V. Ramón Vallejo

Labile and recalcitrant pools of carbon and nitrogen in organic matter decomposing at different depths in soil: an acid hydrolysis approach

The quality of soil organic matter (OM) depends on its distribution among labile and recalcitrant pools and the quality of each pool considered. OM quality is assumed to decrease as decomposition proceeds, but to verify this assumption it is necessary to define quality in operative terms. Here we study the change in OM quality during decomposition of mixtures of four plant materials (Medicago sativa whole ground plants, and ground litter of Eucalyptus globulus, Quercus ilex and Pinus halepensis) with a mineral red earth, incubated at different depths (5, 20, and 40 cm) for 2 years. OM quality was evaluated from acid hydrolysis, considering three pools: (a) Labile Pool I, obtained by hydrolysis with 5 N H2SO4 at 105 °C for 30 min; (b) Labile Pool II, obtained by hydrolysis with 26 N H2SO4 at room temperature overnight, then with 2 N H2SO4 at 105 °C for 3 h; and (c) Recalcitrant Pool, the unhydrolyzed residue. In agreement with previously published results, the recalcitrant C/total OC (RIC), and recalcitrant N/total N (RIN) ratios are regarded as indicators of global OC and N quality. In addition, in Labile Pools I and II, the ratio carbohydrate C/polyphenol C is used as indicator of OC quality. The main findings obtained by applying this approach can be summarized as follows: (1) In undecomposed plant materials, initial RIC ranged from 25% to 60% (Medicago and Pinus mixtures, at extreme values). Throughout decomposition, RIC values increased strongly (Medicago mixtures), slightly (Eucalyptus), or were roughly maintained (Quercus and Pinus), suggesting that strong decreases in OC quality occur only for easily decomposable plant materials. (2) Initial RIN values were between 15% and 30%, i.e., much lower than RIC ones. In contrast with the behaviour of RIC, the RIN values strongly increased in all cases, or, in other words, N quality clearly decreased for all plant materials, owing not only to a lower mineralization of the recalcitrant N, but also to a net incorporation of N to this pool. The amount of incorporated N is significantly related to the initial lignin content of the incubated plant material. Such incorporation seems to occur during wet periods; in contrast, its relationship with temperature was hardly detectable. No similar phenomenon was detected for recalcitrant C. (3) The 13C-CPMAS-NMR spectra of the recalcitrant pool showed prominent peaks in the 0–45 ppm region, which corresponds to the alkyl C and accounts for up to 50% of the total unhydrolyzable C in Quercus mixtures. In contrast, the aromatic zone, 110–160 ppm, was poorly apparent. These features were maintained more or less intact during the 2 years of field incubation, and suggest that lipidic polymers represent a substantial part of the recalcitrant pool. (4) Throughout the decomposition process, the ratio Labile Pool II/Labile Pools I+II decreased for carbohydrates, and increased for phenolic compounds. The use of these ratios is suggested to evaluate the degree of decomposition of plant residues. In Labile Pool I the ratio carbohydrate C/polyphenol C remained the same, whereas for Labile Pool II this ratio decreased strongly, suggesting that the changes in quality may be restricted to a single pool. (5) Samples incubated in upper horizons (5-cm depth) were subjected to a much drier pedoclimate than those incubated at deep layers (20 and 40 cm), resulting in a slower mineralization of both the labile and the recalcitrant pools of C and N. Nevertheless, on a mineralized OC basis, most indicators of quality did not differ statistically between depths. Hence, the drought in the upper horizon retarded the decomposition, but did not result in a different biochemical evolution. Because of its simplicity, chemical fractionation into three pools is a useful approach to characterize biochemical changes in C and N quality during plant residue decomposition.

2 The role of fire in European Mediterranean Ecosystems

Fire is an integral part of many ecosystems, including the Mediterranean ones. However, in recent decades the general trend in number of fires and surface burnt in European Mediterranean areas has increased spectacularly. This increase is due to: (a) land-use changes (rural depopulation is increasing land abandonment and consequently, fuel accumulation); and, (b) climatic warming (which is reducing fuel humidity and increasing fire risk and fire spread). The main effects of fire on soils are: loss of nutrients during burning and increased risk of erosion after burning. The latter is in fact related to the regeneration traits of the previous vegetation and to the environmental conditions. The principal regeneration traits of plants are: capacity to resprout after fire and fire-stimulation of the establishment of new individuals. These two traits give a possible combination of four functional types from the point of view of regeneration after fire, and different relative proportions of these plant types may determine the post-fire regeneration and erosion risk. Field observations in Spain show better regeneration in limestone bedrock type than in marls, and in north- facing slopes than in south-facing ones. Models of vegetation dynamics can be built from the knowledge of plant traits and may help us in predicting post-fire vegetation and long-term vegetation changes under recurrent fires.

