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Abstract Cover cropping is a promising strategy to improve soil health, but it may also trigger greenhouse gas emissions, especially nitrous oxide (N2O). Beyond nitrogen (N) availability, cover crop residue decomposition may accelerate heterotrophic respiration to limit soil O2 availability, hence promote N2O emissions from denitrification under sub-optimal water-filled pore space (WFPS) conditions that are typically not conducive to large N2O production. We conducted a 21-day incubation experiment to examine the effects of contrasting cover crop residue (grass vs legume) decomposition on soil O2 and biogeochemical changes to influence N2O and CO2 emissions from 15N labeled fertilized soils under 50% and 80% WFPS levels. Irrespective of cover crop type, mixing cover crop residue with N fertilizer resulted in high cumulative N2O emissions under both WFPS conditions. In the absence of cover crop residues, the N fertilizer effect of N2O was only realized under 80% WFPS, whereas it was comparable to the control under 50% WFPS. The N2O peaks under 50% WFPS coincided with soil O2 depletion and concomitant high CO2 emissions when cover crop residues were mixed with N fertilizer. While N fertilizer largely contributed to the total N2O emissions from the cover crop treatments, soil organic matter and/or cover crop residue derived N2O had a greater contribution under 50% than 80% WFPS. Our results underscore the importance of N2O emissions from cover crop-based fertilized systems under relatively lower WFPS via a mechanism of respiration-induced anoxia and highlight potential risks of underestimating N2O emissions under sole reliance on WFPS.

Abstract We investigated the individual and synergistic impact of gypsum, elemental sulfur, vermicompost, biochar, and microbial inoculation on soil health improvement in degrading calcareous saline-sodic soils. We developed Linear and nonlinear soil health quantification frameworks to assess the efficacy of remedial practices. The combined inoculated chemical and organic treatments; gypsum + vermicompost and elemental sulfur + vermicompost with 134% (0.29 versus 0.68) and 116% (0.29 versus 0.62) increases in nonlinear index, significantly increased the efficacy of amendments compared with control. An increase in the overall soil health index ranged between 12 to 134%. Microbial inoculation further enhanced the impact of treatments on soil health. Soil health properties included in the indexes explained 29 to 87% of the variance in wheat growth. The findings bring insight into the cost-effective and environmentally sustainable practices to recover degraded saline-sodic soils. Furthermore, the introduced soil health indexes offer a quantitative evaluation of soil remediation strategies.

Denitrifying bioreactors (DNBRs) are widely used to reduce excess nitrate from agricultural drainage. Their performance depends on the physical and chemical properties of the substrate. Common substrate types have been partly reviewed in previous studies. However, few studies have attempted to determine a generalized pattern for the role of substrate type in nitrate removal. This study summarizes 41 types of substrates using a dataset collected from 63 peer-reviewed articles, which include 219 independent DNBR units. The substrates are classified into four groups: ① natural carbon (NC), such as woodchips; ② non-natural carbon (NNC), such as biodegradable polymers (e.g., polycaprolactone (PCL), and waste products (e.g., cardboard); ③ inorganic materials (IMs), such as non-carbon materials (e.g., iron oxide); and ④ multiple materials (MMs), such as a mixture of the above materials. These materials are compared and evaluated through a meta-analysis of nitrate removal rate (NRR; N removal (g∙m−3∙d–1)) and nitrate removal efficiency (NRE). This study reviews substrate performance (NRR and NRE), potential mechanisms, pollution swapping, and cost analysis. Our analysis indicates that woodchips and corncobs are the most cost-effective substrates among NCs. In a comparison of all the studied substrates, MM substrates are recommended as the optimal substrates, especially woodchip-based and corncob-based substrates, which have great potential for improvement. This analysis can assist in optimizing the design of DNBRs to meet the environmental, economic, and practical requirements of users.

