Nitrate and phosphorus transport through subsurface drains under free and controlled drainage

Responses of leached nitrogen concentrations and soil health to winter rye cover crop under no-till corn-soybean rotation in the northern Great Plains

Winter rye (Secale cereale L.) is an important cover crop (CC) in the northern Great Plains (NGP), yet concerns over its establishment under the variable weather conditions of this region are an important limitation for its widespread adoption. This study evaluated the impacts of no-till corn (Zea mays L.)-soybean [Glycine max (L.) Merr.] rotation with winter rye CC established in 2017 on (a) water quality (nitrate-N [NO3 - -N], ammonia-N [NH4 + -N], and total nitrogen [TN]) and (b) soil health parameters at the 0-to-15-cm depth. Data showed that rye CC biomass was 251 kg ha-1 in 2018, 1,213 kg ha-1 in 2019, and 147 kg ha-1 in 2020, coinciding with contrasting growing degree days for rye CC (i.e., 1,458, 2,042, and 794, respectively), as a consequence of variable weather conditions. Water quality was not affected for the periods when rye growth was <300 kg ha-1 . In the season when rye CC had greater biomass (1,213 kg ha-1 ), significant reductions in leached NO3 - -N (19-20%) and TN (8.5-16%) concentrations were observed due to greater N uptake by rye CC (18.8 kg N ha-1 ). Rye CC showed significantly (p ≤ .05) higher microbially active carbon ( ∼13%) and water-extractable organic N(∼11%) than the control treatment. Nonsignificant impacts on soil health indicators due to rye CC showed that the study duration (3 yr) may not be sufficient to see the beneficial impacts of CCs on soils. However, significant reductions in leached NO3 - -N and TN concentrations for one (2019) out of three study years suggest that rye with optimal growth has the potential of reducing N leaching and enhancing soil health for the NGP region.

Analysis of reactive phosphorus treatment by filter materials at the edge of tile-drained agricultural catchments: A global view of the current status and challenges

Phosphorus losses from agriculture have long generated concern due to the ecological impact on surface waters. Here tile-drained agricultural catchments are a critical source for concentrating and transporting phosphorus bioavailable forms or dissolved reactive phosphorus (DRP). Hence, edge-of-field technologies have been introduced to reduce DRP loads. Filter systems have received special attention due to their targeted approach using a permeable filter material (FM) rich in DRP sorbents. This review explores the performance and applicability of FMs in the aforementioned context because of the growing number of studies. An overall analysis revealed that sorption is preferable to precipitation for DRP retention at the edge-of-field, and that FM pH and particle size affect sorption properties and subsequently DRP retention and lifetime. Thus, FMs with predominant amounts of iron and/or aluminium can be recommended. Such materials generally have an appreciable availability of DRP binding sites, strong bonds with DRP and short reaction times, as well as low desorption, which lead to good operation. On the other hand, FMs with predominant amounts of calcium and/or magnesium are restricted to catchments with favourable conditions unless they have optimal reactivity for DRP. The review also found that hydraulic retention time plays a key role in the performance and applicability of FMs, especially in those dependent on precipitation reactions. Therefore, it is crucial that FMs are designed, constructed and managed according to the catchment conditions-including normally varying flow rates and DRP concentrations-in order to ensure successful operation. This reflects in long-term, high and steady net DRP retention along with low costs, thus improving the FM cost-effectiveness, besides discharging non-harmful effluents to aquatic ecosystems.

Paired field and water measurements from drainage management practices in row-crop agriculture

This paper describes a multi-site and multi-decadal dataset of artificially drained agricultural fields in seven Midwest states and North Carolina, USA. Thirty-nine research sites provided data on three conservation practices for cropland with subsurface tile drainage: saturated buffers, controlled drainage, and drainage water recycling. These practices utilize vegetation and/or infrastructure to minimize off-site nutrient losses and retain water in the landscape. A total of 219 variables are reported, including 90 field measurement variables and 129 management operations and metadata. Key measurements include subsurface drain flow (206 site-years), nitrate-N load (154 site-years) and other water quality metrics, as well as agronomic, soil, climate, farm management and metadata records. Data are published at the USDA National Agricultural Library Ag Data Commons repository and are also available through an interactive website at Iowa State University. These multi-disciplinary data have large reuse potential by the scientific community as well as for design of drainage systems and implementation in the US and globally.

