Soil Microbial Biomass C Research Articles

Seasonal changes in soil microbial communities along a fertility gradient of temperate grasslands

This study aimed to: (1) determine whether soil microbial communities along a gradient from intensive (fertilized) to low-input (unfertilized) grassland management, shift in their composition as shown by an increase in the abundance of fungi relative to bacteria and (2) whether these shifts in soil microbial communities vary depending on season. At all sample dates soil microbial biomass-C and -N, and the total abundance of phospholipid fatty acids (PLFA) were highest in unfertilized, undrained treatments and lowest in fertilized and drained grassland. Similarly, microbial activity, measured as CO 2-C respiration, was found to be at its lowest in the most intensively managed grassland. Measures of microbial biomass showed a high degree of seasonality, having summer maxima and winter minima. In contrast, PLFA measures had spring maxima and autumn minima. Seasonal and management differences were also observed within the microbial community. PLFA profiles revealed that most individual fatty acids were highest in the unfertilized treatments, and lowest in fertilized grassland. The fungal-to-bacterial biomass ratio was also highest in the unfertilized and lowest in the fertilized soils, suggesting that higher microbial biomass in former were more due to the growth of fungi than bacteria. As with total PLFA, the abundance of individual fatty acids showed a spring maximum and an autumn minimum. Seasonal differences in PLFA patterns were shown to be related to soil mineral-N and soil moisture contents. Factors controlling shifts in microbial community structure between sample dates and sites are discussed in relation to other studies. A critical assessment of the different measures of microbial biomass is also given. Overall, the findings of this study support the thesis that fungi play a more significant role in soil biological processes of low-input, unfertilized grasslands, than in intensively managed systems.

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 m 3 m −3 for net N mineralization during 24 days of incubation, 0.51±0.22 m 3 m −3 for specific respiratory activity of SMBC, 0.60±0.07 m 3 m −3 for cumulative C mineralization during 24 d of incubation, and 0.76±0.27 m 3 m −3 for SMBC. Selecting a common WFPS level of 0.5 m 3 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.

Carbon and nitrogen mineralization of added particulate and macroorganic matter

Particulate organic matter (POM) is more sensitive than total SOM to changes in management practices and, accordingly, may indicate changes in soil quality. A soil incubation study was conducted to determine the effects of added POM (75 to 250 μm size fraction), or macroorganic matter (MOM, 250 to 2000 μm size fraction) on C and N mineralization and microbial C and N content. A 1 kg composite made from 16 predominantly silt loam soils was amended with 10 g of POM, MOM or MOM ground to a reduced size of 75 to 250 μm (GMOM). The MOM amendment equaled 4.55-fold and POM equaled 1.60-fold of total MOM and POM found in the composite soil. Carbon mineralization of MOM and POM after 8 weeks was approximately 9 and 4%, respectively of the total MOM and POM-C added. Reducing the size of MOM to 75 to 250 μm did not affect mineralization. Nitrogen mineralization was slightly greater in the amended soils after 8 weeks and equaled 5 to 6% of the MOM or POM-total N added. Contribution of POM to total mineralized N from soil organic matter (SOM) in the composite soil was proportional to the POM content in SOM or approximately 12%. Amended soils had 25 to 42% more biomass-C than the control soil 2 weeks after amendment application. After 8 weeks, the amended soils contained about 32% more biomass-C. This increase in biomass-C at 8 weeks accounted for approximately 2% of the added C. At 8 weeks, microbial biomass-N in GMOM-, MOM- and POM-amended soils was about 56, 46 and 14% higher, respectively, than in the control soil. These increases were approximately 8% of the MOM-N added and 2% of the POM-N added. Increases in POM resulted in increases in soil respiration and microbial biomass-C and N, which also are suggested indicators of soil quality. Therefore, POM may be a suitable soil quality indicator that provides similar information as soil respiration or microbial biomass determinations.

