Influence of cropping and fertilization on soil macropore characteristics in a long-term field study

It is known that soil organic carbon (SOC) is positively correlated with soil aggregation and total porosity. Similarly simple relationships between SOC content and abundance of pores in specific diameter ranges seem however elusive. In this study, we used X-ray tomography to investigate if this may be explained by treatment-specific differences in biopore forming agents, in other words soil faunal communities and root growth. We therefore compared the pore and biopore network characteristics in the topsoil of an ongoing long-term field experiment in Ultuna, Sweden, which was started in 1956. We selected three contrasting treatments that had led to significantly different SOC contents, ranging from 0.9 to 2.1% in weight, namely: a bare fallow and two cropped plots with two different fertilization treatments, mineral N-fertilizing with Ca(NO₃)₂ and farm yard manure (FYM). Sixteen undisturbed soil cores were sampled in eight small (ø 22.5 mm; height 65.5 mm) and eight large (ø 65.5 mm; height 74.8 mm) columns from each treatment, respectively (48 samples in total). The results of our study are in line with empirical knowledge that soil treatments associated with increased carbon contents exhibit larger porosities. However, we observed that the abundance and size distribution of biopores at different scales exhibited treatment-specific differences that cannot be explained with SOC content differences alone. Instead, they must have been caused by distinct root morphologies (or complete absence of roots) and/or by differences in soil faunal communities. Our study demonstrates that simple relationships between soil organic matter content and soil macropore network properties must not be taken for granted.