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Sulphur cycling in soil-plant-animal systems under grazed, irrigated pastures in Canterbury, New Zealand and its implication on pasture sulphur requirements

Nguyen, Minh Long
Fields of Research
A laboratory study was conducted to investigate whether the zinc acetate-sodium acetate (Zn-Na acetate) solution that has often been used in the Iohnson-Nishita method (1952) to trap hydrogen sulphide (H₂S) and allow more sensitive sulphur (S) determination using methylene blue rather than the bismuth sulphide colour development of H₂S trapped in sodium hydroxide (NaOH) solution, could be used to quantitatively determine ³²S and ³⁵S (radioactive S) in the same trapping solution. Variable results were obtained, indicating that NaOH was the more suitable solution. The applicability of methylene blue development for H₂S-trapped in NaOH solution was also investigated. Results obtained showed that a delay in methylene blue development of more than 30 minutes after the removal of H₂S-trapping NaOH solution from the Iohnson-Nishita digestion-distillation unit caused a significant reduction in methylene blue intensity. A comparison was made between toluene and dioxane as liquid scintillants for assaying ³⁵S trapped in NaOH solution as H₂S. Toluene was found to improve the counting efficiency of ³⁵S in NaOH solution by 23-27%. The specificity of tin-hydrochloric acid (Sn-HCl) mixture in reducing elemental sulphur (S°) to H₂S in the Iohnson-Nishita method was tested by using acetone extracts of ground (<1mm) faeces, urine, pasture herbage and K₂SO₄ solution. It was found that when these extracts were analysed using Sn-HCl as the reducing mixture, an insignificant amount of S was analysed as Sn-HCl reducible S, except for 2% (10µg S ml⁻¹) of acetone-soluble urinary S. A preliminary survey was undertaken at Winchmore Irrigation Research Station and Templeton Research Station (New Zealand Ministry of Agriculture and Fisheries) to investigate the effects of long-term superphosphate applications on the soil and herbage S status of irrigated, sheep-grazed pastures in Canterbury, New Zealand. Results obtained from the Winchmore trial, situated on a Lismore stony silt loam soil (Udic Ustochrept) receiving annual superphosphate applications (0, 188 and 376 kg ha⁻¹) for 35 years, to provide annual S inputs of 0, 21 and 42 kg S ha⁻¹ respectively, showed that more than 95% of total S in the topsoil (0-75mm) was in organic S (So) and the remainder in readily-soluble and adsorbed sulphate (SO₄²⁻) forms. Increasing annual S inputs from 0 to 21 kg S ha⁻¹ significantly enhanced accumulations of total soil S, hydriodic acid (HI)-reducible S and carbon (C)-bonded soil S. However, further accumulations did not occur when annual S inputs were increased from 21 to 42 kg S ha⁻¹. It was estimated that 50% of this additional (21kg S ha⁻¹) applied S was lost by leaching beyond the main pasture rooting zone (0-300mm). Soil So was found to be stabilised independently from C and nitrogen (N) in the soil organic matter. The accumulation of soil So reached a steady-state after 25-27 years of superphosphate application, whereas C and N attained an earlier steady-state of 9-12 and 3-4 years respectively. Observed changes in soil C-bonded and HI-reducible S over 35 years were not adequately explained by the McGill and Cole (1981) S cycling model. The relatively high proportion of the C-bonded S fraction (50-60% of So) found in all treatments, especially after 15 years of pasture development, probably originated from the microbial incorporation of soil SO₄²⁻ and HI-reducible S into C-bonded S and a contribution from the return of plant and animal S residues. Significant amounts of C-bonded S and HI-reducible S were found in the subsoils (75-300mm) and the proportion of soil So as HI-reducible S increased with depth, thus suggesting that leaching of these soil So fractions, especially HI-reducible S, might have occurred beyond the topsoil (0-75mm) depth. At the Winchmore trial, superphosphate applications of 188 kg ha⁻¹ year⁻¹ were found to provide adequate S for pasture herbage. This was also the case at the Templeton site, situated on a similar soil type (Templeton silt loam; Udic Ustochrept) of low SO₄²⁻ (<15%) retentive