Farming systems and nutrient legacy (stubble residue impact n key nutrients (N P and S))

1New South Wales Department of Primary Industries; University of New England
2New South Wales Department of Primary Industries
3Western Sydney University
4Department of Science, Information Technology and Innovation

Take home messages

  • Crop residue incorporation in soil at 10 tonnes per hectare can enhance the supply of major nutrients [nitrogen (N), phosphorus (P) and sulphur (S)] to the value of $150 to $300 per hectare, depending on soil type and management practices. In the long term, however, about half the amount of crop residue input and nutrient $ value is expected.
  • Crop residues have the potential to release more available P and S from the soil nutrient reserves than the total amount of P and S added to soil through the residues.
  • Canola residues cause higher soil carbon (C) mineralisation and consequently provide greater N, P and S supply following its incorporation in soil, than wheat residues.
  • Tillage increases soil C mineralisation and available N, P and S supply from organic matter in soil, compared to no-till and perennial pasture.
  • Growers must however consider advantages and disadvantages of tillage before changing systems (such as enhanced nutrient supply versus preservation of soil structure and C). It is essential that enhanced nutrient supply via tillage and crop residue input before sowing are to be aligned to meet early crop demand to minimise nutrient losses, while supporting crop productivity and profitability.

Background

Soil organic matter (SOM) is comprised of progressively decomposing plant, animal and microbial residues, including living microbial biomass, and charcoal (all less than 2 mm). Above ground plant biomass such as stubble is termed “crop residues”. Both SOM and crop residues are important resources for sustainable and profitable agriculture. In addition to their positive impacts on key soil quality indicators (such as soil structure, soil water holding capacity, and soil biological activity), both SOM and crop residues may serve as important sources of nutrients [such as nitrogen (N), phosphorus (P) and sulphur (S)] and hence have a direct economic value. These nutrients are mainly released in plant available forms following biological decomposition of SOM and crop residues – a process known as ‘mineralisation’. Crop residues can interact with existing SOM via enhanced microbial activity to further increase soil C mineralisation and release N, P and S; this process is known as ‘positive priming’. Some soil mineral bound nutrients such as P can also be released in plant available forms via certain chemical mobilisation processes, which may be enhanced during decomposition of crop residues in soil systems. It is yet not clear how much organic C mineralisation and crop nutrient supply from these biological and chemical processes vary with crop residue type and soil type under different tillage farming systems. To harness the benefits of nutrient release from tillage and crop residue input to soil systems, growers need to understand:

  • The nutrient supply value of SOM and crops residues.
  • How management practices (such as contrasting tillage systems) influence mineralisation of SOM and crop residues and hence nutrient supply; and
  • The influence of crop residues on the net release of nutrients from soil and existing SOM.

Methodology

Two long-term (16 to 46 years) field sites were studied: (i) Condobolin (CW NSW) and (ii) Hermitage (SE Qld). The soil types were: (i) a red soil (Red Chromosol) at Condobolin and (ii) a cracking clay soil (Vertosol) at Hermitage. Site description is presented in Table 1.

Table 1. Site description

Location

Condobolin Agricultural Research Station (NSW)

Hermitage Research station (Qld)

Coordinates

Latitude 33°05'19"S, Longitude 147°08'58"E (195 m above sea)

Latitude 28°12′S, Longitude 152°06′E (470 m above sea)

History

Trial established in 1998

Trial established in 1968

Climate

Hot, semi-arid climate; non-seasonal precipitation

Subtropical; summer-dominant precipitation

Soil texture

Sandy clay loam

Clay

The selected treatments at the Condobolin site were:

  1. CT = Conventional tillage (with the short fallow wheat/undersown pasture phase in 2013 and transitioning to pasture phase in 2014);
  2. RT = Reduced tillage (with the long fallow wheat/undersown pasture phase in 2013 transitioning to pasture in 2014);
  3. NT = No till (after barley in 2013 and transitioning to pulse phase in 2014); and
  4. PP = Perennial pasture.

