Earthworm density and biomass in relation to plant diversity and soil properties in a Palouse prairie remnant

Earthworms are important soil animals in grassland ecosystems and are considered to be important to soil quality. The overall impact of earthworms on soil properties and plant diversity, however, depends on earthworm species, functional group and the type of ecosystem. The primary purpose of this study was to document the relationship among earthworms, key soil properties and native and exotic plant diversity in the little studied, Palouse prairie grassland (Idaho, USA). A secondary objective was to determine the effectiveness of three methods commonly used to sample earthworms. A hillslope characterized by Palouse prairie vegetation, well-expressed, hummocky (mounded) topography and known to support both exotic and native earthworm species was selected for study. The hillslope was divided into three zones [annual-dominated (AD), mixed (MX) and perennial-dominated (PD)] based on characteristics of the inter-mound plant communities described in previous research. Total earthworm biomass in the MX zone (53.5 g m⁻²) was significantly greater than in the PD zone (14.7 g m⁻²) (P = 0.0384), but did not differ from the AD zone. Earthworm density ranged from 52 to 81.1 individuals m⁻² but was not significantly different across zones. Total C and N at 0 to 10 and 30 to 50 cm depths were significantly greater in the AD and PD zones as compared to the same depths in the MX zone. Soil textural class was silt loam within all zones and the soil silt fraction was positively correlated with total exotic earthworm density (R = 0.783, P = 0.0125) and biomass (R = 0.816, P = 0.0072). Native earthworms were only found in the zone with the greatest total and native plant diversity (PD). Total soil C and N were not correlated to earthworm density, but soil total C and N were significantly negatively correlated with exotic plant density, which indicates that invasive plants may be decreasing soil total C (R = −0.800) and N (R = −0.800). Calculated earthworm densities using data from the electroshocker were generally lower than those based on the hand-sorting method. Electroshocking, however, created lower disturbance and was the only method that resulted in the collection of the deep-burrowing, native species Driloleirus americanus.     

Effects of bioenergy crop cultivation on earthworm communities—A comparative study of perennial (Miscanthus) and annual crops with consideration of graded land-use intensity

Energy crops are of growing importance in agriculture worldwide. This field study aimed to investigate earthworm communities of different intensively cultivated soils during a 2-year period, with special emphasis on annual and perennial energy crops like rapeseed, maize, and Miscanthus. These were compared with cereals, grassland, and fallow sites. Distribution patterns of earthworm abundance, species, and ecological categories were analysed by constrained ordination procedures (redundancy analysis; CANOCO) using a set of environmental variables as predictors, such as CN value of harvest residues, SOC and N_t content, soil pH, soil texture, and land-use intensity. The latter was determined by principal component analysis using average soil coverage and intensity of tillage, weed control, and fertilisation as input variables. It was clearly found that land-use intensity was the dominant regressor for earthworm abundance and total number of species. The diversity of earthworm communities was especially enhanced and showed a more balanced species composition in extensively managed soils under grassland, fallow, and Miscanthus. For the total number of species, Miscanthus (5.1 ± 0.9) took a medium position and neither differed significantly from intensively managed rapeseed (4.0 ± 0.9), cereals (3.7 ± 1.1), and maize sites (3.0 ± 1.4), nor from grassland (6.8 ± 1.5) and fallow (6.4 ± 1.0) sites. Total earthworm abundance ranged between 355 (±132) and 62 (±49) individuals m⁻² in fallow and maize sites, respectively.Interestingly, Miscanthus had quite positive effects on earthworm communities although the CN value of harvest residues was very high. It is recommended that Miscanthus may facilitate a diverse earthworm community even in intensive agricultural landscapes.     

Identifying earthworm's organic matter signatures by near infrared spectroscopy in different land-use systems in Tabasco,Mexico

