1,720,990 research outputs found
Recommended from our members
Improving Nitrogen Management in California Rice Systems: A Synergy Between Remote Sensing Technology and Fundamental Agronomy
Full text linkImproving nitrogen (N) management is critical to maximizing the productivity and sustainability of our agroecosystems. Improving N management requires an understanding of crop N status and yield potential early enough in the growing season when changes to N management can influence yields. However, given the lack of tools currently available to accurately assess crop N status, farmers continuously face the challenge of determining whether their crops require additional N fertilizer. The recent emergence of remote sensing technology has provided a promising alternative that can provide farmers the information they need in an accurate and timely manner. Several studies have demonstrated the potential of remote sensing technology to accurately assess the health and vigor of vegetation at the landscape scale, however few have explored how this technology can be utilized to inform sustainable crop management at the farm scale. This knowledge gap is what inspired this research and led us to investigate how remote sensing technology can be utilized to improve in-season N management in California rice systems.In California, where more than 200,000 ha of flooded rice (Oryza sativa) is cultivated annually, the recommended N management strategy is for farmers to apply the average seasonal N fertilizer requirement prior to flooding and planting as aqua-ammonia injected into the soil. On-farm studies have reported that N fertilizer applied in this manner is efficiently utilized by the crop as it remains protected from denitrification and ammonia volatilization losses until the crop needs it. At panicle initiation (PI), it is recommended to assess crop N status to determine if additional N fertilizer inputs are required as top-dress. The current tools available to assess rice N status include the SPAD chlorophyll meter and Leaf Color Chart, but these tools are not often used as they are time consuming and subjective. Thus, most top-dress N applications take place without evaluating crop N status; possibly resulting in inefficiencies due to over application.
Our goal was to improve N management in California rice systems by developing a sensor-based decision support tool that could guide California rice farmers in their mid-season top-dress N management. This was pursued through N response trials that were established over a 4-yr. period across fourteen on-farm locations throughout the Sacramento Valley rice growing region of California. At PI, Normalized Difference Vegetation Index (NDVI) was measured using both a proximal crop sensor and a multispectral aerial sensor, and Normalized Difference Red-Edge Index (NDRE) was measured only using an aerial sensor. After NDVI and NDRE measurements, biomass was sampled destructively and then top-dress N fertilizer was applied. At maturity, rice plants were harvested to quantify grain yield.
In the first chapter, our objective was to determine which N status parameter is best assessed by NDVI at PI and how accurately NDVI at PI can predict grain yield. The N status parameters quantified in this study were aboveground biomass, plant N concentration, and total N uptake. Quadratic linear regression models were developed to describe the relationship between each N status parameter and NDVI, and a simple linear regression model was developed to describe the relationship between grain yield and NDVI. Our results showed that PI N status was best assessed by NDVI when quantified as total N uptake and that NDVI at PI was positively correlated with grain yield. However, our results also showed that NDVI saturated once crop N uptake exceeded a certain threshold, suggesting alternative indices that do not saturate may provide a basis for a better assessment.
In the second chapter, our objective was to compare the sensitivity of aerially sensed NDVI and NDRE to proximally sensed NDVI for assessing rice crop status when quantified as PI N uptake and grain yield. In order to make direct comparisons across the three indices, the raw values from each index were normalized by calculating the Sufficiency-Index (SI). Quadratic-plateau linear regression models were developed to describe the relationship between each SI and PI N uptake and linear mixed effects models were developed to describe the relationship between each SI and grain yield. Our results showed that aerial NDRE SI and proximal NDVI SI were similarly sensitive at assessing PI N uptake and grain yield, whereas aerial NDVI SI was poorly sensitive. The difference in sensitivity among the three indices was attributed to the relative amount of saturation of each index. Our finding that both the aerial NDRE SI and proximal NDVI SI measured PI rice crop status effectively provides a unique advantage for end-users as it allows them the flexibility to choose the sensor most suitable for their goals.