The PP represents the least-disturbed system and is selected as a reference treatment for comparison with the mixed cropping and pasture phases (CT and RT), or the continuous cropping phase system (NT). Basal fertiliser was applied to each crop phase at 50 kg MAP per hectare. For further details of the management practices at the Condobolin site, see http://cwfs.org.au/2015/12/10/cwfs-aog-soil-carbon-results-summary (accessed: 02.07.16).

The selected treatments at the Hermitage site, comprising mainly wheat cropping phases, were:

NT = No till stubble retained with 90 kg urea-N per hectare; and

CT = Conventional tillage stubble retained with 90 kg urea-N per hectare.

See http://www.regional.org.au/au/asssi/supersoil2004/s10/oral/1609_wangw.htm (accessed: 02.07.16) for further details of the Hermitage trial site.

In Tables 2 and 3, we report relevant soil properties of the two field sites. Total C and nutrient contents of the crop residues used in this study are presented in Table 4.

Table 2. Stocks of total carbon (C), nitrogen (N), phosphorus (P) and sulphur (S), and mineral N, P and S, in Red Chromosol (0–10 cm) at Condobolin. Values in brackets are standard errors (n =3).

Treatments

Total C

Total N

Total P

Total S

Before incubation (day zero)

Mineral N

Mineral P

Mineral S

tonnes per hectare

kilograms per hectare

CT

17.6 (0.9)

1.5 (0.0)

0.6 (0.0)

0.28 (0.01)

4.3 (1.2)

29.8 (5.7)

12.9 (0.5)

RT

18.6 (1.2)

1.6 (0.1)

0.6 (0.0)

0.26 (0.01)

2.3 (0.8)

40.7 (2.5)

3.1 (2.1)

NT

18.9 (0.9)

1.7 (0.0)

0.7 (0.0)

0.26 (0.01)

18.1 (1.5)

21.3 (0.8)

5.4 (0.3)

PP

20.4 (1.7)

1.7 (0.1)

0.6 (0.1)

0.30 (0.04)

4.6 (1.2)

10.6 (2.2)

23.1 (1.1)

Table 3. Stocks of total carbon (C), nitrogen (N), phosphorus (P) and sulphur (S), and mineral N, P and S, in Vertosol (0–10 cm) at Hermitage. Values in brackets are standard errors (n =3).

Treatments

Total C

Total N

Total P

Total S

Before incubation (day zero)

Mineral N

Mineral P

Mineral S

tonnes per hectare

kilograms per hectare

 tonnes per hectare

kilogram per hectare

CT

22.4 (1.1)

1.3 (0.1)

 1.2 (0.0)

0.23 (0.02)

17.6 (1.3)

110.6 (14.6)

17.4 (0.7)

NT

21.7 (1.0)

1.5 (0.1)

1.1 (0.0)

0.19 (0.02)

15.0 (1.8)

63.9 (5.4)

21.2 (2.1)

 Table 4. Carbon and mineral nutrient content in crop residues.

Crop residue type

Total carbon

Total nitrogen

Total phosphorus

Total sulphur

kilogram per tonne

Wheat stem

454.3

3.7

1.8

0.3

Canola stem

448.7

9.1

2.1

2.0

Soil sampling, laboratory incubation and analyses

Soil samples were collected from the topsoil layer (0–10 cm) in May 2014 at the crop pre-sowing stage from contrasting tillage treatments at both field sites and also from the perennial pasture system at the low pasture growth stage at Condobolin. After gently sieving the soils to less than six millimetres, the soils were incubated with or without incorporation of chopped (less than two millimetres) wheat or canola crop residues, for one and four months under controlled soil moisture and temperature conditions. Mineral nitrogen, phosphorus and sulphur in soil were measured before and after incubation. The main aim was to quantify the nutrient supply potential of SOM and crop residues (wheat and canola) across contrasting soil types and management practices. Crop stubble residues were added to the soil at the relatively high loading rate of 10 tonnes per hectare, which can be expected in some regions in productive seasons with good rainfall.

