Influence of soil-urea ratio on N2O emissions and nitrite accumulation in a calcareous silt loam soil   Dongli Liang1, Richard Engel2, and Rosie Wallander2 (1College of Resources and Environment, Northwest A & F University, Yangling, Shaanxi, China, 712100) (2Department of Land Resources and Environmental Sciences, Montana State University, Bozeman, Montana, 59717) Abstract Results and discussion Nest placements of fertilizer are frequently used in developing countries for the production of some crops. Previous research has indicated that placement of urea prills into concentrated zones (e.g. nests) may enhance N2O emissions over more diffuse applications (e.g. broadcast). Two studies were established to better understand this effect. In Experiment I, urea prills (1.3 g) were added to PVC pots containing 6.3 kg Amsterdam silt loam (pH 7.8) soil in a single (SN), double (DN), and triple nest (TN) arrangements, plus a broadcast placement to create a gradient of decreasing fertilizer to soil dilutions. In Experiment II, urea prills were added to jars (930 mL) containing 500 g of Amsterdam soil at six rates (0, 46, 92, 184, 368, 736 mg N kg-1). Maximum N2O fluxes were similar for the three nests and broadcast in Experiment I, but cumulative emissions were enhanced as the ratio of fertilizer to soil in the nest became more concentrated. Over the 46 d experimental period cumulative emissions were equivalent to 45.2, 36.5, 25.0 and 17.5 mg N2O-N m-2 for the SN, DN and TN, and broadcast treatments, respectively. In Experiment II, cumulative N2O emissions increased with urea rates as anticipated and were highly correlated with soil nitrite concentrations. Small amounts of nitrite were present in the soil up to 6 d following fertilization in the 46, 92, 184, 368 mg N kg-1 treatments. For the 736 mg N kg-1 rate, considerable nitrite (15-206 ug NO2-Ng-1 soil) was present over the entire 21 d incubation period. The results of this study indicate applying a high urea rate to a small soil volume, i.e. nest, may lead to inhibition of nitrifying bacteria. Under these conditions N2O emissions and fraction of applied fertilizer N lost as N2O are enhanced over diffuse urea applications. Experiment I: N2O production from urea placements and fertilized controls Introduction Fig. 2 N2O fluxes over the experimental period from (a) different urea placements (double nest omitted for clarity), and (b) fertilized controls. Note difference in scales! Nitrogen fertilizer inputs have been identified as a major source of N2O emissions from agricultural soils (Bouwman, 1990; Clayton et al., 1997; Mosier and Kroeze, 2000). Trends in fertilizer N use suggest this affect will continue into the future. Globally, fertilizer N consumption increased from 31.8 million MT in 1970/1971 to 90.9 million MT in 2005/2006 (IFA, 2007). Fertilizer N use has been projected to grow to 115.3 million MT by 2020 (Bumb and Baanante, 1996) as a result of a growing world population and the need to secure food production to meet the rising demand. Much of the recent growth in world N fertilizer consumption occurred in developing countries, and was accounted for by urea. Urea currently comprises approximately 50% of current global N consumption (IFA, 2007). Hand placement of urea in concentrated zones such as nests is a common practice in developing countries for the production of some row crops. Placement of urea in concentrated zones, such as band and nests, has been promoted to improve crop use efficiency (Yadvinder-Singh et al., 1994) but limited work has been done on the impact of this practice on N2O emissions. A previous investigation conducted in our lab indicated that placement of urea in localized areas, i.e. subsurface band and nest treatments, resulted in higher fertilizer induced N2O emissions than broadcast surface and broadcast incorporated treatments. Also, a laboratory investigation by Tenuta and Beuchamp (2000) indicated emissions of N2O from urea were enhanced as granule size was increased. The objective of this study was to compare N2O emissions from a constant urea N amount applied in three different nest