Seasonal conditions
The period encompassing the application and then cessation of supplementary nutrients onto residues (Phase 2 to 4, 2006 to 2019) was generally characterised by below-average growing season rainfall (mid-May to November) which equalled or exceeded the long-term mean (364 mm) in only 3 of the 14 years (2010, 2015 and 2016) (Fig. 1). Spring rainfall (September and October) which has a significant impact on crop yield potential due to its coincidence with crop reproductive phases, was especially low in many seasons. Crops producing high early biomass early in the season may suffer significant yield penalties when spring conditions are dry in the process known as ‘haying-off’ whereby soil water is exhausted by the vigorous early growth and crops suffer significant stress during the grain filling stage. Risks of “haying-off” are often cited by growers as a reason for conservative application of N fertiliser. In contrast, the rainfall in the periods following residue incorporation until the time of sowing (approximately February to mid-May) was generally near to, or above average, except for two years (2013 and 2018). Together with the warm summer temperatures (mean minimum 10°C, mean maximum 28°C) this provided good conditions for the decomposition of the incorporated residues by soil microorganisms prior to sowing in most years.
Crop growth, yield and quality and residue loads
A summary of the crop growth, yield and residue levels for the + and – supplementary nutrient treatments is provided in Table 4. Crop establishment was unaffected by supplementary nutrient additions to the residue in any year (except for a small reduction in wheat in 2011), and plant populations established were at or above recommended levels for all crops throughout the experimental period. Where measured, there was a tendency towards higher early vegetative biomass when supplementary nutrients were added to residues, which was significant in some years and persisted after supplementary nutrient application ceased in 2014. Despite these intermittent effects on early biomass production there was little impact of supplementary nutrient addition on final crop biomass (2012 only) and no effect on the total biomass grown during Phases 3 or 4. There were few significant effects of supplementary nutrients on grain yield, mostly evident in the machine harvested yield, though the trends were similar in hand-harvested yield. The significant effects on yield were both positive (2014, 2016) and negative (2011, 2019) and occurred both during the period of supplementary nutrient application (Phase 2, 3; 2011, 2014) and after it had ceased (Phase 4; 2016, 2019). Notably there was a significant negative impact of (prior) nutrient addition on grain yield and protein in 2019, five years after supplementary nutrient addition had ceased. This occurred under conditions of an extreme spring drought (Fig. 1) providing evidence of ’haying off‘. Despite these individual year impacts, there was no significant effect on the total grain yield removed over the 8 years of Phase 3 and 4. In contrast to grain yield, there were consistent significant increases in grain N content in response to the supplementary nutrient addition in Phase 2 and 3 during the period of supplementary nutrient addition, which also persisted in some years during Phase 4 (2017, 2019). This was most extreme during the very dry 2019 season where an increase in grain N content was observed. As grain N can be diluted in higher yielding crops, the amount of N removed in grain combines impacts on grain yield and N content. Supplementary nutrients resulted in higher grain N removal in 2011, 2012, 2013 and 2017 but with less N removal in 2019. The effect of supplementary nutrients on the amount of crop residue produced and remaining increased in only 1 year of 9 (2012), and there was no effect on the total residue returned over the 8 years.
Table 4
Summary of plant responses to addition of supplementary nutrients to incorporated residue at Harden long-term experiment. Supplementary nutrients were added to incorporated residue after harvest from 2007 to 2011 (phase 2 as published in Kirkby et al. 2016) and continued to 2014 (phase 3), when addition was ceased and the residual impact was measured on crops to from 2015 to 2019 (phase 4). Significant effects of the nutrient treatments for each and parameter are shown as; ᵻ (P < 0.1), * (P < 0.05), ** (P < 0.01), *** (P < 0.001). Note: Grain N % of lupin (2014) (shown in italic) were not measured but assumed for N removal estimations. Values shown as (-) where not determined.
