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Fallow nitrate-N accumulation rates in central Qld cropping soils

Michael Braunack, Richard Routley and Maurice Conway

Abstract

Accumulation of nitrate-N (NO3-N) in soil profiles resulting from the net effect of nitrogen (N) mineralisation and a range of other N transformation processes was sufficient to satisfy crop N demand in the early years of cropping on the vertosols of NE Australia. After a number of years of cropping, the capacity of soils to provide plant available N declines as inherent soil N reserves are depleted and crops become responsive to applied N fertilisers in many cases. In central Qld, the responsiveness of major cropping soils to N fertilisers varies widely and growers and advisors require robust indicators of the likelihood of response when determining N fertiliser rates.

In this study profile NO3-N was monitored at monthly intervals over fallows ranging between 3 and 12 months in duration at 11 sites representing the major cropping soils in central Qld. Rates of NO3-N accumulation, and the relationship between soil organic carbon levels and NO3-N accumulation were examined.

Nitrate-N accumulation rates in Open downs soils were low and not significantly different from zero. Nitrate-N accumulation rates in Brigalow scrub soils ranged between 0.02 and 0.62 kg N/ha/day with an average rate of 0.34 kg N/ha/day for the 0 - 90 cm soil profile depth. Soil organic carbon concentration in the 0 - 10 and 10 - 20 cm soil depth increments was positively related to N accumulation rates and explained around 50% of the variation in those rates. Month to month variation in soil NO3-N levels was high which reinforces the need for rigorous sampling strategies when sampling paddocks for NO3-N for nitrogen budgeting purposes.

Key Words

Nitrogen, mineralisation, soil carbon, farming systems

Introduction

Profitable grain production can be sustained on the vertosols of the north east Australian grain production region for many years after initial clearing and cultivation without the use of nitrogen (N) fertilisers (Strong et al, 1996). As age of cultivation increases and inherent soil N fertility declines, grain yield response to applied N becomes more likely, and the use of N fertilisers becomes an economic proposition in many systems. Nitrogen budgeting techniques, based on matching anticipated crop N demand with soil N supply (as determined by pre-plant soil testing) have been widely used to determine N fertiliser application rates.

In the central highlands region of Central Qld, grain production is carried out on two broad soil types known locally as ‘Open downs’ (epicalcareous, self-mulching, black vertosols) and ‘Brigalow scrub’ (endocalcareous, self-mulching, black or brown vertosols) (Isbell, 1996), with the local soil names reflecting the native vegetation communities occurring on these soils. Previous studies (Routley et al, 2006) and much anecdotal evidence (Spackman and Garside, 1995) suggest that a positive economic response to N fertiliser application is more likely to occur on Open downs soils than on Brigalow scrub soils. Brigalow scrub soils have higher inherent soil organic carbon (SOC) levels than Open downs soils and as a result more nitrate-N is released in a given time period as a result of mineralisation and nitrification processes.

Whilst soil sampling to determine nitrate-N (NO3-N) levels prior to planting will quantify the level of mineral N present at the time of sampling, differing mineralisation rates post sampling - pre plant, and in-crop, introduce error to the N budgeting process (Routley et al, 2006). We believe that this error is the primary reason for inconsistent responses to N fertiliser application that have been observed on Brigalow scrub soils in central Qld. As N fertiliser prices increase, there is increased incentive to accurately determine soil N supply so that appropriate N fertiliser rates can be selected. Growers and their advisers require simple indicators of the N mineralisation potential of various soils in order to increase the accuracy of N fertiliser recommendations derived through the N budgeting process.

The purpose of the work described in this paper is to further quantify fallow N accumulation rates in the cropping soils of central Qld and to determine the extent to which soil organic carbon levels can be used as a predictor of fallow N accumulation.

Methods

Soil NO3-N levels during fallow periods were monitored on a monthly basis for the duration of the fallow at 11 sites in the central highlands region of central Qld during 2006 and 2007. The sites selected represented the two major cropping soils in central Qld as described above, that is, Open downs (D, five sites) and Brigalow scrub (S, six sites). Fallow starting date and the duration of the fallow period monitored at each site is given in Table 1.

