David A. Zuberer

Active fractions of organic matter in soils with different texture

Summary-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 (745% 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 SGC were not affected by soil texture. The ratio of BSR-to-SOC averaged 1.4 + 0.4 mg mineralizable C g-r SOC d-r. 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 activit was 29 mg mineralizable C g-’ SMBC d-’ with 10% clay and 11 mg mineralizable C g-’ SMBC d- Y wrth 40% clay. The C-to-N ratio of the mineralizable fraction was 10 f 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. Published by Elsevier Science Ltd

Tillage-induced seasonal changes in soil physical properties affecting soil CO2 evolution under intensive cropping

Crop management practices impact soil productivity by altering the soil environment, which in turn affects microbial growth and decomposition processes that transform plant-produced C to soil organic matter (SOM) or CO2. Reduced tillage increases SOM in the long term, but there is limited information on the in situ seasonal changes in soil physical and biological properties that affect SOM dynamics. Our objectives were to: (i) determine the effect of tillage (conventionally disked (CT) and no tillage (NT)) in a sorghum (Sorghum bicolor (L.) Moench.)-wheat (Triticum aestivum L.)/soybean (Glycine max (L.) Merr.) 2-year rotation sequence and a wheat/soybean double-cropping sequence on the seasonal dynamics and soil depth distribution of gravimetric soil water content (SWC), soil temperature, bulk density (BD) and water-filled pore space (WFPS); and (ii) relate soil CO2 evolution to changes in these physical properties. Treatments had been in place for 9 years at the beginning of sampling. The soil was a Weswood silty clay loam (fine, mixed, thermic Fluventic Ustochrept) located in southcentral Texas. Soil CO2 evolution, using the static chamber method with alkali absorption, and physical properties were measured 57 times during a 2-year period. Soil water content with NT was greater than with CT at the 0–50 mm depth during the fallow period of sorghum-wheat/soybean. In contrast to the surface, SWC at the 50–125 mm depth with CT was greater than or equal to that with NT in all crop sequences. Tillage reduced soil BD, especially at the 50–125 mm depth in all crop sequences, but exhibited large seasonal dynamics at the 0–50 mm depth. Greater temporal variation in SWC and BD occurred due to tillage effects. Soil temperature at 50 mm depth at sunrise averaged 1.2°C greater with NT than with CT during May, September and November, perhaps due to reduced heat loss with residue cover. The mean rate of soil CO2 evolution with CT was 1.55, 1.95 and 2.45 g CO2-C m−2 d−1 in sorghum-wheat/soybean, sorghum-wheat/soybean and wheat/soybean, respectively. The corresponding soil CO2 evolution with NT was 9% greater, 12% greater, and not different compared with CT, respectively. Large seasonal differences in soil CO2 evolution occurred with respect to tillage regime. Soil CO2 evolution was highly related to soil temperature, moisture and temperature-moisture interactions. Regression models of soil CO2 evolution on soil temperature, moisture and day of the season explained from 65 to 98% of the temporal variation, depending upon crop sequence, tillage regime and season. Day of the season was related to non-linear residue decomposition and a polynomial function that expressed crop root respiration and microbial respiration due to rhizodeposition. There were significant tillage and crop sequence interactions with soil temperature and moisture, suggesting that C budgets of agroecosystems derived from climatic data alone could be misleading. Conversion from CT to NT increased C sequestration in soil, but soil under NT released the same or more C as CO2, depending upon crop sequence, suggesting that the dynamics of C sequestration/mineralization had changed during the 10-year period.