Microbial biomass C and alkaline phosphatase activity in two compost amended soils

Addition of compost from various sources and of different maturity may affect the soil biochemical properties. A field study was conducted to evaluate the effect of different composts, spring-applied alone or in combination with ammonium nitrate (AN), on microbial biomass C (MBC) and alkaline phosphatase activity (APA) in two soils cropped with spring wheat (Triticum aestivum L. 'Messier') in eastern Quebec, Canada. The experiment was conducted in 1994 and 1995 at different sites on a Kamouraska clay (Orthic Humic Gleysol) and a Saint-André sandy loam (Fragic Humo-Ferric Podzol). Treatments included composts at 180 kg N ha−1, composts at 90 kg N ha−1 supplemented with AN, AN at 90 kg N ha−1, and an unfertilized control. Soil MBC and APA were measured 30 d after compost application and at wheat harvest. Additional sampling was made the following spring. Generally, larger MBC and APA values were found at wheat harvest in soils treated with composts alone than with AN alone or unfertilized. These effects were related to soil C content and climatic conditions. Compost type affected soil biochemical properties which could be attributed to the total C supply and material maturation state. Compost addition constitutes an efficient short-term way to promote soil microbial biomass and enzyme activity in cold climates. Key words: Compost, fertilizer, microbial biomass, soil enzyme, wheat

Soil C and N turnover after incorporation of chopped maize, barley straw and blue grass in the field: Evaluation of the DAISY soil–organic-matter submodel

The DAISY soil–organic-matter submodel was evaluated against independent data from a 1 year field study with incorporation (0–15 cm) of chopped barley straw, blue grass and maize into a sandy loam soil. Investigated parameters were soil respiration, soil mineral N, soil microbial biomass-C and N (SMB-C and N) dynamics and the predicted decomposition of the added organic matter (AOM) measured as light particulate organic matter (LPOM>100 μm, ρ<1.4 g cm −3). Significant differences between values predicted from the model and measured values of soil respiration, soil mineral N, and soil microbial biomass-C and N were observed in all treatments. However, the model predictions for the unamended soil and for the soil receiving barley straw were better than those for the two other treatments. Discrepancies, in the blue grass and in the maize treatment, led to suggestions for model improvements. A distinct short-term pulse of SMB growth observed immediately after incorporation of the plant materials was not predicted fully. However, the difference between the measured and the predicted SMB pools did not induce a complementary difference for the mineral N pool. Soil microbial residues (SMR), temporarily protected against recycling via the microbial turnover and mineralisation, are discussed as a possible sink for the N from SMB. The predicted dynamics of the less labile AOM1-pool (initialised as water inextractable AOM) was correlated with the measured amounts of LPOM from the added plant materials. A slight overestimation of measured LPOM by AOM1 in the initial period after incorporation of AOM was followed by a slight underestimation later on. This trend might be attributed to the assumed constant C/N-ratio and turnover rate during the simulated decay of AOM1, contrasting reality in which LPOM is changing, e.g. C/N-ratio and lignin content. The simple initial partition of AOM into a water extractable part (AOM2) and a water inextractable part (AOM1), both parameterised with predetermined turnover rates and utilisation efficiencies, calls for re-evaluation. Suggestions are made to include the concept of SMR and the changing composition of LPOM into the DAISY model.

Microbial response to repeated applications of low concentrations of pentachlorophenol in an alfisol under pasture