At each site a 10 m x 10 m sampling area was defined so as to minimise the effect of spatial variability in soil organic carbon and nitrate levels. At each sampling date, three soil cores were taken from within the sampling area, divided into depth increments of 0-10, 10-20, 20-30, 30-60, 60-90 and 90-150 cm and bulked for analysis. Nitrate-N concentration was determined using automated colorimetry on a 1:5 soil:water extract and organic carbon was oxidised using an acidified dichromate solution and the absorbtion of the resultant chromic ionswas determined by colorimetry (methods 7B1 and 6A1 in Rayment and Higginson 1992). Nitrate-N content (kgN/ha) for each increment was determined by adjusting N concentration for soil bulk density.

Daily Nitrogen Accumulation Rate (DNAR, kg NO3-N/ha/day) was estimated as the slope of the linear regression of profile NO3-N (kg N/ha) against time at the 11 sites. Regression analysis was also used to examine relationships between soil organic carbon levels and DNAR for individual soil depth increments and for the profile as a whole. Regression parameters and other statistics were determined using linear regression with groups routines in Genstat 9.2 (Payne et al, 2006).

Results

There were differences in soil organic carbon (SOC) levels between the soils at the start of the monitoring period, with the Brigalow scrub soils having higher levels than the Open downs soils (Table 1). This reflects the original vegetation occurring before these soils were developed for agriculture with the scrub soils original vegetation being predominantly Brigalow and the downs soils being open grassland.

Table 1. Site details and starting soil organic carbon (SOC) levels. D = Open downs soils, S = Brigalow scrub soils

Site

D1

D2

D3

D4

D5

S1

S2

S3

S4

S5

S6

Fallow start

Jun 06

Jun 06

Jun 06

Jun 06

Oct 06

Jun 06

Jun 06

Oct 06

Oct 06

Jun 06

Nov 06

Number of monthly samples

7

7

3

12

8

12

12

8

8

12

7

SOC (%

(0-10cm)

0.82

0.73

0.82

0.70

0.70

1.34

1.28

1.34

1.28

1.16

1.16

SOC (%)

(10-20cm)

0.71

0.63

0.71

0.67

0.67

1.13

1.10

1.13

1.10

0.99

0.99

The DNAR of all Downs soils as a group was 0.095 kgN/ha/day (Table 2) although this value was not significantly different to 0 (p=0.05). The DNAR at individual downs soil sites were all low and variable. (Table 2 and Fig 1a). The large negative DNAR observed at site D3 is likely due to N immobilisation as a result of decomposition of large volumes of stubble from the preceding sorghum crop. The DNAR of all Brigalow scrub soils as a group was 0.336 kgN/ha/day (Table 2) and significantly different to 0 (p=0.05). DNAR at individual Brigalow scrub soil sites varied between 0.019 and 0.620 (Table 2 and Fig 1b).

Figure 1 illustrates the overall trends in profile NO3-N levels throughout the monitoring period for each site, as well as the high level of month to month variability that was observed.

Table 2. Daily nitrogen accumulation rate (DNAR, kg/ha/day) for each site, and for soil types.

(* = value significantly different to 0 at p=0.05; ** at p=0.01

Site

DNAR (kg/ha/day)

se

Site

DNAR

(kg/ha/day)

se

D1

0.041

0.272

S1

0.328*

0.126

D2

-0.103

0.272

S2

0.178

0.126

D3

-0.720

1.04

S3

0.620**

0.235

D4

0.042

0.126

S4

0.595*

0.235

D5

0.091

0.235

S5

0.019

0.126

     

S6

0.599*

0.288

All Open downs soils

0.095

0.198

All Brigalow scrub Soils

0.336*

0.142

Fig 1a. Profile nitrate nitrogen (NO3-N)
– Open downs soils

Fig 1b. Profile nitrate nitrogen (NO3-N)
– Brigalow scrub soils

The relationship between SOC levels in the 0 – 10 and 10 – 20 cm soil layers and DNAR for all sites is illustrated in Figure 2.

