Soil nematode community,organic matter,microbial biomass and nitrogen dynamics in field plots transitioning from conventional to organic management

Dynamics of soil bulk density, organic matter, microbial biomass, nitrogen, and nematode communities were assessed for a period of 4 years in field plots transitioning from conventional to organic farming practices. A rotation of soybeans, corn, oats and hay was used as an organic transitioning strategy and the conventional farming system had a corn and soybean rotation for comparison. Organic corn received raw straw pack beef manure and poultry compost at the rate of 27 and 28 Mg/h, respectively, and organic oats received raw straw pack beef manure and poultry compost at the rate of 18 and 1.8 Mg/h, respectively, while conventional plots received synthetic fertilizers. All crops in the organic system received primary tillage (chisel plow, disked and tined) whereas only corn received primary tillage in the conventional system but soybeans were no-till. Weed control was mechanical (twice diskings, rotary hoeings and row cultivation) in the organic system whereas herbicides were used in the conventional system. Soil bulk density did not differ in the two systems over a 4-year period but organic farming had slightly higher organic matter, mineral associated organic matter and particulate organic matter. Conventional system had more N in the mineral pools as indicated by higher NO₃⁻-N whereas organic system had higher N in the microbial biomass indicating shifts in nitrogen pools between the two systems. Bacterivore nematodes were more abundant in the organic than the conventional system for most of the study period. In contrast, the conventional system had significantly higher populations of the root lesion nematode, Pratylenchus crenatus, than the organic system after completion of the rotation cycle (transition period) in spring 2004. The organic hay plots had the lowest populations of P. crenatus compared to corn, soybeans and oats. Nematode faunal profile estimates showed that the food webs were highly enriched and moderately to highly structured and the decomposition channels were bacterial in both systems. The lack of differences in structure index between the organic and conventional systems is probably due to the excessive tillage in the organic farming system, which may have prevented the build up of tillage-sensitive omnivorous and predatory nematodes that contribute to the structure index. We conclude that transition from conventional to organic farming can increase soil microbial biomass-N and populations of beneficial bacterivore nematodes while simultaneously reducing the populations of predominant plant-parasitic nematode, P. crenatus. Our findings also underscore the potential benefits of reducing tillage for the development of a more mature soil food web.     

Soil organic matter in soil depth profiles: Distinct carbon preferences of microbial groups during carbon transformation

This study investigates how carbon sources of soil microbial communities vary with soil depth. Microbial phospholipid fatty acids (PLFA) were extracted from 0–20, 20–40 and 40–60 cm depth intervals from agricultural soils and analysed for their stable carbon isotopes (δ¹³C values). The soils had been subjected to a vegetation change from C3 (δ¹³C≈?29.3‰) to C4 plants (δ¹³C≈?12.5‰) 40 years previously, which allowed us to trace the carbon flow from plant-derived input (litter, roots, and root exudates) into microbial PLFA. While bulk soil organic matter (SOM) reflected ≈12% of the C4-derived carbon in top soil (0–20 cm) and 3% in deeper soil (40–60 cm), the PLFA had a much higher contribution of C4 carbon of about 64% in 0–20 cm and 34% in 40–60 cm. This implies a much faster turnover time of carbon in the microbial biomass compared to bulk SOM. The isotopic signature of bulk SOM and PLFA from C4 cultivated soil decreases with increasing soil depth (?23.7‰ to ?25.0‰ for bulk SOM and ?18.3‰ to ?23.3‰ for PLFA), which demonstrates decreasing influence of the isotopic signature of the new C4 vegetation with soil depth. In terms of soil microbial carbon sources this clearly shows a high percentage of C4 labelled and thus young plant carbon as microbial carbon source in topsoils. With increasing soil depth this percentage decreases and SOM is increasingly used as microbial carbon source. Among all PLFA that were associated to different microbial groups it could be observed that (a) depended on availability, Gram-negative and Gram-positive bacteria prefer plant-derived carbon as carbon source, however, (b) Gram-positive bacteria use more SOM-derived carbon sources while Gram-negative bacteria use more plant biomass. This tendency was observed in all three-depth intervals. However, our results also show that microorganisms maintain their preferred carbon sources independent on soil depth with an isotopic shift of 3–4‰ from 0–20 to 40–60 cm soil depth.     