In the state of Tabasco, South-eastern, Mexico, land-use changes such as the conversion of natural into agricultural systems, modify soil quality and the abundance of soil macrofauna, including earthworms. The aim of this study was to characterize by near-infrared spectroscopy (NIRS) the earthworms’ fingerprint in soil, in six sites including natural and agricultural ecosystems with low and high earthworm biomass and low and high earthworm diversity, in order to identify specific wavelengths that discriminate the presence/abundance of earthworm species and functional groups. The spectral region of 1860–1870 nm was significantly correlated with total earthworm density, particularly at one of the sites (Cedar polyculture; r = 0.8, p < 0.05). Earthworm biomass had a specific NIRS wavelength according to the earthworm species and feeding category: 1820 and1860–1870 nm wavelengths were significantly correlated with Polypheretima elongata (r² = 0.7, p < 0.05; mesohumic species) biomass and 2090 nm for biomass of all Lavellodrilus species (polyhumics). Two species had a much wider spectral range: L. bonampakensis and Dichogaster saliens (an epigeic worm; 1690–2300 nm, r² = 0.7, p < 0.05). Biomasses of Periscolex brachysistis and Diplotrema murchiei were not significantly correlated with any near infrared wavelength spectra analyzed. Combining a maximum of 4 species per wavelength, mesohumic earthworms had a wider wavelength spectrum than polyhumics. Therefore, earthworm species diversity, biomass and abundance are associated with soil quality (as measured by NIR spectra) and this relationship varies with species and ecological category. Sites with lower and higher earthworm diversity have lower and higher soil organic matter quality, respectively, as observed by the wider or narrower spectral range with which earthworm biomasses are correlated.     

Effect of tillage on earthworms over short- and medium-term in conventional and organic farming

Earthworms play an important role in many soil functions and are affected by soil tillage in agricultural soils. However, effects of tillage on earthworms are often studied without considering species and their interactions with soil properties. Furthermore, many field studies are based on one-time samplings that do not allow for characterisation of temporal variation. The current study monitored the short (up to 53 days) and medium term (up to 4 years) effects of soil tillage on earthworms in conventional and organic farming. Earthworm abundances decreased one and three weeks after mouldboard ploughing in both conventional and organic farming, suggesting direct and indirect mechanisms. However, the medium-term study revealed that earthworm populations in mouldboard ploughing systems recovered by spring. The endogeic species Aporrectodea caliginosa strongly dominated the earthworm community (76%), whereas anecic species remained <1% of all earthworms in all tillage and farming systems over the entire study. In conventional farming, mean total earthworm abundance was not significantly different in reduced tillage (153 m⁻²) than mouldboard ploughing (MP; 130 m⁻²). However, reduced tillage in conventional farming significantly increased the epigeic species Lumbricus rubellus from 0.1 m⁻² in mouldboard ploughing to 9 m⁻² averaged over 4 years. Contrastingly, in organic farming mean total earthworm abundance was 45% lower in reduced tillage (297 m⁻²) than MP (430 m⁻²), across all sampling dates over the medium-term study (significant at 3 of 6 sampling dates). Reduced tillage in organic farming decreased A. caliginosa from 304 m⁻² in mouldboard ploughing to 169 m⁻² averaged over 4 years (significant at all sampling dates). Multivariate analysis revealed clear separation between farming and tillage systems. Earthworm species abundances, soil moisture, and soil organic matter were positively correlated, whereas earthworm abundances and penetration resistance where negatively correlated. Variability demonstrated between sampling dates highlights the importance of multiple samplings in time to ascertain management effects on earthworms. Findings indicate that a reduction in tillage intensity in conventional farming affects earthworms differently than in organic farming. Differing earthworm species or ecological group response to interactions between soil tillage, crop, and organic matter management in conventional and organic farming has implications for management to maximise soil ecosystem functions.     

Earthworms as soil quality indicators in Brazilian no-tillage systems

It is well known that earthworm populations tend to increase under no-tillage (NT) practices, but abundances tend to be highly variable. In the present study, data from the literature together with those on earthworm populations sampled in six watersheds in SW Paraná State, Brazil, were used to build a classification of the biological soil quality of NT systems based on earthworm density and species richness. Earthworms were collected in 34 farms with NT aging from 3 to 27 yr, in February 2010, using an adaptation of the TSBF (Tropical Soil Biology and Fertility) Program method (hand sorting of five 20 cm × 20 cm holes to 20 cm depth). Six forest sites were also sampled in order to compare abundances and species richness with the NT systems. Species richness in the 34 NT sites and in the 6 forests ranged from 1 to 6 species. Most earthworms encountered were exotics belonging to the genus Dichogaster (D. saliens, D. gracilis, D. bolaui and D. affinis) and native Ocnerodrilidae (mainly Belladrilus sp.), all of small individual size. In a few sites, individuals of the Glossoscolecidae (P. corethrurus, Glossoscolex sp., Fimoscolex sp.) and Megascolecidae (Amynthas gracilis) families were also encountered, in low densities. Urobenus brasiliensis (Glossoscolecidae) were found only in the forest fragments. In the NT farms, earthworm abundance ranged from 5 to 605 ind m⁻² and in the forest sites, from 10 to 285 ind m⁻². The ranking of the NT soil biological quality, based on earthworm abundance and species richness was: poor, with <25 individuals per m⁻² and 1 sp.; moderate, with ≥25–100 individuals per m⁻² and 2–3 sp.; good, with >100–200 individuals per m⁻² and 4–5 sp.; excellent, with >200 individuals per m⁻² and >6 sp. About 60% of the 34 farms fell into the poor to moderate categories based on this classification, so further improvements to the NT farm's management system are needed to enhance earthworm populations. Nevertheless, further validation of this ranking system is necessary to allow for its wider-spread use.     

