Reduction In Microbial Biomass Research Articles

Extreme CO2 disturbance and the resilience of soil microbial communities

Carbon capture and storage (CSS) technology has the potential to inadvertently release large quantities of CO2 through geologic substrates and into surrounding soils and ecosystems. Such a disturbance has the potential to not only alter the structure and function of plant and animal communities, but also soils, soil microbial communities, and the biogeochemical processes they mediate. At Mammoth Mountain, we assessed the soil microbial community response to CO2 disturbance (derived from volcanic ‘cold’ CO2) that resulted in localized tree kill; soil CO2 concentrations in our study area ranged from 0.6% to 60%. Our objectives were to examine how microbial communities and their activities are restructured by extreme CO2 disturbance, and assess the response of major microbial taxa to the reintroduction of limited plant communities following an extensive period (15–20 years) with no plants. We found that CO2-induced tree kill reduced soil carbon (C) availability along our sampling transect. In response, soil microbial biomass decreased by an order of magnitude from healthy forest to impacted areas. Soil microorganisms were most sensitive to changes in soil organic C, which explained almost 60% of the variation for microbial biomass C (MBC) along the CO2 gradient. We employed phospholipid fatty acid analysis and quantitative PCR (qPCR) to determine compositional changes among microbial communities in affected areas and found substantial reductions in microbial biomass linked to the loss of soil fungi. In contrast, archaeal populations responded positively to the CO2 disturbance, presumably due to reduced competition of bacteria and fungi, and perhaps unique adaptations to energy stress. Enzyme activities important in the cycling of soil C, nitrogen (N), and phosphorus (P) declined with increasing CO2, though specific activities (per unit MBC) remained stable or increased suggesting functional redundancy among restructured communities. We conclude that both the direct (microaerobiosis) and indirect (loss of plant C inputs) effects of elevated soil CO2 flux have significant impacts on the composition and overall structural trajectory of soil microbial populations within disturbed areas.

A meta-analysis of soil microbial biomass responses to forest disturbances

Climate warming is likely to increase the frequency and severity of forest disturbances, with uncertain consequences for soil microbial communities and their contribution to ecosystem C dynamics. To address this uncertainty, we conducted a meta-analysis of 139 published soil microbial responses to forest disturbances. These disturbances included abiotic (fire, harvesting, storm) and biotic (insect, pathogen) disturbances. We hypothesized that soil microbial biomass would decline following forest disturbances, but that abiotic disturbances would elicit greater reductions in microbial biomass than biotic disturbances. In support of this hypothesis, across all published studies, disturbances reduced soil microbial biomass by an average of 29.4%. However, microbial responses differed between abiotic and biotic disturbances. Microbial responses were significantly negative following fires, harvest, and storms (48.7, 19.1, and 41.7% reductions in microbial biomass, respectively). In contrast, changes in soil microbial biomass following insect infestation and pathogen-induced tree mortality were non-significant, although biotic disturbances were poorly represented in the literature. When measured separately, fungal and bacterial responses to disturbances mirrored the response of the microbial community as a whole. Changes in microbial abundance following disturbance were significantly positively correlated with changes in microbial respiration. We propose that the differential effect of abiotic and biotic disturbances on microbial biomass may be attributable to differences in soil disruption and organic C removal from forests among disturbance types. Altogether, these results suggest that abiotic forest disturbances may significantly decrease soil microbial abundance, with corresponding consequences for microbial respiration. Further studies are needed on the effect of biotic disturbances on forest soil microbial communities and soil C dynamics.

Reductions in microbial biomass along disturbance gradients in a semi-natural grassland

Soil disturbance is a common phenomenon in semi-natural sandy grasslands, caused by management practices, livestock trampling and erosion. To better understand ecosystem processes when the surface layers of the soil are disturbed, we investigated the effects of disturbance on different functional microbial groups by the use of signature fatty acid analysis. Four levels of disturbance were investigated in local disturbance gradients in an area with semi-natural sandy grassland. Festuca brevipila Tracey (Poaceae) was used as the study plant to investigate the response of plant and microbial biomass to disturbance. The plant was chosen since it dominated the site and was present at all levels of disturbance. The amount of arbuscular mycorrhizal (AM) fungi in the roots of the study plant was not affected by soil disturbance. The amount of AM fungi, saprotrophic fungi and bacteria in the soil was significantly reduced by increasing level of soil disturbance, coinciding with a reduced vegetation cover and decreasing soil organic matter content. Increasing levels of soil disturbance resulted in elevated soil pH and concentration of inorganic N, whereas the water content and organic matter content was reduced. The shoot N:P ratios of the grass investigated increased with the level of disturbance. We conclude that soil disturbance disfavours fungi due to mechanical interruption of hyphae and through reduced plant biomass. We also conclude that soil disturbance disfavours bacteria in the long run due to reduced organic matter content.