Measurement(s)	subsurface drain flow • Nitrate-N concentration • water table • soil nitrate concentration • precipitation • grain yield
Technology Type(s)	pressure transducers within drainage control structure/outlet • automated flow-proportional sampling • pressure transducers installed inside deep wells of perforated PVC pipes • dry combustion • on-site weather station • mechanical harvest with combine
Factor Type(s)	controlled drianage • drainage water recylcing • saturated buffers
Sample Characteristic - Organism	Zea mays • Glycine max
Sample Characteristic - Environment	agricultural field
Sample Characteristic - Location	United States

Urbanizing with or without nature: pollution effects of human activities on water quality of major rivers that drain the Kumasi Metropolis of Ghana

The effects of urbanization such as population upsurge, increased industrialization, urban agriculture, and rural-urban migration of persons exert pressure on the limited water resources in most cities. This study investigated the impact of human activities on the water and sediment quality of the three main rivers (Wiwi, Subin, and Suntre) in Kumasi, the second-largest city in Ghana. The physicochemical parameters and the concentrations of contaminants, including heavy metals, polycyclic aromatic hydrocarbons, pesticide residues, and microbial loads in the rivers, were linked to the specific human activities at the riverbanks. While all the 37 pesticide residues investigated in river sediments had concentrations below the detection limits (0.005 mg/kg for organochlorines, 0.010 mg/kg for organophosphates, and 0.010 mg/kg for synthetic pyrethroids), the study showed that the sediments are polluted with petrogenic and pyrogenic polycyclic aromatic hydrocarbons. River Subin, the most polluted among the three rivers, recorded benzo[e]pyrene concentrations up to 47,169 µg/kg. The geoaccumulation index and concentration factors show that the rivers are highly contaminated with metals such as cadmium, chromium, mercury, and arsenic and are related to human activities. The microbial quality of the rivers was poor, recording specific microbial loads of 6.8, 4.1, and 1.5 × 10⁷ counts/100 mL respectively for Wiwi, Subin, and the Suntre Rivers. The three water bodies are therefore not suitable for recreational and irrigational purposes.

A Public-Private Partnership to Locate Fields for Implementation and Monitoring of Best Management Practices to Treat Legacy Phosphorus

Legacy nutrients stored in agricultural soils are a substantial component of riverine nutrient discharge contributing to the eutrophication of aquatic ecosystems. These nutrient loads can persist and delay water quality initiatives, for example, those of the Great Lakes Water Quality Agreement which seek to reduce phosphorus (P) loads entering the Western Lake Erie Basin. In this watershed, approximately 5% of fields have P concentrations 2.5-fold greater than the maximum agronomic recommendations for corn and soybeans. Fields with these elevated-P concentrations (&gt;100 mg P kg−1 soil) act as a source of legacy-P and discharge greater P loads. Implementing best management practices to treat runoff from these fields is desirable but finding them has been a challenge as soil test data are proprietary information creating an asymmetric information barrier. To overcome this barrier, we formed a public-private partnership that included agricultural retailers who conduct soil testing for farmers. Agricultural retailers who partnered with this project provided their soil P data and contacted farmers to gauge their interest, maintaining privacy for farmers until they expressed interest. Only 3.8% of soil samples in the provided data had elevated-P concentrations. In many cases, these elevated-P soils were confined to zones within fields, and 13% of fields had at least one elevated-P zone. We pursued these elevated-P fields as research sites for the implementation and monitoring of management practices. The agricultural retailers contacted 77 farmers with surveys, and 25 responded with interest in meeting the research team to discuss the project. Following a preliminary evaluation with the spatial data of fields operated by interested farmers, visits were arranged so that 12 research sites could be located. As indicated through the surveys, discussions with farmers, and soil data, many of the fields had accumulated elevated-P due to historic land-use (livestock, manure, or biosolid application) creating legacy sources. We conclude that public-private partnerships featuring agricultural retailers are a promising tool that may help overcome asymmetric information barriers to finding and managing agricultural fields with legacy-P that that disproportionately contribute to nutrient runoff.

