Urea hydrolysis and inorganic N in a Luvisol after application of fertiliser containing rare-earth elements

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1 X. Xu CSIRO PUBLISHING Australian Journal of Soil Research, 23, 41, Short Communication Urea hydrolysis and inorganic N in a Luvisol after application of fertiliser containing rare-earth elements Xingkai Xu A,B,D, Zijian Wang B, Yuesi Wang A, and Kazuyuki Inubushi C A State Key Laboratory of Atmospheric Boundary Layer Physics and Atmospheric Chemistry, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 129, China. B State Key Laboratory of Environmental Aquatic Chemistry, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 185, China. C Laboratory of Soil Science, Faculty of Horticulture, Chiba University, Matsudo, Chiba , Japan. D Corresponding author; Present address: Laboratory of Soil Science, Faculty of Horticulture, Chiba University, Matsudo, Chiba , Japan; xingkai_xu@hotmail.com Abstract In recent decades, Chinese agriculturists have used rare-earth-containing fertilisers as basal fertilisers together with N fertilisers (e.g. urea). We studied urea hydrolysis and its hydrolysis products in a laboratory experiment using urea-n fertiliser with rare earths at rates from.5 to 5% (w/w). The results indicated that application of rare earths at a high rate could result in a short-term inhibition of urea hydrolysis and an + increase in soil (NH 4 + NO 3 + NO 2 )-N content. When the application rate of rare earths was higher than 5% of the applied urea-n (corresponding to 1 mg/kg soil), soil exchangeable NH + 4 -N content increased significantly following the hydrolysis of the applied urea. Increasing the application rate of rare earths appeared to reduce the content of soil urea-derived (NO 3 + NO 2 )-N. A substantial reduction in soil ph was found immediately after application of rare earths and urea. We conclude that application of rare earths at >1 mg/kg may lead to a substantial increase in the content of urea-derived N in the soil, via the inhibition of urea hydrolysis and nitrification. SR256 Efect et al. of rare earths on ur ea-derived N in soil Additional keywords: nitrification, rare earths, urea. Introduction China contains around 8% of the world s resource of rare earths and is a major producer of them for the world market (Brown et al. 199). Considerable work has shown that the application of rare earths at a suitable rate can improve crop growth and production, as well as resistance to poor growing conditions (e.g. water deficit) (Brown et al. 199; Yu and Chen 1995; Xu 1997; Maheswaran et al. 21). In recent years, millions of tons of fertilisers containing rare earths have been used world-wide to enhance agricultural production (Yu and Chen 1995; Zhu 1999). The widespread application of rare earths may lead to accumulation in soil. This can affect the microbial activity of the soil (Chen and Wang 1986; Wang et al. 1997; Xu and Wang 21). Much attention was paid to the accumulation of rare earths in crops after agricultural application of rare earths (Xu et al. 22, 23). However, so far few experiments have reported the effects of rare earths on available N content in upland soils (Wang et al. 1997; Liu and Wang 21). Upon application of rare earths, the activity of soil urease was reduced substantially (Chen and Wang 1986; Wang et al. 1997; Liu and Wang 21), and the mineralisation of nitrogen and ammonium oxidation in soil weakened (Xu and Wang 21). These studies mainly focussed on the behaviour of native soil nitrogen after application of rare earths. Combined applications of rare earths and N-fertilisers (e.g. urea) are being used in Chinese agriculture to improve crop growth and production (Yu and Chen 1995; Guo 1999; Wen et al. 2). However, little is known of the behaviour of applied fertiliser-n (e.g. urea) in soils after addition of rare earths. Hence, it is important to assess the interaction between rare earths and N-fertilisers in agricultural soils. CSIRO /SR /3/4741

2 742 Aust. J. Soil Res. X. Xu et al. This paper reports the hydrolysis of urea and its hydrolysis products in a soil to which rare earths were added as chloride. The objective of this study is to provide some understanding of the interaction between rare earths and urea. Materials and methods Soil characteristics Soil for this research was a composite surface soil (2 cm) from an upland field near Beijing, China. The soil is a non-calcareous Luvisol (FAO Soil Classification). Main physical and chemical properties of the soil, measured according to Kim (1995), are: total N.1%, total P.86%, total C 1.45%, available N 67.5 mg/kg, available P 72.5 mg/kg, ph 7.2 (soil:water, 1:5), CEC 19.8 cmol/kg, sand 24%, silt 42%, and clay 36%. The surface soil sample was slightly air-dried, and crushed to <5 mm and thoroughly mixed before use. Preparation of solution containing rare earths The rare earths used in the study came from one of the fertilisers containing rare earths, and this was supplied by the Research Center for Agricultural