Fate of Biosolids Pollutants on Disturbed Western Rangelands

Abstract

Biosolids land application has been found to be an environmentally sound and potentially cost effective approach for restoring forage productivity on disturbed western rangelands. In spite of the potential economic and environmental benefits associated with increased forage yield (e.g., higher animal stocking rates, reduced soil erosion etc.), the fate of regulated biosolids pollutants on disturbed and/or marginal landscapes when biosolids application rates exceeds the agronomic rates remains a concern for the agricultural community as well as environmental regulators. In the current study, the vegetative forage yield as well as the regulated pollutants arsenic, cadmium, copper, lead, molybdenum, nickel, selenium and zinc were monitored over an 18 month period following biosolids land application on disturbed western rangeland.By land applying aerobically digested and lime stabilized biosolids on 1/3 acre rangeland test plots at rates of up to twenty times (20X) the estimated agronomic rate, disturbed rangeland forage yields were found to increase from 84.1 lbs/acre (control plots) to over 1800 lbs/acre on plots receiving biosolids. The increase in vegetation on disturbed rangelands not only reduces soil erosion potential but reduces the footprint of agriculture by limiting the land requirement needed to support ranching activities.The soil concentrations of cadmium, lead and molybdenum were found to be below the method detection limits in all test plots even when biosolids were applied at rates equivalent to 20X the agronomic rate. Moreover, the soil concentrations of arsenic, copper, nickel and zinc in test plots were found to be comparable to the control plots even at biosolids application rates significantly greater than the agronomic rate. These findings suggest that, when regulated pollutant levels are compliant with national quality standards, the variability in the background soil mineralogy may be a more important factor in determining soil metal concentration than biosolids land application rates.Of all regulated pollutants evaluated, selenium appeared to have been mobilized within the 18 month test period. In most cases, selenium levels were significantly greater in soil receiving biosolids land application compared to the control plots. Current efforts to analyze forage harvested from sites receiving biosolids land application will help determine the extent to which selenium (as well as other regulated pollutants) is taken up and concentrated within rangeland vegetation.