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Study on Soil Denitrification in Wheat-Maize Rotation System
Available online at www.sciencedirect.com Agricultural Sciences in China 2006, 5(1): 45-49
January 2006
Study on Soil Denitrification in Wheat-Maize Rotation System ZOU Guo-yuanl.2,ZHANG Fu-suo2, JU Xiao-tang2, CHEN Xin-ping2 and LIU Xue-jun2 1
Institute of Plant Nutrition and Natural Resources, Beijing Academy of Agricultural and Forestry Sciences, Beijing 100089, P.R.China Department of Plant Nutrition, China Agricultural University, Beijing 100094, P.R.China
Abstract Soil denitrification was studied in wheat-maize rotation cropping system on an aquic cambisol. Results showed that the N loss amount by denitrification ranged from 4.7 to 9.7 kg per hectare with different levels of nitrogen application and the key stage for denitification was during summer maize-growth-period, especially within 1-2 weeks after fertilizer nitrogen was applied. Similar trend was found between soil N,O production/emission dynamic and denitrification dynamic in the rotation system, which may indicate that mainly N,O is produced in nitrification process. Key words: aquic cambisol, winter wheat, summer maize, denitrification, N,O
INTRODUCTION Ammonia volatilization, nitrification-denitrificationand nitrate leaching are main pathways of nitrogen losses. Nitrate leaching in Northern China mainly happens in seasons with heavy precipitation (Yuan et al. 1995), while ammonia volatilization mainly takes place within a short time after nitrogen fertilization and the amount of nitrogen loss is quite low when fertilizer nitrogen is applied deeply. In the arid areas the percentage of N loss by denitrification will account for the ratio of total N loss, which not only created a lot of interest from soil researchers but also caused a lot of interest from environmental researchers. Some results of different soils from Hebei, Henan and Heilongjiang provinces showed that N loss amount by nitrification-denitrification was no more than 10 kg ha-' (Zhang et al. 2001; Ding et al. 2001; Ding et al. 2004; Ju et al. 2002). The earlier results from Beijing area also could not support the idea that denitrification makes a considerable contribution to the total N loss. In order to demonstrate the
dynamics and rate of denitrification, and their related controlling factors, a continual measurement of denitrification in a.n aquic cambisol in Beijing had been carried out with acetylene inhibition method in a soil core incubation system. As a greenhouse effect gas, N,O originates from the soil mainly from the nitrification-denitrification.Related papers on N,O emission from arid soil in northern China have been frequently reported (Huang et al. 1998; Wang et al. 1994; Wang et al. 1998; Wang et al. 2001). Measurement of denitrification and N,O simultaneously will help to deepen the knowledge of sources of N,O emission, and there are several groups in China working on related areas (Zhang et al. 2001; Ding et al. 2001; Ding et al. 2004; Zou et al. 2001). As a part of above-mentioned study this paper will compare and analyze N,O emission and denitrification in the same soil condition.
MATERIALS AND METHODS Site description The study site (116"12'E, 39'33"
) at the experimen-
This paper is translated from its Chinese version in Scienriu Agriculturu Sinica
ZOU Guo-yuan, Associate Professor, Ph D, Tel: +86-10-88438818. Fax: +86-1048436644, E-mail:gyzou@ 163.com
02006,CAAS. All rights reserved. Publishedby Elsevier Ltd.
ZOU Guo-vuan et a1
46
tal field for Sino-Germany co-operated project is located at Dongbeiwang Farm, in a Beijing suburb. The area has a temperate continental monsoon climate with a mean annual rainfall of 628.9 mm, most of which occurs in the summer. The hottest month is Jul., with mean maximum temperature of 41.6"C. It is an aquic cambisol in this study. The surface soil layer (0-30 cm) has a sand-loamy silt texture, an organic matter content of 2.05, 0.114% total N, 0.106% total P, 1.809% total K, 54 mg kg-' available P and 162 mg kg-' available K.
Winter wheat Jingdong 8 totalling 187 kg ha-' were sown in 15 cm distance-furrow by seeder on Oct. 14, 2000 and harvested on Jun. 15,2001. Summer maize Jingken 114 numbering 60 000 plant ha-' were sown in 70 cm distance furrow on Jun. 20,2001 and harvested on Oct. 3,2001. Fertilizer phosphorus and potassium for each treatment was the same, while multiple superphosphate (P,O,, 180 kg ha-') and potassium chloride (&0,90 kg ha-') were applied once before wheat sowing, and during maize growth season no other fertilizer phosphorus and potassium were applied again.