Application of Rare Earths in China. It is a mixture of rare earths as their nitrates, and has been used in Chinese agriculture. Normally, this mixture is made by extracting the rare earths from their ores using nitric acid, and more than 6% of the mixture is thus in nitrate form and is soluble (Brown et al. 199). To remove nitrate from the mixture, a 4-g portion was dissolved in distilled water and the ph was adjusted to 5.5. Then 4 ml of 4 M oxalate solution was added, and the precipitate formed was filtered and washed with distilled water. The precipitated rare earths oxalate was transformed into oxides in a muffle furnace at 7 C for 4 h, and the oxides were then dissolved in 2 ml hydrochloric acid (1:1). After dissolution, the ph of the solution was adjusted to 5.5 with.1 M HCl and 3 M NaOH, and diluted to 1 ml with distilled water. The prepared stock solution was stored at 4 C before use. The constituents and concentrations of rare earths in the stock solution were analysed by inductively coupled plasma-mass spectrometry (VG PlasmaQuard III, Fisons Instruments, UK). The instrument was operated at a sampling rate of 1. ml/min with a measuring time of 4 s. Indium (In 115 ) was used as an internal standard for calibrating the instrument. The constituents and concentrations of rare earths in the stock solution are shown in Table 1. Soil incubation and application of rare earths The stock solution was diluted to a series of solutions that contained 7.5, 75, 375, and 75 mg rare earths/l, respectively. The ph of each solution was adjusted to 5.5 with.1 M HCl and 3 M NaOH. A 15-g sample of soil in a 25-mL Erlenmeyer flask was treated with a 1-mL solution containing mg of urea (approx. 2 mg N/kg dry soil) and with 2 ml of a solution (ph = 5.5) containing, 7.5, 75, 375, and 75 mg rare earths/l, respectively. The solutions were added drop-wise to moisten the whole soil and mixed thoroughly. Each treatment contained 9 replicates and corresponded to doses of, 1, 1, 5, and 1 µg/g of rare earths in dry soil, respectively. In a control, 1 ml distilled water was used instead of 1-mL solution containing urea, and there was no addition of rare earths. Over a period of 3 days, the soil moisture content was brought up to 2% (approx. 75% field capacity), and the flask was covered to prevent loss of water. Samples were incubated at 25 C. At 12, 24, and 72 h after addition of urea, triplicate samples of each treatment were withdrawn from the incubator. Urea-N and inorganic N, including NH + 4 -N, NO 3 -N, and NO 2 -N in the soil were extracted with 2 M KCl solution containing 5 µg/ml of phenymercuric acetate (soil:solution = 1:1), which can prevent further hydrolysis of urea. The slurry was shaken for 1 h before filtering. Urea was determined by the modified diacetyl monoxime method (Mulvaney and Bremner 1979). Both NH + 4 -N and (NO 3 + NO 2 )-N in the soil extract were measured via titration after steam distillation in the presence of MgO and Devarda s Table 1. Constituents and concentrations of rare earths in the stock solution (mg/l) Y La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Total

3 Effect of rare earths on urea-derived N in soil Aust. J. Soil Res. 743 alloy, and the NO 2 -N by the diazotisation and coupling reaction method (Kim 1995). Fresh soil ph (soil:water = 1:1) was measured by a combination glass electrode and ph meter, over a period of 3 days. To study the effects of the rare earths alone on native soil mineral-n levels, a 15-g sample of the soil in a 25-mL Erlenmeyer flask was treated with 2 ml of a solution containing, 7.5, 75, 375, and 75 mg rare earths/l, respectively. The solutions were added drop-wise to moisten the whole soil and mixed thoroughly. Five treatments each contained 6 replicates and corresponded to doses of, 1, 1, 5, and 1 µg/g of rare earths in dry soil, respectively. Over a period of 3 days, the soil moisture content was brought up to 2%, and the flask was covered to prevent loss of water. Samples were incubated at 25 C. At 24 and 72 h after addition of the rare earths, triplicate samples of each treatment were withdrawn from the incubator. The contents of soil NH 4 + -N and (NO 3 + NO 2 )-N and ph values in different treatments were measured as mentioned above. Calculation and statistical analysis All data were transformed to a dry weight basis. The percentage recovery of the applied urea was calculated from 1 (E C)/F, where E is the urea found in the soil extract, C is the urea in the control, and F the applied urea. Contents of soil urea-n, urea-derived NH + 4 -N, and (NO 3 + NO 2 )-N could be calculated through differences in amount of soil urea-n and inorganic N between the treatment with urea and rare earths, and the control. All data were subjected to analysis of variance, and the means and standard errors were calculated. Significant differences of means among all treatments were analysed by t-test, at a significance level P =.5. Results and discussion Effect of addition of rare earths on urea hydrolysis At 24 h after addition of