Effect of manure and fertilizer application on the distribution of organic carbon in different soil fractions in long-term field experiments

We studied soil organic carbon (SOC) pools using physical fractionation in two long-term arable field experiments in Hungary (Martonvásár and Keszthely), which started in 1950 and 1963, respectively (clay loam and loam soil). Replicate experimental plots of three different manure and fertilizer treatments and one unfertilized control object were sampled in March 2004 at both sites. Samples from all fields were separated into five size and density fractions. Fertilization had a distinct influence on both the OC amount present in two free particulate organic matter (POM) fractions (+19–230% compared to the control treatment) and their relative proportion on the whole-soil OC. This increase in OC was equal or smaller for the POM occluded in microaggregates (53–250 μm) (+16–97% compared to the control treatment), and much smaller for the amount of OC present in the silt + clay sized fraction (+ −2 to 30% compared to the control treatment). The considered SOC fractions were associated with different conceptual SOC pools. Differences in the relative effect of management on the amount of OC present in the fractions consequently resulted in shifts of the relative proportion of the OC present in these conceptual OC pools. The measurability of these changes at the time scale of decades shows that it may be possible to use experimental data such as the data obtained from this study for the development and calibration of SOC models with conceptual OC pools which coincide with physical counterparts.