Earthworm populations and growth rates related to long-term crop residue and tillage management

Conventional tillage creates soil physical conditions that may restrict earthworm movement and accelerate crop residue decomposition, thus reducing the food supply for earthworms. These negative impacts may be alleviated by retaining crop residues in agroecosystems. The objective of this study was to determine the effects of various tillage and crop residue management practices on earthworm populations in the field and earthworm growth under controlled conditions. Population assessments were conducted at two long-term (15+ years) experimental sites in Québec, Canada with three tillage systems: moldboard plow/disk harrow (CT), chisel plow or disk harrow (RT) and no tillage (NT), as well as two levels of crop residue inputs (high and low). Earthworm growth was assessed in intact soil cores from both sites. In the field, earthworm populations and biomass were greater with long-term NT than CT and RT practices, but not affected by crop residue management. Laboratory growth rates of Aporrectodea turgida (Eisen) in intact soil cores were affected by tillage and residue inputs, and were positively correlated with the soil organic C pool, suggesting that tillage and residue management practices that increase the soil organic C pool provide more organic substrates for earthworm growth. The highest earthworm growth rates were in soils from RT plots with high residue input, which differed from the response of earthworm populations to tillage and residue management treatments in the field. Our results suggest that tillage-induced disturbance probably has a greater impact than food availability on earthworm populations in cool, humid agroecosystems.     

Starving the soil of plant inputs for 50 years reduces abundance but not diversity of soil bacterial communities

If soil communities rely on plant-derived carbon, is biodiversity lost when this primary source is removed? Soil microbial and mesofaunal communities at the Rothamsted Highfield site were compared under a mixed grass sward, arable rotation and a section maintained as a bare-fallow for the past 50 years by regular tillage. Organic matter reserves have been degraded and microbial and mesofaunal numbers and mite diversity have declined in this unique bare-fallow site, where fresh carbon inputs have been drastically reduced. However, it supports a species-rich metabolically active bacterial community of similar diversity to that in soil maintained as grass sward. Thus in contrast to soil mesofauna, bacterial diversity (but not abundance) is apparently independent of plant inputs.     

Short-term population dynamics of ammonia oxidizing bacteria in an agricultural soil

Ammonia oxidizing bacteria (AOB) control the rate limiting step of nitrification, the conversion of ammonia (NH₄⁺) to nitrite (NO₂⁻). The AOB therefore have an important role to play in regulating soil nitrogen cycling. Tillage aerates the soil, stimulating rapid changes in soil N cycling and microbial communities. Here we report results of a study of the short term responses of AOB and net nitrification to simulated tillage and NH₄⁺ addition to soil. The intensively farmed vegetable soils of the Salinas Valley, California, provide the context for this study. These soils are cultivated frequently, receive large N fertilizer inputs and there are regional concerns about groundwater N concentrations. An understanding of N dynamics in these systems is therefore important. AOB population sizes were quantified using a real-time PCR approach. In a 15 day experiment AOB populations, increased rapidly following tillage and NH₄⁺ addition and persisted after the depletion of soil NH₄⁺. AOB population sizes increased to a similar degree, over a 1.5-day period, irrespective of the amount of NH₄⁺ supplied. These data suggest selection of an AOB community in this intensively farmed and C-limited soil, that rapidly uses NH₄⁺ that becomes available. These data also suggest that mineralization may play an especially important role in regulating AOB populations where NH₄⁺ pool sizes are very low. Methodological considerations in the study of soil AOB communities are also discussed.     

Impact of cultivar, harvesting time, and seasonal variation on the content of biophenols in olive mill waste

Olive mill waste (OMW) contains substantial amounts of valuable antioxidant biophenols that can be recovered for possible applications in food, pharmaceutical, and cosmetic industries. However, the impact of cultivar, harvesting time, and seasonal variation on the phenolic composition of OMW has not yet been assessed. Total phenols, antioxidant activity, and phenol profiles of OMW extracts from five different Australian-grown cultivars (Barnea, Correggiola, Manzanillo, Mission, and Paragon) were studied at four different harvesting times in the 2004 season. The impact of seasonal variation was assessed by comparing total phenol content, antioxidant activity, and phenol profile of two cultivars (Correggiola and Mission) harvested in the 2004 and 2005 seasons. The phenol content and antioxidant activity at different harvesting times were mainly a function of the olive cultivar. Harvesting time had a quantitative effect rather than a qualitative effect on the phenol profile. Intercultivar and harvesting time variation accounted for a 2-5-fold change in the total phenol and antioxidant capacity, while levels of individual biophenols experienced up to 50-fold change. The phenol content and antioxidant capacity of OMW significantly changed between seasons with different variation patterns for different cultivars.     