Tillage and crop effects on seasonal dynamics of soil CO2 evolution, water content, temperature, and bulk density

Crop management practices impact soil productivity by altering the soil environment, which in turn affects microbial growth and decomposition processes that transform plant-produced C to soil organic matter (SOM) or CO2. Long-term reduced tillage increases SOM, but little is known about the seasonal dynamics of soil CO2 evolution as affected by tillage and crop. The objectives were as follows: (1) To determine the effect of tillage (conventionally disked (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 and soil depth distribution of gravimetric soil water content (SWC), soil temperature, bulk density (BD), and water-filled pore space (WFPS). (2) To relate soil CO2 evolution to changes in these parameters. The soil was a Weswood silty clay loam (fine, mixed, thermic Fluventic Ustochrept) located in southcentral Texas. Soil CO2 evolution, using the static chamber method with alkali absorption, and physical parameters were measured 57 times during 2 years, Soil water content was greater under NT than under CT during the fallow period in all crops, but not always during the growing season. Soil BD was reduced shortly after tillage, but increased to levels observed under NT during wet and cold intervals in the fallow period and during the growing season in all crops. Soil temperature at sunrise was 0.7–1.6°C greater under NT than under CT during spring and autumn months in all crops. Mean soil CO2 evolution was greater during the growing season than during fallow in all crops. Mean soil CO2 evolution during the fallow period was not different between tillage regimes in sorghum (1.25 g CO2-C m−2 day−1) , but was greater under NT compared with CT in wheat (1.88 vs. 1.65 g CO2-C m−2 day−1) and in soybean ( 1.04 vs. 0.86 g CO2-C m−2 day−1). During the growing season, soil CO2 evolution was greater under NT compared with CT in sorghum (2.04 vs. 1.84 g CO2-C m−2 day−1) and in soybean (2.08 vs. 1.58 g CO2-C m−2 dayt1), but was lower under NT compared with CT in wheat (2.09 vs. 2.52 g CO2-C m−2 day−1). Mean Soil CO2 evolution during both periods was related to soil organic C (SOC) under CT, but not under NT. The combined effects of soil temperature, SWC (or WFPS), temperature-water interactions, days after harvest during the fallow period, and days after planting during the growing season explained 73 to 92% of the seasonal variation in soil CO2 evolution depending upon tillage regime and crop. In situ soil CO2 evolution responded to soil temperature and SWC differently depending upon tillage and crop management, suggesting that C budgets of agroecosystems derived from climatic data alone could be misleading. Conversion from CT to NT increased C sequestration in soil, but soil under NT released the same or more C as CO2 depending on crop during Year 9 and Year 10 after initiation, suggesting that the dynamics of C sequestration/ mineralization had changed during the 10 year period.

Seasonal changes in soil microbial biomass and mineralizable c and n in wheat management systems

Crop management strategies can affect the short-term dynamics of the active C and N pools of soil organic matter (SOM) by altering the timing, placement, quantity, and quality of crop root and residue input, as well as nutrient status and environmental conditions (i.e. soil temperature and water content). Our objectives were to quantify seasonal changes in soil microbial biomass (SMB) and mineralizable C and N in continuous wheat (Triticum aestivum L.), continuous wheat-soybean [Glycine max (L.) Merr.j, and wheat-soybean-sorghum [Sorghum bicolor (L.) Moench.] sequences under conventional tillage (CT) and no tillage (NT) with or without N fertilization. Soil classified as a Weswood silty clay loam (fine, mixed, thermic Fluventic Ustochrept) located in southcentral Texas was sampled shortly after planting, during flowering, and following harvest of wheat. Soil microbial biomass C (SMBC) increased by 18% and mineralizable C increased by 37% from planting to flowering when averaged across crop sequence, tillage, and N fertilization regimes. At harvest, SMBC and mineralizable C had returned to the amount at planting in all crop sequences, except in continuous wheat, in which decomposition proceeded without C input during the long fallow. Mineralizable C was 64, 28, and 15% greater at flowering compared to planting under NT and 45, 38, and 29% greater under CT at depths of 0 to 50, 50 to 125, and 125 to 200mm, respectively. The greater increase in mineralizable C near the surface may be related to the abundance of crop roots, rhizosphere products, and more optimal air-filled porosity. With N fertilization, mineralizable N followed the same seasonal pattern as SMBC and mineralizable C. Without N fertilization, mineralizable N did not change during the growing season, despite increased SMBC and mineralizable C at flowering, indicating greater immobilization of N at flowering. Seasonal inputs of crop roots, rhizosphere products, and crop residues significantly altered SMB and mineralizable C and N of this soil, illustrating the dependence of N dynamics on short-term C inputs. Seasonal changes in the active C and N pools of SOM depended upon (i) crop sequence for the quantity, quality, and frequency of C input, (ii) tillage for the depth distribution of added substrates, and (iii) N fertilization for the quantity and quality of substrates. These seasonal changes can alter N availability and conservation.

