Does biochar influence soil physical properties and soil water availability?

Aims

This study aims to (i) determine the effects of incorporating 47 Mg ha^?1 acacia green waste biochar on soil physical properties and water relations, and (ii) to explore the different mechanisms by which biochar influences soil porosity.

Methods

The pore size distribution of the biochar was determined by scanning electron microscope and mercury porosimetry. Soil physical properties and water relations were determined by in situ tension infiltrometers, desorption and evaporative flux on intact cores, pressure chamber analysis at ?1,500 kPa, and wet aggregate sieving.

Results

Thirty months after incorporation, biochar application had no significant effect on soil moisture content, drainable porosity between –1.0 and ?10 kPa, field capacity, plant available water capacity, the van Genuchten soil water retention parameters, aggregate stability, nor the permanent wilting point. However, the biochar-amended soil had significantly higher near-saturated hydraulic conductivity, soil water content at ?0.1 kPa, and significantly lower bulk density than the unamended control. Differences were attributed to the formation of large macropores (>1,200 μm) resulting from greater earthworm burrowing in the biochar-amended soil.

Conclusion

We found no evidence to suggest application of biochar influenced soil porosity by either direct pore contribution, creation of accommodation pores, or improved aggregate stability.     

Wetting properties of fungi mycelium alter soil infiltration and soil water repellency in a γ-sterilized wettable and repellent soil

Soil water repellency (SWR) has a drastic impact on soil quality resulting in reduced infiltration, increased runoff, increased leaching, reduced plant growth, and increased soil erosion. One of the causes of SWR is hydrophobic fungal structures and exudates that change the soil–water relationship. The objective of this study was to determine whether SWR and infiltration could be manipulated through inoculation with fungi. The effect of fungi on SWR was investigated through inoculation of three fungal strains (hydrophilic – Fusarium proliferatum, chrono-amphiphilic – Trichoderma harzianum, and hydrophobic – Alternaria sp.) on a water repellent soil (WR-soil) and a wettable soil (W-soil). The change in SWR and infiltration was assessed by the water repellency index and cumulative infiltration respectively. F. proliferatum decreased the SWR on WR-soil and slightly increased SWR in W-soil, while Alternaria sp. increased SWR in both the W-soil and the WR-soil. Conversely T. harzianum increased the SWR in the W-soil and decreased the SWR in the WR-soil. All strains showed a decrease in infiltration in W-soil, while only the F. proliferatum and T. harzianum strain showed improvement in infiltration in the WR-soil. The ability of fungi to alter the SWR and enmesh soil particles results in changes to the infiltration dynamics in soil.     

Is soil carbon disappearing? The dynamics of soil organic carbon in Java

Research in the soil of the tropics mostly has demonstrated the decline of soil organic carbon (SOC) after conversion of primary forest to plantation and cultivated lands. This paper illustrates the dynamics of SOC on the island of Java, Indonesia, from 1930 to 2010. We used 2002 soil profile observations containing organic carbon (C) analysis in the topsoil, which were collected by the Indonesian Center for Agricultural Land Resources Research & Development from 1923 to 2007. Results show the obvious decline of SOC values from around 2% in 1930–1940 to 0.8% in 1960–1970. However, there has been an increase of SOC content since 1970, with a median level of 1.1% in the year 2000. Our analysis suggests that the human influence and agricultural practices on SOC in Java have been a stronger influence than the environmental factors. SOC for the top 10 cm has shown a net accumulation rate of 0.2–0.3 Mg C ha^?1 yr^?1 during the period 1990–2000. These findings give rise to optimism for increased soil C sequestration in the tropics.     

How do soil fauna and soil microbiota respond to beech forest growth?

The dynamics and performance of soil biota during forest rotation were studied in monoculture beech stands forming a chronosequence of four different age-classes(30,62,111,153 yr).Biomass was monitored in major groups of microflora,microfauna,mesofauna,and macrofauna.Resource availability(litter layer,soil organic mater),biomass of the two dominant decomposer groups(microflora,earthworms)as well as the biomass of mesofauna and microfauna were found to remain quite stable during forest succession.Nevertheles...     

