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Mineralisation of C and N during decomposition of sugarcane and soybean residues

Fiona Robertson

BSES Ltd, 50 Meiers Road, Indooroopilly, Qld 4068, Australia. Present Address: Department of Primary Industries, Hamilton Centre, Private Bag 105, Hamilton, Vic. 3300, Australia. Email: fiona.robertson@dpi.vic.gov.au

Abstract

Green harvesting of sugarcane (Sacharum spp.) produces large quantities of crop residues (trash) of high carbon: nitrogen ratio. Legumes such as soybean are used as green manure crops in sugarcane production systems, and produce residues of low carbon: nitrogen ratio. How these contrasting plant materials interact in terms of decomposition and N availability is not known. The aim of this study was to measure the potential for C mineralisation and N mineralisation or immobilisation when sugarcane and soybean residues and N fertiliser are applied to soil, individually and in combinations. An incubation experiment was conducted where 15N-labelled and unlabelled sugarcane trash, soybean tops, and urea fertiliser were applied to soil in factorial combinations, with a single material in any treatment being 15N-labelled. Mineralisation of C and N were measured for 6 months. Cane trash and soybean tops were good sources of readily decomposable C. Addition of urea had little effect on C mineralisation from soil or residues, except a temporary stimulation during early trash decomposition. Up to 91% urea-N was recovered as soil inorganic N. Soybean tops eventually mineralised 39-67% of their N content. There was evidence of some gaseous loss of mineral N. Trash immobilised all the N mineralised from soil, 80-100% of the N mineralised from soybean tops, and 20-50% of N mineralised from urea. Negligible N was mineralised from trash, and the addition of urea or soybean did not change this. Patterns of C and N mineralisation were generally similar for the amendments alone and in combinations, except where N immobilisation was limited by N availability. By retaining rather than burning sugarcane trash at the end of the crop cycle, significant soil inorganic N may be immobilised and protected from loss.

Key Words

Nitrogen, carbon, mineralisation, immobilisation, crop residue, green manure.

Introduction

Much of the Australian sugarcane (Sacharum spp.) crop is harvested green, which leaves large quantities of crop residues (trash) of high carbon: nitrogen ratio on the soil surface. Legumes such as soybean are increasingly used as green manure crops in sugarcane production systems (in the presence or absence of trash), and produce residues of low carbon: nitrogen ratio. The N in trash is released very slowly during decomposition and is of negligible value to the succeeding ratoon crop (Robertson 2003). Legumes such as soybeans contain 2-4 times more N than trash, and their decomposition can lead to rapid accumulation of mineral N in the soil which, if not taken up by a crop, is liable to be lost by leaching or denitrification. How these contrasting plant materials interact in terms of decomposition and N availability is not known, but is potentially important for crop nutrition, fertiliser requirements, and off-farm loss of N.

The aim of this study was to measure the potential for C mineralisation and N mineralisation or immobilisation when cane and soybean residues and N fertiliser are applied to soil, individually and in combinations.

Methods

Preparation of soil and amendments

Soil (non-calcic brown, Holz and Shields 1985) was collected from the 0-10 cm depth of a sugarcane field at Mackay, Queensland (21.10 S, 149.07 E). Particle size distribution was 56, 27, 17% (sand, silt, clay), total N was 0.064%, total C was 1.12% and pH (water) was 4.56. The soil was dried, crushed and sieved (<2 mm) before use.

Unlabelled and 15N-labelled sugarcane and soybean residues were obtained from glasshouse-grown plants. The plants were fertilised weekly with a solution of either 15N-labelled ammonium sulfate (10 atom% excess) or unlabelled ammonium sulfate, plus a N-free general nutrient solution. The soybeans were allowed to grow for approximately 2 months, then harvested before the development of seed pods. The sugarcane was grown to maturity (approximately 1 year), then all green and senesced leaves were removed from the stalk. All plant material was dried at 70C, to facilitate handling. All sugarcane leaves were combined as the trash fraction, then shredded in a cutter-grinder (fragment size 2-10 cm long and 0.2-1.0 cm wide). Soybean tops and the shredded trash were then cut into ≤ 4 cm-long pieces with scissors. Unlabelled and 15N-labelled urea solutions were prepared containing 10 mg N/mL. Samples of all materials were taken for analysis. The unlabelled and 15N -labelled residues had similar total C and N contents (Table 1), but total N was slightly lower in 15N -labelled than in unlabelled soybean.

