Effects of trifluralin on soil carbon mineralization at different temperature conditions

Trifluralin is a herbicide intensively used in Turkish cotton agriculture. The recommended field dose [(RFD), 480 g active ingredient l^?1], 2 × RFD, 4 × RFD and 6 × RFD of this herbicide were added to virgin (previously no trifluralin applied) and cotton field soils (previously trifluralin applied) from a district (Yumurtal?k, Adana) under Mediterranean climate conditions in order to determine their effects on soil microbial activity as measured by carbon mineralization at the different temperature conditions (20 °C, 25 °C and 30 °C). C mineralization of all samples was determined by the CO₂ respiration method over 30 days (20 °C, 25 °C and 30 °C at constant moist). The ratio (%) of carbon mineralization at all doses of cotton field soil at 30 °C was significantly higher than all other field dose–temperature combinations (P < 0.001). Based on these results, trifluralin is used as a carbon source by soil microorganisms. The herbicide trifluralin was degraded completely in the cotton field but a small fraction remained in the virgin field. This result can be explained by the cotton field soil having both more active microbial populations and more microorganisms adapted to the trifluralin applications than the virgin field.     

Effect of freezing on soil nitrogen mineralization under different plant communities in a semi-arid area during a non-growing season

The influences of winter climate on terrestrial ecosystem processes have been the subject of growing attention, which is necessary to make the predictions about ecological responses to global warming in the future. However, little information can be found about the impacts of a large range of soil temperature fluctuation (e.g. −10 to 5 °C) over winter on the soil nitrogen (N) dynamics in the field. In the present study, we employed an intact soil core in situ incubation technique, and measured soil N mineralization and nitrification rates under three plant communities, i.e. a grassland, a shrub and a plantation, during the non-growing season (October 2004–April 2005) in Inner Mongolia, China. Our results demonstrate the significant effects of different plant communities on soil net N mineralization and the great temporal variations of soil N dynamics during the incubation period. The mean soil net N mineralization rates were 0.93, 0.77 and −1.28 mg N m⁻² d⁻¹, respectively, in the grassland, shrub and plantation. The mean soil NH₄⁺-N in the three plant communities declined by 40%, but the mean soil NO₃⁻-N increased by 190% by the end of the incubation compared with their initial concentrations at the beginning of incubation. The differences in plant communities significantly affected their soil N mineralization rates, accumulations and turnover rates, which followed the order: grassland > shrub > plantation. During the winter time, the studied soils experienced the three phases consisting of mild freezing (−7 to −2 °C soil), deep freezing (approximately −10 °C soil) and freeze–thaw (−2 to 5 °C soil). The results suggest that temporal variations of soil N mineralization are positively affected by the soil temperature and the soil nitrification is dominant in the N transformation process during the non-growing season. Our study indicates that the soil N mineralization over winter can make a substantial contribution to the mineral N pool that plants are able to utilize in the upcoming spring, but may also pose a great risk of mineral N leaching loss if great rainfalls occur during spring and early summer.     

Identifying indicators of C and N cycling in a clayey Ultisol under different tillage and uses in winter

Although tropical and subtropical environments permit two cropping cycles per year, maintaining adequate mulching on the soil surface remains a challenge. In some cases, leaving soils fallow during the winter as an agricultural practice to control pathogens contributes to reduce soil mulching. The aim of this study was to assess attributes associated with C and N cycling in a soil under conventional and no-tillage management, with contrasting uses in winter: black oats (Avena strigosa Schreb) as cover crop or fallow. No-tillage increased total C and N, irrespective the winter crop. Cropping black oats under no-tillage resulted in more microbial biomass C and N, and glutaminase activity (15.2%, 65.2%, and 24%, respectively) than no-tillage under fallow. Under conventional tillage, winter cropping did not affect the attributes under study. Available P was higher in the no-tillage system (9.2–12.3 mg kg⁻¹), especially when cropped with black oats, than in the conventional tillage system (4.8–6.6 mg kg⁻¹). A multivariate analysis showed strong relationships between soil microbiological and chemical attributes in the no-tillage system, especially when cropped with black oats. Soil pH, dehydrogenase and acid phosphatase activities were the most effective at separating the soil use in winter. Microbial N, total N, microbial to total N ratio, available P, metabolic quotient (qCO₂), and glutaminase activity were more effective at separating soil management regimes. The no-tillage system in association with winter oat cropping stimulated the soil microbial community, carbon and nutrient cycling, thereby helping to improve the sustainability of the cropping system.     

