The relationships between erodibility and erosion in a soil treated with two organic amendments

The influence of two organic wastes, cotton gin crushed compost (CC) and beet vinasse (BV) applied for 5 years on a Typic Xerofluvent under dryland conditions near to Sevilla city (Guadalquivir River Valley, Andalusia, Spain) on soil erodibility (K factor of the USLE and RUSLE) and soil loss was studied. CC and BV were applied at rates of 1780, 5340, and 10,680 kg ha⁻¹ (expressed as organic matter content). When CC was applied to the soil, erodibility factor (K) is correlated with soil loss, highlighting a decrease in K and soil loss when increased the dose of CC applied to the soil. In this respect, K decreased 17% in CC-amended soils respect to control soil at the end of the experiment, and soil loss decreased 36% in CC-amended soils respect to control soil at the end of the experiment and for 45 min and 60 mm h⁻¹. However, when BV was applied, soil physical and biological properties decreased. K decreased 6.4% in BV-amended soils respect to control soil at the end of the experiment, and soil loss increased 59.7% in BV-amended soils respect to control soil at the end of the experimental period and for 45 min and 60 mm h⁻¹. We think that this is because the higher level of Na⁺ (and possibly of fulvic acids) in BV increased the exchangeable sodium percentage (ESP) and reduced structural stability of BV-amended soil, leading to higher soil loss. This explains the relatively higher soil loss in BV-amended soils. These results contradict many previous reports in which soil organic matter prevented soil loss. For this reason, the equation of soil erodibility (K factor of USLE and RUSLE) must have in consideration other aspects such as the chemical composition of the soil organic matter as well as the soil structural stability.     

Cultivation effects on temporal changes of organic carbon and aggregate stability in desert soils of Hexi Corridor region in China

Sustainable agricultural use of cultivated desert soils has become a concern in Hexi Corridor in Gansu Province of China, because loss of topsoil in dust storms has been recently intensified. We chose four desert sites to investigate the effects of cultivation (cropping) on (i) soil organic C and its size fractions and (ii) soil aggregate stability (as a measure of soil erodibility). These parameters are of vital importance for evaluating the sustainability of agricultural practices.Total organic C as well as organic C fractions in soil (coarse organic C, 0.1–2 mm; young organic C, 0.05–0.1 mm; stable organic C, <0.05 mm) generally increased with the duration of the cultivation period from 0 (virgin soil, non-cultivated) to more than 30 years (p < 0.05). Compared to total organic C in virgin soils (2.3–3.5 g kg⁻¹ soil), significantly greater values were found after 10 to >20 years of cultivation (6.2–7.1 g kg⁻¹ soil). The increase in organic C in desert soils following prolonged cultivation was mainly the consequence of an increase in the coarse organic C. The increase in total organic C in soil was also dependent on clay content [total organic C = 0.96 + 0.249 clay content (%) + 0.05 cultivation year, R² = 0.48, n = 27, p < 0.001]. This indicates that clay protected soil organic C from mineralization, and also contributed to the increase in soil organic C as time of cultivation increased.There was a significant positive correlation between aggregate stability and total organic C across all field sites. The water stability of aggregates was low (with water-stable aggregate percentage ∼4% of dry-sieved aggregates of size 1–5 mm). There was no consistent pattern of increase in the soil aggregate stability with time of cultivation at different locations, suggesting that desert soils might remain prone to wind erosion even after 50 years of cultivation. Alternative management options, such as retaining harvested crop residues on soil surface and excluding or minimizing tillage, may permit sustainable agricultural use of desert soils.     

Impact of bovine urine deposition on soil microbial activity,biomass, and community structure

