Temporal fluctuations of microbial communities within the crop growing season

Abstract

Abstract Soil microbial communities maintain many ecosystem functions including decomposition and symbiotic nutrient uptake. Prior field studies have focused on either geospatial or relatively large-time scale trends in these communities. Whereas, their small-time scale (e.g., weekly) dynamics in agricultural fields have largely been overlooked. Our objective was to determine the effects and characteristics of short-time scales on five phospholipid fatty acid (PLFA) microbial groups throughout the whole growing season, while under two soil disturbance levels. The microbial community groups included total microbial abundance (TMA), arbuscular mycorrhizal fungi (AMF), total fungi, actinomycetes, and total bacteria. The fungal to bacterial ratio (F:B) was also determined. Soils were sampled twice per week from planting to harvest during two consecutive growing seasons (N\xa0=\xa062) and analyzed for microbial abundances and chemical and physical properties. The study was conducted at an on-farm site with a corn [Zea mays] – soybean [Glycine max] – wheat [Triticum aestivum] rotation and two soil disturbance levels [high (chisel plow) and low (shallow vertical till)] using a randomized complete block design. Data analysis included 1) spatial and temporal coefficients of variations, 2) a mixed linear model to test for time and disturbance level effects, and 3) a Fourier analysis to detect and characterize cyclical patterns via autocorrelation spectral densities. The Fourier analysis also included cross spectral densities and phase lags with a large suite of soil chemical, soil physical, and weather states. Temporal variations of microbial communities were one to threefold greater than their spatial variations. Significant time effects were observed for TMA, AMF, and bacteria, where these communities behaved as sine and consine functions over time. However, strong cyclical patterns were above white noise levels at the experimental plots level for all microbial groups during each growing season. Cross spectral densities indicated that bacteria and actinomycetes tended to cycle at short and long periods (biweekly to whole season) associated with soil physical and weather states, whereas TMA, AMF, fungi, and F:B cycles at long periods (bimonthly to whole season) associated with soil chemical states. These microbial communities in a field setting clearly undergo cyclical patterns over small periods with some modulations from soil disturbance levels, fertilization, and microtopography. Further research on these cyclical and cross-correlated dynamics is warranted to determine the magnitude of influence on ecosystem services and crop productivity.