Molecular dynamics study on the biophysical interactions of seven green tea catechins with lipid bilayers of cell membranes

Molecular dynamics simulations were performed to study the interactions of bioactive catechins (flavonoids) commonly found in green tea with lipid bilayers, as a model for cell membranes. Previously, multiple experimental studies rationalized catechin's anticarcinogenic, antibacterial, and other beneficial effects in terms of physicochemical molecular interactions with the cell membranes. To contribute toward understanding the molecular role of catechins on the structure of cell membranes, we present simulation results for seven green tea catechins in lipid bilayer systems representative of HepG2 cancer cells. Our simulations show that the seven tea catechins evaluated have a strong affinity for the lipid bilayer via hydrogen bonding to the bilayer surface, with some of the smaller catechins able to penetrate underneath the surface. Epigallocatechin-gallate (EGCG) showed the strongest interaction with the lipid bilayer based on the number of hydrogen bonds formed with lipid headgroups. The simulations also provide insight into the functional characteristics of the catechins that distinguish them as effective compounds to potentially alter the lipid bilayer properties. The results on the hydrogen-bonding effects, described here for the first time, may contribute to a better understanding of proposed multiple molecular mechanisms of the action of catechins in microorganisms, cancer cells, and tissues.     

Stabilization mechanisms of organic matter in four temperate soils: Development and application of a conceptual model

Based on recent findings in the literature, we developed a process‐oriented conceptual model that integrates all three process groups of organic matter (OM) stabilization in soils namely (1) selective preservation of recalcitrant compounds, (2) spatial inaccessibility to decomposer organisms, and (3) interactions of OM with minerals and metal ions. The model concept relates the diverse stabilization mechanisms to active, intermediate, and passive pools. The formation of the passive pool is regarded as hierarchical structured co‐action of various processes that are active under specific pedogenetic conditions. To evaluate the model, we used data of pool sizes and turnover times of soil OM fractions from horizons of two acid forest and two agricultural soils. Selective preservation of recalcitrant compounds is relevant in the active pool and particularly in soil horizons with high C contents. Biogenic aggregation preserves OM in the intermediate pool and is limited to topsoil horizons. Spatial inaccessibility due to the occlusion of OM in clay microstructures and due to the formation of hydrophobic surfaces stabilizes OM in the passive pool. If present, charcoal contributes to the passive pool mainly in topsoil horizons. The importance of organo‐mineral interactions for OM stabilization in the passive pool is well‐known and increases with soil depth. Hydrophobicity is particularly relevant in acid soils and in soils with considerable inputs of charcoal. We conclude that the stabilization potentials of soils are site‐ and horizon‐specific. Furthermore, management affects key stabilization mechanisms. Tillage increases the importance of organo‐mineral interactions for OM stabilization, and in Ap horizons with high microbial activity and C turnover, organo‐mineral interactions can contribute to OM stabilization in the intermediate pool. The application of our model showed that we need a better understanding of processes causing spatial inaccessibility of OM to decomposers in the passive pool.     

Modified technique to assess the wettability of soil aggregates: comparison with contact angles measured on crushed aggregates and bulk soil

Wettability parameters determined for individual soils often show a considerable variation depending on the kind of sample (aggregated or homogeneous material) and the method used. To investigate the causes of this variation, we assessed wettability of both intact and crushed aggregates and bulk soil using different methods. Wettability of intact aggregates was characterized by a modified technique where the specific infiltration rates of water and a completely wetting liquid were used to define a repellency index. Contact angles were determined on crushed aggregates and bulk soil using the Wilhelmy plate and capillary rise methods. The repellency index was found to be sensitive to slight differences in wettability and was in good agreement with Wilhelmy plate contact angles. Contact angles measured with the capillary rise method showed a strong deviation from those determined with the Wilhelmy plate method. This can be ascribed to the underlying assumptions of the capillary rise method (i.e. cylindrical and parallel capillaries) resulting in an over‐estimation of contact angle, particularly for the small‐sized particle fraction because of the impact of inertia and pore structure. No significant differences were found between intact and crushed aggregates whereas the bulk soil was slightly more water‐repellent, probably because of a somewhat larger organic carbon content. We conclude that the contact angle determined by the Wilhelmy plate method and the repellency index are appropriate parameters for characterizing soil water repellency because they detected small changes in wettability over a wide range extending from subcritical water repellency to hydrophobicity.     

