Pere Rovira

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.

ZmMYB31 directly represses maize lignin genes and redirects the phenylpropanoid metabolic flux.

Few regulators of phenylpropanoids have been identified in monocots having potential as biofuel crops. Here we demonstrate the role of the maize (Zea mays) R2R3-MYB factor ZmMYB31 in the control of the phenylpropanoid pathway. We determined its in vitro consensus DNA-binding sequence as ACC(T)/(A) ACC, and chromatin immunoprecipitation (ChIP) established that it interacts with two lignin gene promoters in vivo. To explore the potential of ZmMYB31 as a regulator of phenylpropanoids in other plants, its role in the regulation of the phenylpropanoid pathway was further investigated in Arabidopsis thaliana. ZmMYB31 downregulates several genes involved in the synthesis of monolignols and transgenic plants are dwarf and show a significantly reduced lignin content with unaltered polymer composition. We demonstrate that these changes increase cell wall degradability of the transgenic plants. In addition, ZmMYB31 represses the synthesis of sinapoylmalate, resulting in plants that are more sensitive to UV irradiation, and induces several stress-related proteins. Our results suggest that, as an indirect effect of repression of lignin biosynthesis, transgenic plants redirect carbon flux towards the biosynthesis of anthocyanins. Thus, ZmMYB31 can be considered a good candidate for the manipulation of lignin biosynthesis in biotechnological applications.

The maize ZmMYB42 represses the phenylpropanoid pathway and affects the cell wall structure, composition and degradability in Arabidopsis thaliana

The involvement of the maize ZmMYB42 R2R3-MYB factor in the phenylpropanoid pathway and cell wall structure and composition was investigated by overexpression in Arabidopsis thaliana. ZmMYB42 down-regulates several genes of the lignin pathway and this effect reduces the lignin content in all lignified tissues. In addition, ZmMYB42 plants generate a lignin polymer with a decreased S to G ratio through the enrichment in H and G subunits and depletion in S subunits. This transcription factor also regulates other genes involved in the synthesis of sinapate esters and flavonoids. Furthermore, ZmMYB42 affects the cell wall structure and degradability, and its polysaccharide composition. Together, these results suggest that ZmMYB42 may be part of the regulatory network controlling the phenylpropanoid biosynthetic pathway.

Organic carbon and nitrogen mineralization under Mediterranean climatic conditions: The effects of incubation depth

Abstract In a soil profile, temperature and humidity regimes change with depth. Under Mediterranean conditions, upper horizons are more affected by water deficits and drying-rewetting cycles than deep horizons. Our aim was to study how carbon and nitrogen mineralization are affected by depth, and special attention is paid to separating the effects of pedoclimate fromthe effects of other constraints like amount and quality of organic matter. To this end, mixtures of plant + soil material were exposed by incorporation in the field, at depths of 5, 20 and 40 cm, in nylon mesh bags. Mineralization of C and N was studied for 2 y. For all types of plant material studied ( Eucalyptus globulus, Quercus ilex and Pinus halepensis ), mineralization of both carbon and nitrogen was lower at 5 cm. No differences were between 20 and 40 cm. This result, probably as a result of the higher drying of the upper-most horizons, contrasts with the usual findings on this topic. The amounts of both C and N mineralized were lower than expected, probably because plant materials were finely ground, allowing stabilization in the mineral matrix of soil. With the possible exception of Pinus , depth affected the rate of mineralization, not the relation between C and N. It is concluded that, at least under Mediterranean conditions, the pedoclimate in deep layers is more favourable to microbial activity than in upper layers, in which drought is a strong limiting factor. Reduced oxygen availability in the subsoil layers did not inhibit decomposition and mineralization to the same extent as did desiccation in the surface layer. The higher mineralization of C and N usually found in upper horizons may be attributed to the higher amount and quality of organic matter in these horizons, rather than to pedoclimatic constraints.

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.

Decomposition of 13C-labelled plant material in a European 65–40° latitudinal transect of coniferous forest soils: simulation of climate change by translocation of soils

Standard 13 C-labelled plant material was exposed over 2‐3 yr at 8 sites in a north‐south climatic gradient of coniferous forest soils, developed on acid and calcareous parent materials in Western Europe. In addition to soils exposed in their sites of origin, replicate units containing labelled material were translocated in a cascade sequence southwards along the transect, to simulate the eAects of climate warming on decomposition processes. The current Atlantic climate represented the most favourable soil temperature and moisture conditions for decomposition. Northward this climatic zone, where the soil processes are essentially temperature-limited, the prediction for a temperature increase of 38C estimated a probable increase of C mineralisation by 20‐ 25% for the boreal zone and 10% for the cool temperate zone. Southward the cool Atlantic climate zone, (the Mediterranean climate), where the processes are seasonally moisture-limited, the predicted increase of temperature by 1‐28C little aAected the soil organic matter dynamics, because of the higher water deficit. A significant decrease of C mineralisation rates was observed only in the superficial layers recognised in Mediterranean forest soils as ‘xeromoder’ and subject to frequent dry conditions. In the deeper Mediterranean soil organic horizons (the mull humus types), representing the major C storage in this zone, C mineralisation was not aAected by a simulated 28C temperature increase. The temperature eAect is probably counteracted by a higher water deficit. 7 2000 Elsevier Science Ltd. All rights reserved.