Nitrogen (N) from urine excreted by grazing animals can be transformed into N compounds that have detrimental effects on the environment. These include nitrate, which can cause eutrophication of waterways, and nitrous oxide, which is a greenhouse gas. Soil microbes mediate all of these N transformations, but the impact of urine on microbes and how initial soil conditions and urine chemical composition alter their responses to urine are not well understood. This study aimed to determine how soil inorganic N pools, nitrous oxide fluxes, soil microbial activity, biomass, and the community structure of bacteria containing amoA (nitrifiers), nirK, and nirS (denitrifiers) genes responded to the addition of urine over time. Bovine urine containing either a high (15.0 g K⁺ l^?1) or low salt content (10.4 g K⁺ l^?1) was added to soil cores at either low or high moisture content (hereafter termed dry and wet soil respectively; 35% or 70% water-filled pore space after the addition of urine). Changes in soil conditions, inorganic N pools, nitrous oxide fluxes, and the soil microbial community were then measured 1, 3, 8, 15, 29 and 44 days after urine addition. Urine addition increased soil ammonium concentrations by up to 2 mg g d.w.^?1, soil pH by up to 2.7 units, and electrical conductivity (EC) by 1.0 and 1.6 dS m^?1 in the low and high salt urine treatments respectively. In response, nitrate accumulation and nitrous oxide fluxes were lower in dry compared to wet urine-amended soils and slightly lower in high compared to low salt urine-amended soils. Nitrite concentrations were elevated (>3 μg g d.w.^?1) for at least 15 days after urine addition in wet urine-amended soils, but were only this high in the dry urine-amended soils for 1 day after the addition of urine. Microbial biomass was reduced by up to half in the wet urine-amended soils, but was largely unaffected in the dry urine-amended soils. Urine addition affected the community structure of ammonia-oxidising and nitrite-reducing bacteria; this response was also stronger and more persistent in wet than in dry urine-amended soils. Overall, the changes in soil conditions caused by the addition of urine interacted to influence microbial responses, indicating that the effect of urine on soil microbes is likely to be context-dependent.     

陕西省耕地土壤可蚀性因子

[目的]土壤可蚀性因子是计算土壤侵蚀的一个重要因子,对陕西省耕地土壤可蚀性因子展开研究,可为陕西地区的耕地土壤侵蚀计算及评价提供科学依据。[方法]以陕西省9个地区的耕地土壤实测数据为基础,利用通用土壤流失方程USLE(universal soil loss equation)、修订土壤流失方程RUSLE2(revised universal soil loss equation version 2)、侵蚀生产力影响模型EPIC(erosion productivity impact calculator)中可蚀性因子K值的计算公式以及几何平均粒径公式和几何平均粒径—有机质Dg-OM公式,计算不同耕地土壤质地条件下的土壤可蚀性因子。[结果]RUSLE2的极细砂粒转换公式在陕西黄土丘陵沟壑区平均低约14.53%,在陕南地区平均高约32.91%,使用修正公式后平均误差分别为7.81%和13.14%;对比分析K值的估算值与实测值,子洲县实测K值为0.002 69〔(t·hm2·h)/(hm2·MJ·mm)〕,Dg-OM模拟计算均值为0.0297〔(t·hm2·h)/(hm2·MJ·mm)〕;水蚀预报模型WEPP(water erosion prediction project)中的细沟间可蚀性(Ki)和细沟可蚀性(Kr),与USLE的K值相关系数分别为0.738 6和0.607 4。[结论]极细砂粒转换修正公式的计算误差小于RUSLE2模型;Dg-OM模型适合陕西黄土丘陵沟壑区及长武县、杨凌区和安康市典型耕地土壤;WEPP中Ki和Kr,当土壤砂粒含量小于30%,USLE的K值与WEPP的Ki和Kr值有强相关性。     

Rapid estimation of microbial biomass in grassland soils by ultra-violet absorbance

A reliable and simple technique for estimating soil microbial biomass (SMB) is essential if the role of microbes in many soil processes is to be quantified. Conventional techniques are notoriously time-consuming and unreproducible. A technique was investigated that uses the UV absorbance at 280 nm of 0.5 M K₂SO₄ extracts of fumigated and unfumigated soils to estimate the concentrations of carbon, nitrogen and phosphorus in the SMB. The procedure is based on the fact that compounds released after chloroform fumigation from lysed microbial cells absorb in the near UV region. Using 29 UK permanent grassland soils, with a wide range of organic matter (2.9–8.0%) and clay contents (22–68%), it was demonstrated that the increase in UV absorbance at 280 nm after soil fumigation was strongly correlated with the SMB C (r=0.92), SMB N (r=0.90) and SMB P (r=0.89), as determined by conventional methods. The soils contained a wide range of SMB C (412–3412 μg g⁻¹ dry soil), N (57–346 μg g⁻¹ dry soil) and P (31–239 μg g⁻¹ dry soil) concentrations. It was thus confirmed that the UV absorbance technique described was a rapid, simple, precise and relatively inexpensive method of estimating soil microbial biomass.     

