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.
Investigations on the rôle of organic matter in plant nutrition
1. As the result of decomposition of green manure in a swamp soil, the coarse fractions (including coarse and fine sand) of the surface layer (first 3 inches) decreased by 11·2 per cent. while the fine ones (silt+clay) increased by 9·8 per cent. in the course of 43 days. The effect was less pronounced in the manured soil maintained at 60 per cent. saturation, while no alteration was to be noticed in the case of the unmanured specimens. 2. Treatment with dilute hydrochloric acid prior to dispersion had no appreciable effect on the estimates of mechanical composition obtained in the different cases. Loss in solution was also comparatively small. 3. The silica-sesquioxide ratio increased slightly as the result of decomposition of green manure in swamp soil. The composition of the fine fractions was also altered, the total percentages of the oxides of iron, aluminium and silicon increasing from 90·5 to 99·3 in the course of 71 days. 4. The practical significance of the above and related observations has been discussed.