Assessing the Population and live weight of earthworms affected by chemical and organic fertilizers in the soil of an orchard

Earthworms, due to their beneficial role in agroecosystems, are used as indicator species for monitoring the impact of pollutants, changes in soil structure and agricultural practices. The purpose of this study was to investigate the impact of chemical fertilizers management on the density and live weight of earthworms in the soil of an orchard.
Research was carried out in an orchard in Isfahan province. One square meter area was considered as experimental plot. After plowing and grooving at a depth of 5 centimeters per plot, common fertilizers were applied, based on the soil test. Chemical fertilizers such as urea, ammonium sulfate, diammonium phosphate, solupotas, NPK (15, 5, 25), zinc chelate, iron chelate and cow manure were used as experimental treatments with control (no fertilizer). Abundance and live weight of worms were evaluated in two stages of time and in 2 depths of 0-20 and 20-40 cm of soil. In the first stage, 20 days after the addition of fertilizers, and in the second stage, 60 days after fertilization, earthworms were sampled and their number and biomass changes were measured. Treatments were applied in three replications and the data were analyzed using two-way ANOVA (using the GLM process) by SPSS software. In addition, the means were tested with the least significant difference (LSD) at the probability level of 5%.
The assessment of the worms was accomplished at two different depths and at two different times. Sampling in 20th day of experiment showed that at soil depths of 0-20 cm, urea, ammonium sulfate, diammonium phosphate, macro fertilizer NPK and iron chelate had significant negative effect on the abundance and weight of earthworms (p≤0.05). Meanwhile, cow manure (OM) and NPK treatments increased the population and biomass of earthworms. In the depth of 20 to 40 cm, the negative role of urea, ammonium sulfate, diammonium phosphate, NPK fertilizer and iron chelate was decreased, and the NPK maintained its positive role. On the 60th day of sampling, at 0-20 cm depth, urea, ammonium sulfate, diammonium phosphate and iron fertilizer reduced the population and biomass of worms in the soil. However, NPK, OM and zinc chelate (Zn) treatments increased the number of earthworms. At a lower depth (20-40 cm) on the 60th day, the earthworm population was not affected by any of the inorganic fertilizers and nitrogen and phosphate fertilizers not only had no negative effect on the activity of earthworms but also significantly increased their biomass (p≤0.05).
The results showed that chemical fertilizers could have a significant effect on soil organisms, including earthworms. These effects could vary according to the nature and extent of fertilization, as well as the depth of fertilizer usage. Accordingly, in order to reduce the negative effects of chemical fertilizers on soil organisms, it is advisable to reduce their application in shallow soils and when applying, organic fertilizers should be used along with chemical fertilizers.