Soil losses due to cassava and sweet potato harvesting: A case study from low input traditional agriculture

Soil loss due to crop harvesting (SLCH) has been established as an important soil erosion process that has significantly contributed to soil degradation in highly mechanised agriculture. This has stimulated the need to investigate the importance of this process of erosion under low input agriculture where, until now, only water and tillage erosion are known as important phenomena causing soil degradation. This study was conducted in Eastern Uganda with the following objectives: (1) to assess the amount of soil lost due to the harvesting of cassava roots and sweet potato tubers under low input agriculture, (2) to look into the factors that influence variations in these soil losses, and (3) to estimate the amount of plant nutrients lost due to SLCH for cassava and sweet potato. Soil sticking to roots and tubers was washed and the soil suspension oven dried to estimate the amount of soil lost after harvesting. Mean annual soil loss for cassava was 3.4 tonnes ha⁻¹ and for sweet potato was 0.2 tonnes ha⁻¹. Ammonium acetate lactate extractable soil nutrient losses for cassava were N = 1.71 kg ha⁻¹ harvest⁻¹, P = 0.16 kg ha⁻¹ harvest⁻¹, K = 1.08 kg ha⁻¹ harvest⁻¹ and for sweet potato were N = 0.14, P = 0.01 kg ha⁻¹ harvest⁻¹, K = 0.15 kg ha⁻¹ harvest⁻¹. Difference in soil loss due to crop harvesting for cassava and sweet potato could be due to: (1) smaller yields of sweet potato leading to smaller soil losses on an area basis, (2) smoother skin and less kinked morphology of sweet potato that allowed less soil to adhere, and (3) the fact that sweet potato is planted in mounds which dry out faster compared to the soil under cassava. Soil moisture content at harvesting time and crop age were significant factors that explained the variations in the soil lost at cassava harvesting. Soil loss under cassava justifies the need to conduct further investigations on this process of soil erosion under low input agriculture.     

Nitrogen fertilization and cropping systems effects on soil organic carbon and total nitrogen pools under chisel-plow tillage in Illinois

Agricultural soils can be a major sink for atmospheric carbon (C) with adoption of recommended management practices (RMPs). Our objectives were to evaluate the effects of nitrogen (N) fertilization and cropping systems on soil organic carbon (SOC) and total N (TN) concentrations and pools. Replicated soil samples were collected in May 2004 to 90 cm depth from a 23-year-old experiment at the Northwestern Illinois Agricultural Research and Demonstration Center, Monmouth, IL. The SOC and TN concentrations and pools, soil bulk density (ρ_b) and soil C:N ratio were measured for five N rates [0 (N₀), 70 (N₁), 140 (N₂), 210 (N₃) and 280 (N₄) kg N ha⁻¹] and two cropping systems [continuous corn (Zea mays L.) (CC), and corn–soybean (Glycine max (L.) Merr.) rotation (CS)]. Long-term N fertilization and cropping systems significantly influenced SOC concentrations and pools to 30 cm depth. The SOC pool in 0–30 cm depth ranged from 68.4 Mg ha⁻¹ for N₀ to 75.8 Mg ha⁻¹ for N₄. Across all N treatments, the SOC pool in 0–30 cm depth for CC was 4.7 Mg ha⁻¹ greater than for CS. Similarly, TN concentrations and pools were also significantly affected by N rates. The TN pool for 0–30 cm depth ranged from 5.36 Mg ha⁻¹ for N₀ to 6.14 Mg ha⁻¹ for N₄. In relation to cropping systems, the TN pool for 0–20 cm depth for CC was 0.4 Mg ha⁻¹ greater than for CS. The increase in SOC and TN pools with higher N rates is attributed to the increased amount of biomass production in CC and CS systems. Increasing N rates significantly decreased ρ_b for 0–30 cm and decreased the soil C:N ratio for 0–10 cm soil depth. However, none of the measured soil properties were significantly correlated with N rates and cropping systems below 30 cm soil depth. We conclude that in the context of developing productive and environmentally sustainable agricultural systems on a site and soil specific basis, the results from this study is helpful to strengthening the database of management effects on SOC storage in the Mollisols of Midwestern U.S.     

