Organic Waste Nitrogen and Phosphorus Dynamics under Dryland Agroecosystems

Organic waste beneficial-use programs effectively recycle plant nutrients when applied at agronomic rates. Our objectives were to determine: biosolids nitrogen (N) fertilizer equivalency; biosolids N mineralization during years of above and below average precipitation and long-term N mineralization; which soil phosphorus (P) phases dominate following years of biosolids application; and the potential increased environment risk of P when applying an agronomic N rate or excessive rate of biosolids. To address questions related to N dynamics, we utilized research results collected between 1993 and 2004 from a site in Eastern Colorado that received 0, 1, 2, 3, 4, and 5 dry tons biosolids A -1 . To address questions related to P dynamics, results collected between 1982 and 2003 from a second Eastern Colorado site which received 0, 3, 6, 12, and 18 dry tons biosolids A -1 were used. First-year biosolids N mineralization rates were estimated at 25-32% and 21-27%, respectively; long-term first-year N mineralization rate ranged between 27-33%. Based on wheat-grain N uptake, we found that an application rate of 1 dry ton biosolids A -1 supplied about 20 lbs N A -1 . Based on the Colorado P index risk assessment, biosolids applied at agronomic N rates would not force producers to alter application strategies. However, based on this risk assessment, biosolids over-application would force land application rates to be based on crop P requirements. Previous results showed that a minimum of 3 cropping cycles were necessary to reduce soil P concentrations to levels considered less likely to cause environmental degradation. A future reduction in water availability may force some Idaho growers to shift from irrigated to dryland cropping systems. Coupled with the increased production of dairy waste, land applicators will need to find new means to protect natural resources under dryland conditions. Results from our studies have the potential to improve nutrient use efficiency and minimize environmental risk associated with dryland organic waste land application. INTRODUCTION Between 2001 and 2005, the number of dairies in Idaho decreased by 15%; however, the number of dairy cows increased by nearly 24% (Holley and Church, 2006). Results imply that confined animal feeding operations (CAFOs) are becoming larger. Concurrently, the demand for water in the Western US has increased due to a number of factors such as drought, industrial demand, and population growth. Thus, in the future, Idaho producers will most likely be asked to provide high-yielding, high-quality crops to support the dairy industry, while being faced with a reduction in available water. Many reduced-water use or dryland crops will be grown on soils in close proximity to CAFOs. These soils will most likely receive greater quantities of animal waste as compared to soils located at greater distances from the CAFO, forcing producers to follow strict nutrient management plans. Best management practices will be coupled with waste applications based on agronomic requirements of crops to be grown. Organic waste beneficial-use programs have been shown to effectively recycle plant nutrients when applied at agronomic rates. The USEPA (1993) 40 CFR Part 503 regulations for beneficial use of biosolids (sewage sludge) promotes recycling of this material on some crop lands since it is an excellent source of several plant nutrients such as N and P. However, mismanagement of N and P can lead to environmental issues such as ground/surface water contamination and waterway eutrophication. Thus, nutrient availability, transport, and fate concerns have arisen when organic wastes such as biosolids have been applied to dryland agroecosystems. For continuous land application programs under dryland conditions, an important environmental quality and protection question is: How much N will be supplied by biosolids? Short and long-term answers to this question are important because most states require biosolids, as with other organic wastes, to be applied at the agronomic rate for N. And, because biosolids organic N concentrations are much greater than inorganic N forms, one may ask the following questions: What is the apparent first-year N mineralization rate in dryland agroecosystems? What is the first-year N mineralization rate during years of increased precipitation as compared to that during years of drought? In some situations, application of biosolids at an agronomic-rate must be based on P rather than N availability. For example, concerns about agricultural-P pollution of surface water prompted the state of Maryland to require P-based agronomic rates (Shober and Sims, 2003). Applying agronomic rates of biosolids and many other organic materials based on N equivalency leads to soil P accumulation, since the P amounts applied exceed crop removal (Shober and Sims, 2003). Consequently, tracking labile P levels is crucial in any organic waste beneficial-use program. Because organic waste application based on N equivalency tends to oversupply P, one may ask the following questions: Is the excess soil P an environmental concern? If errors were made in agronomic rate calculations and over-application occurred, what would the repercussions be in terms of excess soil P? The objectives of this research were to assess the 12-year or 20-year impact of repeated, increasing biosolids applications on the: 1) biosolids N equivalency; 2) N mineralization rate of biosolids applied to a dryland winter wheat fallow agroecosystem over 6 years of above average precipitation, 6 years of below average precipitation, and over the 12 year study period; 3) total recovery of biosolids applied P; 4) dominant inorganic soil P phases; and 5) potential of increased environment risk of P when applying an agronomic N rate or excessive rate of biosolids. MATERIALS AND METHODS Nitrogen Field Study The N field study began in the summer of 1993 on plots approximately 20 miles east of Brighton, CO. From 1993 to 2004, anaerobically digested biosolids were collected prior to land application and analyzed for organic N, ammoniacal nitrogen (NH4-N), and nitrate nitrogen (NO3-N) (Table 1). Every