Mulching treatment for postfire soil conservation in a semiarid ecosystem

Fire may generate soil degradation and accelerate erosion processes, depending among other factors, on the regeneration capacity of the ecosystem. Conservation measures will be beneficial in those fragile systems where a high degradation hazard is estimated. Mulching treatment was evaluated in order to establish its efficiency in protecting soil and preventing runoff generation in a semiarid area affected by a wildfire. Straw mulch was applied at 200 g m‐2 in three replicated plots. Paired mulched and control plots were established in a three block experimental design. Runoff, sediment yield, plant cover, and dynamics of soil physical properties were measured in these plots over a 2‐year period. Most of the plant regeneration after the fire resulted from resprouting of the dry grassland species, dominated by Brachypodium retusum. Total plant cover was only 50% two years after passage of the fire. Runoff and sediment yield were significantly greater from control plots. Soil loss from control plots was abou...

Examination of thermal and acid hydrolysis procedures in characterization of soil organic matter

Abstract Differential thermogravimetry (DTG), differential scanning calorimetry (DSC), and stepwise thermogravimetry (STG), together with two acid hydrolysis methods (hydrolysis with hydrochloric acid in a single step, and hydrolysis with sulfuric acid in two steps), were evaluated to determine the quality of four plant materials (Medicago sativa, Eucalyptus globulus, Quercus ilex, and Pinus halepensis) before and after mixing with a red earth. These quality indices were then compared with the same materials in the field, whether their decomposition could be predicted. All the thermal methods gave poor results. In both DTG and DSC, the presence of the mineral matrix gave rise to strong distortions in the spectra. Since the spectrum of any mixture is not simply the sum of the spectra of the two components (organic matter + mineral matter), these distortions could not be corrected by simply subtracting the spectrum of the red earth alone. STG trials also gave poor results, because the presence of the mineral matrix greatly increased the quality indices, and reduced the ability of the method to distinguish between organic matter qualities. In view of our results, the usefulness of thermal methods in the characterization of soil organic matter would seem to be restricted to certain organic horizons (L, F, and perhaps H). In contrast, methods based on acid hydrolysis were comparatively more satisfactory. Their resolution (ability to distinguish organic matter qualities) was much higher than that of thermal methods. However, they were able to distinguish carbon more accurately than nitrogen. The sulfuric acid method, unlike the hydrochloric acid method, was affected by the presence of a mineral matrix. While both methods could be improved, in their present form they seem to operate as good predictors of carbon and nitrogen mineralization.

CO2 efflux from a Mediterranean semi-arid forest soil. I. Seasonality and effects of stoniness.

We studied the seasonality of total soil CO2efflux and labeled C-CO2 released from 14Clabeled straw incubated in the H horizon of asemi-arid Mediterranean forest soil. Fieldmeasurements were carried out over 520 days in aseries of reconstructed soil profiles with and withouta gravel layer below the H horizon. We monitored soilclimate and related this to soil CO2 efflux.Seasonal variations in soil CO2 efflux in asemiarid Mediterranean forest were mainly related tochanges in soil temperature. In spite of drought, highrespiration rates were observed in mid summer. Highsoil CO2 efflux in hot and dry episodes wasattributed to increases in soil biological activity.The minimum soil CO2 efflux occurred in latesummer also under dry conditions, probably related toa decrease in soil biological activity in deephorizons. Biological activity in organic layers waslimited by water potential (Ψ) in summer and bytemperature in winter. Rewetting a dry soil resultedin large increases in soil CO2 efflux only at hightemperatures. These large increases represented asignificant contribution to the decomposition oforganic matter in the uppermost horizons. Soilbiological activity in the uppermost horizons was moresensitive to changes in soil Ψ and hence tosummer rainstorms than the bulk soil microbialactivity. The presence of a layer of gravel improvedboth moisture and temperature conditions for thedecomposition of organic matter. As a result, soilCO2 efflux increased in soils containing rockfragments. These effects were especially large for theorganic layers.

Perspectives in dryland restoration: approaches for climate change adaptation

Reforestation efforts in dryland ecosystems frequently encounter drought and limited soil productivity, although both factors usually interact synergistically to worsen water stress for outplanted seedlings. Land degradation in drylands (e.g. desertification) usually reduces soil productivity and, especially, soil water availability. In dry sub-humid regions, forest fires constitute a major disturbance affecting ecosystem dynamics and reforestation planning. Climate change projections indicate an increase of drought and more severe fire regime in many dryland regions of the world. In this context, the main target of plantation technology development is to overcome transplant shock and likely adverse periods, and in drylands this is mostly related to water limitations. In this paper, we discuss some selected steps that we consider critical for improving success in outplanting woody plants, both under current and projected climate change conditions including: (1) Plant species selection, (2) Improved nursery techniques, and (3) Improved planting techniques. The number of plant species used in reforestation is increasing rapidly, moving from a reduced set of well-known, easy-to-grow, widely used species, to a large variety of promising native species. Available technologies allow for reintroducing native plants and recovering critical ecosystem functions for many degraded drylands. However, climate change projections introduce large uncertainties about the sustainability of current reforestation practices. To cope with these uncertainties, adaptive restoration approaches are suggested, on the basis of improved plant quality, improved techniques for optimizing rain use efficiency in plantations, and exploring native plant species, including provenances and genotypes, for their resilience to fire and water use efficiency.

