Temperature sensitivity of organic matter decomposition in two boreal forest soil profiles

Controversial conclusions from different studies suggest that the decomposition of old soil organic matter (SOM) is either more, less, or equally temperature sensitive compared to the younger SOM. Based on chemical kinetic theory, the decomposition of more recalcitrant materials should be more temperature sensitive, unless environmental factors limit decomposition. Here, we show results for boreal upland forest soils supporting this hypothesis. We detected differences in the temperature sensitivity 1) between soil layers varying in their decomposition stage and SOM quality, and 2) inside the layers during a 495 day laboratory incubation. Temperature sensitivity increased with increasing soil depth and decreasing SOM quality. In the organic layers, temperature sensitivity of decomposition increased during the early part of a 495 day laboratory incubation, after respiration rate and SOM quality had notably decreased. This indicates that decomposition of recalcitrant compounds was more temperature sensitive than that of the labile ones. Our results imply that Q₁₀ values for total heterotrophic soil respiration determined from short-term laboratory incubations can either underestimate or overestimate the temperature sensitivity of SOM decomposition, depending on soil layer, initial labile carbon content and temperature range used for the measurements. Using Q₁₀ values that ignore these factors in global climate models provides erroneous estimates on the effects of climate change on soil carbon storage.     

Influence of earthworms and leaf litter on edaphic variables in sewage-sludge-treated soil microcosms

Summary A series of 48 greenhouse soil microcosms were established and treated with combinations of sewage sludge, Acer saccharum leaves, and the earthworms Eisenia fetida and Lumbricus terrestris. The microcosms were incubated at constant moisture for 110 days. Samples were then taken and analysed for sludge depth, organic-matter content, and waterstable soil aggregates. Weights of surface leaves and weights and numbers of surviving earthworms were determined for each microcosm. L. terrestris significantly reduced sludge depth and the surface organic-matter content of microcosm soil and significantly increased percentages of 4-mm diameter water-stable aggregates. Leaf litter also significantly reduced sludge depth and increased 4-mm water-stable aggregates. E. fetida inhibited surface feeding by L. terrestris, reduced its 110-day survival rate, and inhibited the production of 4-mm water-stable aggregates in L. terrestris treatments. Numbers of E. fetida increased in L. terrestris treatments. Sludge depth, organic-matter content and water-stable aggregates were not significantly different from controls in E. fetida treatments.     

Impact of landspread sewage sludge and earthworm introduction on established earthworms and soil structure

Summary Sewage sludge was applied to twelve 4-m² plots in two forest (mixed hardwood, Norway spruce plantation) site and one old field site. The earthworm Eisenia fetida was introduced to half the control and half the treated plots. Earthworm populations were sampled by formalin extraction and hand-sorting five times in the year following treatment. One year after treatment, soil samples were wet-sieved and water-stable aggregate size-class arrays were determined.The dominant earthworm in the study site, Lumbricus terrestris, increased in density and mean individual biomass in response to sludge treatment in mixed hardwood and old field plots. In the Norway spruce plots, L. terrestris increased in individual biomass but decreased in density following sludge application. The density of the introduced E. fetida rapidly declined in all control plots. One year after introduction, E. fetida was found only in the sludge-treated Norway spruce plot. The introduction of E. fetida with sludge decreased the density and biomass of L. terrestris in the hardwood plots.Sludge treatment increased the percentages of 4-mm diameter water-stable aggregates in old field and hardwood plots. The addition of E. fetida with sludge in the hardwood plots generated no increase in 4-mm water-stable aggregates. In the old field, sludge + E. fetida increased the 4-mm water-stable aggregates. Little change in water-stable aggregates in response to either treatment combination was seen in the Norway spruce site.     

