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.
Inorganic and organic N pools in soils burned or heated: immediate alterations and evolution after forest wildfires
Abstract Variation in the inorganic and organic N contents and in the resistance of organic N to step-wise acid hydrolysis (four steps of increasing hydrolytic intensity) was analysed in burned soils sampled (0–5 and 5–10 cm depth) at different times after high severity forest wildfires, as well as in one soil heated in the laboratory at 150, 210 or 350 °C. Ammonium content increased in the 0–5 and 5–10 cm soil layers immediately after burning and after heating at 150, 210 or 350 °C. The positive effect of burning on NH4 + -N persisted for several months and was particularly intense 1 year after burning. Two, 5 and 10 years after wildfire, NH4 + -N in burned soils was similar or slightly higher than in the corresponding unburned soils. Nitrite and nitrate were very low or undetectable in most burned and unburned samples analysed. Organic N strongly decreased in one of the soils immediately after wildfire and in the soil heated at 350 °C; conversely, in the other recently burned soil and in samples heated at 150 and 210 °C, organic N was slightly higher or similar to that in the corresponding unburned soils. During the 2 years after wildfire, organic N changed little and in soils sampled 5 and 10 years after burning the organic N contents were close to those of the unburned control soils. Independent of the variation in organic N content, the lability of organic N dramatically changed after burning or heating, this change remaining for at least 2 years after wildfire. The amount of N solubilised by acid hydrolysis strongly decreased in both 0–5 and 5–10 cm soil layers after wildfire and in soils heated at 210 or 350 °C. This decrease was mainly due to the loss of labile N (solubilised after hydrolysis with 1 and 3 N HCl), while the hydrolysable N obtained with 6 N HCl changed little or even increased. On the contrary, the amount and/or percentage of N resistant to acid hydrolysis (residual N) increased in recently burned soils and in soils heated at 210 or 350 °C (except in the 5–10 cm layer of one soil). In the months following burning, differences in the lability of organic N between the burned and unburned soils decreased, especially in the 0–5 cm soil layer, although 2 years after wildfire the effect of burning on the organic N compounds was still detectable. In soils sampled 5 or 10 years after wildfire, the amounts and percentages of the residual N and of different fractions of hydrolysable N were similar to those of the unburned soils. The relationship between the reduction in hydrolysable N and the alteration of N mineralisation (previously studied) was also analysed.
NUCLEIC ACID BASES IN SOILS AND THEIR ASSOCIATION WITH ORGANIC AND INORGANIC SOIL COMPONENTS
Purines and pyrimidines were determined by ion-exclusion chromatography in 15 soil samples of widely differing pedological origins and in humic fractions isolated from these soils. The nucleic acid bases tended to concentrate in humins and fulvic acids (FA) rather than in humic acids. N in purines and pyrimidines constituted up to 7.4% of the soil-N and up to 18.6% of the N in a FA. The distribution of nucleic acid bases in soils and humic fractions was similar. Guanine, cytosine and adenine were detected in all samples, and thymine in most, while uracil and 5-methyl cytosine were found less frequently. The predominance of guanine and cytosine suggested a microbial DNA origin for the nucleic acid bases. Evidence was obtained that the nucleic acid bases interacted not only with humic materials but also with inorganic soil components such as clays and hydrous oxides. Our data show that purines and pyrimidines accounted for up to 16% of the unidentified N in a soil and for up to 40% of the N in a FA so far n...