Overgrazing and soil carbon dynamics in the western Chaco of Argentina

Very little is known about the effect of overgrazing on carbon loss from soil in semi-arid savannas and woodlands of South America. Soil carbon parameters were measured in a 10,000 ha restoration project in the western Chaco of Argentina (24°43′S and 63°17′W). Three situations were compared: highly restored (HRS), moderately restored (MRS) and highly degraded (HDS). Soil and litter samples were recovered in the dry and wet seasons. SOC and CO₂–C values decreased from the HRS (7.0 kg m⁻² and 130 g m⁻²) to the HDS (1.5 kg m⁻² and 46 g m⁻²) whereas the C mineralization rate increased toward the less restored sites (0.96–2.29). Surface-litter C was similar in both sites under restoration (260 and 229 g m⁻²), being non-existent at the HDS. Leaves from woody species dominated surface-litter in the HRS, whereas grass material was predominant in the MRS. During the wet season, the SOC decreased, whereas both CO₂–C and C mineralization rate increased. The magnitude of the between-season differences was highest at the HDS (62% in SOC, 55% in CO₂, and 80% in C mineralization rate). We estimated that C loss since introduction of cattle into the forest was 58 Mg ha⁻¹, reaching a total of 2×10¹⁵ g at for the entire Chaco. These values are higher than those caused by the conversion of savannas and other ecosystems into agriculture or cultivated pastures. The amount of C fixed in the highly restored site (275 g ha⁻¹ per year) indicates that the Chaco soils have a significant potential as atmospheric carbon sinks.     

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.     

Soil moisture effects on infectivity and persistence of the entomopathogenic nematodes Steinernema scarabaei,S. glaseri,Heterorhabditis zealandica,and H. bacteriophora

We tested the effect of soil moisture on the performance of four entomopathogenic nematodes species that have recently shown promise for the control of white grubs, i.e., Heterorhabditis bacteriophora, H. zealandica, Steinernema scarabaei, and S. glaseri. Experiments for all four nematodes were conducted in sandy loam, for S. scarabaei also in loamy sand and silt loam. Infectivity was tested by exposing third-instar Japanese beetle, Popillia japonica, to nematodes in laboratory experiments and determining nematode establishment in the larvae and larval mortality. Nematode infectivity was the highest at moderate soil moistures (−10 to −100 kPa), and tended to be lower in wet (−1 kPa) and moderately dry (−1000 kPa) soil. In dry soil (−3000 kPa), only S. scarabaei showed some activity. S. scarabaei was active from −1 to −3000 kPa in all soil types but the range of highest activity was wider in loamy sand (−1 to −1000 kPa) than in loamy sand and silt loam (−10 to −100 kPa). Persistence was determined in laboratory experiments by baiting nematode-inoculated soil with larvae of the greater wax moth, Galleria mellonella. For both Heterorhabditis spp. persistence was short at −10 kPa, improved slightly at −100 kPa, significantly at −1000 kPa, and was the highest at −3000 kPa. Both Steinernema spp. persisted very well at −10 kPa. However, S. glaseri persistence was the shortest at −10 kPa but did not differ significantly at −100 to −3000 kPa, whereas S. scarabaei persistence was not affected by soil moisture. Our observations concur with previous observations on the effect of soil moisture on entomopathogenic nematodes but also show that moisture ranges for infectivity and persistence vary among species. Differences among species may be based on differences in size and behavioral and physiological adaptations.     