Columns of an Alfisol under permanent pasture were polluted by repeated additions of pentachlorophenol (PCP) (7 mg l −1) to levels of 102 and 510 mg Kg −1, to simulate a dynamic diffuse pollution. PCP was rapidly sorbed to the soil organic matter, and was only slightly degraded. Measurements of soil microbial biomass-C revealed a 25% decrease in total biomass-C caused by both leaching and PCP toxicity. Microbial biomass-C measurements performed on soil fractions showed that only microorganisms located in the outer compartment of the aggregates were affected. Microorganisms protected by soil micro-aggregates were not affected, suggesting that they were not in contact with PCP, which was thus unavailable for biodegradation. Three gram negative bacterial strains (Si, C3 and C2), able to use PCP as a sole carbon and energy source, were isolated after 0, 1 and 3 months of PCP enrichment respectively, and were identified as Pseudomonas (Si) and Acinetobacter (C3 and C2). In liquid degradation tests, the strains C2 and C3 degraded 60% of PCP within 26 days whereas the Pseudomonas degraded only 25%. A specific immuno-labeling of the three strains permitted to show that repeated PCP additions to soil had a positive, negative or absence of effect on the populations C2, C3 and Si respectively.

Soil carbon and nitrogen dynamics as affected by long-term tillage and nitrogen fertilization

Quantifying seasonal dynamics of active soil C and N pools is important for understanding how production systems can be better managed to sustain long-term soil productivity especially in warm subhumid climates. Our objectives were to determine seasonal dynamics of inorganic soil N, potential C and N mineralization, soil microbial biomass C (SMBC), and the metabolic quotient of microbial biomass in continuous corn (Zea mays L.) under conventional (CT), moldboard (MB), chisel (CH), minimum tillage (MT), and no-tillage (NT) with low (45kgNha–1) and high (90kgNha–1) N fertilization. An Orelia sandy clay loam (fine-loamy, mixed, hyperthermic Typic Ochraqualf) in south Texas, United States, was sampled before corn planting in February, during pollination in May, and following harvest in July. Soil inorganic N, SMBC, and potential C and N mineralization were usually highest in soils under NT, whereas these characteristics were consistently lower throughout the growing season in soils receiving MB tillage. Nitrogen fertilization had little effect on soil inorganic N, SMBC, and potential C and N mineralization. The metabolic quotient of microbial biomass exhibited seasonal patterns inverse to that of SMBC. Seasonal changes in SMBC, inorganic N, and mineralizable C and N indicated the dependence of seasonal C and N dynamics on long-term substrate availability from crop residues. Long-term reduced tillage increased soil organic matter (SOM), SMBC, inorganic N, and labile C and N pools as compared with plowed systems and may be more sustainable over the long term. Seasonal changes in active soil C and N pools were affected more by tillage than by N fertilization in this subhumid climate.

Soil Microbial Biomass and Mineralizable Carbon of Water‐Stable Aggregates

AbstractBiophysical alterations of agricultural soils following adoption of zero tillage (ZT) deserve investigation in order to better understand the processes of soil organic C (SOC) sequestration and turnover. We determined the vertical distribution of soil microbial biomass C (SMBC), C mineralized in 24 d under standard conditions, and basal soil respiration (BSR) in five water‐stable aggregate classes. Four soils (loam, silt loam, clay loam, and clay) from the Peace River region of northern Alberta and British Columbia were sampled following 4 to 16 yr under comparison of conventional shallow tillage (CT) and ZT. Macroaggregates (&gt;0.25 mm) had greater SMBC, more C mineralized in 24 d, and higher BSR than microaggregates at a depth of 0 to 50 mm. Differences between macro‐ and microaggregates in these properties decreased with soil depth. Carbon mineralized in 24 d and SMBC were 9 ± 9% greater (mean of four soils ± standard deviation among soils) under ZT than under CT in macroaggregates, but were 6 ± 11% lower in whole soil due to lower amounts in microaggregates under ZT than under CT. Macroaggregate‐protected SOC to a depth of 200 mm was 6.7 ± 1.9 g m−2 under CT and 9.8 ± 2.6 g m−2 under ZT. Soil organic C in macroaggregates, which had high concentrations of active pools of SOC, appeared to have been shunted into the more stable microaggregate fraction after disturbance with CT. Unlike in temperate, humid climates, decomposition of SOC during the passage from macro‐ to microaggregates may have been limited by the frigid, semiarid climate.

