Distribution and Mineralization of Biosolids Nitrogen Applied to Dryland Wheat K. A. Barbarick,* J. A. Ippolito, and D. G. Westfall ABSTRACT The greatest challenge in land application of biosolids (sewage sludge) for beneficial use is predicting N-mineralization rates so that managers can amend the soil with an agronomic rate of material. We used 11 yr of field-study information frombiosolids addition to dryland hard-red winter wheat (Triticum aestivum L. ’Vona’ or ’TAM107’) to determine not only the distribution of applied N but also to estimate net N mineralization rates. This study is unique since it involves multiple biosolids application in a dryland summer fallow agroecosys- tern. We applied five or six applications of biosolids fromthe cities of Littleton and Englewood, CO(L/E) to Weld loam(Aridic Paleustoll) or Platner loam (Abruptic Aridic Paleustoll) at four locations. This paper focuses on the O (control) andthe 56- or 67-kg N ha - i fertilizer rate, and the 6.7 and 26.8 dry Mg biosolids ha -I rates that we added every crop year. The 6.7 dry Mgbiosolids ha-~ treatment resulted in the following average N distributions: 54% soil residual, 9% grain removal, and 38% unaccounted. At the larger application rate of 26.8 dry Mg biosolids ha -~ treatment, we found the following average N distributions: 35% soil residual, 2% grain removal, and 63% unac- counted. We estimated a net mineralization of 25 to 57%and a first-year N net mineralization of 13 to 43% for 6.7 dry Mg ha- ~ and a net mineralizationof 62 to 78% anda first-year N net mineralization of 41 to 67% for 26.8 dry Mg ha-~. T HE U.S. ENVIRONMENTAL PROTECTION AGENCY (USEPA, 1993) published the 40 CFR Part 503 regulations in 1993to control the beneficial use of biosol- ids (sewage sludge) produced by municipal wastewater- treatment facilities. Their premise was that the wastefacili- ties must apply biosolids to land at an "agronomic rate." Determining this rate is critical in preventing NO3-N leach- ing into groundwater. For most situations, this meant that the biosolids application rate depended on plant N requirement, although P maybe critical in someenviron- ments. This approach appears to be sound, but the major obstacle to implementationis that accurate prediction of N-mineralization rates under long-term multiapplication field conditions is difficult. Generally, webase our esti- mates on various incubation and laboratory techniques. Typically, laboratory predictions of soil N mineraliza- tion are based on incubations devised by Stanford and Smith (1972), which predict N mineralization using first-order kinetics. Molina et al. (1980) expandedthis approach to account for organic N consisting of readily decomposable plus recalcitrant compounds that degrade at different rates. Therefore, they formulated the double exponential model. These methods serve as the founda- tion for soil N-mineralization studies under laboratory conditions. Manyreports on the estimate of N mineralization of biosolids under controlled soil incubations are available. Most of the results are confined to N mineralized within K.A. Barbarick, J.A. Ippolito, and D.G. Westfall, Dep. of Soil and Crop Sciences, Colorado State Univ., Fort Collins, CO80523. Received 31 July 1995. *Corresponding author (kbarbari@ceres.agsci.colostate.edu). Published in J. Environ. Qual. 25:796-801 (1996). the first year after application. Parker arid Sommers (1983) found the following ranges in N-mineralization rates based on 16-wk laboratory incubations of various types of biosolids: 25%of the original organic N for raw and primary, 40% for waste activated, 15% for anaerobically digested, and 8% for composted. The USEPA (1983), with some modification, used the infor- mation from Sommers et al. (1981) and Parker and Sommers (1983) to develop recommendations for their Process and Design Manual. Lindemann and Cardenas (1984) reported total N mineralization rates of 56 to 72 for 32-wk laboratory incubations; and, they indicated that total mineralization (i.e., gross mineralization) increased as the biosolids rate increased. But net mineralization did not. Garau et al. (1986) found that soil type had larger influence on N-mineralization rate than biosolids type or loading rate. Their 16-wk laboratory incubation yielded an average N-mineralization rate of 43 %. Lerch et al. (1992) reported a range in N-mineralization rates of 27 to 55 % during a 12-wk laboratory incubation period with seven different biosolids that had been air dried for <1 yr. They also suggested that the C/N ratio of the biosolids plus the concentration of low molecular weight primary amines could be used to predict N mineraliza- tion. Lindemann et al. (1988) working with freshly amended soil discovered that total N mineralization aver- aged 65 % during a 16-wk incubation and was indepen- dent of soil amendment history (e.g., multiple applica- tions) or fresh-sludge amendment. Haith (1983) developed a model to predict NO3-N leaching occurring following biosolids addition. Sensitiv- ity analyses showed that using first-year mineralization rates of 20 + 5 %changed predictions of NO3-N leaching by three- to eightfold. This model typically would not fit a dryland agroecosystem, since Haith (1983) assumed that percolation would leach all NO3-N unused by the crop below the root zone. Some field studies have yielded estimates of N mineral- ization from waste additions. Kelling et al. (1977) used soil analyses to show approximately 50%of the organic N applied was mineralized within 3 wks after biosolids application. Magdoff and Amadon (1980) tbund an aver- age of 55 %N mineralization for the first year of applica- tion that compared well to their 17-wk laboratory- incubation results of 54%. In a furrow-irrigated desert soil (Typic Torrifluvent), Artiola and Pepper (1992) estimated an average annual mineralization rate of 65 %. Keeney et al. (1975) suggested an organic N decay series of 15 to 20, 6, 4, and 2%for the first, second, third, and fourth years, respectively. We have evaluated the effect of biosolids application rate on soil properties in a winter wheat (Triticum aesti- vum L. ’Vona’ or °TAM107’)-fallow cropping system Abbreviations: L/E, cities of Littleton and Englewood, CO; LSD0.05, least significant difference at the 0.05 probability level; USEPA, U.S. Environmental Protection Agency. 796 Published July, 1996