Nathan O. Nelson

Nitrogen and phosphorus availability in biochar-amended soils

Biochar, a solid coproduct from the thermochemical production of bioenergy, has been reported to increase nutrient availability in soils through increased cation retention and decreased phosphate adsorption. The objectives of this study were to determine biochar effects on N and P availability in the presence and absence of external nutrient inputs. Biochar was obtained from hydrothermal pyrolysis of corn cobs at 305°C with 20 min of retention time. Biochar was added to two soils at three biochar rates (0, 2, and 20 g/kg) in combination with either two N rates (0 and 100 mg/kg) or two P rates (0 and 20 mg/kg) and incubated for 56 days. Soils were extracted for Mehlich-3 P and KCl-extractable NO3-N and NH4-N at 0, 3, 10, 17, 28, and 56 days after soil amendment. Biochar application at 20 g/kg increased NH4-N concentrations by 1.1 to 4.8 mg kg−1 during the first 10 days and consistently decreased NO3-N recovery by 5 to 10 mg kg−1 for the duration of the study. Biochar decreased Mehlich-3 P concentrations in soil by 0.9 mg kg−1 in the absence of P additions and increased Mehlich-3 P concentrations by 3.3 mg kg−1 when added with a P source. Furthermore, biochar increased Mehlich-3-extractable P by 5.4 mg kg−1 in the soil when applied with N fertilizer. Results indicate that biochar-amended soils may need additional N after biochar addition to maximize crop production.

Applicability of models to predict phosphorus losses in drained fields: a review.

Most phosphorus (P) modeling studies of water quality have focused on surface runoff loses. However, a growing number of experimental studies have shown that P losses can occur in drainage water from artificially drained fields. In this review, we assess the applicability of nine models to predict this type of P loss. A model of P movement in artificially drained systems will likely need to account for the partitioning of water and P into runoff, macropore flow, and matrix flow. Within the soil profile, sorption and desorption of dissolved P and filtering of particulate P will be important. Eight models are reviewed (ADAPT, APEX, DRAINMOD, HSPF, HYDRUS, ICECREAMDB, PLEASE, and SWAT) along with P Indexes. Few of the models are designed to address P loss in drainage waters. Although the SWAT model has been used extensively for modeling P loss in runoff and includes tile drain flow, P losses are not simulated in tile drain flow. ADAPT, HSPF, and most P Indexes do not simulate flow to tiles or drains. DRAINMOD simulates drains but does not simulate P. The ICECREAMDB model from Sweden is an exception in that it is designed specifically for P losses in drainage water. This model seems to be a promising, parsimonious approach in simulating critical processes, but it needs to be tested. Field experiments using a nested, paired research design are needed to improve P models for artificially drained fields. Regardless of the model used, it is imperative that uncertainty in model predictions be assessed.

Wheat and sorghum residue removal for expanded uses increases sediment and nutrient loss in runoff.

Crop residue removal for expanded uses such as feedstocks for cellulosic ethanol production may increase loss of sediment and nutrients in runoff. We assessed on-farm impacts of variable rates of residue removal from no-till winter wheat (Triticum aestivum L.) and plow till grain sorghum [Sorghum bicolor (L.) Moench] on sediment, soil organic carbon (SOC) and nutrient losses in runoff in western Kansas. Five treatments with three replications consisting of removing residues at 0, 25, 50, 75, and 100% after harvest under two tillage levels for wheat (no-till and freshly tilled) and grain sorghum (spring tilled and freshly tilled) were established on 1x2 m plots. Simulated rainfall was applied at 115+/-3 mm h(-1) for 30 min. Compared with plots without residue removal, complete removal increased runoff by 61% in freshly tilled wheat plots, 225% in spring-tilled sorghum plots, and 94% in freshly tilled sorghum plots. Residue removal at rates as low as 50% increased loss of sediment. Complete removal doubled the sediment loss to 14 Mg ha(-1) in tilled wheat, whereas it increased sediment loss from 0.9 to 7.2 Mg ha(-1) in no-till wheat. No-till with 100% residue removal lost as much sediment as freshly tilled wheat plots with 0 or 25% removal. Residue removal at 75 and 100% increased losses of total N, total P, and SOC associated with sediment. Overall, excessive residue removal led to large losses of sediment, sediment-bound SOC, and nutrients in runoff. Furthermore, erosion protection provided by no-till management is lost when residue removal exceeds 25%.

Evaluation of phosphorus indices after twenty years of science and development.

