© 2010 19 th World Congress of Soil Science, Soil Solutions for a Changing World 1 – 6 August 2010, Brisbane, Australia. Published on DVD. 11 Modern isotopic methods to investigate the fate and provenance of C sequestered into soils from livestock derived organic matter J.A.J. Dungait, R. Bol, I.D. Bull, D. Chadwick, W. Amelung, S. Granger, R.P. Evershed Rothamsted Research, UK, Email jennifer.dungait@bbsrc.ac.uk Understanding the fate of dung C in soils is challenging due to the ubiquity of plant-derived organic matter (OM), the source material from which both dung-derived OM and soil organic matter (SOM) predominantly originate. A better understanding of the fate of specific components of this substantial source of OM, and thereby its contribution to C cycling in terrestrial ecosystems, can be achieved through the use of labelled dung treatments. Bulk stable carbon isotope analyses are now used routinely to explore OM matter cycling in soils, and have shown that up to 20% of applied dung C may be incorporated into the surface soil horizons several weeks after application, with up to 8% remaining in the soil profile after one year. However, whole soil δ 13 C values represent the average of a wide range of organic components with varying δ 13 C values and mean residence times in soils. Several stable 13 C isotope ratio mass spectrometric methods have been developed to qualify and quantify different fractions of OM in soils and other complex matrices. Gas chromatography-combustion-IRMS (GC-C-IRMS) analyses have been applied to determine the incorporation and turnover of polymeric plant cell wall materials from C 4 dung into C 3 grassland soils using natural abundance 13 C isotope labelling. The mean residence time of C pools increase with the application of manure. In the Hoosfield Classical Experiment at Rothamsted UK, annual applications of manure at a rate of 35 t ha -1 for 140 years resulted in a three-fold increase in soil organic C levels over that in unfertilised plots, and about 50% higher than unfertilised plots 104 years after manure addition had discontinued. This, and other, long-term studies indicate that manure can play a positive role in increasing soil C stocks in soils. However, other studies have concluded that dung application may have no effect or indeed a negative effect on soil C sequestration. Clearly, the dynamics of incorporation of dung C into soil are not well understood, but are highly variable in both space and time. Natural abundance stable 13 C isotope labelling is one of the few proven techniques available for the examination of soil C dynamics in naturally functioning ecosystems. Isotope ratio mass spectrometry is widely used to determine the difference in natural abundance of 13 C between C 3 (δ 13 C =-32 to -20‰) and C 4 (δ 13 C = -9 to -17‰) vegetation which provides the basis for estimating the contribution of 13 C-enriched C 4 sources to SOM in ecosystems otherwise dominated by C 3 vegetation. Naturally, the δ 13 C values of cattle dung reflect the stable isotope values of their feed, therefore, cattle fed naturally 13 C-enriched C 4 species forage, i.e. Zea mays, produce a useful source of natural abundance 13 C- labelled dung that can be applied as a treatment to soils in C 3 ecosystems to explore cycling of dung C. Bulk δ 13 C values of C 3 and C 4 dung-treated soil can be used in a simple mixing model to estimate dung C incorporation after dung deposition using stable C isotope determinations. A maximum of 20% and 12% dung C was determined in the top 5 cm soil horizon of dung-treated soils after autumn and spring applications. Incorporation of dung C in the spring differed from that in the autumn and was more rapid and fluctuated producing a sigmoid pattern of incorporation in the autumn. In the spring experiment, 8% of the dung derived C remained in the soil after 372 days providing direct evidence for the mechanism for increasing C stocks in soils treated annually with manures. However, due to the diversity in decomposition dynamics and δ 13 C values of individual biochemical components of dung, fluxes in bulk δ 13 C values in C 4 dung treated soil may not imply a total loss or gain of dung C. Different components of plant-derived OM, and therefore dung, have a range of δ 13 C values due to fractionations against the heavier isotope during biosynthesis. Cow dung is a complex mixture of biochemical components that are likely to decompose at different rates. The contribution of different biochemical fractions to dung estimated using gravimetric procedures showed that the major component is undigested lignocellulosic plant cell wall material. Cow dung derived from Lolium perenne and Z. mays forages was estimated as 20-30% hemicellulose, 20-30% cellulose, 7% lignin, 12% crude protein and 3-5% fats using the ‘Forage Fibre Analysis’ procedure. Analyses of dung carbohydrates analysed as alditol acetates