A. Cabaneiro

Organic matter changes immediately after a wildfire in an atlantic forest soil and comparison with laboratory soil heating

The quantity, chemical composition and mineralization kinetics of the organic matter of an acid Humic Cambisol, developed over granite, under Pinus sylvestris L. were determined in 0–5 and 5–10 cm samples collected immediately after a high-intensity wildfire, and compared with those of an unaffected site nearby. Organic matter was characterized by different chemical fractionation methods, and the C mineralization was determined by aerobic incubation. A similar unburnt soil located in the same area was heated at the laboratory at 150, 220, 350 and 490°C to measure the losses of C content; the samples heated at 220 and 350°C were selected to determine chemical changes in organic matter composition. Surface and subsurface soil layers lost about 50% of their C content during wildfire. The C mineralized decreased in the surface layer; however, the percentage of total C mineralized increased in both layers. The cumulative CO2C mineralized fitted a double exponential first-order kinetic model, but the fire affected the kinetic parameters, increasing both the labile pool of the potentially mineralizable C and the mineralization rate constants of the recalcitrant and labile pools. Cellulose + hemicelluloses declined significantly after the burning, whereas lipids did not vary. The fire decreased the amount of unhumified organic matter and the alkali-soluble compounds, particularly humic acids, but there was a net increase of humin. The organic matter bound to Fe and especially to A1 was much higher after the burning. In the soil heated under laboratory conditions the changes observed at 150°C were very low, whereas at 490°C almost all the organic matter disappeared. The changes exhibited by the samples heated at 220°C were the most similar to those observed in the samples from the wildfire. At 220 and 350°C the humification and metal complexation percentages of the organic matter increased, similar to the trend observed in the samples from the wildfire.

Carbon mineralization dynamics in soils after wildfires in two Galician forests

The carbon mineralization dynamics of two Humic Cambisols, developed over granite, one under Pinus sylvestris L. (1740 m a.s.l.) and the other under Pinus pinaster Aiton (140 m a.s.l.), were determined in samples of 0‐5 and 5‐10 cm depth collected after high intensity wildfires. Burnt and unburnt soils were sampled five times over 2 yr after the wildfires to determine changes in C concentration and in potential mineralization activity of the soil organic matter. Soil samples from the same forests unaAected by the fires were used as controls. In both soils the fire resulted in a substantial decrease in the soil carbon concentration. Immediately after the fire, the C mineralization was decreased in the surface layer; however, the percentage of total C mineralized increased in both layers. The evolution of these variables over time depended on the soil and on the layer considered. During the first months after the burning the C mineralization presented values lower than those of the control in both layers of the soil located at higher altitude (M) and in the surface layer of the other soil (R), but values higher than those of the control in the subsurface layer of soil R. For the same period, the C mineralization coeAcient in the surface layer was similar to (M) or lower than (R) that of the corresponding control, whereas in the subsurface layer it was maintained above that of the control in both soils. Two years after the fire, the total C concentration had been recovered in the surface layer of both soils whereas in the subsurface layers its value was still 15‐19% lower than that of the same layer in the corresponding control. At the same time, the C mineralization and the percentage of the total C mineralized in the surface layer of the burnt soils were lower than those in the corresponding unburnt soils. In the subsurface layer, soil M exhibited values of these two mineralization indices higher than those of the control, whereas soil R presented values lower than those of the control from 1 yr after the fire. The cumulative CO2-C evolved by the samples during each incubation fits two kinetic models: a simple and a double exponential first order equation. In most cases the coeAcient of determination (R 2 ) was higher for the double exponential model. The fire aAected the kinetic parameters; the eAect was ephemeral on the labile C pool, which increased its content (C0) and its mineralization rate (k), but more persistent on the recalcitrant fraction, which shows a long-term decrease of its instantaneous mineralization rate (h). According to principal components analysis, the variability of the samples studied is mainly due to diAerences on their organic matter quality and, in a smaller proportion, to diAerences in organic matter concentration. The eAect of fire on these factors, which was more pronounced in the soil with the initially higher C concentration, persisted during the 2 yr study. # 1999 Elsevier Science Ltd. All rights reserved.

