Soil microbial and chemical properties of a minimum and conventionally-tilled wheat-fallow system

Authors: Reardon, Catherine - Kate; Wuest, Stewart; Melle, Caroline; Klein, Ann; Williams, John; McCallum, John; BARROSO, JUDIT - Oregon State University; Long, Daniel

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/7/2019
Publication Date: 7/18/2019
Citation: Reardon, C.L., Wuest, S.B., Melle, C.J., Klein, A.M., Williams, J.D., McCallum, J.D., Barroso, J., Long, D.S. 2019. Soil microbial and chemical properties of a minimum and conventionally-tilled wheat-fallow system. Soil Science Society of America Journal. 83(4):1100-1110. https://doi.org/10.2136/sssaj2018.09.0344.
DOI: https://doi.org/10.2136/sssaj2018.09.0344

Interpretive Summary: In semi-arid and arid regions, the availability and timing of precipitation is the greatest limitation to dryland crop production. Summer fallow is practiced in order store water for the wheat crop the following year, thus stabilizing yields and reducing crop failures. Intensive tillage, which has traditionally been used to control weeds, leaves the land susceptible to wind and water erosion, reduces soil carbon and can be detrimental to soil health. In this study, we compared the effect of less intensive minimum tillage (reduced or sweep tillage) to conventional (disk) tillage in a dryland winter wheat-fallow rotation in the low precipitation zone of the Pacific Northwest region. Soils were collected at two depths, 0-10 cm and 10-20 cm, following harvest in 2016 and 2017 from plots in the wheat and fallow phases of the rotations. Soils were analyzed for soil chemical (concentrations of nitrate, ammonium, phosphate, total C, total N and pH) and biological (fungal and bacterial abundance, and enzyme activity related to C-, N-, and P-cycling) properties. All the measurements excluding pH were greater in the top 0-10 cm compared to the lower depth of soil, and all measurements, excluding pH and nitrate, were greater in the 2016 crop year than 2017. Across both phases, there were no differences between any of the measurements based on tillage intensity. However, in the wheat phase alone, minimum tillage resulted in more fungi in the upper soil depth compared to conventional tillage. The results indicate that crop year and soil depth supersede tillage in driving the activity and abundance of soil microbial communities and chemistry in this low precipitation site.

Technical Abstract: Tillage alters the soil environment and microbial communities responsible for decomposition and nutrient cycling. The objective of this study was to evaluate the effects of tillage intensity (minimum vs. conventional) on the soil chemical and microbial properties of a winter wheat-fallow rotation in the low precipitation zone of the Pacific Northwest. Soils collected for two years at two depths from each of the crop phases (wheat and fallow) were analyzed for pH, nutrient availability, total C and N, fungal and bacterial gene abundance, and enzyme activity related to carbon-, phosphorous-, and nitrogen-cycling. All soil variables excluding soil pH were significantly greater in the top 10 cm soil depth, and except for pH and nitrate, all were greater in 2016 than in 2017. Across the full rotation, tillage did not affect any parameter measured. Rather, crop phase influenced several soil chemistries and arylamidase activity, albeit the effects were not consistent across depth. Fungal abundance was influenced by tillage intensity but only in the top depth under wheat, indicating a phase-specific effect rather than a change persisting across the entire rotation. Soil enzymes were strongly related to total carbon, total nitrogen, and phosphate across the two soil depths, but effects were inconsistent between depths and across phases. Overall, crop phase and year were stronger drivers of soil chemical and microbial properties than tillage intensity. Short term (<4 years) tillage intensification does not have a strong influence on the nutrient cycling capacity of soil, although fungal numbers may decline in the cropped phase.