Field experimental design
Measurement
The experiment was carried out from Oct. 2000 to Oct. 2001 in a wheat-maize cropping system. To study soil denitrification three fertilizer nitrogen levels (Table 1) were arranged, which include CK (no nitrogen fertilizer), CN (conventional nitrogen fertilizer) and ON (optimum nitrogen fertilizer), each with three replicates. To measure N,O emission simultaneouslyan additional experiment was carried out in the conventional nitrogen fertilizer plots (CN-NA). Conventional nitrogen fertilizer indicates the farmer habit of fertilization in which 150 kg ha-' (ammonium carbonate-N) were base broadcast applied and another 150 kg ha-' (urea-N) were broadcasted at shooting stage in winter wheat growth season, while 100 kg ha-' (ammonium carbonate -N) were band applied in the middle of two lines of maize at its thirdleaf stage and another 200 kg ha-' (urea-N) were equally broadcast applied at maize tenth-leaf stage. Optimum nitrogen fertilizer means recommendation for N fertilizer, in which the total N amount will be determined according to crop N need for objective yield, and N min of 060 cm soil layer for wheat and 0-90 cm for maize, Basically nitrogen rate were not equal between different duplicates due to soil variability, therefore nitrogen rate of ON treatment in Table 1 were mean data of different duplicates. Nitrogen fertilizationof CN-NA was same as CN treatment, the sole difference between these two treatments was acetylene inhibition.
Denitrification Denitrification rates under field conditions were measured using the soil-core incubation method (Ryden et al. 1987). In the interim from each plot three intact soil cores (4x175 cm, diameter x depth) were randomly extracted with a wall-holed PVC tube and placed together in the field incubationjar. Then the jars (volume, 2400 mL) were sealed with a rubber stopper that was provided with a sampling port. After replacing the headspace with acid-washed C,H, to a final concentration of 10% v/vI the jars were incubated in holes made in the soil within the experimental field. After 24 h of incubation, the atmosphere in the jars was repeatedly mixed with a 50 mL syringe and the gas removed for analyses of N,O. Air samples were analyzed by gas chromatography (Hewlett-Packard 5 890 I1 ) with an electron capture detector (ECD). The detector temperature was maintained at 330°C. The GC had a backflush system with a stainless steel precolumn (1.84 m, 3.2 mm 0.d.) and an analytical column (3.68 m, 3.2 mm 0.d.) packed with porapak Q, both 80-100 mesh, and held at 90°C oven temperature. The carrier gas (5% CH, in Ar) flow was adjusted to 25 mL min-' through the analytical column. Analytical variability was less than 0.8% for repeated analysis of ambient air compared to an N,O standard of 315 ppbv in compressed air (Deuste Steininger) during 8 h. Nitrogen loss by denitrification was calculated from change of N,O concentration of air sampler in the incubation jar. Time for taking of soil cores depends on many factors such as fertilization, irrigation and precipitation. Generally sampling frequency would be increased after nitrogen fertilization, precipitation and irrigation.
Table 1 Treatments of experiment Treatment CK CN ON CN-NA
Nitrogen fertilizer ( kg N ha-1) 0 600 148 600
02006, CAAS. All rightsresewed. Publishedby Elsevier Ltd.
41
Study on Soil Denitrification in Wheat-Maize Rotation System
N,O emission The pretreatment procedure for measurement of N,O emission was similar to measurement of denitrification, the sole difference was no acetylene addition to incubation jar for measurement of N,O emission. In this study only soil cores taken from plots with CN treatment were chosen to measure N;O emission (CN-NA). Soil NH,+-N, NO,--N content and water filled pore space (WFPS) After 24 h incubation the soil core samples were used to measure NH,+-N, NO,--N content and WFPS. The well mixed, 12-g soil samples were extracted with 100 mL of 0.01 mol L-I CaCl,, then the suspensions were shaken, filtered and lastly were analyzed for NHi-N and NO;-N with TRACCS-2000 Continuous Flow Analytical. Separate soil samples were taken to determine WFPS with oven.