IntroductionPromoting sustainable crop production is enhanced by an effective method to assess soil health. However, soil health assessment is challenging due to multiple interactions among dynamic soil properties (i.e., soil health indicators) across management practices and agroecological regions. We tested several currently popular soil health assessment methods for cropping systems in Tennessee in the southeastern US and found that these methods failed to differentiate Tennessee soil health under long-term conservation and conventional management.Materials and methodsThis study developed a Tennessee weighted soil health index (WSHI) by: 1) selecting a set of management-sensitive soil health indicators, 2) assigning meaningful weights to indicators, and 3) normalizing the scores based on regionally relevant undisturbed natural reference sites. The tested cropping systems treatments were moldboard plow (MP) in continuous soybean (SS), no tillage (NT) in SS, NT with wheat cover (NTW) in SS, no cover and chisel plow (NCCT) in continuous cotton (CC), no cover and no tillage (NCNT) in CC, and hairy vetch cover and no tillage (VCNT) in CC. In addition, two woodlots and one grassland sites in the vicinity of the cropping systems were selected to represent undisturbed natural systems.Results and discussionOut of 22 indicators that proved to be management-sensitive, six were selected as a minimum dataset (MDS). These were particulate organic matter C (POM-C), soil respiration from 4-day incubation (4d CO2), small macroaggregate (0.250-2mm)-associated C (SMA-C), surface hardness (PR15), microbial biomass N (MBN), and bulk density (BD). Measured values of the MDS indicators were transformed into unitless normalized scores (based on the regional range of the indicator), and finally integrated into WSHI scores using a weighted-addition approach. Additionally, the soil health gap (SHG) between the soil health of the regional reference system and different cropping systems was calculated. Results revealed that WSHI strongly differentiated soil health between long-term conservation and conventional managements practices. The WSHI scores for southeastern cropland soils varied as follows: VCNT = NTW > NT > NCNT ≥ NCCT ≥ MP. The SHGs under MP, NCCT, NCNT, NT, NTW, and VCNT were 85.5, 79.9, 68, 45.1, 25.2, and 24.3, respectively, relative to the average WSHI of three undisturbed systems. Results showed that the WSHI approach is effective in more meaningful regional assessment of soil health and SHG can be a potential metric for comparing soil health across agroecological regions.

Long-term no-tillage systems enhance cotton yield resilience to climate extremes through improved soil quality in Tennessee, USA, according to a 29-year rain-fed plot-scale cotton experiment.

Soil microbial transformation of nitrogen (N) in nutrient-limited native C4 grasslands can be affected by N fertilization rate and C4 grass species. Here, we report in situ dynamics of the population size (gene copy abundances) and activity (transcript copy abundances) of five functional genes involved in soil N cycling (nifH, bacterial amoA, nirK, nirS, and nosZ) in a field experiment with two C4 grass species (switchgrass (Panicum virgatum) and big bluestem (Andropogon gerardii)) under three N fertilization rates (0, 67, and 202 kg N ha−1). Diazotroph (nifH) abundance and activity were not affected by N fertilization rate nor grass species. However, moderate and high N fertilization promoted population size and activity of ammonia oxidizing bacteria (AOB, quantified via amoA genes and transcripts) and nitrification potential. Moderate N fertilization increased abundances of nitrite-reducing bacterial genes (nirK and nirS) under switchgrass but decreased these genes under big bluestem. The activity of nitrous oxide reducing bacteria (nosZ transcripts) was also promoted by moderate N fertilization. In general, high N fertilization had a negative effect on N-cycling populations compared to moderate N addition. Compared to big bluestem, the soils planted with switchgrass had a greater population size of AOB and nitrite reducers. The significant interaction effects of sampling season, grass species, and N fertilization rate on N-cycling microbial community at genetic-level rather than transcriptional-level suggested the activity of N-cycling microbial communities may be driven by more complex environmental factors in native C4 grass systems, such as climatic and edaphic factors.

