Earthworms reduce nutrient loss from loess soil slopes under simulated rain

Abstract

Burrowing, feeding, and casting activities of earthworms affect nutrient cycling and hydrological processes that take place in sloping lands. However, the effects exerted by earthworms on nutrient losses associated with runoff and sediment during rain remains unclear, particularly in the Loess Plateau, where soil erosion and nutrient loss have led to severe environmental issues. In this study, our purpose was to quantify the effects exerted by the anecic earthworm species, Metaphire guillelmi, on nutrient loss via runoff and erosion in loess soil slopes. Six artificial grass slopes (mesocosms, 15°), comprising three with earthworms (100 individuals m−2) for 28 days, and three without earthworms, were subjected to simulated rain (90 mm h−1) for 60 min. Our results indicated that earthworms had (i) increased the content of NO3–-N from 8.1 to 151.8 mg kg−1 and OP from 51.0 to 71.8 mg kg−1, but decreased the content of OC from 6.6 to 5.2 g kg−1, TN from 0.76 to 0.62 g kg−1, TP from 0.73 to 0.68 g kg−1 and NH4+-N from 3.7 to 2.4 mg kg−1 at surface 2 cm soil; (ii) increased water-stable macroaggregates from 45.1 to 48.3% and soil saturated water conductivity from 0.07 to 0.28 mm min−1, and (iii) greatly reduced the cumulative runoff from 65.7 to 15.3 mm and sediment occurance from 0.44 to 0.16 kg m−2 on slopes. Consequently, earthworm activities decreased runoff-associated nutrients (decreased NH4+-N from 19.5 to 2.8 mg m−2, NO3–-N from 108.2 to 26.2 mg m−2, and SP from 2.2 to 0.1 mg m−2) loss by 75.8–96.2%, and sediment-bound nutrient (decreased OC from 2.4 to 0.8 g m−2, TN from 0.3 to 0.1 g m−2, TP from 0.3 to 0.1 g m−2) loss by 63.8–68.5%. Hierarchical partitioning and variation partitioning analyses showed that runoff and sediment yields and their interaction with the soil physical characteristics were the dominant pathways via which earthworms regulated nutrient loss during rainfall events. This study highlights the importance of soil fauna in regulating erosion and nutrient loss in the Loess Plateau, and indicates that these factors should be considered in future water and soil conservation management.