A comparison of nickel toxicity to pre-exposed earthworms (Eisenia fetida, oligochaeta) in two different test substrates

Earthworms are soil living organisms of high ecological importance. For that reason, earthworms can be considered as feasible biological indicators for many pollutants in soils. Soils are extremely complex and dynamic systems influenced by a number of different abiotic and biological factors determining the effects of potentially toxic substances. To be able to evaluate the toxicity of a single substance to soil living organisms such as earthworms, soil-related influences and interactions need to be excluded. For that purpose, the use of defined, artificial soil free substrates has been widely established. In this study, a new artificial substrate based on the knowledge gained by the application of different methods, in particular the exposure in reconstituted water resembling natural ground water and an agar medium initially developed to void the guts of earthworms, is proposed. Eisenia fetida have been exposed for 96 h to a medium consisting of 1.5% normal melting agarose added to reconstituted ground water. For validation of the substrate, the effects of nickel toxicity, a substance known to be toxic to E. fetida, were evaluated. In addition, to investigate the possible establishment of resistance or adaptation, as well as cross-resistance, E. fetida individuals with a history of previous exposure (more than 10 generations) to nickel and manganese were exposed to different concentrations of nickel. Both media, one consisting of reconstituted ground water [Kiewiet, A.T., Ma, W.-C., 1991. Effect of pH and calcium on lead and cadmium uptake by earthworms in water. Ecotoxicology and Environmental Safety 21, 32-37] and the other of reconstituted ground water plus agar, seemed equally suited for the evaluation of intrinsic effects of nickel and manganese on E. fetida. With regard to the possible development of resistance, E. fetida did not show an increased tolerance towards nickel in spite of being exposed to elevated levels for more than 10 generations; to the contrary, worms with a history of exposure to nickel showed an increased sensitivity towards nickel. No indications of cross-resistance between manganese and nickel pre-exposed worms were detected.     

Toxicological effects of TiO2 and ZnO nanoparticles in soil on earthworm Eisenia fetida

Nanoparticles (NPs) of TiO₂ and ZnO are receiving increasing attention due to their widespread applications. To evaluate their toxicities to the earthworm Eisenia fetida (Savigny, 1826) in soil, artificial soil systems containing distilled water, 0.1, 0.5, 1.0 or 5.0 g kg⁻¹ of NPs were prepared and earthworms were exposed for 7 days. Contents of Zn and Ti in earthworm, activities of antioxidant enzymes, DNA damage to earthworm, activity of cellulase and damage to mitochondria of gut cells were investigated after acute toxicity test. The results from response of the antioxidant system combined with DNA damage endpoint (comet assay) indicated that TiO₂ and ZnO NPs could induce significant damage to earthworms when doses were greater than 1.0 g kg⁻¹. We found that Ti and Zn, especially Zn, were bioaccumulated, and that mitochondria were damaged at the highest dose in soil (5.0 g kg⁻¹). The activity of cellulase was significantly inhibited when organisms were exposed to 5.0 g kg⁻¹ of ZnO NPs. Our study demonstrates that both TiO₂ and ZnO NPs exert harmful effects to E. fetida when their levels are higher than 1.0 g kg⁻¹ in soil and that toxicity of ZnO NPs was higher than TiO₂.     

Temperature sensitivity of soil organic matter decomposition varies with biochar application and soil type

Biochar application has the potential to improve soil fertility and increase soil carbon stock, especially in tropical regions. Information on the temperature sensitivity of carbon dioxide(CO_2) evolution from biochar-amended soils at very high temperatures, as observed for tropical surface soils, is limited but urgently needed for the development of region-specific biochar management targeted to optimize biochar effects on soil functions. Here, we investigated the temperature sensitivity of soil respiration to the addition of different rates of Miscanthus biochar(0, 6.25, 12.5, and 25 Mg ha~(-1)) in two types of soils with contrasting textures. Biochar-amended soil treatments and their controls were incubated at constant temperatures of 20, 30, and 40℃. Overall, our results show that: i) considering data from all treatments and temperatures, the addition of biochar decreased soil CO_2 emissions when compared to untreated soils;ii) CO_2 emissions from biochar-amended soils had a higher temperature sensitivity than those from biochar-free soils; iii) the temperature sensitivity of soil respiration in sandy soils was higher than that in clay soils; and iv) for clay soils, relative increases in soil CO_2 emissions from biochar-amended soils were higher when the temperature increased from 30 to 40℃, while for sandy soils, the highest temperature responses of soil respiration were observed when increasing the temperature from 20 to 30℃. Together, these findings suggest a significantly reduced potential to increase soil organic carbon stocks when Miscanthus biochar is applied to tropical soils at high surface temperatures, which could be counteracted by the soil-and weather-specific timing of biochar application.     

