Determining the Influence of Soil Microbial Biomass Size on Soil Organic Matter Priming and Plant Residue Decomposition

Summary

This protocol describes a method to determine the influence of ryegrass residue addition on soil organic matter mineralization (i.e., priming effect) as well as explore the changes in soil microbial biomass size induced by soil organic matter priming, which involves artificially changing the size of microbial biomass.

Abstract

Soil microbial biomass is of great importance for soil organic matter (SOM) and residue decomposition. The effects of soil microbial biomass size (MBS) on SOM mineralization are still unclear, especially regarding mineralization in the presence of fresh plant residue input, known as the priming effect (PE). A traditional approach to determining this influence is the collection of soils with contrasting MBS, determination of the SOM mineralization rate, and further exploration of the relationships between MBS and SOM mineralization. In this protocol, the initial MBS is artificially changed in a preliminary experiment. Afterwards, the response of SOM priming with plant residue applications is estimated. Also presented is a detailed protocol for changing the initial size of microbial biomass as well as the determination of residue decomposition and SOM priming. The protocol includes six main steps: sample preparation, determination of optimal glucose level to increase MBS, preincubation experiment, confirmation of MBS changes, incubation experiment, and determination of PE and residue decomposition. The advantage of this methodology is that the relationships between the initial size of soil microbial biomass and SOM priming and plant residue decomposition are easily tested by altering microbial biomass artificially in a preincubation setting. This avoids potential confounding influence on the relationships by other factors, such as various soil nutrients or textures of different soils used in the traditional method.

Introduction

Despite accounting for a small fraction of soil organic matter (SOM)¹, microbial biomass plays a critical role in SOM and plant residue decomposition, and it significantly affects SOM dynamics and pools. Previous studies have shown that microbial communities are of great importance in the process of SOM and residue decomposition^2,3,4. However, the influence of soil microbial biomass size (MBS) on residue decomposition, especially SOM mineralization as affected by residue addition (the priming effect, [PE]), is still unclear. For example, similar SOM mi....

Protocol

1. Sample preparation

1. Randomly collect five samples to a depth of 20 cm from arable soil (Mollic Haploxeralf) and form a composite sample.
2. Remove visible plant residue and sieve soils through a 2 mm sieve.
3. Store soil samples at 4 °C for the incubation experiment.

2. Determination of optimal glucose level

1. Determine soil water-holding capacity (WHC) as described by Rey et al.¹¹.
2. Place soils (~20 g dry weigh.......

Discussion

The protocol provides a method to determine SOM priming from the supply of fresh plant residue as well as its decomposition, which has been reported in previous studies. The creative aspect provided in this protocol is that soil microbial biomass is artificially changed to explore the relationships between the size of microbial biomass and SOM priming. By using the method, only the size of soil microbial biomass is changed, maintaining other soil properties (i.e., soil nutrients, soil texture, etc.) that are similar amon.......

Materials

Name	Company	Catalog Number	Comments
Delta V Advantage IRAM through a GC/C-III interface	Thermo Electron Corp., Bremen, Germany
Gas chromatograph (GC) system	Shimadzu
Gas-tight syringe (20 mL)	Amazon.cn	B01N98U7GX	It can be used to add water and sample gas.
GC combustion isotope ratio mass spectrometer (GC/C-IRMS)	Thermo
JMP 13	Jmp.com
Mason jar	Amazon.cn	B00B80TJUI
Trace GC Ultra gas chromatograph	Thermo Electron Corp., Milan, Italy
Vacuum bottles (Labco bottle)	Ojielabs.com	039W