Hourly and daily rainfall intensification causes opposing effects on C and N emissions, storage, and leaching in dry and wet grasslands

TL;DR: In this paper, the authors investigate the effects of expected twenty-first century changes in hourly and daily rainfall on soil carbon and nitrogen emissions, soil organic matter (SOM) stocks, and leaching using a coupled mechanistic soil biogeochemical model (BAMS2).

Abstract: Climate change is expected to alter hourly and daily rainfall regimes and, in turn, the dynamics of ecosystem processes controlling greenhouse gas emissions that affect climate. Here, we investigate the effects of expected twenty-first century changes in hourly and daily rainfall on soil carbon and nitrogen emissions, soil organic matter (SOM) stocks, and leaching using a coupled mechanistic carbon and nitrogen soil biogeochemical model (BAMS2). The model represents various abiotic and biotic processes involving 11 SOM pools. These processes include fungal depolymerization, heterotrophic bacterial mineralization, nitrification, denitrification, microbial mortality, necromass decomposition, microbial response to water stress, protection, aqueous advection and diffusion, aqueous complexation, and gaseous dissolution. Multi-decadal modeling with varying rainfall patterns was conducted on nine Australian grasslands in tropical, temperate, and semi-arid regions. Our results show that annual $${\text{CO}}_2$$ emissions in the semi-arid grasslands increase by more than 20% with a 20% increase in annual rainfall (with no changes in the rainfall timing), but the tropical grasslands have opposite trends. A 20% increase in annual rainfall also increases annual $${\text{N}}_2{\text{O}}$$ and NO emissions in the semi-arid grasslands by more than 10% but decreases emissions by at least 25% in the temperate grasslands. When subjected to low frequency and high magnitude daily rainfall events with unchanged annual totals, the semi-arid grasslands are the most sensitive, but changes in annual $${\text{CO}}_2$$ emissions and SOM stocks are less than $$5\%$$ . Intensification of hourly rainfall did not significantly alter $${\text{CO}}_2$$ emissions and SOM stocks but changed annual $${\text{NH}}_3$$ emissions in the tropical grasslands by more than 300%.

Summary

Introduction

• Studies based on single and multiple cycles of drying-rewetting experiments have arrived at very different conclusions regarding the carbon sources and mechanisms contributing to the observed CO 2 pulses (Schimel 2018) .
• To this end, the authors aim to quantify the long-term impacts of hourly and daily rainfall variations on carbon and nitrogen emissions, leaching, and storage in grasslands with different seasonal rainfall regimes using a mechanistic model.

BAMS2 reaction network

• To account for the control of nitrogen availability on SOM dynamics, the BAMS1 carbon model described in Riley et al. (2014) was coupled to the nitrogen cycle model developed in Maggi et al. (2008) .
• All microbial functional groups assimilate both carbon and nitrogen for growth, with fungi and bacteria having a C:N ratio of 8 and 5, respectively (Mouginot et al. 2014 ).
• The original stoichiometric parameters of SOM decomposition reactions in BAMS1 (Riley et al. 2014) were recalculated to account for the nitrogen immobilization into microbial biomass (Supplementary Information Table S .1).
• SOM polymers are considered to be non-soluble (in solid phases) organic carbon and do not undergo protection processes.

Biogeochemical and transport solver

• The BAMS2 reaction network (Fig. 1 ) was solved in the general-purpose multi-phase and multi-component bioreactive transport simulator BRTSim-v3.1a (Maggi 2019) .
• Equations used to model the transport of fluids and compounds in aqueous, gaseous, and biological phases are described in detail in Maggi (2019) .
• The function f(S L ) in Eq. 5 describes the reduction of microbial activity as a result of changes in water saturation to account for processes not explicitly modeled, such as physiological stress and substrate diffusion within a soil layer; note that chemical transport across soil layers is explicitly modeled as described above.
• Descriptions of mathematical equations, numerical methods, and solution convergence criteria used in BRTSim-v3.1a are detailed in Maggi (2019) .

Site descriptions

• The BAMS2 reaction network was applied in nine Australian grasslands in tropical, temperate, and semi-arid regions that have distinct seasonal rainfall regimes.
• Site locations were determined based on the Dynamic Land Cover Dataset (Lymburner et al. 2011 ) and the modified KOppen climate classification of the Bureau of Meteorology, Australia (Stern and Dahni 2013) (Table 1 ).
• In contrast, the wet season in the temperate region starts from May to September with lower annual rainfall but a higher number of wet days than the tropical region.
• Historical daily rainfall and temperature data (from 1979 to 2017) at each site were obtained from the CPC US Unified Precipitation data provided by the NOAA/OAR/ESRL PSD, Boulder, Colorado, USA (Xie et al. 2010) , and the Global Historical Climatology Network-Daily dataset (Menne et al. 2012) , respectively.
• Plant water uptake , plant nitrogen uptake (R20-R21), and root exudation (R27) were allocated over the soil depth according to the root distribution.