capacity and receiving annual applications of superphosphate for 15-20 years at 250-300 kg ha⁻¹ year⁻¹. The Templeton site was also found to have adequate soil P, K and optimum soil pH for pasture requirements. This site was therefore chosen for the subsequent field experiment to investigate S cycling in grazed pastures. At the Templeton site, total soil S levels within the 0-300mm soil depth, particularly those of C-bonded S at the of 150-300mm depth, were found to be significantly higher in camp than in non-camp areas, indicating a higher return of excretal S to camp areas. C-bonded S from soil So or animal excreta may also have been leached from soil above the 150mm depth. In both camp and non-camp areas, the proportion of soil So as HI-reducible S increased, while that as C-bonded S decreased with depth (0-300mm), suggesting that greater leaching of the HI-reducible S fraction might have occurred. Amounts of soil So, C-bonded S and HI-reducible S in non-camp areas were similar to those in the superphosphate-treated soils of the Winchmore trial. This indicated that accumulations of soil So fractions at the Templeton site may have reached a steady-state similar to that observed in the Winchmore trial. A two-year rotational grazing experiment with incorporation of ungrazed areas within the grazed pastures was conducted at the Templeton site to examine the effects of sheep grazing and mowing with no clippings returned on the plant-availability and cycling of two S fertilisers (gypsum and S°). Treatments which included a control (no S fertiliser), gypsum (CaSO₄.2H₂O) and S° (particle size distribution of 63% < 1mm; 9% < 0.25mm) were randomly allocated to 48 borders (70 x 10m) in a completely randomised block layout, with four replicates per treatment. These treatments were grazed by sheep at 19 and 25 stock units (SU) ha⁻¹ year⁻¹. Each group of sheep for each treatment grazed rotationally four replicates of that treatment. Non-labelled gypsum and S° were applied to the main grazing area (border strips) of each replicate at 25kg S ha⁻¹, except for 1 ungrazed (0.75 x 1m) and 12 grazed hot spots (10 of 0.25 x 1m and 2 of 1 x 1m), where either ³⁵S-labelled gypsum or S° (specific activity 3967 µCi g⁻¹S) was applied at 25 kg S ha⁻¹. In addition, in each replicate of the S° treatment, an area (0.75 x 1m) of the border strip which had received non-labelled S° was excluded from grazing animals to assess the amount of S° remaining in the soil at different intervals after S° application. Results obtained showed that in the control treatment, there was no significant (P ≤0. 05) reduction in pasture dry matter (DM) production, animal liveweight, wool growth or wool S production for at least a year after S fertiliser was withheld, indicating that the soil So provided sufficient S for plant growth and hence animal production. This was contrary to the short-term pasture S requirement predicted according to the MAF S model using the pasture development index (PDI) and New Zealand Ministry of Agriculture and Fisheries (MAF) soil SO₄²⁻ ‘Quick test' . S° was more readily available to pasture plants in grazed than in ungrazed hot spots, probably because of higher S° oxidation in the grazed hot spots. This was supported by higher concentrations and specific activities of labelled-S in the pasture herbage of grazed hot spots, compared with those in ungrazed hot spots within the first 100 days after ³⁵S-labelled S° application. In addition, amounts of residual So remaining in the soil were lower in grazed pastures than in ungrazed areas, as early as 3 months after the application of non-labelled S°. There was a rapid incorporation of labelled-S into the pasture herbage and the wool of sheep within 100 days of applying either ³⁵S-labelled gypsum or S°, indicating that S° was as effective as gypsum in providing S for pasture growth and also wool S incorporation. ³⁵S-labelled gypsum and also S° were readily incorporated into soil SO₄²⁻ and So fractions less than 40 days after S fertiliser applications. Significant amounts of labelled-S were found in soil SO₄²⁻, HI-reducible S and C-bonded S fractions during this period, even to a soil depth of 450-600 mm, indicating that these soil S fractions were likely to have been leached from the