urea, remaining urea-n recovery in any treatment was less than 1%, indicating a rapid hydrolysis of the applied urea in the soil (Fig. 1). At 12 h after fertilisation, increasing the application rates of the rare earths appeared to enhance remaining urea-n recovery in the soil (Fig. 1). When the application rates of rare earths were higher than 5 mg/kg, a significant increase in the remaining urea-n recovery was found at 12 h, compared with the treatment with urea alone (P <.5). However, at 24 h and 72 h no obvious differences in the N recovery were found among all treatments (Fig. 1). Hence, the use of the rare earths might result in a short-term inhibition of urea hydrolysis in the soil. Normally, the adsorption of rare earths by the soil is rapid (Jones 1997), and they are not easily exchanged by other ions. For a neutral soil with a high CEC value, this adsorption would substantially reduce the active content of rare earths in soil, which may affect the microbial activity of the soil. Soil urease activity appeared to decrease as application rates 6 Urea remaining (% of applied) Urea + Urea + 1 Urea + 1 Urea + 1 Urea + 5 Fig. 1. Remaining urea-n recovery in the soil after addition of rare earths. Vertical bars represent standard errors. Different treatments are shown as urea + application rate of added rare earths (mg/kg of rare earths in dry soil) Time after addition of rare earths and urea (h)

4 744 Aust. J. Soil Res. X. Xu et al. of rare earths increased (Chen and Wang 1986; Liu and Wang 21). The inhibition of urease activity in the soil decreased with time upon application of rare earths (Liu and Wang 21). A substantial increase in soil ph was present immediately following the hydrolysis of the applied urea, thus importantly reducing the active content of rare earths in soil. These effects partly explain a short-term inhibition of urea hydrolysis in the soil upon application of rare earths. Effect of addition of rare earths on soil ph After 12, 24, and 72 h incubation, increasing the application rates of rare earths reduced soil ph upon urea application; the trend was most obvious at 12 h (Fig. 2). A reduction in soil ph after application of the rare earths could be expected to reduce N loss as NH 3. In the absence of urea, no obvious variations in soil ph were found after application of the rare earths alone (data not shown) as a series of added solutions with the same ph. Hence, the variation in soil ph after application of rare earths and urea could result from the effects of the rare earths on the hydrolysis of urea and the nitrification of NH + 4 -N formed in the soil. Before the incubation the starting ph value of the soil was 7.2. At 12 h after application of rare earths and urea, the ph value was above 8.2, due to the hydrolysis of the applied urea in the soil (Fig. 2). When the application rates of the rare earths were higher than 1 mg/kg, the soil ph upon urea application was highest at 24 h (Fig. 2). At an application rate of 1 mg/kg of rare earths in dry soil, the highest value of soil ph as shown in the treatment with urea alone was found at 12 h. This apparently shows that addition of the rare earths at a high rate may retard the hydrolysis of the simultaneously applied urea in the soil. This could lead to urea leaching under upland conditions to some extent and the retention of urea-derived N in soils. At 72 h after urea application, a larger reduction of soil ph than at + 24 h was present (Fig. 2), probably due to the nitrification of the NH 4 formed (Lu et al Urea + Urea + 1 Urea + 1 Urea + 5 Urea Soil ph Time after addition of rare earths and urea (h) Fig. 2. Variation of soil ph after addition of rare earths and urea. Vertical bars represent standard errors. Different treatments are shown as urea + application rate of added rare earths (mg/kg of rare earths in dry soil).

5 Effect of rare earths on urea-derived N in soil Aust. J. Soil Res. 745 NH 4 + -N (mg N/kg soil) (a) 12 h 24 h 72 h 7 7 (b) (NO 3 + NO 2 )-N (mg N/kg soil) h 24 h 72 h Fig. 3. Contents of soil urea-derived (a) exchangeable NH + 4 -N, (b) (NO 3 + NO 2 )-N, and (c) available inorganic N at 12, 24, and 72 h after addition of rare earths and urea. Vertical bars represent standard errors. Available inorganic N (mg N/kg soil) (c) 12 h 24 h 72 h Added rare earths (mg/kg rare earths in dry soil) 1995; Zhu et al. 1997). Although the use of the rare earths at high doses could inhibit nitrification to some extent (Xu and Wang 21), retarding the hydrolysis of the simultaneously applied urea might substantially affect soil ph over a period of 3 days. This can partly explain why soil ph at 72 h is lowest in the 2 treatments causing inhibition of nitrification (Fig. 2). Effect of addition of rare earths on inorganic N accumulation Figure 3 shows the effects of the rare earths on soil urea-derived inorganic N. At the beginning of the incubation, NH + 4 -N was the main product of urea hydrolysis (Fig. 3a). Gradually (NO 3 + NO 2 )-N was formed, due to the nitrification process (Fig. 3b).