Year/Crop | Supplementary Nutrients | Density (plants m− 2) | Total Biomass (g m− 2) | | Grain Yield (g m− 2) | Harvest Index | Grain N (%) | N removed (kg ha− 1) | Straw remaining (g m− 2) | |
Z30 | Anthesis | Harvest | | Quad | Header | | | | | |
Phase 2: Supplementary nutrients applied to stubble (Kirkby et al., 2016) | |
2007 (wheat) | No | - | - | - | 986 | | 290 | 241 | 0.29 | 3.06 | 74 | 695 | |
2008 (wheat) | No | 126 | - | 1304 | 1277 | | 381 | 327 | 0.30 | 2.84 | 93 | 896 | |
Yes | 130 | - | 1190 | 1163 | | 293ᵻ | 225** | 0.25* | 3.06 | 69* | 869 | |
2009 (wheat) | No | 188 | 116 | 934 | 1078 | | 320 | 275 | 0.30 | 2.56 | 70 | 758 | |
Yes | 176 | 275*** | 992 | 1250 | | 310 | 238* | 0.25* | 3.18* | 75 | 940 | |
2010 (canola) | No | 84 | - | - | 827 | | 267 | - | 0.33 | 3.59 | 0 | 560 | |
Yes | 73 | - | - | 1487* | | 433* | - | 0.29*** | 4.20ᵻ | 0 | 1054* | |
2011 (wheat) | No | 155 | 273 | 1311 | 1638 | | 548 | 460 | 0.34 | 2.30 | 106 | 1090 | |
Yes | 133*** | 292 | 1376 | 1490 | | 483 | 424* | 0.32 | 2.67*** | 113* | 1007 | |
Phase 3: Supplementary nutrients applied to stubble | |
2012 (wheat) | No | 104 | 111 | 1051 | 1580 | | 712 | 553 | 0.45 | 2.17 | 120 | 868 | |
Yes | 106 | 131* | 1192ᵻ | 1835ᵻ | | 794 | 582 | 0.43 | 2.42** | 141** | 1041* | |
2013 (wheat) | No | 150 | 202 | 1180 | 905 | | 426 | 383 | 0.47 | 2.14 | 82 | 479 | |
Yes | 155 | 278 | 1198 | 1023 | | 392 | 374 | 0.38** | 2.54** | 95 | 631 | |
2014 (lupin) | No | 51 | - | 618 | 841 | | 260 | 176 | 0.31 | 5.7 | 100 | 581 |
Yes | 51 | - | 703 | 1109 | | 401 | 309 ᵻ | 0.35 | 5.7 | 176 ᵻ | 708 | |
Phase 4: No supplementary nutrients added | |
2015 (canola) | No | 68 | - | 411 | 1173 | | 250 | - | 0.21 | 4.2 | 105 | 923 | |
pre-2015 | 80 | - | 501 ᵻ | 931 | | 171 | - | 0.18 | 4.7 | 80 | 760 | |
2016 (wheat) | No | - | - | - | 1594 | | 724 | 644 | 0.45 | 2.07 | 152 | 870 | |
pre-2015 | - | - | - | 1626 | | 741 | 698 ᵻ | 0.46 | 2.13 | 159 | 885 | |
2017 (wheat) | No | 176 | 195 | 850 | 1169 | | 502 | 462 | 0.43 | 1.65 | 78 | 667 | |
pre-2015 | 185 | 223 ᵻ | 980 ᵻ | 1207 | | 513 | 482 | 0.42 | 1.84** | 90* | 694 | |
2018 (canola) | No | - | - | - | 704 | | 220 | 202 | 0.28 | 4.60 | 89 | 491 | |
pre-2015 | - | - | - | 669 | | 204 | 188 | 0.27 | 4.58 | 78 | 472 | |
2019 (wheat) | No | 177 | 514 | 1426 | 1188 | | 276 | 281 | 0.23 | 2.58 | 69 | 913 | |
pre-2015 | 169 | 572 ᵻ | 1307 | 1117 | | 194* | 211** | 0.17*** | 3.10* | 62 ᵻ | 922 | |
Crop Phase totals | |
Phase 3 (2011–2014) | No | | 4892 | | 1946 | 1572 | | 408 | 3018 | |
Yes | 5457 | | 2070 | 1689 | 525 | 3387 | |
Phase 4 (2015–2019) | No | 5828 | | 1972 | 1589 | 493 | 3864 | |
pre-2015 | 5550 | | 1823 | 1579 | 469 | 3733 | |
Phase 3 + 4 (2011–2019) | No | 10720 | | 3918 | 3161 | 881 | 6882 | |
pre-2015 | 11007 | | 3893 | 3268 | 974 | 7120 | |
Change in FF-C and FF-N concentration
