1 S.Gomes (2016)

Rewetting soils: effects of drying and soil

properties on the magnitude and rapidity of CO2

release

Sergio Gomes

School of Biosciences, University of Nottingham, Sutton Bonington Campus, Loughborough,

Leicestershire, LE12 5RD, UK

May 2016

Keywords: Birch effect; Degassing; Hydrophobicity; Soil Respiration; Water repellency

This paper has been prepared for the Journal Soil Biology & Biochemistry.

Abstract

High effluxes of CO2 can be observed from dry soils following wetting. Elevated soil respiration rates are

a result of increased organic substrate mineralisation from soil microbes; a phenomenon called the Birch

effect. CO2 release from soils is known to increase when rewetting drier soils; however the exact onset of

release is unclear. This experiment aimed to investigate the onset, characteristic and magnitude of CO2

release following rewetting of different soil types over different durations of drying. Soils from an arable

field, grassland and woodland area were collected for this experiment. Release of CO2 was investigated

immediately and 24 hours after rewetting 0, 2, 10, 15 and 28 day dried soils. Moisture content of the soil

was adjusted to 50% water holding capacity for rewetting and hydrophobicity of soils was noted over the

course of drying. Rapid release of CO2 was found in all soils 30 seconds after rewetting, which lasted for 2

minutes. The initial CO2 release was suggested to have originated from degassing and changes in release

between different drying levels were attributed to volumes of water applied during rewetting. The release

of CO2 from soils 24 hours after rewetting was constant and suggested to be a result of elevated soil

respiration. The grassland and woodland soils were found to be hydrophobic, resulting in lower water

infiltration. Respiration from the hydrophobic soils was significantly lower than the hydrophilic arable

soils.

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1. Introduction

Soil is an integral part of the global carbon cycle, having the ability to gain and lose carbon

from a number of transformations and interactions (Schimel and Schaeffer, 2012). Soil acts

as a reservoir of carbon, and has a global store of 2,700 Gt C, where 1,550 Gt C is organic

carbon (Ontl and Schulte, 2012). Carbon is mainly lost from soils as carbon dioxide (CO2)

when it is produced and released through soil respiration (Ryan and Law, 2005; Deng et al.,

2010). The property of a soil has been shown to affect respiration and CO2 release; including

pH, texture, temperature and carbon and water content (Rastogi et al., 2002).

When a dry soil is rewetted, there is a temporary increase in CO2 release from a phenomenon

called the Birch effect (Fierer and Schimel, 2002; Meisner et al., 2015; Evans et al., 2016).

Soil respiration and CO2 emission is higher following drying and rewetting, compared to soils

kept constantly moist (Birch, 1958; Muhr et al., 2008; Xu and Luo, 2011). The increase in

CO2 efflux decreases over time after the wetting event (Fierer and Schimel, 2002; Jarvis et

al., 2007; Unger et al., 2010). The length of time which a soil is dried for also affects the CO2

pulse, and larger pulses have been found when soils have been dried for longer (Evans and

Wallenstein, 2012; Meisner et al., 2015). The Birch effect has been found to occur globally in

a range of environments and climates; including grasslands, forests and deserts (Morgan et

al., 2003).

Despite multiple studies investigating the effect, the exact causes and mechanisms behind this

phenomenon are uncertain. This is due to the potential interaction of responsible factors

which makes the identification and isolation of these mechanisms difficult. The source of the

Birch flush is believed to be of biological and physical origin, and the main hypotheses

explaining the source of the flush are: (1) organic matter and substrates exposed due to the

disruption in soil structure, from the physical action of rewetting, are mineralised by soil

microbes (Miller et al., 2005); (2) upon rewetting, microbes killed during drying within the

soil matrix are decomposed by surviving microbes (Jarvis et al., 2007); (3) the availability of

water from rewetting results in a rapid increase in microbial biomass and respiration

(Lovieno and Bååth, 2008); (4) rewetting causes a rapid change in water potential, inducing a

osmotic stress response in microbial cells. Intracellular solutes are released to prevent cellular

lysis, which can be mineralised and decomposed by soil microbes, releasing CO2 (Fierer and

Schimel, 2002).