Decomposition of 13C-labelled standard plant material in a latitudinal transect of European coniferous forests: Differential impact of climate on the decomposition of soil organic matter compartments

Abstract13C labelled plant material was incubated in situ over 2 to 3 years in 8 conifer forest soils located on acid and limestone parent material along a north-south climatic transect from boreal to dry Mediterranean regions in western Europe. The objectives of the experiment were to evaluate the effects of climate and the soil environment on decomposition and soil organic matter dynamics. Changes in climate were simulated using a north-to-south cascade procedure involving the relocation of labelled soil columns to the next warmer site along the transect.Double exponential, decay-rate functions (for labile and recalcitrant SOM compartments) vs time showed that the thermosensitivity of microbial processes depended on the latitude from which the soil was translocated. Cumulative response functions for air temperature, and for combined temperature and moisture were used as independent variables in first order kinetic models fitted to the decomposition data. In the situations where climatic response functions explained most of the variations in decomposition rates when the soils were translocated, the climate optimised decomposition rates for the local and the translocated soil should be similar. Differences between these two rates indicated that there was either no single climatic response function for one or both compartments, and/or other edaphic factors influenced the translocation effect. The most northern boreal soil showed a high thermosensitivity for recalcitrant organic matter compartment, whereas the labile fraction was less sensitive to climate changes for soils from more southern locations. Hence there was no single climatic function which describe the decay rates for all compartments. At the end of the incubation period it was found that the heat sum to achieve the same carbon losses was lower for soils in the north of the transect than in the south. In the long term, therefore, for a given heat input, decomposition rates would show larger increases in boreal northern sites than in warm temperate regions.The changes in climate produced by soil translocation were more clearly reflected by decomposition rates in the acid soils than for calcareous soils. This indicates that the physicochemical environment can have important differential effects on microbial decomposition of the labile and recalcitrant components of SOM.

Soil responses to fire in Mediterranean forest landscapes in relation to the previous stage of land abandonment

The current paper presents a study on the interaction between land abandonment and soil responses to fire in old agricultural terraced landscapes. The study area, located near the Guadalest reservoir (E Spain), was partially affected by a forest fire in August 1998. We monitored burned and unburned areas as well as two pre-fire stand ages since agricultural abandonment: 8-15 years (dry grassland with young Pinus halepensis) and >35 years (mature pine forest). We analysed soil surface structure, water repellency and infiltrability, and we monitored plant response, runoff and sediment production for a period of 7 years after the fire. Aggregate stability increased with both time-since-abandonment and fire. Water repellency increased with land abandonment but was not affected by fire. Unburned erosion plots produced almost no runoff, even during heavy rainstorms. Fire scarcely modified runoff and erosion rates in recently abandoned terraces. A dry period following fire restricted plant recovery in burned pine forest. Burned forest plots registered runoff and sediment yields one to four orders of magnitude higher than unburned forest plots. In burned pine forest, the maximum sediment production was registered 3 years after the fire, when rainstorms took place and plant cover was still low. Old agricultural terraces colonised by pines were found to be both vulnerable to degradation as a consequence of fire and highly dependent on post-fire rain for their recovery.

Land-use and fire history effects on post-fire vegetation dynamics in eastern Spain

Abstract Question: Is post-fire, medium-term vegetation dynamics determined by land-use or fire history prior to fire? Location: South-facing slope in the Gallinera valley, Alicante province, eastern Spain. Methods: After mapping the land-use and fire history of the study site using photo-interpretation, we sampled vegetation structure on a set of plots representing the most frequent land-use and fire history combinations on an area burned six years before sampling. We studied the effects of land-use history, comparing the one-fire land-use trajectories. We analysed the effects of fire history; comparing one- and two-fire plots for both previously cropped and uncropped areas. Results: Most variables were not significantly different between the earliest abandoned plots (abandoned at least 38 years before the fire) and the uncropped plots. On the most recently abandoned plots (abandoned between one and four years before the fire), the therophyte richness and the ratio of seeder:resprouter richness were significantly greatest. Different fire recurrences did not determine different post-fire vegetation on either the uncropped or the early abandoned plots (all dominated by fire-recruited seeder shrubs). The most recently abandoned plots had a lower resilience to fire. Conclusions: Land-use history and recent pre-fire land use, in particular, determined the post-fire vegetation in the medium term. The vegetation composition converged during secondary succession among land-use histories. Increasing fire recurrence had a small effect on mature plant communities, due to the combination of life-history traits determining the response to fire of the dominant species. Nomenclature: de Bolòs et al. (1990) for vascular plants, except for Quercus ilex ssp. ballota.

Fire Ecology and Post-Fire Restoration Approaches in Southern European Forest Types

In this chapter we start by making an overview of plant adaptations to fire and post-fire response types, which will determine the post-fire management alternatives that might be used in specific forest and shrubland types. Some implications of climate change on fire regimes and plant responses are addressed, and the more fire prone forest types in Southern Europe are identified. We finish by discussing the major post-fire questions and management alternatives faced by forest managers, common to all forest and shrubland types affected by wildfires. These include measures to protect soil and reduce erosion risk, how to manage the burned trees, the use of restoration or conversion, active or indirect restoration, the management of herbivory, alien species, and pests and diseases.