Microbial suppression of in vitro growth of Pythium ultimum and disease incidence in relation to soil C and N availability

Experiments were designed to examine effects of the soil microbial community, C and N availability on in vitro growth of Pythium ultimum and its infection of cotton seedlings by manipulating the stage of cellulose decomposition, size and activity of microbial populations, and N availability. In comparison to the untreated control (CONT), cellulose addition alone (CELL) reduced soil nitrate by 35‐80 fold, but had no significant effect on soil ammonium. Soil microbial biomass C (SMBC) increased over 2 fold in 14 days following cellulose addition, but significantly decreased in the following 10 days due to N limitation. Addition of both cellulose and N (NCELL) resulted in sustained SMBC for 24 days and significantly reducedin vitro P. ultimumgrowth and disease incidence. In vitro growth of P. ultimumand disease severity were consistently reduced in the order: CONT > CELL > NCELL. In vitro growth of P. ultimum was lower in soils previously incubated for 24 days than in those incubated for 14 days, and was most closely correlated to cumulative soil CO2 evolution (CO2T). Correlations between P. ultimum growth rates and NO3-N or total available N were substantial (p < 0.05), but much less significant than those between the growth rates and SMBC, microbial activity measured as CO2 evolution rates or CO2T (p<0.0001). Addition of available N (NH4NO3) and C (glucose) just before the assays did not increase the in vitro growth of P. ultimum or disease severity on cotton seedlings, suggesting that time-dependent microbial processes or microbial metabolites significantly contributed to suppression ofP. ultimum growth. Abbreviations: CELL ‐ cellulose treatment, CO2R ‐ CO 2 evolution rate, CO2T ‐ cumulative CO2 evolution, CONT ‐ controls, GRP ‐in vitro growth rate of P. ultimum, NCELL ‐ cellulose plus N treatment, ORIG ‐ original soils (pre-incubated), SAFS ‐ Sustainable Agriculture Farming Systems project, SMBC ‐ soil microbial biomass C, SMBN ‐ soil microbial biomass N, SOC ‐ soluble organic C, TAN ‐ total available N.

Long‐Term Effects of Tillage and Fertilization on Soil Organic Matter Dynamics

AbstractUnderstanding the effects of long‐term management practices on soil C and N pools and activities is essential for sustaining soil productivity. The objectives of this study were to evaluate long‐term and seasonal changes in soil organic C (SOC), soil microbial biomass C (SMBC) and N (SMBN), and mineralizable C and N in continuous corn (Zea mays L.) under conventional tillage (CT), moldboard (MB), chisel (CH), minimum tillage (MT), and no‐tillage (NT) with low (45 kg N, 10 P kg ha−1) and high (90 N kg, 10 P kg ha−1) N fertilization. An Orelia sandy clay loam (fine‐loamy, mixed, hyperthermic Typic Ochraqualf) in south Texas was sampled before corn planting in February, during pollination in May, and following corn harvest in July. No‐tillage and MT retained more corn residue C input as SOC and SMBC than the more intensive tillage systems. Soil organic C, SMBC, SMBN, and mineralizable C and N were greatest in the surface 0 to 50 mm with NT and MT. Seasonal distributions of SMBC and mineralizable C were consistently greater in reduced‐tillage systems (NT and MT), averaging 22 and 34% greater than plowed treatments at planting, 45 and 53% larger at pollination, and 36 and 34% higher at harvest, respectively, at a depth of 200 mm. The greater amount of crop residues remaining with MT and NT may have provided available substrate for maintenance of the larger SMB pool and the higher C and N mineralization in the 0‐ to 200‐mm depth during the growing season. Higher N fertilization increased seasonal mineralizable C and N, but did not consistently affect SOC and SMB. Reduced tillage systems that promote surface residue accumulation provide an opportunity for increasing sequestration of C and mineralizable nutrients within SMB.

