No-tillage conversion of harvested perennial grassland to annual cropland reduces root biomass, decreases active carbon stocks, and impacts soil biota

Abstract

Landscape conversion to agricultural use historically began with tillage, a practice now known to dramatically and often negatively affect soil properties and ecosystem processes in grassland dominated ecosystems. However, converted landscapes are generally dominated by annual crops and intensive management compared to the diverse, perennial plant communities they replace. As a result, the effects of tillage and changes in plant community composition are often confounded. To control for tillage, we imposed a randomized complete block design on a never-tilled grassland with two treatments: (1) perennial grassland and (2) never-tilled annual cropland converted from grassland using herbicides. Our objective was to determine what changes in soil properties and soil biota occur with the conversion of perennial grassland to annual never-tilled cropland. Soil physical and chemical properties, microbial biomass, bacterial and nematode assemblage structure, and root biomass were measured to a depth of one meter. Without tillage, conversion from perennial grassland species to annual crops reduced belowground root biomass to 43% of prior biomass. Three years after conversion, readily oxidizable carbon and microbial biomass were significantly lower in annual never-tilled cropland versus perennial grassland in the surface 40 cm. No consistent changes in bacterial or nitrogen fixing community composition were detected. Nematode community structure was significantly different between grassland and cropland, and nematode assemblages were dominated by taxa typical for disturbed, nutrient poor conditions. Our results show that even in the absence of tillage and under best management practices, annual cropping can reduce soil carbon and impact soil biota and food webs important in nutrient cycling after just three years.