Basnyat, P.

Campbell, C.A.

De Jong, R.

Desjardins, R.

Lemke, R.

Zentner, R.P.

English

University of Saskatchewan

Effect of fallow frequency, fertilizer, and available water on net N mineralization under wheat grown in the semi-arid Brown Soil Zone

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University of Saskatchewan Research Archive

 Effect of Fallow Frequency, Fertilizer, and Available Water on Net N Mineralization under Wheat Grown in the Semiarid Brown Soil Zone    R.P. Zentner1, C.A. Campbell2, P. Basnyat1, R. DeJong2, R. Lemke3,  and R. Desjardins2    1  Semiarid Prairie Agricultural Research Centre, Swift Current, SK. S9H 3X2.  2  Eastern Cereal and Oilseed Research Centre, Ottawa, ON. K1A 0C6.  3  Saskatoon Research Centre, Saskatoon, SK. S7N 5A8   INTRODUCTION • Nitrogen (N) is the nutrient most limiting to cereal crops in the Canadian Prairies. • The supply of N to crops typically comes from applied N fertilizer and N mineralized from soil organic matter. • Numerous factors influence the process of N mineralization in soil, including available water, temperature, aeration, organic matter content, size of the active N pool, and tillage method. • Producers need accurate methods of estimating net N mineralization in soil in order to determine optimum application rates of N fertilizer. • Inaccurate estimates of N requirements can lead to over- or under-fertilization of crops, which is costly and may be damaging to the environment.  OBJECTIVES (i) determine rates of N mineralization under well-fertilized continuous wheat [Cont W (N+P)], fallow-wheat [F-W (N+P)], and under F-W-W (+P) rotations, and (ii) determine the influence of fallow frequency, N fertilizer rate, and available water on apparent net N mineralization.  MATERIALS AND METHODS Long-Term Crop Rotation Experiment • The Swift Current crop rotation experiment was established in 1967 on a Swinton loam soil (Orthic Brown Chernozem). • The 1976 average organic C concentration in the top 15 cm soil depth under F-W (N+P) and Cont W (N+P) were 1.67 and 1.98%, respectively; in 2003 the corresponding values were 1.92 and 2.14%. • Fertilizer N was applied to designated treatments in accordance with soil NO3-N (0-60 cm depth) levels in individual plots measured the previous fall, and the recommendation guidelines from the soil testing laboratory at the University of Saskatchewan. • On average, wheat grown on fallow received about 8 kg N ha-1, and wheat grown on stubble received about 30 kg N ha-1.  The N fertilizer, as ammonium nitrate, was broadcast and soil incorporated with tillage used to prepare the seedbed. • All treatments received about 10 kg P ha-1 yr-1 applied with the seed in accordance with the general recommendations for the area and crop. • All phases of each rotation were present each year, and each rotation was replicated 3 times. • During the 1967-1984 period, soil nitrate and moisture levels (0-15, 15-30, 30-60, 60-90, and 90-120 cm depths) and above-ground plant N were measured 8 times each year in early spring (prior to planting), at plant emergence, 3-leaf, 5-leaf, shot blade, soft dough, harvest, and again just prior to freeze-up in fall (Table 1).  Lysimeter Experiment • Lysimeters (15 cm diam., 120 cm deep) were placed in Swinton loam soil that had been cropped to wheat the previous year and which contained 18 kg NO3-N in the 0-60 cm depth at seeding.  In 1975, wheat was planted into 140 lysimeters, representing two moisture regimes (rainfed and irrigated), seven N fertilizer rates (0, 20.5, 41, 62, 82, 123, and 164 kg N ha-1), five times of sampling, and two replicates.  All cropped plots received 50 kg P ha-1.  In addition, there were 20 summerfallowed lysimeters, representing two P fertilizer rates (0 and 50 kg P ha-1), five sampling times, and two replicates.   • Soil N and moisture and plant N (roots and above-ground biomass, for cropped plots only) were determined (by destructive sampling) at planting, and then again at 3-leaf, tillering, shot blade, anthesis, and maturity of the wheat crop. • The amounts of distilled water applied to the irrigated treatments were 25.4, 12.7, 38.1 and 101.6 mm during the period seeding to 3-leaf, 3-leaf to tillering, tillering to shot blade, and shot blade to anthesis, respectively.  Data Analysis • Net N mineralized (Nmin) in the soil was calculated for two arbitrarily chosen intervals (harvest to freeze-up, and freeze-up to early spring prior to planting) and also for the spring to harvest period for cropped plots.  We also calculated Nmin for four intervals of the 20-month summerfallow period (harvest to freeze-up, freeze-up to early spring, spring to freeze-up of the fallow year, and second freeze-up to just before planting). • N mineralization for the selected periods was defined as: Nmin = [Soil N(j+1) + Plant N(j►j+1)] – [Soil N(j) + Fert N(j►j+1)]   ….(Eqn 1) where, Soil N refers to NO3-N in the 0-90 cm depth, Plant N is N uptake by the crop between two sampling dates J and J+1, and Fert N is the amount of fertilizer N applied between the two sampling dates.  For fallow areas, Plant N and Fert N were set to zero. • The calculations assume N lost by runoff and denitrification, and that gained by wet and dry depositions were negligible.  The calculations also ignore N lost through leaf loss.  RESULTS AND DISCUSSION Crop Rotation Study • During the 20-month fallow period, net Nmin averaged about 118 kg N ha-1.  Of this about 15 kg ha-1 was mineralized between harvest and first spring, about 93 kg ha-1 between first spring and second fall (Table 2), and about 10 kg ha-1 between second fall and planting. • Under cropped conditions, average net Nmin during the spring to fall period averaged 63, 56, and 53 kg ha-1 for F-(W) (N+P), F-W-(W) (+P), and Cont W (N+P), respectively (Table 2), with the values ranging from a low of 4 to 129 kg ha-1.  As found for plots that were fallowed, Nmin during the 8.5 month fall and winter periods averaged only 15 ± 28 kg ha-1, with very little net Nmin occurring (about 4 kg ha-1) during the 6-week period from harvest to freeze-up.  These results were not unexpected because the previous crop dries out the surface soil where most of the N mineralization occurs, and the very cold conditions during winter would suppress microbial activity. • We found reasonable relationships between net Nmin and total precipitation received between spring and fall for 3 of the 4 rotation phases (Fig. 1), with precipitation accounting for about 42% of the variation in net Nmin. • All systems contained values that appear as outliers, likely reflecting NO3-N losses related to leaching and denitrification during wet years (e.g., 1970, 1974, 1975, 1982) (Campbell et al. 1984), flushes in Nmin due to frequent wetting and drying events (1971, 1973) (Campbell 1978), changes in soil organic matter quantity and quality over time (Biederbeck et al. 1994), and/or unmeasured losses of leaf and root N. • We found no relationship between net Nmin and mean daily temperatures between spring and fall (data not shown). • Increases in soil NO3-N, particularly during the summerfallow phase, were closely associated with tillage events in 63% of the occasions (Table 3).  Tillage influences the soil environment and biological activity by disrupting aggregates and releasing entrapped organic substrates making them more accessible to microbial activity and decomposition (Doran and Smith 1987).  Lysimeter Study • Under more controlled conditions, net Nmin between spring and harvest in cropped lysimeters averaged 40 kg ha-1 under dryland and 73 kg ha-1 under irrigated conditions (Table 4). The negative Nmin values between tillering and anthesis likely reflect the unmeasured loss of leaf and root N. • The fallow systems mineralized 107 kg ha-1 over the spring to harvest period.  