Detecting pollutant sources and pathways: High-frequency automated online monitoring in a small rural French/German transborder catchment

Great temporal and spatial variability of inputs make comprehensive monitoring in small and middle sized rivers difficult. In this study, relevant inputs in a small river were recorded with suitable online monitoring equipment coupled in mobile water quality monitoring stations, the study area being a transborder catchment with French and German (Saarland federal state) subcatchments. In addition to a pronounced spatial variability necessitating a denser net of measuring points this catchment has also to be assessed in the light of different national regulations. To identify individual pollution sources and weigh their relative importance, relevant parameters were recorded over a representative monitoring period of several months: phosphorus (P) as total phosphorus (TP) and total reactive P phosphorus (TRP), nitrate (NO₃-N), ammonium (NH₄-N), total organic carbon (TOC), temperature, oxygen (O₂), pH, turbidity, and electrical conductivity (EC). The recorded data were subjected to adapted interpretation together with other catchment-related factors. In order to retrieve maximum information from the online data sets the relationships among certain parameter pairs were also analysed for both storm events and low flow periods. Comparison of loads at the different monitoring sites could reliably verify the majority of nutrient inputs originating in the French subcatchment. Additional sampling of output channels from sewage treatment works (STWs) in the Saarland subcatchment revealed that inputs from several decentralised STWs do not result in significant loads, as opposed to inputs from one STW in France. Our holistic approach provides a basis for adopting cost-effective measures to reduce loads in small river catchments as well as cross-border harmonisation of environmental policies.

Highly efficient removal of phosphorus from agricultural runoff by a new akadama clay barrier-vegetated drainage ditch system (VDD) and its mechanism

A vegetated drainage ditch (VDD) system is an effective management practice for removing excess phosphorus (P) from agricultural runoff. However, the maximization of P removing efficiency by VDD remains a challenge. In this study, new VDDs with akadama clay barriers (particle size of clay: 1-6 mm; height of barrier: 5-15 cm and length of barrier: 10-90 cm) were designed in lab scale, and the mechanism of phosphate removal by akadama clay was investigated. It was found that a new VDD with akadama clay barriers (particle size:1 mm; height:10 cm and length: 90 cm) exhibited the highest removal efficiency of total P (TP) (97.1%), particulate P(PP) (96.9%), and dissolved P (DP) (97.4%), respectively. The retained P was mainly adsorbed in akadama clay barrier sections, and a low concentration of P was observed in soil sections in the new VDD. The maximum adsorption capacity of phosphate to akadama clay was 5.06 mg/g at 298 K, and XPS analysis indicated that phosphate was adsorbed by the inner-sphere complexation formation with the metal elements (Al, Fe). This study indicates that the new VDD with akadama clay barriers is a promising technique to efficiently remove P from agricultural runoff and significantly minimize the risk of P release into streams through runoff.

Controlled Irrigation and Drainage Reduce Rainfall Runoff and Nitrogen Loss in Paddy Fields

In southern China, the growing period of rice is synchronized with the rainy period, and the loss of nutrients (such as nitrogen) due to unreasonable irrigation and drainage, along with rainfall and runoff, has become the main source of agricultural nonpoint source pollution. The laws of runoff and nitrogen loss in paddy fields under different irrigation and drainage modes are not clear. In this study, field experiments were adopted to observe the runoff and nitrogen loss under typical rainfall and throughout the whole growth period. The results showed that, compared with the traditional irrigation and drainage mode, the controlled irrigation and drainage mode reduced the drainage of two typical rainfall processes by 47.5% and 31.3% and the peak drainage by 38.9% and 14.4%. Compared with those under the traditional irrigation and drainage mode, the average concentrations of total nitrogen, nitrate nitrogen, and ammonium nitrogen under the controlled irrigation and drainage mode were reduced by 22.2%, 22.7%, and 27.8%, respectively, during the whole rainfall process on July 21 and were decreased by 27.1%, 11.4%, and 25.6%, respectively, on August 25. In irrigated rice areas, under the controlled irrigation and drainage mode, drainage was reduced after two intercepts through paddy fields and drainage ditches. The nitrogen concentration in the drainage ditch decreased due to the increase in retention time and the effect of the ditch and field wetland. Compared with the traditional irrigation and drainage mode, the total nitrogen, nitrate nitrogen, and ammonium nitrogen loads of the controlled irrigation and drainage mode were reduced by 69.8%, 65.3%, and 69.7%, respectively.