Earthworm-induced N2O emissions in a sandy soil with surface-applied crop residues

Earlier research with endogeic and epigeic earthworm species in loamy arable soil has shown that both earthworm groups can increase nitrous oxide (N₂O) emissions, provided that crop residue placement matches the feeding strategy of the earthworm ecological group(s). However, it is not yet clear whether these effects also occur in sandy soils which typically contain less soil organic matter and have low soil aggregation levels. Here, we aimed to quantify N₂O emissions as affected by endogeic and/or epigeic earthworm species, and to relate changes in N₂O emissions to earthworm-induced changes in soil properties in a sandy soil. A 90 day mesocosm study was conducted with sandy soil and ¹⁵N-labeled radish (Raphanus sativus cv. Adagio L.) residue applied on top. Treatments included: (i) no earthworm addition, (ii) addition of the endogeic species Aporrectodea caliginosa (Savigny), (iii) addition of the epigeic species Lumbricus rubellus (Hoffmeister), and (iv) both species combined. An additional treatment was included without earthworms and with residue manually incorporated into the soil. L. rubellus significantly increased cumulative N₂O emissions from 228 to 859 μg N₂O–N kg^?1 (F_1,12 = 83.12, P < 0.001), whereas A. caliginosa did not affect N₂O emissions. In contrast to earlier studies in loamy soil, no positive interaction between both species with regard to N₂O emissions was found. This was probably related to high competition for organic resources in the relatively poor soil and a low potential for stable soil aggregate formation (and associated anaerobic microsites) by endogeic worms in sandy soil. ¹⁵N isotope analysis revealed that the activity of L. rubellus significantly increased (F_1,12 = 6.20, P = 0.028) the recovery of ¹⁵N in the 250–8000 μm size fraction, indicating incorporation of crop residues into the mineral soil. When residues were manually incorporated, N₂O emissions were significantly (P < 0.008) lower (509 μg N₂O–N kg^?1) than when incorporated by L. rubellus. The high N₂O emissions in the presence of L. rubellus, when compared to manual mixing, suggest a stimulation of microbial activity and/or changes in the microbial community composition. Insights on the earthworm effects on N₂O emission from such soils are discussed.     

Earthworm populations in relation to soil organic matter dynamics and management in California tomato cropping systems

Earthworms are key regulators of soil structure and soil organic matter (SOM) dynamics in many agroecosystems. They are greatly impacted by agricultural management, yet little is known about how these factors interact to control SOM dynamics. This study sought to explore linkages between agricultural management, earthworms and aggregate associated SOM dynamics through a survey of tomato (Solanum lycopersicum L.) cropping systems in northern California. Earthworms and soil samples were collected between February and April of 2005 from 16 fields under one of three types of residue management: (1) tomato mulch – no postharvest tillage and tomato residues left on the soil surface, (2) cover crop – tomato residues tilled in and leguminous cover crop planted, and (3) bare fallow – tomato residues tilled in and soil surface left exposed throughout the winter. Earthworms were collected via hand-sorting and identified to species, while soils were wet sieved to yield four aggregate size classes: large macroaggregates (>2000 μm), small macroaggregates (250–2000 μm), microaggregates (53–250 μm) and the silt and clay fraction (<53 μm). The combined large and small macroaggregate fraction was then fractionated into coarse particulate organic matter (cPOM; 250 μm), microaggregates within macroaggregates (mM; 53–250 μm) and macroaggregate occluded silt and clay (Msc; <53 μm). The earthworms identified in this survey were composed entirely of exotic species and were dominated by Aporrectodea caliginosa. Earthworm abundance was related to residue management, with the tomato mulch systems averaging 4.5 times greater fresh earthworm biomass than bare fallow (P = 0.024). Aggregate stability and total soil C and N also appeared to be influenced by residue management, such that the tomato mulch system displayed significantly greater mean weight diameters than the bare fallow system (P = 0.049), as well as more than 50% greater total soil C and N (P = 0.049 and P = 0.036; respectively). Earthworm biomass was also found to be positively correlated with total soil C (P = 0.009, R² = 0.39) and N (P = 0.010, R² = 0.039) as well as the proportion of macroaggregate C in the cPOM fraction (P = 0.028, R² = 0.30). Our findings suggest that residue handling and the associated management practices (e.g., tillage, organic vs. conventional agriculture) are important for both earthworm populations and SOM storage. Although earthworms are known to influence SOM in many ways, other factors appear to play a more prominent role in governing aggregate associated SOM dynamics.     

