Inorganic and organic N pools in soils burned or heated: immediate alterations and evolution after forest wildfires

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.This work was funded by the Xunta de Galicia and by the Spanish Ministerio de Educación y Ciencia.Peer reviewe

Incorporation of the degradation products of 14C,15N-glycine in various forms of organic carbon and nitrogen in two acid soils

The incorporation of the degradation products of 14C,15N-glycine in the endogenous organic matter of two Cambisols, one developed over acid schists and the other over gabbros, was studied by aerobic incubation for 42 days at 28°C, of unenriched samples and samples enriched with glycine, and analysis of the C and N organic forms in the samples. The incorporation of exogenous C depended on soil type and on the nature of the added substrate. Thus, the addition of glycine increased the proportion of water-soluble C, stimulated microbial oxidation of unhumified organic matter and favoured the synthesis of early humification compounds at the expense of unmineralized and unhumified organic matter; and also favoured the formation of organo-clay compounds (possibly due to adsorption of the glycine or its metabolites on clay) and (where stabilizing agents were present) of insoluble humin. In both soils, the distribution of endogenous N was unaffected by incubation. 15N from glycine was incorporated mainly among identifiable forms (NH+4, amides, hexosamines and α-amino acids) in the schist soil, which may possibly be related to ammonification being the dominant mineralization process. In the gabbro soil, 15N from glycine was incorporated mainly in hydrolyzable unidentified forms which is probably related to the dominance of nitrification in the mineralization process. The virtual absence of isotoplc excess in the hydrolysis residue and the large isotoplc excesses in the first two hydrolysis stages (the shortest and gentlest) showed that added N was considerably more labile than endogenous N. The step-wise hydrolysis procedure used extracted the more labile soil N much more selectively than does conventional continuous hydrolysis.This research was supported by Conselleria de Education y Ordenacion Universitaria de la Xunta de Galicia (Spain).Peer reviewe

Mineralization of a nitrogen-bearing organic substrate model 14C,15N-glycine in two acid soils

The kinetics of the mineralization of organic matter was studied in two Cambisols, one developed over acid schists and the other over gabbros, by aerobic incubation, for 42 days at 28°C, of unenriched samples and samples enriched with the model substrate 14C,15N-glycine. The schist soil proved to have considerable capacity for mineralization of both endogenous and exogenous C and N. whereas the mineralization capacity of the gabbro soil depended on the source of substrate, being similar to that of the schist soil for the C and N of recently added labile glycine but only between one-third and one-half that of the schist soil for endogenous C and N. In the schist soil N mineralization chiefly involved ammonification: in the gabbro soil nitrification was the dominant process. The response to addition of glycine, which was rapid, intense and brief in the schist soil, was slower, less intense and longer-lasting in the gabbro soil, possibly due to the latter's high silt and Al gel contents making the added substrate temporarily unavailable to micro-organisms. It may be concluded from the data obtained that the mineralization of endogenous C and N in the gabbro soil is chiefly limited by the scarcity of available labile C, which is used by heterotrophic micro-organisms as a readily-assimilable source of energy.This research was supported by Conselleria de Educacibn y Ordenación Universitaria de la Xunta de Galicia (Spain).Peer reviewe

Short-term effects of a wildfire on the nitrogen status and its mineralization kinetics in an Atlantic forest soil