In the final chapter, our objective was to develop a NDVI Response-Index capable of predicting the grain yield response to top-dress N fertilizer applied at PI. The NDVI Response-Index was developed by comparing the NDVI of each field treatment to the NDVI of a N non-limiting plot. At PI, top-dress N fertilizer was applied to every plot, and at maturity grain yield was quantified. A linear mixed effects model was developed to describe the relationship between NDVI Response-Index and grain yield with and without top-dress N. An economic analysis was performed to determine the magnitude of grain yield response required for top-dress N applications to be economically feasible. Based on our results, we found that top-dress N applications become profitable once NDVI Response-Index exceeds 1.07 by PI. The NDVI Response-Index presented here provides a useful tool for farmers to make precise mid-season top-dress N decisions which can result in positive outcomes for both crop productivity and environmental sustainability
Recommended from our members
Assessing Fertilizer Nitrogen Sources and Application Timing for Water-Seeded Rice Systems
Full text linkIn California’s water-seeded rice systems, both high yields and nitrogen (N) use efficiency are achieved when the pre-plant N fertilizer (usually aqueous-ammonia; aqua-N) is injected into a dry seedbed before flooding and planting. However, there are situations where applying N fertilizer in this manner is not possible. The objective of this study was to evaluate N management practices by testing different N sources and application times. Over two growing seasons (three site-years) we evaluated different N sources including aqua-N, granular urea, ammonium sulfate, and three enhanced efficiency nitrogen fertilizers (EENFs). Application times for the EENFs, urea, and ammonium sulfate were one day after flooding and 2 weeks after flooding. Urea and ammonium sulfate were also applied in a 4-way split (ratio of 20:30:30:20 applied every two weeks). In 2021, additional treatments were added with aqua-N, ammonium sulfate, and urea applied before flooding; and urea applications at 3, 4, and 5 weeks after flooding. Grain yield and agronomic nitrogen use efficiency (ANUE) were measured and used to make comparisons. All fertilizer N treatments increased grain yields and ANUE relative to the zero-N control. The EENF treatments performed similarly to or worse than urea applied alone. Splitting urea applications was the best option for applying N fertilizer after the field was flooded. Only one split N treatment was evaluated in this study; this warrants further research on fine-tuning the best N splits for these systems
Dry-Seeding Rice Increases N Losses but Reduces Global Warming Potential Compared to Water-Seeding
Full text linkFlooded rice systems are critical for global food security but contribute significantly to anthropogenic greenhouse gas (GHG) emissions due to high methane (CH4) production in anerobic soils. Herbicide use in conventional rice systems has also created selection pressure for herbicide resistant aquatic weed species that threaten yields. Dry seeding (DS) rice, which in California includes early season drainage events, has been shown to reduce CH4 emissions and shift weed species emergence for improved control compared to continuously flooded water-seeded systems (WS). The effect of these drainage events on nitrogen (N) fertilizer losses and nitrous oxide (N2O) emissions, however, are not well understood. In a two-year study we quantified the effects of early season drainage events utilized in DS rice on global warming potential (GWP) (CH4 + N2O in CO2 eq.), nitrate (NO3-) accumulation, and N fertilizer losses as measured by the difference in crop N-uptake compared to a WS control. Despite 1.06 kg ha-1 more N2O emissions in the DS system the GWP was 4,610 CO2 eq. kg ha-1, a 42% reduction compared to 7,983 CO2 eq. kg ha-1 in the WS system. This was due to a 46% reduction in CH4 in the DS (126 CH4 kg ha-1) relative to the WS (235 CH4 kg ha-1) system. Nitrate accumulation in the DS system amounted to 25.9 kg N ha-1, and subsequent N losses via denitrification likely contributed to the 22.4 kg N ha-1 less crop N-uptake in the DS system. These results suggest that DS rice has potential for improved environmental impact via GWP reductions. Future research should consider the effects of increased pre-plant N application rates and timing for improved N management, a quantification of annual GWP including CO2 emissions, and changes in soil organic carbon stocks in DS rice
Recommended from our members
Effect of a year-long fallow on carbon-nitrogen cycling in California continuous rice systems and perspectives on net system greenhouse gas emissions accounting
Full text linkRice (Oryza sativa L.) is integral in meeting global food and nutritional demand. However, its production is becoming increasingly challenging due to climate change and its associated impacts. In this dissertation, we explore the carbon (C) and nitrogen (N) cycling dynamics in rice systems in the context of climate change. Chapter one examined opportunities for greenhouse gas (GHG) emissions mitigation and accounting in Southeast Asia (SEA) through a systematic review. Chapters two to four examined the effect of a year-long fallow on soil N availability, GHG emissions, and agronomic productivity when introduced to continuous rice systems in California. In chapter one, the review synthesized findings across four main components of net system emissions: (1) field GHG emissions, (2) energy inputs, (3) residue utilization beyond the field, and (4) SOC change. Integrating all four components there were two main takeaways. First, the components of field GHG emissions and SOC change were the biggest opportunities for reducing net system emissions and need to be considered for effective climate change mitigation. Second, the reduction of C inputs through residue removal and increased soil aeration through multiple drainage will lower methane (CH4) emissions but may also decrease SOC stocks over time. Hence, we argue that future research needs to consider cross-component effects to optimize net system emissions, specifically the “stacking” of best management practices for mitigation related to field GHG emissions or SOC change in long-term experiments.