The profiles of FF-C and FF-N for the plus and minus supplementary nutrient addition treatments are shown in Fig. 2 at the end of Phase 2 in 2012 (data redrawn from Kirkby et al. 2016), Phase 3 in 2015, and Phase 4 in 2020. The parameters for the fitted curves in Fig. 2 are provided in Table S2. The increase in FF-C and FF-N with supplementary nutrient addition evident throughout the soil profile after Phase 2 in 2012 persisted in 2015, after the ongoing period of supplementary nutrient addition in Phase 3 (compare Fig. 2a, b with Fig. 2c, d). Although the effect was no longer significant at all individual depths, it remained so in the surface layers, and the fitted curves down the entire profile remained significantly different. In contrast, at the end of Phase 4 in 2020, 5 years after the supplementary nutrient application to residues was ceased, the differences in FF-C and FF-N were absent at depths below 45cm, and the differences in the surface layers had also diminished (compare Fig. 2c, d with Fig. 2e, f). Despite this change in the pattern of differences down the profile, the fitted curves remained significantly different (Table S2), being driven mostly by higher levels of FF-C and FF-N in the plus nutrient treatments persisting in the surface layers (Fig. 2e, f). Thus the increase in FF-C and FF-N established by the addition of supplementary nutrients to residues during Phase 2 and Phase 3 though diminished, persisted for at least 5 years to the end of Phase 4. Throughout the entire period, there was little impact of the supplementary nutrient treatments on the (FF-C):(FF-N) ratio (Table S4). However, consistent with observations by Kirkby et al. (2016), the C:N ratio declined significantly with depth from around 11 to 12 in the surface layers to 5–7 in the deepest layers (Table S4).
Change in FF-C and FF-N stocks
The profiles of FF-C and FF-N stocks for the plus and minus supplementary nutrient addition treatments across the 3 phases (2012, 2015 and 2020) are shown in Fig. 3. The parameters for the fitted curves in Fig. 3 are provided in Table S3. Due to the limited impact of the supplementary treatment on bulk density, the pattern of FF-C and FF-N stocks followed the same patterns as those for the concentrations shown in Fig. 2. The cumulative stocks across the entire soil profile for the same sampling times are shown in Table 5.
Table 5
Cumulative stocks of carbon (C) and nitrogen (N) associated with fine fraction soil to a depth of 1.8m for Harden soil in 2012, 2015 and 2020 following supplementary nutrient addition to incorporated residue nutrient across the phases of the experiment. Data and the difference are presented as the mean ± 1 standard error of paired replicates (n = 4) across experimental blocks.
Year (Phase) | | Fine fraction soil carbona (t C ha− 1) | | Fine fraction soil nitrogenb (t N ha− 1) | |
| | - nutrient | + nutrient | difference | | - nutrient | + nutrient | difference | |
2012 (Phase 2) | | 56.4 ± 3.1 | 66.0 ± 3.9 | 9.6 ± 1.2 | | 7.05 ± 0.36 | 7.87 ± 0.36 | 0.82 ± 0.03 | |
2015 (Phase 3) | | 44.8 ± 2.7 | 54.6 ± 3.6 | 9.8 ± 2.8 | | 4.31 ± 0.35 | 5.31 ± 0.41 | 0.99 ± 0.21 |
2020 (Phase 4) | | 47.9 ± 2.7 | 50.9 ± 3.0 | 3.0 ± 1.9 | | 5.52 ± 0.29 | 5.87 ± 0.19 | 0.35 ± 0.33 |
a Two-way analysis of variance for FFC indicated a significant main effect due to both year (P < 0.001; LSD0.05 =3.5) and nutrient addition (P < 0.001; LSD0.05 =2.8) with no significant interaction.