Effects of soil conditions and drought on egg hatching and larval survival of the clover root weevil (Sitona lepidus)

Soil-dwelling insect herbivores are significant pests in many managed ecosystems. Because eggs and larvae are difficult to observe, mathematical models have been developed to predict life-cycle events occurring in the soil. To date, these models have incorporated very little empirical information about how soil and drought conditions interact to shape these processes. This study investigated how soil temperature (10, 15, 20 and 25 °C), water content (0.02 (air dried), 0.10 and 0.25 g g^?1) and pH (5, 7 and 9) interactively affected egg hatching and early larval lifespan of the clover root weevil (Sitona lepidus Gyllenhal, Coleoptera: Curculionidae). Eggs developed over 3.5 times faster at 25 °C compared with 10 °C (hatching after 40.1 and 11.5 days, respectively). The effect of drought on S. lepidus eggs was investigated by exposing eggs to drought conditions before wetting the soil (2–12 days later) at four temperatures. No eggs hatched in dry soil, suggesting that S. lepidus eggs require water to remain viable. Eggs hatched significantly sooner in slightly acidic soil (pH 5) compared with soils with higher pH values. There was also a significant interaction between soil temperature, pH and soil water content. Egg viability was significantly reduced by exposure to drought. When exposed to 2–6 days of drought, egg viability was 80–100% at all temperatures but fell to 50% after 12 days exposure at 10 °C and did not hatch at all at 20 °C and above. Drought exposure also increased hatching time of viable eggs. The effects of soil conditions on unfed larvae were less influential, except for soil temperature which significantly reduced larval longevity by 57% when reared at 25 °C compared with 10 °C (4.1 and 9.7 days, respectively). The effects of soil conditions on S. lepidus eggs and larvae are discussed in the context of global climate change and how such empirically based information could be useful for refining existing mathematical models of these processes.     

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.     

Bacterial community structure of nirK-bearing denitrifiers and the development of properties of soils in created mitigation wetlands

We investigated the abundance and genetic heterogeneity of bacterial nitrite reductase genes (nir) and soil structural properties in created and natural freshwater wetlands in the Virginia piedmont. Soil attributes included soil organic matter (SOM), total organic carbon (TOC), total nitrogen (TN), pH, gravimetric soil moisture (GSM), and bulk density (D_b). A subset of soil attributes were analyzed across the sites, using euclidean cluster analysis, resulting in three soil condition (SC) groups of increasing wetland soil development (i.e., SC1 < SC2 < SC3; less to more developed or matured) as measured by accumulation of TOC, TN, the increase of GSM, and the decrease of D_b. There were no difference found in the bacterial community diversity between the groups (p = 0.4). NirK gene copies detected ranged between 3.6 × 10⁴ and 3.4 × 10⁷ copies g⁻¹ soil and were significantly higher in the most developed soil group, SC3, than in the least developed soil group, SC1. However, the gene copies were lowest in SC2 that had a significantly higher soil pH (~6.6) than the other two SC groups (~5.3). The same pattern was found in denitrifying enzyme activity (DEA) on a companion study where DEA was found negatively correlated with soil pH. Gene fragments were amplified and products were screened by terminal restriction fragment length polymorphism (T-RFLP) analysis. Among 146 different T-RFs identified, fourteen were dominant and together made up more than 65% of all detected fragments. While SC groups did not relate to whole nirK communities, most soil properties that identified SC groups did significantly correlate to dominant members of the community.     