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CO2 release is known to increase following rewetting, and respiration has been identified to

peak within 24 hours, with higher releases observed from dryer soils (Meisner et al., 2015).

There has been no previous identification of how quickly increases in CO2 release occur

following rewetting. Identifying how quickly CO2 efflux occurs would allow investigation of

the sources and factors responsible, including whether the origins are biological or physio-

chemical. Potential differences in the pattern of CO2 release immediately following

rewetting, and after 24 hours, could identify whether the sources of CO2 release after

precipitation change over time, and what effect drying and soil type have on this.

In this experiment, three different soil types with known properties were dried over time and

rewetted. CO2 release was measured from the moment the soils were rewetted, to identify the

onset of increased efflux, as well as 24 hours after the wetting event. This experiment aimed

to address the questions: (1) how soon after rewetting soil does CO2 release become elevated;

(2) what would the efflux of CO2 from soil immediately after rewetting be, and how does this

differ in characteristic to the secondary release 24 hours later; (3) how do different soils and

soil properties affect CO2 release; (4) how does drying affect CO2 release. It was

hypothesised that: (1) the highest pulse in CO2 release would be found after 24 hours, due to

the peak of the Birch effect; (2) higher pulses of CO2 are found in soils with higher organic

contents; (3) longer drying would yield higher CO2 release.

2. Materials and methods

2.1. Soil

The three soils used in this experiment were collected in February 2016, from a farm 600 m

away from the Sutton Bonington campus at the University of Nottingham, in north

Leicestershire, UK (52°49’48’’N, 1°14’20’’W). The site is elevated 48 m above sea level and

has a temperate climate which receives over 1 mm of rainfall on 115 days a year (Morice et

al., 2012). Average yearly precipitation at Sutton Bonington is 616 mm, which is

significantly lower than the national average of 934 mm (Croxton et al., 2006).

There were three distinct areas at the site, which the soils were collected from; consisting of

arable fields, grassland and deciduous woodland. This paper will refer to these soils as

‘arable’, ‘grassland’ and ‘woodland’ respectively henceforth. The arable soil was a sandy

clay loam from the Dunnington Heath series, whereas the grassland and woodland soils were

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both sandy loams from the Wick series. All three soils were taken from the topsoil (0-20 cm).

These three soils were chosen because they were distinctly different, despite being in close

proximity to each other.

The soils were left to air dry for 24 hours after collection and then sieved to remove pebbles

and vegetation. Brief initial drying allowed the fresh soils to be sieved with greater ease

(Haney et al., 2015). The clay arable soil was passed through a 4 mm aperture sieve, and the

grassland and woodland soil through a 2 mm sieve. The arable soil was passed through a

wider aperture because clay soils are more difficult to sieve due to its tendency to stick and

clump (Spellman, 1999).

The organic matter content (OM), pH and water holding capacity (WHC) of the three soils

were quantified for the experiment (Table 1). OM was determined using the loss on ignition

method, heating oven dried samples at 550 ºC for 6 hours and calculating percentage mass

lost (Vereș, 2002). A pH-electrode was used on soil-water solutions, which was shaken for 30

minutes beforehand to reach equilibrium, to attain the pH of the soils. WHC was identified by

wetting the soils and allowing the gravimetric water content to saturate and drain through a

filter funnel (Haney and Haney, 2010).

Table 1. Properties of the three soils.

Soil Arable Grassland Woodland

OM (%) 4.9 6.0 9.7

pH 6.7 5.3 4.7

WHC (mL/g) 0.5 0.7 1.1

2.2. Experimental setup

After sieving, the soils were spread out and layered thinly on separate trays and left to air dry

in a room at a constant temperature of 21 ºC. The soils were considered ‘fresh’ at this stage

and this was taken to be the point the drying treatment began, and also the control of the

experiment. Samples from each soil type were rewetted and measured immediately for CO2

release at five points over a month; after 0 (fresh), 2, 10, 15 and 28 days of drying. A

secondary reading, 24 hours after the initial wetting, was recorded from the same rewetted

samples for CO2 release. Three repeats for each soil type were taken for every measurement

of CO2 efflux.