Small-scale heterogeneity in carbon dioxide, nitrous oxide and methane production from aggregates of a cultivated sandy-loam soil

Spatial variability in carbon dioxide (CO₂), nitrous oxide (N₂O) and methane (CH₄) emissions from soil is related to the distribution of microsites where these gases are produced. Porous soil aggregates may possess aerobic and anaerobic microsites, depending on the water content of pores. The purpose of this study was to determine how production of CO₂, N₂O and CH₄ was affected by aggregate size and soil water content. An air-dry sandy loam soil was sieved to generate three aggregate fractions (<0.25 mm, 0.25–2 mm and 2–6 mm) and bulk soil (<2 mm). Aggregate fractions and bulk soil were moistened (60% water-filled pore space, WFPS) and pre-incubated to restore microbial activity, then gradually dried or moistened to 20%, 40%, 60% or 80% WFPS and incubated at 25 °C for 48 h. Soil respiration peaked at 40% WFPS, presumably because this was the optimum level for heterotrophic microorganisms, and at 80% WFPS, which corresponded to the peak N₂O production. More CO₂ was produced by microaggregates (<0.25 mm) than macroaggregate (>0.25 mm) fractions. Incubation of aggregate fractions and soil at 80% WFPS with acetylene (10 Pa and 10 kPa) and without acetylene showed that denitrification was responsible for 95% of N₂O production from microaggregates, while nitrification accounted for 97–99% of the N₂O produced by macroaggregates and bulk soil. This suggests that oxygen (O₂) diffusion into and around microaggregates was constrained, whereas macroaggregates remained aerobic at 80% WFPS. Methane consumption and production were measured in aggregates, reaching 1.1–6.4 ng CH₄–C kg⁻¹ soil h⁻¹ as aggregate fractions and soil became wetter. For the sandy-loam soil studied, we conclude that nitrification in aerobic microsites contributed importantly to total N₂O production, even when the soil water content permitted denitrification and CH₄ production in anaerobic microsites. The relevance of these findings to microbial processes controlling N₂O production at the field scale remains to be confirmed.     

Changes in the pore space of soil constructions in physical laboratory models

The dynamics of pore space structure in different filled soil constructions during water infiltration and wetting–drying processes is studied. Model laboratory experiments in columns physically simulate water infiltration after penetration at a rate of 600 mm with free outflow from the lower end of the column followed by multiple drying of soil constructions composed by alternating layers of sand, peat, and A and В horizons of soddy-podzolic soil. In two- and three-dimensional tomographic images, changes in the pore space and the interpenetration of solid phase at the boundaries between individual horizons of soil constructions are analyzed.     

Site-Specific Nitrogen Management in Rice Using Remote Sensing and Geostatistics

Proper doses of nitrogenous fertilizer are most important for rice production system because a large part of the nitrogen may be lost if it is not applied judiciously. A study was conducted covering five blocks of Balasore and two blocks of Bhadrak districts. Soil samples were collected randomly, and field visit was conducted during peak vegetative stage of rice. Two approaches have been used in this study for estimating the site-specific nitrogen (N) requirement in the study area. In one approach, geostatisical analysis and kriging was used to develop the soil test–based N recommendation map by which a minimum of 72 kg N ha^?1 and maximum of 94 kg N ha^?1 were recommended. In a second approach, remote sensing was used and N recommendation map was developed using the moderate-resolution imaging spectroradiometer (MODIS) leaf area index (LAI) and normalized difference vegetation index (NDVI) satellite data, and a minimum requirement of 60 kg N ha^?1 and maximum of 120 kg N ha^?1 was estimated through this approach.