Response of organic matter pools and enzyme activities in particle size fractions to organic amendments in a long-term field experiment

Physical soil fractionation may have significant potential to characterize soil organic matter pools of differing stability. This hypothesis was tested in a long-term field experiment on a Eutric Cambisol soil located in Ultuna, central Sweden. In 1956, the soil (0-20 cm depth) contained 15 g kg(-1) of organic carbon, sand/silt/clay contents of 22%/41%/37% and had a pH of 6.6. Seven treatments, continuous fallow, no nitrogen fertilizers (NoN), Ca(NO3)(2), green manure (green grass), animal manure, peat (sphagnum) and sewage sludge were selected. The application of organic amendments was based on addition of equal amounts of carbon (2000 kg C ha(-1) yr(-1)). Topsoil (0-20 cm) was sampled in October 1998, dispersed using low-energy sonication and then fractionated by a combination of wet sieving and repeated centrifuging (2000-200 mum, 200-63 mum, 63-2 mum, 2-0.1 mum and < 0.1 mum). The contribution of size fractions to the total organic carbon (OC) present in bulk soil differed significantly between treatments and was correlated with bulk OC contents. Specifically, the silt fraction, which yielded in all treatments the highest amount of OC compared to other fractions, showed increasing OC contributions with increasing bulk OC, as did the two sand-sized fractions. The relative contribution of the two smallest sized, and presumably most stable, OC fractions (2 - 0.1 mum and < 0.1 mum) decreased with increasing bulk OC. The 13 C abundances of the latter fractions were less influenced compared to the smaller fractions by OC from the amendments. Cluster analysis of isotopic data of particle separates revealed greatest similarity between green manure and animal manure, Ca(NO3)(2) and NoN treatment, and among fallow, sewage sludge and peat. Xylanase and invertase activities of the bulk soil increased after the long-term addition of green manure, animal manure and sewage sludge. Xylanase activity was mainly located in the sand-sized particles, whereas invertase was associated with the clay and silt fraction. The response of xylanase and invertase to different organic amendments can be easily detected in the sand fraction. 16S rRNA-based analysis showed increasing microbial diversity and abundance with decreasing particle size. Analysis of bacterial diversity and enzyme measurements suggested different metabolic activities in different particle size fractions.