-sectioning the trunk into 30-microm thick sections, from the bark to the functional xylem. Soluble amino compounds were extracted from the sections for analysis. Sap samples from twigs were obtained by centrifugation, and bark samples from twigs were obtained by tissue extraction. In both needles and bark, arginine dominated the amino-N pool. Because arginine concentrations in needles increased with N fertilization, arginine dominance of the amino-N pool in needles was higher in N-fertilized trees than in control trees. In bark, N fertilization resulted in a large increase in glutamine concentration, so that glutamine accounted for a larger proportion of the amino-N pool in bark in N-fertilized trees than in control trees. Glutamine dominated the amino-N pool in wood of control trees. Nitrogen fertilization resulted in an increased proportion of arginine in the wood amino-N pool. We conclude that the composition of the amino-N pools in bark, wood and foliage is highly sensitive to N supply. The composition of the amino-N pools can contribute to the regulation of tree N-nutritional status, which is mediated by shoot to root signalling by long-distance transport of amino compounds.

Free amino acids and protein in Scots pine seedlings cultivated at different nutrient availabilities.

Scots pine (Pinus sylvestris L.) seedlings of a provenance from northern Sweden were cultivated hydroponically for 7 weeks in a climate chamber. The nutrient solution contained either 2.5 (low-N) or 50 (high-N) mg N l(-1) with other essential elements added in a fixed optimal proportion to the nitrogen. After 5 and 7 weeks, the seedlings were analyzed for growth, total nitrogen and other essential nutrients, protein and free amino acids. Low-N seedlings grew more slowly and had higher root/shoot ratios than high-N seedlings. With respect to total nitrogen, the effect of the lower nutrient supply was mainly on the nitrogen content of the whole plant and the allocation of nitrogen among tissues, not on tissue nitrogen concentration. This was also the case for potassium, phosphorus, calcium and magnesium. The proportions by weight among these macronutrients in the whole seedlings were similar in both nutrient regimes. The proportion and concentration of sulfur were significantly lower in low-N seedlings than in high-N seedlings, because of a lower net uptake of sulfur than of other macronutrients. The shoot, needles and stem of low-N seedlings had higher concentrations of free amino acids and lower concentrations of protein than the shoot, needles and stem of high-N seedlings. Arginine dominated the pool of free amino acids in the low-N seedlings, whereas glutamine predominated in the high-N seedlings. We conclude that Scots pine seedlings accumulated soluble nitrogen as arginine when net protein synthesis was limited by factors other than nitrogen availability. Nutritional imbalance, as revealed by growth characteristics and a suboptimal proportion and concentration of sulfur in the seedlings, probably affected synthesis of S-amino acids, resulting in the diversion of assimilated nitrogen to arginine instead of protein.

The relationship between crown size and ring width in Pinus sylvestris L. stems: dependence on indole-3-acetic acid, carbohydrates and nitrogen in the cambial region.

Indole-3-acetic acid (IAA), carbohydrates, total nitrogen and amino acids in the cambial region and bark were measured at the top (10-year-old internode) and bottom (1.3 m) of the main stem of 50-year-old Pinus sylvestris L. trees, having different rates and longitudinal gradients of annual ring width formation. The trees were sampled during the most active period of wood production (June 23, July 15) and at the end of this period (August 23). Trees with a small crown and relatively slow growth rate (S-trees) were compared with trees in the same stand that had a large crown and fast growth rate (F-trees) as a result of thinning and fertilization. The effect of bottom pruning fast-growing trees (pruned F-trees) was also investigated. The F-trees had greater wood production than the S-trees at both the stem top and bottom. The difference was larger at the stem bottom, thus the relative decrease in ring width down the stem was steeper in the S-trees. The amount of sucrose and IAA per cm(2) in cambial region tissues was higher in F-trees than in S-trees. However, the differences in annual ring width between treatments and within trees were not reflected in the concentrations (expressed per gram fresh weight) of these substances, measured either in differentiating xylem, the cambium plus phloem, or in the cambial region as a whole. The concentrations of total nitrogen and amino acids were slightly higher in the F-trees than in the S-trees. Pruning the F-trees reduced wood production, particularly at the stem bottom, inhibited the springtime increase in starch, and decreased the amounts of sucrose and IAA per cm(2) in the cambial region. However, it was evident that the concentrations of sucrose, IAA, amino acid and nitrogen in the cambial region were not related to ring width. During June and July, the concentrations of these substances were generally higher at the stem bottom in the pruned F-trees than in the F-trees. The results provide evidence that it is the activity of the cambium rather than the availability of carbohydrates that determines the allocation of wood production along the tree stem. The results also indicate that tracheid production is not directly related to the IAA concentration in the cambial region.

Seasonal changes in amino acids, protein and total nitrogen in needles of fertilized Scots pine trees.

Seasonal changes in amino acids, protein and total nitrogen in needles of 30-year-old, fertilized Scots pine (Pinus sylvestris L.) trees growing in Northern Sweden were investigated over two years in field experiments. The studied plots had been fertilized annually for 17 years with (i) a high level of N, (ii) a medium level of N, or (iii) a medium level of N, P and K. Trees growing on unfertilized plots served as controls. In control trees, glutamine, glutamic acid, gamma-aminobutyric acid, aspartic acid and proline represented 50-70% of the total free amino acids determined. Arginine was present only in low concentrations in control trees throughout the year, but it was usually the most abundant amino acid in fertilized trees. Glutamine concentrations were high during the spring and summer in both years of study, whereas proline concentrations were high in the spring but otherwise low throughout the year. In the first year of study, glutamic acid concentrations were high during the spring and summer, whereas gamma-aminobutyric acid was present in high concentrations during the winter months. This pattern was less pronounced in the second year of investigation. The concentrations of most amino acids, except glutamic acid, increased in response to fertilization. Nitrogen fertilization increased the foliar concentration of arginine from < 1 micromol g(dw) (-1) in control trees to a maximum of 110 micromol g(dw) (-1). Trees fertilized with nitrogen, phosphorus and potassium had significantly lower arginine concentrations than trees fertilized with the same amount of nitrogen only. Protein concentrations were similar in all fertilized trees but higher than those in control trees. For all treatments, protein concentrations were high in winter and at a minimum in early spring. In summer, the protein concentration remained almost constant except for a temporary decrease which coincided with the expansion of new shoots. Apart from arginine, the amino acid composition of proteins was similar in all treatments.