Cultivation effects on soil biological properties, microfauna and organic matter dynamics in Eutric Gleysol and Gleyic Luvisol soils in New Zealand

Information on the influence of duration of cultivation on soil organic matter (SOM) dynamics is needed by researchers, policy makers and farmers for assessment of global ramifications of carbon (C) sequestration and agricultural sustainabilty. This paper examines the changes in microbial biomass carbon (MBC) and nitrogen (MBN), metabolic quotient (qCO2), microfaunal populations, and C and N mineralisation to assess the effect of cultivation on SOM dynamics. Cultivation, for up to 34 years, of soils previously under permanent ryegrass (Lolium perenne L.) and clover (Trifolium sp.) pastures, resulted in a 30–60% decline in concentration of soil organic C and N, and had a significant influence on soil biological parameters in two New Zealand (Eutric Gleysol, Kairanga silty clay loam and Gleyic Luvisol, Marton silt loam) soil types. Cultivated soils had consistently lower MBC and MBN contents, and higher qCO2 than their counterparts in permanent pastures. Changes in the soil microfauna following cultivation were consistent with changes in microbial biomass. Both the amount and proportion of mineralisable N were also reduced with cultivation. In the pasture soils C mineralisation rates (under laboratory conditions) were twice those (ca. 15 mg (CO2-C) kg−1 soil) in the 5–20 year cultivated soils (ca. 7 mg (CO2-C) kg−1 soil), and were reduced to one-half (ca. 3.5 mg (CO2-C) kg−1 soil) in the 34-year cultivated soil. Over 112 days, the soils lost approximately 2.4–5.0% of their total C, and 1.4–5.0% of total N was mineralised. The percentage C loss also differed with soil type, with Marton silt loam (260 g kg−1, clay) soil losing one and half times as much percentage C as Kairanga silty clay loam (420 g kg−1, clay) soil. Despite greater C mineralisation in soil from pasture, more C and N were conserved in it than in the cultivated soils. As annual C inputs in cultivated soils are smaller than the C decomposition, this would result in a negative soil C balance. The shifts in microbial biomass, its metabolic quotients and soil microfauna appear to be associated with differences in the quantity and ‘quality’ of inputs and SOM decomposition rates, and reflect the land-use change from pasture to continuous cultivation.