Isotopic analysis of respired CO2 during decomposition of separated soil organic matter pools

A detailed understanding of the processes that contribute to the δ¹³C value of respired CO₂ is necessary to make links between the isotopic signature of CO₂ efflux from the soil surface and various sources within the soil profile. We used density fractionation to divide soils from two forested sites that are a part of an ongoing detrital manipulation experiment (the Detrital Input and Removal Treatments, or DIRT project) into two soil organic matter pools, each of which contributes differently to total soil CO₂ efflux. In both sites, distinct biological pools resulted from density fractionation; however, our results do not always support the concept that the light fraction is readily decomposable whereas the heavy fraction is recalcitrant. In a laboratory incubation following density fractionation we found that cumulative respiration over the course of the incubation period was greater from the light fraction than from the heavy fraction for the deciduous site, while the opposite was true for the coniferous site.Use of stable isotopes yielded insight as to the nature of the density fractions, with the heavy fraction solids from both forests isotopically enriched relative to those of the light fraction. The isotopic signature of respired CO₂, however, was more complicated. During incubation of the fractions there was an initial isotopic depletion of the respired CO₂ compared to the substrate for both soil fractions from both forests. Over time for both fractions of both soils the respired δ¹³C reflected more closely the initial substrate value; however, the transition from depleted to enriched respiration relative to substrate occurs at a different stage of decomposition depending on site and substrate recalcitrance. We found a relationship between cumulative respiration during the incubation period and the duration of the transition from isotopically depleted to enriched respiration in the coniferous site but not the deciduous site. Our results suggest that a shift in microbial community or to dead microbial biomass as a substrate could be responsible for the transition in the isotopic signature of respired CO₂ during decomposition. It is likely that a combination of organic matter quality and isotopic discrimination by microbes, in addition to differences in microbial community composition, contribute to the isotopic signature of different organic matter fractions. It is apparent that respired δ¹³CO₂ cannot be assumed to be a direct representation of the substrate δ¹³C. Detailed knowledge of the soil characteristics at a particular site is necessary to interpret relationships between the isotopic values of a substrate and respired CO₂.     

Soil physicochemical and microbial drivers of temperature sensitivity of soil organic matter decomposition under boreal forests

Soil organic matter(SOM)in boreal forests is an important carbon sink.The aim of this study was to assess and to detect factors controlling the temperature sensitivity of SOM decomposition.Soils were collected from Scots pine,Norway spruce,silver birch,and mixed forests(O horizon)in northern Finland,and their basal respiration rates at five different temperatures(from 4 to 28℃)were measured.The Q_(10) values,showing the respiration rate changes with a 10℃ increase,were calculated using a Gaussian function and were based on temperature-dependent changes.Several soil physicochemical parameters were measured,and the functional diversity of the soil microbial communities was assessed using the MicroResp?method.The temperature sensitivity of SOM decomposition differed under the studied forest stands.Pine forests had the highest temperature sensitivity for SOM decomposition at the low temperature range(0–12℃).Within this temperature range,the Q_(10) values were positively correlated with the microbial functional diversity index(H'_(mic))and the soil C-to-P ratio.This suggested that the metabolic abilities of the soil microbial communities and the soil nutrient content were important controls of temperature sensitivity in taiga soils.     

Rhizosphere priming of soil organic matter by bacterial groups in a grassland soil

Plants often impact the rate of native soil organic matter turnover through root interactions with soil organisms; however the role of root-microbial interactions in mediation of the “priming effect” is not well understood. We examined the effects of living plant roots and N fertilization on belowground C dynamics in a California annual grassland soil (Haploxeralf) during a two-year greenhouse study. The fate of ¹³C-labeled belowground C (roots and organic matter) was followed under planted (Avena barbata) and unplanted conditions, and with and without supplemental N (20 kg N ha⁻¹ season⁻¹) over two periods of plant growth, each followed by a dry, fallow period of 120 d. Turnover of belowground ¹³C SOM was followed using ¹³C-phospholipid fatty acid (PLFA) biomarkers. Living roots increased the turnover and loss of belowground ¹³C compared with unplanted soils. Planted soils had 20% less belowground ¹³C present than in unplanted soils after 2 cycles of planting and fallow. After 2 treatment cycles, unlabeled soil C was 4.8% higher in planted soils than unplanted. The addition of N to soils decreased the turnover of enriched belowground ¹³C during the first treatment season in both planted and unplanted soils, however no effect of N was observed thereafter. Our findings suggest that A. barbata may increase soil C levels over time because root and exudate C inputs are significant, but that increase will be moderated by an overall faster C mineralization rate of belowground C. N addition may slow soil C losses; however, the effect was minor and transient in this system. The labeled root-derived ¹³C was initially recovered in gram negative (highest enrichment), gram positive, and fungal biomarkers. With successive growing seasons, the labeled C in the gram negative and fungal markers declined, while gram positive markers continued to accumulate labeled belowground C. The rhizosphere of A. barbata shifted the microbial community composition, resulting in greater abundances of gram negative markers and lower abundances of gram positive, actinobacteria and cyclopropyl PLFA markers compared to unplanted soil. However, the longer-term utilization of labeled belowground C by gram positive bacteria was enhanced in the rhizosphere microbial community compared with unplanted soils. We suggest that the activities of gram positive bacteria may be major controllers of multi-year rhizosphere-related priming of SOM decomposition.     