Rimsulfuron in soil: Effect of persistence on growth and activity of microbial biomass at varying environmental conditions

The research was carried out toascertain the effect of rimsulfuron, a solfonylureaherbicide, on soil microbial biomass growth andactivity. Laboratory experiments were performed in asilty clay loam soil to relate changes of soilmicrobial biomass-C content and global hydrolyticactivity to the rimsulfuron persistence underdifferent conditions of temperature and soil humidity.The results showed that rimsulfuron persistencedepended significantly on temperature, while itremained almost unchanged by humidity changes. A rangeof half-life values from 3.5 to 14.8 days was found ina temperature range from 10 °C to 25 °C,with lower half-lives at higher temperature.Persistence data were processed with the VARLEACHmodel, in order to predict rimsulfuron persistenceunder different environmental conditions. On comparingtreated soils with untreated soil samples, decreasesin the microbial biomass-C content and increases inthe global hydrolytic activity were found to beconnected with rimsulfuron persistence at the variousexperimental conditions. These effects persisted fora short time and, they were evident earlier at highertemperature and more persistent at lower humidity.This behaviour is discussed in terms of rimsulfurontoxicity, with the consequent release of endocellularhydrolytic enzymes from the dead microorganisms. Anequation was derived to calculate the microbialbiomass-C content in response to the variation ofrimsulfuron persistence.

Soil organic matter pools with conventional and zero tillage in a cold, semiarid climate

Conservation of soil organic matter (SOM), especially in high-SOM surface soils of the Canadian Prairies, is essential for maintaining soil quality and regulating soil CO 2 emissions. We determined the depth distribution (0–50, 50–125, and 125–200 mm) of soil organic C (SOC), soil microbial biomass C (SMBC), basal soil respiration (BSR), and net N mineralization in four Boralfs subjected to shallow conventional tillage (CT) and zero tillage (ZT). Tillage regimes were initiated 4, 4, 7, and 16 years prior to these measurements on a limed clay loam, unlimed clay loam, loam, and silt loam, respectively, located in northern Alberta and British Columbia. Soils under ZT were an average of 4% denser and 24% wetter than under CT. SOC content to a depth of 200 mm was 6.5 kg m −2 under CT and 11% greater under ZT in the limed clay loam, but not different between tillage regimes in the unlimed clay loam (7.1 kg m −2), silt loam (5.1 kg m −2), and loam (4.3 kg m −2). BSR averaged 4.0 and 3.1 g CO 2-C per square meter per day under CT and ZT, respectively. The portions of SOC and SMBC as BSR under ZT were lower than under CT. However, net N mineralization averaged 3.4 and 4.1 g m −2 per 24 days under CT and ZT, respectively. The cold, semiarid climate of the Canadian Prairies appears to have hindered any acceleration of decomposition of SOM and crop residues owing to soil disturbance with CT. Therefore, SOC and active soil C pools were not enriched by minimized soil disturbance, unlike results obtained from more temperate and humid climates.

Active fractions of organic matter in soils with different texture

Relationships between soil organic C (SOC), soil microbial biomass C (SMBC), mineralizable C and N during a 21 d incubation, and basal soil respiration (BSR) were evaluated on eight soil types from Texas that varied in soil texture (7–45% clay) and organic matter. The portion of SOC as SMBC increased with increasing clay content, whereas the relationships of mineralizable C and N and BSR to SOC were not affected by soil texture. The ratio of BSR-to-SOC averaged 1.4 ± 0.4 mg mineralizable C g −1 SOC d −1. The amount of mineralizable C and N and BSR per unit of SMBC, however, decreased with increasing clay content, indicating that the soil microbial biomass (SMB) was more active in coarse-textured soils than in fine-textured soils. The average specific respiratory activity was 29 mg mineralizable C g −1 SMBC d −1 with 10% clay and 11 mg mineralizable C g −1 SMBC d −1 with 40% clay. The C-to-N ratio of the mineralizable fraction was 10 ± 3 and not affected by soil texture. The established relationships between active soil organic matter (SOM) fractions and soil texture could be used in models predicting SOM turnover.