For this incubation study, soil samples, with and without incorporation of crop residue, were placed in jars in three separate containers of 35 g each. Moisture was adjusted to 60 % of soil water holding capacity and the soils were then incubated at a constant temperature (22°C). The CO2-C released (mineralisable C) was periodically measured. Mineral N (NH4+-N and NO3--N), mineral P (PO43--P; standard Colwell-P) and mineral S (SO42--S), were extracted by 2 M KCl, 0.5 M NaHCO3 and 0.016 M KH2PO4, respectively, at one and four months. Net nutrient availability for N, P or S following the simultaneous mineralisation (organic to inorganic forms), immobilisation (inorganic to organic forms) and fixation (adsorption on soil minerals and/or precipitation with metal cations) processes during SOM mineralisation over one month or four months was quantified (see below):

(i)                  Net plant-available N = Extractable mineral N1 or 4-month – Extractable mineral Nday0

(ii)                Net plant-available P = Colwell P1 or 4-month – Colwell Pday0

(iii)               Net plant-available S = Extractable mineral S1 or 4-month – Extractable mineral Sday0

The isotopic-C labelled crop residues were used to distinguish sources of residue C mineralisation from the SOM mineralisation. That is, the C released from the added crop residues and existing SOM was separately quantified. This allowed us to quantify the primed mineralisation of C from existing SOM due to the application of residues, which is the difference between C mineralisation in the presence and absence of crop residues. So if there is a positive priming effect of crop residues to enhance SOM mineralisation or mobilise mineral-bound nutrients, there could be additional release of nutrients from soil and existing SOM.

Results and discussion

Impact of tillage on mineralisation of carbon in SOM and crop residues in contrasting soils

Over four months, 1.3 to 1.9% (depending on tillage treatment) of C in SOM was released in the red soil at Condobolin, and 1.8 to 2.0% was released in the cracking clay soil at Hermitage, across different management practices (Figure 1a-d). There was a greater soil C mineralisation under CT than NT (and PP) at the Condobolin site but not at the Hermitage site. After incorporation of crop residues in the soils from both the sites, soil C mineralisation increased via the positive priming effect, i.e. by 2.6 to 3.7 times after canola residue addition and 1.5 to 2.3 times after wheat residue addition over four months. At Condobolin, the soil C priming was greater under CT and decreased as the tillage intensity was reduced (for example, in the RT and NT systems). Further, the residue C mineralisation was also greater under CT than RT and/or NT systems at Condobolin (Figure 2). However, at Hermitage, the soil C priming effect and mineralisation of added residue C were similar under both CT and NT (Figures 1 and 2). These results are consistent with the well-established paradigm that less soil disturbance in no-till and perennial pasture systems may stabilise/retard microbial activity and enhance protection of SOM and residues C in stable soil aggregates, consequently decreasing their accessibility to microbial attack, compared to conventional tillage, particularly in the red soil. However, in the cracking clay soil, the swelling-shrinking nature of the clay mineral may override the effect of tillage on C mineralisation, plus the decomposing organic compounds may be highly stabilised by clay minerals in the soil (Curry et al. 2007).  

Interactive effect of crop residue and tillage on the nutrient supply potential in contrasting soils

The results provide useful information on the fertiliser value of SOM and crop residues (wheat or canola) in terms of N, P and S supply in the red soil and the cracking clay soil under contrasting tillage systems.

