Uncertainty in phosphorus fluxes and budgets across the US long-term agroecosystem research network.

Phosphorus (P) budgets can be useful tools for understanding nutrient cycling and quantifying the effectiveness of nutrient management planning and policies; however, uncertainties in agricultural nutrient budgets are not often quantitatively assessed. The objective of this study was to evaluate uncertainty in P fluxes (fertilizer/manure application, atmospheric deposition, irrigation, crop removal, surface runoff, and leachate) and the propagation of these uncertainties to annual P budgets. Data from 56 cropping systems in the P-FLUX database, which spans diverse rotations and landscapes across the United States and Canada, were evaluated. Results showed that across cropping systems, average annual P budget was 22.4 kg P ha-1 (range = -32.7 to 340.6 kg P ha-1 ), with an average uncertainty of 13.1 kg P ha-1 (range = 1.0-87.1 kg P ha-1 ). Fertilizer/manure application and crop removal were the largest P fluxes across cropping systems and, as a result, accounted for the largest fraction of uncertainty in annual budgets (61% and 37%, respectively). Remaining fluxes individually accounted for <2% of the budget uncertainty. Uncertainties were large enough that determining whether P was increasing, decreasing, or not changing was inconclusive in 39% of the budgets evaluated. Findings indicate that more careful and/or direct measurements of inputs, outputs, and stocks are needed. Recommendations for minimizing uncertainty in P budgets based on the results of the study were developed. Quantifying, communicating, and constraining uncertainty in budgets among production systems and multiple geographies is critical for engaging stakeholders, developing local and national strategies for P reduction, and informing policy.

Field-scale nutrient loss assessment following cover crop and manure rate change.

Eutrophication due to elevated nitrogen (N) and phosphorus (P) loss from croplands remains one of the most pressing water quality issues throughout the world. Understanding the effect of implementing conservation management practices is critical for meeting nutrient reduction goals as well as informing conservation programs and policies. A before-after-control-impact (BACI) analysis was used to evaluate the individual and combined effect of cover crops and manure application rate on discharge and nutrient loss using six water years (WY2014-WY2019) of measured data across four distinct drainage zones (1X-NCC; 1X-CC; 2X-NCC; 2X-CC) within an Ohio, USA, crop production field. White mustard significantly reduced mean monthly nitrate (NO3--N) concentration regardless of manure application rate (i.e., 65 m3 ha-1 and 130 m3 ha-1). However, neither the use of white mustard, doubling manure rate, or the combination of the two had a significant impact on mean monthly drainage discharge, dissolved-reactive P (DRP), or total P (TP) loss. Seasonal analysis confirmed that NO3--N concentration in the cover crop zones was signficantly less in fall, winter, and spring. However, significant increases in spring discharge, NO3--N, DRP, and TP loads as well as TP concentration were noted with cover crop and greater manure rate treatments. These findings confirm that cover crops have a reducing effect on NO3--N concentration but may not have any effect on addressing P concerns. Further research is warranted; however, this study highlights that the resource concern (e.g., N or P) should be considered prior to implementing cover crops as a conservation management practice.

Subsurface phosphorus and nitrogen loss following liquid dairy manure and commercial fertilizer application on a clay soil in northwest Ohio.

Nutrient source has been the focus of much debate regarding the re-eutrophication of Lake Erie, despite that only 20% of nutrients applied to crops in the Western Lake Erie Basin (WLEB) originate from organic sources. However, limited data and assessments exist on the subsurface tile drainage water quality comparison between organic (liquid dairy manure) and commercial (mono-ammonium phosphate [MAP]) sources in crop production systems. Subsurface tile drainage, dissolved reactive phosphorus (DRP) and total phosphorus (TP) losses in tile drainage discharge following equal phosphorus (P) based applications of liquid dairy manure and MAP were assessed using a before-after control-impact design and 4 years of data from a paired field system located in northwest Ohio. Nitrate-nitrogen (NO3 - -N ) and total nitrogen (TN) losses were also examined to supplement the P findings; however, due to dissimilar nitrogen application rates, losses were assessed in a different context. No significant differences (p > 0.05) were detected in drainage discharge volumes or TP loads between the control and impact sites. However, statistically significant increases (p < 0.05) were measured for mean daily DRP, NO3 - -N, and TN loads from the dairy manure site. While significant, mean daily DRP differences between commercial (MAP) and liquid dairy manure treatments were only on the order of 0.01 g DRP ha-1 . Assuming current manure application extent and rates, when accumulated annually across the WLEB watershed, these losses are equivalent to less than 1% of target loads. These findings also help to inform nutrient management stewardship as it relates to nutrient source. Furthermore, additional research across a range of soil characteristics and cropping managements should be explored as well as the impacts of other livestock manure nutrients.