Detection and quantification of the nematophagous fungus Hirsutella minnesotensis in soil with real-time PCR

A real-time PCR assay was developed to quantify in soil the fungus Hirsutella minnesotensis, an important parasite of secondary-stage juvenile (J2) of the soybean cyst nematode. A primer pair 5′-GGGAGGCCCGGTGGA-3′ and 5′-TGATCCGAGGTCAACTTCTGAA-3′ and a TaqMan probe 5′-CGTCCGCCGTAAAACGCCCAAC-3′ were designed based on the sequence of the ITS region of the rRNA gene. The primers were highly species-specific. The PCR reaction system was very sensitive and able to detect as few as 4 conidia g^?1 soil. Regression analysis showed similar slopes and efficiency on DNA from pure culture (y = ?3.587x + 41.017, R² = 0.9971, E = 0.9055) and from Log conidia g^?1 soil (y = ?3.855x + 37.669, R² = 0.9139, E = 0.8172), indicating that the real-time PCR protocol can reliably quantify H. minnesotensis in the soil. The real-time PCR assay was applied to 20 soil samples from soybean fields, and compared with a parasitism assay. The real-time PCR assay detected H. minnesotensis in six of the soils, whereas the parasitism assay detected H. minnesotensis in the same six soils and three additional soils. The real-time PCR assay was weakly correlated (R² = 0.49) with the percentage of parasitized J2 in the six soils, indicating that different types of soil may interfere the efficiency of the real-time PCR assay, possibly due to the effect of soil types on efficacy of DNA extraction. The parasitism assay appeared to be more sensitive than real-time PCR in detecting presence of H. minnesotensis, but real-time PCR was much faster and less costly and provided a direct assessment of fungal biomass. Using the two assays in combination can obtain more complete information about the fungus in soil than either assay alone. Hirsutella parasitism was widespread and detected in 13 of the 20 field soils, indicating that these fungi may contribute to suppressiveness of soybean cyst nematode in nature and likely have high biological control potential for the nematode.     

Molecular and physiological bacterial diversity of a semi-arid soil contaminated with different levels of formulated atrazine

A semi-arid soil treated with different concentrations of formulated atrazine in a laboratory experiment was studied over 45 days, by different biological and molecular parameters (bacterial enumeration (cfu), community level physiological profiles (CLPPs) measured by Biolog^® and denaturing gradient gel electrophoresis (DGGE)), to study the bacterial community diversity.Formulated atrazine was almost totally degraded at different concentrations after this incubation time. The number of colony forming units (cfu) for soils with 100 and 1000 mg kg⁻¹ atrazine was significantly (p ≤ 0.05) higher than for the control, 1 and 10 mg kg⁻¹ treatments. DGGE banding patterns showed that regardless of time elapsed, concentrations of 10, 100 and 1000 mg kg⁻¹ atrazine in soil, affected the bacterial community compared to control and 1 mg kg⁻¹.The Shannon diversity index (H′) based on CLPP data showed a significant (p ≤ 0.05) decrease at atrazine concentrations of 100 and 1000 mg kg⁻¹. The Shannon diversity indices for different guilds of source carbon and the parameters K and r (based on the kinetics of colour formation rather than on the degree of colour development) were related to guilds of carbon substrates and atrazine concentration at a sampling time. The parameter K was very sensitive to atrazine effects on microbial communities.These biological and molecular parameters can be used to monitor changes in soils treated with atrazine at different concentrations, even when the pesticide is degraded.     

The temperature dependence of dicyandiamide (DCD) degradation in soils: A data synthesis