Does an earthworm species acclimatize and/or adapt to soil calcium conditions? The consequences of soil nitrogen mineralization in forest soil

Calcium (Ca)-rich food can increase feeding of Lumbricidae. Earthworms can be genetically differentiated at a small spatial scale and acclimatize to the local environment during growth. Soil feeding and subsequent cast production by earthworms affects soil N mineralization. Here, we hypothesized that soil feeding and subsequent cast production by Lumbricidae species increases with high soil Ca content and that this increase is stronger in worms from high-Ca soil. We also hypothesized that changes in the soil feeding of Lumbricidae species along with the Ca content affects the soil N mineralization via changes in the cast production. Using a geophageous earthworm species (Eisenia japonica) originated from two different Ca environments (calcareous soil and sedimentary soil), we investigated cast production and soil N mineralization in three soils (sedimentary soil, sedimentary soil with Ca addition, and calcareous soil). The soil feeding of E. japonica from both origins did not always increase despite the high soil Ca content. We suggest that both the Ca content and other soil conditions (e.g., soil C:N) might be major factors in increasing soil feeding by E. japonica. Furthermore, the influence of Ca addition on cast production varied according to the earthworm origin. As expected, these differences in cast production are linked to soil N mineralization (especially nitrification). In summary, our study suggests that the acclimatization and/or adaptation of Lumbricidae species to local environmental factors not only soil Ca content explains spatially heterogeneous soil N mineralization in forest soil.     

A disconnect between O horizon and mineral soil carbon – Implications for soil C sequestration

Changing inputs of carbon to soil is one means of potentially increasing carbon sequestration in soils for the purpose of mitigating projected increases in atmospheric CO₂ concentrations. The effect of manipulations of aboveground carbon input on soil carbon storage was tested in a temperate, deciduous forest in east Tennessee, USA. A 4.5-year experiment included exclusion of aboveground litterfall and supplemental litter additions (three times ambient) in an upland and a valley that differed in soil nitrogen availability. The estimated decomposition rate of the carbon stock in the O horizon was greater in the valley than in the upland due to higher litter quality (i.e., lower C/N ratios). Short-term litter exclusion or addition had no effect on carbon stock in the mineral soil, measured to a depth of 30 cm, or the partitioning of carbon in the mineral soil between particulate- and mineral-associated organic matter. A two-compartment model was used to interpret results from the field experiments. Field data and a sensitivity analysis of the model were consistent with little carbon transfer between the O horizon and the mineral soil. Increasing aboveground carbon input does not appear to be an effective means of promoting carbon sequestration in forest soil at the location of the present study because a disconnect exists in carbon dynamics between O horizon and mineral soil. Factors that directly increase inputs to belowground soil carbon, via roots, or reduce decomposition rates of organic matter are more likely to benefit efforts to increase carbon sequestration in forests where carbon dynamics in the O horizon are uncoupled from the mineral soil.     

Resource, biological community and soil functional stability dynamics at the soil–litter interface

The interface between decaying plant residues and soil is a focus for soil ecological processes because of resources from the residues diffusing into the soil, and microfauna that proliferate in the adjacent soil. Given that the recovery of soil function following disturbance depends on immigration, colonization and establishment of exotic organisms from adjacent un-disturbed habitats, and the availability of bio-available resources, we hypothesized that the soil–litter interface could contribute to soil functional stability. In laboratory pot trials, soil was separated into two parts by a mesh bag with the inner section amended, or not amended, with rice straw; an outer layer of unamended soil, adjacent to the litter (1.5 cm thick, either heated or not), provided a soil–litter interface. This enabled us to examine the dynamics of dissolved organic carbon (DOC), mineral nitrogen, microbial biomass carbon (MBC), nematode assemblages and functional stability during 35 days incubation. Either 1 mm or 5 μm meshes were used, which allowed nematodes to migrate (SR1) or not (SR5) through the mesh to the soil–litter interface; thus also enabling us to evaluate the role of nematodes in soil functional stability. Higher DOC and MBC but lower mineral nitrogen concentrations were found at the soil–litter interface. Heating increased the availability of soil resources such as mineral nitrogen and DOC, but decreased the MBC and total nematode abundance in the soil. The soil–litter interface was characterized by a higher abundance of nematodes, particularly microbivores, regardless of mesh aperture or disturbance. The difference in nematode abundance between SR1 and SR5 indicated that nematode propagation, due to resource diffusion and nematode migration through the mesh, contributed to the changing numbers of microbivorous nematodes depending on incubation time. The soil functional stability was calculated as a relative change in the functioning of short-term barley decomposition. Soil functional resistance, defined as the instantaneous effect of disturbance on decomposition measured on the first day, was highest in the SR5 treatment. However, soil functional resilience, defined as the recovery of soil function over the whole incubation period (35d), was highest in the SR1 treatment, which is most probably attributed to the functioning of microbivorous nematodes. Our results suggest that small-scale spatial heterogeneity, due to organic residue decomposition, can help maintain soil functions following disturbance.     