Model of apparent and real priming effects: Linking microbial activity with soil organic matter decomposition

The most frequently used models simulating soil organic matter (SOM) dynamics are based on first-order kinetics. These models fail to describe and predict such interactions as priming effects (PEs), which are short-term changes in SOM decomposition induced by easily available C or N sources. We hypothesized that if decomposition rate depends not only on size of the SOM pool, but also on microbial biomass and its activity, then PE can be simulated. A simple model that included these interactions and that consisted of three C pools - SOM, microbial biomass, and easily available C - was developed. The model was parameterized and evaluated using results of ¹²C-CO₂ and ¹⁴C-CO₂ efflux after adding ¹⁴C-labeled glucose to a loamy Haplic Luvisol. Experimentally measured PE, i.e., changes in SOM decomposition induced by glucose, was compared with simulated PE. The best agreement between measured and simulated CO₂ efflux was achieved by considering both the total amount of microbial biomass and its activity. Because it separately described microbial turnover and SOM decomposition, the model successfully simulated apparent and real PE.The proposed PE model was compared with three alternative approaches with similar complexity but lacking interactions between the pools and neglecting the activity of microbial biomass. The comparison showed that proposed new model best described typical PE dynamics in which the first peak of apparent PE lasted for 1 day and the subsequent real PE gradually increased during 60 days. This sequential decomposition scheme of the new model, with immediate microbial consumption only of soluble substrate, was superior to the parallel decomposition scheme with simultaneous microbial consumption of two substrates with different decomposability. Incorporating microbial activity function in the model improved the fit of simulation results with experimental data, by providing the flexibility necessary to properly describe PE dynamics. We conclude that microbial biomass should be considered in models of C and N dynamics in soil not only as a pool but also as an active driver of C and N turnover.     

Earthworm impacts on soil organic matter and fertilizer dynamics in tropical hillside agroecosystems of Honduras

Earthworms are important processors of soil organic matter (SOM) and nutrient turnover in terrestrial ecosystems. In agroecosystems, they are often seen as beneficial organisms to crop growth and are actively promoted by farmers and extension agents, yet their contribution to agroecosystem services is uncertain and depends largely on management. The Quesungual slash-and-mulch agroforestry system (QSMAS) of western Honduras has been proposed as a viable alternative to traditional slash-and-burn (SB) practices and has been shown to increase earthworm populations, yet the effect of earthworms on soil fertility and SOM in QSMAS is poorly understood. This study examined the role of Pontoscolex corethrurus in QSMAS by comparing their influence on aggregate-associated SOM and fertilizer dynamics with their effects under SB and secondary forest in a replicated field trial. Both the fertilized QSMAS and SB treatments had plots receiving additions of inorganic ¹⁵N and P, as well as plots with no inorganic N additions. Earthworm populations were manipulated in field microcosms at the beginning of the rainy season within each management treatment via additions of P. corethrurus or complete removal of existing earthworm populations. Microcosms were destructively sampled at harvest of Zea mays and soils were wet-sieved (using 53, 250 and 2000 μm mesh sizes) to isolate different aggregate size fractions, which were analyzed for total C, N and ¹⁵N. The effects of management system were smaller than expected, likely due to disturbance associated with the microcosm installation. Contrary to our hypothesis that earthworms would stabilize organic matter in soil aggregates, P. corethrurus decreased total soil C by 3% in the surface layer (0-15 cm), predominantly through a decrease in the C concentration of macroaggregates (>250 μm) and a corresponding depletion of C in coarse particulate organic matter occluded within macroaggregates. Earthworms also decreased bulk density by over 4%, but had no effect on aggregate size distribution. Within the two fertilized treatments, the QSMAS appeared to retain slightly more fertilizer derived N in smaller aggregate fractions (<250 μm) than did SB, while earthworms greatly reduced the recovery of fertilizer N (34% decrease) in both systems. Although management system did not appear to influence the impact of P. corethrurus on SOM or nutrient dynamics, we suggest the lack of differences may be due to artificially low inputs of fresh residue C to microcosms within all management treatments. Our findings highlight the potential for P. corethrurus to have deleterious impacts on soil C and fertilizer N dynamics, and emphasize the need to fully consider the activities of soil fauna when evaluating agroecosystem management options.     

