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2.4. La Administración local en la encrucijada
5.5.1 Pasture yield and S uptake
Pasture growth and S uptake in both the small plot and microplot were comparable with no dry matter yield and S uptake responses to the applied fertilizer treatments. The applied fertilizer S was taken up at the expense of soil derived S. This was probably because the soil used in these studies had sufficient labile S for optimum pasture growth and because SSP has been applied annually to the paddock to maintain the optimum P fertility level. In this situation, S concentrations in mixed herbage throughout the trial period range from 0.39-0.47%, well above the critical level of 0.28-0.30% (Sinclair et al., 1 985). lt has been suggested that for a paddock at this fertility level, applications of SSP can be withheld every two or three years without any depression in dry matter yield (Smith, 1 976; Ledgard et al., 1 99 1 ). In this respect the majority of S taken up by plants was derived from the mineralization of organic soil S built up from previous fertilizer applications. Maintenance S fertilization is needed only to replace losses by animal transfer and leaching (Sinclair and Saunders, 1 984) because pasture development has reached a stage where S i mmobilization i nto soil o rganic matter equals organic matter mineralization.
I n terms of S u ptake and dry matte r yield, the performance of the microfi n e S 0 was comparable to SSP, but oxidation of this very fine particle size S0 was very fast and probably
complete within 25 days after application.
5.5.2 Short term fate of the fertilizer sulphur
In this experiment, interest was focused mainly on the immediate fate of fertilizers in the top 1 0 cm soil layer since most plant root activities impact on this layer. A previous study using 35s in this soil revealed that about 90% S taken up by pasture plants was mostly derived from this layer (Horne et al., 1 992, u npublished data). Thus most fertilizer S moved beyond this layer may be considered a loss from the immediate (short term) cycling pool.
Labelled 35s SSP was taken up by pasture mainly during the first 60 days and amounted to 6 to 7% of applied 35s in each cut. Thereafter very small amounts were taken up. This pattern of isotope uptake is common for biologically active nutrients like S (Kennedy and Till, 1 981 b; Gregg and Goh, 1 982; Goh and Gregg, 1 982a) because microbially mediated immobilization and mineralization serve to remove 35s from the available pool and dilute it with unlabelled S as time progresses. This is discussed more fully below. Plant uptake of 35s during the earlier stages of the experiment (first 60 days) was greater in SSP fertilized cores but for the remainder of the experiment, 35s uptake was larger (60-1 50 days) in the S0 cores. This probably reflects the slow release of 35s by oxidation in the first 30 days and the greater amount of 35s remaining in the s0 fertilized soil cores from 60-1 50 days.
5.5.2.2 Recovery of 35 S labelled fertilizer in the top 10 cm of soil
Losses of 35s during the first 30 days of the direct labelling experiment were larger than expected and are unlikely to be completely explained by leaching loss because they represent S leaching losses from the SSP treatment exceeding 20 kg S ha-1 in the first 30 days even when the highest 35s specific activity of the CaP-S pool is considered (i.e. 35s added/ fertilizer plus initial CaP-S at day 0).
Although the large loss of 35s remains largely unexplained by the measurement made in this experiment, the distribution of 35s in soil S forms and i n different soil depths provides useful information. A larger amount of 35s from the labelled SSP could not be accounted for in the top 10 cm of soil (soil plus herbage) as compared to the labelled microfine S0 . By 30 days <3% of the S0 remaining in the soil was S0. The greater retention of 35so probably resulted from its faster rate of incorporation into soil organic S rather than the non-susceptibility of S0 to leaching. With coarser S materials (0.075-0.1 50 mm particle size) the non-susceptibility to leaching may be a more important mechanism in reducing S lost and is studied in Chapter 7.
5.5.2.3 Transformation of 35 S in top 10 cm of soil layer
As discussed above as both experiments proceeded the activity of CaP-35s decreased in all soils and during the first 30 days there was a marked transformation of CaP-35s to organic S. Larger amounts of 35s labelled fertilizers remained as organic 35s in the top 0-3 cm soil layer and this may be attributed to and/or associated with larger root and microbial activity taking
place i n this surface horizon. Almost two-thirds of the 35s remaining in the top soil was transformed into carbon-bo nded S (as shown i n Table 5 .6) . This trend in organic 35s partitioni ng agrees with results of other i nvestigators who observed t hat the rate of i ncorporation of radioactive 35s into organic S was highest in the soil surface layer which contains larger amounts of organic residues (Swank and Fitzgerald, 1 984; Schindler et al.,
1 986; David and M itchell, 1 987) where larger amounts of carbon-bonded S were formed (Strickland et al. , 1 987) . In these pasture soils, pasture roots decrease logarithmically down
the profile (Williams, 1 988) . Gregg (1 976) also fou nd that larger amounts of labelled 35s gypsum were incorporated into organic 35s in the i mproved pasture soils where larger amounts of organic matter had accumulated.
