Microbial phosphorus processing in a gradient of agricultural soil development following mining activity

Open-cast lignite mining significantly disrupts cultivated soils. Restoration and re-cultivation processes enable the conversion of these disturbed areas back into productive land. These processes involve mixing original topsoil (~20%) with parent material loess (~80%), diluting the organic carbon (C) and nitrogen (N) pools, as well as the soil's biological parameters. To restore soil fertility and physical structure, Phase I includes the cultivation of alfalfa to replenish C and N, re-establish biological functions, and the addition of mineral fertiliser (N:P:K, 15:15:15 kg ha^-1). Following two to three years of Phase I, the restoration transitions to Phase II for three to five years, with an initial application of green waste compost (30 t ha^-1) and annual basal mineral fertiliser (N:P:K, 200:80:60 kg ha^-1). Phase III then involves returning the land to farmers with a typical rotation including sugar beet-winter wheat and a mix of organic and mineral fertilisation.

Previous studies have shown that soil C recovery and several key biological functions have only partially recovered, even after more than 50 years since re-cultivation. However, the evolution of P cycling, especially microbial-mediated P cycling, along this gradient remains unknown. This study aims to investigate interactions between soil P, soil microorganisms, and soil properties that affect microbial P cycling and P availability to plants following mining activity.

Hedley fractionation was employed to estimate various P pool sizes, while ion-exchange kinetics (IEK) assessed P exchangeability and reactivity in eight soils (soils restored from 2022 – year 0 – Phase I, 2020 – year 2 – Phase I, 2018 – year 5 – Phase II, 2014 – year 9 – Phase III, 2006– year 17 – Phase III, 1979 – year 44 – Phase III and 1964 – year 59 – Phase III and an original soil undisturbed). In three key soils (year 0 - Phase I; year 59 - Phase III; original undisturbed soil), ¹⁸O-labeled water was used in incubation to determine the degree of ¹⁸O integration within microbial biomass and in various P fractions.

In Phase I, a decrease in the relative size of the most labile-P pool was observed. In Phases II and III, this proportion increased, notably with a larger NaOH-extractable-P increase. P exchangeability decreased during Phase I, then significantly increased in older soils, surpassing that of the original undisturbed soil. Preliminary results indicate microbial P processing is highly correlated with total soil organic C. For instance, microbial P processing was nearly non-existent in newly formed soil (organic C: 0.54 g kg^-1) and was found to be twice as low in 59-year-old soil (organic C: 1.24 g kg^-1) compared to the original soil (organic C: 1.62 g kg^-1).

The current findings demonstrate that despite measured P levels surpassing those of the original soil in the oldest soils, biologically-driven P cycling has not fully recovered more than 50 years after soil re-cultivation.