Successional Effects of No-Till Cover Cropping with Black Oat (Avena strigosa) vs. Soil Solarization on Soil Health in a Tropical Oxisol

Black oat (Avena strigosa) is a cover crop with great potential for weed suppression and erosion control while conserving soil moisture. Little is known about the potential of black oat for enhancing the soil food web structure and the ecosystem services in tropical Oxisols. Two-year field trials were conducted in Hawaii to compare three pre-plant treatments: (1) black oat (BO) as a pre-plant cover crop followed by no-till practice (previously managed by cover crop and cash crop rotation and conservation tillage for 7 years); (2) bare ground (BG) followed by conventional tillage (previously managed by conventional tillage and cash crop planting for 7 years); (3) conventional tilling of bare ground followed by soil solarization (SOL) (previously fallow with weeds for 5 years then summer solarization and cash crop planting for 2 years). Various soil properties and the soil food web structure using nematodes as soil health indicators were monitored throughout the subsequent corn (Zea mays) crops. SOL served as a negative control pre-plant treatment known to manage plant-parasitic nematodes but be destructive to the soil food web. No-till cropping with BO resulted in higher levels of volumetric soil moisture, field capacity, and soil organic matter, and supported a fungal-dominated decomposition pathway in trial I and more structured nematode communities than BG and SOL in trial II. This study provides evidence that no-till cover cropping with black oat improves the soil water conservation and soil food web structure following a continuous conservation tillage system in tropical Oxisols if the black oat biomass is high (36 tons/ha). However, no-till cropping with BO in Oxisol decreased the soil macroporosity and increased the soil bulk density, which were not favorable outcomes for water infiltration. On the other hand, SOL following conventional tillage was successful in generating lethal temperatures to suppress plant-parasitic nematodes and increased water infiltration in both years but was destructive to the soil food web and reduced the soil organic matter and soil moisture in both years, even when solarization failed to generate lethal temperatures in the second year.