Effects of Semiarid Wheat Agriculture on Soil Microbial Properties: A Review

H. Rodgers, J. Norton, L. Van Diepen
University of Wyoming
INTRODUCTION
Semiarid grain agriculture: our most important crops in our most vulnerable landscapes
  • Wheat constitutes 1/5th of the global food supply [3]
  • Most wheat is grown in semiarid lands, which are deteriorating under the pressures of population growth and climate change [4]
Soil microorganisms are critical for sustainable and resilient food production
  • Microbe-rich soils both mitigate climate change (by sequestering carbon) and adapt to it (by resisting extreme events such as floods and drought)
  • Soil microbes show promise as soil health indicators because they respond to management change much more rapidly than soil chemistry or structure [1-2]
  • The relationships between soil microbial properties and management practices are poorly understood in semiarid environments
RESEARCH QUESTIONS
  1. Which management practices consistently support microbial soil health in semiarid wheat agriculture?
  2. How can microbial properties be used to indicate changes in soil health & carbon sequestration, and crop yield?
 
METHODS
We reviewed 60 research papers published 2000-2020 that analyzed soil microbial properties in semiarid non-irrigated wheat fields under different management systems.
 
Figure 1. The reviewed studies took place in seven countries and were organized into sections on fertilization, tillage, and cropping system.
 
RESULTS
Reduced tillage, increased cropping intensity, and organic matter inputs consistently support soil microbial health
 
REDUCED TILLAGE
  • Prevents erosion of microbe-rich topsoil and improves soil moisture retention and temperature regulation
  • Increases microbial biomass and activity in the top 10 or 15 cm [5, 7]
  • Stratifies both bacteria and fungi by depth [8-9]
  • Fungi are more affected by tillage but take longer to recover (4+ years) after tillage than bacteria
  • Representative study: conversion to no-till increased wheat yield (by 20%), the activities of four enzymes, microbial biomass (by 50%), and SOC (by 25%) in 0-10cm after 15 years [5]
INCREASED CROPPING INTENSITY (REDUCED FALLOW)
  • Improves SOC, microbial activity, and fungi
  • Fungi and fungi:bacteria ratio are particularly reliable indicators of increasing C storage due to changes in cropping [11]
  • In drier areas, reducing fallow from every other year to every third year may provide as many benefits as eliminating fallow entirely
  • Legumes increase microbial activity and especially microbial groups associated with N fixation and cycling, and these effects can last several years
  • Great Plains continuous wheat systems have higher SOC (by 17%), fungal biomass (by 300%), and aggregate stability (by 200%) than wheat-fallow, on average [11]
FERTILIZATION & AMENDMENT
  • Fertilization (particularly chemical fertilizers) impact bacteria more than fungi, decreasing fungi:bacteria ratio [13-14]
  • Organic amendments (compost, manure, or biosolids) provide increasing SOC and fungi as well as bacteria, and these effects can last more than a decade in semiarid environments [15]
DISCUSSION & CONCLUSIONS
Microbes are sensitive soil health indicators
  • When sampled correctly, microbes reliably respond to management changes and can predict changes in soil health, C sequestration, and yield sometimes years before these changes are significant
  • Microbial and fungal biomass, enzyme activity, glomalin, and fungi:bacteria most reliably indicated soil health change in the reviewed studies [5-6, 10, 12]
  • Microbial community structure & diversity is sensitive to management but not easily interpretable [1, 2]
 
Figure 2. The most common microbial analyses performed by the reviewed studies were microbial biomass, gene sequencing, and enzyme activities.
 
Sampling and analyses must be tailored to the system being studied
 
STUDY TYPE SAMPLING RECOMMENDATION
Tillage Sample at multiple depths to detect changes in microbial stratification
Fertilization & Amendment
Analyze fungi and bacteria separately. Bacteria and enzyme activities are more sensitive to nutrient status than fungi, but a brief abundance in N-cycling bacteria and reduction in F:B does not indicate long-term soil health improvement
Cropping System

Analyze fungi for a more reliable indicator of increasing SOC. Fungi are more sensitive to plant dynamics and SOC than bacteria [14, 16-18].

Monitor soil moisture, as increases in cropping intensity may not have the desired effect if soil water is depleted

Table 1. Sampling recommendations for the different management systems reviewed.
  • Microbial properties can vary widely over time, so whenever possible, microbial indicators should be considered in relation to a nearby reference system considered “healthy” based on the research objectives
  • Longer-term experiments may choose to focus on fungal properties as more reliable, stable soil health indicators, whereas experiments less than five years or based mostly on soil nutrient differences may need to rely on bacterial indicators and labile C pools
Future research that links short-term microbial change to long-term yield and SOC change in agricultural fields can help understand which microbial analyses best indicate long-term change in specific systems.
 
REFERENCES

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