Dicyandiamide (DCD, C₂H₄N₄) is a nitrification inhibitor that has been studied for more than 80 years. However, there are few papers that have examined the use of DCD on dairy farms where cattle graze pasture and where urine is the primary form of nitrogen (N) deposited onto soils. After DCD was applied (10 kg DCD ha^?1) with bovine urine (700–1200 kg N ha^?1) to five soils throughout New Zealand, the reduction in direct nitrous oxide (N₂O) emissions was significant and remarkably consistent (71 ± 8%, average ± standard error). The application of DCD to these soils occurred in autumn and winter; daily average soil temperature (T) was reported but these data were not further analysed. Perusal of the literature suggested no consensus on the temperature dependence of DCD degradation in soils. Based on published data from controlled-environment studies of soils sampled in four countries, we quantified the relation between T and the time for DCD concentration in soils to decline to half its application value (t_½) as t_½ (T) = 168e^?0.084T with parameter standard errors of ±16 d and ±0.011 d^?1, respectively (n = 16). For example, at 5 °C a 1 °C increase in T reduced t_½ from 110 to 101 d whereas at 25 °C the reduction was 20–19 d. Analysing T data from the New Zealand trials using our t_½ (T) function, over 43–89 d when direct N₂O emissions from treated plots became indistinguishable from the controls, the estimated percentage of applied DCD remaining in the soil averaged 43 ± 10%. These calculations suggested the apparently remaining DCD was ineffective with respect to direct N₂O emissions. In the absence of measurements, explanations for this interpretation included vertical displacement of the DCD and sorption onto organic matter in soils. The consistent DCD efficacy from these trials corresponded with T generally <10 °C, so it is suggested as an application criteria for the reduction of direct N₂O emissions from pastoral soils subjected to urine excretion by grazing cattle.     

Partitioning soil surface CO2 efflux into autotrophic and heterotrophic components,using natural gradients in soil δ13C in an undisturbed savannah soil

We used natural gradients in soil and vegetation δ¹³C signatures in a savannah ecosystem in Texas to partition soil respiration into the autotrophic (Ra) and heterotrophic (Rh) components. We measured soil respiration along short transects from under clusters of C₃ trees into the C₄ dominated grassland. The site chosen for the study was experiencing a prolonged drought, so an irrigation treatment was applied at two positions of each transect. Soil surface CO₂ efflux was measured along transects and CO₂ collected for analysis of the δ¹³C signature in order to: (i) determine how soil respiration rates varied along transects and were affected by localised change in soil moisture and (ii) partition the soil surface CO₂ efflux into Ra and Rh, which required measurement of the δ¹³C signature of root- and soil-derived CO₂ for use in a mass balance model.The soil at the site was unusually dry, with mean volumetric soil water content of 8.2%. Soil respiration rates were fastest in the centre of the tree cluster (1.5 ± 0.18 μmol m^?2 s^?1; mean ± SE) and slowest at the cluster–grassland transition (0.6 ± 0.12 μmol m^?2 s^?1). Irrigation produced a 7–11 fold increase in the soil respiration rate. There were no significant differences (p > 0.5) between the δ¹³C signature of root biomass and respired CO₂, but differences (p < 0.01) were observed between the respired CO₂ and soil when sampled at the edge of the clusters and in the grassland. Therefore, end member values were measured by root and soil incubations, with times kept constant at 30 min for roots and 2 h for soils. The δ¹³C signature of the soil surface CO₂ efflux and the two end member values were used to calculate that, in the irrigated soils, Rh comprised 51 ± 13.5% of the soil surface CO₂ efflux at the mid canopy position and 57 ± 7.4% at the drip line. In non-irrigated soil it was not possible to partition soil respiration, because the δ¹³C signature of the soil surface CO₂ efflux was enriched compared to both the end member values. This was probably due to a combination of the very dry porous soils at our study site (which may have been particularly susceptible to ingress of atmospheric CO₂) and the very slow respiration rates of the non-irrigated soils.     

Accumulation of As,Pb, Zn,Cd and Cu and arbuscular mycorrhizal status in populations of Cynodon dactylon grown on metal-contaminated soils

Metal(loid) accumulation and arbuscular mycorrhizal (AM) status of the dominant plant species, Cynodon dactylon, growing at four multi-metal(loid)s-contaminated sites and an uncontaminated site of China were investigated. Up to 94.7 As mg kg^?1, 417 Pb mg kg^?1, 498 Zn mg kg^?1, 5.8 Cd mg kg^?1 and 27.7 Cu mg kg^?1 in shoots of C. dactylon were recorded. The plant was colonized consistently by AM fungi (33.0–65.5%) at both uncontaminated site and metal-contaminated sites. Based on morphological characteristics, fourteen species of AM fungi were identified in the rhizosphere of C. dactylon, with one belonging to the genus of Acaulospora and the other thirteen belonging to the genus of Glomus. Glomus etunicatum was the most common species associated with C. dactylon growing at metal-contaminated sites. Spore abundance in the rhizosphere of C. dactylon growing at the metal-contaminated soils (22–82 spores per 25 g soil) was significantly lower than that of the uncontaminated soils (371 spores per 25 g soil). However, AM fungal species diversity in the metal-contaminated soils was significantly higher than that in the uncontaminated soils. This is the first report of AM status in the rhizosphere of C. dactylon, the dominant plant survival in metal-contaminated soils. The investigation also suggests that phytorestoration of metal-contaminated sites might be facilitated using the appropriate plant with the aid of tolerant AM fungi.     