Most of the Nmin in the fallow treatments occurred during the period prior to the tillering phase of wheat, when evaporation was not yet at a maximum, and after the anthesis phase of wheat when often there was significant precipitation falling on a dry soil surface.  • When expressed on a daily basis, the rate of Nmin was directly proportional to the amount of water received (0.56 kg ha-1 day-1 for dryland and 1.03 kg ha-1 day-1 for irrigated treatments). • N fertilization at normal rates of application (<60 kg ha-1) had little effect on net Nmin (Table 4).  CONCLUSIONS • Our results show that it is possible to develop reasonable predictions of Nmin to improve N fertilizer recommendations; however, direct measurement is highly variable, reflecting the prevailing soil water conditions, amount and timing of rainfall events, soil temperature, soil organic matter content, and frequency of tillage.  N losses associated with denitrification and leaching events, and that associated with leaf loss may also be significant and need to be quantified and included. • Consequently, reliable estimates of Nmin will need to rely more on process-based models such as CERES, LEACHMN, DNDC, CENTURY, and EPIC to integrate these factors and their interactions. • Data collected in these and other long-term studies will provide an invaluable resource to test and validate these process-level models for predicting net Nmin.  REFERENCES Biederbeck, V.O., Janzen, H.H., Campbell, C.A. and Zentner, R.P. 1994. Labile soil organic matter as influenced by cropping practices in an arid environment. Soil Boil. Biochem. 26: 1647-1656.  Campbell, C.A. 1978 Soil organic carbon, nitrogen and fertility In: Schnitzer, M and Khan, SU (eds.) Soil Organic Matter Developments in Soil Science Vol 8, pp 173-272. Amsterdam:Elsevier.  Campbell, C.A., De Jong, R. and Zentner, R.P. 1984. Effect of cropping, summerfallow and fertilizer on nitrate-nitrogen lost by leaching on a Brown Chernozem loam. Can. J. Soil Sci. 64: 61-74.  Doran, J.W. and Smith, M.S. 1987. Organic matter management and utilization of soil and fertilizer nutrients. Pp53-72 In R.F. Follett, J.W.B. Stewart, C.V. Cote and J.F. Power (eds) Soil fertility and organic matter as critical components of production systems. SSSA Special Pub. No 19. Soil Sc. Soc. Inc. Amer. Soc. Agron. Inc., Madison, Wis.   Table 1. Eighteen-yr (1967-1984) mean plant N, soil NO3-N in 0-90 cm depth, and total Nz at 8 sampling times between spring and fall, used to estimate net Nmin   F-W-(W)y (+P)  F-(W)y (N+P)  Cont W (N+P) Stage of growthx Average date of sampling Days (F)-W-Wy (+P) NO3-N Plant N   NO3-N Total N Plant N  NO3-N Total N Plant N  NO3-N Total N     -----------------------------------------------------------------(kg ha-1)------------------------------------------------------------------------ Spring 14 May    0   64 - 78 78.0 - 126 126.0 -   67   67.0 Emergence 17 may    3   78 - 53 53.0 - 126 126.0 - 108 108.0 3-Leaf 12 June   29   71   3.0 88 91.0     3.7 129 132.7  3.5 108 111.5 5-Leaf 18 June   35   76 11.0 79 90.0   17.5 106 123.5 14.2 112 126.2 Shot-blade 10 July   57   89 25.4 69 94.4   43.3   88 131.3 31.5   81 112.5 Dough 16 Aug   94   87 29.6 83  112.6   50.0   71    121.0 34.5   62     96.5 Harvest 19 Aug   97   89 36.3 43 79.3   54.1   59 113.1 39.8   56    95.8 Fall 16 Oct 155 104 - 58 58.0 -   72   72.0 -   55    55.0 z Average N applied = 7.7 kg ha-1 for F-W-(W) (+P), 6.4 kg ha-1 for F-(W), and 31.6 kg ha-1 for Cont W (N+P). y Values pertain to the rotation phase in parentheses.  