Effect of warming on the temperature dependence of soil respiration rate in arctic,temperate and tropical soils

We examined the response of the temperature coefficient (Q₁₀) for soil respiration rate to changes in environmental temperature through a laboratory incubation experiment. Soil samples were collected from three climatic areas: arctic (Svalbard, Norway), temperate (Tsukuba, Japan) and tropical (Pasoh, Malaysia). The arctic and temperate soils were incubated at 8 °C (control), 12 °C (4 °C warming) and 16 °C (8 °C warming) for 17 days. The tropical soil was incubated at 16 °C (8 °C cooling), 24 °C (control) and 32 °C (8 °C warming). Before and after the incubation experiment, the temperature dependence of soil microbial respiration was measured using an open-airflow method with IRGA by changing the temperature in a water bath. The initial Q₁₀ before the incubation experiment was larger in the soils from higher latitudes: 3.4 in the arctic soil, 2.9 in the temperate soil, and 2.1 in the tropical soil. The response of the microbial respiration rate to change in temperature differed among the three soil types. The temperature dependence of respiration rate in the arctic soil did not change in response to warming by 4 and 8 °C with a Q₁₀ of about 3. On the other hand, the Q₁₀ in the temperate soil decreased with increasing incubation temperature: from 2.8 in soils incubated at 8 °C to 2.5 at 12 °C and 2.0 at 16 °C. In the tropical soil, the Q₁₀ was not changed even by the 8 °C warming with a value of 2.1, whereas the Q₁₀ was increased from 2.1 to 2.7 by the 8 °C cooling. These results suggest that the response of microbial respiration to climatic warming may differ between soils from different latitudes.     

Sclerotium rolfsii dynamics in soil as affected by crop sequences

Crop rotation has been used for the management of soilborne diseases for centuries, but has not often been planned based on scientific knowledge. Our objective was to generate information on Sclerotium rolfsii dynamics under different crop or intercrop activities, and design and test a research approach where simple experiments and the use of models are combined to explore crop sequences that minimize Southern blight incidence.The effect of seventeen green manure (GM) amendments on sclerotia dynamics was analyzed in greenhouse and field plot experiments during two years. The relative densities of viable sclerotia 90 days after winter GM (WGM) incorporation were generally lower than after summer GM (SGM) incorporation, with average recovery values of 60% and 61% for WGM in the field, 66% and 43% for WGM in the greenhouse, and 162% to 91% for SGM in the greenhouse, in 2009 and 2010, respectively. Sclerotia survival on day d after GM amendment was described by the model S_f = S_i × exp(−b × d), relating initial (S_i) and final (S_f) sclerotia densities. Relative decay rates of the sclerotia (b) in SGM amended soil were largest for alfalfa (0.0077 ± 0.0031 day⁻¹) and sudangrass (0.0072 ± 0.0030 day⁻¹). In WGM amended soil, the largest b values were for oat (0.0096 ± 0.0024 day⁻¹), wheat (0.0090 ± 0.0024 day⁻¹) and alfalfa (0.0087 ± 0.0023 day⁻¹).The effect of three cropping sequences (sweet pepper–fallow, sweet pepper–black oat and sweet pepper–onion) on sclerotia dynamics was analyzed in microplot experiments, and the data were used to calibrate the model P_f = P_i/(α + βP_i), relating initial (P_i) and final (P_f) sclerotia densities. Median values for the relative rate of population increase at low P_i (1/α, dimension less) and the asymptote (1/β, number of viable sclerotia in 100 g of dry soil) were 8.22 and 4.17 for black oat (BO), 1.13 and 8.64 for onion (O), and 6.26 and 17.93 for sweet pepper (SwP).By concatenating the two models, sclerotia population dynamics under several crop sequences were simulated. At steady state, the sequence SwP–O–Fallow–BO resulted in the lowest long-term sclerotia density (7.09 sclerotia/100 g soil), and SwP–Fallow in the highest (17.89 sclerotia/100 g soil). The developed methodology facilitates the selection of a limited number of rotation options to be tested in farmers’ fields.     

