Bent Christensen

Turnover of organic matter in differently textured soils: I. Physical characteristics of structurally disturbed and intact soils

Abstract Soil type effects on organic matter turnover are most often ascribed directly to differences in soil clay content. Since soil texture determines the physical characteristics of soil, aggregation and water holding capacity may be more relevant to address in the search for controls of organic matter turnover. Most studies of microbial processes in soils are based on structurally disturbed soil, where the abiotic conditions for the microbial activity may be quite different from those in intact soils. In this study, basic physical characteristics were determined for structurally disturbed and intact soil samples from differently textured soils. Bulk soil was retrieved from 0–20 cm depth at six locations along a textural gradient in an arable field on Weichselian morainic deposits in Denmark. The samples (NA1 to NA6) ranged in clay from 11 to 45% and in silt from 7 to 15%. Clay and silt-sized organomineral complexes were isolated from NA2 soil by ultrasonic dispersion and sedimentation in water. The clay and silt fractions were added individually and in varying proportions to NA1 soil, providing three clay-amended (CL2, CL4 and CL6) and three silt-amended (SI2, SI4 and SI6) soils. All 12 soils were crushed in air dry state to 100 μm). Air diffusivity and permeability measurements showed disturbed soils to have a less continuous and more tortuous pore system than undisturbed reference samples. Water-filled pore space at a critical level of air diffusion potential was significantly higher for undisturbed than for disturbed samples, especially in soils high in clay. Drop cone measurements showed disturbed soils to be structurally weaker than undisturbed ones. Intact and structurally disturbed soils were found to differ significantly in physical properties even after 17 months of soil structure regeneration. Water-filled pore space seems to reflect the potential of available water and aeration status to regulate aerobic microbial activity of structurally disturbed soil, but not of intact field soil.

SIMPLICITY-CORRELATED SIZE GROWTH OF THE NUCLEAR 28S RIBOSOMAL RNA D3 EXPANSION SEGMENT IN THE CRUSTACEAN ORDER ISOPODA

The expansion segments within the eukaryote nuclear 23S-like ribosomal RNA molecule are now well characterized in many diverse organisms. A different base compositional bias, a higher propensity for size variability, and an increased evolutionary rate distinguish these regions from the universally conserved “core” regions of the molecule. In addition, some expansion segments of higher eukaryotes exhibit significant sequence simplicity which is hypothesized to occur by slippage-mediated mutational processes. We describe the discovery of extreme size variation of the D3 expansion segment in the crustacean order Isopoda. Among 11 species D3 varies in size from 180 to 518 nucleotides but maintains a homologous secondary structure. The D3 size is significantly positively correlated to relative simplicity factor (RSF), indicating that growth is most likely by insertion of simple sequences. D3 size and RSF correlate approximately with a morphology-based phylogeny, and within oniscideans RSF increases as more recent divergences occur. The D3 ofArmadillidium vulgare, with an RSF of 1.87, is the highest value recorded for any known expansion segment. Regions of high sequence simplicity in nuclear ribosomal RNA were previously only known from the higher vertebrate lineage. Here we demonstrate that this phenomenon occurs in a more extreme condition within a monophyletic invertebrate lineage. The extreme size changes identified could indicate that expansion segments are an extraneous element in the functioning ribosome.

Decomposability of organic matter in particle size fractions from field soils with straw incorporation

Abstract Soils from two field experiments on straw disposal were fractionated according to particle size using ultrasonic dispersion and gravity-sedimentation in water. Samples of whole soils, clay. silt and sand-size fractions were held for 49 days at 20°C and the CO2 evolution measured on 14 dates by gas chromatography. Recovery of soil solids. C and N was 99, 98 and 93%, respectively. Most of the soil C and N was in the clay ( The decomposition rate constants were higher for the sandy loam than for the loamy sand soil. For both soils, the decomposability of the organic matter decreased in the order: sand > clay ⩾ whole soil > silt. Straw incorporation increased the decomposition rate of whole soil and sand organic matter. whereas the effect of straw on clay and silt respiration was small. Between 58 and 73% of the respiration was from clay, 21–25% from silt and 6–19% from the sand size fraction.