Data analysis and calculation Nitrogen loss by denitrification (or N,O emission for treatment CN-NA) is expressed as equation. Q = (C,-C,)xVxF/(T,-T,) C,: N,O concentration of air sample after incubation; C , : N,O concentration of air sample before incubation; V: Total volume of incubation jar minus soil volume; T,-T,: Incubation time (24 h in this study); F: Unit coefficient. In this study, the total nitrogen loss by denitrifica-
i
tion (or N,O emission for treatment CN-NA) is the nitrogen loss accumulation of every measurement interval, while the nitrogen loss of measurement interval is equivalent to Q times measurement interval.
RESULTS Denitrificationdynamics and fertilizer nitrogen effects Fig. 1 shows dynamics of soil NH,+-N, NO,--N content, WFPS and denitrification rate. Peak of soil NH,+-N content quickly occurred after nitrogen fertilization while its persistent duration was quite short, indicating that nitrification completed quickly (Fig. 1-A). NH,C-N was converted to NO,--N through nitrification and the nitrate has been retained in the soil layer in high content for quite long time (Fig. 1-B). Denitrification rate fluctuated within the rotation duration, while summer maize growth season was the key period for soil denitrification, and nitrogen loss by denitrification mainly happened within 1-2 weeks after fertilizer nitrogen application (Fig. I-D). Denitrification dynamics has same trend with dynamics of soil NH,'-N content, while it has no such relationship with dynamics of soil NO,--N content. Though soil mineral nitrogen content and WFPS were quite high during certain period of winter wheat season (Fig. I-C), denitrification rate was still very low, sug-
lfI
25 20 15 10 5
0
Fig. 1 The dynamic of surface soil NH,+-N, NO, -N, water content and denitrification rate.
4 means fertilization date. The same as below.
02006,GAAS. All rights reserved Publishedby Elsevier Ltd
ZOU Guo-yuan er a1
48
gesting that lower temperature in this stage may limit the biological process. Nitrogen source is an important factor affecting soil denitrification. In this study soil denitrification rate was relatively low when soil mined nitrogen content was lower than that of treatment-CK.Optimized nitrogen fertilization (ON) resulted in lower soil mineral nitrogen content compared with conventional nitrogen fertilization (CN), thereafter denitrification rate of ON was relatively low.
Nitrogen loss amount by denitrification The nitrogen loss amount by denitrification for every measurement interval is estimated by the results that the denitrification rate per unit times measurement interval (Table 2). The results showed that total nitrogen loss amount by denitrification in wheat-maize rotation system for different treatments were between 4.71 and 9.67 kg ha-'. If minus the nitrogen loss from soil the ratio of net N denitrificationto fertilizer N applied,which is 600 kg ha-', then it was 0.83% for treatment CN. Thus the N loss by denitrificationin Beijing arable topsoil is not high according to above analysis. It was also found that maize growth season was a key stage for denitrification with N loss amount by 2.67-7.31 kg ha-' accounting for 56.7-75.6% of annual N loss amount though this duration is not longer than 4 months while wheat growth season cover about 8 months. During maize growth season in Beijing it is hot and humid, always with heavy rainfall resulting in advantageous soil situation for denitrification, while during wheat growth season the arable topsoil is always dry and cold, with relatively low temperature for long time not favoring denitrification. In winter wheat growth season soil denitrification was weak and there was no big difference between different N fertilization treatments, while in summer maize growth season soil denitrification increased and increase of fertilizer N application resulted in the trend of higher N loss by denitrification.Treatment CN raised N loss by denitrification compared with treatment ON
but the difference between these treatments was not significant.
N,O emission Soil N,O is mainly generated from nitrification and denitrification.In different soils the relative importance of these two biological processes in N,O production depends on a series of factors (Bremner 1997) controlling both nitrification rate, denitrification rate, and processes the ratio of N,O/N, produced. In this experiment an additional measurement arranged in CN plots to test N,O emission indicated that the peak of N,O emission took place just after N fertilization especially in maize growth season, implying that nitrification processes may play an important role on N,O emission in arid soil. Fig.:! shows the annual accumulatedN,O emission and N denitrification. The N,O-N emission was 5.66 kg ha-' per year, lower than N loss amount by denitrification which was 9.67 kg ha-' per year, which is different from the results in the last year (Zou et al. 2001). Different fertilizer N application method might induce different results of N loss measured. In last year's experiment based fertilizer N on maize was band applied and soil core was taken from the band for measurement of N,O emission and denitrification, while in this study based fertilizer N on maize was broadcasted and soil core was taken evenly. Soil NO,--N content was
'*[
0
A C N
--o-CN-NA
50
100
150
200
250
300
350
400
Days after soning ofwintcr wheat (d)
Fig. 2 Soil N,O emission and nitrogen loss by denitrification in winter wheat-summer maize rotation system.