Microbial activity in response to water-filled pore space of variably eroded southern Piedmont soils

Potential C and N mineralization and soil microbial biomass C (SMBC) are soil biological properties important in understanding nutrient and organic matter dynamics. Knowledge of soil water content at a matric potential near field capacity is needed to determine these biological properties. The objective of this study was to examine whether adjustment of soil water content to a common level of water-filled pore space (WFPS) may be an acceptable alternative that would require little prior analysis in comparison with adjustment based on matric potential. Potential C and N mineralization and SMBC were determined from 15 variably eroded soils of the Madison–Cecil–Pacolet association (clayey, kaolinitic, thermic Typic Kanhapludults) in response to WFPS. The levels of WFPS to achieve maximum activity and biomass under naturally settled conditions were unaffected by clay content and occurred at 0.42±0.03 m³ m⁻³ for net N mineralization during 24 days of incubation, 0.51±0.22 m³ m⁻³ for specific respiratory activity of SMBC, 0.60±0.07 m³ m⁻³ for cumulative C mineralization during 24 d of incubation, and 0.76±0.27 m³ m⁻³ for SMBC. Selecting a common WFPS level of 0.5 m³ m⁻³ resulted in 96±2%, 97±5%, 97±4%, and 88±10% of the maximum for these four properties, respectively, and was a reasonable compromise when attempting to estimate these properties during simultaneous incubations. Adjusting soil water content based on WFPS was simpler and nearly as reliable as based on matric potential, in which soil water content at −33 kPa varied from 0.16 to 0.30 g g⁻¹.     

Production of carbon dioxide and nitrous oxide in alkaline saline soil of Texcoco at different water contents amended with urea: A laboratory study

Soil of the former lake Texcoco is alkaline saline with pH often >10 and electrolytic conductivity (EC) >70 dS m^?1 with rapidly changing water contents. Little is known how fertilizing this area with urea to vegetate the soil would affect emissions of carbon dioxide (CO₂) and dynamics of N. Texcoco soil with electrolytic conductivity (EC) 2.3 dS m^?1 and pH 8.5 (TEXCOCO A soil), EC 2.0 dS m^?1 and pH 9.0 (TEXCOCO B soil) and 200 dS m^?1 and pH 11.2 (TEXCOCO C soil) was amended with or without urea and incubated at 40% of water holding capacity (WHC), 60% WHC, 80% WHC and 100% WHC, while emissions of nitrous oxide (N₂O) and CO₂ and dynamics of ammonium (NH₄⁺), nitrite (NO₂^?) and nitrate (NO₃^?) were monitored for 7 days. An agricultural soil served as control (ACOLMAN soil). The emission of CO₂ increased in the urea amended soil 1.5 times compared to the unamended soil, it was inhibited in TEXCOCO C soil and was >1.2 larger in soil incubated at 40%, 60% and 80% WHC compared to soil incubated at 100% WHC. The emission of N₂O increased in soil added with urea compared to the unamended soil, was similar in TEXCOCO A and B soils, but was <0.2 mg N kg^?1 soil day^?1 in TEXCOCO C soil and generally larger in soil incubated at 60% and 80% WHC compared to soil incubated at 40% and 100% WHC. The water content of the soil had no significant effect on the mean concentration of NH₄⁺, but addition of urea increased it in all soils. The concentration of NO₂^? was not affected by the water content and the addition of urea except in TEXCOCO A soil where it increased to values ranging between 20 and 40 mg N kg^?1. The concentration of NO₃^? increased in the ACOLMAN, TEXCOCO A and TEXCOCO B soil amended with urea compared to the unamended soil, but not in the TEXCOCO C soil. It decreased with increased water content, but not in TEXCOCO C soil. It was found that the differences in soil characteristics, i.e. soil organic matter content, pH and EC between the soils had a profound effect on soil processes, but even small changes affected the dynamics of C and N in soil amended with urea.     

