Nutrient Assessment of a Dryland Wheat Agroecosystem after 12 Years of Biosolids Applications

Abstract

Biosolids beneficial‐use programs effectively recycle plant nutrients when these waste materials are applied at an agronomic rate. Plant‐nutrient availability questions have arisen relating to Littleton and Englewood (L/E), Colorado Wastewater Treatment Plant bio‐solids applications to dryland wheat (Triticum aestivum L.)‐fallow agroecosystems. What is the long‐term estimated nitrogen equivalency (NE) of the biosolids? Can we estimate long‐term micronutrient distribution with continuous biosolids application? How does plant‐nutrient availability change with continuous application? Before each growing season we added biosolids at rates of 0 to 11.2 dry Mg ha−1 to plots arranged in randomized complete blocks with four replications per treatment. We found 12 years of application (6 applications to two sites in a 2‐yr wheat–fallow rotation) produced N equivalencies, based on wheat‐grain N uptake, of about 9 kg N Mg−1 biosolids. Estimated first‐year mineralization rates were ≈21 to 33%. Since P, Cu, Ni, and Zn grain removal were <1% of biosolids‐applied concentrations, we estimated that tillage‐layer (top 20 cm of soil) concentrations could be predicted within 5% of actual total soil contents based on biosolids additions of these nutrients. Biosolids additions produced linear increases in NH4HCO3‐diethylenetriaminepentaacetic acid (AB‐DTPA) soil extract concentrations of P, Cu, Ni, and Zn. Soils initially were Zn deficient; biosolids application provided plant‐available Zn for dryland wheat. If biosolids agronomic rates were based on P instead of N availability, these soils could not receive biosolids. Also, if the Colorado Phosphorus Index was utilized, agronomic rates would continue to be based on N. Biosolids addition to dryland winter wheat according to N agronomic rates is a feasible method of recycling plant nutrients.