T. Carballas

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.

Soil microbial and extractable C and N after wildfire

Abstract The effect of wildfire on soil microbes and extractable C (Cext) and N (Next) changed with respect to the time from burning and soil depth. Initially, microbial biomass C (Cmic) and N (Nmic) were drastically reduced in the soil surface layer (0–5 cm) and reduced by 50% in the subsurface (5–10 cm), whereas Cext increased by 62% in the surface layer and did not significantly change in the subsurface. These parameters were affected for the following 4 years, during which the average reductions in the soil surface and subsurface layers were, respectively, 60% and 50% for Cmic, 70% and 45% for Nmic, 60% and 40% for the ratio Cmic: organic C (Corg) and 70% and 30% for the ratio Nmic: total N (Ntot), while for Cext the surface layer was the only zone consistently affected and Cext decreased by up to 59%. Immediately after a fire, the Cext : Corg ratio increased by 3.5-fold and 2-fold in the surface and subsurface layers, respectively; thereafter for 2 years, it decreased in the surface layer (by up to 45%) while the effect on the subsurface layer was not consistent. The effect of burning on Next lasted 1 year, in which Next increased by up to 7- and 3-fold in the surface and subsurface layers, respectively, while the average Next : Ntot ratio doubled in the surface layer and increased by 34% in the subsurface. During the time in which each parameter was affected by burning, the soil factor explained a high percentage of variance in the fluctuations of Cmic, Nmic, Cmic : Corg and Nmic : Ntot, while those of Next and Next : Ntot, but not those of Cext and Cext : Corg depended on both the soil and its depth. In the burned soils similar patterns of response were found between the following parameters listed in pairs: Cmic and Nmic; Cmic : Corg and Nmic : Ntot; Cext and Next; and Cext : Corg and Next : Ntot. However, after the fire relationships found previously between the parameters studied and many other soils properties were either no longer evident, or were inverted. Although the addition of cellulose to the burned soil favoured fungal mycelium development and increased Cmic and Cext contents, the negative effect of burning on the microbial biomass and the Cext was not counteracted even under incubation conditions suitable for both microbial growth and C mineralization.

Changes in soil phosphorus and acid phosphatase activity immediately following forest fires

Abstract Three soils affected by wildfires and a soil that had been subjected to a controlled fire were sampled at two depths (0–5 and 5–10 cm), between 1 day and 1 month after burning. In all cases an unburnt soil was used as a control. The controlled fire did not affect acid phosphatase activity and had only very slight effects on P fraction distribution, in good agreement with the low soil temperatures recorded during the fire (never ⪢ 50°C in the upper 5 cm of soil). On the other hand, wildfires strongly reduced acid phosphatase activity and had an intense mineralizing effect on organic P; consequently inorganic P greatly increased and P fractions distribution was profoundly altered. The reductions in acid phosphatase activity and the extent of organic P oxidation in the various soil samples were used to estimate the soil temperatures reached during each fire. We show that there is a close relationship between soil temperatures estimated in this way and changes in P distribution within the various inorganic and organic fractions.

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

Abstract 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 N m = N 0 ((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.

Seasonal changes in microbial biomass and nutrient flush in forest soils

Microbial biomass and N, P, K, and Mg flushes were estimated in spring, summer, autumn, and winter samples of different forest soils. The microbial biomass showed significant seasonal fluctuations with an average distribution of 880±270 μg C g-1 soil in spring, 787±356 μg C g-1 soil in winter, 589±295 μg C g-1 soil in summer, and 560±318 μg C g-1 soil in autumn. The average annual concentrations of C, N, P, K, and Ca in the microbial biomass were 704, 106, 82, 69 and 10 μg g-1 soil, respectively. Microbial C represented between 0.5 and 2% of the organic soil C whereas the percentage of microbial N with respect to the total soil N was two-to threefold higher than that of C; the annual fluctuations in these percentages followed a similar trend to that of the microbial biomass. Microbial biomass was positively correlated with soil pH, moisture, organic C, and total N. The mean nutrient flush was 31, 15, 7, and 4 μg g-1 soil for N, K, P, and Mg, respectively, and except for K, the seasonal distribution was autumn ≥spring ≫ winter ≥summer. The average increase in available nutrient due to the mineralization of dead microbial cells was 240% for N, and 30, 26, and 14% for P, K, and Mg, respectively. There was a positive relationship between microbial biomass and the N, P, K, and Mg flushes. All the variables studied were significantly affected by the season, the type of soil, and the interaction between type of soil and season, but soil type often explained most of the variance.