Adsorption of phosphate on iron-coated sand granules as a robust end-of-pipe purification strategy in the horticulture sector

Nutrient enrichment in water bodies, and its detrimental consequences, are a well known and worldwide environmental problem. Agricultural activities are identified as an important source of diffuse losses of phosphate and nitrate because of the leaching out fertilizers from agricultural fields. This study encompasses the implementation of an end-of-pipe treatment by capturing phosphate from greenhouse effluent, using granular iron-coated sand (ICS) in an adsorption process. ICS is evaluated as a low-cost by-product because of its adsorption capacity and kinetics. The Langmuir isotherm was suitable for describing the adsorption thermodynamics. The adsorption capacity at an equilibrium concentration C_e of 25 mg PO₄-P/L ranged between 1.85 and 3.07 mg PO₄-P/g sorbent. Furthermore, both the pseudo-second-order model (R² = 0.9823) and the Elovich model (R² = 0.9803) showed a good fit with the kinetic data over the time range investigated, indicating that chemisorption is the rate-limiting step controlling the adsorption process. Higher adsorption capacities were observed at lower initial pH. Continuous bench-scale column experiments were performed to verify the adsorption potential of a filter bed under flow-through conditions, and the experimental data were fit to the Bohart-Adams model. Additionally, a discontinuous feeding regime of the column, resulting in intermediate resting periods, was introduced and showed an enhanced adsorption efficiency over a longer period. Finally, a pilot-scale experiment showed the potential of the ICS for the removal of phosphate from greenhouse effluent. The adsorption process, moreover, enables the recovery of phosphate via efficient desorption.

High flow event induced the subsurface transport of particulate phosphorus and its speciation in agricultural tile drainage system

Subsurface storm flow of phosphorus (P), including particulate P, has been recently discussed as an important P transport path in contrast to typical surface runoff events. However, P speciation, and P concentration during storm events has not been extensively investigated; therefore, its contribution to the water quality is not clearly understood. In this study, the physicochemical properties of particulate P in tile water samples during a high flow event were investigated in Midwestern agricultural lands using wet chemical methods, 31P Nuclear Magnetic Resonance spectroscopy and P K-edge X-ray absorptions near edge structure spectroscopy. In slightly alkaline pH tile water, total P was ranging from ∼0.06 to 0.22 mg L^-1, which is significantly greater than dissolved reactive P (DRP) (∼0.02-0.08 mg L^-1). The tile water contains P enriched particulate matters (∼200-660 mg L^-1). Total P in the colloidal fraction was from 1013 to 2270 mg kg^-1. Phosphate and organic P species, especially monoesters, are sorbed in soil colloids like calcite, and iron oxides, and colloids are effective carriers of P in the subsurface transport process during storm events. The results of this study show that storm events can accelerate the subsurface transport of P with soil particles in addition to DRP.

Nutrient control in water bodies: A systems approach

Nutrient pollution is considered a wicked problem because of its many significant economic, social, and environmental impacts that are caused by multiple pollutants originating from a variety of sources and pathways that exist across different temporal and spatial scales. Further adding to the difficulty in managing nutrient pollution is that it is a global, rural, and urban problem. A systems approach can improve nutrient management by incorporating technological, environmental, and societal considerations. This approach can consider valuation of monetized and nonmonetized co-benefits and the inherent consequences that make up a nutrient management program. In this introduction to a special collection of papers on nutrient pollution, we describe several systems frameworks that can be used to support nutrient management and evaluation of system performance as it relates to impacts, then highlight several attributes and barriers of nutrient management that point to the need for a systems framework, and conclude with thoughts on implementing systems approaches to nutrient management with effective community engagement and use of new technologies. This special collection presents results from a USEPA Science to Achieve Results (STAR) initiative to advance solutions to nutrient pollution through innovative and sustainable research and demonstration projects for nutrient management based on a systems approach. These studies evaluate several promising nutrient control technologies for stormwater or domestic wastewater, investigate the effects of agricultural conservation practices and stream restoration strategies on nutrient loads, and discuss several challenges and opportunities-social, policy, institutional, and financial considerations-that can accelerate adoption of reliable technologies to achieve system-level outcomes.

