Gross sulphur mineralisation-immobilisation turnover in soil amended with plant residues

Summary

The rates of sulphur (S) released to and removed from the soil inorganic pool were estimated using the isotopic dilution technique. In an initial study fresh soil was mixed with combinations of two inorganic S levels (0 and 10 µg S g-1 soil) and three plant residues (wheat straw, perennial ryegrass and oilseed rape) and followed over 32 days of incubation. As 35S recovery was inadequate prior to day 2 and re-mineralisation of immobilised 35S occurred after day 8 thereby invalidating the method, estimates of gross S transformation rates should be based on data sampled between days 2 and 8. In the main experiment 16 plant residues with ranges in S contents of 0.08-0.81%, C/S ratios of 50-604 and lignin content of 0.9-10.8 were mixed with soil and carrier-free 35S label. Net turnover rates varied from 58% of S in Persian clover being immobilised to 76% of S in winter cress being mineralised within five days of incubation. Gross S mineralisation varied from 0.9-14.9 µg S g-1 soil d-1 whereas gross immobilisation only varied from 0.5 to 3.1 µg S g-1 d-1. Gross S immobilisation was strongly correlated to the C/S ratio of the plant material (P<0.001) whereas gross S mineralisation showed a weaker, but still significant, correlation with lignin content (P<0.05). The results indicate that immobilisation may predominantly have been a biological process in response to carbon addition while early mineralisation may have been dominated by the biochemical hydrolysis of organic sulphates in the residues. If attention is paid to the various constraints and limitations, isotopic pool dilution using 35S offers a tool that may prove valuable in understanding and modelling soil S turnover.