Soil Organic Matter Pools during Early Adoption of Conservation Tillage in Northwestern Canada

AbstractChanges in soil organic matter (SOM) pools during adoption of reduced (RT) or zero tillage (ZT) can influence soil physical properties, nutrient cycling, and CO2 flux between soil and atmosphere. We determined soil organic C (SOC), soil microbial biomass C (SMBC), basal soil respiration (BSR), and mineralizable N to a depth of 200 mm at the end of 3, 5, and 6 yr after implementation of tillage management on a Falher clay (fine, montmorillonitic, frigid Typic Natriboralf) near Rycroft, Alberta, in a canola (Brassica campestris L.)‐wheat (Triticum Aestivum L.)‐barley (Hordeum vulgare L.)‐fallow cropping system. At the end of 6 yr, SOC was not different among tillage regimes and averaged 8.6 kg m−2. At the end of 3 and 5 yr, SMBC was not significantly different among tillage regimes, but at the end of 6 yr SMBC was 7% greater in RT and 9% greater in ZT than in conventional tillage (CT). Basal soil respiration and mineralizable N at the end of 6 yr were not different among tillage regimes following barley and averaged 2.7 g CO2‐C m−2 d−1 and 5.0 g inorganic N m−2 24 d−1, respectively. However, BSR following fallow was 2.2, 2.5, and 2.6 g CO2‐C m−2 d−1 in CT, RT, and ZT, respectively. Mineralizable N following fallow was 5.8 g inorganic N m−2 (24 d)−1 in RT and ZT and 7.3 g inorganic N m−2 (24 d)−1 in CT. At 0 to 50 mm, there was no significant increase in SOC at the end of 6 yr, a 17 to 36% increase in SMBC, and a 12 to 69% increase in BSR with ZT compared with CT, depending on rotation phase. Relatively small changes in SOM pools with adoption of conservation tillage may be attributable to the large amount of SOM initially present and the cold, semiarid climate that limits SOM turnover.

Seasonal dynamics of active soil carbon and nitrogen pools under intensive cropping in conventional and no tillage

AbstractActive fractions of soil carbon (C) and nitrogen (N) can undergo seasonal changes due to environmental and cultural factors, thereby influencing plant N availability and soil organic matter (SOM) conservation. Our objective was to determine the effect of tillage (conventional and none) on the seasonal dynamics of potential C and N mineralization, soil microbial biomass C (SMBC), specific respiratory activity of SMBC(SRAC), and inorganic soil N in a sorghum [Sorghum bicolor (L.) Moench]‐wheat (Triticum aestivum L.)/soybean [Glycine max (L.) Merr.] rotation and in a wheat/soybean double crop. A Weswood silty clay loam (fine, mixed, thermic Fluventic Ustochrept) in southcentral Texas was sampled to 200 mm depth 57 times during a 2‐yr period. Potential C mineralization was lowest (≈︁2 to 3 g · m−2 · d−1) midway during the sorghum and soybean growing seasons and highest (≈︁3 to 4 g · m−2 · d−1) at the end of the wheat growing season and following harvest of all crops. Addition of crop residues increased SMBC for one to three months. Potential N mineralization was coupled with potential C mineralization, SRAC, and changes in SMBC at most times, except during the wheat growing season and shortly after sorghum and soybean residue addition when increased N immobilization was probably caused by rhizodeposition and residues with low N concentration. Seasonal variation of inorganic soil N was 19 to 27%, of potential C and N mineralization and SRAC was 8 to 23%, and of SMBC was 7 to 10%. Soil under conventional tillage experienced greater seasonal variation in potential C and N mineralization, SRAC, bulk density, and water‐filled pore space than under no tillage. High residue input with intensive cropping and surface placement of residues were necessary to increase the long‐term level of active C and N properties of this thermic‐region soil due to rapid turnover of C input.