Abstract The combination of extended dry periods and high intensity rainfall, common in the southeastern US, leads to greater variability in soil moisture and consequently increases uncertainty to microbial processes pertinent to soil carbon (C) mineralization. However, field-based findings on soil moisture sensitivity to soil C cycling are very limited. Therefore, a field experiment was conducted in 2018 and 2019 on a soybean (Glycine max L.) cropland in the southeastern US with three soil moisture treatments: drought (simulated using rainout-shelter from June to October in each year), rainfed (natural precipitation), and irrigated (irrigation and precipitation). Soil respiration was measured weekly from May to November in both years. Soil samples were collected multiple times each year from 0–5, 5–15, and 15–30 cm depths to determine microbial biomass C (MBC), extractable organic C (EOC), hydrolytic enzyme activities, and fungal abundance. The cumulative respiration under drought compared to other treatments was lower by 32% to 33% in 2018 and 38% to 45% in 2019. Increased MBC, EOC, and fungal abundance were observed under drought than other treatments. Specific enzyme activity indicated fewer metabolically active microbes under drought treatment compared to rainfed and irrigated treatments. Also, maintenance of enzyme pool was observed under drought condition. These results provide critical insights on microbial metabolism in response to soil moisture variation and how that influences different pools of soil C under field conditions.

Interactions between species above- and belowground are among the top factors that govern ecosystem functioning including soil organic carbon (SOC) storage. In agroecosystems, understanding how crop diversification affects soil biodiversity and SOC storage at the local scale remains a key challenge for addressing soil degradation and biodiversity loss that plague these systems. Yet, outcomes of crop diversification for soil microbial diversity and SOC storage, which are key indicators of soil health, are not always positive but rather they are highly idiosyncratic to agroecosystems. Using five case studies, we highlight the importance of selecting ideal crop functional types (as opposed to focusing on plant diversity) when considering diversification options for maximizing SOC accumulation. Some crop functional types and crop diversification approaches are better suited for enhancing SOC at particular sites, though SOC responses to crop diversification can vary annually and with duration of crop cover. We also highlight how SOC responses to crop diversification are more easily interpretable through changes in microbial community composition (as opposed to microbial diversity). We then develop suggestions for future crop diversification experiment standardization including (1) optimizing sampling effort and sequencing depth for soil microbial communities and (2) understanding the mechanisms guiding responses of SOC functional pools with varying stability to crop diversification. We expect that these suggestions will move knowledge forward about biodiversity and ecosystem functioning in agroecosystems, and ultimately be of use to producers for optimizing soil health in their croplands.

Background Fertilizer addition can contribute to nitrogen (N) losses from soil by affecting microbial populations responsible for nitrification. However, the effects of N fertilization on ammonia oxidizing bacteria under C4 perennial grasses in nutrient-poor grasslands are not well studied. Methods In this study, a field experiment was used to assess the effects of N fertilization rate (0, 67, and 202 kg N ha−1) and grass species (switchgrass (Panicum virgatum) and big bluestem (Andropogon gerardii)) on ammonia-oxidizing bacterial (AOB) communities in C4 grassland soils using quantitative PCR, quantitative reverse transcription-PCR, and high-throughput amplicon sequencing of amoA genes. Results Nitrosospira were dominant AOB in the C4 grassland soil throughout the growing season. N fertilization rate had a stronger influence on AOB community composition than C4 grass species. Elevated N fertilizer application increased the abundance, activity, and alpha-diversity of AOB communities as well as nitrification potential, nitrous oxide (N2O) emission and soil acidity. The abundance and species richness of AOB were higher under switchgrass compared to big bluestem. Soil pH, nitrate, nitrification potential, and N2O emission were significantly related to the variability in AOB community structures (p

Native C4 grasses have become the preferred species for native perennial pastures and bioenergy production due to their high productivity under low soil nitrogen (N) status. One reason for their low N requirement is that C4 grasses may benefit from soil diazotrophs and promote biological N fixation. Our objective was to evaluate the impact of N fertilization rates (0, 67, and 202 kg N ha–1) and grass species (switchgrass [Panicum virgatum] and big bluestem [Andropogon gerardii]) on the abundance, activity, diversity, and community composition of soil diazotrophs over three agricultural seasons (grass green-up, initial harvest, and second harvest) in a field experiment in East Tennessee, United States. Nitrogen fertilization rate had a stronger influence on diazotroph population size and activity (determined by nifH gene and transcript abundances) and community composition (determined by nifH gene amplicon sequencing) than agricultural season or grass species. Excessive fertilization (202 kg N ha–1) resulted in fewer nifH transcripts compared to moderate fertilization (67 kg N ha–1) and decreased both richness and evenness of diazotrophic community, reflecting an inhibitory effect of high N application rates on soil diazotrophic community. Overall, cluster I and cluster III diazotrophs were dominant in this native C4 grass system. Diazotroph population size and activity were directly related to soil water content (SWC) based on structural equation modeling. Soil pH, SWC, and C and N availability were related to the variability of diazotrophic community composition. Our results revealed relationships between soil diazotrophic community and associated soil properties, adding to our understanding of the response of soil diazotrophs to N fertilization and grass species in native C4 grass systems.

