No-till cropping systems and N fertilisation influences on organic matter composition of physical fractions of a subtropical Acrisol as assessed by analytical pyrolysis (Py-GC/MS)

Aiming to investigate the influence of no-till cropping systems and nitrogen fertilisation on the organic matter composition of the whole soil, particulate organic matter (POM) and clay-size fraction of a subtropical Acrisol, Curie-point pyrolysis gas-chromatography/mass spectrometry (Py-GC/MS) was employed on samples from a long-term experiment of Southern Brazil. Furans, polyphenol derivatives (benzene, toluene and styrene), N-compounds (pyrrole and indole), lignin moieties (guaiacol and syringol derivatives) and aliphatic compounds were identified on the pyrograms. The whole soil of pigeon pea (Cajanus cajan) + maize (P + M) cropping system contained a higher number of lignin-derivatives and aliphatic moieties than the whole soil of grassland, bare soil and oat (Avena strigosa)/maize (O/M) cropping system. In POM fraction, most of the differences were also related to P + M cropping system, which contained higher styrene, indole and aliphatic biopolymers compared to the other cropping systems. The organic composition of clay fraction was not affected by cropping systems. The nitrogen fertilisation in P + M and O/M cropping systems did not change the quality of organic matter neither in whole soil nor in physical fractions. Lower signal intensities corresponding to lignin derivatives were observed for clay samples compared to those in the POM. The expressive signals of furans in the clay fraction indicate that polysaccharides are important constituents of the organic matter in this fraction. Possibly such labile compounds are being protected by organo-minerals interactions in clay. Curie-point Py-GC/MS analysis seems to be a useful tool for studying the organic matter quality of whole soil and physical fraction samples from different soil management systems.