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Sustainable Agronomic Approaches for Reclaimed Wastewater Utilization: A Focus on Phosphorus Removal, Plant Tissue and Soil Nutrients Retention and Leachate Quality using Biochar and Nitrogen Application Rates under Different Forage Systems
Environmental and Natural Resource Sciences
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Biochar, a compound produced from the pyrolysis of agricultural residue or any other carbon-rich materials, has been investigated for its potential for environmental management especially for climate mitigation stemming from its ability to reduce greenhouse gas emission and increase carbon sequestration in general. Recent discussions have highlighted the potential benefits of utilizing biochar as a soil amendment to improve soil nutrient dynamics, enhance crop production, and contribute to sustainable agriculture. Biochar as a soil amendment has beneficial characteristics for improving soil water holding capacity, soil pH, cation exchange capacity (CEC), carbon-nitrogen (C:N) ratio, and soil bulk density that can mitigate these limitations for agricultural production in arid and semiarid environments. Improvement of crop productivity has been linked to improved nutrient uptake and efficiency as a result of the biochar application. There have been several reviews that have linked such beneficial responses of biochar application to its absorbent properties. This sorption ability of biochar has been demonstrated in different applications, from mitigation of environmental degradation (e.g., water quality maintenance) to agronomic applications such as the retention and recycling of the nutrients in cropping systems particularly phosphorus (P) and nitrogen (N).Frequent use of treated wastewater as an alternate source for agricultural crop irrigation in regions characterized by water scarcity is common. However, while such practice offers important environmental and economic advantages mainly through a continuous and stable supply of water and valuable plant nutrients, excessive levels of soil nutrients, particularly P can also occur, especially over prolonged use resulting in environmental repercussions such as deterioration of water bodies and supplies. Phosphorus-enriched soils are a fundamental problem for the land-based agricultural industry because of P loss through leaching they continue to pose a significant risk to water quality. Reduction of, or removal of P from P-enriched soils has seen little success because of unabated and unintended agronomic practices by farmers or through the continued application of P-rich manures or inorganic P fertilization beyond crop needs. One technology that has been widely utilized is phytoextraction which uses plants to remove, degrade or sequester contaminants or nutrients in soils. It is a suggested solution because when implemented it is cost-effective and environmentally safe. Forages aside from providing feed for ruminants may provide new solutions for environmental management as they possess the ability to accumulate high levels of nutrients in their biomass.Three studies were conducted to evaluate sustainable agronomic approaches for reclaimed wastewater utilization using biochar and nitrogen application rates under different forage systems. The research was conducted at the University of Nevada, Reno Main Experiment Station, Reno, Nevada over a three-year period from 2017 to 2019.The first study evaluated the effects of biochar and nitrogen application rates on herbage accumulation and nutrient removal from a tall fescue-bluegrass mixture. Treatments were three biochar application rates (0, 8.9, 17.8 Mg ha-1) and three N rates (0, 80, 120 kg N ha-1) arranged in a 3 × 3 factorial with four replications each in a randomized complete block design experiment. Data collected were soil moisture, forage biomass, initial and end of season soil samples. Biomass samples were analyzed for plant tissue nutrient concentration and nutrient removal computed based on biomass from each treatment. Responses were considered different at P < 0.05. Averaged across both seasons, soil moisture content was greatest at biochar rate of 17.8 Mg ha-1 (0.282 cm3 cm-3) followed by 8.9 Mg ha-1 (0.242 cm3 cm-3) and the least being the control (0.174 cm3 cm-3). However, biochar application rate did not affect biomass yield, forage tissue P and N concentrations, P and N removal, or interact with the other experimental variables of N rate and year to influence the response variables. There was, however, a linear increase in biomass yield (5.9 to 8.8 Mg DM ha-1), and cumulative P (38.1 to 50.9 kg ha-1) and N (187.8 to 321.1 kg ha-1) removal as N application rate increased from 0 to 120 kg N ha-1. There was a trend for a biochar × N rate interaction on soil P concentration and it tended to be greater for the combinations 8.9 and 17.8 Mg ha-1 biochar rates and 80 and 120 kg N ha-1 rates compared to the unamended control. Even though our study did not reveal a definitive effect of biochar on the major response parameters (biomass, tissue P and N concentrations) evaluated, the trend for a biochar × N rate interaction on soil P concentration offers hope that biochar-amended soils coupled with appropriate N fertilization will be effective in P retention on agricultural landscapes irrigated with treated wastewater. Based on soil moisture, lower levels of irrigation may be possible for forage production when biochar is applied in semiarid environments like Nevada. The second study evaluated biochar and N application rates on tall fescue tissue macro- and microminerals concentrations, soil nutrient status, and leachate quality. Treatments and experiment design were the same as in the first study. Results indicate that forage tissue P concentration was altered by biochar rate × year interaction and a main effect of N rate (P < 0.05). Tissue P concentration was greater at the highest biochar application rates. Biochar application rate also had a positive influence on forage minerals P, K, Na, and Mn as well as forage S and B concentrations. Results suggest that, while biochar application alone did not significantly improve the overall nutrient status of the studied soil, biochar and N supplementation has the potential to influence nutrient content of forages to more than marginally improve the nutritional status of forages for forage-based livestock systems. The third study evaluated the effectiveness of forage systems (FS) of grass and legume monocultures, and their mixtures on herbage accumulation, tissue P concentration, and P removal from a grassland irrigated with treated wastewater. A total of 23 FS using tall fescue [Schedonorus arundinaceus (Schreb.) Dumort], alfalfa (Medicago sativa L.), red clover (Trifolium pratense L.) and white clover (Trifolium repens L.) in monocultures or grass-legume mixtures (25:75, 50:50, and 75:25) based on seeding rate were used. Data collected were biomass, botanical composition, and subsamples from biomass were analyzed for phosphorus concentration to quantify total P removed based on biomass for each treatment. Responses were considered different at P < 0.05. Herbage accumulation (HA), tissue P concentration, and P removal differed among FS and year. Herbage accumulation was similar for the grass monocultures (10.5 Mg ha-1; SE = 1.1) and the majority of the grass-legume mixtures (9.0 Mg ha-1; SE = 1.1) but both systems had greater HA than legumes monoculture (4.3 Mg ha-1; SE = 1.1). Total P removed was least among legume monocultures (34.0 kg P ha-1; SE = 6.2) in this study and generally similar for grass monocultures (67.4 kg P ha-1; SE = 6.2) and grass-legume mixtures of 75:25 (61.7 kg P ha-1; SE = 6.2). The results indicate that grass-legume mixtures that contained at least 50% grass would be suitable options for P removal on P enriched grasslands