Rainfall scenarios

• Numerical experiments were conducted with three rainfall scenarios.
• The weather generator in Chen et al. ( 2010) was modified to generate rainfall time series with varying statistical properties specific for each scenario, whereas no modification was applied to the evapotranspiration time series.
• The authors discuss the possible implication of this simplification below.
• Change in annual cumulative rainfall amount, also known as Scenario 1.
• Rainfall time series were modified so that the annual cumulative rainfall amount (P cum ) ranged within +/-20% of the historical value, while the annual number of wet days (D wet ) remained constant.

Analyses and benchmarking

• Prior to the numerical experiments, baseline simulations (using historical rainfalls) were initialized with SOM concentrations close to the organic carbon content reported in the SoilGrids database (Hengl T et al. 2017 ) and the microbial biomass close to zero.
• The simulations were run for 2000 years for biochemical reactions in the root zone to reach a steady state and to develop a steady microbial biomass profile.
• The outputs of the 2000-year simulations were then used as initial conditions in the numerical experiments.
• Because BAMS2 includes only microbial heterotrophic respiration, CO 2 emissions in the baseline simulations were compared against heterotrophic soil respiration flux (R H ) of 353 natural and unmanaged grasslands reported in the Soil Respiration Database Version 4.0 (SRDB-V4 Bond-Lamberty and Thomson 2018).
• The lag time between two time series was quantified using cross-correlation analysis (function xcorr in Matlab2017a).

Benchmarking of baseline simulations

• In baseline simulations, the semi-arid grasslands, which received the lowest amount and least frequent rainfall, had the lowest CO 2 emissions and SOM inputs (Fig. 2 ).
• CO 2 emissions in these sites were slightly lower than those in the temperate grasslands.
• Other studies argued that a wetter soil would have higher anaerobicity, and therefore should have higher N 2 O emissions (Skiba and Smith 2000) .
• In BAMS2, is the only source of inorganic nitrogen to the soil, mainly coming from N 2 fixation (R19) and mineralization of N-containing monomers (R9-R11).
• The authors note however that, in wet soils that have low concentrations, the nitrifiers may have adapted to a K M value lower than that applied in BAMS2, which was calibrated against temperate soils (Maggi et al. 2008) .

Controls of soil moisture dynamics on C and N emissions

• In all grasslands, the correlation R(S, P) was relatively weak with slightly higher values observed in the tropical grasslands in the wet season.
• Impacts of annual rainfall amount Contrary to the general expectation that increasing annual rainfall (P cum ) would have a larger impact on drier lands, their simulations suggested that both dry and wet grasslands are very sensitive to changes in P cum , and they have distinctive responses (Fig. 4 ), also known as Scenario 1.
• Together with increased water advection at high P cum , the increased biological activity also led to a substantial increase in DOC and DIC leaching to soils below the root zone (Supplementary Fig. S.7a, b) .
• Nitrification and denitrification rates in the temperate grasslands decreased substantially with increasing P cum , leading to the reduction in N 2 O and NO emissions (Fig. 4b, c ).
• Increased water content also decreased the volatilization of ammonia (Fig. 4d ).

Scenario 2: impacts of daily rainfall amount and frequency

• The authors investigated the response of C and N dynamics to variations in daily rainfall amount and frequency by changing the number of wet days D wet in a year while keeping the total annual rainfall constant; that is, a time-series with a smaller D wet value has fewer but larger rainfall events.
• The balance between increased SOM inputs and decomposition caused a slight increase in SOM stocks (<2%, Fig. 5e ) and a substantial increase in DOC and DIC leaching to below the root zone (Supplementary Fig. S.8a, b) .
• The effects of increased rainfall intensity and reduced frequency on nitrogen emissions in the semi-arid grasslands matched relatively well with the numerical-experiments tested in Gu and Riley (2010) .
• Gu and Riley (2010) also found that, when applied with a low total rainfall amount, high intensity and low frequency rainfall events reduced N 2 O emissions in sandy loams soils, but increased NO emissions.
• Big pulses of water diluted and transported inorganic nitrogen out of the root zones, and hence decreased the nitrification and denitrification rates.

