Leachate quantity and climatic conditions
Across treatments, collected leachate amounts averaged 27, 10 and 14% of precipitation plus irrigation in S1, S2 and S3, respectively (Table 2).
Table 2
Total precipitation and irrigation (mm) per period, cumulative lysimeter leachate (mm) and mineral nitrogen (N) load in leachate (kg ha− 1) per period. Lowercase letters represent treatment differences (p < 0.05). F1 = Initial fallow, S1 = 1st year with crops, S2 = 2nd year with crops, S3 = 3rd year with crops, F2 = latter fallow. Control = no N fertilizer, Shallow = shallow N fertilizer placement (0.07 m), Mixed = mixed placement of N fertilizer (half at 0.07 m, half at 0.2 m), Deep = deep placement of N fertilizer (0.2 m).
Period | Precip + Irrigation (mm) | | Control | Shallow | Mixed | Deep |
F1 | 322 | Leachate (mm) | 83 ± 17 | 133 ± 12 | 110 ± 15 | 109 ± 7 |
| | N load (kg ha− 1) | 25 ± 11 | 69 ± 11 | 49 ± 4 | 53 ± 12 |
S1 | 599 | Leachate | 179 ± 10 | 168 ± 6 | 160 ± 17 | 148 ± 5 |
| | N load | 41 ± 11b | 124 ± 12a | 108 ± 13a | 96 ± 21ab |
S2 | 637 | Leachate | 101 ± 27a | 36 ± 6ab | 94 ± 14a | 27 ± 4b |
| | N load | 15 ± 5ab | 11 ± 2ab | 24 ± 4a | 9 ± 2b |
S3 | 642 | Leachate | 97 ± 14 | 114 ± 33 | 79 ± 14 | 74 ± 4 |
| | N load | 14 ± 3 | 46 ± 22 | 15 ± 3 | 20 ± 3 |
F2 | 746 | Leachate | 165 ± 10 | 211 ± 48 | 195 ± 38 | 150 ± 5 |
| | N load | 10 ± 1 | 62 ± 52 | 15 ± 9 | 12 ± 1 |
Over the three periods (S1 – S3) % total leachate quantity relative to total water inputs was lowest in Deep and highest in Control following the pattern of Control > Mixed > Shallow > Deep, corresponding to 20, 18, 17 and 13%.
The Control had significantly higher (p = 0.04) mean cumulative leachate amount (± SE) for the three growing seasons (S1-S3), 377 ± 37 mm H2O, compared to the lowest in deep, 249 ± 12 (Fig. 2).
S1 = 1st year with crops, S2 = 2nd year with crops, S3 = 3rd year with crops, F2 = latter fallow. Control = no N fertilizer, Shallow = shallow N fertilizer placement (0.07 m), Mixed = mixed placement of N fertilizer (half at 0.07 m, half at 0.2 m), Deep = deep placement of N fertilizer (0.2 m). Day 0 = initial fertilization.
Mixed and Shallow were intermediates with mean cumulative leachate of 332 ± 36 and 319 ± 25, respectively. The Deep placement had the lowest quantity of leachate for all periods except for the initial F1 period. Within fertilized treatments, Deep leached 25% less water compared to Shallow and 29% less than Mixed.
During the initial fallow period there were no statistical differences in water flow, although there was some variation between individual lysimeters and overall among the different treatments (Fig. 3, Table 2).
F1 = Initial fallow, S1 = 1st year with crops, S2 = 2nd year with crops, S3 = 3rd year with crops, F2 = latter fallow. Control = no N fertilizer, Shallow = shallow N fertilizer placement (0.07 m), Mixed = mixed placement of N fertilizer (half at 0.07 m, half at 0.2 m), Deep = deep placement of N fertilizer (0.2 m).
Mean leachate amount was somewhat higher in Shallow (133 ± 12 mm) and lowest in the Control (83 ± 17 mm). The Mixed and Deep treatments had intermediate water flow in F1 with 110 ± 15 and 109 ± 7 mm. Total precipitation and irrigation during the F1 period was approximately 322 mm, the lowest quantity of all periods (Fig. 1, Table 2).
In the winter and spring following the first cropping season (S1), the pattern of water flow changed from the preceding F1 period. The S1 period (June 2017-April 2018) was generally wetter than the preceding period, particularly in the autumn and winter months (Fig. 1). The sum of precipitation and irrigation in S1 was 599 mm. Though there were no significant differences in mean cumulative leachate, the treatments in S1 were by amount Control > Shallow > Mixed > Deep corresponding to 179 ± 10, 168 ± 6, 160 ± 17, and 148 ± 5 mm, respectively (Figs. 2 and 3, Table 2).