Table 2. Estimated annual net Nminz in four selected crop rotation phases during the 1967-1984 period, sampled for plant and soil NO3-N 8 times between spring and fall     Year (F)-W-W (+P) F-W-(W) (+P) F-(W) (N+P) Cont W (N+P) Precipitationy  ----------------------------------------(kg ha-1)------------------------------------------ (mm) 1967   55   56  34  24 126 1968   66   72  39  43 183 1969 121   84  65 110 195 1970   55   35  27  19 287 1971   94   32  92  68 141 1972   83   48  19  34 166 1973   43   87  92  19  93 1974   69    4  48  90 275 1975   42   36  99 114 245 1976   88   83  58  75 234 1977 119   40  32  43 246 1978 166   40  73  27 194 1979 121 103  43  32 157 1980 114   58  52 -x 229 1981 188   80 111  51 250 1982   51   93 129  70 332 1983 116   42 -x  12 164 1984   90   21  76  62 186 Average   93   56  63  53   z Based on soil NO3-N measured in 0-90 cm depth. y Precipitation received between spring and fall sampling. x Missing data.   Table 3. Relationship between frequency of tillage and change in Nmin in fallow phase of the (F)-W-(+P) rotation Year Tillage Date Changez in NO3 Proportion of tillage operations resulting in positive change (%) 1969 8/5, 4/7, 22/7, 5/9 (+) (+) (-ve) (+) 75 1970 15/5, 24/6, 24/7, 12/8 (+) (+) (+) (+) 100 1971 7/5, 9/6, 16/7, 5/8 (0) (+) (+) (+)  75 1972 4/5, 12/6, 7/7, 25/8 (+) (+) (0) (+) 75 1973 14/5, 11/7 (0) (+) 50 1974 16/5,  5/7, 2/8 (-ve) (+) (-ve)  33 1975 20/5, 10/7, 10/9 (+) (-ve) (-ve) 33 1976 5/5, 11/6, 6/7, 13/8 (-ve) (-ve) (+) (+) 50 1977 25/5, 4/7, 28/7, 29/8 (+) (+) (-ve) (-ve) 50 1978 22/5, 21/6, 11/7, 8/8 (+) (0) (-ve) (-ve) 25 1979 30/5, 10/7, 31/7, 31/8 (+) (-ve) (+) (+) 75 1980 5/5, 24/6, 22/7 (+) (+) (-ve) 66 1981 11/5, 2/7, 16/7, 4/8, 17/9 (0) (-ve) (+) (+) (+) 60 1982 26/5, 20/7, 10/8, 30/8 (-ve) (+) (+) (+)  75 1983 20/5, 23/6, 26/7, 29/8 (+) (+) (+) (+) 100 1984 9/5, 26/6, 25/7 (+) (-ve) (+)   66 Mean     63 z Change in soil NO3-N associated with each tillage operation: (+) for increase, (-ve) for decrease, and (0) for no change during the associated period.   Table 4. Effect of soil water, N fertilizer and time sampled on apparent net Nmin between spring and harvest for dryland and irrigated treatments      Fallow Rate of N applied (kg ha-1) Period Days Precipitation 0 0 20.5 41 62 82 123 164 Mean cropped Dryland  (mm) ------------------------------- Apparent Nmin  (kg ha-1) ---------------------- 3-Leaf to Tillering 11   41.6  26   7 10 10    3 -19 -16  14   5 Tillering to Shot blade 14     4.6   -4 18   5 14 -12   11    0 -39   7 Shot blade to Anthesis 12     5.0    6  -8  -3  -7    3 -16    2  12   2 Anthesis to Maturity 34 124.0   59 14 17 35   47   14  14  39 26 Total   175.2 107 39 32 49   53   25  16  65 40        Rate of N applied (kg ha-1) Period Days Precipitation +Irrigation   0 20.5 41 62 82 123 164 Mean cropped Irrigated  (mm)  ----------------------- Apparent Nmin  (kg ha-1) --------------------- 3-Leaf to Tillering 11   54.3    3   5 15  26 -2   11  8 10 Tillering to Shot blade 14   42.7  10   3 -2 -18 12 -18  0   4 Shot blade to Anthesis 12 106.6    9   3  1  24   4   -4 -8   6 Anthesis to Maturity 34 124.0  37 63 76  39 40  35 29 46 Total  327.6   59 74 92  89 56  46 37 73  Fig. 1.  Relationship between net Nmin during the growing season and precipitation between spring and fall, for (a) (F)-W-W (+P), (b) F-(W) (N+P), (c) Cont W (N+P) and (d) F-W-(W) (+P) [possible outliers shown with a *] 04080120160200(a) (F) -W-W (+P)1974197519701982y = 0.55 x + 3.1R2 = 0.42Net Nmin(0 to0.9 m) (kg ha-1)Precipitation (mm)Figure 1y = 0.39 x - 20.5R2= 0.42(b) F-(W) (N+P)197319701971120160y = 0.24 x - 0.9R2= 0.43040800 50 100 150 200 250 300(c) Cont W (N+P)1969 19751970y = 0.14 x + 24.5R2= 0.120 50 100 150 200 250 300 350(d) F-W-(W) (+P)197319741979  

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Effect of fallow frequency, fertilizer, and available water on net N mineralization under wheat grown in the semi-arid Brown Soil Zone

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