Compound-specific δC and δH analyses of plant and soil organic matter: A preliminary assessment of the effects of vegetation change on ecosystem hydrology

Here we present δ¹³C and δ²H data of long-chained, even-numbered (C₂₇-C₃₁) n-alkanes from C3 (trees) and C4 (grasses) plants and from the corresponding soils from a grassland-woodland vegetation sequence in central Queensland, Australia. Our data show that δ¹³C values of the C4 grassland species were heavier relative to those of C3 tree species from the woodland (Acacia leaves) and woody grassland (Atalaya leaves). However, n-alkanes from the C4 grasses had lighter δ²H values relative to the Acacia leaves, but showed no significant difference in δ²H values when compared with C3 Atalaya leaves. These results differ from those of previous studies, showing that C4 grasses had heavier δ²H values relative to C3 grasses and trees. Those observations have been explained by C4 plants accessing the more evaporation-influenced and isotopically heavier surface water and tree roots sourcing deeper, isotopically lighter soil water (“Two-layered soil-water system”). By comparison, our data suggest that ecosystem changes (vegetation “thickening”) can significantly alter the soil hydrological characteristics. This is shown by the heavier δ²H values in the woodland soil compared with lighter δ²H values in the grassland soil, implying that the recent vegetation change (increased tree biomass) in the woodland had altered soil hydrological conditions. Estimated δ²H values of the source-water for vegetation in the grassland and woodland showed that both trees and grasses in open settings accessed water with lighter δ²H values (avg. −46‰) compared with water accessed by trees in the woodland vegetation (avg. −7‰). These data suggest that in semi-arid environments the “two-layer” soil water concept might not apply. Furthermore, our data indicate that compound-specific δ²H and δ¹³C analyses of n-alkanes from soil organic matter can be used to successfully differentiate between water sources of different vegetation types (grasses versus trees) in natural ecosystems.     

Thermal stability of soil organic matter pools and their δC values after C3-C4 vegetation change

Carbon isotopic composition of soils subjected to C₃-C₄ vegetation change is a suitable tool for the estimation of C turnover in soil organic matter (SOM) pools. We hypothesized that the biological availability of SOM pools is inversely proportional to their thermal stability. Soil samples from a field plot with 10.5 years of cultivation of the C₄ plant Miscanthus×gigantheus and from a reference plot under C₃ grassland vegetation were analysed by thermogravimetry coupled with differential scanning calorimetry (TG-DSC). According to differential weight losses (dTG) and energy release or consumption (DSC), five SOM pools with increasing thermal stability were distinguished: (I) 20-190 °C, (II) 190-310 °C, (III) 310-390 °C, (IV) 390-480 °C, and (V) 480-1000 °C. Their δ¹³C values were analysed by EA-IRMS. The weight losses in pool I were connected with water evaporation, since no significant C losses were measured and δ¹³C values remained unchanged. The δ¹³C of pools II and III in soil samples under Miscanthus were closer to the δ¹³C of the Miscanthus plant tissues (−11.8‰) compared to the thermally stable SOM pool V (−19.5‰). The portion of the Miscanthus-derived C₄-C in total SOM in 0-5 cm reached 55.4% in the 10.5 years. The C₄-C contribution in pool II was 60% and decreased down to 6% in pool V. The mean residence times (MRT) of SOM pools II, III, and IV were similar (11.6, 12.2, and 15.4 years, respectively), while pool V had a MRT of 163 years. Therefore, we concluded that the biological availability of thermal labile SOM pools (<480 °C) was higher, than that of the thermal stable pool decomposed above 480 °C. However, the increase of SOM stability with rising temperature was not gradual. Therefore, the applicability of the TG-DSC for the separation of SOM pools with different biological availability is limited.     