PHYSICAL AND CHEMICAL CHARACTERIZATION OF FOUR COMPOSTED URBAN REFUSES

Abstract The physical and chemical characteristics of four composted urban refuses (one of them amended with CaCO 3 during the composting process) from Spanish industrial composting plants were studied from the point of view of their use as organic fertilizers. The four composts were very fine in texture with low bulk density and high salinity. The pH was close to neutrality; the organic matter content ranged from 42 to 60% and the C/N ratio from 16 to 22 (7 for the amended urban refuse). Most of the total N was in organic forms; NH 4 + -N largely predominatedNO 3 − -N in the composts, except for the amended composted refuse where the predominant inorganic form was NO 3 − -N. Calcium was the most abundantnutrient followed by K, Na, Mg and P. Most of the Ca and Na were in available forms; available K and Mg were lower and available P very small. Total Al and Fe were extremely abundant followed by Zn, Mn, Pb, Cu, Cr, Ni and Cd. The percentage of available Mn was very high, followed by available Cu and Pb in two of the composts, and available Zn and Cd. Available Al, Fe, Ni and Cr were very low or negligible. Most of the total Zn, important percentages of total Pb, Mn, Al and Fe, but very low proportions of total Cr and Ni and only traces of Cd, were complexed with organic matter; these compounds seemed to be soluble organo-metallic complexes, except part of those formed by Al and Cd that could be stable complexes. Although the four composted refuses were unbalanced with regard to the main nutrients they all had potential agronomic value. Total C contents and C/N ratios in the three non-amended composts were in the range for stabilized composts; however, the NH 4 + -N content seemed to point to the presence of non-stabilized substances.

Thermal resistance to high temperatures of different organic fractions from soils under pine forests

Abstract Thermal resistance of the organic matter from two Humic Cambisols from the NW of Spain, developed over granite and under pine forest, was determined by heating samples of the upper 5 cm of soils at 220 and 350 °C for 30 min. Unheated samples of the same soils were used as control. Lignin, holocellulose, lipids and water-soluble compounds as well as humic substances were determined by different chemical fractionation methods. At 220 °C, the losses of holocellulose exceeded 75%, this being the most vulnerable organic fraction. At this temperature, the lipidic fraction exhibited different thermal resistance in both soils, showing losses of 73% in one case and only 11% in the other. Lignin exhibited the lowest losses, increasing proportionally. Fulvic and humic acids decreased at 220 °C but humin exhibited a net increase. At 350 °C, holocellulose practically disappeared and lignin losses were significant, whereas lipid loss slightly increased compared with the loss observed at 220 °C. All humic substances decreased at 350 °C, but humin was the most resistant humic fraction. Heating the soils at 220 and 350 °C increased the Fe and Al extracted with Na 4 P 2 O 7 and NaOH and the percentage of humification of the remaining soil organic matter.

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. NHinf4sup+−N largely predominated over NOinf3sup--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 Al gel content of >1% showed lower net N mineralization rates than soils with Al 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.

Carbon and nitrogen mineralization in an acid soil fertilized with composted urban refuses

Abstract The C- and N-mineralization kinetics of a Cambisol over granite fertilized with four composted urban refuses (one of them amended with CaCO 3 in the composting process) as well as the degradation kinetics of the wastes themselves were studied. The C-mineralization was determined by incubation of the samples for 42 days at 28°C in a thermostat bath and measurement of the CO 2 evolved from the samples. The N-mineralization was performed by aerobic incubation in an incubator at the same temperature and for the same time as in the case of C. Addition of the composts to the soil significantly increased the C-mineralization rate. About 30% of the organic C of the composts (only 16% in the amended composts) was mineralized after 6 weeks incubation. Most C mineralization took place in the first 3 weeks for only about 3% of the organic C (0·9% in the amended composts) was mineralized between weeks 3 and 6. The amended compost was the only one that stimulated the soil N-mineralization rate. The other composts provoked N-immobilization (or denitrification) during the incubation and although an increase of the net inorganic-N production was detected after week 2, the net N-mineralization rate was almost nil at the end of the incubation. Net nitrification predominated over net ammonification in the soil with or without addition of composts. The non-amended compost behaviour seem to show that they had not undergone enough stabilization; as for the amended composted refuse its singular behaviour could be due to its high carbonate content.

