Soil moisture dynamics alone does not explain differences in growth associated to organic amendment in a reforestation of a semiarid ecosystem

Abstract

The success of reforestation of degraded areas in semiarid environments is constrained by low soil water content and fertility. In order to overcome these limitations, several site preparation techniques previous to plantation have been developed. Organic amendments have been used to improve physical soil properties in order to increase water availability for plants. However, the role of improvement in nutrient availability produced by organic amendment could have been neglected in semiarid vegetation restoration. In this paper, we analyze the effect of two site preparation treatments on soil moisture dynamics of an experimental reforestation with Pinus halepensis Mill. in a semiarid Mediterranean ecosystem. In one treatment (S) only lineal subsoiling (60–80 cm deep) was applied, in the other (SFF) a fresh urban refuse was added at a dose of 10 kg m−1 into the subsoiled furrow. During the first 5 years the growth of pines was significantly higher in SFF. At this time, we studied soil moisture dynamics at 0–20 cm in transects (for 8 months) and in soil profiles at 5, 15, 25 and 45–55 cm depth (for 23 months). Measurements made in two adjacent off-lines unaffected by site preparation were used as covariates. We also measured soil physical properties, macronutrients (N,P,K) and total organic carbon (TOC) content. No significant differences in soil physical properties were found between the amended versus not amended treatment except for hydraulic conductivity and sand fraction content. SFF treatment showed higher hydraulic conductivity, especially in top soil layers (0–10 cm), and lower content of sand fraction. Phosphorus availability was significantly higher in soil amended with organic refuse while no effect was detected in the content of the rest of macronutrients measured. In the transects, a reduction of 30% of the mean soil water content was observed in planting lines (0.14 cm 3 cm−3) in contrast with adjacent off-lines (0.20 cm 3 cm−3); no statistically significant differences between site preparation techniques were found. Soil profiles observations revealed that soil water content was significantly higher in S than in SFF treatment. This suggests that improvement in soil moisture associated to improvement in the physical structure of the soil cannot explain differences in growth. We argue instead this is mostly attributable to improved nutrient availability and microbiological activity associated with the organic amendment. These results stress the importance of a better understanding of the role of nutrients in the restoration of water-stressed ecosystems.