Barley straw decomposition under field conditions: Effect of placement and initial nitrogen content on weight loss and nitrogen dynamics

Abstract The effect of initial straw N content (0.41, 0.57 and 0.92% N) and straw placement (either above the soil surface, on the soil surface, or buried at 5, 10 or 15 cm depth) on barley straw decomposition (weight loss and N content) under field conditions was examined in a coarse sand soil and in a sandy loam soil. The straw, which was enclosed in mesh bags, was exposed on the 20 September and sampled 8 times during the succeeding 14 months. Soil moisture contents were similar at 5,10 and 15 cm; this was also true for moisture contents of straw samples recovered from these depths, whereas straw exposed above or on the soil surface was much drier on the April–July sampling dates. The differences in soil temperatures observed between soil depths did not affect weight losses significantly. During the first month after burial, straw showed an average weight loss of 35%. Subsequent weight loss patterns were similar for straw buried at 5, 10 and 15 cm in the sandy loam. 50% of the straw weight was lost by late spring, and after 1 yr the loss was 80–90%. Weight loss rates were 0.07% day −1 during November–May and 0.22% day −1 during May–December. Straw exposed above the soil surface lost 13% of its weight during the first month and subsequently 0.09% day −1 . Straw exposed on the soil surface showed a somewhat irregular weight loss pattern. Straw with 0.92% N showed a higher first-month weight loss (44%) than straw with 0.41 and 0.57% N (32%), but subsequent weight loss patterns were essentially similar. Seasonal differences in weight loss rates were smaller in the coarse sand than in the sandy loam. Average weight loss rate for straw in the coarse sand was 0.16% day −1 during the January–November period. N was initially leached from straw buried in the coarse sand and from straw placed above or on the soil surface. Leaching losses of N increased with initial straw N content and ranged from 7 to 40%. Subsequent changes in the N content of straw could not be directly related to initial straw N content. Straw buried in the sandy loam immobilized N from burial until spring; changes in N contents followed a similar pattern. At maximum straw N immobilization, N contents had increased by 35 to 47%. Straw then released N at a relatively constant rate and, after 14 months, N losses corresponded to 47–60% of the initial straw N content. In the sandy loam, maximum N immobilization was calculated to be 9–11 kg Nha −1 for a straw application of 5t ha −1 .

Turnover of organic matter in differently textured soils. II. Microbial activity as influenced by soil water regimes

To evaluate the effect of soil texture and soil water content on decomposition of organic carbon (OC), turnover of partially stabilized 14C-labelled ryegrass residues was studied at four matric potentials in twelve differently textured soils of similar origin and cropping history. Six soils were from a naturally occurring clay gradient and had 11, 16, 21, 31, 37 and 45% clay (termed NA1 to NA6). Three clay-amended soils (CL2, CL4, CL6) and three silt-amended soils (SI2, SI4, SI6) were prepared by adding clay or silt sized organomineral complexes extracted from the NA2 soil to a portion of the NA1 soil. After 14C-labelled ryegrass had decomposed for eight months under field-like conditions, soil cores were sampled, adjusted to four matric potentials (-30, -100, -500 and -1500 hPa) and incubated at 20°C for 15 weeks. The content of native soil organic carbon (SOC) in the NA soils was not related to texture. The SOC content increased with clay and silt in the CL and SI soils because of OC contained in the applied size separates. The relative CO2-evolution from CL and SI soils was lower than from the texturally corresponding NA soils, indicating a slower turnover of C supplied with the clay separate than of bulk OC. Differences in the decomposability of native SOC and residues of 14C-ryegrass were better explained by soil moisture parameters than by soil textural composition. Within each set of soils, evolution of CO2 from native SOC was highly correlated with the volumetric water content. The same was true for 14CO2-evolution, but correlations were significantly improved when 14CO2 was related to water retained in soil pores with diameters >0.2 m. This indicated that the water available for the turnover of residues from ryegrass and of native SOC was retained in different fractions of the pore volume. Our study suggested that water was the main factor in controlling turnover of SOC. Texture effects were indirect and expressed through soil structure which in turn defined the soil pore system and thus the ability of the soils to retain water of different availability to the decomposer organisms.