In the second cropping year (S2), nearly every month had both lower precipitation and higher average temperatures compared to the long-term normal (Fig. 1), particularly during the cropping period from May-July 2018. Although it appears that July received sufficient precipitation, the majority occurred late in the month on a single day, when 79 mm out of the monthly total of 82 mm rain fell. Lysimeter leachate quantity was greatly affected by the drought and only two of the four treatments, Control and Mixed placement, were releasing water by February 2019, and in large quantities. Sufficient quantities of water did not flow from all lysimeters until around April 2019. The mean cumulative amount for S2 (± SE) was significantly higher in the Control (101 ± 27 mm) and Mixed placement (94 ± 14 mm) compared with Shallow (36 ± 6 mm) and deep (27 ± 4 mm) (Fig. 3, Table 2). In terms of within-treatment comparisons, the Control treatment water flow in this period was significantly higher than at any other sampling time during S1-S3 with the exception of one sampling occasion immediately following, in December 2019 (Fig. 3). Similarly, leachate quantity in the Mixed placement was significantly higher than all other sampling times in S1-S3.
Rainfall was relatively closer to long-term normal during S3 in comparison with the previous year, although some compensation with irrigation was necessary during May - July 2019 (Fig. 1). Mean cumulative leachate (± SE) in S3 followed the order of Shallow > Control > Mixed > Deep corresponding to 114 ± 33, 97 ± 14, 79 ± 14, and 74 ± 4 mm although there were no significant differences between treatments (Table 2).
Though not significant, the deep placement continued to leach less water than all other treatments into the F2 period despite the absence of crops, although most treatment effects tapered off after December 2020 (Fig. 3). Mean cumulative water flow followed the order of Shallow > Mixed > Control > Deep and corresponded to 210 ± 47, 195 ± 37, 165 ± 10, and 150 ± 5 mm (± SE). Total rainfall in F2 was around 746 mm precipitation over the 16-month period (Fig. 1, Table 2).
N Load and N volume-weighted concentrations
Cumulative N load for all cropping seasons (S1-S3) was lowest in the Control, 70 ± 18 kg N ha− 1, and highest in Shallow, 181 ± 21 kg N ha− 1 with significant differences (p = 0.009) between the two treatments. The Mixed and Deep placements were intermediates with mean cumulative N loads of 147 ± 14 and 124 ± 24 kg ha− 1 respectively, resulting in a reduction of leachate N losses of 21 and 37% compared to shallow.
N losses in leachate during F1 were relatively low in the Control (25 ± 11 kg N ha− 1) compared to the other treatments (69 ± 11, 49 ± 4, 53 ± 12 kg N ha− 1 for Shallow, Mixed, and Deep respectively), even though no fertilizer had been applied to any of the treatments. Nitrogen load steadily increased in all lysimeters after the initial disturbance at the beginning of the setup (Fig. 3) and by the end of F1, treatment differences were nearly significant (p = 0.055). This flush of mineralized N peaked in S1 and began to decline in S2. Total mean N in leachate (± SE) in F1 was 25 ± 11, 69 ± 11, 49 ± 4, and 53 ± 12 kg N ha− 1 for the control, shallow, mixed and deep placements respectively. In S1, total mean leachate N increased to 41 ± 11, 124 ± 12, 108 ± 21, and 96 ± 13 kg N ha− 1 for Control, Shallow, Mixed, and Deep respectively, and there were significant treatment differences where Shallow, Mixed > Deep > Control, with Shallow and Mixed significantly higher than the Control (Fig. 2, Table 2).
Subsequent to S1, average N loads decreased below F1 levels. During the growing season of S2 there was a drought during the critical part of the growing period from May to July in 2018, with low precipitation and high temperatures compared to the long-term normal (Fig. 1). Consequently, there was just one brief period of water flow the following spring (Fig. 3) from late February to April 2019, in which the Mixed placement had the highest leachate N load, significantly higher than Deep (p = 0.04), following the pattern Mixed > Control, Shallow > Deep. S2 mean leachate N loads (± SE) were 15 ± 5, 11 ± 2, 24 ± 4, and 9 ± 2 kg N ha− 1 for Control, Shallow, Mixed, and Deep respectively (Table 2).
