The goal of this project is to understand how long-term (>5 years) implementation of soil health practices affect the distribution of soil P and hydrologic connectivity in artificially-drained soils on the Delmarva Peninsula. The specific objectives of this project are to:
Objective 1: Use the Delaware P Index and the Maryland P Management Tool versions 1 and 2 to determine the risk of edge-of-field P loss. We will run the Delaware P Site Index (PSI) and Maryland P Management Tool (PMT) on Delmarva fields with and without a long-term history of soil health practices to identify fields with a high overall risk of edge-of-field P loss. Ten fields that are identified as high risk will be used in this study. Fields with a long-term history of soil health practices (conservation tillage and/or cover crops) will be paired with fields with little to no history of soil health. Paired fields will have similar soil types and cropping rotations.
Objective 2: Compare the vertical distribution of P in soils with and without a long-term history (>5 year) of soil health practices. Accumulation of soil P near the soil surface can increase the risk of dissolved P losses in runoff. In contrast, accumulation of soil P at depth is suggestive of subsurface P losses in leachate. We will investigate soil P stratification within the topsoil and P accumulation at depth at each study field. Soil cores will be collected from several locations in the field and split into 15 depth increments. Soil samples will be analyzed for soil P using a variety of chemical extraction methods to evaluate differences in soil P dynamics through the soil profile in fields with and without a long-term history (>5 year) of soil health management practices.
Objective 3: Determine how soil health practices impact hydrologic connectivity. Hydrologic flow p due to the formation of preferential flow pathways (large connected poathways that connect soil P sources to drainage ditches result in a high risk of P loss. Without this hydrologic connectivity, there is a very low risk of P loss. Soil health management practices can affect hydrologic connectivityres). We will determine how soil health practices impact hydrologic connectivity by comparing the dry soil aggregates size distribution in soils with and without a history of soil health practices. Soils with a wider size distribution of soil aggregates suggest the formation of preferential flow pathways through which P can leach.
The purpose of this project is to investigate the impact of conservation tillage (including no-till, ridge till, strip till, and vertical tillage) and cover crops on subsurface legacy P loss on the Delmarva Peninsula, where historical application of poultry litter at nitrogen (N)-based rates resulted in the accumulation of soil P (Toor and Sims, 2015). This “legacy” P, which is present at concentration that exceed crop needs, is continuously lost from soils in runoff and leachate during storm events. Legacy P losses are linked to eutrophication and hypoxia in sensitive water bodies like the Chesapeake Bay (Sharpley et al. 2009). Drawdown of soil P through crop harvest is estimated to take several decades (Fiorellino et al., 2017). As such, farmers need strategies to control legacy P loss from soils in the short-term (Sharpley et al., 2013; Qin and Shober, 2018).
One such strategy to address legacy P losses is to promote adoption of soil health practices such as conservation tillage or cover crops. Most soil health practices can mitigate sediment-bound P in runoff and dissolved P losses from newly applied manure (Dodd and Sharpley, 2016; Qin and Shober, 2018). However, soil practices often increase infiltration and soil aggregation, which can lead to the formation of preferential flow pathways. Subsurface leaching through preferential flow pathways was identified as a major mechanism for legacy P loss on the Delmarva Peninsula, where flat, sandy, poorly drained soils are often artificially-drained through open ditch networks (Kleinman et al., 2007; Vadas et al., 2007). Delmarva farmers may implement soil health practices to improve soil conditions and reduce erosion losses, but these practices may not be effective at controlling subsurface legacy P losses. Adoption of soil health practices may even increase subsurface P losses as documented by some researchers (Kleinman et al, 2009; Christianson et al., 2016).
This study aims to investigate how conservation tillage and cover crops affect P distribution and hydrologic connectivity (i.e., P stratification within surface soils and P accumulation at depth) within legacy P soils on the Delmarva Peninsula. Both soil properties impact the risk of subsurface P loss. This project addresses two of the “key themes in sustainable agriculture”: 1) the reduction of environmental and health risks in agriculture and 2) the conservation of soil, the improvement of water quality and the protection of natural resources. The results from this study will improve our understanding of how soil health practices impact subsurface legacy P loss on the Delmarva Peninsula. This information will help practitioners increase the effectiveness of current and future soil health practices in controlling subsurface P losses.
Due to a wet winter so far, we have yet to sample for this study. We plan to sample before the end of January 2020. Below are our proposed methods:
Objective 1: Use the Delaware P Index and the Maryland P Management Tool versions 1 and 2 to determine the risk of edge-of-field P loss for study field selection.
We will run the Delaware PSI and Maryland PMT tools on 20-30 working agricultural fields on the Delmarva Peninsula. Targeted fields will be managed in a grain crop rotation (e.g., corn, full season soybean or corn, wheat, double-crop soybean). Among these fields, we will identify 10-15 fields with a long-term history (>5 years) of soil health practices (i.e., conservation tillage, with or without cover crops) and 10-15 fields with little to no history of soil health practices. Ideally, we will pair fields with similar soil series, soil characteristics (e.g., acidic soil pH, agronomic excessive Mehlich 3 P concentrations), management (e.g., irrigation, drainage intensity, application of P manures or fertilizers, and crop rotation), and historical application of P fertilizers and manures.
