Recent adoption of national rules for organic crop production have stimulated greater interest in meeting crop N needs using manures, composts, and other organic materials. This study was designed to provide data to support Extension recommendations for organic amendments. Specifically, our objectives were to (i) measure decomposition and N released from fresh and composted amendments and (ii) evaluate the performance of the model DECOMPOSITION, a relatively simple N mineralization/immobilization model, as a predictor of N availability. Amendment samples were aerobically incubated in moist soil in the laboratory at 22 degrees C for 70 d to determine decomposition and plant-available nitrogen (PAN) (n = 44), and they were applied preplant to a sweet corn crop to determine PAN via fertilizer N equivalency (n = 37). Well-composted materials (n = 14) had a single decomposition rate, averaging 0.003 d(-1). For uncomposted materials, decomposition was rapid (>0.01 d(-1)) for the first 10 to 30 d. The laboratory incubation and the full-season PAN determination in the field gave similar estimates of PAN across amendments. The linear regression equation for lab PAN vs. field PAN had a slope not different from one and a y-intercept not different than zero. Much of the PAN released from amendments was recovered in the first 30 d. Field and laboratory measurements of PAN were strongly related to PAN estimated by DECOMPOSITION (r(2) > 0.7). Modeled PAN values were typically higher than observed PAN, particularly for amendments exhibiting high initial NH(4)-N concentrations or rapid decomposition. Based on our findings, we recommend that guidance publications for manure and compost utilization include short-term (28-d) decomposition and PAN estimates that can be useful to both modelers and growers.

Simple, reliable estimates of biosolids N availability are needed to develop land application programs that will benefit crops without risk of excess nitrate leaching. This study was conducted to assess the effect of biosolids processing on plant available nitrogen (PAN) release during the first and second growing seasons after application. We compared 14 sources of biosolids and a range of inorganic N rates in two replicated field experiments on established tall fescue (Festuca arundinacea Schreb.). The biosolids encompassed a range of treatment and dewatering/drying processes. A single biosolids application was made in May of the first year, and tall fescue yield and N uptake were measured by harvest for the next two growing seasons. Inorganic N was split across multiple applications each year. Fertilizer efficiency regression equations were developed for the inorganic N treatments, and used to calculate biosolids PAN from N uptake data. Year 1 PAN was similar across a range of biosolids treatment processes. For nonlagoon biosolids, PAN averaged 37 ± 5% of total biosolids N. Lagoon biosolids PAN ranged from 8 to 25% of total N, with the oldest, most stable biosolids having the lowest PAN. Year 2 PAN averaged 13 ± 2% for nonlagoon biosolids, excluding heat‐dried materials, which were lower (5 to 8%). Our calculations indicated that about half of the Year 2 PAN became available during the cool season, suggesting that winter cover cropping may be needed to reduce the potential for nitrate leaching loss in summer annual cropping systems.

Repeated applications of biosolids provide long-term benefits by increasing soil organic matter and N supply but can cause excess accumulation of P. Our objective was to determine the residual effects of repeated surface applications of biosolids on N availability and fate, tall fescue ( Schreb.) response, soil C, and P. A field experiment was started in 1993 to compare two biosolids products, each applied at three rates (6.7, 13.4, and 20.1 Mg ha yr), with synthetic N fertilizer (0 and 403 kg N ha yr as ammonium nitrate). Treatments were surface applied for 10 yr, followed by a 9-yr residual period where all plots received a reduced rate of inorganic N (202 kg N ha yr). Annual measurements included forage yield, N uptake, and soil nitrate N. Soil samples collected in 2002 and 2011 were analyzed for total C and N and Bray-1 P. Cumulative apparent N recoveries in harvested grass (1993-2010) were 51% for biosolids N and 72% for ammonium nitrate. Net fall soil nitrate N summed for the period 1993-2002 ranged from <1 to 3% of N applied. The N applied that was accounted for in forage and soil averaged 74% for biosolids and 73% for ammonium nitrate. Soil C increased in the biosolids treatments, and the increase was equivalent to 27% of biosolids C. Bray-1 P remained at excessive levels (338-629 mg P kg soil) 9 yr after the last biosolids application.