Soil organic nitrogen composition in Pinus forest acid soils: variability and bioavailability
Abstract The total N content in the acid forest soils studied ranged between 0.41% and 1.43%, and in more than 98% was composed of organic N. Total hydrolysable organic N, hydrolysable unknown N (HUN) and α-aminoacidic N represented around 70%, 34% and 20% of the organic N, respectively, and varied in wide ranges. The percentages of amidic N and of the organic N compounds solubilised to NH4+ were approximately 6% and 5%, respectively, and ranged in narrow intervals. Aminoglucidic N reached a maximum of 3.8% of the organic N and was undetectable in some of the samples analysed. Most of the hydrolysable N, HUN and α-aminoacidic N was solubilised with 1 N and 3 N HCl, while a high amount of the compounds recovered as NH4+ (60%) was obtained with 6 N HCl. The distribution of aminoglucidic N in the four fractions of increasing hydrolytic intensity was very irregular. The organic N composition in the 0 to 5-cm and 5 to 10-cm layers was not significantly different. The variation among samples was determined mainly by the organic N compounds less resistant to acid hydrolysis (hydrolysable N and HUN less resistant to acid hydrolysis, amidic N and labile ammoniacal N) and by all α-aminoacidic N fractions. Aminoacidic N was positively correlated with electrical conductivity and negatively correlated with exchangeable Al. The net N mineralisation over 10 weeks of incubation was positive in all the soil samples analysed. The inorganic N content after the incubation and the microbial N content were positively correlated with other variables – mainly with amidic N and α-aminoacidic N, as well as with HUN and the hydrolysable N less resistant to hydrolysis.
Modified method for the fractionation of soil organic nitrogen by successive hydrolyses
Abstract Three methods of fractionating organic N by acid hydrolysis with HC1 were compared using two test soils: conventional continuous hydrolysis, the authors' previous step-wise method and a modification of this method designed to improve the recovery of α-aminoacid N and reduce the percentage of “unknown” N. The modification consists in the inclusion of a rapid hydrolysis with conc. HC1 and conc, propionic acid in a reaction bomb. In all three methods, trisodium citrate was employed to prevent the flocculation of Fe/Al gels during the neutralization of the hydrolyzates. The recommended method is the modified step-wise procedure. With respect to the continuous method this procedure reduced “unknown” N (residual N + hydrolyzable unknown N + NH 4 + N), slightly improved the recovery of α-aminoacids and allowed the determination of an additional form of N (amidic N); and with respect to the original step-wise method it not only retained all the latter's advantages but also doubled the recovery of α-aminoacids while reducing the quantity of unknown N. The sequential solubilization of each form of organic N in the two test soils is discussed on the basis of the data obtained with the proposed modified step-wise method.
Claves sistemáticas de suelos
Libro de 388 paginas, publicado en Madrid por el Consejo Superior de Investigaciones Cientificas en 1952. Incluye 3 figuras en las paginas 31, 32 y 33m; ademas incorpora XXVI (26) laminas a todo color excepto la I y la II.