Table 1. C and N contents of amendments.

Amendment

Total C (%DM)

Total N (%DM)

C:N Ratio

15N Atom % excess

Trash-15N

36.1

0.55

66

6.04

Trash-unlabelled

35.4

0.52

68

0

Soybean-15N

34.0

1.18

29

1.39

Soybean-unlabelled

33.7

1.35

25

0

Urea-15N

-

-

 

9.80

Urea-unlabelled

-

-

 

0

The experiment

Amendments were mixed with 250 g of air-dry soil and placed in plastic pots (8.5 cm diameter) with holes on the base but lined with nylon cloth (1 mm mesh) to prevent soil loss. The unlabelled and 15N-labelled trash, soybean and urea were combined as shown in Table 2 to create 15 experimental treatments containing one 15N-labelled amendment, and one treatment containing unamended soil.

Table 2. Experimental treatments, and total C and N applied.

Treatment

Amendments

Total N applied (mg/pot)

Total C applied (mg/pot)

C:N ratio applied

1

CaneTrash-15N

30

1986

66

2

SoybeanTops-15N

32

918

29

3

Urea-15N

66

0

-

4

CaneTrash-unlabelled

29

1947

68

5

SoybeanTops-unlabelled

36

910

25

6

Urea-unlabelled

66

0

-

7

CaneTrash-15N + SoybeanTops-unlabelled

67

2895

43

8

CaneTrash-15N + Urea-unlabelled

96

1986

21

9

CaneTrash-unlabelled + SoybeanTops-15N

60

2865

48

10

CaneTrash-unlabelled + Urea-15N

95

1947

21

11

SoybeanTops-15N + Urea-unlabelled

98

918

9

12

SoybeanTops-unlabelled + Urea-15N

102

910

9

13

CaneTrash-15N + SoybeanTops-unlabelled + Urea-unlabelled

133

2895

22

14

CaneTrash-unlabelled + SoybeanTops-15N + Urea-unlabelled

126

2865

23

15

CaneTrash-unlabelled + SoybeanTops-unlabelled + Urea-15N

131

2857

22

16

Soil only

0

0

18

Amendments were applied at rates commonly encountered under field conditions: trash at 10 t dry matter (DM)/ha (5.5 g/pot), soybean at 5 t DM/ha (2.7 g/pot), and urea at 120 kg N/ha (66 mg/pot). Enough pots were prepared for 4 replicates, and 4 destructive sampling times. Deionised water was added to bring the soil to approximately 80% of field capacity (30% gravimetric water content). The pots were placed inside heavy-duty zip-seal plastic bags (left open) and moved to a constant-temperature room at 25C, in the dark. Soil water was replenished at approximately weekly intervals. On 15 occasions (starting on days 6, 10, 15, 21, 24, 31, 36, 42, 48, 52, 58, 69, 91, 95, and 120 after set-up), CO2 evolved from the pots was collected using alkali traps (vials containing 18 mL of 1 M sodium hydroxide suspended 1-2 cm above the pot surface, and the plastic bags sealed for 2-3 days). On 4 occasions (days 17, 43, 91, and 121), a complete set of pots was destructively sampled - the pot contents were thoroughly mixed and analysed for water and inorganic N content. The soils were then dried at 40C, crushed to <2 mm and retained for further analyses. On day 96, the remaining pots were leached, in an attempt to simulate the depletion of inorganic N under field conditions through plant uptake and leaching (pots mounted in the neck of a 1-L glass jar and leached with 200 ml of deionised water). The leachates were discarded.

Calculations and statistical analysis

Cumulative C mineralisation during the experiment (ΔC) was estimated from the area under the curve of CO2 accumulation against time. Mineralisation of C due to the amendment (ΔCA) was calculated as ΔC from amended soil minus ΔC from unamended soil. Apparent net mineralisation or immobilisation of N (ΔN) was calculated as soil inorganic N at the end of the incubation period minus soil inorganic N at the start. Apparent net mineralisation or immobilisation of N due to the amendment (ΔNA) was calculated as ΔN from amended soil minus ΔN from unamended soil. Percentage 15N recovery at the end of the incubation period was calculated from:
% 15N recovery = [(Soil 15N atom% excess x mg total N in soil) / mg 15N originally applied ] x 100

Percentage 15N loss (including leached 15N) was calculated as 100 minus % 15N recovery. Total mineralisation of N from all the amendments in any treatment (Nmin) was thus estimated from:
Nmin = ΔN + 15N loss + (Unlabelled N loss)

where Unlabelled N loss is N loss from all the unlabelled amendments in that treatment, calculated by assuming that the percentage loss of each unlabelled amendment was the same as the percentage loss from the corresponding 15N-labelled amendment in the treatment containing the same combination of amendment types.