Transformation of 35s from microfine S0 to organic forms was as fast as that of 35s labelled gypsum in SSP, but the amounts transformed were twice those of the SSP treatments. lt could not be explained why a larger amount of organic 35s occurred in the s0 treated cores. The transformation may be associated with or occur concurrently with the microbial oxidation processes. During these processes autotrophs derived energy from S0 oxidation to fix carbon (Alexander, 1 977). More detailed studies are required to determine the impact of S oxidizing microbes on the form of organic S formed. However, it is of interest to note that the capacity of soil to incorporate fertilizer S into organic forms differs with different fertilizer forms. In addition, it has been shown that the soil capacity for incorporation of S into organic forms varies with different soils (Autry and Fitzgerald, 1 991 ). This was also indicated by the survey of pasture soil conducted by Jackman (1 964a, 1 964b) and relates to the climatically controlled biological productivity of the site and the ability of the soil to form organo-mineral complexes from plant and animal residues.
As a nutrient conservation point of view, it may be considered that S0 is more preferable than SSP, since larger amount of S were retained in organic forms which appeared to be slowly mineralized as the experiment proceeded.
Although larger amo u nts of o rganic 35s occurred initially in the s0 treated cores, the proportions of 35s remaining in the soil which were subsequently transformed into the organic fraction were about the same for both fertilizers (Table 5.8). More than 90% of the remaining 35s was transformed into organic S. This i ndicates the importance of the transformation processes and suggests that it is possible that more 35s from the SSP may be transformed into organic 35s if losses by leaching can be reduced.
5.5.3 Comparison between labelled fertilizer and Inverse dilution techniques
Theoretically, the inverse dilution technique is employed to detect changes in the labile pool or exchangeable pool of nutrients in a system when a treatment is applied after the labile pool or the exchangeable pool has been labelled and a maximum equilibrium has been attained. Change in the labile pool can be measured through plant uptake. Only the changes of the native soil pools can be quantified. But a quantification of changes of the applied treatment (e.g. fertilizers) cannot be achieved if the treatment is concurrently transformed into an inactive form (organic forms). Shedley (1 982) employed this technique (inverse dilution) in a study of oxidation of s0 and fou nd t hat a large amount of the oxidized S (su lphate form) was transformed into organic forms. The author considered that using changes of soil sulphate levels to estimate S oxidation is inaccurate and underestimated the oxidation rates.
I n general , results of the inverse dilution technique were consistent with the labelling technique . Larger losses of 35s occurred in the SSP fertilized cores and more 35s was i ncorporated into organic S in the S0 fertilized cores. Additionally, the inverse dilution technique also revealed changes in the soil organic fractions, ester-so4= and carbon-bonded S. lt appeared that carbon-bonded S was likely to be a greater source of mineralized S. This trend in organic S mineralization has been observed by Freney et al. (1 975), Mclaren and
Swift (1 977), McGill and Cole ( 1 981 ) , Mclaren et a!. (1 988) and Ghani et a!. ( 1 99 1 )
Isotope recovery and dilution data from both techniques remain difficult t o i nterpret without models which are more descriptive of the factors accounting for daily removals of S from the exchangeable pool, in particular leaching losses.
5.5.4 The mlcroplot technique ( undisturbed soil core)
lt is considered that the microplot technique employed i n these studies gave results comparable to the small plot. The coefficient of variation (%C.V.) among treatments was small. Some investigators e mployed this technique to study the fate of fertilizer in soil systems, e.g. Peverill et al. ( 1 977); Martin (1 985) ; Destain et al . • (1 989) and Williams et al.
practical difficulties, especially concerning the u niformity of fertilizer application. All of the applied nutrients can be conserved within the microplots and possible run-off of labelled fertilizers is prevented (Martin, 1 985; Destain et al. , 1 989).