Soil organic matter and land degradation in semi-arid area,Israel

Soil organic matter (SOM) was monitored at five research sites along a climatic transect extending from the Judean Mountains (mean annual rainfall 700 mm; annual mean temperature 17 °C) to the Dead Sea (mean annual rainfall < 100 mm; annual mean temperature 23 °C) in Israel. At four sites, representing four climatic regions, Mediterranean (site GIV), semi-arid (site MAL), mildly arid (site MIS) and arid (site KAL), four to eight soil samples were taken four times a year, in January, March, May and September, from 1992 through 1993 and 1994 and in April and August 2000. In the last 2 months soil samples were also taken from another site (MAB) in the semi-arid area. Comparison between the sites along the climatic transect shows that, except for site MAB, SOM increased significantly in both 0–2 cm and 2–10 cm, from the arid site, through the mildly arid site and the semi-arid site, to the Mediterranean site. Analysis of SOM temporal patterns of the two semi-arid sites (MAL and MAB) shows significant change from the normal SOM pattern in both the regional scale and the soil profile scale in one site (MAB). The a-normal pattern of SOM and the low soil aggregate stability at MAB indicates land degradation and it is attributed to overgrazing.     

Soil microbial community response to surfactants and herbicides in two soils

The environmental impacts of herbicides on desirable plants and the soil biota are of public concern. The surfactants that are often used with herbicides are also under scrutiny as potentially harmful to soil biological systems. To address these concerns, we used two soils, a silt loam and a silty, clay loam from south central Missouri, to investigate the impacts of herbicides and surfactants on soil microbial communities using phospholipid fatty acid (PLFA) analysis. The surfactants used in this study were alkylphenol ethoxylate plus alcohol ethoxylate (Activator 90), polyethoxylate (Agri-Dex), and a blend of ammonium sulfate, drift reduction/deposition polymers and anti-foam agent (Thrust). The herbicides were glyphosate, atrazine and bentazon. Surfactants and herbicides were applied to soils at label rate, either alone or combined, to 4000 g soil per pot. The two soils differed in history, texture, some chemical characteristics and several microbial community characteristics. A few of the chemicals altered some of the components of the microbial community after only one application of the chemical at field-rate. The Cole County, MO silt loam showed larger changes in the microbial community with application of treatments. For the Boone County, MO silty clay loam, Activator 90, Agri-Dex and bentazon treatments increased microbial biomass determined by PLFA; Thrust decreased PLFA markers, bacteria to fungi ratio; and Agri-Dex at both rates decreased monounsaturated fatty acids. Changes in the microbial community due to herbicides or surfactants were minimal in this study of a single application of these chemicals, but could be indicators of potential long-term effects. Long-term studies are needed to determine the changes in the microbial community after several years of annual applications of herbicides and surfactants on a wide array of soil types and management practices.     

Soil organic matter stabilization in turfgrass ecosystems: Importance of microbial processing

Biochemical modification of plant materials may contribute considerably to the formation and stabilization of soil organic matter, but its significance remains elusive in turfgrass systems. This study aimed to close this knowledge gap by examining the dynamics of soil organic matter in turfgrass systems as well as its stability using δ¹³C and δ¹⁵N records. Two geographic locations, each containing 3 or 4 turfgrass systems of different ages were used as the study sites because site-associated differences, in particular soil pH (alkaline versus acidic) might cause divergence in microbial processing during organic matter decomposition and resynthesis. We observed that soil C storage was ∼12% greater in the alkaline site than the acidic one. In addition, accumulation rates of soil organic C and N were about 3-fold higher in the alkaline site. Soil organic matter was physically fractionated into light and heavy fractions. Heavy fraction from the alkaline site mineralized more slowly than the acidic one, indicating that soil organic matter was more stable in the alkaline site. Furthermore, the stability of soil organic matter based upon δ¹⁵N records and C-to-N ratio of organic matter was again found to be more stable in the alkaline site than the acidic one. While both soil δ¹³C and δ¹⁵N increased as turfgrass systems aged, rates were greater in the alkaline site than the acidic one. Temporal shifts in soil δ¹³C and δ¹⁵N were attributed mainly to isotope fractionation associated with microbial processes rather than selective preservation of ¹³C- or ¹⁵N-enriched chemical compounds of plant materials. Our results suggested that microbial decomposition and resynthesis played an important role in organic matter stabilization in turfgrass systems and this microbial processing could be managed via microbial activity-regulating factors, such as soil pH.     