Treatment effects were tested by 1-way Analysis of Variance (ANOVA) for repeated measures. Least significant differences (LSD) were calculated when ANOVA indicated significant effects (P<0.05).

Analytical Methods

Soil inorganic N (ammonium + nitrate) was determined by extracting fresh soil in 2 M potassium chloride, followed by automated colorimetric analysis of the extracts (Rayment and Higginson, 1992, method 7C2). Total N and C were determined in dried residues (< 0.5 mm) using a Leco combustion analyser. Carbon dioxide in the sodium hydroxide traps was measured by titration against dilute hydrochloric acid (Zibilske 1994). The 15N content of soil and residues was determined in pulverised samples on a Europa mass spectrometer.

Results

There was good agreement between unlabelled and 15N-labelled amendments in measurements of C mineralisation, soil total N and inorganic N (Fig. 1), thus the aim of producing 15N-labelled and unlabelled residues of similar chemical composition and decomposition characteristics was achieved.

Figure 1. C mineralisation (a), total soil N (b), and inorganic soil N (c) from 15N-labelled and unlabelled amendments. The broken lines shows the 1:1 relationship.

The C mineralisation (Fig. 2) from unamended soil was much less than from all the other treatments except the urea-amended soils. Soil ΔCA (Fig. 3 b,d) was not affected by application of urea on its own. Trash and soybean on their own produced similarly large increases in ΔCA for the first few weeks, then the rate of C mineralisation continued at a lower rate. By day 121, trash had mineralised slightly more C than soybean (780 and 620 mg C/pot, respectively), representing 39% of trash C and 67% of soybean C. Application of urea temporarily increased ΔCA from soybean (to day 10) and trash (to day 58) (Fig. 2). At day 121, the cumulative effect of urea on soybean had disappeared, but cumulative ΔCA from trash was still increased by urea addition (to 1100 mg C/pot, or 57% of trash C) (Fig. 3 b,d). Application of trash and soybean together caused a very large ΔCA, which showed the same pattern of decline with time as that shown by the residues alone. Cumulative ΔCA for trash and soybean together was 1500 mg C/pot, or 52% of the applied C. Urea addition to the trash and soybean mixture increased ΔCA at most of the samplings, and significantly increased cumulative ΔCA (to 1700 mg C/pot, or 61% of residue C).

Figure 2. C mineralisation rate during incubation (ΔC). Bars are LSD (P=0.05) for comparing treatments. Arrow indicates time of leaching.

Apparent net N mineralisation in unamended soil (ΔN) reached a maximum of 4 mg N/pot (Fig. 3c). Trash by itself resulted in net immobilisation, with ΔNA of up to -6 mg N/pot, or 21% of the original trash N content (Fig. 3a,c). Soybean immobilised N to day 17 (ΔNA -1 mg N/pot), then gradually mineralised N to a maximum ΔN of 15 mg N/pot (43% of soybean N content) at day 91. Urea produced a ΔNA of ≤36 mg N/pot during the first 43 days, and this rose to a maximum of 60 mg N/pot (91% of urea applied) at day 91. Addition of trash completely negated the apparent N mineralisation from soybean (maximum ΔNA -2 mg N/pot), and approximately halved apparent mineralisation from urea (maximum ΔNA 36 mg N/pot). Soybean and urea together apparently mineralised only slightly more N than urea on its own (maximum ΔNA 68 mg N/pot, or 67% of applied N). Soybean, trash and urea together (maximum ΔNA 49 mg N/pot, 37% of applied N) mineralised slightly more N than trash and urea together. Large amounts of inorganic N remained in the soil after the leaching on day 96.

Figure 3. C mineralisation and apparent N mineralisation from soil (ΔC, ΔN) and amendments (ΔCA, ΔNA), expressed as mg and as percentage of applied. Bars are LSD (P=0.05) for comparing treatments. Arrow indicates time of leaching.