Extending vegetative cover with cover crops influenced phosphorus loss from an agricultural watershed.

Excess phosphorus (P) from agriculture is a leading cause of harmful and nuisance algal blooms in many freshwater ecosystems. Throughout much of the midwestern United States, extensive networks of subsurface tile drains remove excess water from fields and allow for productive agriculture. This enhanced drainage also facilitates the transport of P, particularly soluble reactive phosphorus (SRP), to adjacent streams and ditches, with harmful consequences. Thus, reducing SRP loss from tile-drained cropland is a major focus of regional and national efforts to curb eutrophication and algal blooms. The planting of cover crops after crop harvest is a conservation practice that has the potential to increase retention of fertilizer nutrients in watersheds by extending the growing season and limiting bare ground in the fallow season; however, the effect of cover crops on SRP loss is inconsistent at the field-scale and unknown at the watershed-scale. In this study, we conducted a large-scale manipulation of land cover in a small, agricultural watershed by planting cover crops on >60% of croppable acres for six years and examining changes in SRP loss through tile drains and at the watershed outlet. We found reduced median SRP loss from tiles with cover crops compared to those without cover crops, particularly during periods of critical export from January to June. Variation in tile discharge influenced SRP loss, but relationships were generally weaker in tiles with cover crops (i.e., decoupled) compared to tiles without cover crops. At the watershed outlet, SRP yield was highly variable over all seasons and years, which complicated efforts to detect a significant effect of changing land cover on SRP export to downstream systems. Yet, watershed-scale planting of cover crops slowed cumulative SRP losses and reduced SRP export during extreme events. Overall, this study demonstrates the potential for cover crops to alter patterns of SRP loss at both the field- and watershed-scale.

Cover crops differentially influenced nitrogen and phosphorus loss in tile drainage and surface runoff from agricultural fields in Ohio, USA.

Nitrogen (N) and phosphorus (P) loss from crop production agriculture is transported to adjacent and downstream water bodies, resulting in negative environmental impacts including harmful and nuisance algal blooms. Cover crops are a conservation management practice that replaces bare soil with vegetation outside of the cash crop growing season, purportedly reducing N and P loss by increasing water and nutrient demand in agroecosystems. In this study, we compared nitrate (NO3--N), total N (TN), dissolved reactive P (DRP), and total P (TP) loads in subsurface (tile) drainage and surface runoff from fields with cover crop management (CC) and fields without cover crop management (NoCC) using continuous monitoring data from 40 agricultural fields located throughout northcentral Ohio, United States (US). We found that average monthly tile NO3--N and TN loads from CC fields were ~50% less than NoCC fields, while average monthly tile discharge, DRP, and TP loads did not differ between CC and NoCC fields. Cover crops also did not significantly influence average monthly surface metrics. Cover crops reduced monthly totals of tile NO3--N and TN loads by ~1.0-2.6 kg N ha-1 from January to June (winter and spring), coinciding with critical periods of nutrient loss from agroecosystems in the midwestern US, but increased monthly totals of tile DRP (by 0.4-12.1 g DRP ha-1) and TP (by 1.2-31.6 g TP ha-1) loads during some months. We found similar patterns at the annual time scale whereby CC fields had lesser cumulative annual totals of tile NO3--N and TN but greater cumulative annual totals of tile DRP and TP. These results show that the influence of cover crops on N loads, but not P, were consistent across temporal scales of examination, demonstrating that cover crops effectively increased N demand and mitigated N losses from agricultural fields. The variable influence of cover crops on P loads underscores the need for greater understanding of the factors and mechanisms that control P loss in systems that include cover crop management. Furthermore, these findings stress the importance of identifying and selecting conservation management practices tailored to the natural resource concern.

Effect of winter cover crops on soil nutrients in two row-cropped watersheds in Indiana.

The midwestern United States is a highly productive agricultural region, and extended crop-free periods in winter/spring can result in nitrogen (N) and phosphorus (P) losses to waterways that degrade downstream water quality. Planting winter cover crops can improve soil health while reducing nutrient leaching from farm fields during the fallow period. In this study, we used linear mixed effects models and multivariate statistics to determine the effect of cover crops on soil nutrients by comparing fields with cover crops (n = 9) versus those without (n = 6) in two Indiana agricultural watersheds: the Shatto Ditch Watershed, which had >60% of croppable acres in winter cover crops, and the Kirkpatrick Ditch Watershed, which had ∼20%. We found that cover crops decreased soil nitrate-N by >50% and that the magnitude of reduction was related to the amount of cover crop biomass. In contrast, cover crops had variable effects on water extractable P and Mehlich III soil test P. Finally, cover crop biomass significantly increased soil N mineralization and nitrification rates, demonstrating that cover crops have the potential to supply bioavailable N to cash crop after termination. Our study showed that widespread implementation of winter cover crops holds considerable promise for reducing nutrient loss and improving soil health. The degree to which these results are generalizable across other systems depends on factors such as climate, soil characteristics, and past and current agronomic practices.