The Delaware PSI and Maryland PMT are field level risk assessment tools that farmers and consultants can use to determine the risk of edge-of-field P losses. These tools identify critical source areas, where P sources (soil P, manure P, or fertilizer P) occur in areas with hydrologic connectivity. The Delaware P Site Index (PSI) and Maryland Phosphorus Management Tool (PMT) require knowledge of past management practices (e.g., fertilizer/manure application method, rate and timing, crop rotation, and soil health practices), as well as information about soil and field characteristics (e.g., slope of field, distance to surface water body, soil test P). This information will be obtained from certified crop advisors, grower records, field visits, and the Web Soil Survey. The field information will be used to calculate the relative risk of edge-of-field P losses using the Delaware PSI (Sims et al., 2016) and the Maryland PMT (Coale, 2008).
Once all paired fields (with and without soil health practices) have been rated using the Delaware PSI and the Maryland PMT, we will select 5 sets of fields for more in-depth soil characterization. Ideally, we will select fields with a ‘high or very high’ risk for edge-of-field P losses using these P indices.
Objective 2: Compare the vertical distribution of P in soils with and without a long-term history (>5 year) of soil health practices.
At each study field, 5 composite soil cores will be collected after fall harvest and prior to spring planting (i.e., Fall 2019). The composite soil cores (each consisting of soil from three soil cores) will be collected along a transect perpendicular to a drainage ditch. Soil cores will be collected using a Giddings probe (10 cm diameter) to a depth of 1 m. Before compositing, soil cores will be divided into approximately 15 depth increments (ranging from 2.5-10 cm), with smaller increments (2.5 or 5 cm) taken near the soil surface and the depth to seasonal high-water table (which will be determined prior to sampling using the USDA-NRCS SoilWeb application in Google Earth). Soil cores will be composited by depth, air-dried, ground, sieved (2 mm), and bagged prior to analysis.
In order to investigate P stratification and P accumulation at depth, each soil sample will be extracted (Self-Davis, 2009) and analyzed for water extractable P (WEP; Murphy and Riley, 1962) by a spectrophotometer (GENESYS™ 30 Visible Spectrophotometer). Using the WEP concentrations for each composite soil core above and below the water table, we will determine the average WEP concentration at the average seasonal high-water table (WEPWT) for each study field, which can be used as a proxy for subsurface P loss risk. The WEPWT concentration for each study field will be determined by averaging the WEPWT concentrations of the field’s composite soil cores. The calculated WEPWT concentrations of study fields with long-term soil health practices will be compared to the WEPWT concentrations of study fields without soil health practices.
In addition, soils will be sent to the University of Delaware Soil Testing Lab to undergo a routine soil analysis using standard soil testing procedures (NECC-1312, 2011). The routine soil test consists of pH (1:10 soil/deionized water), organic matter (loss on ignition-LOI), the degree of P saturation, and a Mehlich 3 extraction. Following a Mehlich 3 extraction, soil test P, calcium (Ca), magnesium (Mg), manganese (Mn), zinc (Zn), copper (Cu), iron (Fe), boron (B), sulfur (S), sodium (Na), and aluminum (Al) concentrations will be analyzed by inductively coupled plasma optical emission spectroscopy (ICP-OES).
Lastly, composite samples from four depth increments (e.g., 0-2.5, 2.5-5, and the depth increments above and below the seasonal high-water table) from two locations in each study field (80 samples) will undergo a modified Hedley sequential chemical fractionation (Sui et al., 1999) to determine the operational soil P pools. The modified Hedley sequential chemical fractionation consists of sequentially extracting 0.5 g of each soil sample with a) 30 mL of DI water (water extractable P), b) 30 mL of 0.5 M NaHCO3 (labile P), c) 30 mL 0.1 M NaOH (moderately labile P), and d) 30 mL 0.1 M HCl (readily insoluble P). The extracts generated during each step of the fractionation procedure will be analyzed for P, Ca, Mg, Mn, Zn, Cu, Fe, B, S, Na and Al by ICP-OES at University of Delaware Soil Testing Lab.
Objective 3: Determine how soil health practices impact hydrologic connectivity.
Three additional soil cores will be collected at each study field to determine the size distribution of dry aggregates according to the procedure documented by Kemper and Rosenau (1986). Soil samples for dry aggregate analysis will be collected along the same depth increments as the soil cores for chemical analysis to 30 cm depth. When the soil is somewhat dry, a square-cornered spade will lift a soil sample from a desired depth then be placed on a tray and air dried before analysis. Once the soil samples are air-dried, each soil sample will be moved from the tray to a RoTap Sieve Shaker for analysis (sieve sizes 4 mm, 2 mm, 1 mm, 0.5 mm, and 0.250 mm). Dried soils will be shaken for 6 to 10 minutes and then the aggregates in each sieve will be weighed.
Statistical Analysis (all Objectives)
Comparisons of soil parameters and P Index scores between study fields with long-term soil health practices and study fields without soil health practices will be completed using paired t-tests in the R programming language with a significance level of alpha = 0.05.
Due to a wet winter so far, we have yet to sample for this study. We plan to sample before the end of January 2020.