Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/29/2013
Publication Date: 10/25/2013
Citation: Cogger, C.G., Bary, A.I., Kennedy, A.C., Fortuna, A. 2013. Long-term crop and soil response to biosolids applications in dryland wheat. Journal of Environmental Quality. 42:1872–1880. Interpretive Summary: A two-year wheat-fallow rotation is practiced in semi-arid regions of the Pacific Northwest and Great Plains of the United States to capture fallow-year moisture for the next crop of wheat. Wheat-fallow rotation accelerates the decline in soil organic C compared with annual crops, because of decreased residue and increased erosion during fallow. Researchers and farmers have sought means of improving soil organic C status in wheat-fallow rotations, such as conservation tillage. Organic amendments are a way to restore organic matter in depleted soils. This experiment was conducted to determine the long-term response of crop and soil to biosolids applications in a dryland wheat-fallow rotation. Agronomic rates of biosolids produced equivalent or greater grain yields than anhydrous ammonia, and also increased soil organic matter. Repeated applications of biosolids increase soil N supply, suggesting that lower rates are needed to avoid N and yield loss. Excess P may become a future water quality risk, depending on site-specific P transport factors. Use of environmental P indices to identify P loss, adoption of conservation tillage to reduce erosion, rotation of biosolids applications to sites with low soil test P levels, and development of wastewater treatment processes that reduce biosolids P will enhance assist the safe use of biosolids applications to grain-fallow rotations, and allow continued C and N benefits associated with biosolids use. Agronomic biosolids applications are an effective and low-cost tool to increase soil carbon and improve soil health in soils depleted of organic matter after years of grain-fallow rotation. This long-term experiment provides growers, land managers and scientists and regulators valuable information on biosolids management.
Technical Abstract: Biosolids have the potential to improve degraded soils in grain-fallow rotations. Our objectives were to determine if repeated use of biosolids in wheat-fallow rotations could (i) supply adequate but not excessive N for grain production and (ii) maintain or increase soil C, without creating high risk of P loss. A replicated on-farm experiment was established in 1994 in Douglas County, WA, comparing anaerobically digested biosolids with anhydrous ammonia N and a zero-N control. Biosolids were applied at 5, 7, and 9 Mg ha-1 every fourth year (2 crop cycles) and incorporated 10 cm deep, while anhydrous ammonia plots received 56 kg ha-1N every second year. Grain yield and protein were determined for each crop. Soil chemical, biological, and bulk density analyses were made in 2012. The medium and high biosolids rates significantly increased grain yield (3.63 vs. 3.13 Mg ha-1) and protein (103 vs. 85 g kg-1) compared with anhydrous ammonia averaged over all crops. Soil C (0- to 10- cm depth) was 16.9 g kg-1 for the medium biosolids rate (34 Mg ha-1 cumulative biosolids applied), significantly higher than for anhydrous ammonia (9.4 g kg-1). The medium biosolids rate had significantly lower bulk density and higher extractable P than anhydrous ammonia. Mineralizable N increased from 52 mg kg-1 for anhydrous ammonia to 156 mg kg-1 for the medium biosolids rate. The medium and high biosolids rates significantly increased bacteria to fungi ratios, Gram-negative bacteria, and anaerobic bacteria markers compared with anhydrous ammonia and zero-N. Biosolids can be an agronomically beneficial and environmentally sound management practice in wheat-fallow systems.