Tillage and Crop Effects on Seasonal Soil Carbon and Nitrogen Dynamics

AbstractSeasonal changes in the active fractions of soil C and N are important in understanding mineralization‐immobilization processes that influence soil N availability. Our objective was to determine the effect of tillage (conventional tillage [CT] and no tillage [NT]) in monoculture sorghum [Sorghum bicolor (L.) Moench], wheat (Triticum aestivum L.), and soybean [Glycine max (L.) Merr.] on the seasonal dynamics of in situ inorganic soil N, potential C and N mineralization, and soil microbial biomass C (SMBC). A Weswood silty clay loam (fine‐silty, mixed, thermic Fluventic Ustochrept) located in south‐central Texas was sampled to a depth of 200 mm 57 times during a 2‐yr period. In situ soil NO3− accumulated at a rate of ≈0.03 g N m−2 d−1 following harvest when temperature and moisture were adequate. Potential soil C mineralization was 35, 39, and 53% greater under NT than under CT when averaged across months in wheat, sorghum, and soybean, respectively. However, following harvest of each crop, potential soil C mineralization increased at a faster rate under CT than under NT and differences between tillage regimes were reduced for several months, which caused temporary N immobilization in CT. Periods of net N immobilization at the end of the growing season and beginning of the fallow period were often associated with an increasing SMBC pool. Large seasonal changes in active soil C and N pools could be related to rhizodeposition, crop residue addition, and the long fallow period. The type, placement, and timing of residue input significantly altered the seasonal dynamics of the active soil C and N pools and their effect on soil N availability.

Decomposition of lupine biomass by soil microorganisms in developing mount St. Helens' pyroclastic soils

Legumes like Lupinus lepidus and L. latifolius affect soil C and N concentrations and microbial activity in Mount St. Helens' pyroclastic deposits. Concentrations of total Kjeldahl-N (TKN), total organic-C (TOC) and water soluble-C (H 2OC) were measured in soil from under live L. lepidus (LULE), live L. latifolius (LULA) and dead L. lepidus (DEAD), and in bare soil (BARE). Soil microbial biomass-C estimated by substrate-induced respiration (SIR-C), metabolic quotient ( qCO 2) and C and N mineralization from lupine biomass-amended and non-amended soils were also estimated. The greatest concentrations of TKN, TOC and SIR-C were observed near the surface. In surface soil (0–5 cm), LULA soil contained the highest concentrations of TKN, TOC and SIR-C followed by DEAD, LULE and BARE plant-soil types, respectively. The concentrations of H 2OC were relatively constant among different plant-soil types and depths. Changes in qCO 2 with depth varied with plant-soil type. While TOC was strongly and linearly correlated with TKN, the relationship between SIR-C and both TKN and TOC was curvilinear and suggested a carrying capacity for SIR-C of about 400 kg microbial biomass-C ha −1. In an incubation study, the patterns of cumulative net respiration were similar to those observed for TKN and TOC (i.e. LULA > DEAD > LULE > BARE). Soils amended with L. latifolius biomass had a higher rate of net N mineralization than soils amended with L. lepidus biomass. Low amounts of TOC, TKN and soluble C, together with zero net N mineralization in some amended soils, suggest that microbial communities in Mount St. Helens' soils are N limited. Competition for inorganic-N between microorganisms and plants may thus be an important controlling mechanism for succession.