Grass Invasions Across a Regional Gradient are Associated with Declines in Belowground Carbon Pools

The composition of plant communities everywhere now likely comprises alien as well as native species, and those aliens that become invasive have wide-ranging impacts on the structure and function of recipient ecosystems. These impacts include perturbations to soil carbon (C) cycling, but the direction and magnitude of impacts are species and climate dependent, making it difficult to generalize whether a specific invader will promote losses or gains in soil C stocks. Generalizations of a specific invader’s impacts are necessary; however, because the range of an invader can encompass thousands of square kilometers, meaning their effects can have broad, regional consequences. To quantify broad-scale and context-dependent impacts of a specific invader, multi-site investigations that capture and measure local and regional environmental heterogeneity are necessary. Using this approach, we show that a widespread grass invader of forest understories is associated with declines in soil C during infilling (spreading within the invaded range). Across the 36 study sites, total soil C stocks declined (P = 0.113) by approximately 12% (estimated mean ± SD, uninvaded: 2,429 ± 512.9 vs. invaded: 2,140 ± 520.7 g C m−2). The decline in total soil C is driven by a significant (P = 0.047) reduction in the native-derived, mineral-associated soil C fraction. This fraction, whose mass and slow turnover makes it an important C store, is approximately 15% lower in invaded (estimated mean ± SD: 1,560 ± 400.4 g C m−2) than uninvaded plots (1,826 ± 398.1 g C m−2). Notably, declines in this C fraction are only apparent at 21 of the sites, reflecting how environmental heterogeneity in other variables (specifically pH, soil moisture, and clay content) are important to quantify to determine invader impacts across a region. The 26% decline in microbial biomass with invasion (P = 0.011; estimated mean ± SD, uninvaded: 10.05 ± 1.79 vs. invaded: 7.40 ± 1.80 g C m−2) is also dependent on site characteristics (pH), and reductions are greater where the invader occurs at higher densities. Reductions in microbial biomass and soil C with invasion suggest that grass invasion will alter soil C cycling and decrease forest-C stores across the study region, although invader effects at a specific-site will be dependent on environmental context.

Rapid soil organic matter loss from forest dieback in a subalpine coniferous ecosystem

Forest dieback caused by climate-change associated stresses and insect outbreaks has emerged as a global concern, and the biogeochemical consequences of this phenomenon need to be elucidated. We measured biological and chemical traits of soil beneath live trees or trees recently killed by a mountain-pine-beetle outbreak in a subalpine coniferous forest in the Front Range of Colorado. We focused on the top 5 cm of mineral soil just beneath the O horizon and measured microbial biomass, soil invertebrate abundance and composition, and soil chemical characteristics. With the termination of inputs from rhizodeposition, mycorrhizal fungal turnover and fine root turnover, soil total carbon (C) and total nitrogen (N) in the mineral soil at three sites decreased by 38–49% and 26–45%, respectively. Tree mortality was associated with reduced soil microbial biomass but soil nematode and microarthropod densities were unchanged. Nematode trophic structure was altered with an increased proportion of bacterial feeders. Soil inorganic N concentrations were inversely correlated to microbial C:N ratios. Tree death was associated with increased soil pH, a possible loss of calcium (Ca 2+), but an accumulation of soil inorganic N, largely as NH 4 +. Our results suggest that forest dieback results in rapid C and N loss from surface mineral soils and that the accumulation of soil inorganic N, the reduction in microbial biomass, and the more bacterial-based soil food web increase the potential of enhanced N loss from affected ecosystems.