Response of soil organic matter fractions and composition of microbial community to long-term organic and mineral fertilization

The effects of organic and mineral fertilization on four soil organic matter (SOM) fractions (non-protected, physically protected, chemically protected, and biochemically protected) and microbial community composition were investigated by sampling soil of a 35-year-long fertilization experiment. The SOM fractions were investigated by combined physical and chemical approaches, while microbial community composition was determined by phospholipid fatty acid analysis (PLFA). Organic C (SOC) was primarily distributed within the microaggregate-protected particulate organic matter (iPOM) and the hydrolysable and non-hydrolysable silt-sized (H-Silt, NH-Silt) fractions, which accounted for 11.6–16.9, 23.4–28.9, and 25.4–30.6% of the total SOC content, respectively. The contributions of these “slow” fractions (iPOM, H-Silt, NH-Silt) to the increased SOC were 178–293, 118–209, and 85–109% higher after long-term sole manure or manure in combination with inorganic N fertilization compared with unfertilized soil (control). The combination of manure and mineral fertilizers increased the coarse and fine non-protected C (cPOM and fPOM) contents much more (34.1–60.7%) than did manure alone. PLFAs, bacteria, G (+) bacteria, and actinomycete abundances were the highest in soil with manure, followed by soil treated with manure combined with mineral N. The addition of inorganic and organic fertilization both altered the microbial community composition compared with the control. All SOM fractions contributed to 81.1% of the variance of the PLFAs-related microbial community composition by direct and indirect effects. The change in coarse unprotected particulate organic matter (cPOM) was the major factor affecting soil microbial community composition (p < 0.001). Our study indicates that physical, chemical, and biochemical protection mechanisms are important in maintaining high SOC level after the addition of manure. A close linkage between soil microbial community composition and cPOM suggests that C availability is an important factor for influencing microbial composition after long-term inorganic and organic fertilization.     

Soil organic matter, microbial biomass and enzyme activities in a tropical agroforestry system

 The effects of growing trees in combination with field crops on soil organic matter, microbial biomass C, basal respiration and dehydrogenase and alkaline phosphatase activities were studied in soils under a 12-year-old Dalbergia sissoo (a N₂-fixing tree) plantation intercropped with a wheat (Triticum aestivum) – cowpea (Vigna sinensis) cropping sequence. The inputs of organic matter through D. sissoo leaf litter increased and crop roots decreased with the increase in tree density. Higher organic C and total N, microbial biomass C, basal soil respiration and activities of dehydrogenase and alkaline phosphatase were observed in treatments with tree-crop combination than in the treatment without trees. Soil organic matter, microbial biomass C and soil enzyme activities increased with the decrease in the spacing of the D. sissoo plantation. The results indicate that adoption of the agroforestry practices led to an improved organic matter status of the soil, which is also reflected in the increased nutrient pool and microbial activities necessary for long-term productivity of the soil. However, tree spacing should be properly maintained to minimize the effects of shading on the intercrops. Received: 21 February 1997     

Effects of woody debris, microtopography, and organic matter amendments on the biotic community of constructed depressional wetlands