year, biosolids were hand-applied to 6 ft by 56 ft plots at rates equal to 0, 1, 2, 3, 4, and 5 dry tons A -1 , hand raked to improve uniformity, and incorporated to a depth of ~ 8 inches. Every year, urea fertilizer (46-0-0) was hand applied to non-biosolids plots at rates of 0, 20, 40, 60, 80, and 100 lbs A -1 . These rates bracket those commonly used on dryland wheat in Colorado (typically 2-3 dry tons biosolids A -1 ; 40 lbs N fertilizer A -1 ). Four replications of all treatments were used in a randomized complete block arrangement. Grain samples were collected from all cropping years, and grain N concentration was determined by dividing protein content found with a Dickey John GAC III ® near infra-red analyzer by 5.7. Grain N uptake (Nu) was determined by Nu = Nc*Y*1000, where Nc = grain N concentration and Y = grain yield. Linear regression analyses were then completed for the effects of biosolids and N fertilizer rates on Nu for each harvest, and for the total grain yield and cumulative Nu for the first and second 6-year period, and over the 12 yrs of study. The first 6-year period (1993-1998) were years of above average precipitation; the second 6-year period (1999-2004) experienced drought conditions. We took the average intercept for each material’s linear regression model and completed a second set of regression analyses where the intercept for the biosolids and the N fertilizer models were set to the average intercept of both. This approach allowed us to equate the N fertilizer to the biosolids regression equation. Nitrogen fertilizer equivalency (NE) was then found by calculating the ratio of the slope of the biosolids curve to the slope of the N curve as: NE = Bslope/Nslope. Plant available N (Np) from the USEPA (1983) calculation was next determined, assuming an application rate of 1 dry ton biosolids A -1 and a first-yr mineralization rate of 20%: Np = [NNO3 + Kv(NNH4) + 0.20(No)] + residual, where NNO3 = biosolids NO3-N content, Kv = NH4-N volatilization factor (assumed to be a range of 0 for complete loss to 1.0 for complete recovery of NH4-N), NNH4 = biosolids NH4-N content, No = biosolids organic N content, and residual = residual No from previous two biosolids applications. Using NE, Np, and assuming a 20% first-yr mineralization rate (USEPA, 1983), the effective N mineralization rates (Mr) for the first and second 6-year periods, and over the entire 12-year period was determined using Mr = (NE*0.20)/Np. Phosphorus Field Study The P field study began in the summer of 1982 on plots approximately 15 miles east of Brighton, CO. Every other year from 1982 to 2002, anaerobically digested biosolids were handapplied to 12 ft by 56 ft plots at rates equal to 0, 3, 6, 12, and 18 dry tons A -1 , hand raked to improve uniformity, and incorporated to a depth of 8 inches. Biosolids were not applied in 1998 due to a potential land sale. Biosolids were collected every year prior to land application, and analyzed for total P (Table 1). The 18 dry tons A -1 application rate was discontinued in 1992 because it was deemed excessive in terms of many soil parameters (N, P, micronutrients). In a previous study, we utilized the 18 dry tons A -1 plots to determine the time necessary for P to be reduced to concentrations which would lower environmental risk (Barbarick and Ippolito, 2003). Four replications of all treatments were used in a randomized complete block arrangement. Yearly and cumulative masses of biosolids-borne P applied for each application rate were determined (i.e. P inputs). Yearly wheat grain samples were collected, digested with concentrated HNO3, and analyzed for P. Yearly and cumulative masses of grain-P removed were determined based on P content and yield of grain. Phosphorus contained within wheat straw was assumed to be returned to the soil during conventional tillage practices. The potential soil P accumulation was estimated as the difference between the amount of biosolids P added and the amount of P removed in grain. Soil samples were collected from the 0-8 and 8-24 inch depths from each plot following the 2003 wheat harvest. Soils were air-dried, crushed to pass a 0.08 inch sieve, and total P determined using a 4 M HNO3 digest. The actual increase in total soil P (0to 24 inch depth) was calculated from the difference between the 1982 and 2003 total soil P concentrations. Dominant inorganic soil P mineral phases (soluble, aluminum (Al)-bound, iron (Fe)-bound, occluded, calcium (Ca)-bound) were determined in the soil surface (0-8 inches) and subsurface (8-24 inches). Finally, a P risk index assessment was used, based on, among other factors, soil AB-DTPA test P, to determine P risk associated with agronomic or excessive biosolids application rates. Values for soil AB-DTPA test P are typically half of soil Olsen test P values commonly used for Idaho soils. RESULTS AND DISCUSSION Nitrogen Field Study Twelve years of biosolids applications produced N equivalencies, based on winter wheatgrain N uptake, of about 20 lbs N A -1 (Table 1). A dryland winter wheat crop typically requires about 40 lbs N A -1 ; thus, approximately 2 dry tons biosolids A -1 would meet the crop N needs. Estimated biosolids first-year N mineralization rates over 6 years of above average precipitation were 25 to 32%, while over 6 years of below average precipitation were 21-27%. Over the 12 year study period, estimated first year N mineralization rates ranged from 27-33%. In Washington, Cogger et al. (1998) found that dryland winter wheat recovered 11to 44% of biosolids-borne N. Using 12-week laboratory incubations, Lerch et al. (1992) found a 55% mineralization for the L/E biosolids. He et al. (2000) reported 48% N mineralization from pelletized biosolids. Results from our research can aid land applicators in determining first-year N release from this organic waste under dryland conditions. Erring on the side of conservatism (i.e. greater first-year N mineralization), organic waste applicators could calculate and supply the crop N needs while protecting the environment against off-site N transport. Table 1. Organic N, NH4-N, and NO3-N in biosolids applied from 1993 to 2004 on the N field study plots. Biosolids-borne P applied from 1982 to 2003 on the P field study plots.