The effect of fire on microbial biomass: a meta-analysis of field studies

Soil microbes regulate the transfer of carbon (C) from ecosystems to the atmosphere and in doing so influence feedbacks between terrestrial ecosystems and global climate change. Fire is one element of global change that may influence soil microbial communities and, in turn, their contribution to the C dynamics of ecosystems. In order to improve our understanding of how fire influences belowground communities, we conducted a meta-analysis of 42 published microbial responses to fire. We hypothesized that microbial biomass as a whole, and fungal biomass specifically, would be altered following fires. Across all studies, fire reduced microbial abundance by an average of 33.2% and fungal abundance by an average of 47.6%. However, microbial responses to fire differed significantly among biomes and fire types. For example, microbial biomass declined following fires in boreal and temperate forests but not following grasslands fires. In addition, wildfires lead to a greater reduction in microbial biomass than prescribed burns. These differences are likely attributable to differences in fire severity among biomes and fire types. Changes in microbial abundance were significantly correlated with changes in soil CO2 emissions. Altogether, these results suggest that fires may significantly decrease microbial abundance, with corresponding consequences for soil CO2 emissions.

Climate change feedbacks to microbial decomposition in boreal soils

Boreal ecosystems store 10–20 % of global soil carbon and may warm by 4–7 °C over the next century. Higher temperatures could increase the activity of boreal decomposers and indirectly affect decomposition through other ecosystem feedbacks. For example, permafrost melting will likely alleviate constraints on microbial decomposition and lead to greater soil CO2 emissions. However, wet boreal ecosystems underlain by permafrost are often CH4 sources, and permafrost thaw could ultimately result in drier soils that consume CH4, thereby offsetting some of the greenhouse warming potential of soil CO2 emissions. Climate change is also likely to increase winter precipitation and snow depth in boreal regions, which may stimulate decomposition by moderating soil temperatures under the snowpack. As temperatures and evapotranspiration increase in the boreal zone, fires may become more frequent, leading to additional permafrost loss from burned ecosystems. Although post-fire decomposition could also increase due to higher soil temperatures, reductions in microbial biomass and activity may attenuate this response. Other feedbacks such as soil drying, increased nutrient mineralization, and plant species shifts are either weak or uncertain. We conclude that strong positive feedbacks to decomposition will likely depend on permafrost thaw, and that climate feedbacks will probably be weak or negative in boreal ecosystems without permafrost. However, warming manipulations should be conducted in a broader range of boreal systems to validate these predictions.

Grazing-induced reduction of natural nitrous oxide release from continental steppe

Atmospheric concentrations of the greenhouse gas nitrous oxide (N(2)O) have increased significantly since pre-industrial times owing to anthropogenic perturbation of the global nitrogen cycle, with animal production being one of the main contributors. Grasslands cover about 20 per cent of the temperate land surface of the Earth and are widely used as pasture. It has been suggested that high animal stocking rates and the resulting elevated nitrogen input increase N(2)O emissions. Internationally agreed methods to upscale the effect of increased livestock numbers on N(2)O emissions are based directly on per capita nitrogen inputs. However, measurements of grassland N(2)O fluxes are often performed over short time periods, with low time resolution and mostly during the growing season. In consequence, our understanding of the daily and seasonal dynamics of grassland N(2)O fluxes remains limited. Here we report year-round N(2)O flux measurements with high and low temporal resolution at ten steppe grassland sites in Inner Mongolia, China. We show that short-lived pulses of N(2)O emission during spring thaw dominate the annual N(2)O budget at our study sites. The N(2)O emission pulses are highest in ungrazed steppe and decrease with increasing stocking rate, suggesting that grazing decreases rather than increases N(2)O emissions. Our results show that the stimulatory effect of higher stocking rates on nitrogen cycling and, hence, on N(2)O emission is more than offset by the effects of a parallel reduction in microbial biomass, inorganic nitrogen production and wintertime water retention. By neglecting these freeze-thaw interactions, existing approaches may have systematically overestimated N(2)O emissions over the last century for semi-arid, cool temperate grasslands by up to 72 per cent.

Sequential biodegradation of TNT, RDX and HMX in a mixture

We describe TNT's inhibition of RDX and HMX anaerobic degradation in contaminated soil containing indigenous microbial populations. Biodegradation of RDX or HMX alone was markedly faster than their degradation in a mixture with TNT, implying biodegradation inhibition by the latter. The delay caused by the presence of TNT continued even after its disappearance and was linked to the presence of its intermediate, tetranitroazoxytoluene. PCR–DGGE analysis of cultures derived from the soil indicated a clear reduction in microbial biomass and diversity with increasing TNT concentration. At high-TNT concentrations (30 and 90 mg/L), only a single band, related to Clostridium nitrophenolicum, was observed after 3 days of incubation. We propose that the mechanism of TNT inhibition involves a cytotoxic effect on the RDX- and HMX-degrading microbial population. TNT inhibition in the top active soil can therefore initiate rapid transport of RDX and HMX to the less active subsurface and groundwater.

