Mountain Scholar :: Browsing by Author "Conant, Richard, committee member"

Browsing by Author "Conant, Richard, committee member"

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Open Access
Crop residue: a hero's journey from biomass to soil carbon in eastern Colorado dryland crop rotation systems
(Colorado State University. Libraries, 2019) Schnarr, Cassandra, author; Schipanski, Meagan, advisor; Ham, Jay, committee member; Conant, Richard, committee member; Tatarko, John, committee member
Crop residues play a vital role in reducing the potential for wind erosion of agricultural soils in arid and semi-arid regions. The residues act via three modes: reducing wind speed, acting as a physical impediment to wind reaching the soil surface, and as an organic matter input to spur aggregation and aggregate stability. The interactions of crop residues, crop rotation systems, and wind erosion factors were studied at three long-term agricultural research sites along an evapotranspiration gradient near Sterling, Stratton, and Walsh, Colorado. The sites have a 30-year history of dryland, no-till management, and are divided into different cropping system intensities that vary in the frequency of summer fallow periods in the rotation. Crop rotations studied here include wheat (Triticum aestivum)-fallow, wheat-corn (Zea mays) – fallow, and continuously cropped plots with small grains and forage crops including foxtail millet (Setaria varidis) and forage sorghum (Sorghum bicolor). Forage crop and wheat residues were tracked over two growing seasons (2015 and 2016) to estimate the length of time before soil surface cover fell below a 30% threshold and to create models for residue persistence. Decomposition Days (DD), a calculation that factors in temperature and rainfall to estimate cumulative conditions that favor decomposition, was used to normalize time scales following harvest across sites and years. Wheat residue covered 82% of the soil surface following harvest and summer forage crops covered 56%. Wheat persisted longer, taking 62.5 DD to fall to the 30% cover threshold, forage crop residue remained above the threshold for 16.6 DD. The decline of forage crop residue cover followed an exponential decay model. Wheat residue surface cover had a longer, slower decline and fit a quadratic decay model. Wheat stem heights were taller following harvest and heights declined at a similar or faster rate than forage crops. To assess rotation legacy impacts on soil erodibility, soils were sampled in May 2015 and tested for dry aggregate size distribution, dry aggregate stability, and carbon distribution by size classes and between cropping intensities. No differences were found in the amount of erodible aggregate size fraction (<0.84mm) by cropping system intensity. The site with the highest amount of clay in the soil displayed a significant difference in aggregate stability by crop rotation, with wheat-fallow rotations having stability of 2.96 ln J/Kg and continuously cropped systems having 2.80 ln J/Kg. Carbon distribution did not differ by crop rotation but did differ by size class at the site with the highest potential evapotranspiration and lowest clay content where the largest aggregates contained the highest proportion of carbon. Every phase (i.e., rotation year) of each of the crop rotation systems were represented each year. There was a significant difference in mean erodible fraction and aggregate stability by cropping phase at the time of sampling at the site with the highest clay content. Taken together, the crop residue and soil aggregate portions of the study indicate that the reliable and consistent prevention of wind erosion by crop system intensity may be more dependent upon annual crop residue surface cover than longer-term management impacts on soil aggregation properties. The differences in aggregate stability by crop type could be due to the impacts of active root systems at the time of sampling. More investigation is warranted into the influence of active root systems on macro dry aggregates and whether dry aggregate stability properties differ by season. Further study into the application of residue biomass decay models to residue soil cover, particularly in crops with multiple layers of residue is also indicated.