6 746 Aust. J. Soil Res. X. Xu et al. NH 4 + -N (mg N/kg soil) (a) 24 h 72 h 25 (b) (NO 3 + NO 2 )-N (mg N/kg soil) Fig. 4. Contents of soil (a) exchangeable NH + 4 -N, (b) (NO 3 + NO 2 )-N, and (c) available inorganic N at 24 and 72 h after application of rare earths alone. Vertical bars represent standard errors. Available inorganic N (mg N/kg soil) (c) Added rare earths (mg/kg rare earths in dry soil) The rate of urea hydrolysis clearly affected the amount of inorganic N that was derived from the applied urea. Figure 3a shows that the NH 4 + -N accumulated in the soil after application of rare earths and urea. The highest NH 4 + -N content in any treatment was found at 24 h after the incubation, which was in accordance with a rapid hydrolysis of the applied urea (Fig. 1). At 12, 24, and 72 h, increasing the application rates of the rare earths might enhance the NH 4 + -N content in the soil (Fig. 3a). This is in contrast to the variation in soil (NO 3 + NO 2 )-N content after addition of rare earths and urea (Fig. 3b). The consistently higher NH 4 + -N content in the treatment with rare earths and urea may seem to be

7 Effect of rare earths on urea-derived N in soil Aust. J. Soil Res. 747 Table 2. Soil NO 2 -N formation after addition of urea and rare earths (mg N/kg soil) Values in the table are means of 3 replicates, and standard errors are shown in parentheses. Within each column, means followed by the same letters are not significantly different by t-test at P =.5 Added rare earths Time after addition of urea (h) (mg/kg) (rare earths in dry soil) (.18)d 6.43(.19)c 37.1(2.81)b (.26)d 6.51(.33)c 36.74(2.55)b (.15)c 6.3(.31)bc 37.56(1.25)b (.4)b 5.43(.21)b 35.5(2.29)b 1.53(.9)a 4.5(.8)a 29.85(.15)a contradictory, because upon application of the rare earths one might observe a short-term inhibition of urea hydrolysis. An inhibition of nitrification of the NH 4 + -N formed (Fig. 3) would partly explain the contradiction. In addition, in the absence of urea, application of the rare earths at >5 mg/kg significantly enhanced the content of native soil NH 4 + -N, and the nitrification was inhibited (Fig. 4a, b). When the application rates of rare earths increased up to 1 mg/kg, at 12 h the accumulation of soil NO 2 -N significantly decreased (P <.5) (Table 2). Furthermore, at 72 h the NO 2 -N content in the soil still remained the lowest at a rate of 1 mg/kg (P <.5). The reduction of the NO 2 indicated that the first step of the nitrification process was more significantly inhibited than the second step, when the rare earths were added to the soil. Xu and Wang (21) showed that at a high rate, the use of rare earths might effectively inhibit the oxidation of exogenous NH 4 + -N in the soil slurry. Figure 4b shows that application of rare earths at a high rate can significantly reduce the formation of native soil (NO 3 + NO 2 )-N. Figure 3c shows the available inorganic N accumulation in the soil amended with rare earths. In all treatments, the content of soil urea-derived inorganic N was 23-fold greater at 24 h than at 12 h. However, a minor variation for all treatments in the N pool was found from 24 h to 72 h. This completely resulted from the hydrolysis of the applied urea in the soil (Fig. 1). Over a period of 3 days, increasing the application rates of rare earths enhanced the content of soil urea-derived inorganic N (Fig. 3c). At the application rate of 1 mg/kg of rare earths in dry soil, a substantial increase in the soil urea-derived inorganic N was found (P <.5), in comparison with that in the control. In the absence of urea, increasing the application rates of rare earths enhanced contents of native soil inorganic N (Fig. 4c). Under the experimental conditions, contents of soil urea-derived inorganic N were calculated via differences in amount of soil inorganic N between the treatment with urea and rare earths, and the control. Hence, the content of soil urea-derived inorganic N was, to some extent, overestimated after addition of rare earths, especially at the rate of 1 mg/kg. At a high rate, the use of the rare earths can perhaps cause inorganic N release from soil microbes. In conclusion, application of the rare earths at >1 mg/kg might induce a short-term inhibition of urea hydrolysis in the soil, and retard the nitrification of the NH 4 + -N formed. Total inorganic N increased as the application rates of rare earths increased. A substantial reduction in soil ph occurred after 72 h in all treatments. We conclude that application rate of rare earths at >1 mg/kg can lead to a substantial increase in the content of urea-derived