Table 2 Nitroeen losses bv denitrification for different treatments Ikp h a ~ 9 Treatment CK CN ON
Winter wheat season 2.04f 0.15 a 2.36 f 0.40 a 2.16 f 0.19 a
Summermaize season 2.67f0.10a 7.31 f 1.34 b 5.82 f 2.65 ab
Total amount 4.71 2 0.24 a 9.67 f 1.44 b 7.98 +. 2.64 ab
02006,CAAS. All rights resewed. Publishedby Elsevier Ltd.
Study on Soil Denitrification in Wheat-Maize Rotation System
kept high for long time in fertilization band and the relatively high NO,--N content may inhibit N,O reduction, inducing higher ratio of N,O/N,, thereafter resulting in higher results of N,O emission in last year’s experiment. Due to lack of systematic study, certain causal information is presently insufficient.
DISCUSSION Soil denitrificationincludes both biological and chemical denitrificationprocesses, of which biological processes play a main role and is of greater interest. All factors controlling microbial biomass and activity affect soil denitrification. The major factors controlling soil denitrification are WFPS and temperature in field condition. It has been found that there is a close relation between denitrification and combined effect of soil moisture and temperature in estimation of these factors (Aulakh et al. L983a; Aulakh et al. 1983b; Aulakh et al. 1984; Aulakh and Rennie 1985). In Beijing area it is hot and humid in summer when maize grows, and cold and dry in winter and spring when wheat grows, correspondingly higher rate of denitrification in maize growth season and lower rate of denitrification in wheat growth season may be attributed to the climate characteristics in this area. Quantitative measurement of N loss by denitrification in arid soil is important to understand the fate of fertilizer N in soil and relative importance of each loss. Unfortunately directly measured and systematic data of N loss by denitrification in China are still in the initial stages. Results of this study show that N loss by denitrification is not serious in cropping rotation system and it is not a key factor accounting for N losses compared with other methods of loss in the same area. Long-term measurement of denitrification losses had been arranged on the same field and results for two years show that denitrification dynamics and annual losses were quite similar. Change of N fertilizationmethods in the following year may have affected denitrification and N,O emission, therefore further research on denitrification combined with fertilizationmethods would provide more important information.
References Aulakh M S, Rennie D A, Paul E A. 1983a. The effect of various clover managementpractices on gaseous N losses and mineral
49
N accumulation. Canadian Journal of Soil Science, 63,593605. Aulakh M S, Rennie D A, Paul E A. 1983b. Field studies on gaseous nitrogen losses from soils under continuous wheat versus a wheat-fallow rotation. Plant and Soil, 75, 15-27. Aulakh M S, Rennie D A, Paul E A. 1984. Gaseous nitrogen losses from soils under zero-till as compared with con vent i on a1- ti 11 manage men t s y stems . Jo u rn a 1 of Environmental Quality, 13, 130-136. Auiakh M S, Rennie D A. 1985. Gaseous nitrogen losses from conventional and chemical summer fallow. Canadian Journal of Soil Science, 65, 195-203. Bremner J M. 1997. Sources of nitrous oxide in soils. Nutrient Cycling in Agroecosystems, 49,7-16. Ding H, Cai G X, Wang Y S, Chen D L. 2001. Nitrificationdenitrification losses of nitrogen fertilizer and N,O emission from maize-chao soil system in north China plain. Scientia Agricultura Sinica, 34,416-421. (in Chinese) Ding H, Wang Y S. 2004. Denitrification losses of nitrogen fertilizer and N,O emission from different crop-black soil systems in northeast China. Journal of Agro-Environment Science, 23, 323-326. (in Chinese) Huang G H, Chen G X, Zhang Z M, Wu J, Huang B, Cleemput 0 V. 1998. N 2 0 emission in maize field and its mitigation. Acta Scientiae Circumstantiae, 18,344-349. (in Chinese) Ju X T, Liu X J, Zou G Y, Wang Z H, Zhang F S. 2002. Evaluation of nitrogen loss way in winter wheat and summer maize rotation system. Scientia Agricultura Sinica, 35, 1493-1499. (in Chinese) Ryden J C, Skinner J H, Nixon D J. 1987. Soil core incubation system for the field measurement