Loss of 15N from trash, soybean, and urea in the various mixtures is shown in Fig. 4. Trash by itself lost ≤11% of its original 15N content. Addition of soybean and/or urea did not significantly change this (Fig. 4a). The 15N loss from soybean alone was ≤18% until day 91, and increased to 40% at day 121 (Fig. 4b). The 15N loss from soybean was not significantly affected by addition of urea, but was reduced (to ≤16%) at day 121 by addition of trash or trash plus soybean. The 15N loss from urea alone gradually increased to 15% by day 91, and had increased to 43% by day 121 (Fig. 4c). Soybean addition slightly increased 15N loss from urea (to 20%) until day 43, but reduced 15N loss at day 121. Trash or trash plus soybean addition had no effect on 15N loss from urea until day 121, when 15N loss was reduced to ≤12%.

Figure 4 Loss of 15N from labelled trash, soybean and urea, decomposing alone or with other unlabelled amendments. Bars are LSD (P=0.05) for comparing treatments. Arrow indicates time of leaching.

Calculated total N mineralisation (Nmin) (Fig. 5) was greatest in the soybean + urea and the urea treatments. The absolute quantity of N mineralised was greatest with soybean + urea (45-86 mg/pot, 45-86%), but the proportion of amendment-N mineralised was greatest with urea alone (39-72 mg/pot, 59-110%). The Nmin from soybean alone increased gradually from day 17 to 121, to a maximum of 20 mg/pot and 63%. Trash alone immobilised N throughout the incubation (Nmin –2 to 0 mg/pot). Trash addition markedly reduced Nmin from the other amendments at most sampling times.

Figure 5. Calculated N mineralisation (Nmin) expressed as mg (a) and as percentage of applied (b).

Bars are LSD (P=0.05) for comparing treatments. Arrow indicates time of leaching.

Discussion

Trash and soybean tops were both significant sources of labile C, with 39-67% of their original C being mineralised after 121 days (Fig. 3). The pattern of C mineralisation in these residues, however, was different. Soybean-C was more labile, with rapid mineralisation during the first few weeks, followed by slower mineralisation as the labile C supply became depleted. Trash-C was mineralised more slowly than soybean at first, but mineralisation was sustained for longer. The peak in C mineralisation, representing decomposition of the most accessible residue components, occurred within 10-15 days (Fig. 2).

Urea had no effect on C mineralisation from soil. Application of urea to plant residues would not be expected to increase C mineralisation unless decomposition was limited by N availability (Alexander 1977). Thus, cumulative C mineralisation from soybean was not increased by urea. However, the initial stimulation of C mineralisation from soybean by urea (first 10 days) suggested that decomposition of the most labile C fraction was limited by N supply. Mineralisation of C from trash was apparently N-limited for the first 58 days (Fig. 2), the effect of which was still evident in cumulative ΔC at day 121 (Fig. 3). This confirmed other findings that N addition to trash may stimulate C mineralisation during the early stages (<2 months) of decomposition, but had no effect on C mineralisation over 8 months (F. Robertson unpublished data). Urea applied to trash and soybean together caused a similar increase in ΔC during this incubation experiment.

Apparent net mineralisation of N estimated from ΔN and ΔNA suggested that soil N mineralisation was <14 g N/g soil, and that trash immobilised all the inorganic N in the soil (Fig. 3). The soybean initially caused net immobilisation, but >40% of the soybean-N was finally mineralised, providing additional soil inorganic N of 51 g N/g soil. The increase in soil inorganic N caused by urea was surprisingly slow, accounting for only about half of the applied urea on day 43 (Fig. 3). This may have been due to slow hydrolysis or to microbial immobilisation of hydrolysed urea. However, most (91%) of the applied urea was later recovered as inorganic N.

The ΔNA from soybean and from urea were very much reduced by the addition of trash. This net immobilising effect of trash was much greater than for trash alone, reflecting the greater N availability in the mixtures (apparent immobilisation –58 to –81 g N/g soil, assuming soybean and urea mineralised the same alone and in combination with trash). Trash had a similar effect on ΔNA in the Trash + Soybean + Urea treatment (apparent immobilisation –61 g N/g soil). The ΔNA in mixtures was mostly similar to the total ΔNA of the same amendments decomposing alone, suggesting that microbial utilisation of the amendments was not limited in the mixtures.