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All rewetting during the experiment adjusted the soils to 50% WHC. This ensured that the

water content of the soils when CO2 efflux was measured would be constant, and a controlled

variable. The dry soil weight was also kept constant throughout the experiment, at 3.5 g.

The hydrophobicity of the soils was taken in the experiment, using the water droplet

penetration time method (Contreras et al., 2008). The time taken for droplets of water to

infiltrate the soils was recorded and 5 seconds was used as the threshold to determine

hydrophobicity (Doerr, 1998). Hydrophobicity was measured alongside rewetting, to identify

if it changed over time. Three repeats, per soil, were taken each time.

2.3. Rewetting

The dried soils were placed in 45 mL chambers, which were connected to a soil respiration

measurer, set up in an airtight circuit system. The soils were rewetted to 50% WHC, using a

needle syringe to squirt water through a plastic plug at the top of the chambers. The plug

allowed rewetting from injection, whilst maintaining the airtight circuit system. The needle

was injected into the centre of the soil samples within the chamber.

The water content of the soils decreased as soils dried over the experiment. The fresh soils at

the start of the experiment were at 25% WHC, but after 10 days of drying, negligible amounts

of moisture remained in the soils (<1%). This meant that drier soils required higher volumes

of water to readjust to 50% WHC when rewetted. The weight of soil put into the chambers

also changed over the experiment, due to changing water content. The water content present

in the soils was determined by oven heating at 105 ºC (Berney et al., 2011). The moisture

content which was already present in the soils was quantified to determine how much soil

was required to maintain 3.5 g of equivalent dry soil weight, as well as how much water was

needed to re-wet the soils to 50% WHC.

2.4. Measurements

A multi-channel infra-red gas analyser (ADC Bioscientific EGA60, Hoddesdon) was used to

measure the CO2 release from soils in the experiment. The chambers which contained the

rewetted soils had two spigots, which were connected to both the input and output pumps of

the gas analyser with tubing. This maintained an airtight circuit of air, preventing CO2 from

escaping, or contamination from outside sources of CO2. The total volume of the circuit,

containing the tubing, chamber and internal volume of the analyser was 105 mL. Prior to all

measurements, a chamber of soda lime was introduced into the circuit to purge all CO2 within

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the looped system, and removed after this was achieved. Soda lime reacts with CO2,

removing it from air (Andrews, 2005). This ensured the circuit was free of CO2 before all

measurements and ensured CO2 recorded in the experiment originated from the soil.

Cumulative CO2 releases from soils were measured immediately after rewetting, and 24 hours

later. The soils were left in the chambers between the initial and second reading the following

day. The chambers were left with the spigots exposed and unconnected to the circuit,

preventing excessive CO2 build up. The gas analyser sampled CO2 every 10 seconds, for 10

minutes, at a pump speed of 40 mL min-1

, providing high resolution and real time response

curves. Rates of release were determined from differences in cumulative levels over the

duration of sampling and adjusted per of gram of soil.

2.5 Statistical analyses

One way ANOVAs were used to investigate variables over drying, and factorial ANOVAs to

compare interaction effects between soil type and drying on CO2 release and hydrophobicity

(Minitab 17). Post hoc Tukey tests were used to determine which differences in treatments

and variables were significant. P = 0.05 was used as the threshold of significance.

3. Results

3.1. Hydrophobicity

The water droplet penetration time (WDPT) was significantly different between soil type (P

< 0.001), and across drying (P < 0.001); these effects also interacted significantly (P < 0.001)

(Figure 1). The grassland soil was observed to have the longest WDPT up until 15 days, after

which the woodland soil experiences the longest. The WDPT of the arable soil did not

significantly change over time, however it did increase in the grassland (P = 0.008) and

woodland soil (P = 0.001). The increase in WDPT in grassland soils was minimal after 10

days, but continued to increase in woodland soils.

The arable soil was the only soil which was not considered hydrophobic (Figure 1). The

average WDPT in arable soils were the shortest, while grassland soils were 16 times longer

(P < 0.001), and woodland soils 14 times longer (P < 0.001). There was no significant

difference found in the average WDPT between grassland and woodland soils.