Nutrient release in the ‘soil only’ incubation study (i.e. without crop residue incorporation):

  1. Plant-available N. Depending on management practices, SOM supplied 12–23 kg N per hectare over one month (short-term mineralisation), which increased to 15–36 kg N per hectare over four months in the red soil (Condobolin, NSW) (Figure 3a,d). In the cracking clay soil (Hermitage, Qld), SOM supplied 8–14 kg N per hectare over one month and 22–24 kg N per hectare over four months (Figure 4a,d). Thus, mineral N continued to be released over four months during microbial decomposition of SOM. Consistent with the pattern of C mineralisation, net mineralisable N was greater in the CT than other systems with reduced or no tillage intensity (RT, NT or PP), particularly over the first month.
  2. Plant-available P. SOM supplied 10–22 kg P per hectare in the red soil (Figure 3b,e), and 23–56 kg P per ha in the cracking clayey soil over one month (Figure 4b,e). However, over four months, the net available P was negative (i.e. immobilised), ranging between 4 and −31 kg P per hectare at Condobolin and between -13 and -101 kg P per hectare at Hermitage. At Condobolin, RT resulted in the greatest net P immobilisation and PP the lowest, whereas at Hermitage, CT versus NT had the greatest net P immobilisation. This pattern could be due to microbial immobilisation and/or fixation of inorganic P by clay minerals and metal cations in the soils (Malik et al., 2012).
  3. Plant-available S. SOM released 5–22 kg S per hectare over one month (Figure 3c,f), with CT releasing more than NT and PP at the Condobolin site (red soil). However, this S released via SOM mineralisation in the short term in the red soil was immobilised over four months, possibly by microorganisms. In the cracking clay soil (Hermitage site), no S was released over one and four months (Figure 4c,f).

Nutrient release in the ‘soil plus crop residue’ study:

Plant available N only slightly increased following addition of wheat or canola residues over four months in both soils (Figures 3 and 4), likely due to microbial N demand for enzyme synthesis. On the other hand, plant available P and S increased considerably over four months following addition of the crop residues in both soils. The effect of the residue inputs on the P and S release was greater in the cracking clay soil than the red soil.

These results suggest that crop residue incorporation may assist in offsetting the immobilisation process of both P and S in the contrasting soils, particularly after considerable degradation and enhanced interaction of the residues with SOM and soil minerals over time. Firstly, microbial activity and mineralisation of existing SOM may have been stimulated after the input of the crop residues in the soils (Figure 1). Secondly, as N availability was limited over four months, microbial communities would have enhanced degradation of SOM due to soil ‘N mining’, thus facilitating additional release of nutrients (such as P and S) from soil. Furthermore, simpler organic molecules released during decomposition of crop residue over four months may have contributed to mobilising nutrients (particularly P) bound to the soil through desorption and dissolution mechanisms (Guppy et al., 2005). These processes have the potential to enhance nutrient release from the soil reserve beyond the direct input of nutrients via crop residues, and our study suggests that the residue-incorporated CT or RT system are likely to have greater potential to release nutrients in the soils than NT.

The summarised results on the magnitude of nutrient release from the wheat or canola residue incorporated soils over four months are presented in Table 5.

Table 5. Nutrient release following wheat or canola residue incorporation (at 10 tonnes per hectare)

kilogram per hectare 

Red soil 
(Condobolin, New South Wales)

Cracking clay soil
(Hermitage, Queensland)

Wheat

Canola

Wheat

Canola

Nitrogen 

30

30

25

25

Phosphorus

30

30

90

100

Sulphur

30

40

35

55

Note: for simplicity, results are averaged across the contrasting tillage systems. See the detailed results in Figures 3 and 4.