a Two-way analysis of variance for FFN indicated a significant main effect due to both year (P < 0.001; LSD0.05 =0.43) and nutrient addition (P < 0.001; LSD0.05 =0.30) with no significant interaction.
The cumulative data show that the difference in FF-C and FF-N stocks established at the end of Phase 2 (9.6 t ha− 1 and 0.82 t ha− 1, respectively) were effectively maintained during the ongoing application of nutrients in Phase 3 (to 9.8 t ha− 1and 0.99 t ha− 1) but declined to 3.0 t ha− 1 for FF-C and 0.35 t ha− 1for FF-N after 5 years without supplementary nutrient application (i.e., at the end of Phase 4 in 2020). Despite the overall decline in the absolute FF-C and FF-N stocks in both treatments in Phase 3 presumably linked to climatic or overall negative nutrient balance, the differences generated by the supplementary nutrients persisted.
Nitrogen mass balance
The N mass balance for Phase 2, 3 and 4 is shown in Fig. 4, and was calculated using N inputs (Table 3) and N removal in harvested grain (Table 4) to calculate the net N addition in each treatment over time. The results show the plus supplementary nutrient treatment diverging from a neutral to a net positive N balance during Phase 2 and into Phase 3, while the minus nutrient treatment remained in a net negative balance during the same period. Both treatments declined in N balance in 2014 and 2015 as no N fertiliser was added to the lupin crop in 2014, and only 7 kg N ha− 1 to the subsequent canola crop. This sharp decline in N balance in both treatments was arrested from 2016 onwards during Phase 4 when the top-dressed N added to the cereal crops was better matched the removal in grain. The decline in the N balance after 2014 in Fig. 4 does not account for the likely N-fixation by the lupin crop, which if effective, could have been as high as 140 kg N ha− 1 (based on 20 kg N per t above ground biomass, Peoples and Craswell 1992). Differences in the levels of soil mineral N generated by the supplementary nutrient treatments could have generated differences in the fixed N between the nutrient treatments, so it is difficult to be definitive about the effects on the N balance. In addition to this uncertainty the level of potential leaching loss at the site is also not known although the estimates of leaching at the site based on APSIM simulations (Lilley et al. 2019) suggest significant leaching events (> 50 kg N ha− 1 ) were only likely in the wet years of 2010, 2012 and 2016 (rainfall shown in Fig. 1). Differential N leaching may have occurred between the treatments if higher levels of mineral N were present in the soil at the time of the high rainfall.
Economic analysis
A simple economic analysis comparing the cost of the supplementary fertiliser with the potential change in income derived by yield increases or payments for sequestered soil C (@$40 t− 1) was conducted and is summarised in Table 6. The fertiliser cost was calculated for both the actual costs used (Starter15 @$500 ha− 1), but also using the cheapest equivalent forms of fertiliser to supply only the levels of N, P and S required to sequester the C (see Table S5).
By 2015 supplementary nutrients (as Starter15) amounted to a cost of $1109 ha− 1 with an extra 9.8 tha− 1of soil C being sequestered (35.9 t CO2 equivalent @$40 t− 1) worth $1,436 ha− 1. Crop yields in the period 2007 to 2015 were changed by a net increase in value up to 2015 of +$837 ha− 1. Consequently, by 2015 the use the Starter15 as a nutrient source generated a return of $2273 ha− 1 from additional grain yield and C credit income. This is a profit difference after 8 years of $1164 ha− 1 for an investment of $1109 or a 1.05 return on investment. Using the optimised fertiliser strategy reduced the cost to $687, increasing the profit to $1586 ha− 1 for a 1.43 return on investment. Notably the payment for soil C @$40 t− 1 CO2 equivalent is the larger driver of the profit given the lower overall and variable annual effect of the treatment on crop yield during these seasons.