Open Access
Modular modeling and its applications in studies of grazing effects
(Colorado State University. Libraries, 2016) Miao, Zhongqi, author; Boone, Randall, advisor; Conant, Richard, committee member; Ocheltree, Troy, committee member
Grazing is an important ecosystem process that can affect the grazing system at different levels. Overall grazing effect can be a combination of various direct and indirect effects. It is difficult to study grazing with all of the effects considered. To have a better knowledge of grazing effects and animal-plant interactions, modeling is one important pathway to achieve this goal. People usually use a diversity of approaches when modeling grazing based on different objectives, which makes model evaluations and comparisons difficult. With modular modeling, where different model components are regarded as separate and standardized modules, this situation can be changed. An example model is developed using a modular approach. It included most of the grazing effects and switches that can turn these effects on and off. This model was designed to be capable for applications with different hypothesis and objectives. It is expected to be clearer for people who are not familiar to models to make comparisons and evaluations of grazing effects. To test the feasibility of the model, a theoretical experiment on compensatory behavior in grassland production and a realistic simulation on plant-animal interactions in Qinghai-Tibetan plateau, China, are conducted. The results of these two applications demonstrate the benefits of using modular modeling in studies of grazing effects.
Open Access
Residue-derived carbon transformations with altered residue management in an irrigated corn system in Colorado
(Colorado State University. Libraries, 2019) Leichty, Sarah I., author; Cotrufo, M. Francesca, advisor; Stewart, Catherine, advisor; Conant, Richard, committee member
Soil plays an essential role in storing carbon (C) and nutrients regulating climate and sustaining food production. As such, it is important to understand how human activities affect C cycling in soil. Agricultural lands, particularly croplands, comprise some of the most managed areas of the world. We have the potential to positively impact C sequestration in croplands by understanding how residue management affects the C mineralization as well as the formation and stabilization of soil organic C (SOC). No-till (NT), which leaves residue on the surface, has been proposed as the superior residue management for building SOC due to a decreased rate of residue and existing SOC decomposition. Conventional tillage (CT), which breaks aggregates while incorporating residue into the soil, is thought to result in less SOC storage through increased residue and SOC decomposition, and soil erosion. However, NT does not always result in higher SOC storage than CT when measuring total profile stocks and the gains might not be stable in a changing climate. I studied a semi-arid, irrigated, corn system where gains in surface SOC but losses at depth had been observed because of conversion to NT. I studied the effect of altered residue management by mimicking NT and CT residue placement and soil disturbance and followed the residue-derived C into soil respiration (CO2) and physically defined SOC pools. I applied isotopically labeled (13C) residue either on the soil surface (NT) or incorporated into the soil (CT), which allowed me to track losses and transformations of residue C under different residue placement. A third treatment with surface-applied residue following soil disturbance tested the effect of disturbing aggregates during tillage. I collected soil respiration measurements and determined its isotopic composition for twelve months following residue addition to understand how residue location and soil disturbance affected the rate of total CO2 efflux, and of their component fractions (i.e., derived from residue vs. derived from soil). As reported in Chapter 2, I found that incorporated residue had higher residue-derived respiration in the first six months ("off season"), but surface-applied residue had higher residue-derived respiration during the later six months ("growing season"). Growing season respiration rates were higher and driven largely by warmer temperature and more available water (i.e. irrigation events) than in the off season. Thus, the growing season trend dictated the full year trend with surface residue more vulnerable to loss as CO2 efflux compared to incorporated residue that was likely protected from decomposition by the soil matrix presumably from either aggregation or mineral associations. Following, I tracked the fate of the remaining residue C by measuring SOC pools with varying levels of stability and report these results in Chapter 3. I harvested incorporated and surface-applied residue treatments six months after residue addition. I found that incorporated residue had more residue-derived C in bulk soil compared to surface-applied residue treatments. The larger pool of residue SOC within the incorporated residue treatment was due to increased residue decomposition compared to slower surface residue decomposition. Based on the two pathways of SOC formation proposed in Cotrufo et al., (2015a), residue C was tracked into both minerally-associated and particulate organic matter (MAOM and POM respectively). We hypothesized more MAOM formation due to the importance of the DOC-microbial pathway at the initial stages of decomposition with a smaller pool of POM due to the physical transfer path becoming important later in decomposition. After