Graduation date: 1986

A agricultura é a atividade humana que mais afeta o meio ambiente. Imensas quantidades de insumos agrícolas são aplicados sobre o solo e grande parte destes degrada os recursos hídricos. Para uma investigação adequada do efeito destes insumos, estudam-se as propriedades hidráulicas do solo, que influem no transporte de solutos neste meio. Medir tais propriedades e modelar os parâmetros correlatos são tarefas extremamente complexas, devido ao tempo requerido, dinheiro, instrumentação e escala. As metodologias convencionais inferem as propriedades hidráulicas em amostras que estão em equilíbrio, através de técnicas invasivas e sob restrições especiais. Esta tese contribui com a ciência do ambiente, via ciência do solo, propondo um novo método de estudo da infiltração da água na região não-saturada do solo, utilizando a tomografia computadorizada (TC). O tomógrafo foi aqui desenvolvido e construído. A TC, neste trabalho, mediu a umidade (teta) durante o fluxo não-saturado e, através da solução numérica da equação de Richards e do modelo de Rossi-Nimmo, obtiveram-se a curva de retenção, a sortividade, k(teta) e a difusividade D(teta). Resultados qualitativos, como imagens 2D e 3D, e resultados quantitativos demonstraram a boa correlação do método proposto com o método tradicional de medida da curva de retenção. Amostras de solo estrurado foram analisadas em laboratório e em campo. / Agriculture is the human activity that most affects the environment. Huge amounts of chemicals are applied on the soil. Pesticides percolation and runoff degrades water resources. Thus, soil hydraulic properties must be known due to their influence on solute transport. The measurement of these properties and the modelling of related parameters are often difficult, if not impossible, due to the involved time, money, instrumentation, and scale. Traditional methodologies infer hydraulic properties in samples that are in equilibrium, through invasive techniques and under some special constraints. This thesis contributes with environmental science, via soil science, as it proposes a new method to study the infiltration in the unsaturated zone of soil, by means of CT. The scanner was developed and constructed in this work. The proposed methodology uses profiling CT to measure the water content (theta) during the water flow, and by means of numerical solution of Richards equation and Rossi-Nimmo model water retention, sorptivity, hydraulic conductivity k(theta), and diffusivity D(theta) are obtained. Qualitative results, as 2D and 3D images, are presented and the quantitative results of water retention show good correlation of the proposed method with the conventional tensiometers method. Structured soil column samples are analyzed in the field and in laboratory.

Made available in DSpace on 2014-06-11T19:30:25Z (GMT). No. of bitstreams: 0 Previous issue date: 2008-01-17Bitstream added on 2014-06-13T19:19:12Z : No. of bitstreams: 1 marques_rr_dr_botfca.pdf: 1561857 bytes, checksum: f962228d29c966c9acd2c4e111c35a0d (MD5) / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / Estudos relacionados ao cultivo de amendoim no sistema de semeadura direta são escassos, principalmente relacionados com aplicação superficial de calcário e gesso. A aveia branca é uma das alternativas para o cultivo de inverno no Brasil, por produzir grãos para alimentação humana, forragem e adubo verde, diversificando a atividade agrícola, no entanto são inexistentes na literatura informações a respeito da calagem e gessagem superficiais nesta cultura. Em função do exposto, o presente trabalho teve por objetivo avaliar o comportamento das culturas de amendoim e aveia branca cultivadas no sistema semeadura direta, em área com aplicação superficial de calcário e gesso, bem como as conseqüências no movimento de bases no perfil do solo, na concentração de cátions solúveis na parte aérea, na nutrição e produtividade das culturas. O experimento foi conduzido nos anos agrícolas de 2004/2005 e 2005/2006 na Fazenda Experimental Lageado da Faculdade de Ciências Agronômicas – Campus de Botucatu, num Latossolo Vermelho distrófico. Foi utilizado o delineamento experimental em blocos casualizados com parcelas subdivididas e quatro repetições. As parcelas foram constituídas pela aplicação de quatro níveis de calcário dolomítico com PRNT = 71,2. As doses de calcário foram definidas de acordo com a análise química do solo na profundidade 0-0,2m para elevar a saturação por bases a 70% (2.000 kg ha- 1), metade da dose recomendada (1.000 kg ha-1), o dobro da dose recomendada (4.000 kg ha-1) e uma testemunha (sem aplicação). As subparcelas foram constituídas pela ausência e presença da aplicação de 