Carbon pulses but not phosphorus pulses are related to decreases in microbial biomass during repeated drying and rewetting of soils

Drying and rewetting cycles are known to be important for the turnover of carbon (C) in soil, but less is known about the turnover of phosphorus (P) and its relation to C cycling. In this study the effects of repeated drying-rewetting (DRW) cycles on phosphorus (P) and carbon (C) pulses and microbial biomass were investigated. Soil (Chromic Luvisol) was amended with different C substrates (glucose, cellulose, starch; 2.5 g C kg −1) to manipulate the size and community composition of the microbial biomass, thereby altering P mineralisation and immobilisation and the forms and availability of P. Subsequently, soils were either subjected to three DRW cycles (1 week dry/1 week moist) or incubated at constant water content (70% water filled pore space). Rewetting dry soil always produced an immediate pulse in respiration, between 2 and 10 times the basal rates of the moist incubated controls, but respiration pulses decreased with consecutive DRW cycles. DRW increased total CO 2 production in glucose and starch amended and non-amended soils, but decreased it in cellulose amended soil. Large differences between the soils persisted when respiration was expressed per unit of microbial biomass. In all soils, a large reduction in microbial biomass (C and P) occurred after the first DRW event, and microbial C and P remained lower than in the moist control. Pulses in extractable organic C (EOC) after rewetting were related to changes in microbial C only during the first DRW cycle; EOC concentrations were similar in all soils despite large differences in microbial C and respiration rates. Up to 7 mg kg −1 of resin extractable P (P resin) was released after rewetting, representing a 35–40% increase in P availability. However, the pulse in P resin had disappeared after 7 d of moist incubation. Unlike respiration and reductions in microbial P due to DRW, pulses in P resin increased during subsequent DRW cycles, indicating that the source of the P pulse was probably not the microbial biomass. Microbial community composition as indicated by fatty acid methyl ester (FAME) analysis showed that in amended soils, DRW resulted in a reduction in fungi and an increase in Gram-positive bacteria. In contrast, the microbial community in the non-amended soil was not altered by DRW. The non-selective reduction in the microbial community in the non-amended soil suggests that indigenous microbial communities may be more resilient to DRW. In conclusion, DRW cycles result in C and P pulses and alter the microbial community composition. Carbon pulses but not phosphorus pulses are related to changes in microbial biomass. The transient pulses in available P could be important for P availability in soils under Mediterranean climates.

Interactive effects of wildfire and permafrost on microbial communities and soil processes in an Alaskan black spruce forest

AbstractBoreal forests contain significant quantities of soil carbon that may be oxidized to CO2 given future increases in climate warming and wildfire behavior. At the ecosystem scale, decomposition and heterotrophic respiration are strongly controlled by temperature and moisture, but we questioned whether changes in microbial biomass, activity, or community structure induced by fire might also affect these processes. We particularly wanted to understand whether postfire reductions in microbial biomass could affect rates of decomposition. Additionally, we compared the short‐term effects of wildfire to the long‐term effects of climate warming and permafrost decline. We compared soil microbial communities between control and recently burned soils that were located in areas with and without permafrost near Delta Junction, AK. In addition to soil physical variables, we quantified changes in microbial biomass, fungal biomass, fungal community composition, and C cycling processes (phenol oxidase enzyme activity, lignin decomposition, and microbial respiration). Five years following fire, organic surface horizons had lower microbial biomass, fungal biomass, and dissolved organic carbon (DOC) concentrations compared with control soils. Reductions in soil fungi were associated with reductions in phenol oxidase activity and lignin decomposition. Effects of wildfire on microbial biomass and activity in the mineral soil were minor. Microbial community composition was affected by wildfire, but the effect was greater in nonpermafrost soils. Although the presence of permafrost increased soil moisture contents, effects on microbial biomass and activity were limited to mineral soils that showed lower fungal biomass but higher activity compared with soils without permafrost. Fungal abundance and moisture were strong predictors of phenol oxidase enzyme activity in soil. Phenol oxidase enzyme activity, in turn, was linearly related to both 13C lignin decomposition and microbial respiration in incubation studies. Taken together, these results indicate that reductions in fungal biomass in postfire soils and lower soil moisture in nonpermafrost soils reduced the potential of soil heterotrophs to decompose soil carbon. Although in the field increased rates of microbial respiration can be observed in postfire soils due to warmer soil conditions, reductions in fungal biomass and activity may limit rates of decomposition.