Impact factors and mechanisms of dissolved reactive phosphorus (DRP) losses from agricultural fields: A review and synthesis study in the Lake Erie basin

Dissolved Reactive Phosphorus (DRP) losses from agricultural fields promote algae growth in water bodies, and may increase the risk of Harmful Algal Blooms (HABs). Using existing data from the Lake Erie Basin, we applied multiple regression analysis to better understand the impacts of both site-specific conditions (e.g., soil types/properties) and management practices (e.g., Agricultural Conservation Practices [ACP]) on annual DRP losses in subsurface and surface runoff. Results showed that soil properties significantly impact DRP losses. Greater DRP losses were associated with increased soil pH and Soil Test Phosphorus (STP). By contrast, soil organic matter (SOM) was inversely correlated with DRP losses. Soil clay content was also inversely correlated with DRP subsurface losses, but had no impact on DRP surface losses. The ACPs evaluated had varied effectiveness on DRP loss reduction. Cropping systems involving soybean could reduce DRP subsurface losses, whereas winter cover crops could cause unintended DRP subsurface losses. Cropping systems involving soybean and cover crops, however, had no impact on DRP surface losses. In addition, no-till and conservation tillage also enhanced DRP losses compared to conventional tillage, particularly for soils with high SOM and/or high clay content. Precipitation amount and fertilizer application rate significantly increased DRP surface losses as expected. Fertilizer application rate, however, had no impact on DRP subsurface losses. The impact of precipitation amount on DRP subsurface losses depends on STP levels. Precipitation amount significantly increases DRP subsurface losses when STP is higher (>41 mg kg-1 in this analysis). The optimal STP level for crop growth is 30 to 50 mg kg-1. Results from this study help us to better understand DRP losses and the effectiveness of ACPs for controlling them. We suggest taking soil surveys and soil tests into consideration when designing and/or implementing ACPs to manage DRP losses. Furthermore, the method we used for this study could be applied to other agricultural regions to investigate impacts of site-specific conditions and management practices on water quality.

Concurrent transport and removal of nitrate, phosphate and pesticides in low-cost metal- and carbon-based materials

Low-cost magnesium- and/or carbon-based materials have a great potential to remove soluble contaminants from surface and ground water. This study examined mechanisms that control the removal of nitrate, phosphate and pesticides (tricyclazole, malathion and isoprothiolane) during their transport through calcined magnesia (MgO) and corn stalk biochar. Various miscible column breakthrough experiments were carried out and morphology and crystallographic structures of reactive materials were examined. Approximately 96% (78,950 mg-NO₃^-/kg) and 48% (27,455 mg-NO₃^-/kg) of nitrate were removed from biochar and MgO columns, respectively. Chemical adsorption dominated nitrate removal during early phase (i.e., <11 PVs for biochar and <100 PVs for MgO, respectively), and microbial denitrification dominated during the following phase. 92% of the applied phosphate (6168 mg-PO₄^3-/kg) was removed in MgO column, while much less in biochar column (4%, 347 mg-PO₄^3-/kg). Mineral surface analyses confirmed that electrostatic attraction, ligand exchange, and chemical precipitation were responsible for phosphate removal. For the three pesticides, biochar exhibited larger removal capacity (1260-2778 mg/kg) than MgO (28-2193 mg/kg) due to the functional groups on biochar. The removal of pesticides based on their physico-chemical properties. Malathion had highest removal rate (98-100%), attributing to chemical sorption and bio-degradation, followed by isoprothiolane (47-79%) and tricyclazole (6-64%).

Reactive Transport and Removal of Nutrients and Pesticides in Engineered Porous Media

Agricultural nonpoint pollution has been recognized as a primary source of nutrients and pesticides that contaminate surface water and groundwater. Reactive materials have great potential to remove nutrients and pesticides from agricultural drainage water. In this study, we investigated the reactive transport and removal of coexisting nitrate, phosphate, and three pesticides (tricyclazole, isoprothiolane, and malathion) by iron filings and natural ore limestone through column experiments under saturated flow conditions. Breakthrough results showed that 45.0% and 35.8% of nitrate were removed by iron filings and limestone during transport, with average removal capacities of 2670 mg/kg and 1400 mg/kg, respectively. The removal of nitrate was mainly due to microbial denitrification especially after 131–154 pore volumes (≈30 d), whereas reduction to ammonia dominated nitrate removal in iron filings during early phase (i.e., <21.7 d). The results showed that 68.2% and 17.6% of phosphate were removed by iron filings and limestone, with average removal capacities of 416.1 mg/kg and 155.2 mg/kg, respectively. Mineral surface analyses using X-ray diffraction (XRD) and scanning electron microscope (SEM) coupled with energy-dispersive X-ray analysis (EDX) suggested that ligand exchange, chemical precipitation, and electrostatic attraction were responsible for phosphate removal. Chemical sorption was the main mechanism that caused removals of 91.6–100% of malathion and ≈27% of isoprothiolane in iron filings and limestone. However, only 22.0% and 1.1% of tricycalzole were removed by iron filings and limestone, respectively, suggesting that the removal might be relevant to the nonpolarity of tricyclazole. This study demonstrates the great potential of industrial wastes for concurrent removal of nutrients and pesticides under flow conditions.

