Soil food web structure during ecosystem development after land abandonment

The re-establishment of natural species rich heathlands on abandoned agricultural land is a common land use change in North-West Europe. However, it can take several decades to reestablish natural species rich heathland vegetation. The development rate has found to depend both on soil food web composition and on soil processes. We measured the soil food web composition in three ex-arable fields abandoned 2, 9 and 22 years ago and in a heathland. To characterize food structure, we defined four trophic levels and a root, fungal and bacterial energy channel.Wehypothesized that with increasing time since abandonment, i.e. field age, (1) the basic resource level biomass, i.e. soil organic matter (SOM) and roots, will increase and thereby also that of biomasses at higher trophic levels, (2a) the root energy channel biomass will decrease, (2b) the fungal energy channel biomass will increase, and (2c) the bacterial energy channel biomass will not change. The results showed that biomasses at the basic resource level and at the first trophic level indeed increased with field age, but not the biomasses at higher trophic levels. It is not clear what the cause of the lack of increase in higher trophic levels was, possibly top-down or bottom-up forces played a role. The root energy channel biomass decreased and the fungal channel increased as hypothesized, but the bacterial channel biomass also increased with field age. The increase of the bacterial channel biomass contradicted the hypothesis, but agreed with the observed increase in SOM quantity and lack of decrease in SOM quality. On overall, results show that changes in belowground food webs lag behind changes of the aboveground vegetation. Such time lags may hamper secondary vegetation succession. Understanding those time lags may therefore help to develop management schemes improving land conversion processes.