8 748 Aust. J. Soil Res. X. Xu et al. N in the soil. This suggests that the combined application of rare earths and N-fertilisers (e.g. urea) might enhance total inorganic N. Acknowledgments This project was financially supported partly by the Knowledge Innovative Program of the Chinese Academy of Sciences (approved no. KZCX1-SW-1B) and by the National Natural Science Foundation of China (agreement No and 29974). We also acknowledge financial support from the Fengqiu Agro-Ecological Experimental Station of the Chinese Academy of Sciences (agreement No. 2165) and from the K. C. Wong Education Foundation (Hong Kong). References Brown PH, Rathjen AH, Graham RD, Tribe DE (199) Rare earth elements in biological systems. In Handbook on the physics and chemistry of rare earths. (Eds KA Jr Gschneidner, L Eyring) Vol. 13, pp (Elsevier Science: Amsterdam) Chen ZZ, Wang XM (1986) Effect of rare earth element on soil enzyme activities and relationship between soil enzymes and rhizosphere nutrition of winter wheat planted on yellow brown earth. Journal of Anhui Agricultural College 21(suppl.), Guo BS (1999) Recent research advance of rare earth in the field of biology. Chinese Rare Earth 2, Jones DL (1997) Trivalent metal (Cr, Y, Rh, La, Pr, Gd) sorption in two acid soils and its consequences for bioremediation. European Journal of Soil Science 48, Kim HT (1995) Soil sampling, preparation and analysis. (Marcel Dekker: New York) Liu DF, Wang ZJ (21) Influence of rare earth elements on chemical transformation of nitrogen in agricultural soil. Chinese Journal of Applied Ecology 12, Lu RS, Shi ZY, Lai QW (1995) Research on nutrients degeneration in red soil (II)-transformations of urea and ammonium carbonate in red soil. Chinese Journal Soil Science 26, Maheswaran J, Meehan B, Reddy N, Peverill K, Buckingham S (21) Impact of rare earth elements on plant physiology and productivity. (Rural Industries Research & Development Corporation: Victoria) Mulvaney RL, Bremner JM (1979) A modified diacetyl monoxime method for colorimetric determination of urea in soil extracts. Communication in Soil Science and Plant Analysis 1, Wang ZJ, Lu P, Liu DF (1997) Initial assessment of the effects of applying rare earths on soil integrity. In Proceedings of the 2nd Sino-Dutch Workshop on the Environmental Behaviour and Toxicology of Rare Earth Elements and Heavy Metals. (Eds R Guicherit, WZ Zhu) pp (The Netherlands Organization for Applied Sciences: Delft) Wen HY, Peng RZ, Chen XW (2) Application of rare earth compound fertilizer in some crops in central Yunnan. Chinese Rare Earths 21, 554. Xu XK (1997) Application of the rare-earth fertilizers for agricultural sustainable development. Research Development for Resource, Environment and Ecology Network 8, Xu XK, Wang ZJ (21) Effect of lanthanum and mixtures of rare earths on ammonium oxidation and mineralization of nitrogen in soil. European Journal of Soil Science 52, Xu XK, Zhu WZ, Wang ZJ, Witkamp GJ (22) Distributions of rare earths and heavy metals in field-grown maize after application of rare earth-containing fertilizer. The Science of the Total Environment 293, Xu XK, Zhu WZ, Wang ZJ, Witkamp GJ (23) Accumulation of rare earth elements in maize plants (Zea mays L.) after application of mixtures of rare earth elements and lanthanum. Plant and Soil 252, Yu Z, Chen M (1995) Rare earth elements and their application. (Metallurgical Industry Press: Beijing) Zhu WZ (1999) Advanced inductively coupled plasma mass spectrometry analysis of rare earth elements: environmental application. (AA Balkema Publishers: Rotterdam) Zhu ZL, Wen QX, Freney JR (1997) Nitrogen in soils of China. (Kluwer Academic Publishers: Dordrecht, The Netherlands) Manuscript received 1 May 22, accepted 11 December 22