Effect of cattle slurry fractions on nitrogen mineralization in soil

The mineralization kinetics of nitrogen in acid soils, and their modification by the addition of an organic fertilizer (cattle slurry), were studied by incubating a humic cambisol for 36 weeks using a method based on that of Keeney & Bremner (1967). The cumulative curve of the quantity of nitrogen mineralized in soil not given fertilizer departs significantly from Stanfords theoretical model, which predicts linear dependence of nitrogen mineralized upon √t. The observed kinetics are interpreted as due to the superposition of two mineralization processes involving different substrates. The cumulative mineralized nitrogen curves for soil samples enriched with the various slurry fractions likewise reflect complex kinetics involving at least two main substrates. Consideration of the net mineralized nitrogen shows that F,, the solid fraction with the highest C/N ratio, clearly induced immobilization of nitrogen during the first 130 days of incubation, and analysis of the NO 3 /NH 4 ratio suggests that this immobilization was probably at the expense of nitrate. F 3 , the liquid fraction, first induced a brief period of mineralization and then stabilized nitrogen levels, giving rise to a reduction in net mineralized nitrogen. The addition to the soil of F 2 , the semi-liquid fraction, produced results intermediate between those of the other two fractions. In conclusion, the increase in organic nitrogen in the soil after addition of cattle slurry depends in the short term on the liquid and semi-liquid fractions, whereas long-term effects involve both the stable residues of these fractions and the more solid fraction. The labile fraction of the pool of mineralizable N benefits more than the recalcitrant fraction, and the time constants of the mineralization process are reduced.

Microbial biomass and C mineralization in agricultural soils as affected by atrazine addition

This study examines the effects of atrazine on both microbial biomass C and C mineralization dynamics in two contrasting agricultural soils (organic C, texture, and atrazine application history) located at Galicia (NW Spain). Atrazine was added to soils, a Humic Cambisol (H) and a Gleyic Cambisol (G), at a recommended agronomic dose and C mineralization (CO2 evolved), and microbial biomass measurements were made in non-treated and atrazine-treated samples at different time intervals during a 12-week aerobic incubation. The cumulative curves of CO2–C evolved over time fit the simple first-order kinetic model [Ct = Co (1 − e−kt)], whose kinetic parameters were quantified. Differences in these parameters were observed between the two soils studied; the G soil, with a higher content in organic matter and microbial biomass C and lower atrazine application history, exhibited higher values of the total C mineralization and the potentially mineralizable labile C pool than those for the H soil. The addition of atrazine modified the kinetic parameters and increased notably the C mineralized; by the end of the incubation the cumulative CO2–C values were 33–41% higher than those in the corresponding non-added soils. In contrast, a variable effect or even no effect was observed on the soil microbial biomass following atrazine addition. The data clearly showed that atrazine application at normal agricultural rates may have important implications in the C cycling of these two contrasting acid soils.

Carbon- and nitrogen-containing compounds in composted urban refuses

Abstract The composition of the organic matter of four composted urban refuses (one of them amended with CaCO 3 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. NH 4 + -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.

Obtainment of a carbon-13-, nitrogen-15-labelled burnt soil

Abstract The restoration of the C and N cycles in the soil-plant system is a basic step for the reclamation of burnt soils. To evaluate accurately the efficacy of restoration techniques, it is necessary to use isotopic tracers and, therefore, a 13C-, 15N-labelled burnt soil should be made available. The present paper describes a technique for obtaining a 13C-, 15N-labelled burnt soil by burning a labelled forest soil in the laboratory.