Rate of recycling of sulfur from urine, feces and litter applied to the soil surface

The recycling of S from plant litter, dung and urine is an important process for supplying S for pastures. A pot experiment was conducted where 35S-labelled litter (25% white clover/38% ryegrass/21% weed) and S-35-labelled urine and faeces collected from sheep fed the same herbage as was used as litter was surface applied to pots and the fate of the applied S was followed for 100 days with ryegrass as the test plant. In camp soil, 45% of the S applied in urine was taken up by ryegrass plants within 12 days of application. In non-camp soil, the uptake of urine-S was about 20% over the same period. Cumulative uptake of 35S from urine in camp soil was subsequently restricted, with a maximum of 60% eventually measured in plants after 100 days. Mean rates of release of S (0-37 days) from litter and faeces was respectively 16.2 and 4.5 mg g-1 day-1. The calculated half-times from S in the two materials were respectively 43 and 154 days under controlled environmental conditions with adequate moisture. Litter S followed organic matter (OM) decomposition, but faecal S release was initially more rapid than faecal OM decomposition. There was little S release from faeces after day 25. Rather, S was immobilized in faeces during the 25-100 day period. The decomposition of litter and faeces was divided into an initial rapid process during which soluble S and more labile S was released, followed by a slower process involving the release of S from tissues more resistant to mineralization. The uptake of 35S from labelled materials was initially more rapid than would be expected for total S released from the added litter and faeces and the 35Suptake effect was short-lived relative to the continued effect of added material on total S uptake. The preferential uptake of 35S from the surface-applied material appears to be due to limited root development at the early stages of the experiment. Movement of 35S into the soil organic matter pool was very rapid; 58.4% of urine S was in the soil organic matter fraction in the non-camp soil by day 6. The amount of applied S in the organic matter equilibrated at about day 75. The accumulation of applied S from the materials added was greater than that recorded in previously reported studies for inorganic sulfate (e.g. about 50%). Soil P and S status had little effect on rates of release of S. from the applied materials, however, the effect of the camp and non-camp soil on total S recycling was markedly different as a result of the different amounts of plant growth and thus S uptake in the two soils. The decomposition of litter indicated peak rates of S release at two specific times over the 100 days and indicated successional changes in micro-organism activity. With faeces, the experiment was not continued for sufficiently long to show micro-organism effects.