2.100 kg ha-1 de gesso agrícola. Como cultura de verão foi utilizado o cultivar de amendoim (IAC 886) nos dois anos de condução do experimento em toda a área experimental e como cultura de inverno foi utilizado o cultivar de aveia branca (IAC 7) nos dois anos... / There have been few studies about peanut crop in no-tillage system mainly about superficial liming and gypsum application. White oat is an alternative for winter cropping in Brazil to be used as human food, forage and green manure; nevertheless, there is no information about superficial liming and gypsum application for this crop. Therefore, the objective of this work was to evaluate peanut and white oat yield as affected by superficial liming and gypsum application in no-tillage system as well as base movement in soil profile, concentration of water-soluble cations in the shoot of plants and plant nutrition. The experiment was carried out in the growing seasons of 2004/2005 and 2005/2006 in Botucatu, São Paulo State, Brazil, on a dystrophic Red Latosol. The experimental design was the completely randomized block with subdivided plots and four replications. The main plots consisted of four dolomite limestone levels with neutralization power of 71.2. Limestone levels were determined by soil chemical analysis in the 0-0.20m depth; the aim was to apply the recommended dose to raise base saturation up to 70% (2000 kg ha-1), half of the recommended dose (1.000 kg ha-1) and twice the recommended dose (4000 kg ha-1) besides the control, with no application. Subplots consisted of areas with and without gypsum application (2000 kg ha-1). Peanut (cultivar IAC 886) and white oat (IAC 7) consisted of the summer and winter crops cultivated in two growing seasons, respectively. The results of the present research showed that superficial liming affected soil acidity and increased Ca and Mg levels in the soil profile along with gypsum application, which contributed for the effects to be observed in deeper layers. Then, it was found that superficial liming showed residual effects and allowed fine lime particles to move down in the soil profile... (Complete abstract click electronic access below)

Conservation tillage systems are increasingly adapted replacing conventional turnover moldboard plowing practices worldwide. This is part of a sustainable intensification of agriculture to meet future global food demand while at the same time sustaining environmental resources. The choice of tillage system affects soil structure and thereby also soil hydraulic properties (SHP) such as the water retention characteristic (WRC) and the hydraulic conductivity characteristic (HCC). Effects of agricultural management on SHP have been widely studied in the past decades. Thereby, temporal variations were identified as a major source of variability in the quantification of soil pore space and SHP. Such variability is introduced by tillage creating a loose soil matrix that eventually settles due to gravity, wetting-drying cycles and temperature fluctuations but also variable soil organic matter distributions in the soil and biological activity. Past efforts to model soil water dynamics showed that consideration of time-variable SHP may significantly improve simulation results. This involves both the seasonal variability as well as long-term land-use changes from conventionally to untilled soil. A prerequisite for such an approach is the periodic quantification of the WRC and HCC in the field and laboratory. In addition to the direct provision of modeling parameters, the quantification of WRC and HCC over time yields information on soil structural changes in the shape of a soil pore size distribution (PSD). The evolution of derived PSDs can be modeled and with that, the evolution of SHP might be predicted. However, there is little data available and the processes happening over one cropping season or between land-use changes need to be better understood. The aim of this dissertation was to shed light on soil pore space and associated hydraulic property changes on a long-term (23 years) tillage experiment in Eastern Germany. Three treatments with varying tillage intensity were investigated: conventional tillage with a turnover moldboard plow (CT), reduced mulch tillage with a cultivator (RT) and no tillage with direct sowing (NT). The soil was a Haplic Luvisol with silt loam texture. Objectives were twofold: • Objective 1) was to quantify the temporal variability in PSD over one winter wheat cropping season by frequently measuring SHP. Soil physical quality of the three treatments was assessed using this data. • Objective 2) was to characterize the soil structural differences between the treatments by relating hydraulic