Table 6
Costs of supplementary fertiliser and effects on annual crop yield and the value of the increase in soil carbon (C) measured at the end of Phase 2, 3 and 4.
Year (Crop) | Actual fertilisera (kg ha− 1) | Cost @ $500 t− 1 | Optimized fertiliserb ($ ha− 1) | Yieldc change (t ha− 1) | Value of yield changed ($ ha− 1) | Soil C difference measured (t ha− 1) | Value @ $40 t− 1 CO2 equivalentse | |
Phase 2: Supplementary nutrients applied to stubble (Kirkby et al. 2016) | |
2007 (wheat) | - | - | | - | - | | | |
2008 (wheat) | 273 | $137 | $90 | -1.0* | - $290* | | | |
2009 (wheat) | 350 | $175 | $105 | -0.4* | -$116* | | | |
2010 (canola) | 364 | $182 | $106 | + 1.6 | +$885 | | | |
2011 (wheat) | 454 | $227 | $133 | -0.4* | -$116* | 9.6 | $1,406 | |
Phase 3: Supplementary nutrients applied to stubble | |
2012 (wheat) | 420 | $210 | $123 | + 0.3 | +$87 | | |
2013 (wheat) | 301 | $150 | $90 | -0.1 | -$29 | | |
2014 (lupin) | 56 | $28 | $40 | + 1.3* | +$416* | 9.8 | $1,436 |
Phase 4: No supplementary nutrients added | |
2015 (canola) | 0 | 0 | 0 | -0.8* | -$442* | | | |
2016 (wheat) | 0 | 0 | 0 | + 0.6* | +$174* | | | |
2017 (wheat) | 0 | 0 | 0 | + 0.2 | +$58 | | | |
2018 (canola) | 0 | 0 | 0 | -0.1 | -$55 | | | |
2019 (wheat) | 0 | 0 | 0 | -0.7* | -$203* | 3.0 | $439 | |
Total to 2015 | | $1,109 | $687 | | -$106 (+$837) | | | |
Total to 2020 | | $1,109 | $687 | | -$577 (+$369) | | | |
a starter15 fertilizer = 14.3% N, 12% P, 10.5% S
b nutrient costs based on optimized use of fertilizer products (see Supplementary Information Table S5)
cd * indicates significant yield difference (P < 0.05)
d assumes long-term grain prices of $290 t− 1 (wheat); $553 t− 1 (canola), $320 t− 1 (lupin)
e based on prices at https://www.renewableenergyhub.com.au/market-prices/
In the period after nutrient addition ceased (2015–2019, Phase 4) there was no further cost of supplementary nutrients, and additional crop yield income overall up to 2019 was reduced to $369 ha− 1, and the difference in soil FF-C by 2020 had declined from 9.8 tha− 1 to 3.0 t ha− 1 worth only $439. Consequently, the position in 2019 was either a loss of $301 ha− 1 (using Starter 15 (14:12.7:0:11, NPKS) as a fertiliser source) or a small profit difference of $121 using optimised fertiliser strategy. In summary, using an optimised fertiliser strategy, supplementary nutrient addition to residue can generate reasonable profits, primarily from payments for sequestered soil C rather than yield benefits, but these may be quickly eroded when nutrient addition to residue ceased for a period of 5 years.
The costs of fertiliser and the prices paid for grain and for soil C are all subject to market fluctuations which will influence the economic outcomes from these scenarios. For example, in 2021/22 the cost of fertilisers more than doubled and grain prices also increased significantly while the prices paid for C remained relatively static. To examine the sensitivity of the profit difference outcomes to these variations, we adjusted the price paid for fertiliser up by 50% and 100% (which in reality occurred in 2021), and adjusted the price paid for C and for grain by -50% and + 50% (Table 7).