To increase wetland acreage and biodiversity, Delaware agencies constructed >220 depressional wetlands. During construction, agencies included amendments thought to increase biodiversity. Because the efficacy of amendments is unknown, we investigated their effects on macroinvertebrate and vegetative communities. We selected 20 standardized wetlands (five contained coarse woody debris (CWD) and microtopography amendments (land surface ridges and furrows), five had neither, five had CWD only, and five had microtopography only). Additionally, 12 wetlands had received organic matter amendments (i.e., straw). Insect richness (P = 0.010; r² = 0.16), insect biomass (P = 0.023; r² = 0.13), intolerant insect biomass (P = 0.033, r² = 0.03), Ephemeroptera biomass (P = 0.027; r² = 0.12), and Odonata biomass (P = 0.046; r² = 0.10) increased with CWD volume. Obligate plant percent cover increased with microtopographic variation (P = 0.029; r² = 0.120). Although organic matter amendments did not increase percent soil organic matter (t_13.7 = −1.16, P = 0.264), total (P = 0.027; r² = 0.12), native (P = 0.036; r² = 0.11), and facultative (P = 0.001; r² = 0.24) plant richness increased with percent soil organic matter. To enhance biodiversity, constructed wetlands should contain CWD, but additional research is needed to understand the benefits of microtopography and organic matter amendments.     

Soil microbial community patterns related to the history and intensity of grazing in sub-montane ecosystems

Long-term variations in the frequency and intensity of sheep (Ovis aries) grazing have led to the development of ubiquitous plant successional transitions in sub-montane regions of the UK. In this study, we measured a range of soil microbial properties across these successional transitions in three biogeographic regions of the UK, to establish how gradients of grazing-influence (in terms of the history and intensity of sheep grazing) alter the biomass, activity, and structure of soil microbial communities. We also measured soil physicochemical variables to relate changes in soil microbial community arrangement along these grazing-related successional transitions to key soil properties. Our results from three locations show that microbial communities of soils display some consistent and ‘broad-scale’ trends along successional transitions that are related to the history and intensity of grazing. We show that microbial biomass of soil is maximal at low-to-intermediate levels of grazing influence and that the phenotypic evenness (a component of diversity) of the microbial community declines as the intensity of grazing increases. We also provide evidence that soil microbial communities of heavily grazed sites are dominated by bacterial-based energy channels of decomposition, whereas in systems that are less intensively grazed, or completely unmanaged, fungi have a proportionally greater role. Further studies are needed to establish the significance of these changes in relation to soil-level ecosystem processes of decomposition and nutrient cycling. The data show that human disturbances can have profound effects on the biomass and structure of the soil communities that regulate soil processes in these ecosystems and that these effects are consistent across sites.     

Soil microbial biomass, dehydrogenase activity, bacterial community structure in response to long-term fertilizer management

This study describes the effects of balanced versus nutrient-deficiency fertilization on soil microbial biomass, activity, and bacterial community structure in a long-term (16 years) field experiment. Long-term fertilization greatly increased soil microbial biomass C and dehydrogenase activity, except that the P-deficiency fertilization had no significant effect. Organic manure had a significantly greater (P<0.05) impact on the biomass C and the activity, compared with mineral fertilizers. Microbial metabolic activity (dehydrogenase activity per microbial biomass C) was significantly higher (P<0.05) under balanced fertilization than under nutrient-deficiency fertilization. General bacterial community structure was analyzed by PCR-denaturing gradient gel electrophoresis (DGGE) targeting eubacterial 16S rRNA gene. Mineral fertilization did not affect the DGGE banding pattern, while specific DGGE band was observed in organic manure-fertilized soils. Phylogenetic analysis showed that the change of bacterial community in organic manure-fertilized soil might not be because of the direct influence of the bacteria in the compost, but because of the promoting effect of the compost on the growth of an indigenous Bacillus sp. in the soil. We emphasize the importance of balanced-fertilization, as well as the role of P, in maintaining soil organic matter, and promoting the biomass and activity of microorganisms.     