Microbial biomass, functional capacity, and community structure after 12 years of soil warming

We examined the effect of chronic soil warming on microbial biomass, functional capacity, and community structure in soil samples collected from the Soil Warming Study located at the Harvard Forest Long-term Ecological Research (LTER) site. Twelve years of chronic soil warming at 5 °C above the ambient temperature resulted in a significant reduction in microbial biomass and the utilization of a suite of C substrates which included amino acids, carbohydrates, and carboxylic acids. Heating significantly reduced the abundance of fungal biomarkers. There was also a shift in the mineral soil microbial community towards gram positive bacteria and actinomycetes.

Effect of microbial inoculation during vermicomposting of different organic substrates on microbial status and quantification and documentation of acid phosphatase

In this experiment, three microbial strains were inoculated in two different organic wastes to study their effect on the humic acids content, acid phosphatase activity and microbial properties of the final stabilized products. Pyrophosphate extract of vermicomposts were analyzed through polyacrylamide gel electrophoresis to study the nature of a isozymes in different treatments. Results suggested that vermicomposting increased humic acids content and acid phosphatase activity in organic substrates and microbial inoculation further enhanced the rate of humification and enzyme activity. Although humic acids content in different microorganism-inoculated vermicomposts were statistically at par, acid phosphatase activity in these treatments was significantly (P<0.05) different. Results revealed that microbial respiration was increased due to vermicomposting, but a reduction in microbial biomass was recorded after stabilization of organic wastes. Although vermicomposting increased the value of microbial quotient (qCO(2)), microbial inoculation did not show any significant effect on qCO(2). The zymogram revealed that two isozymes of acid phosphatase (group II and group III) were present in all vermicompost samples and higher acid phosphatase activity in fungi-inoculated vermicomposts might be due to the presence of an additional isozyme (group I) of acid phosphatase.

Impacts of urea N addition on soil microbial community in a semi-arid temperate steppe in northern China

Nitrogen (N) addition has been well docu- mented to decrease plant biodiversity across various terrestrial ecosystems. However, such generalizations about the impacts of N addition on soil microbial communities are lacking. This study was conducted to examine the impacts of N addition (urea-N fertilizer) on soil microbial communities in a semi-arid temper- ate steppe in northern China. Soil microbial biomass carbon (C), biomass N (MBN), net N mineralization and nitrification, and bacterial and fungal community level physiological profiles (CLPP) along an N addition gradient (0- 64 gNm �2 year �1 )w ere measured. Three years of N addition caused gradual or step increases in soil NH4-N, NO3-N, net N mineralization and nitrification in the early growing season. The reductions in microbial biomass under high N addition levels (32 and 64 g N m �2 year �1 ) are partly attributed to the deleterious effects of soil pH. An N optimum between 16 and 32 g N m �2 year �1 in microbial biomass and functional diversity exists in the temperate steppe in northern China. Similar N loading thresholds may also occur in other ecosys- tems, which help to interpret the contrasting observa- tions of microbial responses to N addition.

Dynamics of residue decomposition and N 2 fixation of grain legumes upon sugarcane residue retention as an alternative to burning