Influence of temperature and vegetation cover on soluble inorganic and organic nitrogen in a spodosol

In this study, the influence of temperature and vegetation cover on soluble inorganic and organic nitrogen in a spodosol from north east Scotland was investigated. Firstly, soil cores were incubated at 5, 10 and 15°C for up to 8 weeks. Net mineralisation was observed at all temperatures with larger rates observed at higher temperatures. In contrast, water extractable dissolved organic nitrogen (DON) displayed no clear trend with time and showed little response to temperature. Secondly, intact cores of the same soil, with and without vegetation, were leached with artificial rain for 6 weeks at 6.5 and 15°C. Temperature and the presence of vegetation interacted to have a significant (P<0.01) effect on the concentration of NO₃⁻ in leachates; highest concentrations were observed in leachates from cores without vegetation at 15°C, whereas lowest concentrations were observed in leachates from cores with vegetation at 6.5°C. In contrast, concentrations of DON and dissolved organic carbon (DOC) were significantly (P<0.001) higher in leachates from cores with vegetation than without vegetation and were not affected by temperature. The cumulative amounts of DON and DOC leached from the cores with vegetation were 4 and 2.5 times greater, respectively, than those leached from the cores without vegetation. Comparison of soil solution (extracted by centrifugation at 0–5 and 5–10 cm depth) after leaching for 6 weeks, showed that the upper layer contained more than twice the amount of DON than the 5–10 cm layer and that the difference in concentration between the two depths was enhanced in the presence of vegetation. The results indicate that vegetation is an important source of DON and DOC. However, the removal of vegetation did not lead to an increase in the quantity of total dissolved nitrogen (TDN) in soil water, but resulted in a change in the dominant N fraction from DON to NO₃⁻. In addition, the results show that DON, in both the incubated and leached cores, did not change as inorganic N was mineralised. This suggests that if water extractable DON was acting as a source of NH₄⁺ or NO₃⁻, then it was being replenished by, and in equilibrium with, a large reserve of organic N. Evidence of such a pool was indirect in the form of additional DON (equivalent to 2 g N m⁻²) being extracted by 0.5 M K₂SO₄.     

Winter wheat carbon exchange in Thuringia,Germany

Eddy covariance measurements and estimates of biomass net primary production (NPP) in combination with soil carbon turnover modelled by the Roth-C model were used to assess the ecosystem carbon balance of an agricultural ecosystem in Thuringia, Germany, growing winter wheat in 2001. The eddy CO₂ flux measurements indicate an annual net ecosystem exchange (NEE) uptake in the range from −185 to −245 g C m⁻² per year. Main data analysis uncertainty in the annual NEE arises from night-time u¹ screening, other effects (e.g. coordinate rotation scheme) have only a small influence on the annual NEE estimate. In agricultural ecosystems the fate of the carbon removed during harvest plays a role in the net biome production (NBP) of the ecosystem, where NBP is given by net ecosystem production (NEP=−NEE) minus non-respiratory losses of the ecosystem (e.g. harvest). Taking account of the carbon removed by the wheat harvest (290 g C m⁻²), the agricultural field becomes a source of carbon with a NBP in the order of −45 to −105 g C m⁻² per year. Annual carbon balance modelled with the Roth-C model also indicated that the ecosystem was a source for carbon (NBP −25 to −55 g C m⁻² per year). Based on the modelling most of carbon respired resulted from changes in the litter and fast soil organic matter pool. Also, the crop and management history, particularly the C input to soil in the previous year, significantly affect next year’s CO₂ exchange.     

Effects of vegetation on soil microbial C,N, and P dynamics in a tropical forest and savanna of Central Africa