Soil organic matter and the extracellular microbial matrix show contrasting responses to C and N availability

An emerging paradigm in soil science suggests microbes can perform ‘N mining’ from recalcitrant soil organic matter (SOM) in conditions of low N availability. However, this requires the production of extracellular structures rich in N (including enzymes and structural components) and thus defies stoichiometric expectation. We set out to extract newly synthesised peptides from the extracellular matrix in soil and compare the amino acid (AA) profiles, N incorporation and AA dynamics in response to labile inputs of contrasting C/N ratio. Glycerol was added both with and without an inorganic source of N (10% ¹⁵N labelled NH₄NO₃) to a soil already containing a large pool of refractory SOM and incubated for 10 days. The resulting total soil peptide (TSP) and extracellular pools were compared using colorimetric methods, gas chromatography, and isotope ratio mass spectrometry. N isotope compositions showed that the extracellular polymeric substance (EPS) contained a greater proportion of products formed de novo than did TSP, with hydrophobic EPS-AAs (leucine, isoleucine, phenylalanine, hydroxyproline and tyrosine) deriving substantially more N from the inorganic source provided. Quantitative comparison between extracts showed that the EPS contained greater relative proportions of alanine, glycine, proline, phenylalanine and tyrosine. The greatest increases in EPS-peptide and EPS-polysaccharide concentrations occurred at the highest C/N ratios. All EPS-AAs responded similarly to treatment whereas the responses of TSP were more complex. The results suggest that extracellular investment of N (as EPS peptides) is a microbial survival mechanism in conditions of low N/high C which, from an evolutionary perspective, must ultimately lead to the tendency for increased N returns to the microbial biomass. A conceptual model is proposed that describes the dynamics of the extracellular matrix in response to the C/N ratio of labile inputs.     

From the litter layer to the saprolite: Chemical changes in water-soluble soil organic matter and their correlation to microbial community composition

Organic matter content and chemistry is vital to the structure and function of soil systems, but while organic matter is recognized as biogeochemically important, its chemical interaction with soil processes is not well understood. In this study we used fluorescence spectroscopy, which has been used extensively for understanding the role of organic matter in aquatic systems, to identify chemical changes in organic matter with depth in a soil system. Soil was collected from nine different pits in a first-order montane catchment in the Colorado Front Range. The water-soluble soil organic matter was extracted from each sample and fluorescence and UV–vis spectroscopy was used to analyze its chemical character. While organic matter chemistry had little correlation with landscape location and local vegetation, there were noticeable consistent trends between soil horizon and organic matter chemistry in each pit. Total organic matter decreased with depth and became less aromatic with increasing depth. This less aromatic material in the saprolite also had a greater microbial signature. The redox character of the organic matter accompanied this change in source and molecular structure, with more oxidized character corresponding with organic matter with more microbial input and more reduced character corresponding to organic matter with more plant input. A concurrent investigation of the microbial population of the same soil samples also showed microbial population composition varying more with soil depth than landscape position, and depth changes in microbial diversity occurred concomitantly with depth changes in organic matter chemistry.     

The contribution of soil organic matter fractions to carbon and nitrogen mineralization and microbial community size and structure

The aims of this study were to: (i) assess the impact of hay and fertilizer application on organic matter (OM) fractions (dissolved organic matter (DOM), light fraction organic matter (LFOM, <1.0 g cm⁻³), heavy fraction OM (HFOM, <1.7 g cm⁻³)), carbon (C) and nitrogen (N) cycling processes and microbial community size and structure, and (ii) quantify the role of OM fractions to C and N cycling. Soil was collected in 2001 from a field experiment to which grass hay (1996) and/or fertilizer (1995 and 1999) had previously been applied. DOM-C (P<0.05) and DOM-N (P=0.07) were significantly higher in control and fertilized soil than hay and hay+fertilized soil. LFOM and HFOM C and N contents and C/N ratios were significantly (P<0.05) higher in hay+fertilized and hay amended soil than in control and fertilized soil. Potentially mineralizable-N (PMN), microbial biomass-C (MB-C), microbial biomass-N (MB-N) and microbial respiration (CO₂) were not affected by fertilizer and/or hay application. Gross N mineralization (Gross Min) and gross nitrification (Gross Nit) rates were significantly (P<0.05) higher in fertilized, hay, hay+fertilized soil than control soil. However, there was no significant difference between treatments in gross N immobilization rates. Results reported here highlight the importance of a labile fraction of the DOM pool to N and C cycling as its removal significantly (P<0.05) reduced PMN, MB-N, Gross Min and Gross Nit compared with whole soil in most or all treatments. In soil where DOM+LFOM were removed PMN was significantly (P<0.05) lower, but MB-C, Gross Min and Gross Nit was significantly (P<0.05) higher than in DOM removed soil. This suggests that LFOM plays an important role as a sink for mineral-N. Total soil phospholipid fatty acid (PLFA) concentration was significantly (P<0.05) higher in hay amended than control, fertilized and hay+fertilized soil. Principal components analysis was able to clearly discriminate between control, fertilized, hay+fertilized and hay amended soil. Soil amended with hay or fertilizer had a microbial community structure which differed from that of the control or hay+fertilized soils. Redundancy analysis with Monte Carlo permutation tests revealed that PLFA profiles were strongly correlated to differences in Gross Min, Gross Nit, MB-N, MB-C, MB-C/N ratio, total soil C and total soil C/N ratio. The results of this research suggest that changes in microbial structure are related to aspects of soil C and N pools and cycling.     

