Modelling C and N mineralisation in soil food webs during secondary succession on ex-arable land

Abstract

The rate of secondary succession after land abandonment depends on the interplay between aboveground and belowground processes. Changes in vegetation composition lead to altered amounts and composition of soil organic matter (SOM) with consequences for the abundance and functioning of the soil food web. In turn, soil food web structure determines the mineralisation rate of nutrients that can be taken up by plants. This study analyses changes in the C and N mineralisation rates along with soil food web structure during secondary succession after land abandonment. In a previous study, changes in soil food web structure and SOM quantity and quality were measured at different stages of secondary succession on abandoned arable fields (abandoned for 2, 9 and 22 years and a heathland, which is the assumed target of the secondary succession). Based on these measurements we expected the C and N mineralisation rates to increase during secondary succession. The key hypothesis is that with a description of the soil food webs in terms of quantified biomasses, natural death rates, energy conversion efficiencies and diets enables a calculation of C and N mineralisation rates in soils. The basic assumptions connected to this hypothesis are that on a time-scale of years the population sizes are in steady state. We also calculated mineralisation rates per trophic level and energy channel. Based on the same measurements we expected that the contributions by the lower trophic level groups will increase as well as the mineralisation rates by bacterial and fungal energy channels. Measured C and N mineralisation indeed increased during the 22-year period of abandonment. The calculated C and N mineralisation rates showed the same trend after land abandonment as the measured values. Calculated contributions to mineralisation of organisms at trophic level 1 increase during secondary succession following land abandonment. The fungal decomposition channel contributed more to N mineralisation than the bacterial decomposition channel, whereas both channels contributed equally to C mineralisation rates. Direct contributions by higher trophic levels to mineralisation decreased during secondary succession. However, higher trophic levels were direct important for N mineralisation and indirect for both C and N mineralisation due to their effect on biomass turnover rates of groups at lower trophic levels. The increasing total N mineralisation rate of the soil food web, however, does not benefit plants, as during succession plant species that mainly grow under high nutrient availability are replaced by species that can grow in nutrient poor condition.