Nitrate Runoff Contributing from the Agriculturally Intensive San Joaquin River Watershed to Bay-Delta in California

Nitrogen loading from agricultural landscapes can trigger a cascade of detrimental effects on aquatic ecosystems. Recently, the spread of aquatic weed infestations (Eichhornia crassipes, Egeria densa, Ludwigia spp., and Onagraceae) in the Sacramento-San Joaquin Delta of northern California has raised concerns, and nitrogen loading from California’s intensive farming regions is considered as one of the major contributors. In this study, we employed the Soil and Water Assessment Tool (SWAT) to simulate nitrogen exports from the agriculturally intensive San Joaquin River watershed to the Delta. The alternate tile drainage routine in SWAT was tested against monitoring data in the tile-drained area of the watershed to examine the suitability of the new routine for a tile nitrate simulation. We found that the physically based Hooghoudt and Kirkham tile drain routine improved model performance in representing tile nitrate runoff, which contributed to 40% of the nitrate loading to the San Joaquin River. Calibration results show that the simulated riverine nitrate loads matched the observed data fairly well. According to model simulation, the San Joaquin River plays a critical role in exporting nitrogen to the Delta by exporting 3135 tons of nitrate-nitrogen annually, which has a strong ecological implication in supporting the growth of aquatic weeds, which has impeded water flow, impairs commercial navigation and recreational activities, and degrades water quality in Bay-Delta waterways. Since nitrate loadings contributed by upstream runoff are an important nutrient to facilitate weed development, our study results should be seen as a prerequisite to evaluate the potential growth impact of aquatic weeds and scientific evidence for area-wide weed control decisions.

Soil and Crop Management Practices to Minimize the Impact of Waterlogging on Crop Productivity

Waterlogging remains a significant constraint to cereal production across the globe in areas with high rainfall and/or poor drainage. Improving tolerance of plants to waterlogging is the most economical way of tackling the problem. However, under severe waterlogging combined agronomic, engineering and genetic solutions will be more effective. A wide range of agronomic and engineering solutions are currently being used by grain growers to reduce losses from waterlogging. In this scoping study, we reviewed the effects of waterlogging on plant growth, and advantages and disadvantages of various agronomic and engineering solutions which are used to mitigate waterlogging damage. Further research should be focused on: cost/benefit analyses of different drainage strategies; understanding the mechanisms of nutrient loss during waterlogging and quantifying the benefits of nutrient application; increasing soil profile de-watering through soil improvement and agronomic strategies; revealing specificity of the interaction between different management practices and environment as well as among management practices; and more importantly, combined genetic, agronomic and engineering strategies for varying environments.

Enhanced removal of phosphate from aqueous solutions using a modified sludge derived biochar: Comparative study of various modifying cations and RSM based optimization of pyrolysis parameters

Different biochars produced by the impregnation of Mg, Ca, Al, Cu, and Fe were compared for the phosphate (P) uptake capacity and the effect on solution pH. Among them, Ca- and Mg-rich biochars demonstrate better sorption ability to P and have less effect on pH change. The optimum conditions of the pyrolysis processes were determined using response surface methodology. Comparison of the P removal efficiency of these two biochars under optimum conditions imply the superior adsorption capability of Ca-rich biochar. According to XRD analysis, calcite is the dominant mineral on the biochar surface, indicating the potential of Ca-rich biochar for P removal by adsorption and precipitation. Predictive second-order kinetic and linear Langmuir isotherm models could adequately interpret the P sorption process for optimized Ca-rich biochar. The maximum P sorption capacity of Ca-rich biochar of 153.85 mg/g is superior to other adsorbents reported in literature.