six months of residue decomposition, residue C was greater in POM than MAOM fractions for all treatments (either marginally or significantly), indicating the importance of POM formation pathways in both surface-applied and incorporated residue treatments. After accounting for differences in decomposition, I found that incorporated and surface-applied SOC formation efficiencies of residue decomposition (i.e., the fraction of residue C decomposed that resulted in SOC versus being lost through mineralization) were similar when SOC pools were summed. However, the SOC formation efficiency of surface-applied residue with soil disturbance was 40% lower than either surface-applied or incorporated residue, indicating that soil disturbance inhibited SOC formation when residue was surface applied. It is surprising that surface-applied residue POM formation was high in the short term because surface residue had to enter the soil matrix over time unlike incorporated residue. The high amount of POM likely formed through biotic transformation by soil fauna in contrast to POM protection likely through aggregation in the incorporated residue treatment. Thus, it appears that POM formation was more important than MAOM formation for both surface-applied and incorporated residue management. My results show that intact NT systems, particularly with earthworms and fungal networks may efficiently transfer POM C belowground during the initial six months of decomposition. At 12 months, I predict that protection of incorporated residue will be more pronounced compared to surface residue, since surface residue appears to be vulnerable to decomposition after irrigation events. Longer-term results (~3 years) will elucidate whether CT residue management continues to store more residue SOC than NT, potentially explaining why irrigated, NT systems might not consistently store more SOC. Climate change will likely increase the use of irrigation to meet crop water needs, thus we need to continue exploring the complexities of SOC formation and stabilization within irrigated NT and CT in order to sequester C for the health of our soil and planet.
Open Access
Sensitivity of microbial community physiology to soil moisture and temperature in an old field ecosystem
(Colorado State University. Libraries, 2011) Steinweg, Jessica Megan, author; Wallenstein, Matthew, advisor; Conant, Richard, committee member; Paul, Eldor, committee member; Stromberger, Mary, committee member
To view the abstract, please see the full text of the document.
Open Access
Soil heterogeneity in agricultural and natural ecosystems: relationships between anaerobic activity, organic matter, nutrients, and greenhouse gases
(Colorado State University. Libraries, 2017) Brewer, Paul E., author; von Fischer, Joseph, advisor; Calderón, Francisco, committee member; Conant, Richard, committee member; Cotrufo, M. Francesca, committee member; Wallenstein, Matthew, committee member
Many soil biogeochemical processes are difficult to predict, in part, due to the spatial heterogeneity of physical, chemical, and biological components of soil. Understanding how heterogeneity forms and affects biogeochemical processes is important because of the ultimate impacts on nutrient availability, carbon storage, and climate change. Oxygen and soil organic matter are two key components of soil microbial habitat, so I performed research to determine how the heterogeneity of each affect ecosystem functions. Oxygen can be absent in soil aggregates, litter patches, rhizospheres, and the guts of soil fauna, and when this occurs in unsaturated soils with oxic pore air these areas are referred to as anoxic microsites. The formation, persistence and impact of anoxic microsites are poorly characterized because these microsites are difficult to measure, especially across large areas that define ecosystem level processes. I studied what factors cause them to form and persist and how they affect C and N cycling and GHG fluxes. I performed focused, mechanistic laboratory studies of natural and agricultural soils, as well as field-scale studies of anoxic microsite effects in agricultural systems. In multiple studies, I circumvented the limitations and problems related to measuring soil oxygen or reduction-oxidation (redox) potentials at sub-millimeter scales instead by using gross CH4 production as an indicator of anoxic microsite presence and activity. I used two relatively recent methodological approaches to make gross CH4 measurements, CH4 stable isotope pool dilution for laboratory measurements and a CH4 process and transport model for field studies. I found that methanogenesis correlated with respiration, soil moisture, plant presence, and agricultural practice both in laboratory and field studies, indicating that the distribution of anoxic microsites is altered by climatic and land use factors in ways that are similar to the large-scale anoxic zones of wetlands. Methanogenesis was associated with elevated NH4+ concentrations and N2O flux, but lower NO3- concentrations. These relationships are consistent with slower nitrification and greater denitrification, so measurements of methanogenesis may be a useful proxy for other anaerobic processes. I also found evidence that consistent upland methanogenesis may stimulate methanotrophy (i.e., gross CH4 consumption) over the course of years, counterintuitively leading to an increase in net CH4 uptake. Finally, redox potential was not as strong an indicator of methanogenesis as expected, so I join others in concluding that redox potential may not be a desirable method for quantifying anoxic