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Figure 1. Water droplet penetration time of the arable, grassland and woodland soils after 0, 2, 10, 15 and

28 days. Dotted bars denote hydrophobic soils. Average values, with standard error mean error bars, are

shown (n = 3).

3.2. Immediate pulse

All rewetting yielded immediate pulses of CO2 which was characterised by a short term

increase in the rate of release. This pulse peaked at around 80 seconds after rewetting, and

ended after 150 seconds (Figure 2). The highest rates of release were found in woodland soils

and the lowest in arable soils. The onset of CO2 release after rewetting was around 30

seconds in all soils, and was not found to be significantly different between soil types. No

relationship between drying and the onset of release was found either, as no drying level was

significantly different to the control.

0

2

4

6

8

10

12

14

0 2 10 15 28

Wat

er

dro

ple

t p

en

etr

ati

on

tim

e (

s)

Days dried

Arable

Grassland

Woodland

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Figure 2. Rates of CO2 release immediately following rewetting to 50% WHC, in soils dried for different

durations in arable (A), grassland (B) and woodland soils (C). The data point means, with standard error

mean error bars, are presented (n = 3).

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The cumulative CO2 in the first 150 seconds represent the release from the initial pulse

observed after rewetting. This release was lowest in the control soils, and generally increased

with greater drying; peaking after 10 days (Figure 3). Cumulative CO2 release in the first 150

seconds was found to be significantly different between the three soils (P < 0.001), and over

time (P < 0.001); these effects also significantly interacted (P = 0.04). While the greatest

release were consistently from the woodland soils, grassland had higher releases than the

arable in the first 2 days, after which higher releases were observed from the arable soils.

Release from woodland soils was, on average, 48% higher than the arable (P < 0.001), and

42% higher than the grassland (P < 0.001). There was no significant difference in the average

CO2 efflux between arable and grassland soils.

Cumulative CO2 release significantly changed with greater drying in the arable (P < 0.001)

and grassland soil (P = 0.05), but no significant change was found in the woodland soil. A

post hoc Tukey test found two significantly different groups in the arable soil, consisting of

the control and 2 day soil, and the 10, 15 and 28 day soil (P < 0.05). Soils within the same

group were not significantly different to each other.

Figure 3. The average cumulative release of CO2 in the first 150 seconds after rewetting in arable,

grassland and woodland soils over drying. Hatched bars denote significant difference to 0 day control (P <

0.05). Error bars are standard error means (n = 3).

3.3. Release 24 hours post rewetting

The release of CO2 24 hours after the initial rewetting event was distinctly different in nature

to the release observed immediately following wetting. There was no pulse or short term

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increase and peak in CO2 efflux, as seen right after wetting. The rate of CO2 release 24 hours

after was constant and no changes in release rates were observed during the 10 minute

sampling period.

Release rates increased with drying, compared to the control, but were not noticeably

different after 2 days (Figure 4). There was a significant difference in the rates observed

across the different soils (P < 0.001) and drying treatments (P < 0.001); these effects also

significantly interacted (P < 0.001).

The average rate of CO2 release in arable soils was 90% higher than in grassland soils (P <

0.001) and 118% higher than woodland soils (P < 0.001). There was no significant difference

found between the average rate of release in grassland and woodland soils. A post hoc Tukey

test found all drying levels in the arable soils to be significantly different to the control, but

none were significantly different to each other (P < 0.05).

Figure 4. Average rates of CO2 release across all soils 24 hours after initial wetting. Hatched bars denote

significant difference to the 0 day control (P < 0.05). Error bars are standard error means (n = 3).

4. Discussion

4.1. Hydrophobicity and soil properties

The three different soils all had different OM contents, pH and water droplet penetration

times. While OM content and pH were fixed properties of the soil that did not change, water

repellency was seen to increase in the grassland and woodland soils over time.

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4.1.1. OM content and hydrophobicity and pH

The grassland and woodland soils were found to be hydrophobic unlike the arable soil, and

this could be attributed to their higher organic matter content. Soils with higher organic

matter contents tend to be more water repellent (Vogelmann et al., 2013). Filamentous fungi

living in the soil matrix exude hydrophobic organic proteins, known as hydrophobins, which

can increase soil aggregation, but also water repellency (Chenu and Cosentino, 2011).