Soil microbial respiration is one of the largest sources of carbon (C) emissions to the atmosphere in terrestrial ecosystems, which is strongly dependent on multiple environmental variables including soil moisture. Soil moisture content is strongly dependent on soil texture, and the combined effects of texture and moisture on microbial respiration are complex and less explored. Therefore, this study examines the effects of soil moisture on the mineralization of soil organic C Soil organic carbon in three different soils, Ultisol, Alfisol and Vertisol, collected from mixed forests of Georgia, Missouri, and Texas, United States , respectively. A laboratory microcosm experiment was conducted for 90 days under different moisture regimes. Soil respiration was measured weekly, and destructive harvests were conducted at 1, 15, 60, and 90 days after incubation to determine extractable organic C (EOC), phospholipid fatty acid based microbial community, and C-acquiring hydrolytic extracellular enzyme activities (EEA). The highest cumulative respiration in Ultisol was observed at 50% water holding capacity (WHC), in Alfisol at 100% water holding capacity, and in Vertisol at 175% WHC. The trends in Extractable Organic Carbon were opposite to that of cumulative microbial respiration as the moisture levels showing the highest respiration showed the lowest EOC concentration in all soil types. Also, extracellular enzyme activities increased with increase in soil moisture in all soils, however, respiration and EEA showed a decoupled relationship in Ultisol and Alfisol soils. Soil moisture differences did not influence microbial community composition.

Abstract Soil organic matter (SOM) in managed grasslands have economic and environmental benefits. This experiment evaluated a stockpiled winter grazing system with two summer management treatments (grazing or hay harvest) and three forage species treatments: tall fescue (TF, Schedonorus arundinaceus Schreb.), switchgrass (SG, Panicum virgatum L.), and mixed big bluestem–Indiangrass [BBIG, Andropogon gerardii Vitman–Sorghastrum nutans (L.) Nash]. Soil was sampled on 18 dates (January 2016–July 2017) at two depths (0–5 and 5–15 cm) in 15 paddocks in central Tennessee. Total organic carbon and total nitrogen concentrations in 0–5‐cm samples were greater in grazed paddocks relative to hay harvest, and greater in TF relative to BBIG and SG. Summer grazing also resulted in greater 0–5‐cm permanganate‐oxidizable carbon (POXC) and 5–15 cm hot‐water extract ultraviolet absorbance at 254 nm (A254). Hot‐water extractable carbon, A254, and POXC concentrations were reduced in SG soils compared with TF and BBIG. Summer hay harvests, compared with grazing, reduced hot‐water extractable C/N in both soil horizons in TF. The interactions between management and plant species suggests contrasting nutrient cycling associated with TF and the morphologically different native grasses BBIG and SG. This study represents the first observations of soil impacts within stockpiled grazing systems and the first observations of grazed native grass species in the southeastern United States.