Enhancement of arbuscular mycorrhizal fungus (Glomus versiforme) on the growth and Cd uptake by Cd-hyperaccumulator Solanum nigrum

Arbuscular mycorrhizal fungus (AMF) can enhance plant growth and resistance to toxicity produced by heavy metals (HMs), affect the bioavailability of HMs in soil and the uptake of HMs by plants, and thus has been emerged as the most prominent symbiotic fungus for contribution to phytoremediation. A greenhouse pot experiment was conducted to assess the effect of Glomus versiforme BGC GD01C (Gv) on the growth and Cd accumulation of Cd-hyperaccumulator Solanum nigrum in different Cd-added soils (0, 25, 50, 100 mg Cd kg⁻¹ soil). Mycorrhizal colonization rates were generally high (from 71% to 82%) in Gv-inoculated treatments at all Cd levels. Gv colonization enhanced soil acid phosphatase activity, and hence elevated P acquisition and growth of S. nigrum at all Cd levels. Moreover, the presence of Gv significantly increased DTPA-extractable (phytoavailable) Cd concentrations in 25 and 50 mg Cd kg⁻¹ soils, but did not affect phytoavailable Cd in 100 mg Cd kg⁻¹ soil. Similarly, inoculation with Gv significantly increased Cd concentrations of S. nigrum in 25 and 50 mg Cd kg⁻¹ soils, but decreased Cd concentrations of the plants in 100 mg Cd kg⁻¹ soil. Overall, inoculation with Gv greatly improved the total Cd uptakes in all plant tissues at all Cd levels. The present results indicated that S. nigrum associated with Gv effectively improved the Cd uptake by plant and would be a new strategy in microbe-assisted phytoremediation for Cd-contaminated soils.     

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.     

Survival and persistence of genetically modified Sinorhizobium meliloti in soil

Studies were conducted to evaluate the survival and persistence of Sinorhizobium meliloti 104A14 and two acid phosphatase-negative mutants in Kirkland (fine, mixed, thermic Udertic Paleustolls) silt loam soils with various fertility levels, and to assess the impact of inoculation on nodule occupancy and soil microbial community structure in the inoculated alfalfa (Medicago sativa L.) rhizosphere. Recovery of the inoculated strains was 100% (in the order of 10⁸ cells g⁻¹ soil) immediately following inoculation to soils, but decreased from 10⁸ cells g⁻¹ soil to undetectable levels in a nutrient-poor soil within 32 days. In a nutrient-rich soil, approximately 2–3% (4.7–7.43×10⁶ cells g⁻¹ soil) of the mutants and 23% (5.84×10⁷ cells g⁻¹ soil) of the wild-type inocula persisted for more than 64 days. Survivability and persistence of the wild-type S. meliloti were significantly greater than that of the genetically modified acid phosphatase negative mutants in all the soils tested. The persistence and nodule occupancy of the introduced S. meliloti in sterile and non-sterile soils were also tested for two repeated alfalfa growth periods in the same plant growth units, with a 1 month interval in between and no additional inoculation for the second period. Nodule occupancy of the introduced S. meliloti in non-sterile soils ranged from 30 to 60% for the first period and 85 to 100% for the second period. Our results suggest that survival and persistence of S. meliloti was enhanced by alfalfa cultivation and increased soil fertility, but impaired by mutation of acid phosphatase genes regardless of phosphorus nutritional levels. Moreover, inoculation with genetically modified S. meliloti strain 104A14 promoted indigenous bacterial growth in soil (increased bacterial population from 1.4×10⁶ to 4.3×10⁶ cells g⁻¹ soil), but not the growth of fungi and yeast. However, inoculation of the wild-type S. meliloti or genetically modified mutants did not result in significant changes in microbial community structure as indicated by EP indices and ratios of r/K strategists.     