The forest–savanna transition zone is widely distributed on nutrient-poor oxisols in Central Africa. To reveal and compare the nutrient cycle in relation to soil microbes for forest and savanna vegetation in this area, we evaluated seasonal fluctuations in microbial biomass carbon (MBC), nitrogen (MBN), and phosphorus (MBP) for 13 months as well as soil moisture, temperature, soil pH levels, and nutrients for both vegetation types in eastern Cameroon. Soil pH was significantly lower in forest (4.3) than in savanna (5.6), and soil N availability was greater in forest (87.1 mg N kg⁻¹ soil) than in savanna (32.9 mg N kg⁻¹ soil). We found a significant positive correlation between soil moisture and MBP in forest, indicating the importance of organic P mineralization for MBP, whereas in savanna, we found a significant positive correlation between soil N availability and MBP, indicating N limitation for MBP. These results suggest that for soil microbes, forest is an N-saturated and P-limited ecosystem, whereas savanna is an N-limited ecosystem. Additionally, we observed a significantly lower MBN and larger MB C:N ratio in forest (50.7 mg N kg⁻¹ soil and 8.6, respectively) than in savanna (60.0 mg N kg⁻¹ soil and 6.5, respectively) during the experimental period, despite the rich soil N condition in forest. This may be due to the significantly lower soil pH in forest, which influences the different soil microbial communities (fungi-to-bacteria ratio) in forest versus savanna, and therefore, our results indicate that, in terms of microbial N dynamics, soil pH rather than soil substrate conditions controls the soil microbial communities in this area. Further studies should be focused on soil microbial community, such as PLFA, which was not evaluated in the present study.     

Evaluation of an optimal extraction method for measuring d-ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO) in agricultural soils and its association with soil microbial CO2 assimilation

Assimilating atmospheric carbon (C) into terrestrial ecosystems is recognized as a primary measure to mitigate global warming. Ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO) is the dominant enzyme by which terrestrial autotrophic bacteria and plants fix CO₂. To investigate the possibility of using RubisCO activity as an indicator of microbial CO₂ fixation potential, a valid and efficient method for extracting soil proteins is needed. We examined three methods commonly used for total soil protein extraction. A simple sonication method for extracting soil protein was more efficient than bead beating or freeze–thaw methods. Total soil protein, RubisCO activity, and microbial fixation of CO₂ in different agricultural soils were quantified in an incubation experiment using ¹⁴C-CO₂ as a tracer. The soil samples showed significant differences in protein content and RubisCO activity, defined as nmol CO₂ fixed g⁻¹ soil min⁻¹. RubisCO activities ranged from 10.68 to 68.07 nmol CO₂ kg⁻¹ soil min⁻¹, which were closely related to the abundance of cbbL genes (r = 0.900, P = 0.0140) and the rates of microbial CO₂ assimilation (r = 0.949, P = 0.0038). This suggests that RubisCO activity can be used as an indicator of soil microbial assimilation of atmospheric CO₂.     

The influence of soluble carbon and fertilizer nitrogen on nitric oxide and nitrous oxide emissions from two contrasting agricultural soils

Contradictory effects of simultaneous available organic C and N sources on nitrous oxide (N₂O), carbon dioxide (CO₂) and nitric oxide (NO) fluxes are reported in the literature. In order to clarify this controversy, laboratory experiments were conduced on two different soils, a semiarid arable soil from Spain (soil I, pH=7.5, 0.8%C) and a grassland soil from Scotland (soil II, pH=5.5, 4.1%C). Soils were incubated at two different moisture contents, at a water filled pore space (WFPS) of 90% and 40%. Ammonium sulphate, added at rates equivalent to 200 and 50 kg N ha^?1, stimulated N₂O and NO emissions in both soils. Under wet conditions (90% WFPS), at high and low rates of N additions, cumulative N₂O emissions increased by 250.7 and 8.1 ng N₂O–N g^?1 in comparison to the control, respectively, in soil I and by 472.2 and 2.1 ng N₂O–N g^?1, respectively, in soil II. NO emissions only significantly increased in soil I at the high N application rate with and without glucose addition and at both 40% and 90% WFPS. In both soils additions of glucose together with the high N application rate (200 kg N ha^?1) reduced cumulative N₂O and NO emissions by 94% and 55% in soil I, and by 46% and 66% in soil II, respectively. These differences can be explained by differences in soil properties, including pH, soil mineral N and total and dissolved organic carbon content. It is speculated that nitrifier denitrification was the main source of NO and N₂O in the C-poor Spanish soil, and coupled nitrification–denitrification in the C-rich Scottish soil.     