Combined application of biochar and nitrogen (N) fertilizer has the potential to reduce N losses from soil. However, the effectiveness of biochar amendment on N management can vary with biochar types with different physical and chemical properties. This study aimed to assess the effect of two types of hardwood biochar with different ash contents and cation exchange capacity (CEC) on soil N mineralization and nitrous oxide (N2O) production when applied alone and in combination with N fertilizer. Soil samples collected from a temperate pasture system were amended with two types of biochar (B1 and B2), urea, and urea plus biochar, and incubated for 60 days along with soil control (without biochar or urea addition). Soil nitrate N, ammonium N, ammonia-oxidizing bacteria amoA gene transcripts, and N2O production were measured during the experiment. Compared to control, addition of B1 (higher CEC and lower ash content) alone decreased nitrate N concentration by 21% to 45% during the incubation period while the addition of B2 (lower CEC and higher ash content) alone increased the nitrate N concentration during the first 10 days. Biochar B1 also reduced the abundance of amoA transcripts by 71% after 60 days. Compared to B1 + urea, B2 + urea resulted in a significantly greater initial increase in soil ammonium and nitrate N concentrations. However, B2 + urea had a significantly lower 60-day cumulative N2O emission compared to B1 + urea. Overall, when applied with urea, the biochar with higher CEC reduced ammonification and nitrification rates, while biochar with higher ash content reduced N N2O production. Our study demonstrated that biochar has the potential to enhance N retention in soil and reduce N2O emission when it is applied with urea, but the specific effects of the added biochar depend on its physical and chemical properties.

Abstract Seasonal flooding following heavy rain deposits large amounts of sediments on productive lands in the lower parts of fields in western Tennessee. The deposited sediments have high proportion of sand particles that negatively affect soil physiochemical properties, which make the soil uneconomic for farming. Soil amendments such as biochar, a by‐product of renewable energy production from organic waste materials, have the potential to remediate sandy soil left after flood events and improve crop yields by increasing water holding capacity and soil nutrient content. The objective of this project was to evaluate the effect of biochar produced from two types of hardwood feedstocks on water retention and corn (Zea mays L.) growth in sandy soil. A greenhouse experiment quantifying the growth of corn was conducted in a randomized complete block design with five replications. Sandy soil was amended with three biochar rates (0, 10, and 20% by volume) under two irrigation levels. The irrigation levels were control and a dry treatment based on the past 10 yr of rainfall data. Biochar application greatly improved water retention in the flooded sandy soil. Biochar increased soil K and P concentration. However, at the end of the study, corn growth was not different in biochar amended and non‐amended sandy soil. This research demonstrated that biochar as a soil amendment has the potential to improve quality attributes of poor soil, such as the soil water and nutrient concentration in a previously flooded sandy soils.

Abstract Weed communities may potentially reduce row‐crop yields resulting in a loss of revenue for producers. Studies have reported a decrease in the efficacy of chemical methods of weed control due to increased weed resistance. With the goal of reducing reliance on chemical weed control and promoting sustainable crop production in mind, we evaluated different winter cover crop (CC) types (single‐, double‐, and multi‐species) and CC planting methods (broadcasting vs. drilling) on CC and weed groundcover and row‐crop yield in a no‐till corn (Zea mays L.)–soybean [Glycine max (L.) Merr.] rotation system in Tennessee from 2013 to 2017. Our results showed that winter CC reduced spring weed ground coverage by 64–76% compared with winter fallow with no herbicide application. A five‐species CC mixture increased soybean yields by 6 to 8.5% across seeding method treatments compared with other CC treatments and winter fallow. This yield increase was more visible in a drought year. Corn, grown in years with normal weather patterns, did not show a yield response to CC types. From this study, cover cropping was found to be an effective strategy for weed control compared with leaving the field fallow without herbicide application following row‐crop harvest; however, no consistent species‐specific trends were observed on weed suppression and row‐crop yield. Future research should focus on understanding CC species composition and biomass production in double‐ and multi‐species mixtures from this field experiment to draw robust conclusions on agronomic responses of individual CC species and planting methods.