The 15N lost from the pots comprised soluble N that was removed by leaching at day 96, and gaseous N lost through denitrification or volatilisation. As the pots were not leached until day 96, and there was no drainage from the pots after watering, the 15N lost at the first 3 samplings must have been through denitrification or volatilisation. It should be recognised, however, that some of the errors associated with the 15N results were quite large, particularly for soybean. Loss of 15N from trash was small (<11%), and not affected by the presence of any of the other amendments (Fig. 4). Loss of 15N from soybean reached 40%, with up to half of this apparently lost in gaseous forms. Loss of 15N from urea was 43%, with one third of it apparently gaseous loss. Addition of soybean increased loss of urea-15N at the first 3 samplings, perhaps because the greater labile C supply stimulated denitrification. Addition of trash markedly reduced 15N loss from soybean and urea at the end of the experiment, probably because mineralised N was protected from being leached by being immobilised.

The 15N data is useful for understanding the mineralisation-immobilisation behaviour of individual components in the decomposing mixtures. Of greater agronomic importance, though, is the net mineralisation or immobilisation of the complete mixture (Nmin), as this is what determines N supply to the crop. The Nmin results (Fig. 5) suggested that 63% of soybean N was mineralised at the end of the experiment. The Nmin from trash was negative. Urea Nmin accounted for 93-110% of applied N. Trash addition reduced Nmin from soybean and urea. Soybean and urea together had the greatest Nmin.

This study demonstrated the capacity of trash to immobilise large quantities of inorganic N from soil, urea, or soybean tops. This characteristic could be used to advantage by sugarcane growers. e.g. by retaining rather than burning the trash at the end of the crop cycle, significant soil inorganic N may be immobilised and protected from loss.

It should be recognised, however, that this study relates only to the first 6 months after application of the amendments, and to a soil with no history of trash or soybean addition. Patterns of C and N mineralisation from the current amendments would, of course, eventually change with time. Furthermore, mineralisation from old residues accumulated from several annual applications may influence mineralisation from the current amendments.

Conclusions

Cane trash and soybean tops were good sources of readily decomposable C. Addition of urea had little effect on C mineralisation from soil or residues, except a slight stimulation in the first 2 months of trash decomposition. Urea produced the greatest increase in soil inorganic N, 85-110% being recovered. Soybean tops eventually mineralised 40-70% of their N content. There was evidence of some gaseous loss of N mineralised from soybean and urea. Trash immobilised all the N mineralised from soil, 80-100% of the N mineralised from soybean tops, and 20-50% of N mineralised from urea. Negligible N was mineralised from trash, and the addition of urea or soybean did not change this. Patterns of C and N mineralisation were generally similar for the amendments alone and in combinations, except where N immobilisation was limited by N availability.

Acknowledgements

I thank Kylee Sankowsky and Patricia Nelson for their part in this work. The work was funded by the CRC for Sustainable Sugar Production, now defunct, and BSES Ltd.

References

Alexander M (1977) ‘Introduction to Soil Microbiology.’ (John Wiley & Sons: New York)

Bundy LG, Meisinger JJ (1994) Nitrogen availability indices. In ‘Methods of Soil Analysis. Part 2. Microbiological and Biochemical Properties’. (Eds Weaver RW, Angle S, Bottomley P, Bezdicek D, Smith S, Tabatabai A, and Wollum A) pp. 951-984. (Soil Science Society of America Inc.: Madison, Wisconsin, USA)

Holz GK, Shields PG (1985) ‘Mackay sugarcane land suitability study. Part 1. Land resource inventory.’ Land Resource Bulletin QV85001. (Department of Primary Industries: Brisbane)

Rayment GE, Higginson FR (1992) ‘Australian Laboratory Handbook of Soil and Water Chemical Methods.’ (Inkata Press: Melbourne)

Robertson F (2003) ‘Sugarcane Trash Management: Consequences for Soil C and N.’ (CRC for Sustainable Sugar Production: Townsville) 39 pp.

Zibilske LM (1994) Carbon mineralisation. In ‘Methods of Soil Analysis. Part 2, Microbiological and Biochemical Properties’. (Eds Weaver RW, Angle S, Bottomley P, Bezdicek D, Smith S, Tabatabai A, and Wollum A) pp. 835-863. (Soil Science Society of America Inc: Madison, USA)

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