No-till annual wheat increases plant productivity, soil microbial biomass, and soil carbon stabilization relative to intermediate wheatgrass in a Mediterranean climate

As interest in the production of perennial grain crops increases, it is critical to quantify differences in above ground biomass (AGB) productivity and soil carbon stabilization potential between annual and perennial grain crops, and to understand the factors determining these relationships. This study measured the effects of crop type and nitrogen (N) fertilization on AGB, soil microbial biomass, and soil carbon while controlling for the effect of tillage in rainfed annual and perennial grain crops in a Mediterranean climate. AGB and associated soil samples from tilled annual wheat (Triticum aestivum), no-till annual wheat, and intermediate wheatgrass [Kernza (R) (Thinopyrum intermedium)] (IWG) were collected annually across three years under various N fertilization rates. We quantified AGB yields, soil microbial biomass (via phospholipid fatty acid analysis), permanganate oxidizable carbon (POXC) and mineralizable soil carbon (MinC). Across N rates and years, IWG yielded approximately 25% less AGB than tilled and no-till annual wheat (p <= 0.001). The lack of soil disturbance in no-till wheat and IWG contributed foremost to greater soil microbial biomass in the topsoil by year three of the experiment (p < 0.1). Crop type and the associated AGB productivity had the greatest influence on whether microbial communities trended toward stabilizing versus mineralizing soil carbon. More specifically, higher soil carbon stabilization processes were primarily driven by the increased plant productivity of tilled and no-till annual wheat compared to IWG. Within the annual wheat systems, lack of tillage also contributed to soil carbon stabilization, but primarily in the top 30 cm of soil. Compared to the other treatments, no-till annual wheat resulted in 25% (IWG) and 36% (tilled annual wheat) greater soil microbial biomass in the top 30 cm by the final year of the experiment and had higher soil carbon stabilization potential than IWG at 0-15 and 15-30 cm and tilled annual wheat at 0-15 cm across time (p <= 0.01). This was achieved by matching tilled annual wheat in aboveground biomass productivity and IWG in lack of soil disturbance. Results from this study suggest that no-till annual wheat is better suited than tilled annual wheat or IWG to increase soil microbial biomass and soil carbon stabilization in shallow soil, while maintaining stable aboveground biomass yields over time in a Mediterranean climate.