Endogeic and anecic earthworm abundance in six Midwestern cropping systems

Endogeic and juvenile anecic earthworm abundance was measured in soil samples and anecic populations were studied by counting midden numbers at the sites of two long-term cropping systems trials in South-central Wisconsin. The three grain and three forage systems at each site were designed to reflect a range of Midwestern USA production strategies. The primary objectives of this work were to determine if the abundance of endogeic or anecic earthworms varied among cropping systems or crop phases within a cropping system and were there specific management practices that impacted endogeic or anecic earthworm numbers. The earthworms present in the surface soil were: Aporrectodea tuberculata (Eisen), A. caliginosa (Savigny), A. trapezoides (Dugés); and juvenile Lumbricus terrestris (L.). True endogeic abundance was greatest in rotationally grazed pasture [188 m^?2 at Arlington (ARL) and 299 m^?2 at Elkhorn (ELK)], and smallest in conventional continuous corn (27 m^?2 at ARL and 32 m^?2 at ELK). The only type of anecic earthworm found was L. terrestris L. There was an average of 1.2 middens per adult anecic earthworm and the population of anecics was greatest in the no-till cash grain system (28 middens m^?2 at ARL, 18 m^?2 at ELK) and smallest in the conventional continuous corn system (3 middens m^?2 at ARL, 1 m^?2 at ELK). Earthworm numbers in individual crop phases within a cropping system were too variable from year-to-year to recommend using a single phase to characterize a whole cropping system. Indices for five management factors (tillage, manure inputs, solid stand, pesticide use, and crop diversity) were examined, and manure use and tillage were the most important impacting earthworm numbers across the range of cropping systems. Manure use was the most important management factor affecting endogeic earthworm numbers; but no-tillage was the most important for the juvenile and adult anecic groups and had a significantly positive influence on endogeic earthworm counts as well. The pesticides used, which were among the most commonly applied pesticides in the Midwestern USA, and increasing crop diversity did not have a significant effect on either the endogeic or anecic earthworm groups in this study. Consequently, designing cropping systems that reduce tillage and include manure with less regard to omitting pesticides or increasing crop diversity should enhance earthworm populations and probably improve sustainability.     

Earthworms,soil mineral nitrogen and forage production in grass-based hayfields

This study was designed to address how earthworm activity influences soil mineral nitrogen (N), plant N uptake and forage yield in grass-based hayfields. Earthworm populations were reduced by applying carbaryl pesticide to the experimental field plots every 2-weeks, effectively eliminating the earthworms for up to 12-weeks from May to August. Grass yields and tissue N concentrations were measured every 2 weeks, and the soil mineral N concentration determined at the final harvest. Reducing earthworm populations for up to 12-weeks did not affect grass yield or N uptake. However, regression analysis showed that plots with undisturbed earthworm populations had higher soil N by 0.8 kg N ha^?1 per week, representing mineralization of about 10 kg N ha^?1 during the 12-week study. This was a fraction of the fertilizer N recommendation (75 kg N ha^?1) for grass-based hayfields in this region. Therefore, the increase in soil mineral N from earthworm activity was small, relative to the N requirements of the hayfield.     