Arbuscular mycorrhizal fungi affects water repellency in soils through this release and their

abundance have been found to be lower in arable soils, compared to woodland soils

(Stromberger, 2005; Rillig et al., 2010). The grassland was found to be more hydrophobic

than the woodland soil, despite having a lower OM content (Table 1); however not all organic

matter within a soil is intrinsically hydrophobic (Ahmed et al., 2015). High organic matter

content does not only affect water repellency, but it also tends to decrease soil pH (Rukshana

et al., 2010). The arable soil had the highest pH and the lowest OM content (Table 1).

4.1.2. WDPT over drying

WDPT increased with drying in grassland and woodland soils, while this trend was reversed

in the arable soils. Hydrophobic and hydrophilic soils react differently to wetting as soil

moisture content changes. Wetting of dry hydrophilic soils is rapid due to high attraction and

water potential difference between the soil and water (Vogelmann et al., 2013). The opposite

occur in intrinsically hydrophobic soils, which becomes increasingly water repellent as a soil

dries (Quyum, 2000).

4.1.3. Lower water saturation in hydrophobic soils

The hydrophobicity of the grassland and woodland soils likely meant that these soils were not

rewetted to 50% WHC in the experiment. There was visible pooling of water droplets on the

grassland and woodland samples after rewetting, with sections of the soil staying dry;

characteristic of hydrophobic soils. This creates preferential flow, reducing infiltration and

water availability (Goebel et al., 2011). This was more prominent in the drier, more

hydrophobic, soils.

4.2. Immediate CO2 release

Highest cumulative and rates of CO2 release in the first 150 seconds were found in woodland

soils, and lowest from arable soils. The higher rates of release observed in woodland and

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grassland soils were also accompanied by much larger ranges in rates measured across

repeats; represented by the large error bars in Figure 2. This is likely due to these soils being

hydrophobic in nature, resulting in uneven wetting across repeats.

4.2.1. Rapid and short pulse

Highest rates of release from rewetted soils are found immediately following rewetting

(Borken et al., 2003; Meisner et al., 2015). Mineralisation of organic substrates in microbial

respiration is the source of CO2 release from rewetted soil (Rastogi et al., 2002; Fierer and

Schimel, 2002; Guntiñas et al., 2013). The pattern of the CO2 pulse observed immediately

after rewetting in the experiment does not suggest this to be the origin of the efflux. The time

frame of the CO2 pulse observed from the experiment is rapid and this flux is seen from 30

seconds (Figure 2).

The initial CO2 pulse ceased 150 seconds after rewetting, which is not expected if it was due

to elevated respiration. The Birch flush is directly related to organic substrate availability,

which increases microbial growth (Rousk and Bååth, 2007; Herron et al., 2009). Increased

CO2 release from the Birch effect tends to last several days after rewetting (Fierer and

Schimel, 2002). Elevated levels of release finally return to baseline rates after the increased

population of soil microbes can no longer be supported due to substrate depletion (Reischke

et al., 2014). The end in the CO2 pulse seen in the first 150 seconds does not point towards

the Birch effect, as substrates would unlikely be completely mineralised in such a short time.

The rate of release captured is also much higher than what is expected from soil respiration.

4.2.2. Degassing

The timeframe and rapidity of the initial CO2 release after rewetting could suggest it

originated from degassing. Degassing is the fastest response to wetting in dry soils, and

usually occurs minutes after rewetting (Yiqi and Zhou, 2010). This time frame fits with the

pulse observed, as opposed to the Birch effect which is a release over a much longer period.

Degassing describes the release of air which is displaced as water infiltrates through soil

pores (Yiqi and Zhou, 2010). The air displaced from the water is concentrated in CO2, built

up from microbial respiration. The CO2 pulse observed could therefore represent the

displacement of air trapped within soil pores as water infiltrates through the samples, with

most air degassed after 150 seconds, represented by the end in the pulse. Meisner et al.,

(2015) also found the greatest release of CO2 immediately after rewetting; however

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cumulative release over the first 4 hours was recorded, instead of every 10 seconds in this

study, so it was not able to able to identify the short 150 second pulse seen in this experiment.

