Seasonal changes in bulk density under semiarid patchy vegetation: the soil beats

We investigated the temporal variation of soil bulk density (BD) in a semiarid Macrochloa tenacissima grassland where BD plays a key role in the soil–plant feedback mechanisms that are responsible for the patchy distribution of vegetation. We used the core method to analyse the soil BD in two experimental sites in microsites that were representative of the spatial heterogeneity of soils and vegetation. The study was performed over two years, in winter (three times) and summer (twice). We studied the changes in BD in relation to the rainfall and to the soil properties that describe or affect the soil structural conditions, such as soil moisture, particle-size, clay mineralogy, the contents of various organic carbon fractions and certain microaggregation indices. We found a consistent temporal variation in BD, which was lower in winter (1.22±0.02g/cm3, average±SEM) than in summer (1.33±0.02g/cm3) and depended on the rainfall in the weeks before sampling. The greatest change was observed during the first year of study, when the wettest winter and the driest summer occurred. The variation of BD was most pronounced in the mounds developed under senescent plants (+16.7% average increase from winter to summer), intermediate under adult plants and in the residual mounds (+8.9%) and was weak in the soils of the bare areas or under young plants (+5.1%). This implies that the microtopography should get steeper in winter and flatter in summer. The largest changes in BD were correlated with variables representing organic matter and its action in adhering soil particles. We observed consistent temporal variations in BD that were spatially heterogeneous and related to wet conditions in the soils studied. Research is needed to establish the mechanisms responsible for the changes in BD, which may include greater activity of roots and fauna during the wet season and the enrichment in labile organic fractions that are hygroscopic and can bind soil particles into temporary aggregates. Spatially heterogeneous temporal variation in BD must be taken into account in inventories of soil nutrients, water or carbon. Moreover, it implies an increased porosity under vegetated patches just when the soil receives the most rainfall, resulting in a positive feedback of plant growth and mound formation. This mechanism might be added to those already described for plant–soil feedbacks in dry lands. •We studied soil bulk density in microsites representing the soil heterogeneity.•Consistent t