Wheat and barley straw decomposition under field conditions: Effect of soil type and plant cover on weight loss, nitrogen and potassium content

Abstract The decomposition of wheat and barley straw buried in a coarse sand and a sandy loam soil was followed under field conditions using chopped straw enclosed in mesh bags. The straw was buried at 10 cm depth in early autumn. Plots were either kept fallow or covered in plants during the summer and autumn following straw burial. Samples were recovered on 10 dates over 15 months and were analyzed for weight loss, C, N and K content. Results obtained were corrected for organic matter and nutrients introduced to the straw by soil entering the mesh bags. Soil temperatures and soil moisture contents were determined. The plant cover reduced soil temperatures, soil moisture contents and straw weight loss rates during the summer. The overall weight loss pattern was similar for the two straw types. In both soils, the straw showed an initial weight loss of 30% during the first month after burial. After 6 months (spring), the straw had lost about 50% of its weight. The weight loss then proceeded faster in fallow than in the planted plots. Seasonal changes in weight loss rates were relatively small. The absolute weight loss rates, determined by a simple linear regression model, ranged from 0.10% day−1in planted plots of the coarse sand to 0.17% day−1 in uncovered plots of the sandy loam. For planted plots, weight losses after 15 months of exposure were 67% in the coarse sand and 78% in the sandy loam. Corresponding values for fallowed plots were 75 and 92%, respectively. Initial straw N contents differed, but overall N dynamics was less influenced by straw type than by soil type. In the sandy loam, net immobilization began just after straw burial and reached a maximum in the spring, where 3 mg N g−1 initial straw was immobilized. The N release was more rapid in fallowed than in planted plots. In the coarse sand, N was initially leached from the straw, and the immobilization of N was less extensive than in the sandy loam. Further, the release of N from the straw was grossly similar in fallowed and planted plots. For all treatments, the net release of N from the straw began when the C-to-N ratio was between 28 and 35. Most of the straw K content was leached during the first month after straw burial. Subsequent straw K content was not affected by initial K content or plant cover.

Calibration and validation of the soil organic matter dynamics of the Daisy model with data from the Askov long-term experiments

Abstract Daisy is a semi-mechanistic model that simulates crop production, soil water and C and N dynamics in agro-ecosystems. In this study, the soil organic matter (SOM) submodel of Daisy was calibrated and validated with data from a series of long-term (30–100xa0yr) experiments dominated by cereal cropping systems. The experiments were conducted under temperate conditions at the Askov Experimental Station, Denmark. The recalibration of the Daisy model improved the model simulations considerably. The RMSE between simulations and observations of the experiments used for calibration improved from 33 to 19, whereas this measure improved from 32 to 9.0 for the experiments used for validation. The original calibration of the model appears to be based upon too low an input of organic matter (especially because rhizodeposition is ignored), too low an efficiency with which added organic matter is converted into more stable forms of SOM, and too low an overall decomposition rate. Our study suggests that the fraction of SOM involved in medium-term turnover is substantially larger than previously thought and inferred by most SOM turnover models. This may warrant a recalibration of current simulation models, and stresses the importance of using long-term experiments with widely differing treatments for model evaluation. Long-term treatments with widely contrasting initial C contents and annual C inputs, especially bare fallow treatments, appear to be valuable resources in this respect.