A Humic Cambisol developed over granite under Pinus pinaster Sol. located in the Atlantic climate zone, which had been affected by a high intensity wildfire, was studied 1 month after burning. The soil had a very rich organic matter A horizon, 30 cm deep. The effects of the fire on the N status and N mineralization capacity were estimated comparing the surface (0–5 cm) and subsurface (5–10 cm) layers from the burnt soil with the corresponding layers from the same unbumt soil. N mineralization kinetics were determined by aerobic incubation at 28°C for 11 weeks. The fire increased the total N content in the surface layer but not in the subsurface. Total inorganic N, which was mainly in the form of NH+4-N, increased after the burning in both layers, whereas NO−3-N content, which was very low, only increased in the subsurface layer. The fire increased the N mineralization capacity, but did not modify organic N mineralization behaviour. Ammonification largely predominated over nitrification in both the unburnt and the burnt soils. N mineralization kinetics followed the first order equation Nm = N0((1 − e−kt) but the fire affected the kinetic parameters. The potentially-mineralizable N decreased and the kinetic constant increased in the burnt samples showing that the wildfire reduced the mineralizable organic-N reserves and increased the mineralization rate, thus predicting a rapid depletion of the labile organic N. The temporary ability of the burnt soil to supply available N is recommended to be used to grow an early crop to avoid physical soil degradation.This research was supported by Conselleria de Education y Ordenacion Universitaria de la Xunta de Galicia and by Comision Interministerial de Ciencia y Tecnologia (CICYT), Spain.Peer reviewe

Nitrogen mineralization and its controlling factors in various kinds of temperate humid-zone soils

The N mineralization capacity of 41 temperate humid-zone soils of NW Spain was measured by aerobic incubation for 15 days at 28°C and 75% of field capacity. The main soil factors affecting organic N dynamics were identified by principal components analysis. Ammonification predominated over nitrification in almost all soils. The mean net N mineralization rate was 1.63% of the organic N content, and varied according to soil parent materials as follows: soils on basic and ultrabasic rocks < soils over acid metamorphic rocks < soils developed over sediments < soils over acid igneous rocks < soils on limestone. The N mineralization capacity was lower in natural soils than in cropped soils or pastures. The accumulation of organic matter (C and N) seems to be due to poor mineralization which was caused, in decreasing order of importance, by high exchangeable H-ion levels, high Al and Fe gel contents and, to a lesser extent (though more markedly in cropped soils), by silty clay texture and exchangeable Al ions.Peer reviewe

Comparison between a chemical and a biological method to determine the N mineralization potential of temperate-humid region soils

The potentially mineralizable organic N of 33 different soils was estimated by a chemical test (hot extraction with 2N KCl) and the values compared with those previously obtained by a biological method (aerobic incubation in the laboratory). On average, the organic N solubilized by the chemical procedure was significantly lower than that mineralized by a two weeks aerobic incubation for all the soils as a whole. The same was true for soils developed over acid rocks and over sediments. However, the values obtained for the soils developed over limestone and basic rocks were similar by both methods. The values obtained by both methods were not significantly correlated neither when considering all soils together nor when considering different groups according to soil management or parent material. Significant correlations between both methods were only found when the soils were separated into two groups according to their organic N content: soils with less than 400 mg N 100 g−1 soil and soils with more than 400 mg N 100 g−1 soil. The organic N solubilized by the chemical procedure was significantly correlated with the hexosamine-N content; however, it was not correlated with the factors that control the biological mineralization of the organic N, except with the soluble Al content. Therefore, the chemical extraction did not seem to address the biologically active N pool in a selective way.Peer reviewe

Humic substances in a catena of estuarine soils: distribution of organic nitrogen and carbon

This paper describes the organic fractions of a catena comprising two eutric fluvisols, one (F1) in the intertidal zone of an estuary and the other (F2) on its bank, and a distric cambisol (C) that is not affected by seawater. The humus of the fluvisols has features inherited from the terrestrial humus together with others acquired during its evolution in the marine environment. All the samples had little unhumified organic matter, a fair amount of humin and a major fraction that is soluble in alkalis. Exposure to seawater changes the kind of organo-metallic complexes present and alters the relative solubility of humic and fulvic acids in alkaline reagents, thereby modifying the degree of polymerization of the humus. The distribution of organic nitrogen also changes with exposure to seawater: ammonium content gradually rises, amide content remains practically constant and aminoacids fall gradually, while hexosamines, hydrolysable unknown nitrogen and hydrolysable organic nitrogen all decrease sharply.Peer reviewe