Controls on subsurface nitrate and dissolved reactive phosphorus losses from agricultural fields during precipitation-driven events.

The magnitude of nitrogen (N) and phosphorus (P) exported from agricultural fields via subsurface tile drainage systems is determined by site-specific interactions between weather, soil, field, and management characteristics. Here, we used multiple regression analyses to evaluate the influence of 29 controls of precipitation event-driven discharge, nitrate (NO3--N) load, and dissolved reactive P (DRP) load from subsurface tile drains, leveraging a unique dataset of ~7000 precipitation events observed across 40 agricultural fields (n = 190 site years) instrumented to collect continuous water quality samples. We calculated marginal effects of significant controls and assessed the modifying influence of event rainfall, duration, and intensity, and antecedent precipitation. Tile discharge was strongly and positively influenced by previous 7-day precipitation and total rainfall and negatively influenced by daily temperature and tile spacing. Both tile NO3--N and DRP loads were positively influenced by transport and source variables, including event discharge and total fertilizer applied as well as soil test P (STP) in the case of tile DRP load; factors with the strongest negative influence on tile NO3--N and DRP loads were related to time of year. The strength and direction of both positive and negative controls also varied with precipitation characteristics. For example, the positive influence of event discharge on nutrient loads lessened as event duration, event intensity, and previous 7-day precipitation increased, while the positive influence of N and P sources strengthened, particularly in response to extreme (or maximum) events. Results here demonstrate the predominant role of transport and source controls while accounting for interactive effects among site-specific characteristics and underscore the importance of storm dynamics when managing N and P loss from agricultural fields.

Legacy phosphorus concentration-discharge relationships in surface runoff and tile drainage from Ohio crop fields.

Legacy phosphorus (P) in agricultural soils can be transported to surface waters via runoff and tile drainage, where it contributes to the development of harmful and nuisance algal blooms and hypoxia. However, a limited understanding of legacy P loss dynamics impedes the identification of mitigation strategies. Edge-of-field data from 41 agricultural fields in northwestern Ohio, USA, were used to develop regressions between legacy P concentrations (C) and discharge (Q) for two P fractions: total P (TP) and dissolved reactive P (DRP). Tile drainage TP concentration (CTP ) and DRP concentration (CDRP ) both increased as Q increased, and CTP tended to increase at a greater rate than CDRP . Surface runoff showed greater variation in C-Q regressions, indicating that the response of TP and DRP to elevated Q was field specific. The relative variability of C and Q was explored using a ratio of CVs (CVC /CVQ ), which indicated that tile drainage TP and DRP losses were chemodynamic, whereas losses via surface runoff demonstrated both chemodynamic and chemostatic behavior. The chemodynamic behavior indicated that legacy P losses were strongly influenced by variation in P source availability and transport pathways. In addition, legacy P source size influenced C, as demonstrated by a positive relationship between soil-test P and the CTP and CDRP in both tile drainage and surface runoff. Progress towards legacy P mitigation will require further characterization of the drivers of variability in CTP and CDRP , including weather-, soil-, and management-related factors.

Water Flow and Biofilm Cover Influence Environmental DNA Detection in Recirculating Streams.

The increasing use of environmental DNA (eDNA) for determination of species presence in aquatic ecosystems is an invaluable technique for both ecology as a field and for the management of aquatic ecosystems. We examined the degradation dynamics of fish eDNA using an experimental array of recirculating streams, also using a "nested" primer assay to estimate degradation among eDNA fragment sizes. We introduced eDNA into streams with a range of water velocities (0.1-0.8 m s-1) and substrate biofilm coverage (0-100%) and monitored eDNA concentrations over time (∼10 d) to assess how biophysical conditions influence eDNA persistence. We found that the presence of biofilm significantly increased initial decay rates relative to previous studies conducted in nonflowing microcosms, suggesting important differences in detection and persistence in lentic vs lotic systems. Lastly, by using a nested primer assay that targeted different size eDNA fragments, we found that fragment size altered both the estimated rate constant coefficients, as well as eDNA detectability over time. Larger fragments (>600 bp) were quickly degraded, while shorter fragments (<100 bp) remained detectable for the entirety of the experiment. When using eDNA as a stream monitoring tool, understanding environmental factors controlling eDNA degradation will be critical for optimizing eDNA sampling strategies.