The P Index was proposed as a nutrient management tool in 1992 and has been implemented as such for the past decade. However, lack of water quality improvement in agricultural watersheds and discrepancies in P loss ratings between P indices have raised questions about continued use of the P Index. In response to these concerns, a symposium was held as part of the 2011 ASA, CSSA, SSSA annual meetings. This symposium produced a special collection of seven papers describing the role of P indices in P management, evaluation of P indices, new models for assessing P loss, methods to improve P indices, and changes in producer behavior resulting from P Index use. The objectives of this introductory paper are to provide background on the P Index concept, overviews of the special collection papers, and recommendations for future P Index evaluation and development research. The papers in this special collection conclude that P indices can provide accurate assessments of P loss but must be evaluated appropriately. Evaluation will require compiling large regional P loss datasets at field and small watershed scales. Simulation models may be used to generate P loss estimates; however, models must be calibrated and validated to ensure their accuracy. Further development of P indices will require coordinated regional efforts to identify common P Index frameworks and standardized interpretations. Stringent P Index evaluations will expand the utility of P indices for critical source area identification and strategic best management practice implementation by regulatory, education, and scientific communities alike.

Conservation practice effectiveness in the irrigated Upper Snake River/Rock Creek watershed

The Upper Snake River/Rock Creek Conservation Effects Assessment Project was initiated in 2005 to determine the effectiveness of conservation practices in an irrigated watershed. Our objectives were to determine water and salt balances and water quality effects of using sprinkler rather than furrow irrigation in the Twin Falls irrigation tract in southern Idaho. Data from the current study were compared with earlier studies conducted from 1968 to 1971. Irrigation water diverted from the Snake River supplied 73% and 83% of the hydrologic input to this 82,000 ha (202,000 ac) watershed in 2005 and 2006, respectively, with approximately 40% flowing back to the Snake River through furrow irrigation runoff, unused irrigation water, and subsurface drainage. Net suspended sediment loss decreased from 460 kg ha-1 (400 lb ac-1) during the 1971 irrigation season to 220 kg ha-1 (190 lb ac-1) in 2005 and 10 kg ha-1 (9 lb ac-1) in 2006 by switching from furrow to sprinkler irrigation, applying polyacylamide, and installing sediment ponds. The relative amount of sprinkler irrigation in a subwatershed did not correlate with the total loss of suspended sediment for July 2005 and 2006 (r = 0.12). The lack of correlation was primarily due to extremely high sediment concentrations in two of the five subwatersheds, possibly due to furrow irrigation management. Two potential concerns identified during this initial analysis were an accumulation of total salts in the watershed and increased nitrate concentrations in four return flow streams compared to earlier studies. Future analyses will determine the effects of specific practices with this watershed.

Modification and validation of GLEAMS for prediction of phosphorus leaching in waste-amended soils

Excess phosphorus applied to soils with low P adsorption capacities can enter surface water via leaching and subsurface transport, thereby negatively impacting water quality. Computer simulation models can be used to describe the effects of management practices on P leaching losses, provided the models are appropriately validated. The objectives of this research were to modify and validate P subroutines in the GLEAMS (Groundwater Loading Effects of Agricultural Management Systems) model to more accurately reflect P sorption and desorption, and then use the modified model to determine crop and waste management effects on long-term P leaching losses below the root zone of a grazed pasture with >20-year history of swine lagoon liquid application and considerable P buildup in the soil profile. GLEAMS was modified with the Langmuir equation to partition labile P between adsorbed and solution phases. The modification improved predictions of percolate P concentrations and soil P accumulation in acid sandy soils receiving waste-based P additions. The modification also increased model sensitivity to changes in crop and P management. The modified model predicted that P-based swine lagoon liquid applications would decrease P leaching by >20 kg ha-1 year-1 compared to N-based applications. Eliminating all P applications decreased the predicted P leaching losses by less than 1 kg ha-1 year-1 compared to P-based swine lagoon liquid application. Results show that P can continue leaching from P-saturated soils even in the absence of P additions.

Calibration of the APEX Model to Simulate Management Practice Effects on Runoff, Sediment, and Phosphorus Loss

Process-based computer models have been proposed as a tool to generate data for Phosphorus (P) Index assessment and development. Although models are commonly used to simulate P loss from agriculture using managements that are different from the calibration data, this use of models has not been fully tested. The objective of this study is to determine if the Agricultural Policy Environmental eXtender (APEX) model can accurately simulate runoff, sediment, total P, and dissolved P loss from 0.4 to 1.5 ha of agricultural fields with managements that are different from the calibration data. The APEX model was calibrated with field-scale data from eight different managements at two locations (management-specific models). The calibrated models were then validated, either with the same management used for calibration or with different managements. Location models were also developed by calibrating APEX with data from all managements. The management-specific models resulted in satisfactory performance when used to simulate runoff, total P, and dissolved P within their respective systems, with > 0.50, Nash-Sutcliffe efficiency > 0.30, and percent bias within ±35% for runoff and ±70% for total and dissolved P. When applied outside the calibration management, the management-specific models only met the minimum performance criteria in one-third of the tests. The location models had better model performance when applied across all managements compared with management-specific models. Our results suggest that models only be applied within the managements used for calibration and that data be included from multiple management systems for calibration when using models to assess management effects on P loss or evaluate P Indices.