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

Abstract 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.

Arbuscular Mycorrhiza and Petroleum-Degrading Microorganisms Enhance Phytoremediation of Petroleum-Contaminated Soil

While plants can phytoremediate soils that are contaminated with petroleum hydrocarbons, adding microbes to remediate contaminated sites with petroleum-degrading microorganisms and arbuscular mycorrhizal fungi (AMF) is not well understood. The phytoremediation of Arabian medium crude oil (ACO) was done with a Lolium multiflorum system inoculated with an AMF (Glomus intraradices) and a mixture of petroleum-degrading microorganisms—the bacterium, Sphingomonas paucimobilis (Sp) and the filamentous fungus, Cunninghamella echinulata (Ce, SpCe)—or with a combination of microorganisms (AMF + SpCe). Based on an earlier study on screening plants for phytoremediation of ACO, L. multiflorum (Italian ryegrass) was selected for its tolerance and rapid growth response (Alarcón, 2006). The plants were exposed to ACO-contaminated soil (6000 mg kg−1) for 80 d under greenhouse conditions. A modified Long Ashton Nutrient Solution (LANS) was supplied to all treatments at 30 μg P mL−1, except for a second, higher P, control treatment at 44 μg P mL−1. Inoculation with AMF, SpCe, or AMF + SpCe resulted in significantly increased leaf area as well as leaf and pseudostem dry mass as compared to controls at 30 μg P mL−1. Populations of bacteria grown on a nitrogen-free medium and filamentous fungi increased with AMF + SpCe and SpCe treatments. The average total colonization and arbuscule formation of AMF-inoculated plants in ACO-contaminated soil were 25% and 8%, respectively. No adverse effects were caused by SpCe on AMFcolonization. Most importantly, ACOdegradation was significantly enhanced by the addition of petroleum-degrading microorganisms and higher fertility controls, as compared to plants at 30 μg P mL−1. The highest ACOdegradation (59%) was observed with AMF + SpCe. The phytoremediation of ACO was also enhanced by single inoculation of AMF or SpCe. The effect of AMF and petroleum-degrading microorganisms on plant growth and ACOdegradation was not attributable to differences in proline, total phenolics, nitrate reductase levels, or variation in plant–gas exchange.

Comparison of microbiological methods for evaluating quality and fertility of soil

Routine soil testing procedures that are rapid and accurate are needed to evaluate C and N mineralization in agricultural soils in order to determine soil quality and fertility. Laboratory methods were compared for their usefulness in determining soil microbial biomass and potential activity in a Weswood silty clay loam (fine, mixed, thermic Fluventic Ustochrept) subjected to long-term tillage, crop sequence, and N-fertilizer management practices. The methods included basal soil respiration, net N mineralization during a 10-day incubation, soil microbial biomass C with the chloroform fumigation-incubation technique with and without subtracting a control value, soil microbial biomass N with the chloroform fumigation-incubation technique, substrate-induced respiration, and arginine ammonification. All methods were highly correlated with each other and, therefore, appear to adequately reflect soil microbial biomass and potential activity under laboratory conditions. The longer incubation times used with the basal soil respiration, N mineralization, and microbial biomass C and N assays resulted in higher correlations and lower variation among replications compared to the shorter incubation times used with substrate-induced respiration and arginine ammonification. The relatively rapid procedural time (3 h) required for the latter two assays could make these methods more attractive for routine soil testing, although multiple assays on the same sample may be necessary because these methods are less precise than the incubation methods that require 10 days.