Organic carbon sequestration in earthworm burrows

Earthworms strongly affect soil organic carbon cycling. The aim of this study was to determine whether deep burrowing anecic earthworms enhance carbon storage in soils and decrease C turnover. Earthworm burrow linings were separated into thin cylindrical sections with different distances from the burrow wall to determine gradients from the burrow wall to the surrounding soil. Organic C, total N, radiocarbon (¹⁴C) concentration, stable isotope values (δ¹³C, δ¹⁵N) and extracellular enzyme activities were measured in these samples. Anecic earthworms increased C stocks by 270 and 310 g m^?2 accumulated in the vertical burrows. C-enrichment of the burrow linings was spatially highly variable within a distance of millimetres around the burrow walls. It was shown that C accumulation in burrows can be fast with C sequestration rates of about 22 g C m^?2 yr^?1 in the burrow linings, but accumulated C in the burrows may be mineralised fast with turnover times of only 3–5 years. Carbon stocks in earthworm burrows strongly depended on the earthworm activity which maintains continuous C input into the burrows. The enhanced extracellular enzyme activity of fresh casts was not persistent, but was 47% lower in inhabited burrows and 62% lower in abandoned burrows. Enzyme activities followed the C concentrations in the burrows and were not further suppressed due to earthworms. Radiocarbon concentrations and stable isotopes in the burrow linings showed an exponential gradient with the youngest and less degraded organic matter in the innermost part of the burrow wall. Carbon accumulation by anecic earthworm is restricted to distinct burrows with less influence to the surrounding soil. Contrary to the initial hypothesis, that organic C is stabilised due to earthworms, relaxation time experiments with nuclear magnetic resonance spectroscopy (NMR) did not reveal any enhanced adsorption of C on iron oxides with C stabilising effect. Our results suggest that earthworm activity does not substantially increase subsoil C stocks but burrows serve as fast ways for fresh C transport into deep soil horizons.     

Role of soil water content in the carbon and nitrogen dynamics of Lumbricus terrestris L. burrow soil

We evaluated the role of soil water content in controlling C and N dynamics within the drilosphere created by the anecic earthworm Lumbricus terrestris (L.). Mesocosms (volume = 3.1 l) were each amended with corn litter and three earthworms. Control treatments received no earthworms and no other earthworm species were present in the soil. WET and DRY treatments received a total of 9.25 cm and 3.25 cm of water, respectively. Water was added on weeks 1, 3, 7, and 10 at a rate of 2.0 cm per mesocosm for WET treatments and 0.5 cm per mesocosm for DRY treatments. Mesocosms were sampled destructively after incubation at 18–20 °C for 0, 3, 7, and 13 weeks. The water content of WET burrow soil ranged from 0.12 g g⁻¹ to 0.18 g g⁻¹ and was significantly higher than in the DRY treatment throughout the incubation period. The live weight of earthworms was significantly higher in the WET treatment only on week 13, whereas litter consumption was significantly lower in the DRY treatment for week 13. Carbon mineralization, measured as CO₂ evolved after a 24-h incubation, was consistently higher in WET than in DRY burrow soil. Effects of differences in soil water content were also apparent for biomass C and metabolic quotient. Soil water content did no affect the total C concentration of burrow soil. DRY burrow soil had consistently lower levels of nitrate than WET soil throughout the experiment. Lower levels of ammonium and inorganic N were observed for WET burrow soil on weeks 3 and 7. Water content did not have a significant effect on burrow soil total N. We concluded that the water content of the drilosphere affects both C and N dynamics and can affect the speciation of inorganic N; yet, the effects of soil water content do not appear to result from differences in the feeding activities of anecic earthworms.     

Influence of carbon amendments on soil denitrifier abundance in soil microcosms

It is known that carbon (C) amendments increase microbial activity in anoxic soil microcosm studies, however the effects on abundance of total and denitrifier bacterial communities is uncertain. Quantitative PCR was used to target the 16S rRNA gene for the total bacterial community, the nosZ functional gene to reflect a broad denitrifier community, and functional genes from narrow denitrifier communities represented by Pseudomonas mandelii and related species (cnorB_P) and Bosea/Bradyrhizobium/Ensifer spp. (cnorB_B). Repacked soil cores were amended with varying amounts of glucose and red clover plant tissue (0–1000 mg C kg^? 1 of soil) and incubated for 96 h. Carbon amendment significantly increased respiration as measured by cumulative CO₂ emissions. Inputs of red clover or glucose at 1000 mg C kg^? 1 of soil caused increased abundance in the total bacteria under the conditions used. There was about an approximate 2-fold increase in the abundance of bacteria bearing the nosZ gene, but only in treatments receiving 500 or 1000 mg C kg^? 1 of soil of glucose or red clover, respectively. Additions of ≥ 500 mg C kg^? 1 soil of red clover and ≥ 250 mg C kg^? 1 of glucose increased cnorB_P-gene bearing denitrifiers. Changes in abundance of the targeted communities were related to C availability in soil, as indicated by soil respiration, regardless of C source. Applications of C amendments at rates that would occur in agricultural soils not only increase microbial activity, but can also induce changes in abundance of total bacterial and denitrifier communities in studies of anoxic soil microcosms.     