Initial soil organic carbon stocks govern changes in soil carbon: Reality or artifact?

Changes in soil organic carbon (SOC) storage have the potential to affect global climate; hence identifying environments with a high capacity to gain or lose SOC is of broad interest. Many cross-site studies have found that SOC-poor soils tend to gain or retain carbon more readily than SOC-rich soils. While this pattern may partly reflect reality, here we argue that it can also be created by a pair of statistical artifacts. First, soils that appear SOC-poor purely due to random variation will tend to yield more moderate SOC estimates upon resampling and hence will appear to accrue or retain more SOC than SOC-rich soils. This phenomenon is an example of regression to the mean. Second, normalized metrics of SOC change—such as relative rates and response ratios—will by definition show larger changes in SOC at lower initial SOC levels, even when the absolute change in SOC does not depend on initial SOC. These two artifacts create an exaggerated impression that initial SOC stocks are a major control on SOC dynamics. To address this problem, we recommend applying statistical corrections to eliminate the effect of regression to the mean, and avoiding normalized metrics when testing relationships between SOC change and initial SOC. Careful consideration of these issues in future cross-site studies will support clearer scientific inference that can better inform environmental management.     

Can composition and physical protection of soil organic matter explain soil respiration temperature sensitivity?

The importance of soil organic matter (SOM) in the global carbon (C) cycle has been highlighted by many studies, but the way in which SOM stabilization processes and chemical composition affect decomposition rates under natural climatic conditions is not yet well understood. To relate the temperature sensitivity of heterotrophic soil respiration to the decomposition potential of SOM, we compared temperature sensitivities of respiration rates from a 2-year long soil translocation experiment from four elevations along a ~3000 m tropical forest gradient. We determined SOM stabilization mechanisms and the molecular structure of soil C from different horizons collected before and after the translocation. Soil samples were analysed by physical fractionation procedures, ¹³C nuclear magnetic resonance (NMR) spectroscopy, and differential scanning calorimetry (DSC). The temperature sensitivity (Q ₁₀) of heterotrophic soil respiration at the four sites along the elevation transect did not correlate with either the available amount of SOM or its chemical structure. Only the relative distribution of C into physical soil fractions correlated with Q ₁₀ values. We therefore conclude that physical fractionation of soil samples is the most appropriate way to assess the temperature sensitivity of SOM.     

Is vegetation composition or soil chemistry the best predictor of the soil microbial community?

With the species composition and/or functioning of many ecosystems currently changing due to anthropogenic drivers it is important to understand and, ideally, predict how changes in one part of the ecosystem will affect another. Here we assess if vegetation composition or soil chemistry best predicts the soil microbial community. The above and below-ground communities and soil chemical properties along a successional gradient from dwarf shrubland (moorland) to deciduous woodland (Betula dominated) were studied. The vegetation and soil chemistry were recorded and the soil microbial community (SMC) assessed using Phospholipid Fatty Acid Extraction (PLFA) and Multiplex Terminal Restriction Fragment Length Polymorphism (M-TRFLP). Vegetation composition and soil chemistry were used to predict the SMC using Co-Correspondence analysis and Canonical Correspondence Analysis and the predictive power of the two analyses compared. The vegetation composition predicted the soil microbial community at least as well as the soil chemical data. Removing rare plant species from the data set did not improve the predictive power of the vegetation data. The predictive power of the soil chemistry improved when only selected soil variables were used, but which soil variables gave the best prediction varied between the different soil microbial communities being studied (PLFA or bacterial/fungal/archaeal TRFLP). Vegetation composition may represent a more stable ‘summary’ of the effects of multiple drivers over time and may thus be a better predictor of the soil microbial community than one-off measurements of soil properties.     

