Cumulative and Residual Effects of Repeated Biosolids Surface Applications On Tall Fescue Response, N Fate, Soil C, and Nutrients.

Repeated applications of biosolids can provide long term benefits by increasing soil organic matter, and detrimental effects through excess accumulation of some nutrients, such as phosphorus.  Our objective was to determine the residual effects of repeated surface applications of biosolids on nitrogen availability and fate, tall fescue response, soil C, and accumulation of P.  We began a field experiment in 1993, comparing two types of biosolids applied at three rates (0.5 X, 1X, and 1.5X agronomic rate), inorganic N (1X agronomic rate), and a zero-N control.  Treatments were surface applied for 10 years (1993-2002) followed by a 9-year residual period (2003-2011) where all plots received inorganic N at 0.5X agronomic rate.  Annual measurements included forage yield (6 harvests/yr), N uptake, and soil nitrate-N.  Soil samples collected from 0-8, 8-15, and 15-30 cm depths in 2002 and 2011 were analyzed for total C and N, and selected elements.  Cumulative apparent N recovery in harvested grass (1993-2010) was 50-54% of N applied for the biosolids treatments, and 72% for the inorganic N treatment.  Cumulative net fall soil nitrate N (subject to leaching) ranged from <1 to 3% of total N applied for the biosolids treatments.  Total N accounted (including changes in soil N and cumulative net fall soil nitrate N) averaged 74% for the biosolids and inorganic N treatments.  Soil C at 0-8 cm was 30 to 41 g kg-1 for the biosolids treatments compared with 23 g kg-1 for inorganic N in 2002, and 31 to 36 g kg-1 for biosolids compared to 28 g kg-1 for inorganic N in 2011.  Smaller, but significant biosolids effects on soil C were measured in the 8-15 and 15-30 cm depths.  Bray-1 P accumulated to excessive levels at all depths for all biosolids rates and remained excessive 9 years after the last biosolids application.