In the third growing season (S3), precipitation was higher, particularly in October-December, relative to the previous drought year, resulting in a different pattern in leachate N loads, where Shallow and Deep had greater N loads compared to Control and Mixed, although not significantly. Total mean N loads (± SE) were 14 ± 3, 46 ± 22, 15 ± 3, and 20 ± 3 kg N ha− 1 for Control, Shallow, Mixed, and Deep respectively (Table 2). An individual lysimeter in the Shallow treatment leached N at levels 5 times greater than all other lysimeters, and continued to leach high levels of N well into F2 the following year (Figs. 2 and 3). In the last period when no fertilization or cropping had occurred, the N load in the majority of lysimeters declined. Cumulative treatment N load means for F2 (± SE) for the Control, Shallow, Mixed, and Deep were 10 ± 1, 62 ± 52, 15 ± 9, and 12 ± 1 kg N ha− 1 respectively.
Greenhouse gas fluxes
Only about 9% of N2O fluxes had a p-value within acceptable range, and of those about half surpassed the variability of the detection limit of the GC (max ppm – min ppm < GC detection limit). Likewise, 25% of CH4 fluxes had a p-value < 0.05, and of those only one was below the GC detection limit.
Photosynthetic CO2 uptake in the Control was greatest in the earlier stages of plant growth (Fig. 4) in both S1 and S2 with the Shallow treatment following a similar trend or with somewhat less uptake than the Control.
S1 = 1st year with crops, S2 = 2nd year with crops. Control = no N fertilizer, Shallow = shallow N fertilizer placement (0.07 m), Mixed = mixed placement of N fertilizer (half at 0.07 m, half at 0.2 m), Deep = deep placement of N fertilizer (0.2 m).
The Mixed and Deep treatments tended to have greater CO2 uptake later in the growing period relative to Control and Shallow. In 2017 (S1), the Control had the greatest CO2 uptake but the following year was the lowest and nearly half that of the fertilized treatments. Total uptake (sum of negative fluxes) in S1 was 549, 477, 487 and 472 mg CO2-C m− 2 h− 1 for Control, Shallow, Mixed, and Deep respectively. In S2 the pattern of total uptake changed, and was 366, 618, 646, and 654 mg CO2-C m− 2 h− 1 for Control, Shallow, Mixed, and Deep respectively.
Biomass yield and N balance
Nitrogen use efficiency (NUE) was high in all fertilized treatments in all seasons due to the high crop N output relative to the fertilization rate (Table 3, Fig. 5).
Table 3
Nitrogen (N) balance components per cropping season, N use efficiency (NUE) (%), agronomic efficiency of N (AEN) (kg kg-1), and the recovery efficiency of N (REN) (%). N balance values are presented as treatment mean value (kg N ha− 1) ± standard error. Lowercase letters represent significant treatment differences within the given period (p < 0.05). S1 = 1st year with crops, S2 = 2nd year with crops, S3 = 3rd year with crops. Control = no N fertilizer, Shallow = shallow N fertilizer placement (0.07 m), Mixed = mixed placement of N fertilizer (half at 0.07 m, half at 0.2 m), Deep = deep placement of N fertilizer (0.2 m).
| S1 | S2 | S3 |
Control | Shallow | Mixed | Deep | Control | Shallow | Mixed | Deep | Control | Shallow | Mixed | Deep |
N inputs and outputs (kg N ha− 1) |
Seeds | 8 | 8 | 8 | 8 | 8 | 8 | 8 | 8 | 8 | 8 | 8 | 8 |
Fertilization | | 100 | 100 | 100 | | 100 | 100 | 100 | | 100 | 100 | 100 |
Harvested grain | -118 ± 17 | -141 ± 14 | -157 ± 9 | -155 ± 13 | -80 ± 4b | -127 ± 9a | -158 ± 12a | -157 ± 5a | -95 ± 6b | -151 ± 25ab | -145 ± 16ab | -190 ± 5a |
Harvested straw | -27 ± 2b | -38 ± 6ab | -43 ± 2a | -43 ± 1a | -13 ± 1b | -30 ± 3a | -26 ± 4a | -31 ± 1a | -18 ± 2b | -32 ± 1a | -29 ± 2a | -30 ± 2a |
Leachate load | -41 ± 11b | -124 ± 12a | -108 ± 13a | -96 ± 21ab | -15 ± 5ab | -11 ± 2ab | -24 ± 4a | -9 ± 2b | -14 ± 3 | -46 ± 22 | -15 ± 3 | -20 ± 3 |
N surplus | -179 | -195 | -200 | -185 | -100 | -60 | -101 | -89 | -119 | -121 | -80 | -132 |
NUE % | n.a. | 179 | 200 | 198 | n.a. | 157 | 184 | 188 | n.a. | 183 | 174 | 220 |
AEN (kg kg− 1) | n.a. | 0.23 | 0.39 | 0.36 | n.a. | 0.47 | 0.78 | 0.77 | n.a. | 0.56 | 0.5 | 0.95 |
REN (%) | n.a. | 34 | 55 | 53 | n.a. | 64 | 92 | 95 | n.a. | 70 | 60 | 107 |
S1 = 1st year with crops, S2 = 2nd year with crops, S3 = 3rd year with crops. Control = no N fertilizer, Shallow = shallow N fertilizer placement (0.07 m), Mixed = mixed placement of N fertilizer (half at 0.07 m, half at 0.2 m), Deep = deep placement of N fertilizer (0.2 m).