Does soil organic carbon quality or quantity govern relative temperature sensitivity in soil aggregates?

Soil aggregates govern soil organic carbon (SOC) sequestration. But, sparse understanding about the process leads to inaccuracy in predicting potential of soil to stabilize C in warming world. We appraised effects of 43 years of fertilization on relative temperature sensitivity of SOC decomposition (Q₁₀) in soil aggregates to know whether SOC quality or quantity governs Q₁₀. Treatments were: fallow, control, 100% recommended dose of nitrogen (N), N and phosphorus (NP), N, P and potassium (NPK), and NPK + farmyard manure (FYM) (NPK + FYM). Macroaggregates, microaggregates and silt + clay (s + c) fractions were incubated for 16 weeks at 25, 35 and 45 °C, SOC quality (R₀) and Q₁₀ were computed. SOC mineralization from macro- and micro- aggregates were 34 and 28% higher than s + c across the treatments. The s + c fraction of NPK + FYM had ~ 41, 40 and 24% higher C decay rate than NPK plots at 25, 35 and 45 °C, respectively. For s + c fraction Q₁₀ increased over other aggregates. Mean Q₁₀ of s + c fraction was ~ 18.3 and 17.5% higher than macro and micro-aggregate-C, respectively. R₀ was the lowest for NPK + FYM, suggesting long-term manuring with balanced NPK significantly enhance recalcitrance of C. We observed Q₁₀ of macroaggregates and s + c fraction is controlled by C quality but C quantity governs Q₁₀ of microaggregates in Vertisol. Specifically, microaggregates of NPK + FYM were more temperature sensitive, and could be vulnerable to C loss. Hence, practices facilitating microaggregate formation should be avoided. Thus, we recommend manure application for facilitating C sequestration.

     

urce, biological community and soil functional stability dynamics at the soil–litter interface

The interface between decaying plant residues and soil is a focus for soil ecological processes because of resources from the residues diffusing into the soil, and microfauna that proliferate in the adjacent soil. Given that the recovery of soil function following disturbance depends on immigration, colonization and establishment of exotic organisms from adjacent un-disturbed habitats, and the availability of bio-available resources, we hypothesized that the soil–litter interface could contribute to soil functional stability. In laboratory pot trials, soil was separated into two parts by a mesh bag with the inner section amended, or not amended, with rice straw; an outer layer of unamended soil, adjacent to the litter (1.5 cm thick, either heated or not), provided a soil–litter interface. This enabled us to examine the dynamics of dissolved organic carbon (DOC), mineral nitrogen, microbial biomass carbon (MBC), nematode assemblages and functional stability during 35 days incubation. Either 1 mm or 5 μm meshes were used, which allowed nematodes to migrate (SR1) or not (SR5) through the mesh to the soil–litter interface; thus also enabling us to evaluate the role of nematodes in soil functional stability. Higher DOC and MBC but lower mineral nitrogen concentrations were found at the soil–litter interface. Heating increased the availability of soil resources such as mineral nitrogen and DOC, but decreased the MBC and total nematode abundance in the soil. The soil–litter interface was characterized by a higher abundance of nematodes, particularly microbivores, regardless of mesh aperture or disturbance. The difference in nematode abundance between SR1 and SR5 indicated that nematode propagation, due to resource diffusion and nematode migration through the mesh, contributed to the changing numbers of microbivorous nematodes depending on incubation time. The soil functional stability was calculated as a relative change in the functioning of short-term barley decomposition. Soil functional resistance, defined as the instantaneous effect of disturbance on decomposition measured on the first day, was highest in the SR5 treatment. However, soil functional resilience, defined as the recovery of soil function over the whole incubation period (35d), was highest in the SR1 treatment, which is most probably attributed to the functioning of microbivorous nematodes. Our results suggest that small-scale spatial heterogeneity, due to organic residue decomposition, can help maintain soil functions following disturbance.     