Effects of cropping and tree density on earthworm community composition and densities in central Cameroon

Earthworms can have positive effects upon crop growth in the tropics. If soils are to be managed sustainably, then more attention should be paid to the effects of cultivation and cropping practices upon earthworms. When forest vegetation is cleared, slashed, burned and land is tilled and cultivated, earthworm abundance, diversity and activity are reduced. Conversely, retaining trees in agroecosystems may maintain earthworm populations during the cropping phase.Here, we assessed the impact on earthworm species diversity and densities of crop cultivation in the understorey of timber plantations thinned to two tree densities and compared these with uncropped, undisturbed timber plantation controls. The plots were reassessed after two and a half years of fallow to see whether populations had recovered. The experiment was in central Cameroon.Seventeen earthworm species were recorded from Eudrilidae subfamilies Eudrilinae and Pareudrilinae, Ocnerodrilidae and Acanthodrilidae, most of which were endemics. This included two new species from two new genera from the sub-family Pareudrilinae, one new species from one new genus of Ocnerodrilidae, two new species of Dichogaster and one new species of Legonodrilus. Ten species were epigeic, six were endogeic and one was anecic.Generally, earthworm densities were lower in cropped plots than in the undisturbed plantation control. The most abundant species was a Legonodrilus sp. nov. with average densities of 49 individuals m⁻² in the crop phase and 80 ind. m⁻² in the fallow phase. By the fallow phase, densities in the low tree density (120 ind. m⁻²) were higher than in the high density (40 ind. m⁻²). The densities of the epigeic Acanthodrilidae were significantly reduced to 7 ind. m⁻² in the cropped plots compared with 42 ind. m⁻² in the control plots. The effects of cropping were thus species-specific and more work is required to identify which of these endemics are the ecosystem engineers in the system.     

Deep-burrowing earthworm additions changed the distribution of soil organic carbon in a chisel-tilled soil

We investigated the influence of earthworms on the three-dimensional distribution of soil organic carbon (SOC) in a chisel-tilled soil. By burrowing, foraging, and casting at the surface and throughout the soil, anecic earthworms such as Lumbricus terrestris L. may play a major role in regulating the spatial distribution of organic matter resources both at the surface and within the soil. In the fall of 1994, we manipulated ambient earthworm communities, which were without deep burrowing species, by adding 100 earthworm individuals m⁻² in spring and fall for 3 years. Overall, the biomass of L. terrestris was increased with earthworm additions and total earthworm biomass declined compared with ambient control treatments. To investigate the spatial variability in soil organic carbon due to this shift in earthworm community structure, we sampled soil on a 28×24 cm grid from the surface to 40 cm in four layers, 10 cm deep. Samples were analyzed for total carbon. We found that additions of anecic earthworms significantly increased average soil organic carbon content from 16.1 to 17.9 g C kg⁻¹ for the 0–10 cm soil, and from 12.4 to 14.7 g kg⁻¹ at 10–20-cm depth, and also changed the spatial distribution of soil organic carbon from uniform to patchy, compared with the ambient treatment.     

Surface soil responses to silage cropping intensity on a Typic Kanhapludult in the piedmont of North Carolina

Although reduced tillage itself is beneficial to soil quality and farm economics, the amount of crop residues returned to the soil will likely alter the success of a particular conservation tillage system within a farm operation. We investigated the impact of three cropping systems (a gradient in silage cropping intensity) on selected soil physical, chemical, and biological properties in the Piedmont of North Carolina, USA. Cropping systems were: (1) maize (Zea mays L.) silage/barley (Hordeum vulgare L.) silage (high silage intensity), (2) maize silage/winter cover crop (medium silage intensity), and (3) maize silage/barley grain—summer cover crop/winter cover crop (low silage intensity). There was an inverse relationship between silage intensity and the quantity of surface residue C and N contents. With time, soil bulk density at a depth of 0–3 cm became lower and total and particulate C and N fractions, and stability of macroaggregates became higher with lower silage intensity as a result of greater crop residue returned to soil. Soil bulk density at 0–3 cm depth was initially 0.88 Mg m⁻³ and increased to 1.08 Mg m⁻³ at the end of 7 years under high silage intensity. Total organic C at 0–20 cm depth was initially 11.7 g kg⁻¹ and increased to 14.3 g kg⁻¹ at the end of 7 years under low silage intensity. Stability of macroaggregates at 0–3 cm depth at the end of 7 years was 99% under low silage intensity, 96% under medium silage intensity, and 89% under high silage intensity. Soil microbial biomass C at 0–3 cm depth at the end of 7 years was greater with low silage intensity (1910 mg kg⁻¹) than with high silage intensity (1172 mg kg⁻¹). Less intensive silage cropping (i.e., greater quantities of crop residue returned to soil) had a multitude of positive effects on soil properties, even in continuous no-tillage crop production systems. An optimum balance between short-term economic returns and longer-term investments in improved soil quality for more sustainable production can be achieved in no-tillage silage cropping systems.     