Conclusions

  1. The results demonstrate that SOM has a fertiliser value and can release plant-available N, P and S, which increases with tillage.
  2. Mineral N could continue to be released during SOM decomposition over a four-month period, while mineral P and S could also be released over a one-month period under ideal soil moisture and temperature conditions.
  3. Our data suggest that any short-term (one month) released mineral P and S could be locked up in the soils via microbial immobilisation or adsorption on soil minerals over the longer term (four months), particularly if there is a lack of plant C input (e.g. during a fallow period).
  4. On the other hand, crop residue incorporation has the potential to counteract nutrient immobilisation in soil. For example, the residue-incorporated soils released substantial amounts of nutrients, particularly P and S, following a significant decay of residues in the soil, which was greater under conventional and/or reduced tillage than no-till.
  5. Growers however must be aware of the usual ‘pros and cons’ of tillage systems (for example, soil structure decline) before changing systems.
  6. It may be prudent to gently work crop residues into the soil, for example, in the topsoil (5 to 10cm deep) to expedite residue C mineralisation and the release of nutrients from the residues and soil reserves.
  7. Where possible the timing of tillage and crop stubble incorporation should be aligned to meet early crop nutrient demand.
  8. Our data suggest that crop residues have the potential to enhance the supply major nutrients [N, P, S] in soil to the value of $3 to $6 billion in Australia ($0.75 to $1.5 billion in NSW). These calculations are based on (a) nutrient costs of $1 per kg N, $3 per kg P and $1 per kg S; (b) crop residue incorporation at a relatively higher rate i.e. 10 tonnes per hectare, which can be expected in a productive region or season with good rainfall; and (c) ~20 million hectares of agricultural lands under some forms of crop stubble management in Australia and ~5 million hectares in NSW (Australian Bureau of Statistics, http://www.abs.gov.au/).
  9. As this study was performed under ideal soil moisture and temperature conditions in the laboratory, it is recommended that the economic value of nutrient supply from crop residues requires validation under field situations with variability in environmental factors (low/high temperature, low/high rainfall), soil type, and the stubble residue yield.

Resources

An earlier version of this GRDC update paper: Harnessing the benefit of crop residues and tillage 

References

Curry KJ, Bennett RH, Mayer LM, Curry A, Abril M, Biesiot PM, Hulbert MH (2007). Direct visualization of clay microfabric signatures driving organic matter preservation in fine-grained sediment. Geochimica et Cosmochimica Acta 71: 1709–1720.

Malik MA, Marschner P, Khan KS (2012). Addition of organic and inorganic P sources to soil – Effects on P pools and microorganisms. Soil Biology & Biochemistry 49: 106–113.

Guppy CN, Menzies NW, Moody PW, Blamey FPC (2005). Competitive sorption reactions between phosphorus and organic matter in soil: a review. Australian Journal of Soil Research 43: 189–202.

Acknowledgements

Funding for this work was provided through the GRDC Project DAN00169. The research undertaken as part of this project is made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC, the author would like to thank them for their continued support. Thanks to Luke Beange and Abigail Jenkins for reviewing an early draft of the paper, and Central West Farming Systems for providing access to their long-term trial.

Contact details

BP Singh
NSW Department of Primary Industries
Ph: 02 4640 6406
Mob: 0410130711
Email: bp.singh@dpi.nsw.gov.au

Appendix

Figure 1. Cumulative percent of soil carbon (C) mineralised in the presence and absence of residues, over four months of laboratory incubation. Wheat (left panel) and canola (right panel) stem residues were incorporated in the red soil (top panel) and cracking clay soil (bottom panel) under differently managed cropping systems at Condobolin and Hermitage, except in the PP system. See expanded abbreviation of treatments (CT, RT, NT and PP) in the methodology section. Least significant differences at P ≤ 0.05 were LSD0.05 = 1.30 and 0.77 for soil C mineralised in the red soil and the cracking clay soil, respectively.   

Figure 1. Cumulative percent of soil carbon (C) mineralised in the presence and absence of residues, over four months of laboratory incubation. Wheat (left panel) and canola (right panel) stem residues were incorporated in the red soil (top panel) and cracking clay soil (bottom panel) under differently managed cropping systems at Condobolin and Hermitage, except in the PP system. See expanded abbreviation of treatments (CT, RT, NT and PP) in the methodology section. Least significant differences at P ≤ 0.05 were LSD (0.05)= 1.30 and 0.77 for soil C mineralised in the red soil and the cracking clay soil, respectively.