topsoil (0-75mm). Estimated leaching losses for ³⁵S-labelled gypsum (44 %) were significantly higher that those for ³⁵S- labelled S° (36%) over the 343 day period. However, these losses were not affected by the increase in stocking rates (SR's). Over 80-84% of urinary Sand 14-20% of faecal S deposited by sheep during the year were found in the SO₄²⁻ and ester SO₄²⁻ fractions. Approximately 18 to 19% of excretal S was estimated to be transferred to camp areas under both SR's studied. This excretal S transfer was comparable to that (12.5%) estimated by the MAF S model for intensive rotational grazing systems (Sinclair and Saunders, 1984). The size of the S cycling pool in grazed hot spots was not affected by forms of S fertiliser applied or by an increase in the annual SR, although it was higher in grazed than in ungrazed hot spots. In both grazed and ungrazed hot spots, 30-50% of the S cycling pool was in the residue 5 pool and approximately 90% of the total soil S was not involved in S cycling. This indicated that factors which affect the cycling rate of the residue S pool are likely to influence pasture S requirements. A laboratory experiment was conducted to investigate the effects on S° oxidation of surface-applied S°, mixing S° with soil and the application of sheep excreta to soils, using 50g topsoil (0-75mm) from different soil types (Horotiu sandy loam, Typic Vitrandept; Mahoenui silt loam, Typic Dystrochrept; Templeton silt loam, Udic Ustochrept) and sterilised sand, each incubated separately with sheep urine (12ml; 1240 µg S ml⁻¹), ground (<1mm) sheep faeces (2.5 g; 4905 µg S g⁻¹) and 5° (37mg; < 0.10m) either singly, or in different combinations at 27°C for different periods of time up to 12 weeks. Results obtained showed that over the 12 week incubation period, amounts of Ca(H₂PO₄)₂-extractable S were 2 to 3 times higher when S° was mixed with the soil rather than surface applied. In addition, amounts of Ca(H₂PO₄)₂-extractable soil S were significantly higher in the presence of sheep faeces, suggesting that faeces enhanced S° oxidation. This enhancement was more pronounced when S° was mixed with the soil. It was more noticeable in Horotiu and Mahoenui soils than in Templeton soil, probably because of differences in soil biological activity. Urine yielded inconsistent effects on the levels of Ca(H₂PO₄)₂-extractable soil S. In sterilized sand, there was no increase in Ca(H₂PO₄)₂- extractable soil S after So application unless either urine or faeces was added, indicating that sheep excreta probably introduced S° oxidising micro-organisms to the sand and also provided nutrients for these micro-organisms. Increases in Ca(H₂PO₄)₂ -extractable soil S were linearly related (R2 = 0.99***; *** = P:S≤0.001) to decreases in the amounts of S° remaining in the soil after S° application, indicating that Ca(H₂PO₄)₂-extractable soil S could be used to measure S° oxidation in laboratory incubation studies. A laboratory study was conducted to investigate the extent of water-solubility of different faecal S fractions in sheep faeces by shaking ground (< 1mm) faeces with water (1:5 ratio of faecal DM:water) for 30 minutes. Results obtained showed that a significant proportion of faecal S (14.7 - 25%) was extracted by water and most of this (70%) was in the organic S fraction. Water-extractable inorganic faecal 5 was considered to have partly originated from faecal SO₄²⁻ and ester SO₄²⁻ (r = 0.45** - 0.63***; ** = P≤0.01; *** = P≤0.001). Some of the water-soluble faecal S might have also originated from faecal C-bonded S, as shown by a significant correlation between faecal organic S and water-extractable organic faecal S (r = 0.53 - 0.5**; ** = P≤0.01) or water-extractable inorganic faecal S (r = 0.40 - 0.41*; * = P≤0.05). The release of different faecal S fractions from faecal pellets and ground (<1mm) faeces in the presence and absence of sheep urine over a 38 day period was also investigated by incubating these materials with sterilised sand at 20-25°C in the presence of either distilled water or a prepared 2.5% aqueous soil solution obtained from the Templeton silt loam soil as a source of soil micro-organisms (9.5 x 10⁵ isolate ml⁻¹). More than 95% of faecal SO₄²⁻ and ester SO₄²⁻ and 17-35% of faecal C-bonded S were estimated