Respiratory substrate availability plays a crucial role in the response of soil respiration to environmental factors

We examined the response of the temperature coefficient (Q₁₀) for soil respiration to changes in soil temperature and soil moisture through a laboratory incubation experiment. Two types of soils differing in vegetation and moisture status were collected and incubated under two temperatures (10 and 30 °C) and two soil moisture regimes (35 and 75% of water holding capacity, WHC) for 5 weeks. Before and after the incubation experiment, the temperature coefficient of soil respiration was measured using soda-lime method by changing temperature in a water bath. For both soils, the mean Q₁₀ values of the respiration rate were 2.0 in the 30 °C and 2.3 in the 10 °C soil treatments. Higher temperature with lower soil moisture treatment significantly decreased the Q₁₀ value, whereas lower temperature with higher soil moisture treatment significantly enhanced the Q₁₀ value (ANOVA, p < 0.05). These results indicate that soils became less sensitive to temperature when incubated under higher temperature with higher moisture conditions, and more sensitive in lower temperature with higher moisture conditions.There was a significant correlation (r² = 0.67, p < 0.05) between water-soluble carbon (WSC) and soil respiration rate. However, the correlation between soil respiration rate and microbial biomass carbon (MBC) was weak (r² = 0.27, p > 0.05). Although incubation temperature and moisture accounted for 40 and 29% (as r² × 100%), respectively, of variations in Q₁₀, soil water-soluble carbon content alone could have explained 79% of the variation, indicating that the availability of respiratory substrate, rather than the pool of soil microorganisms, played a crucial role in the response of the temperature coefficient to environmental factors. These results suggest that biotic factors should also be taken into consideration when using the Q₁₀ function to predict the response of soil respiration to global warming.     

Improved soil structure and citrus growth after inoculation with three arbuscular mycorrhizal fungi under drought stress

In a controlled potted experiment, citrus (Poncirus trifoliata) seedlings were inoculated with three species of arbuscular mycorrhizal (AM) fungi, Glomus mosseae, G. versiforme or G. diaphanum. Two soil-water levels (ample water, −0.10 MPa; drought stress, −0.44 MPa) were applied to the pots 4 months after transplantation. Eighty days after water treatments, the soils and the citrus seedlings were well colonized by the three AM fungi. Mycorrhizal fungus inoculation improved plant biomass regardless of soil-water status but decreased the concentrations of hot water-extractable and hydrolyzable carbohydrates of soils. Mycorrhizal soils exhibited higher Bradford-reactive soil protein concentrations than non-mycorrhizal soils. Mycorrhizas enhanced >2 mm, 1–2 mm and >0.25 mm water-stable aggregate fractions but reduced 0.25–0.5 mm water-stable aggregates. Peroxidase activity was higher in AM than in non-AM soils whether drought stressed or not, whereas catalase activity was lower in AM than non-AM soils. Drought stress and AM fungus inoculation did not affect polyphenol oxidase activity of soils. A positive correlation between the Bradford-reactive soil protein concentrations, soil hyphal length densities, and water-stable aggregates (only >2 mm, 1–2 mm and >0.25 mm) suggests beneficial effects of the AM symbiosis on soil structure. It concluded that AM fungus colonization enhanced plant growth under drought stress indirectly through affecting the soil moisture retention via glomalin's effect on soil water-stable aggregates, although direct mineral nutritional effects could not be excluded.     

Turnover of low molecular weight dissolved organic C (DOC) and microbial C exhibit different temperature sensitivities in Arctic tundra soils