Earthworms in Chromolaena odorata (L.) King and Robinson (Asteraceae) fallows along a chronosequence: Changes in community structure and identification of persistent and indicator species

The species Chromolaena odorata (Asteraceae) is a notorious invasive shrub spreading throughout West and Central Africa and as such, there is a need to determine its environmental impact, particularly on soil biodiversity and functioning. Indeed, soil organisms such as earthworms are known to strongly influence soil properties and biogeochemical cycles. This study, conducted in Central Côte d’Ivoire, aims to investigate the temporal dynamics of earthworm communities in C. odorata fallows of different ages and to identify associated indicators and persistent species. Three distinct classes of fallows identified by local farmers, were considered: young (1–3 years, C1), medium-aged (4–8 years, C2) and old (>9 years, C3). Each of the classes included four plot replicates where earthworms were sampled using the Tropical Soil Biology and Fertility (TSBF) 25 cm × 25 cm × 30 cm soil monolith method. The study of earthworm communities was focused on density, biomass, diversity and complementarity. Indicator values (IndVals) were used to identify indicator species of the classes of fallows. The shrub exerted a mixed influence on earthworms depending on the functional group, with litter feeders and polyhumics declining over time as a result of a reduction of the litter availability on the soil surface. The species richness was significantly greater in C1 than in the other classes although the Shannon–Weaver's index did not vary significantly. However, a cluster analysis performed on densities highlighted marked differences between C2 and the two other classes in terms of community composition. Indicator species were found for C1 and C2. The geophagous Millsonia omodeoi has emerged as a persistent species as its density and biomass steadily increased so that it became the dominant species in old fallows. The roles of litters and soil parameters in influencing earthworm communities are discussed.     

Nitrogen mineralization in the ruderal sub-alpine communities in Mount Uludağ, Turkey

Nitrogen mineralization and nitrification in the soil of sub-alpine ruderal community of Mount Uludağ, Bursa, Turkey was measured for 1 year, under field conditions with Verbascum olympicum and Rumex olympicus being the dominant pioneer species under dry and wet sites, respectively. Seasonal fluctuations were observed in N mineralization and nitrification. The net N mineralization and nitrification were high in early summer and winter, due to high moisture. The annual net N mineralization rate (for the 0–15 cm soil layer) was higher under R. olympicus (188 kg N ha⁻¹ yr⁻¹) than under V. olympicum (96 kg N ha⁻¹ yr⁻¹). A significant positive correlation between net N mineralization and soil organic C (r² = 0.166), total N (r² = 0.141) and water content (r² = 0.211) was found. Our results indicate that N mineralization rate is high in soils of ruderal communities on disturbed sites and varies with dominant species and, a difference in net N mineralization rate can be attributed to organic C, total N and moisture content of soils.     

What are the limits of the drilosphere? An incubation experiment using Metaphire posthuma

The near infrared reflectance spectroscopy (NIRS) method was used in the present study to compare earthworm-made soil aggregates to aggregates found in the surrounding bulk soil. After initially assessing the daily cast production of Metaphire posthuma, boxes with soil incubated with M. posthuma and control soils were subjected to wetting in order to reorganize the soil structure. After two months of incubation, soil aggregates produced by earthworms (casts and burrows), soil aggregates that were appeared to be unaffected by earthworms (bulk soil without visible trace of earthworm bioturbation from the earthworm treatment) and soil aggregates that were entirely unaffected by earthworms (control – no earthworm – treatment) were sampled and their chemical signatures analyzed by NIRS. The production of below-ground and surface casts reached 14.9 g soil g worm^?1 d^?1 and 1.4 g soil g worm^?1 d^?1, respectively. Soil aggregates from the control soils had a significantly different NIRS signature from those sampled from boxes with earthworms. However, within the earthworm incubation boxes the NIRS signature was similar between cast and burrow aggregates and soil aggregates from the surrounding bulk soil. We conclude that the high cast production by M. posthuma and the regular reorganization of the soil structure by water flow in and through the soil lead to a relatively homogenous soil structure. Given these results, we question the relevance of considering the bulk soil that has no visible activity of earthworm activity as a control to determine the effect of earthworms on soil functioning.     