Effect of soil characteristics on N mineralization capacity in 112 native and agricultural soils from the northwest of Spain

N mineralization capacity and its main controlling factors were studied in a large variety (n=112) of native (forest, bush) and agricultural (pasture, cultivated) soils from several climatic zones in Spain. The available inorganic N content, net N mineralization, and net N mineralization rate were determined after 6 weeks of aerobic incubation. NH~-N largely predominated over NOg-N (ratio near 10:1) except in some agricultural soils. Net N mineralization predominated (83% of soils) over net N immobilization, which was more frequent in agricultural soils (25%) than in native soils (9%). In forest soils, both net N mineralization and the net N mineralization rate were significantly higher than in the other soil groups. The net N mineralization rate of pasture and cultivated soils was similar to that of bush soils, but available inorganic N was lower. The net N mineralization rate decreased in the order: soils over acid rocks>soils over sediments>soils over basic rocks or limestone; moreover, the highest net N mineralization and available inorganic N were found in soils over acid rocks. The highest N mineralization was found in soils with low C and N contents, particularly in the native soils, in which N mineralization increased as the C:N ratio increased. N mineralization was higher in soils with a low pH and base saturation than in soils with high pH and base saturation values, which sometimes favoured N immobilization. Soils with an A1 gel content of >1% showed lower net N mineralization rates than soils with A1 gel contents of <1%, although net N mineralization and available inorganic N did not differ between these groups. The net N mineralization rate in silty soils was significantly lower than in sandy and clayey soils, although soil texture only explained a low proportion of the differences in N mineralization between soils.This research was supported by Consellerfa de Educaci6n of the Xunta de GaliciaPeer reviewe

Loss of organic matter in Atlantic forest soils due to wildfires. Calculation of the ignition temperature

The effect of burning on the organic matter of three forest soils, which had been affected by wildfires within one month prior to sampling, was studied using a differential scanning calorimeter (DSC). The soils were cambisols under Pinus spp. located in Galicia (NW Spain). Thermograms of the soils were carried out between 50 and 600°C. The combustion enthalpies of the soils were determined from the thermograms. By comparing the combustion enthalpy data of the burnt soils and that of the corresponding unburnt soils, the loss of organic matter due to the burning was calculated. The ignition temperature for each soil was also estimated from the thermograms.This research was supported by Conselleria de Educación y Ordenación Universitaria de la Xunta de Galicia and by the Comisión Interministerial de Ciencia y Tecnologia (CICYT), Spain.Peer reviewe

Carbon- and nitrogen-containing compounds in composted urban refuses

The composition of the organic matter of four composted urban refuses (one of them amended with CaCO3 in the composting process) from Spanish industrial composting plants was studied. Fundamental components and C-bearing compounds (humic substances) were assessed by classical fractionation methods; N-bearing compounds were determined by acid step-wise hydrolyses. Cellulose and hemicelluloses largely predominated over lignin, which was the second important component, followed by water-soluble compounds, the content of lipids being very low. Water-soluble compounds represented 3–14% of the organic C. Alkali-soluble compounds were quite low since humic compounds (humic and fulvic acids) only represented about 17% of the organic C (24% in the amended compost); humic acids predominated over fulvic acids, the most polymerized compounds being higher than the less polymerized ones. The predominant fraction (about 70% of the organic C) was the insoluble one which comprised unhumified compounds, insolubilizable humin, microbial humin (1·4–9·0% of the organic C) and residual humin (20–40% of the organic C). The extraction percentage was very low whereas the degree of humification was close to 50%. About 85% of the organic N was hydrolysable. Hydrolysable unidentified-N was the predominant organic N form, followed by α-aminoacid-N. NH4+-N from organic compounds and hexosamine-N were very small and amide-N was not detectable. According to the percentage of humification the four composted refuses were stabilized materials.This research was supported by the COTOP, Xuntade Galicia, Spain.Peer reviewe