Joan Romanyà

Effects of slash burning on soil phosphorus fractions and sorption and desorption of phosphorus

Abstract Soil P fractions and P sorption-desorption characteristics were studied 7 months after clearfelling and slash burning at a mixed Eucalyptus forest site in eastern Australia. Depending on the fire intensity, three different microsites were generated: unburnt, burnt and intensely burnt (ashbed). Phosphorus fractions were extracted from the soil with NH 4 F (0.03 N) + HCl (0.025 N) (Bray I), NaHCO 3 (0.5 N), NaOH (0.5 N), and H 2 SO 4 (1 N). Adsorption isotherms were obtained by equilibrating soils with solutions having concentrations of P, and desorption of the adsorbed P was studied by extracting the soils with Bray I extract. The effects of fire on soil P were greatest in the surface soil horizons and depended upon fire intensity. Ashbed soils differed from unburnt soils for P fractions and P sorption and desorption characteristics. Labile inorganic P (Bray I) increased from less than 1 mg kg −1 in the unburnt soil to 5–13 mg kg −1 in the ashbed. Inorganic P (NaOH and H 2 SO 4 extractions) increased markedly after fire, especially in the surface layers. The increase in labile organic P (NaHCO 3 -extractable) contrasted with a decrease in total organic P (H 2 SO 4 ) and less labile organic P (NaOH) in ashbed soils, suggesting marked transformation of organic P pools after intense fire. After incubation for 2 months, labile organic P in the unburnt soil increased, whereas large decreases were observed in ashbeds and surface (0–5 cm) burnt soils. The ashbed soil showed an increase in sorption capacity in the 0–5 cm soil layer, but the sorbed P was generally less tightly bound to the solid phase. Seedling growth and foliage P concentrations were greatest in ashbed soils. Harvesting and burning increased the spatial distribution of soil P in the field. The ashbed and burnt microsites represented 19% and 18% respectively, of the surface area of the slash burnt coupe, and about 8 kg P ha −1 was deposited in ash.

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.

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.

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.

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.

Nitrogen supply rate in Scots pine (Pinus sylvestris L.) forests of contrasting slope aspect

We studied Nitrogen (N) transformations in Pinus sylvestris forest stands in the foothills of the SE Pre-Pyrenees (NE Spain). Plots were selected in two contrasting aspects (two plots per aspect) and N supply rate was measured by the resin-core incubation technique once every three months. N leaching through litter layers (L and F horizons) was evaluated by 5 zero-tension lysimeters in each plot. NH4+-N, NO3--N and soluble organic-N were determined in all solutions. N supply rate showed a clear seasonal pattern. Ammonification and nitrification were segregated in space and in time. While ammonification showed a peak in spring, nitrification was higher in summer. There was evidence suggesting that nitrification occurs mostly in A1 horizon. Nitrification rates differed significantly among plots. N supply rate was 12.7–23.5 kg N·ha-1·yr-1 but it did not differ between aspects or plots. Inorganic-N leached through litter layers was 14–17 kg N·ha-1·yr-1, and represented a high proportion of N supply rate. Organic-N leached through litter layers (27.8–37.0 kg N·ha-1·yr-1) was higher than leached inorganic-N. However, in most cases organic-N did not represent a high proportion of changes in soluble organic-N pools in H and A1 horizons (about 240 kg N·ha-1·yr-1). This large decrease in soluble organic-N was much greater than the increase in inorganic-N. The possible fate of these large amounts of organic-N is discussed.

Nitrogen and phosphorus leaching from the forest floor of a mature Pinus radiata stand

Abstract In a mature Pinus radiata plantation we studied the amounts and forms of nitrogen and phosphorus leached from the forest floor paying special attention to seasonal changes. Forest floor leachates were collected using zero-tension lysimeters placed underneath the Oa horizon. A large amount of the N and P leached throughout the year leached during short periods of time (61 % of the P and 51 % of the N in two months). The amount of litter and its quality (C, N and P concentration and the C:N, C:P and N:P ratios) were not related to spatial variability in litter leachates. The seasonal pattern of inorganic P concentration and leaching suggested that peaks could be attributed to increases in throughfall that were not completely retained by organic layers. Organic forms accounted for a high proportion of the N and P leached, especially for the former.

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.

Carbon and nitrogen stocks and nitrogen mineralization in organically managed soils amended with composted manures.

The use of composted manures and of legumes in crop rotations may control the quality and quantity of soil organic matter and may affect nutrient retention and recycling. We studied soil organic C and N stocks and N mineralization in organically and conventionally managed dryland arable soils. We selected 13 extensive organic fields managed organically for 10 yr or more as well as adjacent fields managed conventionally. Organic farmers applied composted manures ranging from 0 to 1380 kg C ha yr and incorporated legumes in crop rotations. In contrast, conventional farmers applied fresh manures combined with slurries and/or mineral fertilizers ranging from 200 to 1900 kg C ha yr and practiced a cereal monoculture. Despite the fact that the application of organic C was similar in both farming systems, organically managed soils showed higher C and similar N content and lower bulk density than conventionally managed soils. Moreover, organic C stocks responded to the inputs of organic C in manures and to the presence of legumes only in organically managed soils. In contrast, stocks of organic N increased with the inputs of N or C in both farming systems. In organically managed soils, organic N stocks were less mineralizable than in conventional soils. However, N mineralization in organic soils was sensitive to the N fixation rates of legumes and to application rate and C/N ratio of the organic fertilizers.

CO2 efflux from a Mediterranean semi-arid forest soil. II. Effects of soil fauna and surface stoniness

Many forest soils in the Mediterranean basin areshallow and contain high amounts of gravel in theorganic layers. Recent studies on soil organic matteraccumulation have shown high amounts of organic matteroccurring mainly in soils with high levels ofstoniness at the soil surface. The gravel layer mayaffect the microclimatic conditions of the soilsurface and probably the distribution and activity ofsoil fauna.In order to quantify the combined effects soil fauna(epigeic macrofauna and earthworms) and stoniness onthe release of soil CO2, we performed a threefactor field experiment by using a series ofreconstructed soil profiles. Factors 1 and 2 consistedof the exclusion/presence of soil epigeic macrofaunaand earthworms, and factor 3 of the presence/absenceof a gravel layer intermingled with the H horizon. Weincubated 14C straw in the H horizon and carriedout three 40 mm rainfall simulations.Soil respiration primarily depended on the season. Theeffects of soil fauna were generally small and did notcoincide with periods of high faunal activity. Thelargest effects of both earthworms and soil epigeicfauna were found after wetting the soil in summer. Theeffects of the earthworms were concentrated in themineral soil while the effects of the epigeic faunawere concentrated in the H horizon and mainly arosetowards the end of the experiment. This suggests thatthe effects of epigeic fauna may have beenunderestimated due to the length of the experiment.The gravel layer increased the effect of faunaprobably by creating more favorable microclimaticconditions. The accumulation of organic matter insoils with high levels of stoniness cannot beexplained by the effect of gravel on soil microclimatenor by its effect on the activity of soil fauna.