Nutrient availability and interactions between soil arthropods and microorganisms during decomposition of coniferous litter: a mesocosm study

Summary The functional roles of the fungivorous collembolan Tomocerus minor and the detritivorous isopod Philoscia muscorum during the decomposition of Pinus nigra needles were studied in mesocosms filled with two different types of F1 litter, obtained from two different forest soils. The effects of the animals on the availability of K⁺, Ca²⁺, NO _inf3 ^sup- , NH _inf4 ^sup+ , and PO _inf4 ^sup3- and on the respiration, dehydrogenase, and cellulase activity of microorganisms were measured over one growing season. The animals were introduced into the F1 litter in three densities. The most important animal effect was a buffering effect, in that addition of the animals increased nutrient availability and microbial activity where the corresponding values in control mesocosms without animals were low, and decreased the nutrient availability and microbial activity where control values were high. This effect occurred for both species and was most evident in the substrate with the highest temporal fluctuations. The effects on nutrient availability are attributed to an animal effect on the activity of and successional stage reached the microbial community, with NH _inf4 ^sup+ availability seen as the most important factor. The concept of functional groups in relation to these animal effects is discussed.     

Use of C-labelled plant materials and ergosterol, PLFA and NLFA analyses to investigate organic matter decomposition in Antarctic soil

The relationship between organic matter decomposition and changes in microbial community structure were investigated in Antarctic soils using ¹³C-labelled plant materials. Soils with and without labelled Deschampsia antarctica (a native Antarctic grass) were incubated for 42 days and sampled at 0, 7, 14, 21, 28 and 42 days. Changes in microbial community structure were assessed using phospholipid fatty acid analysis (PLFA) and an analysis of the fatty acids associated with the neutral lipid fraction (NLFA). These studies showed that there were no significant changes in PLFA or NLFA profiles over time suggesting no change in microbial community structure during residue decomposition. There was a marked increase however, in ergosterol levels in these soils indicative of growth of the fungal biomass. Analysis of this ergosterol using gas chromatography-mass spectrometry confirmed the transformation of the plant residue by showing the incorporation of ¹³C-plant C into the ergosterol. This incorporation of ¹³C into the ergosterol increased over the incubation period. Importantly, these changes associated with fungal growth were not evident in the analysis of either the PLFA or NLFA fractions thus questioning the reliability of such approaches for studying changes in microbial communities associated with the decomposition of plant residues.     

Increase in soil pH due to Ca-rich organic matter application causes suppression of the clubroot disease of crucifers

Clubroot disease of cruciferous plants caused by the soil-borne pathogen Plasmodiophora brassicae is difficult to control because the pathogen survives for a long time in soil as resting spores. Disease-suppressive and conducive soils were found during the long-term experiment on the impact of organic matter application to arable fields and have been studied to clarify the biotic and abiotic factors involved in the disease suppression. The fact that a large amount of organic matter, 400 t ha⁻¹ yr⁻¹ farmyard manure (FYM) or 100 t ha⁻¹ yr⁻¹ food factory sludge compost (FSC), had been incorporated for more than 15 yr in the suppressive soils and these soils showed higher pH and Ca concentration than the disease conducive soil led us to hypothesize that an increase in soil pH due to the long-term incorporation of Ca-rich organic matter might be the primary cause of the disease suppression. We have designed a highly reproducible bioassay system to examine this hypothesis. The suppressive and conducive soils were mixed with the resting spores of P. brassicae at a rate of 10⁶ spore g⁻¹ soil, and Brassica campestris was grown in a growth chamber for 8 d. The number of root hair infections was assessed on a microscope. It was found that the incorporation of FYM and FSC at 2.5% (w/w) to the conducive soil suppressed the infection and that the finer particles (?5 mm) of FSC inhibited the infection and increased soil pH more effectively. Neutralization of the conducive soil by Ca(OH)₂, CaCO₃ and KOH suppressed the infection, but the effectiveness of KOH was less than those of Ca(OH)₂ and CaCO₃. Acidification of the suppressive soils by H₂SO₄, promoted the infection. The involvement of soil biota in the disease suppression was investigated using the sterilized (γ-ray irradiation) suppressive soils with respect to soil pH. The γ-ray irradiation promoted the infection at pH 5.5, but no infection was observed at pH 7.4 irrespective of the sterilization status. All these observations suggest that soil pH is a major factor in disease suppression by organic matter application and that Ca and soil biota play certain roles in the suppression under the influence of soil pH.     