conductivity over a wide soil moisture range to other soil physical and chemical properties. For Objective 1), undisturbed soil cores (250 cm3) were taken over one winter wheat cropping cycle on five occasions from December 2015 to after the harvest in August 2016. Those soil cores were used to determine the saturated hydraulic and the WRC as well as the HCC in the laboratory. The data was parametrized with the bimodal Kosugi and Mualem model. Soil physical quality was assessed by the relative field capacity and air capacity as suggested in recent literature. Results showed that tilled soil, i.e. CT and RT, exhibited a distinct bimodal PSD with a structural and a textural mode. However, this structural mode was temporally instable and diminished after the winter and throughout the early growing season. Likely processes behind those changes were wetting-drying cycles, rainfall impact and freeze-thaw cycles. Shortly before and after the harvest some of the structural mode was restored which was probably induced by decomposing organic matter mixed into the topsoil from the previous winter wheat harvest during stubble breaking. Described changes were evident in decreases of transmission pores (⌀ 50 - 500 µm) during winter and increases during summer. Untilled soil, i.e. NT, tended towards a unimodal PSD with less transmission but more storage (⌀ 0.5 - 50 µm) pores. Temporally this soil was rather inert. This was attributed to natural compaction in absence of annual tillage for more than 20 years. Soil physical quality varied with the changes in PSD. Water availability was not an issue. Overall, the soil physical quality indicators for soil aeration were outside of an optimal range for indicators for most of the time. For Objective 2), field infiltration measurements were conducted with a hood (tension) infiltrometer to obtain (near-) saturated hydraulic conductivity. Soil cores were taken to quantify unsaturated hydraulic conductivity. Other properties for correlation and multiple regression analysis were bulk density, the bubbling pressure, organic C, as well as macro- and mesoporosity. X-ray µCT imaging on undisturbed soil cores from CT and NT treatments gave additional information on soil pore metrics. Results pointed towards a distinctly different soil structure between tilled and untilled soil. Near-saturated hydraulic conductivity of tilled soil was negatively correlated with bulk density as well as macro- and mesoporosity. None of the properties was meaningful for untilled soil. Imaging results confirmed the hypothesis, that (near-) saturated hydraulic conductivity on NT is governed by few well-connected large pores, while the soil matrix is comparably dense conducting only small amounts of infiltrating water. On tilled soil, the overall porosity is relevant for water transmission. Large continuous pore systems, however, get destroyed by annual tillage. In summary, the study showed distinct differences in soil structure and inherently also SHP between conservation and conventional tillage treatments. Differences in SHP, both in (near-) saturated hydraulic conductivity as well as WRC and HCC were large between some occasions. Therefore, this study confirmed the notion that on arable soils one-off measurements of SHP are not enough for their proper quantification. This was especially true for tilled soil. Modeling tasks over one cropping period, i.e. for example for irrigation schedules, will make periodic measurements necessary, i.e. unless an accurate modeling of the PSD becomes feasible. Current restraints are that most PSD models only consider a short-term post-tillage loss of porosity while a restored macropore system is not accounted for. In contrast to CT and RT, NT soil was temporally stable. While water retention was improved, (near ) saturated hydraulic conductivity was overall lower than on tilled soil. Correlation and regression analysis in combination with X-ray µCT explained some of the differences observed by tension infiltration measurements. Results highlighted that for arable soil, tillage treatments and probably other agricultural management practices, need to be considered when developing pedotransfer functions for an accurate estimation of SHP.:Table of Contents Declaration of conformity I Acknowledgements II Table of Contents IV List of Figures VII List of Tables XI Nomenclature XIII Abstract XV Zusammenfassung XVIII 1 Introduction 1 1.1 The sustainable development agenda and conservation tillage 1 1.2 Soil structure and soil hydraulic properties 3 1.3 Effects of conservation tillage on soil hydraulic properties 5 1.4 Temporal variability of soil hydraulic properties 8 1.5 Objectives and hypotheses 10 1.6 Structure