Todd W. Andraski

Measuring Water-Extractable Phosphorus in Manures to Predict Phosphorus Concentrations in Runoff

Water-extractable phosphorus (WEP) in manures can influence the risk of phosphorus (P) losses in runoff when manures are land applied. We evaluated several manure handling and extraction variables to develop an extraction procedure for WEP that will minimize pre-analysis manure-sample-handling effects on WEP measurements. We also related manure WEP determinations to runoff dissolved reactive phosphorus (DRP) concentrations found in previously conducted field simulated rainfall experiments using the same manures to evaluate WEP as a predictor of P runoff losses. Dairy and poultry manure WEP concentrations increased with manure-to-water extraction ratio and shaking time. Relative to fresh manures, drying and grinding dairy manures before analysis usually decreased WEP concentrations, while WEP in poultry manures was often increased. Pre-analysis handling effects on WEP were minimized at the 1:1000 extraction ratio with a 1-h shaking time. Relationships between manure WEP and runoff DRP concentrations were strongly influenced by season of year and WEP extraction procedure. The best prediction of DRP concentration in spring runoff experiments was with manure WEP concentration at the 1:1000 extraction ratio. With fall runoff studies, DRP concentrations were best predicted with WEP application rate rather than concentration. These seasonal differences can be explained by the greater percentage of rainfall that ran off in the fall compared to the spring. For all studies, runoff DRP concentrations were strongly related (r2 = 0.82) to the ratio of runoff to rainfall volumes, confirming that models need to take runoff hydrology into account as well as manure WEP in P-loss risk assessments.

Manure composition and incorporation effects on phosphorus in runoff following corn biomass removal.

Greater demand for corn ( L.) stover for bioenergy use may lead to increased corn production acreage with minimal surface residue cover, resulting in greater risk for soil erosion and phosphorus (P) losses in runoff. A rainfall simulation study was conducted to determine the effects of spring-applied dairy cow () manure (none, in-barn composted, and exterior walled-enclosure pit) with >200 g kg organic solids content following fall corn biomass removal with and without incorporation (chisel plow [CP] and no-till [NT]) on sediment and P in runoff. Runoff was collected from a 0.83-m area for 60 min following the onset of rainfall simulation (76 mm h), once in spring and once in fall. Runoff dissolved reactive P (DRP) and dissolved organic P (DOP) concentrations were positively correlated with manure P rate and were higher in NT compared with CP. Conversely, sediment and particulate P (PP) concentrations in runoff were inversely correlated with manure P rate (and manure solids) and were higher in CP compared with NT. Runoff volume where no manure was applied was higher in NT than in CP in spring but similar in fall. The addition of manure reduced runoff volumes by an average of 82% in NT and 42% in CP over spring and fall. Results from this study indicate that surface application of dairy manure with relatively high solids content may reduce sediment and PP losses in runoff without increasing the risk of increased DRP and DOP losses in the year of application where corn biomass is harvested.

Scale-of-measurement effects on phosphorus in runoff from cropland

Phosphorus (P) risk loss assessment tools such as P indices are usually developed from small-plot scale data showing the relationships between various site and management variables and runoff P losses. Little information is available on how small-plot runoff composition compares with field-scale measurements. This study was conducted to compare runoff volume and composition measurements at the field scale with those obtained from natural runoff at the small-plot (1 m2 [10.8 ft2]) scale. Sediment, dissolved P, and total P in natural runoff from small plots located in two fields (7.2 and 12 ha [17.8 and 29.6 ac]) were compared with similar measurements from the fields over an 18-month period. Runoff from small-plot rainfall simulations in both fields was also analyzed for P and sediment. The fields, cropped with either corn (Zea mays L.) or alfalfa (Medicago sativa L) interseeded with bromegrass (Bromus inermis), were located on a Tama silt loam soil (fine-silty, mixed, superactive, mesic Typic Argiudoll) in southwest Wisconsin (42°42′ N, 90°22′W). Statistical analysis using repeated measures showed no significant differences between the two scales of measurements for dissolved P concentrations in runoff. Total P concentrations in small-plot runoff were greater than those in field runoff. Runoff volume and dissolved P concentrations were greater in winter than in summer, but summer runoff had higher sediment concentrations. Small plots had greater cumulative runoff volumes per unit area in both seasons compared to the fields. The dissolved P concentration relationships between the two scales of measurement for individual runoff events were very good in the corn field (r2 = 0.90) but not in the alfalfa field (r2 = 0.09). Sediment P enrichment ratios varied by crop and were similar in the small-plot and field runoff. Cumulative runoff-dissolved P concentrations were strongly related (r2 = 0.95) to average soil-test P at both scales. The agreement of small-plot and field runoff-dissolved P concentrations during the 18-month measurement period supports use of small-plot data in P loss risk assessment tools.