Effects of liming on organic matter decomposition and phosphorus extractability in an acid humic Ranker soil from northwest Spain

Summary A laboratory incubation experiment was carried out over 17 weeks to determine the effect of liming on soil organic matter. The amount of lime as calcium hydroxide [Ca(OH)₂] required to completely neutralise exchangeable Al was found to be five times the standard lime requirement. This large amount of lime had a limited overall effect on the short-term stability of soil organic matter, causing the release of 1300 g g^-1 of C (1.7% total soil C) above the control during the incubation. Liming may have altered the potential availability of soil organic matter and organic P, as shown by a marked reduction in the extractability of soil organic P with sodium bicarbonate and sodium hydroxide. The latter was unlikely to be due to the formation of calclium-P artefacts, and may be attributed to the combined chemical effects of added calcium hydroxide and precipitation of exchangeable Al on the nature and solubility of soil organic constituents and organomineral complexes. The addition of lime increased the degradation of added oak leaf litter by 50%, from 3.2 to 4.7 mg g^-1, as determined by CO₂ evolution. The enhanced litter degradation indicated increased microbial activity in limed soil, but this improvement had only minor effects on the stability of native organic matter. This study highlights the need for further research into the relationships between the chemical nature of organic P in soil and the physical, chemical, temporal, and agronomic factors that control its turnover and availability.     

Factors controlling decomposition of soil organic matter in fallow systems of the high tropical Andes: A field simulation approach using C- and N-labelled plant material

N-rich (C:N=27) and N-poor (C:N=130) wheat straw, labelled with ¹⁴C and ¹⁵N, was incubated for 2 yr in two major ecosystems of the upper elevation belt of cultivation in the high Andes: the moist Paramo (precipitation=1329 mm, altitude=3400 m asl, Andes of Merida, Venezuela) and the dry Puna (precipitation=370 mm, altitude=3800 m asl, Central Altiplano, Bolivia). The experiment was installed in young (2 yr) and old (7 yr) fallow plots. The following soil analyses were performed at nine sampling occasions: soil moisture, total-¹⁴C and -¹⁵N, and Microbial Biomass (MB)-¹⁴C and -¹⁵N. The measured data were fitted by the MOMOS-6 model (a process based model, with five compartments: labile and stable plant material, MB, and labile (HL) and stable humus (HS)) coupled with the SAHEL model (soil moisture prediction) using daily measured and/or predicted meteorological data. The aim was to understand how (1) the climatic conditions, (2) the quality of plant material, (3) the fallow age and (4) the soil properties affect the cycling of C and N within the soil organic matter system.The fallow age (2 and 7 yr) did not affect the measured data or the model predictions, indicating that in these systems the decomposition potential is not affected by fallow length. During the short initial active decomposition phase, the labile plant material was quickly exhausted, enabling a build up of MB and of HL. During the low activity phase, that covered 4/5 of the time of exposure, the MB size decreased slowly and the HL pool was progressively exhausted as it was reused by the MB as substrate. The HL compartment was directly or indirectly the major source for the inorganic ¹⁵N production. If the C:N ratio of the added plant material increased, the model predicted (1) a reduction of the decomposition rates of the plant material (essentially the stable plant material) and (2) an increased mortality of the MB which increased the production of HL (microbial cadavers and metabolites). Thus the essential effect of the slower decomposition due to the N-poor plant material was a higher accumulation of C and N in the HL and its slower recycling by the MB during the low activity phase. The labelling experiment allows to understand the higher soil native organic matter content in Paramo soils compared to Puna. The large sequestration of organic matter generally observed in the Paramo soils can be explained by two abiotic factors: the unfavourable soil microstructure and the accumulation of free aluminium linked to the climatic and acid soil conditions, inhibiting the microbial activity physically and chemically.