configurations (single, double, triple) to simulate varying soil to fertilizer volume ratios (Experiment I), and a second study (Experiment II) where varying doses of urea were applied to a constant soil volume to determine how soil and urea ratio affect soil N2O and nitrification. N2O fluxes rose and returned quickly to background levels for broadcast placement. N2O production was prolonged (SN > DN > TN) by placing urea in more concentrated nests (Fig. 2a). Flux was minimal for NaNO3 indicating nitrification was the primary process responsible for N2O production. The highest flux occurred with NaNO2 (Fig. 2 b). Fig. 5 Soil (a) pH, (b) ammonium-N, (c) nitrite-N, and (d) nitrate-N as affected by different urea rates during 21 day incubation period (n=4). Table 2. Spearman rank correlation coefficients for N2O emissions with several soil parameters for treatments Urea rates (n=28) (mg N kg-1) All (n=168) variable 46 92 184 368 736 pH -0.263 0.889** 0.643** 0.545** 0.472** -0.249 -0.106 NH4+ 0.347 0.969** 0.836** 0.735** 0.790** 0.186 0.801** NO3- -0.201 -0.961** -0.819** -0.843** -0.789** -0.301 NO2- 0.378 ** 0.958** 0.913** 0.947** 0.876** 0.250 0.757** NO3-+NO2- -0.166 -0.956 ** -0.791** -0.797** -0.756 ** -0.152 0.317** Nmin -0.119 -0.621** -0.280 -0.502** -0.410 * 0.115 0.772** Table 1. Accumulation and daily N2O emission for different treatments during experimental period (46 days) Trt Cumulative N2O-N loss Maximum N2O emission Average N2O emission T1/2cumulated N2O emission fertilizer induced emission Applied N as N2O-N mg N m-2 µg N2O-N m-2 d-1 day mg N2O-N m-2 % control 1.4a* 40a 30a 17.2b - NaNO2 64.9d 6377c 1411d 7.4a 63.5d 0.32d NaNO3 1.5a 82a 33a 20.9b 0.10a 0.00a BC 16.9b 2707b 368b 5.67a 15.5b 0.09b SN 44.5c 2284b 967c 18.2b 43.1c 0.22c DN 36.6c 2327b 795c 12.8b 35.2c 0.18bc TN 24.8b 2068b 538b 10.0a 23.4bc 0.12b Methods *,** indicate significant correlations at the 0.05 and 0.01 levels of probability Nmin = NH4+ + NO2- + NO3- Exp I – Greenhouse pot study (1) PVC pots (19.5 cm dia x 30 cm high) – see Figure 1 (2) Soil - Amsterdam silt loam soil (6300 g), pH 7.8 (3) Treatments (7 total) a. urea placements (4) broadcast incorporated (BC) single concentrated nest (SN) double diluted nest (DN) triple diluted nest (TN) b. fertilized controls – broadcast applied (2) NaNO2 NaNO3 c. unfertilized control (4) Fertilizer N rate = 200 kg N ha-1 equivalence, or 1.3 g urea prills/pot (5) Experimental design - 5 replications in a RCBD (6) Soil moisture = 0.21 kg H2O kg-1 soil, distilled water added every 2 days to replace evaporation losses (7) Gas sampling conducted daily basis during experiment (55 d) by placing lids on top of PVC pots and removing 25 mL of head space. Aliquots were transferred to evacuated Extainers (13 mL and gases were analyzed on a GC equipped with a 63Ni electron capture detector. Soil pH significantly increased in the first 3 days for all treatments as a result of urea hydrolysis. Progressively higher pH values were observed on 3 day as urea concentration increased (Fig 5a) The concentration of ammonium diminished with time, but at the 736 mg N kg-1 rate disappearance of ammonium was prolonged (Fig. 5b). The high concentration of urea created conditions restrictive to nitrifying bacteria. This is evidenced further in the soil nitrite vs. time and soil nitrate vs. time profiles (Fig. 5c, d). Significant quantities of soil nitrite were present throughout the entire study at the 736 mg N kg-1 urea rate. Nitrification contributed to the majority of N2O production as it is positively correlated with ammonium and nitrite, and negatively correlated with nitrate (Table 2). For the many of the urea rates, N2O emissions were most strongly correlated with soil nitrite levels. * Values within a column followed by the same letter are not significantly different at P<0.05. N2O emission from unfertilized control and NaNO3 were consistently low throughout the experimental period, while NaNO2 produced the highest N2O emission flux (=278 ug N2O-N m-2 h-1 on day 6) and cumulative N2O loss. Maximum N2O flux activity from urea occurred at 5, 7 10 and 12 days for the BC, TN, DN and