of the dissertation 12 2 Materials and methods 15 2.1 Study area 15 2.1.1 Tillage experiment Lüttewitz (‘Schlag Gasthof’) 15 2.1.2 Treatments and agricultural management 16 2.2 Sample design 20 2.3 Field measurements 22 2.3.1 Hood infiltrometer measurements 22 2.3.2 Analysis of hood infiltrometer measurements 24 2.3.3 Macropore stability indicator 24 2.3.4 Undisturbed and disturbed soil sampling 25 2.4 Laboratory measurements 26 2.4.1 Saturated hydraulic conductivity 26 2.4.2 Water retention and hydraulic conductivity characteristic 26 2.4.3 Other soil properties 27 2.5 Model fitting procedure 28 2.5.1 Bimodal models for the water retention and hydraulic conductivity characteristic 28 2.5.2 Parametrization to quantify changes in the pore size distributions and pore volume fractions 29 2.5.3 Parametrization to infer unsaturated hydraulic conductivity for variability analysis 31 2.6 Capacitive soil physical quality indicators 32 2.7 Relationship between imaged pore metrics and field near-saturated hydraulic conductivity 32 2.8 Statistical analysis 33 3 Results 35 3.1 Rainfall patterns 35 3.2 Field (near-) saturated hydraulic conductivity 36 3.3 Threshold pore radius 37 3.4 Laboratory saturated hydraulic conductivity 38 3.5 Unsaturated hydraulic conductivity 39 3.6 Soil pore size distributions and pore volume fractions over one cropping season 40 3.7 Capacitive soil physical quality indicators 45 3.8 Correlation and linear regression of hydraulic conductivity with other soil properties 46 3.9 Other soil properties 49 3.9.1 Bulk density 49 3.9.2 Soil organic carbon and nitrogen 50 3.10 Imaged soil structure and hydraulic conductivity 52 3.10.1 Comparison of hydraulic conductivity obtained through three methods in Spring 2018 52 3.10.2 Soil pore metrics 52 3.10.3 Correlation between hydraulic conductivity and pore metrics 53 4 Discussion 55 4.1 Soil pore size distributions over one cropping cycle 55 4.1.1 Soil pore size distribution is bimodal on tilled soil and varies with time 55 4.1.2 Summary Objective 1) Hypotheses A and B 58 4.2 The effects of a changing pore system on soil physical quality 59 4.2.1 Suboptimal soil physical quality indicators change with time 60 4.2.2 Summary Objective 1) Hypothesis C 62 4.3 Tillage effects on variability of hydraulic conductivity 62 4.3.1 (Near ) saturated hydraulic conductivity 62 4.3.2 Unsaturated hydraulic conductivity 64 4.3.3 Summary: Objective 2) Hypothesis D and E 65 4.4 Factors influencing water transmission and its temporal variation 65 4.4.1 Soil properties partly explain variability in hydraulic conductivity on CT 65 4.4.2 Imaged pore metrics explain differences in field hydraulic conductivity 67 4.4.3 Summary Objective 2) Hypothesis F 68 5 Summary and outlook 69 References 73 Appendix 93

Crop management practices, such as soil tillage and crop residue management, are land management activities with potentially large impacts on carbon (C) cycling and climate at the global scale. Improvements in crop management practices, such as conservation tillage or 'no-till' (NT), have been proposed as climate change mitigation measures because such practices may alter C cycles through increased sequestration of soil C in agricultural soils. Despite their potential importance, regional to global scale data are lacking for many crop management practices, and few studies have evaluated the potential impact of the full range of crop management practices on C cycling and climate at the global scale. However, monitoring of crop management practices is crucial for assessing spatial variations in management intensity and informing policy decisions. Inclusion of crop management practices in Earth system models used for assessing global climate is a key requirement for evaluating the overall effects of different crop management practices on C cycling and their potential to mitigate climate change. Studies in this dissertation seek to address these issues by: (1) evaluating the efficacy of remote sensing methods for monitoring differences in soil tillage and crop residue management practices in Iowa; (2) incorporating soil tillage practices into an Earth system model and assessing the potential for soil C sequestration and climate change mitigation through adoption of NT practices; (3) assessing the historical impact of including the full range of crop management practices (residue harvest, grain harvest, soil tillage, irrigation, and fertilization) on changes in C cycling associated with land use and land cover change (LULCC) to crops in an Earth system model. The remote sensing study found that performance of the minimum Normalized Difference Tillage Index (minNDTI) method for assessing differences in tillage and