Effects of early irrigation and compost addition on soil and vegetation of a restored semiarid limestone quarry are undetectable after 13 years

Semi-arid environments are strongly limited by water and nutrients, which hinders their recovery after anthropogenic disturbances. Application of compost and irrigation can improve soil fertility and enhance vegetation growth during the restoration of these environments. However, these restoration techniques may also favor the establishment of opportunistic communities and arrest natural succession. Mid and long-term assessments (>10 years) of the improvements in soil conditions and water availability after ecological restoration are scarce, although this is particularly important given the slow dynamics characterizing these environments. We assessed the effect of two levels of irrigation and compost addition (full-factorial design) in a limestone quarry under semi-arid conditions after a 13 years-period. These treatments were applied in addition to topsoiling, hydroseeding and plantation of woody species in the entire study area. Whereas the latter restoration treatments produced an overall increase in soil carbon content and salinity during this period (78% more soil carbon, over 200 mu S/cm [nearly four times] more salinity), watering and compost addition did not affect the accumulation of soil organic carbon (at 0-10 cm and 10-20 cm) or soil salinity. In contrast to the observed short-term effects, water and compost addition did not affect the cover, biomass and diversity of the herbaceous layer, or the survival and growth of planted woody species 13 years after the treatments ended. Fading treatment effects occurred despite substantial changes in vegetation structure and composition between year 1 and year 13 (25% more herbaceous species, 13% less biomass, up to 80% decline in woody seedling survival). Conversely, similar to what we observed 13 years ago, the survival of woody plants was affected by the composition of the herbaceous layer, with species-specific responses to their cover and composition. Our study illustrates temporal shifts in the effects of two commonly applied restoration treatments, and highlights the need for midand long-term monitoring programs to properly assess the effectiveness of restoration actions.

Automated summary

Semi-arid environments face challenges in recovery after disturbances, with compost and irrigation potentially improving soil fertility during restoration efforts. However, long-term assessments (>10 years) of these restoration techniques are limited, and short-term effects may not be sustained over time, as seen in a 13-year study where water and compost addition did not impact herbaceous layer vegetation growth. Long-term monitoring is essential to properly evaluate the effectiveness of restoration actions.