Burning of sugarcane residues contributes to air pollution and sugarcane producers have been forced to abandon it. The change from burning to residue retention is likely to alter the cycling of nutrients. Additionally, there is often a time gap of 6–8 months between two different sugarcane cycles during which legumes could be planted. Thus, the objective of this study was to assess the effects of burning, mulching or incorporation of sugarcane residues on residue decomposition and N mineralization (sugarcane residue management period) and subsequently upon ploughing (legume period) on N dynamics, N 2 fixation, development and nutrient yields of groundnut and soybean grown between two sugarcane cycles on a sandy soil in Northeast Thailand. Soil microbial biomass N increased when sugarcane residues were incorporated instead of burned or surface applied at 14 days after initiation of cane residue management. Thereafter, high net N mineralization was accompanied by a reduction in microbial biomass N, indicating that mineralized N was derived from microbial N turnover. However, upon ploughing after 96 days the different previous sugarcane residue management strategies had no significant ( P > 0.05) effect on net mineral N and microbial biomass N during the subsequent legume period. Although, 15N enrichment in control reference plants and plant N uptake indicated significant N immobilization effects persisting into the legume crop phase, the proportion of N derived from N 2 fixation (%Ndfa) or amount of N 2 fixed were not significantly different between sugarcane residue management treatments. Soybean fixed more N 2 (78%Ndfa, 234 kg N fixed ha −1) than groundnut (67%Ndfa, 170 kg N fixed ha −1) due to its larger N demand and a poorer utilization of soil N (64 kg N ha −1 vs. 85 kg N ha −1). Groundnut led to a positive soil N balance while that of soybean was negative due to its high nitrogen harvest index. Legume residues returned 61 and 146 kg N ha −1 to the soil for soybean and groundnut, respectively, compared to only 34–39 kg N ha −1 by fallow weeds. Sugarcane residue retention improved soil organic carbon and N content. The results suggested that although a change from burning to sugarcane residues retention led to alterations in N cycling and improved soil organic matter it did not significantly affect N 2 fixation due to the uniforming action of ploughing and the extended time gap between sugarcane residue incorporation and legume planting.

Evaluating Solarization and Cultivated Fallow for Johnsongrass (Sorghum halepense) Control and Nitrogen Cycling on an Organic Farm

ABSTRACT A two-year field study evaluating efficacy of two weed control techniques on the noxious weed johnsongrass (Sorghum halepense L. Pers.) was conducted in Lexington KY. The weed control treatments were soil solarization, accomplished by covering the soil with clear plastic for 8 weeks, and cultivated bare fallow. Both treatments, along with a control treatment (mowing), were applied during the summers of 2003 and 2004 to a field heavily infested with johnsongrass (40–50% coverage). Effects of solarization on soil microbial activity and nitrogen (N) cycling were also evaluated. In both years, half of each treatment plot was lightly tilled 8 months after initial treatment application, and johnsongrass populations were characterized 2 months later. At the end of the experiment, the solarized-untilled treatment had lower johnsongrass populations than the cultivated-tilled and control treatments. Shallow tillage resulted in significantly more johnsongrass in the cultivated treatment in 2003, and in both cultivated and solarized treatments in 2004, compared with their untilled counterparts. At the conclusion of the experiment the johnsongrass populations were considerably reduced in both treatments compared with the original infestation. Solarization resulted in significant increases of both NH4-N and NO3-N in the soil during the time the plastic was in place, likely due to release of labile NH4-N from soil microbial biomass and subsequent nitrification. Substrate-induced respiration rates were also reduced during solarization, indicating a reduction of microbial biomass. However, these soil indicators were not persistent and should not prohibit the use of this weed management technique by organic growers.

Response of microbial activities in two contrasting soils to 4-nonylphenol treated with biosolids

In the application of biosolids on agricultural lands, 4-nonylphenol (4-NP) in soils is an important environmental concern because of its associated estrogenic risk to animals and human beings. Incubation experiments that involved the mixing of two contrasting soils (A: calcareous sandy soil; B: acidic clayey soil) and biosolids in 4-NP were performed to examine the effect of 4-NP on the rate of production of CO 2, the mineralization of N and the microbial biomass, by considering the biodegradation of 4-NP for the evaluation of soil health. The experimental results indicated that the half-life ( t 1/2) of 4-NP increased with the supplementary concentration of 4-NP (80, 160 and 240 mg kg −1) in the two soils, and the t 1/2 values in the soil A are always lower than that in soil B. The 4-NP supplement in the biosolids reverses C mineralization in soil B more than it does in soil A, but it reverses N mineralization in soil A more than in soil B. The aeration status and microbial population of the biosolids treated soils are key factors in determining the time course of 4-NP degradation associated with the microbial activities. The 4-NP was biodegraded mainly by bacteria, and the effect on C and N mineralization of 4-NP input is determined by a balance of the reductions in microbial biomass C (MBC) and N (MBN). After destruction in microbial cell membrane and protein structures by the 4-NP, C and N mineralization, MBC and MBN were subsequently followed by a final decline phase for the later period of incubation.