C and N turnover in structurally intact soils of different texture

The turnover of native and applied C and N in undisturbed soil samples of different texture but similar mineralogical composition, origin and cropping history was evaluated at –10 kPa water potential. Cores of structurally intact soil with 108, 224 and 337 g clay kg-1 were horizontially sliced and 15N-labelled sheep faeces was placed between the two halves of the intact core. The cores together with unamended treatments were incubated in the dark at 20°C and the evolution of CO2-C determined continuously for 177 d. Inorganic and microbial biomass N and 15N were determined periodically. Net nitrification was less in soil amended with faeces compared with unamended soil. When adjusted for the NO3-N present in soil before faeces was applied, net nitrification became negative indicating that NO3-N had been immobilized or denitrified. The soil most rich in clay nitrified least N and 15N. The amounts of N retained in the microbial biomass in unamended soils increased with clay content. A maximum of 13% of the faeces 15N was recovered in the microbial biomass in the amended soils. CO2-C evolution increased with clay content in amended and unamended soils. CO2-C evolution from the most sandy soil was reduced due to a low content of potentially mineralizable native soil C whereas the rate constant of C mineralization rate peaked in this soil. When the pool of potentially mineralizable native soil C was assumed proportional to volumetric water content, the three soils contained similar proportions of potentially mineralizable native soil C but the rate constant of C mineralization remained highest in the soil with least clay. Thus although a similar availability of water in the three soils was ensured by their identical matric potential, the actual volume of water seemed to determine the proportion of total C that was potentially mineralizable. The proportion of mineralizable C in the faeces was similar in the three soils (70% of total C), again with a higher rate constant of C mineralization in the soil with least clay. It is hypothesized that the pool of potentially mineralizable C and C rate constants fluctuate with the water content and in the long term result in similar total C content irrespective of texture.

Phospholipid fatty acid profiles and C availability in wet-stable macro-aggregates from conventionally and organically farmed soils

Abstract Whole soil samples and four aggregate size classes (2–8 mm, 1–2 mm, 0.5–1 mm and 0.25–0.5 mm) from organically or conventionally farmed sandy loam soils were compared with respect to texture, C content and C mineralization potential, microbial biomass C and phospholipid fatty acid (PLFA) composition. The PLFA concentration of organically farmed soils (44–56 nmol g−1 dry wt.) was larger than in soils under conventional management (28–32 nmol g−1 dry wt.) and correlated with biomass C. Principal component analyses demonstrated only minor differences between whole soil samples with respect to PLFA composition. The texture of soil fractions obtained by wet-sieving deviated strongly from the texture of whole soil, paticularly in the 0.25–0.5 mm and 0.5–1 mm size classes. These fractions also appeared to include some non-aggregate particulate organic matter. The C mineralization during a 13-week incubation increased significantly with decreasing aggregate size class in four of the six soils. Biomass C declined during the incubation, and the decline in most cases could account for the C mineralized. No consistent differences were observed between conventionally and organically farmed soils or between aggregate size classes with respect to taxonomic composition or physiological status of the microbial community.

Decomposability of barley straw: Effect of cold-water extraction on dry weight and nutrient content

Abstract Mature spring barley ( Hordeum vulgare L.) straw, cut to a length of 5 cm, was successively extracted with cold water. The influence of leaching on dry weight, nutrient content, and decomposability was investigated in fresh and in partially-decomposed straw. The loss in dry weight was 6% after one extraction, increasing to 9% after three successive extractions. Incubating the straw between extractions increased losses of dry matter and N, whereas losses of ash, K, P, Mg and Ca were not affected by the intervening incubations. Three successive extractions removed 58% of the ash, 87% of the K, 59% of the P, 34% of the Mg and 25% of the Ca. The loss of N increased from 20% without intermittent incubations to 43% for straw samples incubated at 15°C between the extractions (total incubation period, 66 days). The respiration losses of untreated straw samples incubated for 203 days at 5 and 15°C were 9 and 23%, respectively. Extracting the straw before incubation reduced initial respiration losses. After 203 days, the cumulative respiration loss of leached straw incubated at 15°C was 11% of initial straw dry weight, whereas the loss at 5°C was similar to that of untreated straw.