residue management was below average compared to previous studies, even when using imagery from both Landsat 8 and Sentinel-2A sensors. Accurate assessment of these practices using minNDTI was hindered by issues with image quality and inability to obtain sufficient cloud-free, time series imagery during the critical planting window. Remote sensing research aimed at obtaining regional to global scale data on tillage and residue management practices is likely to continue to face these issues in the future, but further research should incorporate additional sensors and assess the efficacy of the minNDTI method for multiple locations and years. Adoption of NT practices in the Community Land Model, which is the land component of the Community Earth System Model, resulted in a cumulative soil C sequestration of 6.6 – 14.4 Pg C from 2015 – 2100 under a future climate change scenario (Representative Concentration Pathway 8.5), and cumulative soil C sequestration was equal to approximately one year of present-day fossil fuel emissions. Adjusting for areas where NT is already practiced had minor impacts on cumulative soil C storage, reducing gains in soil C from NT adoption by 0.4 – 0.9 Pg C globally. These results indicate that soil C sequestration and potential for climate change mitigation through NT may be more limited than has been anticipated elsewhere. Soil C sequestration via NT adoption was highest in temperate regions of developed countries with high initial soil C contents, indicating these areas should be targeted for NT adoption. Simulating the full range of crop management practices in the Community Land Model resulted in an increase in C emissions due to LULCC of 29 – 38 Pg C compared to scenarios with generic crops and model defaults. Individual crop management practices with the largest impact on LULCC emissions were crop residue harvest (18 Pg C), followed by grain harvest (9 Pg C) and soil tillage (5 Pg C). Although implementation of crop residue harvest and soil tillage was extreme in this study, these results imply that Earth system models may underestimate emissions from LULCC by excluding the full range of crop management practices. Studies in this dissertation corroborate the importance of crop management practices for C cycling and climate, but further research on these management practices is needed in terms of data collection, improving process-level understanding, and inclusion of these practices in Earth system models. / Doctor of Philosophy

This study measured sediment, nutrient and pesticide edge-of-field and deep percolation losses from conventional tillage (CT), strip tillage (ST), and no-till (NT) treatments in a burley tobacco production system. The field experiment results show that the CT treatment yielded more total runoff, 93.6 mm, than ST and NT. Compared to the CT treatment, ST reduced the total mass loss of NO₃⁻ by 37 percent, NH₃ by 54 percent, PO₄³⁻ by 12 percent, TP by 65 percent and TSS by 64 percent. The NT treatment reduced the total mass of NO₃⁻ by 49 percent, NH₃ by 46 percent, PO₄³⁻ by 17 percent, TP by 73 percent and TSS by 77 percent. Field experiment results showed that, respectively, the ST and NT treatments yielded 77 and 82 percent less chlorpyrifos, an insecticide, mass loss in runoff compared to CT. With respect to flumetralin loss, a growth inhibitor, the NT treatment yielded 30 percent less and ST yielded 6.8 percent more flumetralin mass, compared to CT. The percolated water results show that compared to the CT treatment, ST resulted in 33 percent less NH₃, 7.5 percent less TKN, and 39 percent less PO₄³⁻ total mass. The NT treatment yielded 41 percent less NH₃ and 20 percent less TKN total mass loss compared to the CT treatment. The NT treatment had the most NO₃⁻, PO₄³⁻, and TP mass loss below the root zone, however NT also had the most percolated water by 53 mm (838 L). The ST treatment yielded 77 percent less, and NT yielded 82 percent less chlorpyrifos mass loss compared to the CT treatment. Compared to CT, the NT treatment resulted in 30 percent less and ST yielded 7 percent more flumetralin mass in percolated water. The GLEAMS model was used to simulate runoff, nutrient, sediment and pesticide losses from the same three tillage practices evaluated in the field experiment. The model results showed that for runoff volume, TSS, nitrogen, chlorpyrifos and flumetralin in runoff, the conventional tillage practice generated greater losses than the conservation tillage practices. Compared to the field experiment results, GLEAMS under predicted nitrogen and phosphorus in percolated water. The conservation tillage practices simulated in GLEAMS were effective in minimizing the loss of agricultural pollutants. / Master of Science