How different reforestation approaches affect red soil properties in southern China

AbstractSignificant differences were found in soil physicochemical and biological effects between various forest restoration approaches of a hilly red‐soil region, southern China. Soil quality was the highest in natural secondary forest (0·95), while in sites revegetated with tea‐oil camellia (Camellia oleifera), Chinese fir (Cunninghamia lanceolata) and slash pine (Pinus elliottii) and control sites, integrated soil quality indices were 0·68, 0·55, 0·36 and 0·04 respectively. The lower soil quality of plantations and controls resulted from increased disruption of soil physical structure, lower quality litter fall, lower litter fall production, reduction in microbial biomass, decline of microbial function and loss of soil nutrients due to periodically artificial tending and accelerated soil erosion. Improvement in soil physicochemical properties and enhanced soil microbial function at a natural secondary forest site demonstrated the inherent restoration of these soils. Hence, natural restoration, as well as reducing human disturbance, is a better approach to improving soil properties than returning farmland to planted woodlands. Copyright © 2005 John Wiley &amp; Sons, Ltd.

NITRAPYRIN DELAYS DENITRIFICATION ON MANURED SOILS

Excessive application of manure may lead to NO3− leaching to groundwater and fluxes of nitrogen oxides to the atmosphere. Nitrification inhibitors such as nitrapyrin (N-serve; 2-chloro-6-(trichloromethyl)pyridine) may help to conserve manure N in the root zone by limiting NO3− supply to denitrifiers. The objective of this study was to test the effect of nitrapyrin on the timing and amounts of denitrification and N2O fluxes in manured soils under conditions favorable to denitrification. The study consisted of a laboratory incubation of soils under aerobic conditions. Three agricultural soils and a sand were included in the study, all with high moisture and initial NO3−-N content. Each soil received three treatments: 1) manure plus nitrapyrin (190 mg nitrapyrin kg−1 soil), 2) manure alone (0.15 mg manure N g−1), and 3) soil alone controls. Nitrapyrin was mixed with the manure before addition to soil. Destructive samplings were carried out weekly for 10 weeks. At each sampling, soil-extractable mineral N, microbial biomass N, denitrified N, and N2O fluxes were measured. Nitrapyrin was effective in reducing nitrification, thus enhancing soil NH4+-N accumulation and possibly reducing the potential for nitrate leaching. Although nitrapyrin was effective in reducing nitrification in manured soils, the effect on soil mineral N and potential N supply to plants varied across soils because of the interaction between nitrification, denitrification, and N immobilization. Neither manure nor nitrapyrin consistently affected net N mineralization in the five different soil types. Microbial N immobilization and/or denitrification were strong sinks of N that reduced net N mineralization. Nitrapyrin did not affect cumulative denitrification, but some soils had delayed denitrification when nitrapyrin was added. Manure had a strong effect on N2O fluxes and denitrified N in some soils, but the effects of nitrapyrin were inconsistent. Nitrapyrin significantly reduced microbial N immobilization in two agricultural soils. The observed reductions in microbial biomass may affect N availability beyond the time frame of the experiment because less N will be available for remineralization.

Application of Organic and Inorganic Fertilizer for Sustainable Maize and Cowpea Yields in Nigeria

A pot experiment evaluating the influence of five rates each of compost, inorganic fertilizer (MFert), and combination of both fertilizers (OBF) on maize (Zea mays L.), performance was conducted in the greenhouse using three cropping cycle. This was followed by a field trial lasting four cropping seasons at Ilora in the derived savanna and at Ibadan in the forest zones of Nigeria. Maize followed by cowpea was used as test crops on the field. Application of the fertilizers led to increases in maize and cowpea grain yields and improved the nutrient element concentrations in maize leaf tissue. Agronomic effectiveness of compost and OBF on maize relative to MFert was 88 and 118%, at Ilora and 81 and 97% at Ibadan. There was an increase of 56 and 89% Relative Agronomic Effectiveness (RAE), for compost and OBF over MFert in the residual study on cowpea. Also, the soil macro and micro nutrient status was highly improved in comparison with the MFert at postharvest period. The optimum rates of the organic fertilizers for sustainable maize and cowpea yield were between 5 and 10 t ha−1. The agronomic effectiveness of the residual organic fertilizers was higher than that of MFert. Optimum rate of OBF was 5 t ha−1. Application of compost significantly increased total microbial biomass in the soil. Also, compost and OBF application led to a fairly high population of Azotobacter (N-fixing bacteria) and thus its biomass. The MFert, however caused a sharp reduction in microbial biomass in the soil. The organic fertilizer improved aggregate stability index and permeability of the soil. On the whole, the positive effects obtained from OBF and compost in this study favor the recycling of farm wastes for sustainable crop production.