SN t treatments, respectively. The magnitude of the maximum emission flux was not significantly affected by placement, but the date of maximum flux activity was delayed for the more concentrated placements. Also, the date when half of the emission production was achieved (T1/2) was delayed by placing urea in a concentrated placement. Cumulative N2O losses from SN and DN were significant higher than from TN and BC. Cumulative N2O loses rose as the ratio of soil to urea decreased because the high concentration of urea prolonged emission activity. References Fig. 1. Schematic of PVC pot cross-section (top). Configure of nest placements (bottom) . All nests are 5 cm below surface and total amount of urea applied is constant (SN=DN=TN). Bouwman AF, 1990. Exchange of greenhouse gases between terrestrial ecosystems and the atmosphere. In: Bouwman A. F. (ed). Soils and the greenhouse effect. John Wiley Sons Ltd. Chichester, p 61-128. Bumb, B. and C. Baanante. 1996. World trends in fertilizer use and projections to 2020. 2020 Brief No. 38 (International Food Policy Research Institute, Washington, DC, USA), World Resources, 1998-99. Available online at http://www.ifpri.org/2020/briefs/number38.htm (verified Sept. 2007). Clayton H, T.P. McTaggar,, J. Parker, L. Swan, and K.A. Smith. 1997. Nitrous oxide emission from fertilized grassland - A year study of the effects of N fertilizer form and environmental conditions. Biol Fertil Soils 25: 252-260. International Fertilizer Industrial Association (IFA), 2007. Total fertilizer consumption statistics by region from 1970/71 to 2005/06 [Online] Available at http://www.fertilizer.org/ifa/statistics/indicators/tablen.asp (verified 1 Sept. 2007). Mosier, A and C. Kroeze, 2000. Potential impact on the global atmospheric N2O budget of the increased nitrogen input required to meet future global food demands. Chemosphere: Global Change Sci. 2:465-473. Tenuta M. and E.G. Beauchamp. 2000. Nitrous oxide production from urea granules of different sizes. J. Environ. Qual. 29: 1408-1413. Yadvinder-Singh, S.S.Malhi, M. Nyborg, and E.G. Beachamp. 1994. Large granules, nest or bands: Method of increasing efficiency of fall-applied urea for small cereal grains in North America. Fert. Res. 38:61-87. Experiment II: Urea-N:soil ratios (jar study) Experiment II - Lab jar study Urea-N was applied at six rates (0, 46, 92, 184, 368, and 736 mg N kg-1 soil) to Mason canning jars (1 L) containing 500 g of dry Amsterdam soil and packed to a bulk density of 1.40 g cm-3 to create different ratios of soil to urea. (2) Soil moisture = 0.21 kg H2O kg-1 soil, kept constant by placing parafilm over jars with holes for gas exchange (3) Experimental layout - the experiment began with 168 jars or 28 replicates of each treatment. (4) Beginning 3 d following fertilization, 24 jars (6 treatments x 4 replicates) were removed for destructive sampling and soil NH4+-N, NO2--N, NO3--N and pH were analyzed. This process was repeated every 3 d for a total of 7 times over 21 d. (5) Gas samples were collected daily from 24 jars for N2O and CO2 analysis over the 21 d incubation period. Fig. 3 Soil N2O emissions from different urea rates Fig. 4 The relationship between maximum N2O emissions or mean N2O emission and urea-N rates N2O emission fluxes increased and the maximum N2O flux was delayed with increasing urea-N:soil ratios. N2O production is particularly prolonged at the highest urea-N rate (Figure 3) The magnitude of the maximum emission flux increased linearly with urea-N rate (Figure 4 – gray circles) Mean emission flux over 55 days increased with urea-N rate according to a quadratic relationship (Figure 4 – red circles). The higher production of N2O is particularly apparent above 368 mg urea-N kg-1. This indicates that a progressively higher fraction of fertilizer N is lost as N2O as urea-N concentrations exceed this level. Acknowledgments The authors wish to express their thanks to the USDA - National Research Competitive Grants Program for supporting our work on trace gas emissions from soils (Air quality program: 2004-35112-14233; Soil and soil biology program: 2004-35107-14951).