This experiment could suggest that the highest release observed in the first 4 hours in the

study by Meisner et al., (2015) was due to the accumulation of degassed CO2 after rewetting.

Meisner et al., (2015) stated a decoupling in microbial growth and respiration in the first 4

hours, which saw low microbial growth but high soil respiration with high releases of CO2.

This decoupling between microbial growth and respiration has also been observed in other

investigations (Lovieno and Bååth, 2008; Blazewicz et al., 2014). This study suggests this to

be a result of degassing, explaining the high releases of CO2, yet low growth in soil microbes,

as degassing is a physical phenomenon not mediated by biological activity.

4.2.3. Degassing between soil types

The higher releases seen in the woodland and grassland soil can be assumed to be a result of

greater degassing. The woodland soil had the highest CO2 release, which is likely due to it

having the highest WHC, so was rewetted with the greatest volume of water. The grassland

soil was also rewetted more than the arable due to a higher WHC, and also saw higher

releases. The grassland and woodland soil had higher organic matter contents, which

contributes to greater soil aggregation; increasing porosity and water infiltration

(Franzluebbers, 2002). These soils would be expected to have a greater number of pores

capable of trapping gas, and the increased infiltration through this system could have led to

greater degassing (Rastogi et al., 2002). Woodland and grassland soils have higher respiration

rates than arable soils (Ritz et al., 2006). Higher respiration rates along with greater porosity

could have meant these soils trapped more CO2 within pores, which would have yielded a

greater pulse upon rewetting through degassing.

4.2.4. Degassing over time

As the soils were dried, the volume of water added to the soils to rewet them to 50% WHC

increased, and this correlates to the increasing cumulative CO2 captured over time. After 10

days, the water content in the soil remained negligible. The volume of water added to rewet

soils was the same after this point. The grassland and woodland soil were influenced by the

uncontrolled variable of hydrophobicity which changed over time. The arable soils were the

only samples which were consistently rewetted to 50% WHC. There were no significant

differences found in the release of CO2 in arable soil after the first 150 seconds of rewetting in

soils after 10, 15 and 28 days. The Birch flush is known to increase in soils dried more

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extensively (Jarvis et al., 2007; Schimel et al., 2010). The same release found in the arable

soils after 10 days therefore further suggests the source of the initial flush is degassing. It is

suggested that the initial release of CO2 is not affected by drying, but the volume of water

added and the subsequent degassing. If the efflux was from the Birch flush, we would expect

to see an increase in CO2 release over time. Meisner et al., (2015) found drying to not affect

release immediately after rewetting. The same release observed is likely down to the fact

these soils were rewetted with the same volume of water, which resulted in equal degassing.

4.3. Secondary reading after 24 hours

No spike or fluctuation in the rate of release was seen in the secondary measurements of CO2

release, unlike immediately after rewetting.

4.3.1. Constant respiration

CO2 release from the Birch effect is known to last for days and peaks 24 hours after rewetting

(Meisner et al., 2015). The constant release and the fact respiration is known to be elevated

from the Birch effect a day after rewetting suggests that the secondary effluxes recorded from

the samples originated from soil respiration. Sampling was taken over 10 minutes both

immediately after rewetting and a day after. Changes in respiration rate are gradual and

would not be not drastically fluctuate in such a short time span. This reinforces the suggestion

that the initial release was not from respiration, but from degassing, and that the secondary

constant release identified was from the Birch effect.

4.3.2. Hydrophobicity and respiration

Deciduous woodland soils are known to have higher respiration rates than grassland soils,

while arable soils have the lowest (Ritz et al., 2006). On average, the arable soil yielded the

highest respiration rates, while the woodland was generally the lowest (Figure 4). This could

be down to the fact that the arable soils were the only soils completely rewetted, as they were

not hydrophobic. Greatest respiration would be expected to be seen in samples which had

been evenly re-saturated to 50%WHC. Reduced water availability in parts of the grassland

and woodland samples could have led to these dry sections exhibiting very low CO2 releases.