Effects of inorganic fertilizer and manure on soil archaeal abundance at two experimental farms during three consecutive rotation-cropping seasons

Soil archaeal population dynamics at two experimental sites of the same clay-loam type in Ottawa and Woodslee, Ontario, were investigated to determine fertilizer and manure effects following their different long-term crop rotation and fertilization schemes. Phylogenetic analysis of cloned soil archaeal 16S rRNA gene libraries of both sites identified them with group 1.1b of Thaumarchaeota. The gene population dynamics subtly varied in the order of 10⁷ copies g⁻¹ soil when monitored by quantitative real-time PCR during three growing seasons (2007–2009). In Ottawa, where plots were amended with dairy-farm manure, soil thaumarchaeal gene abundance was double of the unamended plots. At the Woodslee N-P-K-fertilized plots, it remained at least 30% fewer than that of the unfertilized ones. These cultivated plots showed soil carbon limitation while the fertilized ones were low in soil pH (ca. 5.5). Surface soils from an unfertilized sod plot and an adjacent deciduous forest had higher total carbon content (C:N ratio of 9 and 11, respectively). Their thaumarchaeal gene abundance varied up to 4.8 × 10⁷ and 7.0 × 10⁷ copies g⁻¹ soil, respectively. The former value was also attained at the manure-amended plots in Ottawa, where the C:N ratio was just below 10. Where soil pH was above 6.0, there was a weak and positive correlation between soil total C and the estimated gene abundance. Such gene population dynamics consistently demonstrated the stimulating and suppressive effects of dairy-farm manure (Ottawa site) and inorganic fertilizers (Woodslee site), respectively, on soil thaumarchaea. At both sites archaeal amoA and 16S rRNA gene abundance were similarly affected. Archaeal amoA gene abundance also outnumbered bacterial amoA abundance, suggesting that ammonia-oxidizing archaea might be dominant in these soils. Only minor crop effects on gene population dynamics were detected.     

Responses of soil dissolved organic matter to long-term plantations of three coniferous tree species

Tree species have significant effects on the availability and dynamics of soil organic matter. In the present study, the pool sizes of soil dissolved organic matter (DOM), potential mineralizable N (PMN) and bio-available carbon (C) (measured as cumulative CO₂ evolution over 63 days) were compared in soils under three coniferous species — 73 year old slash (Pinus elliottii), hoop (Araucaria cunninghamii) and kauri (Agathis robusta) pines. Results have shown that dissolved organic N (DON) in hot water extracts was 1.5–1.7 times lower in soils under slash pine than under hoop and kauri pines, while soil dissolved organic C (DOC) in hot water extracts tended to be higher under slash pine than hoop and kauri pines but this was not statistically significant. This has led to the higher DOC:DON ratio in soils under slash pine (32) than under hoop and kauri pines (17). Soil DOC and DON in 2 M KCl extracts were not significantly different among the three tree species. The DOC:DON ratio (hot water extracts) was positively and significantly correlated with soil C:N (R² = 0.886, P < 0.01) and surface litter C:N ratios (R² = 0.768, P < 0.01), indicating that DOM was mainly derived from litter materials and soil organic matter through dissolution and decomposition. Soil pH was lower under slash pine (4.5) than under hoop (6.0) and kauri (6.2) pines, and negatively correlated with soil total C, C:N ratio, DOC and DOC:DON ratio (hot water extracts), indicating the soil acidity under slash pine favored the accumulation of soil C. Moreover, the amounts of dissolved inorganic N, PMN and bio-available C were also significantly lower in soils under slash pine than under hoop and kauri pines. It is concluded that changes in the quantity and quality of surface litters and soil pH induced by different tree species largely determined the size and quality of soil DOM, and plantations of hoop and kauri pine trees may be better in maintaining long-term soil N fertility than slash pine plantations.