Misidentification of soil bacteria by fatty acid methyl ester (FAME) and BIOLOG analyses

Abstract Fatty acid methyl ester (FAME) analysis is commonly used by soil scientists as a sole method for identifying soil bacteria. We observed discrepancies with this method for identifying certain species of bacteria. Therefore, we used carbon substrate oxidation patterns (BIOLOG) and some simple physical and chemical tests to determine the extent of these discrepancies. Identification with FAME profiles gave false positives for Arthrobacter globiformis, Micrococcus kristinae, and M. luteus, and identification with BIOLOG patterns gave a false positive identification for A. globiformis. A visual check and Gram stain are recommended when FAME analysis identifies soil isolates as M. kristinae or M. luteus, and an additional spore formation test is recommended when FAME and BIOLOG analyses identify isolates as A. globiformis.

Microalgal productivity, community composition, and pelagic food web dynamics in a subtropical, turbid salt marsh isolated from freshwater inflow

Carbon entering the food web originating from microalgal productivity may be as important to salt marsh consumers as carbon originating from vascular plant production. The objective of this study was to further our understanding of the role played by microalgae in salt marshes. We focused on microalgal productivity, community dynamics, and pelagic food web linkages. Across three consecutive springs (2001–2003), we sampled the upper Nueces Delta in southeast Texas, United States; a shallow, turbid system of ponds and elevated vegetated areas stressed by low freshwater inflow and salinities ranging from brackish (11) to hypersaline (300). Despite high turbidity and low external nutrient loadings, microalgal productivity was on the order of that reported for vascular plants. Primary productivity in surface waters ranged from 0 to 2.02 g C m−2 d−1 and was usually higher than primary productivity associated with the benthos, which ranged from 0 to 1.14 g C m−2 d−1. This was likely due to high amounts of wind-driven resuspended sediment limiting production at greater depths. Most of the water column microalgal biovolume seemed to originate from the benthos and was comprised mostly of pennate diatoms. But true phytoplankton taxa were also observed, which included cryptomonads, chlorophyhtes dinoflagellates, and cyanobacteria. Succession from r-selected to K-selected taxa with the progression of spring, a common phenomena in aquatic systems, was not observed. Codominance by both potentially edible and less edible taxa was found. This was likely due to decreased grazing pressure on r-selected taxa as salinity conditions became unfavorable for grazers. In addition to a decoupled food web, reduced primary and net productivity, community respiration, and microalgal and zooplankton population densities were all observed at extreme salinities. Our findings suggest that a more accurate paradigm of salt marsh functioning within the landscape must account for microalgal productivity as well as production by vascular plants. Because the value of microalgal productivity to higher trophic levels is taxa specific, the factors that govern microalgal community structure and dynamics must also be accounted for. In the case for the Nueces Delta, these factors included wind mixing and increasing salinities.

Impacts of Cropping Systems and Long-Term Tillage on Soil Microbial Population Levels and Community Composition in Dryland Agricultural Setting

Few studies have used molecular methods to correlate the abundance of specific microbial taxonomic groups with changes in soil properties impacted by long-term agriculture. Community qPCR with 16S rRNA gene sequencing to examine the effects of long-term crop-management practices (no-till vs. conventional tillage, and continuous wheat (Triticum aestivum L.) vs. sorghum-wheat-soybean rotation (Sorghum bicolor L. Moench-Triticum aestivum L.-Glycine max L. Merr) on bacterial and fungal relative abundances and identify the dominant members of the soil microbial community. The qPCR assays revealed that crop rotation decreased bacterial copy numbers, but no-till practices did not significantly alter bacteria or fungi relative to conventional tillage. Cyanobacteria were more abundant while Actinobacteria were less numerous under continuous wheat. Acidobacteria and Planctomycetes were positively correlated with soil microbial biomass C and N. This study highlights ways cropping systems affect microbial communities and aids the development of sustainable agriculture.