of denitrification using acetylene-inhibition. Soil Biology & Biochemistry, 19,753757. Wang G C, Du R, Wang Y F, Liu G R, Wang Y S, Chen Z Z, Lu D R. 1998. Characteristics of N,O and CH, fluxes and their seasonal variations from Inner Mongolia grassland. Acta Agrestia Sinica, 6,306-3 11. (in Chinese) Wang S B, Song W Z, Su W H, Zeng H H, Zhang Y M, Wang Z P. 1994. N,O emission from winter wheat fields. AgroEnvironmental Protection, 13,210-212. (in Chinese) Wang X K, Zhuang Y H, Li C S. 2001. The distribution pattern of N,O emissionfrom agriculturalsoil in China. Acta Ecologica Sinica, 21, 1225-1232. (in Chinese) Yuan F M, Chen Z M, Yao Z H, Zhou C S, Fu G M, Song Y L, Li X P. 1995. NO,--N transformation accumulation and leaching loss in surface layer of chao-soil in Beijing. Acta Pedologica Sinica, 32, 388-399. (in Chinese) Zhang Y M, Dong W X, Zeng J H, Chen D L. 2001. Dynamic of denitrification rate and N,O flux in maize field. Chinese Journal of Eco-Agriculture, 9,70-72. (in Chinese) Zou G Y, Zhang F S, Chen X P, Li X H. 2001. Nitrificationdenitrification and N,O emission from arable soil. Soil and Environmental Sciences, 10, 273-276. (in Chinese) (Edited by ZHAO Qi)
82006,CAAS. All rightsresewed. Publishedby ElsevierLtd.
January 2006
Study on Soil Denitrification in Wheat-Maize Rotation System ZOU Guo-yuanl.2,ZHANG Fu-suo2, JU Xiao-tang2, CHEN Xin-ping2 and LIU Xue-jun2 1
Institute of Plant Nutrition and Natural Resources, Beijing Academy of Agricultural and Forestry Sciences, Beijing 100089, P.R.China Department of Plant Nutrition, China Agricultural University, Beijing 100094, P.R.China
Abstract Soil denitrification was studied in wheat-maize rotation cropping system on an aquic cambisol. Results showed that the N loss amount by denitrification ranged from 4.7 to 9.7 kg per hectare with different levels of nitrogen application and the key stage for denitification was during summer maize-growth-period, especially within 1-2 weeks after fertilizer nitrogen was applied. Similar trend was found between soil N,O production/emission dynamic and denitrification dynamic in the rotation system, which may indicate that mainly N,O is produced in nitrification process. Key words: aquic cambisol, winter wheat, summer maize, denitrification, N,O
INTRODUCTION Ammonia volatilization, nitrification-denitrificationand nitrate leaching are main pathways of nitrogen losses. Nitrate leaching in Northern China mainly happens in seasons with heavy precipitation (Yuan et al. 1995), while ammonia volatilization mainly takes place within a short time after nitrogen fertilization and the amount of nitrogen loss is quite low when fertilizer nitrogen is applied deeply. In the arid areas the percentage of N loss by denitrification will account for the ratio of total N loss, which not only created a lot of interest from soil researchers but also caused a lot of interest from environmental researchers. Some results of different soils from Hebei, Henan and Heilongjiang provinces showed that N loss amount by nitrification-denitrification was no more than 10 kg ha-' (Zhang et al. 2001; Ding et al. 2001; Ding et al. 2004; Ju et al. 2002). The earlier results from Beijing area also could not support the idea that denitrification makes a considerable contribution to the total N loss. In order to demonstrate the
dynamics and rate of denitrification, and their related controlling factors, a continual measurement of denitrification in a.n aquic cambisol in Beijing had been carried out with acetylene inhibition method in a soil core incubation system. As a greenhouse effect gas, N,O originates from the soil mainly from the nitrification-denitrification.Related papers on N,O emission from arid soil in northern China have been frequently reported (Huang et al. 1998; Wang et al. 1994; Wang et al. 1998; Wang et al. 2001). Measurement of denitrification and N,O simultaneously will help to deepen the knowledge of sources of N,O emission, and there are several groups in China working on related areas (Zhang et al. 2001; Ding et al. 2001; Ding et al. 2004; Zou et al. 2001). As a part of above-mentioned study this paper will compare and analyze N,O emission and denitrification in the same soil condition.