Soil microbial community response to variation in vegetation and abiotic environment in a temperate old-growth forest

Changes of soil microbial community caused by the heterogeneity of abiotic and biotic environment were studied in the reserve Dobroč, Slovakia. Data on vegetation, microclimate, soil properties and microbial activity were collected on two linear transects crossing both the core of the reserve and the buffer zone.In contrast to expectations, the variation of most environmental variables was comparable or even higher in the buffer zone than in the old-growth forest. Beta diversity was much higher in the natural forest, which coincided with differentiation patterns of trees and understory plants.Mantel correlations between microbial community indicators and environmental variables showed that soil chemistry and vegetation diversity were the most important determinants of microbial activity. Redundance analysis of microbial data identified potassium content, plant richness and influence of fir as the drivers of functional group composition. Inconsistency of correlations of microbial community characteristics with environmental variables indicates that different processes associated with the demography and functions of microbiota are driven by different environmental factors. Amount and variety of substrates available for decomposition seems to influence microbial community more than microclimate.     

Soil organic matter: Distribution,genesis, and management to reduce greenhouse gas emissions

In this paper we describe the accumulation of soil organic matter (SOM) during pedogenesis and the processes that can lead to the emission of greenhouse gases (CO₂, CH₄, N₂O) to the atmosphere via SOM decomposition and denitrification. We discuss the role of management on SOM accumulation and loss, and the potential for controlling emission or comsumption of greenhouse gases by soils. We conclude that under current climate conditions there are global scale opportunities to reduce greenhouse gas emissions from soils and increase the indirect sequestration of greenhouse gases in soils through improved soil management.     

Soil organic matter decomposition driven by microbial growth: A simple model for a complex network of interactions

Priming effects are expressions of complex interactions within soil microbial communities. Thus, we aimed at building a microbial population growth model which could deal with different substrates, resources and populations. Our model divides the decomposition/growth process at the population level in two stages, mimicking mechanisms taking place at molecular and cellular scales: (1) the first stage is a reversible process whereby microbial biomass capture their substrate to form a complex within definite proportions; (2) the second stage is the irreversible rate-limiting utilization of substrate per se. It is supposed to be a first order process with respect to the quantity of complex. We put these assumptions into equations using an analogy with chemical reactions at equilibrium. We show that this model (1) provides a mathematical formalism that bridges the gap between first order decay of substrates and Monod kinetics; (2) sets constraints on the possible combinations of microbial functional traits, yielding microbial strategies in agreement with observations; (3) allows to model both positive and negative priming effects, and more generally complex interactions between the various components of a soil system. This model is designed to be used as a kernel in any soil organic matter model.     

