Biosolids and manures provide nutrients and organic matter to soils but can also pollute runoff. Phosphorus (P) is the limiting nutrient for eutrophication in most fresh water systems, and states are developing strategies to reduce P losses from cropland amended with manure and biosolids.
The water extractable P (WEP) of manures and biosolids has been proposed as a predictor of potential to enrich drainage and surface runoff P. A rainfall simulation study was conducted to investigate the relationship between the percent extractable P (PWEP = [WEP ÷ PT]
× 100) and runoff P levels. Five P-sources [fresh dairy manure, anaerobically digested biosolids produced without Fe or Al addition, aerobically digested biosolids (elevated Al), anaerobically digested biosolids (elevated Fe), and composted biosolids (elevated Fe)] were surface applied
at 112 kg P ha-1 to runoff trays containing an acid-shale and a calcareous soil, each with high and low soiltest P background levels (four soils in all). Trays were sloped at 3% and subjected to 30-min rainfall events (intensity = 7.1 cm hr−1). Two rainfall events
were performed, separated by a 48- hour rest period between events. Collected runoff was analyzed for total mass, runoff dissolved P (RDP), and total (sediment plus dissolved) runoff P (TRP). For all soil treatments, the trend of RDP (mg L−1) consistently followed the pattern:
dairy manure >> biosolids without Al or Fe > biosolids with Al >composted biosolids with high Fe ≈ biosolids cake with high Fe. The effect of soil type on RDP was found to be statistically different only for the acid-shale high-P (Mehlich-3 P = 520-ppm) material. Runoff
DP for all soils was highly correlated (r2 > 0.98) with the PWEP of the P-source. Average runoff DP from high-Fe biosolids (both cake and compost) was not statistically different from unamended soil control treatments. Total RP (mg L−1) was also significantly
greater for dairy manure than for biosolids. As P-based nutrient management is implemented for land application, it is imperative that policies reflect different runoff P potentials of biosolids and manures. Moreover, WEP is a simple test that could be developed for routine use as a measure
of P-source availability in the P site indices being developed in several states.
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