Table 7
Cumulative profit diiference resulting from the application of supplementary nutrients to crop residues after 8 years (2015, end of Phase 3) and in 2020 (end of Phase 4) after a subsequent 5 years without nutrient application to residues. Profit outcomes were generated using long-term average (LTA) prices for grain and fertiliser and a carbon (C) price of $40 t− 1 CO2 equivalents (as described in Table 6) and are shown in bold text. The effect of changing the C price (+/- $20 t− 1), changing the LTA grain prices by -50% and + 50% and increasing the LTA fertiliser price by 50% and 100% on the profit difference are shown.
| | | Carbon price $CO2 t− 1 equivalence |
| | | $20 t− 1 (-50%) | | $40 t− 1 (current) | | $60 t− 1 (+ 50%) |
Years / Phase | | | Grain price | | Grain price | | Grain price | |
| | | -50% | LTA | +50% | | -50% | LTA | +50% | | -50% | LTA | +50% |
2015 End of Phase 3 (after 8 years of nutrient application) | |
Fertiliser price | LTA | | $499 | $868 | $1286 | | $1167 | $1586 | $2004 | | $1885 | $2304 | $2722 | |
| + 50% | | $106 | $525 | $943 | | $824 | $1243 | $1661 | | $1542 | $1961 | $2379 | |
| + 100% | | -$238 | $181 | $599 | | $480 | $899 | $1317 | | $1198 | $1617 | $2035 | |
2020 End of Phase 4 (5 years after nutrient application ceased) | |
Fertiliser price | LTA | | -$283 | -$99 | $85 | | -$63 | $121 | $305 | | $156 | $340 | $524 | |
| + 50% | | -$626 | -$442 | -$258 | | -$406 | -$222 | -$38 | | -$187 | -$3 | $181 | |
| + 100% | | -$970 | -$786 | -$605 | | -$750 | -$566 | -$382 | | -$531 | -$347 | -$163 | |
The analysis shows that in 2015 at the end of Phase 3, after 8 years of nutrient addition, the cumulative profit difference of $1586 ha− 1 using LTA grain and fertiliser prices and the current C price ($40 t− 1) could vary from as low as $480 ha− 1 when fertiliser prices were high and grain prices low, up to $2004 ha− 1 with LTA fertiliser prices and higher grain price. Increasing the price paid for C to $60 t− 1 increased the cumulative profit at average fertiliser and grain prices (to $2304 ha− 1), with the profit difference ranging from $1198 ha− 1 to $2722 ha− 1 across different price scenarios. A decrease in C price to $20 t− 1 halved cumulative profit at average fertiliser and grain prices (to $868 ha− 1) with a range of -$238 to $1286 ha− 1. In 2015 after 8 years of fertiliser addition, there was little risk of economic loss under any of the price scenarios. However, given the investment in fertiliser ranged from $687 ha− 1 to $1374 ha− 1 under different scenarios, the return on investment (ROI) ranged from − 0.17 to 1.87 at $20 t− 1 for C, 0.35 to 2.92 at $40 t− 1, and 0.87 to 3.96 at $60 t− 1. The risks of relatively small returns on investment (< 1.00) over an 8-year period may discourage growers from adopting the practice especially at low C prices.
In 2020 after 5 years without addition of supplementary fertiliser to the crop residues there were risks of substantial losses (-$970 ha− 1) at high fertiliser prices and low C grain prices and little chance of profit under any scenario even with a C price of $40 t− 1. The risk of losses remained likely even as C prices increased, and especially if fertiliser prices increased above the LTA. Even at the highest C price of $60 t− 1 and with LTM fertiliser and grain prices, the cumulative profit difference of $340 in 2020 represented a ROI of only 0.49. It is clear that an extended period without ongoing nutrient addition to the residue can lead to a reduction in the original amounts of additional C sequestered by nutrient addition, eroding potential profits from sequestered C which has important implications with respect to when C credits may be paid.