MATERIALS AND METHODS Site description The study site (116"12'E, 39'33"
) at the experimen-
This paper is translated from its Chinese version in Scienriu Agriculturu Sinica
ZOU Guo-yuan, Associate Professor, Ph D, Tel: +86-10-88438818. Fax: +86-1048436644, E-mail:gyzou@ 163.com
02006,CAAS. All rights reserved. Publishedby Elsevier Ltd.
ZOU Guo-vuan et a1
46
tal field for Sino-Germany co-operated project is located at Dongbeiwang Farm, in a Beijing suburb. The area has a temperate continental monsoon climate with a mean annual rainfall of 628.9 mm, most of which occurs in the summer. The hottest month is Jul., with mean maximum temperature of 41.6"C. It is an aquic cambisol in this study. The surface soil layer (0-30 cm) has a sand-loamy silt texture, an organic matter content of 2.05, 0.114% total N, 0.106% total P, 1.809% total K, 54 mg kg-' available P and 162 mg kg-' available K.
Winter wheat Jingdong 8 totalling 187 kg ha-' were sown in 15 cm distance-furrow by seeder on Oct. 14, 2000 and harvested on Jun. 15,2001. Summer maize Jingken 114 numbering 60 000 plant ha-' were sown in 70 cm distance furrow on Jun. 20,2001 and harvested on Oct. 3,2001. Fertilizer phosphorus and potassium for each treatment was the same, while multiple superphosphate (P,O,, 180 kg ha-') and potassium chloride (&0,90 kg ha-') were applied once before wheat sowing, and during maize growth season no other fertilizer phosphorus and potassium were applied again.
Field experimental design
Measurement
The experiment was carried out from Oct. 2000 to Oct. 2001 in a wheat-maize cropping system. To study soil denitrification three fertilizer nitrogen levels (Table 1) were arranged, which include CK (no nitrogen fertilizer), CN (conventional nitrogen fertilizer) and ON (optimum nitrogen fertilizer), each with three replicates. To measure N,O emission simultaneouslyan additional experiment was carried out in the conventional nitrogen fertilizer plots (CN-NA). Conventional nitrogen fertilizer indicates the farmer habit of fertilization in which 150 kg ha-' (ammonium carbonate-N) were base broadcast applied and another 150 kg ha-' (urea-N) were broadcasted at shooting stage in winter wheat growth season, while 100 kg ha-' (ammonium carbonate -N) were band applied in the middle of two lines of maize at its thirdleaf stage and another 200 kg ha-' (urea-N) were equally broadcast applied at maize tenth-leaf stage. Optimum nitrogen fertilizer means recommendation for N fertilizer, in which the total N amount will be determined according to crop N need for objective yield, and N min of 060 cm soil layer for wheat and 0-90 cm for maize, Basically nitrogen rate were not equal between different duplicates due to soil variability, therefore nitrogen rate of ON treatment in Table 1 were mean data of different duplicates. Nitrogen fertilizationof CN-NA was same as CN treatment, the sole difference between these two treatments was acetylene inhibition.
Denitrification Denitrification rates under field conditions were measured using the soil-core incubation method (Ryden et al. 1987). In the interim from each plot three intact soil cores (4x175 cm, diameter x depth) were randomly extracted with a wall-holed PVC tube and placed together in the field incubationjar. Then the jars (volume, 2400 mL) were sealed with a rubber stopper that was provided with a sampling port. After replacing the headspace with acid-washed C,H, to a final concentration of 10% v/vI the jars were incubated in holes made in the soil within the experimental field. After 24 h of incubation, the atmosphere in the jars was repeatedly mixed with a 50 mL syringe and the gas removed for analyses of N,O. Air samples were analyzed by gas chromatography (Hewlett-Packard 5 890 I1 ) with an electron capture detector (ECD). The detector temperature was maintained at 330°C. The GC had a backflush system with a stainless steel precolumn (1.84 m, 3.2 mm 0.d.) and an analytical column (3.68 m, 3.2 mm 0.d.) packed with porapak Q, both 80-100 mesh, and held at 90°C oven temperature. The carrier gas (5% CH, in Ar) flow was adjusted to 25 mL min-' through the analytical column. Analytical variability was less than 0.8% for repeated analysis of ambient air compared to an N,O standard of 315 ppbv in compressed air (Deuste Steininger) during 8 h. Nitrogen loss by denitrification was calculated from change of N,O concentration of air sampler in the incubation jar. Time for taking of soil cores depends on many factors such as fertilization, irrigation and precipitation. Generally sampling frequency would be increased after nitrogen fertilization, precipitation and irrigation.