DenNit – Experimental analysis and modelling of soil N2O efflux in response on changes of soil water content, soil temperature, soil pH, nutrient availability and the time after rain event

To quantify the effects of soil temperature (T_soil), and relative soil water content (RSWC) on soil N₂O emission we measured N₂O soil efflux with a closed dynamic chamber in situ in the field and from soil cores in a controlled climate chamber experiment. Additionally we analysed the effect of soil acidity, ammonium, and nitrate concentration in the field. The analysis was performed on three meadows, two bare soils and in one forest. We identified soil water content, soil temperature, soil nitrogen content, and pH as the main parameters influencing soil N₂O emission. The response of N₂O emission to soil temperature and relative soil water content was analysed for the field and climate chamber measurements. A non-linear regression model (DenNit) was developed for the field data to describe soil N₂O efflux as a function of soil temperature, soil moisture, pH value, and ammonium and nitrate concentration. The model could explain 81% of the variability in soil N₂O emission of all individual field measurements, except for data with short-term soil water changes, namely during and up to 2 h after rain stopped. We validated the model with an independent dataset. For this additional meadow site 73% of the flux variation could be explained with the model.     

Potential soil P mobilisation capacity–method development and comparison of rhizosphere soil from different crops

Background

Phosphorus (P) deficiency is wide-spread in agricultural soils. In light of increasing P fertilizer costs, it is of interest to assess the capacity of soil microbes to mobilise native soil P and added P. There is currently no method to assess P mobilisation in situ.

Methods

The soil P mobilisation potential was assessed by incubating low P soil for up to 30?days with poorly available P sources; C and N were added to increase microbial activity and ensure that only P was limiting microbial growth.

Results

The increase in microbial P from day 0 to day 15 showed that microbes were able to mobilise P from FePO₄ and phytate. The P mobilisation potential (sum of microbial and resin P) of the rhizosphere soil decreased in the following order: faba bean > chickpea and white lupin > wheat. After 10?days, up to 80% of the mobilised P was microbial P, whereas after 30?days, almost all P mobilised was resin P.

Conclusions

The method developed in this study is useful assessing not only potential of a soil to mobilise P but also, by using different poorly available P sources, the mechanisms of P mobilisation.     

Bioavailability of Cd in a soil–rice system in China: soil type versus genotype effects

Human exposure to toxic heavy metals via dietary intake is of increasing concern. In this regard, the bioavailability of heavy metals in the soil–crop system is considered to be a key factor controlling plant uptake and, therefore, public health risk through food chain transfer [J. Environ. Sci. Health B 34(4) (1999) 681]. In 2002, a pot experiment was conducted to elucidate the relative significance of soil types and rice genotype on the bioavailability, uptake and partitioning of Cd by two rice cultivars with distinct affinity for Cd. The results indicated that the total uptake of Cd and accumulation in grain was dependent on both soil type and genotype effects. Cd spiking enhanced high Cd uptake and partitioning in grain. Inherent differences in soil type effecting Cd bioavailability were less significant under the Cd spiking regime as compared to non-Cd spiking. In the case of Soil-P, with low Cd bioavailability as indicated by the comparatively lower MgCl₂ extractableCd, differences in metal affinity between genotypes dominated uptake. Conversely, inherent differences in soil type affecting Cd bioavailability dominated uptake in the low metal affinity cultivar treatments. Under the experimental conditions evaluated, the positive interaction between soil type and genotype results in elevated levels of Cd in rice grain with the Cd values exceeding the Chinese food guideline limit of 0.2 mg kg^–1. The results indicated that Cd bioavailability and plant uptake is dependent on soil chemical and physical properties affecting Cd mobility, rice genotype and soil pollution status. The results further suggested that caution should be paid to rice production with the new high metal affinity genotypes on soils with inherent Cd bioavailability as with acidic Red Soils of Jiangxi Provinces, China.     