microsites. I also studied the effects of the spatial distribution of soil organic matter in the form of litter patches in soil. In a laboratory incubation, I manipulated the size and number of litter patches and soil moisture in a uniform mineral soil matrix. I found that dry soils with litter that was aggregated into larger patches exhibited greater rates of decomposition and nutrient availability, but that in wetter soils there were few effects of litter distribution. This complements my studies of anoxic microsites by showing that not only the presence of soil microsites, but variation in their size and distribution can also alter soil processes. In summary, my dissertation research concentrated on the causes and biogeochemical consequences of anoxic microsites and heterogeneity of organic matter in agricultural and natural ecosystems. My findings have increased our understanding of soil heterogeneity and the potential for it to cause significant changes in nutrient availability, decomposition, and greenhouse gas fluxes.
Open Access
The effects of soil structure on soil organic matter: a mechanistic approach
(Colorado State University. Libraries, 2022) Even, Rebecca, author; Cotrufo, M. Francesca, advisor; Conant, Richard, committee member; Paustian, Keith, committee member
Two key factors theorized to affect soil organic carbon (SOC) dynamics are type of plant carbon (C) inputs and soil structure (i.e., soil aggregation), both are influenced by management practices and are considerably intertwined. Research surrounding these factors has increased in the last several decades as the threat of climate change has forced policy makers to find natural based solutions to rising CO2 levels in Earth's atmosphere. Given that soil acts as the largest terrestrial C pool but has lost substantial amounts of C due to land use change and unsustainable agriculture, focus has shifted towards identifying better ways to manage arable lands that improve SOC storage. Among the conventional management practices tillage is likely the most studied, because of its damage to soil structure, leading to soil C losses. However, while research centered on tillage effects on soil aggregation and SOC cycling is vast, few studies explore how plant C input type (i.e., soluble versus structural) and disturbance (i.e., tilling) together affects SOC in soils with different degrees of aggregation. We examined the effects of soil texture, disturbance, and plant input type on soil aggregation, C mineralization, and formation and persistence of plant input-derived SOC to better understand the mechanisms by which soil aggregates help form and protect SOC, specifically as particulate and mineral associated organic carbon (POC and MAOC). POC and MAOC are expected to be formed by distinct pathways, respectively from structural and soluble inputs. Because of their different mechanisms of protection, POC and MAOC are also expected to respond differently to plant inputs and management practices, like tilling, that disturb soil aggregates. We aimed to parse the formation and persistence of POC and MAOC by adding 13C labeled plant residue separated into soluble and structural plant constituents to determine how these physically distinct plant compounds contribute to either pool when soil is intact or disturbed. In an in-lab incubation using 13C enrichment, we traced SOC over the course of one year in a factorial design with four factors: soil type*disturbance*plant input*harvest. Our results showed, as expected, that hot-water extractable (HWE) plant inputs contributed substantially to MAOC while structural plant components (SPC) inputs preferentially formed POC. Interestingly, we found that disturbance resulted in less HWE mineralized to CO2 and more MAOC formation in the highly aggregated (HA) soil suggesting that increased mineral surface area caused more efficient dissolved OM sorption. Moreover, HWE-derived MAOC persisted in both the undisturbed (U) and disturbed (D) HA soils but not in low aggregation (LA) soils, indicating that persistence of MAOC is dependent on soil type and aggregation (i.e., soil physical structure). Although we did not observe significant differences in aggregate-occluded POC (oPOC) formation between HAD and LAD soils, we did see higher oPOC persistence in HAD soil compared to LAD soil. Greater accumulation oPOC in HAD from day 22 to the end of the incubation suggests, again, that soil type influences the persistence of POC through occlusion in aggregates. To corroborate this, we also found that LAD soil had the highest CO2 mineralization of SPC plant inputs as SPC was left more unprotected in the soil with a low capacity to aggregate. Disturbance did not affect microbial biomass in either HA or LA soils. We saw more plant-derived microbial biomass C from HWE inputs compared to SPC inputs in the bulk soil, indicating that HWE inputs are assimilated into microbial biomass, thus incorporated into SOC with higher efficiency. Lastly, there was a significant drop in % plant-derived microbial biomass C in the bulk soil overtime, as expected. However, because the % HWE-derived MAOC persisted in HA soils regardless of disturbance, we illustrated the importance of microbial necromass in addition to direct DOC sorption for SOC stabilization as MAOC. Overall, my study provides mechanistic understanding for the role of soil structure and aggregation on POC and MAOC formation and persistence which can help improve the representation of these processes in models, to provide better predictions of SOC changes with changes in management practices affecting disturbance.