In chapters two to four, we examined the effect of fallow interjection on continuous rice systems in California. Due to winter droughts leading to water restrictions or spring rains leading to prevented planting, growers have been forced to fallow their lands. As such, the norm of continuous rice cropping is challenged and the year-long fallow will have important effects on soil N fertility, GHG emissions, and agronomic productivity. There were two main treatments – continuous rice (CR) and fallow rice (FR; rice following a year-long fallow). In chapter two, we evaluated crop uptake of soil N (N uptakesoil) and fertilizer N (N uptakefertilizer) using 15N-enriched ammonium sulfate. Examining the sources of crop N uptake, N uptakesoil in the FR treatment was 16.7 kg N ha-1 higher than the CR treatment at maturity but N uptakefertilizer was similar. These results indicate that N uptakesoil was primarily responsible for lower N uptake in CR. Soil phenols, which have been documented to accumulate in continuously flooded rice systems and stabilize soil N, were greater in CR than FR in both the research station study and the regional survey study. Together, higher phenol levels and lower N uptakesoil in CR provide mechanistic evidence that the introduction of a season-long fallow to continuous rice systems enhances soil N availability by reducing organic substrate recalcitrance.
In chapter three, we quantified GHG emissions (CH4 and N2O) of the systems, including the year-long fallow (F). In the summer, FR had a 45-53% reduction in cumulative CH4 emissions compared to FR in two of three years. F had no CH4 emissions in the summer. Summer N2O emissions were low for all three treatments. Summer global warming potential (GWP) accounted for more than 96% of annual GWP in CR (13,937 kg CO2 eq ha-1) and FR (9,236 kg CO2 eq ha-1). For F, the winter season accounted for 94% of the annual GWP (413 kg CO2 eq ha-1) due to N2O emissions. Additionally, we showed that particulate organic carbon (POC) and mineral-associated organic carbon (MAOC) levels in CR and FR were similar in the year with no treatment effect on CH4¬ emissions.
In the final chapter, we evaluated the agronomic productivity of the two systems. Maximum observed yields did not differ between CR and FR, averaging 14.0 Mg ha-1 in 2021, 12.6 Mg ha-1 in 2022, and 9.6 Mg ha-1 in 2023. Based on quadratic regressions of yield response to N, the agronomic optimum N rate (AONR) was higher for CR in all years. Where no fertilizer N was applied, FR yielded higher than CR, averaging a difference of 2.9 Mg ha-1. The yield differences at 0 kg N ha-1 can be attributed to soil N availability, where FR averaged 31.6 kg N ha-1 more soil N uptake than CR at maturity. Apparent fertilizer nitrogen recovery efficiency (FNRE) did not differ between treatments and averaged 59.8%. Stem rot, caused by Sclerotium oryzae, was more severe in CR than in FR, having averaged severity indexes of 3.7 and 3.1 respectively. Based on differences in soil N uptake and FNRE, the N rate can be reduced by roughly 50 kg N ha-1 for fields following a fallow compared to continuously cropped fields, allowing growers to maintain yields with lower inputs
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
- …