Mid-Atlantic crop producers are increasingly transitioning to soil conservation methods that include reducing or eliminating tillage and planting high residue cover crops. These practices are associated with an increase in moderate to severe damage to field crops by slugs. Conserving, and even enhancing, natural enemy populations is a desirable way to manage slug infestations because remedial control measures are limited. To better understand how cover crop usage and tillage practices affect slug and natural enemy populations, 43 Virginia fields with different combinations of tillage practices and cover crop use were intensively sampled in 2018 and 2019. Fields were sampled over a six-week period during the early planting season when slugs are most problematic. Shingle traps and pitfall traps were used to sample slugs and natural enemies, respectively. To determine how multiple farming practices, soil composition, landscape features, and field history affect slug feeding injury to seedling plants, over 1,000 hectares of commercial production fields in the Shenandoah Region of Virginia were scouted for slug feeding injury to seedling plants. Corresponding crop producers were then surveyed on management methods. Our goal was to determine if slug feeding risk could be predicted by a single factor and or a combination of factors. Behavioral assays were performed with a common slug pest, Deroceras laeve, to determine if this species prefers feeding on maize, soybean, daikon radish, crimson clover, rye, or hairy vetch leaf tissue. Our sampling study found that cover crop use and conservation tillage type did not affect slug presence and damage, but that these factors affected various slug predators in different ways. We also observed that fields with more Phalangiidae and total predators overall had fewer slugs. Average slug feeding injury in both years was low and no factor or interaction of factors in our broader survey affected slug feeding injury ratings in fields. Behavioral assays indicated that slugs fed more on soybean tissue compared with maize, slugs consumed less maize when it was offered with hairy vetch or crimson clover, and slugs consumed less soybean when it was offered with hairy vetch or daikon radish. / Master of Science in Life Sciences / Invasive slug species the gray field slug, Deroceras reticulatum (Muller), and the marsh slug, Deroceras laeve, are major pests of maize and soybean grown in Virginia no-till systems. Slug feeding causes injury to young plants that, when severe, can reduce crop yield and necessitate replanting a field. Chemical control options are limited, and farmers may not be willing to use tillage as a control measure. Conserving arthropod slug predators may suppress pest populations below economic levels. Research is needed to understand how farming methods such as tillage type (e.g., no-till, strip tillage, vertical tillage) and cover crop use affect slugs and their predators. Our study investigated 1) how tillage type and cover crop use affected slug and slug predators, 2) if certain farming practices and field characteristics can be used to predict slug injury, and 3) if slugs preferred feeding on commonly-used cover crop species when offered with maize or soybean seedlings. Commercial production fields with different combinations of no tillage or reduced tillage, and planted with or without cover crops, were sampled to evaluate how these farming practices affect slugs and slug predators. Shingle traps and frames were used before planting and during early plant growth stages to collect and identify slug species; pitfall traps were used throughout the growing season to collect and identify slug predator species. To determine if slug feeding risk could be predicted, a survey of commercial production fields in the Shenandoah area was conducted and used to identify factors, if any, that influence slug feeding. Laboratory assays were used to determine feeding preference by offering slugs opportunity to feed on leaf tissue from a maize or soybean seedling or a mature cover crop species. We found that tillage type and use of cover crops did not affect the abundance of slugs, but that they did influence predator populations. The highest number of total predators were found in fields with reduced tillage, cover crops, and no insecticide use prior to crop emergence. Harvestmen were potentially the most impactful slug predator in the region. Field surveys suggested that no tested factor or interaction of factors affected slug injury to plants in commercial fields. Finally, we observed that slugs fed differently on soybean and maize tissue depending on cover crop species present.