Can commercial soil microbial treatments remediate plant–soil feedbacks to improve restoration seedling performance?

Seedling performance is often a limiting factor in ecological restoration. Changes in the soil microbial community generated by invasive plants contribute to seedling failure. A method to remediate invasive species‐induced changes to the soil microbial community that results in increased native species seedling performance and decreased invasive species seedling performance could have a large impact on the success of many restoration efforts. In a greenhouse experiment, we first examined the changes in the soil microbial community created by invasive compared to native grasses. Then, we investigated four microbial treatments (bacterial inoculant, fungal inoculant, fungicide, and bactericide/fungicide) to remediate microbial plant–soil feedbacks (PSFs) created by invasive species Bromus inermis and Poa pratensis and increase the performance of natives Andropogon gerardii, Elymus canadensis, Pascopyrum smithii, and Schizachyrium scoparium. We found that the PSF mitigation treatments had some context‐dependent utility for restoration. For example, all of the treatments decreased the performance of B. inermis and fungal inoculant decreased the performance of P. pratensis. However, no single treatment increased the performance of all natives. Fungicide increased the performance of A. gerardii and E. canadensis in soil previously occupied by B. inermis and the performance of S. scoparium in soil previously occupied by P. pratensis. If validated in the field, PSF mitigation treatments may have utility for restoration practitioners.     

What limits nitrate uptake from soil?

Abstract. An accepted view, that unless nitrate concentrations in the soil solution are very low (e.g. below 0.1–0.2 mol m^?3) the growth of high-yielding crops is not limited by the availability of nitrogen, is challenged. Conventional analyses of nutrient supply and demand, based on calculations of apparent inflow rates (uptake rates per unit total root length) are invalid. Apparent inflow rates are inversely proportional to root length. The convention of using total root length grossly overestimates the fraction of the root system active in nutrient uptake. Consequently, inflow rates based on total root lengths underestimate the true values, indicating unrealistically low nutrient concentration differentials between bulk soil and root surfaces required to drive uptake. An alternative method of analysis is suggested. This is based on total nutrient uptake rather than on inflow rate. Measurements of the former do not depend on estimates of active root length and can be made directly and reliably. The method was applied to data obtained from a pot experiment using spring wheat (Triticum aestivum L., cv. Wembley) grown in soil without nitrogen fertilizer (N₀) or with nitrogen fertilizer equivalent to 200kg N ha^?1 (N⁺). Soil nitrate concentrations calculated using the conventional method based on total root length, suggested that any increases in concentration above those measured in the N⁰ treatment should not have resulted in increased uptake and growth. However, the N⁺ plants were always bigger than those in the No treatment, refuting this suggestion. Theoretical uptakes of nitrogen (calculated initially on the basis of a fully active root system) were adjusted, by reducing the effective root length incrementally, until the theoretical uptake matched the measured net uptake of nitrogen. The mean fractions of the root systems likely to have been involved in nitrate uptake were 11% and 3.5% of the total lengths of root in the N⁰ and N⁺ treatments, respectively.     

How do roots penetrate strong soil?

The mechanical and physiological bases for root growth against high mechanical impedance are reviewed. The best estimates of maximum axial root growth pressure (_max) in completely impeded pea roots appear to be from 0.5 to 0.6 MPa, which results from a turgor pressure of about 0.8 MPa. When roots are incompletely impeded, a range of responses has been reported. Roots do not change elongation rate in a simple mechanical way in response to changes in mechanical impedance. Instead, ethylene might play a key role in mediating an increase in root diameter and a decrease in elongation rate. These changes persist for some hours or days after impedance is removed. Differences between species in their ability to penetrate strong soil layers are not related to differences in _max, but appear to be due to differences in root diameter. In rice, differences between cultivars in the ability of their roots to penetrate strong wax layers are not related to their elongation rates through uniformly strong media. Differences between species or cultivars in their ability to penetrate strong layers may be due to differences in the tendency of roots to deflect or buckle when they grow from a weak to a strong environment.