Genetic engineering has been used to decrease the lignin content and to change the lignin composition of switchgrass (Panicum virgatum L.) to decrease cell wall recalcitrance to enable more efficient cellulosic biofuel production. Previous greenhouse and field studies showed that downregulation of the gene encoding switchgrass caffeic acid O-methyltransferase (COMT) and overexpression of the switchgrass PvMYB4 (MYB4) gene effectively improved ethanol yield. To understand potential environmental impacts of cultivating these transgenic bioenergy crops in the field, we quantified the effects of field cultivation of transgenic switchgrass on soil organic carbon (SOC) dynamics. Total and active SOC as well as soil respiration were measured in soils grown with two COMT-downregulated transgenic lines (COMT2 and COMT3), three MYB4-overexpressed transgenic lines (L1, L6, and L8), and their corresponding non-transgenic controls. No differences in total SOC, dissolved organic carbon (DOC), and permanganate oxidizable carbon (POXC) were detected between transgenic and non-transgenic treatments for both COMT (10.4–11.1 g kg−1 for SOC, 60.0–64.8 mg kg−1 for DOC, and 299–384 mg kg−1 for POXC) and MYB4 lines (6.89–8.21 g kg−1 for SOC, 56.0–61.1 mg kg−1 for DOC, and 177–199 mg kg−1 for POXC). Soil CO2-carbon (CO2-C) production from the COMT2 transgenic line was not significantly different from its non-transgenic control. In contrast, the COMT3 transgenic line had greater soil CO2-C production than its non-transgenic control (210 vs. 165 µg g−1) after 72 days of laboratory incubation. Combining the improvement in ethanol yield and biomass production reported in previous studies with negligible change in SOC and soil respiration, COMT2 could be a better biofuel feedstock than COMT3 for environmental conservation and cost-effective biofuel production. On the other hand, MYB4 transgenic line L8 produced more biomass and total ethanol per hectare while it released more CO2-C than the control (253 vs. 207 µg g−1). Long-term in situ monitoring of transgenic switchgrass systems using a suite of soil and environmental variables is needed to determine the sustainability of growing genetically modified bioenergy crops.

The North Central Education and Research Activity Committee (NCERA-59) was formed in 1952 to address how soil organic matter formation and management practices affect soil structure and productivity. It is in this capacity that we comment on the science supporting soil quality and associated soil health assessment for agricultural lands with the goal of hastening progress in this important field. Even though the suite of soil quality indicators being applied by U.S. soil health efforts closely mirrors the “minimum data set” we developed and recommended in the mid-1990s, we question whether the methods or means for their selection and development are sufficient to meet current and emerging soil health challenges. The rush to enshrine a standard suite of dated measures may be incompatible with longer-term goals. Legitimate study of soil health considers soil change accrued over years to decades that influence on- and off-site function. Tailoring of methods to local conditions is needed to effectively apply and interpret indicators for different soil resource regions and land uses. Adherence to a set suite of methods selected by subjective criteria should be avoided, particularly when we do not yet have adequate data or agreed upon interpretive frameworks for many so-called “Tier 1” biological indicators used in soil health assessment. While pooling data collected by producer-groups is one of the most exciting new trends in soil health, standardizing methods to meet broad inventory goals could compromise indicator use for site or application-specific problem solving. Changes in our nation's research landscape are shifting responsibility for soil stewardship from national and state government backed entities to public-private partnerships. As a result, it is critical to ensure that the data needed to assess soil health are generated by reproducible methods selected through a transparent process, and that data are readily available for public and private sector use. Appropriate methods for engagement need to be applied by public-private research partnerships as they establish and expand coordinated research enterprises that can deliver fact-based interpretation of soil quality indicators within the type of normative soil health framework conceived by USDA over 20 years ago. We look to existing examples as we consider how to put soil health information into the hands of practitioners in a manner that protects soils' services.

Soil organic carbon (SOC) content is strongly influenced by agricultural management practices. Leveraging three long-term field experiments in Tennessee, this study analyzed the effect of crop rotation, tillage, and cover crops on SOC and permanganate-oxidizable C (POXC) at various soil depth increments (0–2.5, 2.5–5, 5–7.5, 7.5–10, 10–15, 15–22.5, 22.5–30, 30–45, and 45–60 cm), as well as at the 0- to 15-cm profile. Corn ( L.) and cotton ( L.)–based systems showed increased SOC from 0 to 22.5 cm compared with soybean [ (L.), Merr.]–based systems. Additionally, no-till systems accumulated more SOC than tilled systems from 0 to 5 cm, but cover cropping showed no effects. Results also showed a significant positive relationship between SOC and POXC ( < 0.0001, = 0.93). This study revealed that SOC and POXC are strongly influenced by management practices in the surface shallower depth increments of southeastern US croplands.

