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

MSCAGR (Soil Science) / Department of Soil Science / Soil resilience is the ability of a soil to recover its function or capacity after applied stress such as crusting. Some soils have high potential for recovery while others have poor resilience. Soils with poor resilience are much more vulnerable to degradation. Many soils in South Africa are susceptible to crust formation, which affects many soil surface properties and processes and hence productivity. The objectives of this study were to demonstrate how soil resilience to crust formation is affected by conventional tillage and conservation agriculture in selected soils in South Africa. Soil samples were collected from four different soils (Hutton, Shortland, Glenrosa and Dundee) using PVC pipes with the length of 20 cm and diameter of 5cm and scanned using micro xray computed tomography for total pores. Total porosity from Luvisols, Ferrosols, Leptsols and Fluvisols under both conventional tillage and conservation agriculture was used to find soil resilience index. Soil crusting was influenced by both soil texture and clay mineralogy. The dominance of kaolinitic mineral caused the soil to be more stable as compared to soil dominated by quartz. Luvisols, Ferrosols and Leptsols were more stable and had aggregate stability of 57%, 69,5% and 32,7%, respectively. On the other hand, Fluvisols had poor aggregate stability with the value of 14,2%. Total porosity was in the order of 34,3%>32,2%>23,5%>16,3% for Ferrosols, Luvisols, Leptsols and Fluvisols, respectively. Soil crusting influenced the total porosity. Tillage practices had influence on soil crust formation hence, total porosity of the soils. Total porosity was higher under conservation agriculture as compare to conventional tillage. Resilience total porosity was in the order of 37,5> 23,9> 4,1> -30,1 on Luvisols, Ferrosols, Leptsols and Fluvisols, respectively. Soil resilience to crust formation was influenced by tillage practices. Soil resilience of Luvisols, Ferrosols and Leptsols can be achieved through conservation agriculture however, soil resilience of Fluvisols can be achieved through conventional tillage. / NRF

The theoretical section of this thesis addresses corn characteristics and growing. Subsequently, it describes particular tillage technologies dividing them into conventional mouldboard ploughing technology, minimum tillage and introduces strip-till technology as a separate chapter. The major part describes the founding and evaluation of a 2016 field trial for study of different tillage technologies in terms of impact on silage corn growth quality, yield and physical characteristics of soil. The trial was based on five different technologies in two terms - strip tillage, ploughing and minimum tillage with two distinct tools at three depths. For trial evaluation, following parameters were observed: plant count, weight and height, dry matter content and yield and soil penetrometer resistance. The trial outcomes are commented and compared with trial results of other authors.

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To combat herbicide resistance among weeds, non-herbicide methods of control, such as cover crops, are becoming widely adapted. Experiments were conducted to determine how to effectively establish and manage cover crops in order to suppress tall waterhemp and Italian ryegrass and to assess their overall impact on soybean growth and yield. Various cover crop establishment methods were evaluated, and it was determined that interseeding at the R7 growth stage of soybean was the least effective method for proper cover crop establishment. Biomass data demonstrated that interseeding created the least amountof cover crop biomass, with no differences found among the other establishment methods that included drilling and sowing broadcast. At soybean planting timing, treatments with tillage had greater control of tall waterhemp than those without tillage. Wheat was shown to have the greatest weed suppressive capabilities, largely due to its ability to create high levels of residual biomass. Daikon radish produced the least biomass residue and had the poorest tall waterhemp control. The termination experiment of Elbon rye determined that treatments with rolling could impact soybean emergence and plant height largely due to dense biomass that lay flat onto the soil surface

Farmers have been disrupting the carbon cycle ever since humans started converting forests to agricultural lands. But are there farming practices that can be applied to increase the carbon storage in the soil and subsequently counteract increasing carbon dioxide levels in the atmosphere? In this study I investigate if soil organic matter (SOM) and soil organic carbon (SOC) change with longer interruption between tillage events. The study was conducted by studying SOM concentrations and SOC pools in eight fields with different time since tillage (1 to 14 years). I found that SOM concentrations increased in the O horizon of the studied soil in response to increased time since tillage. Here, SOM concentrations were on average around 13 % one year after tillage, while fourteen-year-old farmland had a concentration around 15 %. In similar, SOC pool increased from around 0.1 kg C m-2 in the O horizon of 1 year old soil to 0.33 kg C m-2 14 years after tillage. While both SOM concentrations and SOC pools increased in the O horizon over time since tillage, the SOM concentration and SOC pools decreased in the subsoil. I found no net sequestering of SOC in response to less frequent tillage in comparison to more frequency tillage. My conclusion is that limiting tillage to 14-year cycles is not enough to increase carbon sequestration.

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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