Soil Organic Carbon, Microbial Biomass, and Mineralizable Carbon and Nitrogen in Sorghum

AbstractQuantifying changes in soil microbial biomass and mineralizable C and N is important in understanding the dynamics of the active soil C and N pools. Our objectives were to quantify long‐term and seasonal changes in soil organic C (SOC), soil microbial biomass C (SMBC) and N (SMBN), and mineralizable C and N in continuous sorghum [Sorghum bicolor (L.) Moench] and sorghum‐wheat (Triticum aestivum L.)/soybean [Glycine max (L.) Merr.] sequences under conventional tillage (CT) and no tillage (NT) with and without N fertilization. A Weswood silty clay loam (fine, mixed, thermic Fluventic Ustochrept) in south‐central Texas was sampled after planting in April, during flowering in June, and following sorghum harvest in August. More crop residue C input was retained as SOC and SMBC under NT than under CT. Soil organic C, SMBC, SMBN, and mineralizable C and N were greatest at a depth of 0 to 50 mm under NT. Mineralizable C and SMBC averaged 18% greater in rotation than in monoculture, probably due to greater C input via crop roots and residues in rotation and a shorter fallow. Mineralizable N with N fertilization was 36% greater in continuous sorghum but not different in rotated sorghum. Mineralizable C and SMBC increased an average of 5%, but mineralizable N decreased 41% from planting to flowering, probably due to rhizodeposition. From planting to post‐harvest, mineralizable C and SMBC increased 9% but mineralizable N decreased 15% due to crop residue addition. Soil N availability was reduced by plant additions in the short term but enhanced in the long term.

Temporal changes in soil microbial biomass carbon in an arable soil. Consequences for soil sampling

Sugar beet, winter wheat and winter barley were planted within a crop rotation on an arable soil with conventional soil management. Soil samples were taken monthly from different depths of the whole plough layer (0–10, 10–20 and 20–30 cm) during a 56 month period. The samples were analysed for microbial biomass carbon using the substrate-induced respiration technique. Temporal changes in the amount of microbial biomass carbon were observed. Within a year, microbial biomass-C varied from low values (−15% of total mean) in winter to high values (+15% of total mean) in summer. Relative deviations from the annual means were calculated for each month in the year to demonstrate these fluctuations. Temporal changes in microbial biomass-C depended on the sources of sample variation (5 years, 3 crops, 3 sampling depths). The highest relative deviation from the annual mean microbial biomass-C was attributable to the factor “year”. Less variations were caused by “crops” and “sampling depth”. Soil microbial biomass-C remained constant during frost periods. From the observed temporal changes, recommendations for a suitable date for soil sampling are given, which allows a representative estimation of the mean annual microbial biomass-C content in arable soils.

Long‐Term Changes in Soil Carbon and Nitrogen Pools in Wheat Management Systems

AbstractCrop management strategies that alter the timing, placement, quantity, and quality of crop residue input can affect the size, turnover, and vertical distribution of the active and passive pools of soil organic matter (SOM). Our objectives were to quantify long‐term changes in soil organic, soil microbial biomass (SMB), and mineralizable C and N in continuous wheat (Triticum aestivum L.), continuous wheat/soybean [Glycine max (L.) Merr.], and wheat/soybean‐sorghum [Sorghum bicolor (L.) Moench.] sequences under conventional tillage (CT) and no tillage (NT) with and without N fertilizer. A Weswood silty clay loam (fine, mixed, thermic Fluventic Ustochrept) in southcentral Texas was collected from a 9‐yr field study. Soil microbial biomass C (SMBC) and N (SMBN) were determined with the chloroform fumigation‐incubation method and mineralizable C and N were determined from 10‐d aerobic incubations at 25°C. More crop residue C input was retained as soil organic C (SOC), SMBC, and mineralizable C under NT than under CT. Soil organic C, SMBC, and mineralizable C at a depth of 0 to 50 mm were 33 to 125% greater under NT than under CT. Increasing cropping intensity increased SOC up to 22%, SMBC up to 31%, and mineralizable C up to 27% under NT. Differences in crop management systems significantly altered SMB and the associated mineralizable N level, which supplies crops with mineral N. High clay content soils of central Texas can be effectively managed to increase the active and passive pools of SOM using minimal fallow with NT.