to be released over a 38 day incubation period. Significantly more S was found to be released from urine-treated faeces (28.9 - 42.7 µg S g⁻¹ day⁻¹) than from urine-untreated faeces (25.5 - 28.5 µg S g⁻¹ day⁻¹). This enhancement was more pronounced when ground faeces instead of faecal pellets were used. The addition of soil solution to urine-treated faeces instead of distilled water significantly increased the release of faecal inorganic S, while significantly reduced the release of faecal organic S, suggesting that faecal organic S was mineralised to SO₄²⁻ by introduced soil micro-organisms. The addition of soil solution was found to enhance the estimated net mineralisation of organic S from both urine-treated and untreated faeces by 4.2 - 4.6 and 2.0 µgS g⁻¹ faeces day⁻¹ respectively. However, the estimated net faecal S mineralisation rate in faecal pellets (0.8 - 6.1 µg S g⁻¹ day⁻¹) was close to that in ground faeces (1.1 - 6.0 µg S g⁻¹ day⁻¹). The release of faecal organic and inorganic S from ground faeces was adequately described by the double exponential model, taking into account the time of incubation and the decreasing rate of faecal S release with time and also by the non-delayed, two-pooled model (labile and potentially-labile pools). However, in faecal pellets, the release patterns of faecal inorganic and organic S were best described by the delayed leaching and the non-delayed leaching one-pooled (labile pool) models respectively. In order to investigate effects of available soil moisture (ASM) at the time of urine application and the amount of applied irrigation water on the fate of urinary S in soil-plant pasture systems, a field plot trial of 271 days duration was conducted on irrigated pastures at Templeton (Templeton silt loam) by applying ³⁵S-labelled urine (1.8 litres, 1300 µg S ml⁻¹ and 250 µCi g⁻¹ S) to field plots (600 x 600 mm) o at a rate equivalent to that normally occurring in urine patches (150ml in 0.03 m²). Results obtained showed that 55 to 90% of ³⁵S-labelled urine was incorporated into soil SO₄-S, ester SO₄-S and C-bonded S fractions within the major rooting zone (0-300mm), even within 27 days after urine application. Labelled-S was found in these soil S fractions to a considerable depth (0-500 or 0- 900mm) during a 271 day period, indicating that these soil S fractions and/or ³⁵S-labelled urine were leached beyond the major rooting zone. Estimated total leaching losses of applied urinary S (68-70 %) beyond a depth of 300mm over the 271 day period were comparable to those (75 %) predicted from the MAF S model for this Templeton soil. Lower amounts of labelled-S were found beyond the 300mm soil depth when urinary S was applied to the soil at 25 % available water holding capacity (AWHC) rather than at field capacity, indicating that urinary S in soils at field capacity might not have sufficient opportunity to be adsorbed by soil particles, penetrate into micropores within soil aggregates or be immobilised by soil micro-organisms. Estimated leaching losses of urinary S were similar in soils with estimated 50% and 75% AWHC. Different amounts of irrigation water (estimated full water deficit (WD), ½WD and 2WD) imposed 7 days after urine application did not affect the total recovery of labelled-S in herbage, roots and soil (0-300mm). In order to assess the adequacy of the MAF S model for predicting pasture maintenance S requirements, quantitative estimates of S inputs and outputs according to the model were compared with those measured in the field and laboratory experiments conducted on Templeton silt loam. It was found that differences between predicted values and those measured in the field did not amount to more than 5 kg S ha⁻¹, indicating that the MAF S model is appropriate for predicting a pasture maintenance S requirement for the site studied. The predicted short-term S requirement (3.2 - 11.2 kg S ha⁻¹) for - this site, using the PDI and the MAF soil SO₄²⁻ 'Quick test' did not agree with field data, which showed no significant pasture DM reduction for at least one year after withholding SO₄²⁻ containing (superphosphate) fertiliser. Thus the MAF S model may not be appropriate for assessing short-term pasture S requirements.
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