Temperature and substrate effects on C & N mineralization and microbial community function of soils from a hybrid poplar chronosequence

Substrate input as well as climatic factors affect C and N cycling and microbial properties in forest soils. We used a microcosm approach to investigate the response of CO₂ efflux, net N mineralization, and microbial community-level physiological profile (CLPP) to temperature (5 vs. 15 °C) and substrate (with and without sucrose addition) addition in surface mineral soils collected from 4-, 6-, 13-, and 15-year old (ages in 2007) hybrid poplar (Populus deltoides × Populus × petrowskyana var. Walker) stands in northern Alberta. In the early stage of incubation (0–2 h), CO₂ efflux was higher at 5 °C than at 15 °C with little effect from substrate addition, while 24 h after the addition of substrate, CO₂ efflux became higher under the 15 °C incubation. After 72 h incubation, temperature and substrate addition effects on CO₂ efflux subsided and CO₂ efflux rates tended to converge among the treatments. Net N mineralization was significantly affected by substrate addition and stand age, while rates of net ammonification were higher at 5 °C than at 15 °C. Net N mineralization occurred without sucrose addition while net immobilization occurred with sucrose addition. The soil from the youngest stand had the lowest N mineralization rate among the stands for each corresponding substrate-incubation temperature treatment. We used Ecoplates from Biolog™ to study sole-carbon-source-utilization profiles of microbial communities at the end of the incubation. Principal component analysis of C utilization data separated microbial communities with respect to substrate addition, incubation temperature and stand age. Our data showed that organic matter mineralization and microbial substrate utilization were affected by incubation temperature, substrate availability and stand age, indicating that the responses of microbial communities in the studied hybrid poplar plantations to temperature changes were strongly mediated by labile C availability and stand development.     

Relationships between soil pH and microbial properties in a UK arable soil

Effects of changing pH along a natural continuous gradient of a UK silty-loam soil were investigated. The site was a 200 m soil transect of the Hoosfield acid strip (Rothamsted Research, UK) which has grown continuous barley for more than 100 years. This experiment provides a remarkably uniform soil pH gradient, ranging from about pH 8.3 to 3.7. Soil total and organic C and the ratio: (soil organic C)/(soil total N) decreased due to decreasing plant C inputs as the soil pH declined. As expected, the CaCO₃ concentration was greatest at very high pH values (pH > 7.5). In contrast, extractable Al concentrations increased linearly (R² = 0.94, p < 0.001) from below about pH 5.4, while extractable Mn concentrations were largest at pH 4.4 and decreased at lower pHs. Biomass C and biomass ninhydrin-N were greatest above pH 7. There were statistically significant relationships between soil pH and biomass C (R² = 0.80, p < 0.001), biomass ninhydrin-N (R² = 0.90, p < 0.001), organic C (R² = 0.83, p < 0.001) and total N (R² = 0.83, p < 0.001), confirming the importance of soil organic matter and pH in stimulating microbial biomass growth. Soil CO₂ evolution increased as pH increased (R² = 0.97, p < 0.001). In contrast, the respiratory quotient (qCO₂) had the greatest values at either end of the pH range. This is almost certainly a response to stress caused by the low p. At the highest pH, both abiotic (from CaCO₃) and biotic Co₂ will be involved so the effects of high pH on biomass activity are confounded. Microbial biomass and microbial activity tended to stabilise at pH values between about 5 and 7 because the differences in organic C, total N and Al concentrations within this pH range were small. This work has established clear relationships between microbial biomass and microbial activity over an extremely wide soil pH range and within a single soil type. In contrast, most other studies have used soils of both different pH and soil type to make similar comparisons. In the latter case, the effects of soil pH on microbial properties are confounded with effects of different soil types, vegetation cover and local climatic conditions.