Multispecies cover cropping has become popular in recent years because of the multiple ecosystem benefits compared with single- or double- species cover cropping. However, scientific studies on the effects of multispecies cover cropping—especially in the southern United States—are limited. A field study was initiated in 2013 at the University of Tennessee’s Research and Education Center in Milan, TN, to assess the agronomic and soil responses from single-, double-, and multispecies cover cropping in corn ( L.)–soybean [ (L.) Merr.] systems. After 3 yr, we found that a multispecies mixture of legumes, grasses, and spp. significantly increased soybean yield, gravimetric soil water content, and soil inorganic nitrogen as compared to the less-diverse treatments and a no-cover control. However, after 3 yr, cover cropping did not increase soil organic carbon. Although multispecies cover cropping exhibited a positive effect on yield and some soil properties after 3 yr, we plan to continue collecting multiple years of data from this field trial.

Enzyme dynamics are being incorporated into soil carbon cycling models and accurate representation of enzyme kinetics is an important step in predicting belowground nutrient dynamics. A scarce number of studies have measured activation energy (Ea) in soils and fewer studies have measured Ea in arctic and tropical soils, or in subsurface soils. We determined the Ea for four typical lignocellulose degrading enzymes in the A and B horizons of seven soils covering six different soil orders. We also elucidated which soil properties predicted any measurable differences in Ea. β-glucosidase, cellobiohydrolase, phenol oxidase and peroxidase activities were measured at five temperatures, 4, 21, 30, 40, and 60°C. Ea was calculated using the Arrhenius equation. β-glucosidase and cellobiohydrolase Ea values for both A and B horizons in this study were similar to previously reported values, however we could not make a direct comparison for B horizon soils because of the lack of data. There was no consistent relationship between hydrolase enzyme Ea and the environmental variables we measured. Phenol oxidase was the only enzyme that had a consistent positive relationship between Ea and pH in both horizons. The Ea in the arctic and subarctic zones for peroxidase was lower than the hydrolases and phenol oxidase values, indicating peroxidase may be a rate limited enzyme in environments under warming conditions. By including these six soil types we have increased the number of soil oxidative enzyme Ea values reported in the literature by 50%. This study is a step towards better quantifying enzyme kinetics in different climate zones.

BACKGROUND: Physico-chemical sorption onto soil minerals is one of the major processes of dissolved organic carbon (OC) stabilization in deeper soils. The interaction of DOC on soil solids is related to the reactivity of soil minerals, the chemistry of sorbate functional groups, and the stability of sorbate to microbial degradation. This study was conducted to examine the sorption of diverse OC compounds (D-glucose, L-alanine, oxalic acid, salicylic acid, and sinapyl alcohol) on temperate climate soil orders (Mollisols, Ultisols and Alfisols). METHODOLOGY: Equilibrium batch experiments were conducted using 0-100 mg C L(-1) at a solid-solution ratio of 1∶60 for 48 hrs on natural soils and on soils sterilized by γ-irradiation. The maximum sorption capacity, Q(max) and binding coefficient, k were calculated by fitting to the Langmuir model. RESULTS: Ultisols appeared to sorb more glucose, alanine, and salicylic acid than did Alfisols or Mollisols and the isotherms followed a non-linear pattern (higher k). Sterile experiments revealed that glucose and alanine were both readily degraded and/or incorporated into microbial biomass because the observed Q(max) under sterile conditions decreased by 22-46% for glucose and 17-77% for alanine as compared to non-sterile conditions. Mollisols, in contrast, more readily reacted with oxalic acid (Q(max) of 886 mg kg(-1)) and sinapyl alcohol (Q(max) of 2031 mg kg(-1)), and no degradation was observed. The reactivity of Alfisols to DOC was intermediate to that of Ultisols and Mollisols, and degradation followed similar patterns as for Ultisols. CONCLUSION: This study demonstrated that three common temperate soil orders experienced differential sorption and degradation of simple OC compounds, indicating that sorbate chemistry plays a significant role in the sorptive stabilization of DOC.