Low water availability substantially reduces respiration (Yuste et al., 2007). Pathways

between microorganisms and substrates can be limited in dry soils and mineralisation can

only occur when microbes come into physical contact with organic substrates (Chenu and

Stotzky, 2002). Rewetting soils break up soil structure and expose organic matter, as well as

15 S.Gomes (2016)

redistributing and increasing mobility of microbes to substrates (Ouyang and Li, 2013).

Water repellency is therefore seen as a factor which reduces soil respiration and CO2 release.

4.3.3. pH and respiration

The arable soil also had a relatively neutral pH, while the woodland soil was the most acidic

(Table 1). Lower rates of respiration are generally found in soils with a lower pH (Angert et

al., 2015). Low pH can decrease growth rates of microbes and the availability of organic

substrates, reducing overall respiration (Rousk et al., 2009). The effect of pH, along with

water repellency is likely to explain for the differences in soil respiration between soil types.

4.3.4. Drying soils and respiration

All arable soils which were dried had significantly higher respiration rates than the control

(Figure 4). Drying was seen to increase respiration but there was no significant difference

between the 2, 10, 15 and 28 day soils. This suggested that drying significantly increased

respiration in soils rewetted to 50% WHC, but the duration of drying itself did not affect

release rates. As a soil dries, the water potential decreases, and soil microbes must actively

accumulate solutes to reduce their internal water potential to prevent water loss and survive

(Schimel et al., 2010). Growth ceases in microbes over drying and some may die, but

extracellular enzymes continue to degrade organic matter (Manzoni et al., 2014). The action

of enzymes and the increasing number of dead microbes both contribute to an increasing

accumulation of degradable material in the soil matrix. Upon rewetting, surviving microbes

release the solutes accumulated during drying, to readjust their internal matric potentials

(Mikha et al., 2005). Decomposition of largely abundant biodegradable material from

respiration of surviving microbes results in rapid growth and the CO2 efflux responsible for

the Birch effect (Butterly, 2008; Sun et al., 2015). Increased drying is known to increase the

Birch response due to the increased accumulation of biodegradable material (Clifford et al.,

1998). Differences in respiration rates have been observed between soils dried over multiple

weeks and months, but not between soils dried for shorter durations (Jarvis et al., 2007;

Meisner et al., 2015). No significant differences found in this experiment could be because 28

days is not long enough for the build-up in degradable solutes to yield significantly different

flushes.

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4.4. Suggested further research

This study found evidence and possible suggestions to explain why high initial CO2 release

and the pulse observed in the first 150 seconds after rewetting dry soils originates from

degassing. Despite this, further research is suggested to investigate this. A rewetting

experiment could be conducted on two sets of soils, with one set placed in a vacuum prior to

rewetting, while the other is not. Soil type, quantity, and volume of water in rewetting would

be kept constant. If lower CO2 release is seen in soils with prior placement in a vacuum, it

would be attributed to less CO2 present in soil pores due to prior removal in the vacuum,

supporting the degassing theory.

5. Conclusions

The results from this experiment showed that CO2 release after wetting was rapid, occurring

around 30 seconds after irrigation. The source of efflux after rewetting also changed over

time. There was an immediate pulse of CO2 after rewetting, suggested to originate from

degassing, and a steady elevated rate of respiration 24 hours after. Drying was not shown to

have a significant effect on the CO2 release in this experiment which was not expected.

Differences in the immediate pulse of CO2 were more likely due to the volume of water

added upon rewetting, instead of the effect of drying itself on soils. Drying was seen to

increase respiration compared to soils which were not dried at all. Despite this, the length of

drying did not significantly increase rates when soils were dried over 28 days. Rates of CO2

release were much higher immediately following rewetting than after 24 hours. The soils with

higher organic matter contents were not shown to yield uniformly higher rates of release in

this experiment. The role of hydrophobicity, affected by organic matter content, influenced

CO2 release in soils. Hydrophobic soils experienced lower infiltration upon rewetting,

reducing water availability. This reduced decomposition of soil organic matter and

subsequent CO2 release.

Acknowledgements

I would like to thank Professor Karl Ritz for his support and guidance throughout this study,

as well as the staff in the department of Biosciences at the University of Nottingham, which I

have had the pleasure of working and learning from.

17 S.Gomes (2016)

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