Call number: LD2668 .T4 1985 J638 / Master of Science

A two year study was initiated in 2001 on a 2.4 ha site of mostly St. Amable loamy sand and shallow loamy sand at the Macdonald Campus Research Farm. This study sought to assess the effects of long term tillage and residue practices on soil physical properties and to relate these properties to their influence on total carbon, total nitrogen and soil microbial biomass-carbon. The site was set up as a factorial experiment with three tillage practices (no till---NT; reduced till---RT; and conventional till---CT) and two residue practices (with residue (grain corn), +R; without residue (silage corn), -R). Soil physical properties measured were bulk density, macroporosity at -6 kPa, saturated hydraulic conductivity (Ksat ), dry aggregate distribution, soil moisture and soil temperature. Wheel versus non-wheel track bulk density was also determined in the NT plots. Total porosity, soil water characteristic curves and pore volume distribution were derived from field samples. Crop yield was also assessed. Tillage alone had an effect on total porosity, bulk density, pore diameter and macroporosity. No individual physical property was significantly affected by the residue treatments; however, the tillage and residue interaction was significant for Ksat, soil moisture and soil temperature. Dry aggregate distribution was not affected by either tillage, residue or the combination of the two. Tillage affected total carbon and total nitrogen in the 0--0.10 m depth. Soil microbial biomass-carbon was affected by residue inputs. Total carbon and nitrogen were influenced by the bulk density, but none of the other soil physical properties. Soil microbial biomass-carbon was not influenced by any of the physical properties. Tillage and residue practices had no significant effect on crop yield for either year.

The impact of tillage and residue management on soil microorganisms was studied over the maize (Zea mays L.) growing season in southwestern Quebec. Tillage and residue treatments were imposed on a sandy loam to loamy sand soil in fall 1991. Treatments consisted of no till, reduced tillage, and conventional tillage with crop residues either removed from (-R) or retained on (+R) experimental plots, laid out in a randomized complete block design. Soil microbial biomass carbon (SMB-C), soil microbial nitrogen (SMB-N) and phospholipid fatty acid (PLFA) concentrations were measured four times, at two depths (0--10 cm, 10--20 cm), over the 2001 growing season. Sample periods were: May 7th (pre planting), June 25 th, July 16th, and September 29th (prior to corn harvest). The effect of time was of a greater magnitude than those attributed to tillage or residue treatments. While SMB-C showed no seasonal change (160 mug C g-1 soil); SMB-N was responsive to mineral nitrogen fertilizer; and PLFA data showed an increase in fungi and total PLFA throughout the season. PLFA profiles showed better distinction between sampling period, and depth, than treatments. Of the two treatments, the effect of residue was more pronounced than that of tillage, with increased SMB-C and SMB-N (6.1% and 96%) in +R plots compared to -R plots. This study illustrated that measuring soil quality based on soil microbial components must take into account seasonal changes in soil physical, chemical conditions, and nutrient supply.