N surplus, the difference between N inputs to and outputs from the system, was highest in all treatments in S1 and lowest in S2, the latter of which occurred during a drought and resulted in the lowest N outputs from almost all components for all treatments.
In S1, N losses via leachate were very high (Fig. 3, Table 2), even in the control, and leachate accounted for the second-highest output from the system after harvested grain N (Table 3). Although yields were similar among treatments in S1 (Table 4), due to the difference in grain and straw N uptake, the Mixed and Deep treatments had higher outputs in grain and straw, the latter significantly higher, compared with the control (p = 0.02).
Table 4
Grain yield (tons ha− 1), straw and grain N content (% N). Lowercase letters represent treatment differences within the given period (p < 0.05). S1 = 1st year with crops, S2 = 2nd year with crops, S3 = 3rd year with crops. Control = no N fertilizer, shallow = shallow N fertilizer placement (0.07 m), mixed = mixed placement of N fertilizer (half at 0.07 m, half at 0.2 m), deep = deep placement of N fertilizer (0.2 m).
Period | Parameter | Control | Shallow | Mixed | Deep |
S1 | Grain yield | 6.4 ± 0.3 | 6.2 ± 0.4 | 6.7 ± 0.5 | 6.6 ± 0.5 |
| Grain % N | 1.83 ± 0.20b | 2.26 ± 0.14ab | 2.38 ± 0.08a | 2.29 ± 0.07ab |
| Straw % N | 0.53 ± 0.02 | 0.69 ± 0.08 | 0.72 ± 0.02 | 0.71 ± 0.03 |
S2 | Grain yield* | 5.8 ± 0.2b | 6.1 ± 0.7ab | 7.7 ± 0.6a | 7.5 ± 0.2ab |
| Grain % N | 1.4 ± 0.03b | 2.1 ± 0.09a | 2.0 ± 0.02a | 2.1 ± 0.03a |
| Straw % N | 0.30 ± 0.02b | 0.43 ± 0.02a | 0.45 ± 0.02a | 0.50 ± 0.02a |
S3 | Grain yield | 6.7 ± 0.3b | 7.9 ± 1.5ab | 8.3 ± 1.1ab | 10.1 ± 0.2a |
| Grain % N | 1.41 ± 0.02b | 1.97 ± 0.09a | 1.77 ± 0.07a | 1.89 ± 0.02a |
| Straw % N | 0.22 ± 0.02b | 0.34 ± 0.03a | 0.29 ± 0.02ab | 0.27 ± 0.01ab |
*excluding outlier lysimeter 13 (Shallow treatment) |
Additionally, mixed placement had significantly higher grain N content compared to the control (p = 0.045) (Table 4).
Both AEN, a measure of grain N uptake efficiency that accounts for N uptake in the non-fertilized control, and REN, where the additional N in straw is incorporated into the calculation, followed a similar trend over time, with the greatest treatment differences occurring in the final S3 season. This trend is similar to the trend in grain yield between the Mixed and Deep treatments (Table 4). During S1 and S2, both Mixed and Deep treatments had similar yields and crop N uptake, and thus similar system losses in the form of crop N, and were greater than Shallow placement in both years. However, in S3, when there was neither drought nor an excess of mineralized N as in the previous years, Mixed placement had intermediate yields and the lowest crop N uptake among the fertilized treatments, although leachate losses were as low as the control (Table 3).
In the cumulative N balance (sum of S1-S3) (Fig. 5), the Deep placement had the greatest N surplus but the highest amount and proportion (83%) of N losses from harvested crops relative to total N losses among the fertilized treatments. An opposite trend was observed in the Shallow placement, where crop N losses were lowest but leaching losses were highest, while Mixed placement was an intermediate to Deep and Shallow. The Control had the second highest N surplus, primarily due to no fertilizer input, and each N loss component was the lowest among all treatments. However, the proportion of N losses in the Control were similar to the Deep placement, but approximately 1.7 times lower in each component.