Calcareous soils are frequently typified by a low availability of plant nutrients due to poor solubility of these elements at high pH. Calcicole plants have recently been shown to release organic acids in response to the nutrient deficient conditions prevailing in these soils. It has been speculated, however, that the efficiency of this nutrient mobilization mechanism may be significantly reduced by microbial degradation of the organic acids. In conventional methods, root exudate degradation is typically determined by the addition of 14 C‐radiolabeled substrates to soil and subsequent tracking of their fate with time by trapping evolved 14 CO 2 in a strong alkali trap. However, in calcareous soils, 14 CO 2 and H 2 14 CO 3 produced by microbial decomposition may become trapped as Ca(H 14 CO 3 ) 2 The aim of this study was to develop and validate an experimental procedure for the accurate quantification of 14 C‐labeled substrate degradation rates in calcareous soils. Conventional methods for determining 14 C‐labeled substrate decomposition rates in calcareous soils are inaccurate due to incomplete recovery of 14 CO 2 Up to 49% of the 14 CO 2 produced during microbial degradation of 14 C‐labeled organic acids (malate, oxalate, citrate) was trapped as carbonate in this calcareous soil (pH 7.58). For an acid soil (pH 4.32) no detectable amount of 14 CO 2 was trapped. We describe a simple, accurate, and reliable method, which includes a postincubation HCl addition, for the accurate determination of 14 CO 2 ‐evolution and substrate degradation in calcareous soils.
No abstract is provided for this article.
Dissolved organic nitrogen (DON) forms a significant pool of bioavailable N in the water column (62%) and in sediments of Ria Formosa lagoon (53%). We assessed the uptake rates of inorganic and organic nitrogen and its interactions in the seagrass Zostera marina , and further explored the possibility of seagrasses to use complex organic substrates (peptides). Uptake rates by leaves and roots were quantified in choice‐uptake experiments where plants were exposed to mixed N solutions containing both 15 N inorganic (ammonium + nitrate) and 13 C 15 N organic (alanine + trialanine) nitrogen at field‐relevant concentrations, and compared with uptake rates of single inorganic or organic N forms. Ammonia was the preferred N source, but plants preferred DON to nitrate. DON uptake was significantly higher through roots than leaves, coinciding with the one‐order of magnitude higher concentration of DON in the sediment than in the water. Not only amino acids, as reported elsewhere for other seagrasses, but also peptides constitute relevant N sources for Z. marina (10% and 4% of the total N uptake). Seagrasses may thus compete with microbes for organic substrates at an earlier stage of protein degradation in the N cycle than previously thought. Because no interactions occurred between inorganic and organic N sources, the total N uptake by Z. marina was higher when both sources were present, showing that organic nitrogen is a complementary rather than alternative source of nitrogen. The uptake of organic nitrogen should be included in future studies assessing the total N budgets of seagrass meadows.
Irradiation of Ru3(CO)12 hexane in the presence of triphenylphosphine gives undecacarbonyl (triphenylphosphine)- triangulo -triruthenium in addition to the previously obtained Ru(CO)4PPh3 and Ru(CO)3(PPh3)2. X-Ray structure determination of the cluster compound reveals a single isomer in which there is a triangle of ruthenium atoms with the triphenylphosphine equatorially substituted and all the carbonyl groups terminally sited. The Ru-Ru distances are 2.907(3), 2.876(3) and 2.875(3)Å, with the longest distance cis to the triphenylphosphine ligand. The crystals are monoclinic space group C2/c with a 22.30(2)Å, b 16.34(1)Å, c 17.42(1)Å, B 103.84(4)° and Z 8. The structure was refined to R 7.2% for 1773 observed counter amplitudes.
As a prelude to remote sensing of rhizomania, hyper-spectral leaf reflectance and multi-spectral canopy reflectance were used to study the physiological differences between healthy sugar beets and beets infested with Beet necrotic yellow vein virus. This study was conducted over time in the presence of declining nitrogen levels. Total leaf nitrogen was significantly lower in symptomatic beets than in healthy beets. Chlorophyll and carotenoid levels were reduced in symptomatic beets. Vegetative indices calculated from leaf spectra showed reductions in chlorophyll and carotenoids in symptomatic beets. Betacyanin levels estimated from leaf spectra were decreased at the end of the 2000 season and not in 2001. The ratio of betacyanins to chlorophyll, estimated from canopy spectra, was increased in symptomatic beets at four of seven sampling dates. Differences in betacyanin and carotenoid levels appeared to be related to disease and not nitrogen content. Vegetative indices calculated from multi-spectral canopy spectra supported results from leaf spectra. Logistic regression models that incorporate vegetative indices and reflectance correctly predicted 88.8% of the observations from leaf spectra and 87.9% of the observations for canopy reflectance into healthy or symptomatic classes. Classification was best in August with a gradual decrease in accuracy until harvest. These results indicate that remote sensing technologies can facilitate detection of rhizomania.