Figure 2 Relation between soil organic carbon (%) and daily nitrogen accumulation rate (DNAR) for the 0-10 and 10-20 cm depths (data from all sites) (Both regression coefficients significantly different to 0 at P=0.05).

Discussion and Conclusion

A range of factors interact to determine the degree of accumulation and profile distribution of NO3-N during fallow periods on cropping soils (Angus et al, 2006). These factors include:

  • N mineralisation and nitrification resulting from decomposition of soil organic matter
  • Soil moisture and temperature conditions through their effect on mineralisation rates
  • N immobilisation resulting from breakdown of high C:N ratio crop residues
  • N losses due to denitrification events
  • N losses and redistribution due to leaching, and
  • N uptake by weeds

As a result, predicting fallow NO3-N accumulation with any degree of accuracy is difficult. The results from this study indicate that net NO3-N accumulation during fallow periods was close to zero in the Open downs soils examined. The average accumulation rate observed of 0.095 kg N/ha/day, or 34 kg N/ha/year is comparable with rates reported in other studies (Routley et al 2006). These rates are small compared with the N demand of typical cereal crops produced in the region (typically 100 kg N/ha, H Cox pers comm.) and it would appear that N fertiliser rates on these soils could be targeted at supplying the total crop N demand.

The NO3-N accumulation rates observed on Brigalow scrub soils were much higher and varied extensively among individual sites. Accumulation rates expressed on an annual basis ranged between 7 kg N/ha and 226 kg N/ha and averaged 122 kg N/ha. For a typical 7 month fallow period, such as would occur between two wheat or sorghum crops, the average N accumulation observed on Brigalow scrub soils would supply around 70 kg N/ha or 70 % of the N requirement of typical grain crops. Fallow NO3-N accumulation is therefore a major contributor to crop N supply on these soils and the extent and variability of N accumulation needs to be accounted for in N budgeting calculations.

Soil organic carbon levels in the 0-10 or 10-20 cm soil layers can be used to give a broad indication of the N accumulation potential of a particular soil, however, the degree of precision achieved is not sufficient to allow predictions based on SOC levels to replace direct measurements via soil sampling in N budgeting calculations, particularly on soils with relatively high SOC levels (>1%).

The high level of month-to-month variability in soil NO3-N levels observed in this study is likely due to a number of factors including variations in temperature and soil moisture, as well as sampling error, and may reflect the relatively small number of soil cores taken at each sampling site and date. This finding reinforces the need to take sufficient soil cores to achieve the required degree of accuracy and precision when sampling for N budgeting purposes (Dalgliesh & Foale, 1998).

References

Angus JF, Bolger TP, Kirkegaard JA and Peoples MB (2006) Nitrogen mineralisation in relation to previous crops and pastures. Australian Journal of Soil Research 44, 355-365.

Dalgliesh, NP and Foale, MA (1998). A guide to soil sampling. In Soil Matters. Eds NP Dalgliesh & MA Foale. pp. 17-46, CSIRO, Australia.

Isbell RF (1996) The Australian Soil Classification. CSIRO, Melbourne. 143p.

Payne, RW et al (2006) The Guide to GenStat Release 9 Part 2: Statistics. VSN International, Hertfordshire.

Rayment,G.E. and Higginson F, R (1992). Australian Laboratory Handbook of soil and water chemical methods. Inkata Press Melbourne.

Routley RA, Sullivan A, Braunack M, Spackman G and Conway M (2006). Components of long-term nitrogen balance in central Qld grain cropping systems, In Groundbreaking stuff. Proceedings of the 13th Australian Agronomy Conference, 10-14 September 2006, Perth, Western Australia. Australian Society of Agronomy.

Spackman, GB and Garside, AL (1995). Major factors affecting grain production in central Qld and their implications for sustainable grain farming systems. GRDC, Canberra.

Strong W, Dalal R, Weston E, Cooper J, Lehane K, King A and Chicken C (1996) Sustaining productivity of a Vertisol at Warra, Queensland, with fertilisers, no-tillage or legumes. 2. Long-term fertiliser nitrogen needs to enhance wheat yields and grain protein. Australian Journal of Experimental Agriculture 36, 665-674.

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