Polar ecosystems are currently experiencing some of the fastest rates of climate warming. An increase in soil temperature in High Arctic regions may stimulate soil permafrost melting and microbial activity, thereby accelerating losses of greenhouse gases. It is therefore important to understand the factors regulating the rates of C turnover in polar soils. Consequently, our aims were to: (1) assess the concentration of low molecular weight (MW) dissolved organic carbon (DOC) in soil, (2) to investigate the temperature-dependent turnover of specific low MW compounds, and (3) to analyse the influence of substrate concentration on C cycling. Microbial mineralisation of labile low MW DOC in two High Arctic tundra soils was investigated using soil solutions spiked with either ¹⁴C-labelled glucose or amino acids. Spiked solutions were added to the top- and sub-soil from two ecosystem types (lichen and Carex dominated tundra), maintained at three temperatures (4–20 °C), and their microbial mineralisation kinetics monitored. ¹⁴CO₂ evolution from the tundra soils in response to ¹⁴C-glucose and -amino acid addition could best be described by a double first order exponential kinetic equation with rate constants k₁ and k₂. Both forms of DOC had a short half-life (t_1/2) in the pool of microbial respiratory substrate (t_1/2 = 1.07 ± 0.10 h for glucose and 1.63 ± 0.14 h for amino acids; exponential coefficient k₁ = 0.93 ± 0.07 and 0.64 ± 0.06 h^?1 respectively) whilst the second phase of mineralisation, assumed to be C that had entered the microbial biomass, was much slower (average k₂ = 1.30 × 10^?3 ± 0.49 × 10^?4 h^?1). Temperature had little effect on the rate of mineralisation of ¹⁴C used directly as respiratory substrate. In contrast, the turnover rate of the ¹⁴C immobilized in the microbial biomass prior to mineralisation was temperature sensitive (k₂ values of 0.99 × 10^?3 h^?1 and 1.66 × 10^?3 h^?1 at 4 and 20 °C respectively). Concentration-dependent glucose and amino acid mineralisation kinetics of glucose and amino acids (0–10 mM) were best described using Michaelis–Menten kinetics; there was a low affinity for both C substrates by the microbial community (K_m = 4.07 ± 0.41 mM, V_max = 0.027 ± 0.005 mmol kg^?1 h^?1). In conclusion, our results suggest that in these C limiting environments the flux of labile, low MW DOC through the soil solution is extremely rapid and relatively insensitive to temperature. In contrast, the turnover of C incorporated into higher molecular weight microbial C pools appears to show greater temperature sensitivity.     

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.     

Long-term management impacts on soil C,N and physical fertility: Part II: Bad Lauchstadt static and extreme FYM experiments

Manure is a source of plant nutrients and can make a valuable contribution to soil organic matter (SOM). Two experimental sites were studied on a Halpic Phaeozem soil near Bad Lauchstadt in Germany. The first experiment, called the static experiment, commenced in 1902. The impact of fresh farmyard manure (FYM) (0, 20 and 30 t ha⁻¹ 2 year⁻¹) combined with P, K and N fertiliser application on total organic C (C_T), labile C (C_L), non-labile C (C_NL), total N (N_T), mean weight diameter (MWD) and unsaturated hydraulic conductivity (K_unsat) was investigated. The second experiment commenced in 1984 and investigated the effect of extreme rates of fresh FYM applications (0, 50, 100 and 200 t ha⁻¹ year⁻¹) and cropping, or a continuous tilled fallow on the same soil properties. At both sites a nearby grassland site served as a reference. On the static experiment, FYM application increased all C fractions, particularly C_L, where application of 30 t ha⁻¹ 2 year⁻¹ increased C_L by 70% compared with no FYM application. Fertiliser additions to the static experiment had a positive influence on C fractions while N_T increased from both FYM and fertiliser application. MWD increased as a result of FYM application, but did not reach that of the grassland site. Both fertiliser and FYM application increased K_unsat (10 mm tension) on the static experiment. In the second experiment application of 200 t ha⁻¹ year⁻¹ of FYM increased concentrations of C_L by 173% and of C_NL by 80%, compared with no FYM application to make them equivalent to, or greater than the grassland site. A continuously tilled fallow resulted in significant decreases in all C fractions, N_T and MWD compared with the cropped site, while K_unsat (10 mm tension) was increased on the 0 and 50 t ha⁻¹ year⁻¹ treatments as a result of a recent tillage. There was no difference in K_unsat between the cropped and the continuous tilled fallow at FYM applications of 100 and 200 t ha⁻¹ year⁻¹. There were similar significant positive correlations of all C fractions and N_T with MWD on both experimental sites but the relationships were much stronger on the extreme FYM experiment. Weaker relationships of C fractions and N_T with K_unsat (10 mm tension) occurred for the static experimental site but these were not significant for the extreme FYM experimental site. The strongest relationship between C fractions and K_unsat was with C_L. This research has shown that applications of FYM can increase SOM and improve soil physical fertility. However, the potential risk of very high rates of FYM on the environment need to be taken into consideration, especially since the application of organic materials to soils is likely to increase in the future.