Microbial-based soil quality indicators in irrigated and rainfed soil portions of Mediterranean olive and peach orchards under sustainable management

The main objective of this study was to apply microbial indicators of soil quality in drip-irrigated olive and peach orchards managed with sustainable agricultural practices. Soil characterization was carried out in different areas of the orchards along the row, under the drippers (Rdr), and along the inter-row, rainfed (IRrf), to evaluate the effects of irrigation. Two parameters were followed during one year: a) a biochemical soil indicator (Nc/Nk ratio) based on soil N/C turnover and soil enzyme activities, and b) the abundance of three important N-cycling genes (nifH, amoA and nosZ). Localized irrigation caused higher values of water content in the Rdr areas, compared to IRrf. The Nc/Nk ratio exhibited all the attributes of a reliable soil fertility indicator, being generally higher in irrigated Rdr areas. The abundance of nifH and amoA in the soil showed a trend similar to Nc/Nk, being affected by higher soil water content, while nosZ abundance was generally insensitive to irrigation. Both Nc/Nk and gene abundances, much more than the measured chemical, biochemical and molecular soil parameters considered alone, can give a precise idea on N and C soil dynamics, that in turn, affect soil quality and fertility.