Hydrothermal conversion of corn cobs and crude glycerol

The effect of operating parameters including reaction temperature, retention time, and biomass content on bio-oil yield from hydrothermal conversion of corn cobs was investigated. The highest bio-oil yield of 23.9% on the basis of biomass dry weight was obtained at 305°C, 20 min retention time, and 10% biomass content. At these conditions, the effect of crude glycerol on bio-oil yield and quality was studied. Bio-oil yield based on the total weight of corn cobs and crude glycerol remained almost constant at approximately 24% when the ratio of crude glycerol/corn cobs was below 3. When more crude glycerol was added, bio-oil yield dramatically increased to 36.3%. H2 molar percentage in the gas product increased from 11.1% to 27.5% as the crude glycerol/corn cobs ratio increased from 0 to 5. Bio-oil quality in terms of density and viscosity was also enhanced; however, oxygen content in bio-oil increased from 15.5% to 19.9%.

Multisite Evaluation of APEX for Water Quality: II. Regional Parameterization

Phosphorus (P) Index assessment requires independent estimates of long-term average annual P loss from fields, representing multiple climatic scenarios, management practices, and landscape positions. Because currently available measured data are insufficient to evaluate P Index performance, calibrated and validated process-based models have been proposed as tools to generate the required data. The objectives of this research were to develop a regional parameterization for the Agricultural Policy Environmental eXtender (APEX) model to estimate edge-of-field runoff, sediment, and P losses in restricted-layer soils of Missouri and Kansas and to assess the performance of this parameterization using monitoring data from multiple sites in this region. Five site-specific calibrated models (SSCM) from within the region were used to develop a regionally calibrated model (RCM), which was further calibrated and validated with measured data. Performance of the RCM was similar to that of the SSCMs for runoff simulation and had Nash-Sutcliffe efficiency (NSE) > 0.72 and absolute percent bias (|PBIAS|) < 18% for both calibration and validation. The RCM could not simulate sediment loss (NSE < 0, |PBIAS| > 90%) and was particularly ineffective at simulating sediment loss from locations with small sediment loads. The RCM had acceptable performance for simulation of total P loss (NSE > 0.74, |PBIAS| < 30%) but underperformed the SSCMs. Total P-loss estimates should be used with caution due to poor simulation of sediment loss. Although we did not attain our goal of a robust regional parameterization of APEX for estimating sediment and total P losses, runoff estimates with the RCM were acceptable for P Index evaluation.

Polyethersulfone membrane filters for sampling soil water from In situ soils and intact soil columns for phosphate analysis

Abstract Porous plates or cups are commonly used to collect soil solution samples in field studies or from intact soil columns. Some commonly used materials for porous plates may adsorb soil solution constituents such as phosphorus (P). An alternative to using a porous plate is to use a membrane filter with a known pore size and bubble point. The objective of this study was to evaluate the utility of polyethersulfone membranes (pore size 0.45 µm and bubble point >200 kPa) to extract soil solution from in situ soils and intact soil columns for phosphate analysis. In situ soil solution samplers were constructed from modified reusable polysulfone membrane filter holders equipped with polyethersulfone membranes (47 mm diameter). A −10 kPa vacuum was maintained in the samplers, which enabled soil solution collection at soil water potentials of 0 to −4 kPa in loamy sand, 0 to −10 kPa in sandy loam, and 0 to −12 kPa in sandy clay loam soils. In a laboratory study, soil solution samplers continued to hold a vacuum to −77 kPa soil water potential. Soil solution samplers were further evaluated in a field study at 45‐, 90‐, and 135‐cm depths in two soils. Samplers operated with relatively few difficulties for the first 12 months of field evaluation. Membranes apparently dried during periods of low soil water potential but increases in soil moisture were sufficient to rewet the membrane. Sampler failures in the field increased during 13–18 months because aged vacuum tubing and root interferences with samplers at 45 cm. Improvements in sampler design may improve the durability for implementation in long‐term field experiments. Membrane filters worked near flawlessly to maintain unsaturated conditions in intact soil columns. The filter units facilitated easy collection of soil water from the intact soil columns without altering the chemical composition of the percolate.