Figure 2. Cumulative percent of residue carbon (C) mineralised, over four months of laboratory incubation. Wheat (left panel) and canola (right panel) stem residues were incorporated in the red soil (top panel) and cracking clay soil (bottom panel) under contrasting tillage systems at Condobolin and Hermitage. See expanded abbreviation of treatments (CT, RT, NT) in the methodology section. Least significant differences at P ≤ 0.05 were LSD0.05 = 6.3 and 2.2 for residue C mineralised in the red soil and the cracking clay soil, respectively.

Figure 2. Cumulative percent of residue carbon (C) mineralised, over four months of laboratory incubation. Wheat (left panel) and canola (right panel) stem residues were incorporated in the red soil (top panel) and cracking clay soil (bottom panel) under contrasting tillage systems at Condobolin and Hermitage. See expanded abbreviation of treatments (CT, RT, NT) in the methodology section. Least significant differences at P ≤ 0.05 were LSD (0.05)= 6.3 and 2.2 for residue C mineralised in the red soil and the cracking clay soil, respectively.

Figure 3. Impact of tillage practices at the Condobolin site (NSW) on net availability of nitrogen (a, d), phosphorus (b, e) and sulphur (c, f) released from soil only, and soil plus added wheat (left panel) and canola (right panel) residues over one month and four months of laboratory incubation. See expanded abbreviation of treatments (CT, RT, NT and PP) in the methodology section for the Condobolin site. Least significant differences at P ≤ 0.05 were LSD0.05 = 11.3, 16.9 and 11.8 for the net availability of nitrogen, phosphorus and sulphur, respectively, over one month; and LSD0.05 = 13.7, 14.2 and 11.5 for the net availability of nitrogen, phosphorus and sulphur, respectively, over four months in the red soil.

Figure 3. Impact of tillage practices at the Condobolin site (NSW) on net availability of nitrogen (a, d), phosphorus (b, e) and sulphur (c, f) released from soil only, and soil plus added wheat (left panel) and canola (right panel) residues over one month and four months of laboratory incubation. See expanded abbreviation of treatments (CT, RT, NT and PP) in the methodology section for the Condobolin site. Least significant differences at P ≤ 0.05 were LSD (0.05) = 11.3, 16.9 and 11.8 for the net availability of nitrogen, phosphorus and sulphur, respectively, over one month; and LSD (0.05) = 13.7, 14.2 and 11.5 for the net availability of nitrogen, phosphorus and sulphur, respectively, over four months in the red soil.

 Figure 4. Impact of tillage practices at the Hermitage site (Qld) on net availability of nitrogen (a, d), phosphorus (b, e) and sulphur (c, f) released from soil only, and soil plus added wheat (left panel) and canola (right panel) residues over one month and four months of laboratory incubation. See expanded abbreviation of treatments (CT, NT) in the methodology section for the Hermitage site. Least significant differences at P ≤ 0.05 were LSD0.05 = 9.5, 18.0 and 8.0 for the net availability of nitrogen, phosphorus and sulphur, respectively, over one month; and LSD0.05 = 7.8, 30.0 and 8.9 for the net availability of nitrogen, phosphorus and sulphur, respectively, over four months in the cracking clay soil.

Figure 4. Impact of tillage practices at the Hermitage site (Qld) on net availability of nitrogen (a, d), phosphorus (b, e) and sulphur (c, f) released from soil only, and soil plus added wheat (left panel) and canola (right panel) residues over one month and four months of laboratory incubation. See expanded abbreviation of treatments (CT, NT) in the methodology section for the Hermitage site. Least significant differences at P ≤ 0.05 were LSD (0.05) = 9.5, 18.0 and 8.0 for the net availability of nitrogen, phosphorus and sulphur, respectively, over one month; and LSD (0.05) = 7.8, 30.0 and 8.9 for the net availability of nitrogen, phosphorus and sulphur, respectively, over four months in the cracking clay soil.

GRDC Project code: DAN00169