Open Access
The role of organic matter chemistry in iron redox transformations, sorption to iron oxides, and wetland carbon storage
(Colorado State University. Libraries, 2018) Daugherty, Ellen E., author; Borch, Thomas, advisor; Barisas, George, committee member; Conant, Richard, committee member; Neilson, James, committee member
Organic carbon comprises a versatile and complex class of compounds that influence water quality, soil health, fate and transport of environmental contaminants, biogeochemical cycles, and climate change. Key to predicting the responses of these systems and processes to environmental change is a molecular-level understanding of how organic carbon reacts with other components of soil and water. Yet due to its complexity and that of the systems in which it is found, organic carbon dynamics remain poorly understood. In both terrestrial and aquatic environments, the reactivity and biological necessity of iron and carbon link the biogeochemical cycling of these elements. Complexation of iron by dissolved organic carbon molecules alters its solubility and oxidation-reduction behavior and may explain the persistence of reduced iron (Fe(II)) in oxic aquatic environments. By examining the coordination environment of Fe(II) complexed by dissolved organic matter (DOM) and evaluating the effects of complexation on Fe(II) oxidation, I determined that the majority of Fe(II)–DOM complexes were characterized by coordination with citrate-like ligands, which were unlikely to inhibit oxidation by molecular oxygen. Nonetheless, association with reduced organic matter could extend the lifetime of Fe(II) in oxic environments by several hours. In soils and sediments, iron minerals act as effective sorbents of organic matter, preserving substantial amounts of carbon from microbial decomposition. These interactions have increasingly been recognized as important components of carbon sequestration, yet the effects of temperature on sorption behavior remain unknown. Through several batch and continuous flow experiments, I demonstrated a positive relationship between temperature and sorption of DOM on iron oxide surfaces. The temperature sensitivity of sorption behavior varied among riverine, peat, and soil DOM types, with riverine natural organic matter sorbing and desorbing the most at all temperatures. Analyses of effluents also revealed preferential sorption of aromatic compounds during the initial stages of sorption. In soils, organic matter quantity and composition are determined primarily by the balance between plant productivity and microbial decomposition, which are in turn dependent upon climate, temperature, hydrology, nutrient availability, and soil composition. Wetlands store disproportionately large amounts of carbon, yet the processes controlling storage are poorly understood. I investigated how different environments created by the hydrology and geomorphic setting of two wetland types, depressional and slope, impacted soil organic carbon storage and composition. Results showed a prevalence of aliphatic structures in depressional wetlands, especially in deeper soils, suggestive of anaerobic decomposition processes. By comparison, carbon in slope wetlands was dominated by labile plant carbohydrates in surface soils and aromatic compounds at depth, a likely indication of less anaerobic conditions. These results demonstrate divergent pathways of organic matter processing in different hydrogeomorphic environments. In total, this work contributes to more mechanistic understandings of important carbon dynamics that influence carbon and iron cycling, climate change, and environmental health.
