Management induced organic matter differentiation in grassland and arable soil: a study using pyrolysis techniques

Differences in agricultural management and land use lead to differences in soil structure, soil organic matter (SOM) dynamics and composition. We investigated the SOM composition at three depth layers in a permanent pasture (PP), an organic arable (OA) and a conventional arable (CA) field within one soil series in marine loam deposits in The Netherlands. Both arable fields were in the grass phase of the rotation. The chemical composition of SOM was determined by a combination of conventional pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) and of thermally assisted hydrolysis and methylation (THM) with tetramethylammonium hydroxide (TMAH). In PP, SOM was composed of relatively little decomposed, mainly grass-derived material comprising polysaccharides, lignin, aliphatic compounds (extractable lipids, cutin, suberin) and proteins. With depth, plant-derived constituents decreased, whereas microbial and humified material predominated. Both arable soils contained mainly strongly humified plant material and microbially altered proteineous material that showed heterocyclic N-compounds together with alkylbenzenes and phenols upon pyrolysis. With THM small traces of plant-derived alkanols and cutin/suberin were observed in the arable soils. The upper layers of OA contained little lignin, which can only be derived from the grass vegetation or manure inputs since last ploughing (2 years before) since it was not found in the whole plough layer. Overall SOM composition is therefore hardly affected by organic farming compared to conventional management. The differences in SOM content and composition between the pasture and arable fields can be ascribed to differences in input and depth distribution of fresh organic materials. But a difference in physical protection of easily mineralisable SOM between pasture and arable soils is also likely to contribute.