Biosolids and Dairy Manure as Nitrogen Sources for Prairiegrass on a Poorly Drained Soil

Accurate prediction of plant‐available N release from municipal and agricultural byproducts is needed to optimize crop yields and minimize nitrate leached to groundwater. We conducted a 4‐yr study to determine plant‐available N release from two byproducts: dewatered biosolids (stabilized solids from municipal waste‐water treatment) and dairy manure. The Buckley loam soil (fine‐loamy, mixed, nonacid, mesic Typic Humaquepts) at our study site had a perched water table 0 to 24 in. below the surface from mid‐November through mid‐ May. Biosolids and manure were surface‐applied to established prairiegrass [ Bromus unioloides (Willd.) H.B.K. cv. Grasslands Matua; syn. B. willdenowii Kunth] for 2 yr (1991–1992). Plant‐available N released from biosolids or manure was measured for two additional years (1993–1994) after we terminated application. Cumulative apparent N recovered by grass harvest for 1991,1992, and 1993 was 77, 74, and 65% for urea; 27, 29, and 38% for biosolids; and 17, 28, and 36% for manure. Biosolids or manure applied in 1991–1992 increased soil nitrate recovered after a summer of chemical fallow in 1994. The additional nitrate recovered in 1994 represented approximately 6% of the biosolids or manure N applied in 1991–1992. Our study demonstrated increased soil N mineralization rates following 2 yr of biosolids or manure application. Farm management plans for biosolids and manure use must account for the residual effects of previous applications. Research Question Byproducts from livestock production and municipal waste treatment can provide nutrients and organic matter for sustainable cropping systems. Organic byproducts such as biosolids (stabilized solids from municipal wastewater treatment) and dairy manure are sources of N. Management plans for biosolids and dairy manure use are usually based on N availability. A number of factors complicate estimates of the N‐supplying capacity of biosolids and dairy manure. These include the effect of previous biosolids and manure applications on current season N availability, and the potential for denitrification loss at poorly drained sites. The primary objective of our research was to determine crop N recovery from biosolids and dairy manure on a poorly drained soil in comparison with urea, a source of rapidly available N. Literature Summary Soil organic N accumulated from previous biosolids or manure applications generally reduces the need for current season N inputs in perennial grass cropping systems. However, a number of factors affect quantity and timing of N availability in a perennial grass cropping systems. These factors include the loss of ammonia N from the soil surface, organic N mineralization rates that vary depending on environmental conditions, and the storage of N in unharvested stubble, crowns, and roots of a perennial grass crop. The potential for N loss via denitrification is generally high for poorly drained soils. Loss of N via denitrification reduces the ampunt of inorganic N available for crop uptake. The bacteria responsible for the denitrification process require: N in the nitrate form, available organic carbon, and anaerobic conditions. Organic amendments like manure or biosolids generally accelerate denitrification by increasing the supply of carbon and nitrate, and by increasing the probability of anaerobic soil conditions via increased soil respiration rates. Predicting the timing and quantity of denitrification loss is difficult because of the many variables involved. Soil drainage class (an estimate of depth to a seasonal water table) has been routinely used as a predictor of denitrification loss, because anaerobic conditions are more likely on poorly drained soils. Study Description The soil at our study site has a high seasonal water table during late fall and early spring. The soil surface horizon (0–12 in.) is underlain by a slowly permeable, compact volcanic mudflow. Most grass roots are confined to the surface horizon, because the compact mudflow severely restricts root growth. Even with tile drainage, a perched water table persists at 0 to 24 in. below the surface from mid‐November through mid‐May. We applied biosolids and dairy manure for 2 yr (1991–1992) at three application rates to prairiegrass, a cool‐season grass recently introduced for forage production in our region. We chose prairiegrass for this study because we thought its growth in late fall and early spring would be well‐suited to capture N released from biosolids or manure applications. Annual total N applied in 1991–1992 ranged from 240 to 1018 lb/acre per yr for biosolids, 145 to 727 lb/acre per yr for manure, and was 240 lb/acre per yr for urea. We measured grass yield and N uptake by harvesting grass every 30 to 60 d during the growing season. Plant‐available N released from biosolids or manure was monitored for two additional growing seasons after we terminated biosolids and manure application. Applied Questions How do biosolids, dairy manure, and urea compare as sources of N for perennial grass? Cumulative apparent N recovery (ANR) after 2 yr of application was 29% for biosolids, 28% for manure, and 74% for urea. Thus, for the first 2 yr of application, biosolids N was 39% as effective as urea N and manure N was 38% as effective as urea N. The greater N recovery for urea was expected because all of the N in urea is in rapidly available form, while most of the N in the biosolids and manure is in slow‐release organic form. How does the slow‐release nature of biosolids and dairy manure N affect the production and management of perennial grass crops? The slow‐release N produced by biosolids and manure application can be a major benefit for grass cropping systems on poorly drained soils. In our study, summer biosolids and manure application provided N for fall and early spring grass production. Typically, fresh forage is in short supply in fall and early spring, and fertilizer application equipment traffic is not desirable on the wet soils present at these times of the year. The slow‐release characteristics of biosolids and manure also require managers to reduce N application rates when these materials are repeatedly applied to the same fields. The need to credit N applied previously was demonstrated in our study. In the third year of our study (1993), previous (1991–1992) applications of biosolids or manure were more effective in increasing grass yield than were current season applications of urea. In 1994, previous (1991–1992) applications of biosolids or manure increased soil nitrate recovered after a summer of chemical fallow. Did poor soil drainage affect N availability to the perennial grass crop? Cumulative ANR by prairiegrass on our poorly drained soil for 1991–1992 was similar to that reported in other studies, despite the high water table from late fall through late spring at our site. Soil nitrate tests (0–12 in.) taken after the September grass harvest in 1991–1993 suggested that most of the plant‐available N supplied by biosolids and manure was used by the grass.