Aboveground litter quality changes may drive soil organic carbon increase after shrub encroachment into mountain grasslands

Shrub encroachment into grasslands is ubiquitous but its impact on soil organic C (SOC) remains unclear. In previous work we had observed that shrub encroachment into mesic mountain grasslands increased SOC content. Here we sought the mechanisms of this increase. To this end, we assessed aboveground and belowground production for a conifer shrub (Juniperus communis L), a legume shrub (Cytisus balansae ssp. europaeus (G. López & Jarvis) Muñoz Garmendia) and grass (Festuca eskia Ramond ex DC), together with decomposition rates for both aboveground litter and roots. Belowground C net inputs do not clearly explain SOC increase: grass root production was higher than that of either shrub and the decomposition rate of grass roots was the lowest. Aboveground C net inputs were only slightly greater in shrubs than in grass, but the decomposition rate of litter of both shrubs was much lower than that of grass. The decomposition of conifer litter was N-limited, whereas that of legume shrub litter was P-limited. Thus we conclude that the SOC increases after shrub encroachment into mesic grasslands probably as a result of higher recalcitrance of shrub aboveground litter relative to grass litter.

Modelling the Ecological Vulnerability to Forest Fires in Mediterranean Ecosystems Using Geographic Information Technologies

Forest fires represent a major driver of change at the ecosystem and landscape levels in the Mediterranean region. Environmental features and vegetation are key factors to estimate the ecological vulnerability to fire; defined as the degree to which an ecosystem is susceptible to, and unable to cope with, adverse effects of fire (provided a fire occurs). Given the predicted climatic changes for the region, it is urgent to validate spatially explicit tools for assessing this vulnerability in order to support the design of new fire prevention and restoration strategies. This work presents an innovative GIS-based modelling approach to evaluate the ecological vulnerability to fire of an ecosystem, considering its main components (soil and vegetation) and different time scales. The evaluation was structured in three stages: short-term (focussed on soil degradation risk), medium-term (focussed on changes in vegetation), and coupling of the short- and medium-term vulnerabilities. The model was implemented in two regions: Aragón (inland North-eastern Spain) and Valencia (eastern Spain). Maps of the ecological vulnerability to fire were produced at a regional scale. We partially validated the model in a study site combining two complementary approaches that focused on testing the adequacy of model’s predictions in three ecosystems, all very common in fire-prone landscapes of eastern Spain: two shrublands and a pine forest. Both approaches were based on the comparison of model’s predictions with values of NDVI (Normalized Difference Vegetation Index), which is considered a good proxy for green biomass. Both methods showed that the model’s performance is satisfactory when applied to the three selected vegetation types.

Decomposition of Medicago sativa debris incubated at different depths under Mediterranean climate.

Reports on the dynamics of carbon and nitrogen at different depths in soil profiles usually describe a higher mineralization of both at upper horizons. Nevertheless, under Mediterranean climate, upper soil horizons are strongly affected by drought, especially in summer, and may offer pedoclimatic conditions less favorable to microbial activity than deep soil layers. Therefore, decomposition could be slower in the upper soil layers. To test this hypothesis, mixtures of Medicago sativa ground plants and soil material were incubated in the field, at 5, 20, and 40 cm depth, in nylon mesh bags. Mineralization of carbon and nitrogen was studied for two years. At 5 cm, mineralization of both elements was lower, and no differences were found between 20 and 40 cm. After two years of field incubation, the remaining carbon (as a percentage of the initial content) was 27.95 % +/- 0.88 at 5 cm depth, 19.87% +/- 0.77 at 20 cm, and 18.78 % +/- 1.19 at 40 cm. Mineralization of nitrogen exceeded that of carbon. After two years of field incubation, the remaining nitrogen (as a percentage of the initial content) was 17.62% +/- 3.06 at 5 cm depth, 12.17% +/- 0.94 at 20 cm, and 11.26% +/- 0.99 at 40 cm. The biodegradation rate in upper layers was lower for all biochemical fractions (water-soluble compounds, lipids, polysaccharides, lignin). This contrasts with the usual findings on this topic, but is consistent with our previous results with forest litter samples. Mineralization clearly followed double-exponential kinetics, with a labile and a recalcitrant pool of both carbon and nitrogen. The labile pool of carbon accounted for about 50% of the total initial carbon, whereas that of nitrogen accounted for about 60%. No clear effect of depth on the proportion between the labile and the recalcitrant pool was observed, neither for carbon nor for nitrogen. It was not possible to identify both pools with the biochemical fractions, suggesting that these pools should be interpreted in physical terms (unprotected vs. protected ). No direct effect of depth on the global retention of N was detected.

Recovery of fresh debris of different sizes in density fractions of two contrasting soils

Fresh plant residues are often identified with the light organic matter obtained by fractionation in a dense liquid, but the extent to which residues can be extracted efficiently by densimetric methods has not been widely studied. This paper presents the results of two experiments in which 14C-labelled straw was mixed with two soils of differing texture, and the mixture was subjected to densimetric fractionation. In the first experiment, soil was mixed with bulk milled straw, and in the second, with straw of different sizes (> 200 μm, 200-50 μm, 50-20 μm and < 20 μm). The recovery of straw in the light fraction was low, and decreased with decreasing straw size, reaching a minimum (less than 20 %) for straw < 20 μm. Except for straw < 20 μm, the recovery in the light fraction was lower in soil richest in silt and clay. In the absence of soil (blanks), the recovery of fine straw in the light fraction was lower than in the presence of soil, suggesting that the recovery of fine straw in the light fraction is partly due to its association with light coarse debris. Extractable and polytungstate-soluble fractions accounted for a small proportion of the 14C-activity. These results suggest that densimetric methods are not efficient for recovering fresh plant residues, except in the case of large residues in coarse-textured soils.