Embargo
Tropical forest root characteristics and responses to drying across environmental gradients
(Colorado State University. Libraries, 2024) Longhi Cordeiro, Amanda, author; Cusack, Daniela F., advisor; Ojima, Dennis, committee member; Cotrufo, M. Francesca, committee member; Conant, Richard, committee member
Fine roots represent the interface between plants and soils, and as such regulate all major biogeochemical cycles in terrestrial ecosystems, including tropical forests. Tropical forests play a crucial role in global carbon (C) cycling, largely due to their extensive root biomass and significant soil C stocks. However, these ecosystems have been experiencing more frequent severe droughts across some regions and are predicted to continue experiencing these extreme drought events in the future. This dissertation seeks to contribute to the understanding and synthesis of tropical root responses to drying in varying environmental conditions. In chapter 1, I gave an introduction about the importance of fine roots to ecosystem function and the impacts of drying in tropical forests. In chapter 2, I characterized root biomass, morphology, nutrient content, colonization to 1.2 meters depth as well as and arbuscular mycorrhizal fungal (AMF) to 20 cm depth in 32 plots across four distinct lowland Panamanian forests which are representative of the vast variation in soil fertility and mean annual precipitation (MAP) found across tropical forests. Root characteristics measurements, such as morphology and chemistry, at soil layers deeper than 30 cm have been rarely documented and to the best of knowledge this is the first study in tropical forests. I observed that that some root traits changed with soil depth similarly across sites while others had site-specific variation. I also observed larger variation at the soil surface and that morphological traits, in addition to root biomass can affect soil C stocks. In chapter 3, the effects of experimental and seasonal drying on fine root dynamics were explored using a partial throughfall reduction experiment across the same 32 plots as in chapter 2. I found that chronic drying impacted root biomass, productivity, morphology and arbuscular mycorrhizal fungi (AMF) colonization. Root biomass and characteristics also changed across seasons with different dynamics across depths. Chapter 4 focused on the effects of drought on tropical seedling development in a controlled chamber environment. I observed that drying decreased seedling growth, but high soil fertility and AMF inoculation mitigated these effects. I also observed changes in root morphology, leached C, new C allocation patterns, and aboveground traits in response to drought, but with usually interacting effects with fertility and AMF inoculation. Chapter 5 contributes a tropical root database (TropiRoot 1.0 database) with root data extracted from scientific papers across different countries and continents. Overall, this dissertation provides novel results and insights into the variation in root characteristics among tropical forests and their responses to climatic drying with interacting effects of fertility, symbionts and soil depth effects. It brings novel measurements that have never been published in tropical forest studies. In chapter 2, I found novel results about how different tropical forests had similar patterns of root variation with depth. It indicated differences in resources acquisition at the soil surface (likely for nutrients) and at deeper soil layers (likely for water) that are usually less investigated. I also showed a large variation of roots at surface soil across different forests that may influence forest responses to global change factors. In chapter 3, I supported some results across the literature such as drying decreasing root growth at the soil surface. However, I added new results such as drying decreasing root productivity at deeper soil layers, and changing root morphology and associations with symbionts probably to compensate the lower root growth. All together I observed that drying promoted changes in acquisition strategies and also that fertile forests may respond differently to drying. In chapter 4, I showed some clear tradeoffs in plant traits providing evidence that they are constantly changing in response to the environment. Also, I provided some novel results on the mechanisms, such as nutrient retention, on how mycorrhizal and fertility mitigated some negative effects of drying on plant growth. This aligns with the field study showing some possible resilience in the fertile forests to drying. The findings highlight the complex interactions between root traits and environmental conditions, offering important implications for predicting tropical forest responses to changing moisture and nutrient availability. All these chapters together provided a good understanding on how different forests respond to environmental changes. These impacts on soil C storage, links with root function and possible larger vulnerability of some forests are great topics for future studies.

Browsing by Author "Conant, Richard, committee member"

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