Table 1 Treatments of experiment Treatment CK CN ON CN-NA
Nitrogen fertilizer ( kg N ha-1) 0 600 148 600
02006, CAAS. All rightsresewed. Publishedby Elsevier Ltd.
41
Study on Soil Denitrification in Wheat-Maize Rotation System
N,O emission The pretreatment procedure for measurement of N,O emission was similar to measurement of denitrification, the sole difference was no acetylene addition to incubation jar for measurement of N,O emission. In this study only soil cores taken from plots with CN treatment were chosen to measure N;O emission (CN-NA). Soil NH,+-N, NO,--N content and water filled pore space (WFPS) After 24 h incubation the soil core samples were used to measure NH,+-N, NO,--N content and WFPS. The well mixed, 12-g soil samples were extracted with 100 mL of 0.01 mol L-I CaCl,, then the suspensions were shaken, filtered and lastly were analyzed for NHi-N and NO;-N with TRACCS-2000 Continuous Flow Analytical. Separate soil samples were taken to determine WFPS with oven.
Data analysis and calculation Nitrogen loss by denitrification (or N,O emission for treatment CN-NA) is expressed as equation. Q = (C,-C,)xVxF/(T,-T,) C,: N,O concentration of air sample after incubation; C , : N,O concentration of air sample before incubation; V: Total volume of incubation jar minus soil volume; T,-T,: Incubation time (24 h in this study); F: Unit coefficient. In this study, the total nitrogen loss by denitrifica-
i
tion (or N,O emission for treatment CN-NA) is the nitrogen loss accumulation of every measurement interval, while the nitrogen loss of measurement interval is equivalent to Q times measurement interval.
RESULTS Denitrificationdynamics and fertilizer nitrogen effects Fig. 1 shows dynamics of soil NH,+-N, NO,--N content, WFPS and denitrification rate. Peak of soil NH,+-N content quickly occurred after nitrogen fertilization while its persistent duration was quite short, indicating that nitrification completed quickly (Fig. 1-A). NH,C-N was converted to NO,--N through nitrification and the nitrate has been retained in the soil layer in high content for quite long time (Fig. 1-B). Denitrification rate fluctuated within the rotation duration, while summer maize growth season was the key period for soil denitrification, and nitrogen loss by denitrification mainly happened within 1-2 weeks after fertilizer nitrogen application (Fig. I-D). Denitrification dynamics has same trend with dynamics of soil NH,'-N content, while it has no such relationship with dynamics of soil NO,--N content. Though soil mineral nitrogen content and WFPS were quite high during certain period of winter wheat season (Fig. I-C), denitrification rate was still very low, sug-
lfI
25 20 15 10 5
0
Fig. 1 The dynamic of surface soil NH,+-N, NO, -N, water content and denitrification rate.
4 means fertilization date. The same as below.
02006,GAAS. All rights reserved Publishedby Elsevier Ltd
ZOU Guo-yuan er a1
48
gesting that lower temperature in this stage may limit the biological process. Nitrogen source is an important factor affecting soil denitrification. In this study soil denitrification rate was relatively low when soil mined nitrogen content was lower than that of treatment-CK.Optimized nitrogen fertilization (ON) resulted in lower soil mineral nitrogen content compared with conventional nitrogen fertilization (CN), thereafter denitrification rate of ON was relatively low.
