Effects of Semiarid Wheat Agriculture on Soil Microbial Properties: A Review
- Wheat constitutes 1/5th of the global food supply 
- Most wheat is grown in semiarid lands, which are deteriorating under the pressures of population growth and climate change 
- 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
- Which management practices consistently support microbial soil health in semiarid wheat agriculture?
- How can microbial properties be used to indicate changes in soil health & carbon sequestration, and crop yield?
- 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 
- Improves SOC, microbial activity, and fungi
- Fungi and fungi:bacteria ratio are particularly reliable indicators of increasing C storage due to changes in cropping 
- 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 
- 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 
DISCUSSION & CONCLUSIONS
- 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]
|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
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
- 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
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