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.

Microbial biomass and metabolic activity in four acid soils

Abstract The fumigation method was used to estimate microbial biomass C in four Haplumbrepts developed over different kinds of rock. In order to investigate the relationship between metabolic activity and microbial biomass and population density, CO 2 release from the glucose-enriched and unenriched soils was measured during 28 days of incubation. Biomass C levels lay between 36 and 112 mg 100 g −1 of dry soil, and made up only a small proportion of total soil C (0.77–1.38%). Only a small fraction of this biomass was detected by counting viables, but the microbial population was nevertheless significantly correlated with the biomass determined by fumigation. Among the physico-chemical properties of the soils, microbial biomass and population size were both chiefly affected (favourably) by humidity, total C and N and Al gel content. Metabolic activity was slight, either because part of the micro-organisms are inactive or because of a limited supply of substrate (the organic matter present may be unsuitable as a substrate or protected from microbial attack). Percentage C mineralization was inversely related to organic matter, silt and Al gel contents, and likewise failed to exhibit positive correlation with respiration, the biomass determined by fumigation or the counted population. The metabolic activity of the biomass appeared to depend upon the quality and nature of soil organic matter rather than its quantity, which nevertheless controlled microbial population size. Neither microbial biomass estimates nor viable population counts faithfully reflected metabolic activity in the soils.

Microbial biomass and its contribution to nutrient concentrations in forest soils

Abstract Soil microbial biomass and its contribution to the available Ca, Mg, Na, K and P concentrations were studied in a wide range of forest soils. Biomass C was measured using the chloroform fumigation-incubation technique; N, P, K and Ca in biomass were estimated from biomass C values. Soils were incubated at 25°C for 10 days and the differences between the available nutrient contents in the fumigated and the unfumigated soils (the flush of nutrients) were used to estimate the contribution of the microorganisms to the available nutrient concentrations. Microbial biomass C and N ranged from 282 to 1614 μg C g −1 dry soil (d.s.) and from 42 to 242 μg N g −1 d.s. The average quantities of P, K and Ca in microbial biomass were about 86, 73 and 10 μg g −1 d.s., respectively. The death of microorganisms by CHCl 3 fumigation caused an immediate flush (flush during day 0) of N, K, Na, P, and Mg with mean values of 10, 21, 8, 7 and 4 μg g −1 d.s., respectively. After 10 days of incubation the flush of nutrients (flush during day 10) showed mean values of 36 μg g −1 d.s. for N, and 20, 9 and 6 μg g −1 d.s. for the K, P. and Mg respectively, whereas the Na flush values were not significant in most soils. As for Ca the variability between the soil replicates was higher than the variability caused by the fumigation or incubation. Biomass C showed a significant relationship with the N flush measured in day 10 and with the Mg, K and P flush measured in both days 0 and 10. The results showed that soil microorganisms contained substantial amounts of both C and inorganic nutrients and that their contribution to the pool of available nutrients was large for N, important in the cases of P, K and Mg and not significant for Na.

An intelligent system for forest fire risk prediction and fire fighting management in Galicia

Abstract Over the last two decades in southern Europe, more than 10 million hectares of forest have been damaged by fire. Due to the costs and complications of fire-fighting a number of technical developments in the field have been appeared in recent years. This paper describes a system developed for the region of Galicia in NW Spain, one of the regions of Europe most affected by fires. This system fulfills three main aims: it acts as a preventive tool by predicting forest fire risks, it backs up the forest fire monitoring and extinction phase, and it assists in planning the recuperation of the burned areas. The forest fire prediction model is based on a neural network whose output is classified into four symbolic risk categories, obtaining an accuracy of 0.789. The other two main tasks are carried out by a knowledge-based system developed following the CommonKADS methodology. Currently we are working on the trail of the system in a controlled real environment. This will provide results on real behaviour that can be used to fine-tune the system to the point where it is considered suitable for installation in a real application environment.

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.