Urea fertilisation affected soil organic matter dynamics and microbial community structure in pasture soils of Southern Ecuador

In the mountain rainforest region of the South Ecuadorian Andes natural forests have often been converted to pastures by slash-and-burn practice. With advanced pasture age the pasture grasses are increasingly replaced by the tropical bracken leading to the abandonment of the sites. To improve pasture productivity a fertilisation experiment with urea was established. The effects of urea on soil organic matter (SOM) mineralisation and microbial community structure in top soil (0–5 cm depth) of an active and abandoned pasture site have been investigated in laboratory incubation experiments. Either ¹⁴C- or ¹⁵N-labelled urea (74 mg urea-N kg⁻¹ dw soil) was added to track the fate of ¹⁴C into CO₂ or microbial biomass and that of ¹⁵N into the KCl-extractable NH₄-N or NO₃-N or microbial biomass pool. The soil microbial community structure was assessed using phospholipid fatty acid analysis (PLFA). In a second experiment two levels of ¹⁴C-labelled urea (74 and 110 mg urea-N kg⁻¹ dw soil) were added to soil from 5 to 10 cm depth of the respective sites. Urea fertilisation accelerated the mineralisation of SOC directly after addition up to 17% compared to the non-fertilised control after 14 days of incubation. The larger the amount of N potentially available per unit of microbial biomass N the larger was the positive priming effect. Since in average 80% of the urea-C had been mineralised already 1 day after amendment, the priming effect was strong enough to cause a net loss of soil C. Although the structure of the microbial community was significantly different between sites, urea fertilisation induced the same alteration in microbial community composition: towards a relative lower abundance of PLFA marker characteristic of Gram-positive bacteria and a higher one of those typical of Gram-negative bacteria and fungi. This change was positively correlated with the increase in NH₄, NO₃ and DON availability. In addition to the activation of different microbial groups the abolishment of energy limitation of the microbes seemed to be an important mechanism for the enhanced mineralisation of SOM.     

油蒿群落土壤碳酸钙和脱氢酶对土壤有机质的响应

Little attention has been paid to the role of soil organic matter (OM) in the formation of pedogenic carbonate in desert soils. The relationships among soil OM, soil dehydrogenase activity (DHA), and soil CaCO₃ in a plant community dominated by Artemisia ordosica, located on the eastern boundary of Tengger Desert in the Alxa League, Inner Mongolia, China, were studied to understand whether OM was directly involved in the formation of pedogenic carbonate. The results showed that DHA and CaCO₃ positively correlated with OM content, and DHA, OM, and CaCO₃ were correlated with each other in their spatial distribution, indicating that abundant OM content contributed to the formation of CaCO₃. Therefore, the formation of pedogenic CaCO₃ was a biotic process in the plant community dominated by A. ordosica.     

Properties of dissolved organic matter related to soil organic matter quality and nitrogen additions in Norway spruce forest floors

The quality of dissolved organic matter (DOM) is highly variable and little information is available on the relation of DOM quality to the structure and composition of its parent soil organic matter (SOM). The effect of increasing N inputs to forest soils on the structure and composition of both SOM and DOM also remains largely unclear. Here we studied the release of DOM, its specific UV absorption and two humification indices (HIX) derived from fluorescence spectra from Oa material of 15 North- and Central-European Norway spruce (Picea abies (L.) Karst.) stands. The Oa material was incubated aerobically at 15 °C and water holding capacity over a period of 10 months and extracted monthly with an artificial throughfall solution. Soil respiration was determined weekly. The influence of mineral N inputs on composition of DOM and on respiration rates was investigated on periodically NH₄NO₃-treated Oa samples of eight selected sites. Release of dissolved organic carbon (DOC) from untreated Oa material samples ranged from 0.0 to 58.6 μg C day⁻¹ g C⁻¹ and increased with increasing C-to-N ratio. One HIX and UV absorption of DOM were negatively correlated to the degree of oxidation of lignin-derived compounds and positively to the C-to-N ratio and – HIX only – to the aromatic C content of SOM. Mineral N addition had no distinct effect on respiration rates. In six of eight samples the N-treatment caused an increase in specific UV absorption or one HIX of DOM. However, these effects were not statistically significant. Addition of mineral N did not affect the rates of DOM release. Our results show that properties of SOM largely determine the amount and quality of DOM in forest floors. Changes of DOM quality due to mineral N additions are likely, but we cannot confirm significant changes of DOM release.