Nitrogen loss amount by denitrification The nitrogen loss amount by denitrification for every measurement interval is estimated by the results that the denitrification rate per unit times measurement interval (Table 2). The results showed that total nitrogen loss amount by denitrification in wheat-maize rotation system for different treatments were between 4.71 and 9.67 kg ha-'. If minus the nitrogen loss from soil the ratio of net N denitrificationto fertilizer N applied,which is 600 kg ha-', then it was 0.83% for treatment CN. Thus the N loss by denitrificationin Beijing arable topsoil is not high according to above analysis. It was also found that maize growth season was a key stage for denitrification with N loss amount by 2.67-7.31 kg ha-' accounting for 56.7-75.6% of annual N loss amount though this duration is not longer than 4 months while wheat growth season cover about 8 months. During maize growth season in Beijing it is hot and humid, always with heavy rainfall resulting in advantageous soil situation for denitrification, while during wheat growth season the arable topsoil is always dry and cold, with relatively low temperature for long time not favoring denitrification. In winter wheat growth season soil denitrification was weak and there was no big difference between different N fertilization treatments, while in summer maize growth season soil denitrification increased and increase of fertilizer N application resulted in the trend of higher N loss by denitrification.Treatment CN raised N loss by denitrification compared with treatment ON
but the difference between these treatments was not significant.
N,O emission Soil N,O is mainly generated from nitrification and denitrification.In different soils the relative importance of these two biological processes in N,O production depends on a series of factors (Bremner 1997) controlling both nitrification rate, denitrification rate, and processes the ratio of N,O/N, produced. In this experiment an additional measurement arranged in CN plots to test N,O emission indicated that the peak of N,O emission took place just after N fertilization especially in maize growth season, implying that nitrification processes may play an important role on N,O emission in arid soil. Fig.:! shows the annual accumulatedN,O emission and N denitrification. The N,O-N emission was 5.66 kg ha-' per year, lower than N loss amount by denitrification which was 9.67 kg ha-' per year, which is different from the results in the last year (Zou et al. 2001). Different fertilizer N application method might induce different results of N loss measured. In last year's experiment based fertilizer N on maize was band applied and soil core was taken from the band for measurement of N,O emission and denitrification, while in this study based fertilizer N on maize was broadcasted and soil core was taken evenly. Soil NO,--N content was
'*[
0
A C N
--o-CN-NA
50
100
150
200
250
300
350
400
Days after soning ofwintcr wheat (d)
Fig. 2 Soil N,O emission and nitrogen loss by denitrification in winter wheat-summer maize rotation system.
Table 2 Nitroeen losses bv denitrification for different treatments Ikp h a ~ 9 Treatment CK CN ON
Winter wheat season 2.04f 0.15 a 2.36 f 0.40 a 2.16 f 0.19 a
Summermaize season 2.67f0.10a 7.31 f 1.34 b 5.82 f 2.65 ab
Total amount 4.71 2 0.24 a 9.67 f 1.44 b 7.98 +. 2.64 ab
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Study on Soil Denitrification in Wheat-Maize Rotation System
kept high for long time in fertilization band and the relatively high NO,--N content may inhibit N,O reduction, inducing higher ratio of N,O/N,, thereafter resulting in higher results of N,O emission in last year’s experiment. Due to lack of systematic study, certain causal information is presently insufficient.
DISCUSSION Soil denitrificationincludes both biological and chemical denitrificationprocesses, of which biological processes play a main role and is of greater interest. All factors controlling microbial biomass and activity affect soil denitrification. The major factors controlling soil denitrification are WFPS and temperature in field condition. It has been found that there is a close relation between denitrification and combined effect of soil moisture and temperature in estimation of these factors (Aulakh et al. L983a; Aulakh et al. 1983b; Aulakh et al. 1984; Aulakh and Rennie 1985). In Beijing area it is hot and humid in summer when maize grows, and cold and dry in winter and spring when wheat grows, correspondingly higher rate of denitrification in maize growth season and lower rate of denitrification in wheat growth season may be attributed to the climate characteristics in this area. Quantitative measurement of N loss by denitrification in arid soil is important to understand the fate of fertilizer N in soil and relative importance of each loss. Unfortunately directly measured and systematic data of N loss by denitrification in China are still in the initial stages. Results of this study show that N loss by denitrification is not serious in cropping rotation system and it is not a key factor accounting for N losses compared with other methods of loss in the same area. Long-term measurement of denitrification losses had been arranged on the same field and results for two years show that denitrification dynamics and annual losses were quite similar. Change of N fertilizationmethods in the following year may have affected denitrification and N,O emission, therefore further research on denitrification combined with fertilizationmethods would provide more important information.
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49
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