Discussion

• The BAMS2 model represents the highly complex interplay between many biotic and abiotic mechanisms hypothesized to be important for carbon and nitrogen cycles, including depolymerization, SOM mineralization, microbial mortality, necromass decomposition, N 2 fixation, nitrification, denitrification, protection, advection, and diffusion.
• These mechanisms have different responses to soil water content, and therefore a detailed description of their interactions is pivotal to this study that explicitly aims at assessing the impact of rainfall variability on soil carbon and nitrogen dynamics.
• The authors note however that the determination of model parameter values can be difficult for a model with high complexity, and this can introduce additional uncertainties.
• The authors note that the parameter sensitivity may change after coupling the two models, and therefore a global sensitivity analysis of BAMS2 is needed, and it is the target of their next work.
• Hence, field studies that spanned across time-scales of months may capture only the transient effects.

Conclusions

• The authors present a C-N coupled mechanistic SOM model (BAMS2) to investigate the effects of hourly and daily rainfall variations on soil carbon and nitrogen emissions, stocks, and leaching in grasslands with different seasonal rainfall regimes.
• BAMS2 captured relatively well the Birch effect and the carbon and nitrogen dynamics observed in grasslands, with model outputs falling within the range of field observations compiled in various published databases.
• Dry and wet grasslands responded differently to variations in rainfall patterns and rainfall variability had a different impact on carbon and nitrogen emissions.
• The balance between SOM inputs and decomposition, however, always resulted in increasing SOM stocks with increasing annual rainfall in all grasslands.
• High rainfall amounts can dilute concentrations to below optimal values for nitrification, thus reducing N 2 O and NO emissions in the temperate grasslands.

Full text

Lawrence Berkeley National Laboratory
Recent Work
Title
Hourly and daily rainfall intensification causes opposing effects on C and N emissions,
storage, and leaching in dry and wet grasslands
Permalink
https://escholarship.org/uc/item/0zs3r5jb
Journal
Biogeochemistry, 144(2)
ISSN
0168-2563
Authors
Tang, FHM
Riley, WJ
Maggi, F
Publication Date
2019-07-30
DOI
10.1007/s10533-019-00580-7
Peer reviewed
eScholarship.org Powered by the California Digital Library
University of California

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$-,>
$?=&
/&
$9
-
(93,>
@9
-

(93$
A
$
A++>
B"($$(9
C
@

&$
=&
&8'-+++1D-+;A:%

8%-+;-:/
$$&
82-+;A1B-+;5:
8E-+;F:7
&3&
&8C-++A1G
-+;61?-+;6:8G-++61(
-+;F:$82/*=<-+;A1
*-+;,:89
-

5
9
x
1
-++,1B-+;-:<7
&$$
C8(9:
7$
9(9
<<$
8(-++6:$8<-+;-:
9
-
97
=/9
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
$383
HBirch eectI*;4,F1#-+;+1J-+;+:(
$/7&
&&$
$$&9
-
8(-+;F:
$$$
8B;4F6:$<$$8&/EK-+;-:
$8-+;5:$
8C-+;-:7&
/L7
$&//$
@$&
(9&
829:$82@:8#-+;F:9
-
M7$
$$8**-+++:1
=2@
<$7
3&$
8*-+;-13*;4441

<C-++4:J
$&&
$$8E!-+;+1(
-+;F:&$$(9
<$$<$
$82?3-+;-:3$8##N-+;5:
8-+;6:8**-++,1(3
;44,:8#;44-1
(7;4F,1!-+++:$&
3&/
8-++F1((-++5:<
82?3-+;-:@L
$
$3$$&
97$
8#=;4441#-++5:$
$&&8C
-+;-1*-+;,:=
7&(9
&=/
&$

*%(;&!
8-+;5:&8-++F:$
&(9
O8*%(-1*%$(9
&-:;;(98&
:&$8$
77:3
$=
&=7
*%(-$3/$&
1
-
991$1
3
7
&%
/

*%(-3

&$(9
*%(;$$!8-+;5:
&8-++F:O
38*%(-;:(98
:1&(9
8
:1&8
A
  
9
-
9
-
:1&$
8F
DEP
:$8B
HET
:/7<
$8B
AOB
:/7<$8B
NOB
:$
8B
DEN
:
@*%(- $(98!;O!F
;:%$$$
$&"F,&
8-+;5:@</8!4O
!;;:<$$
& 
$$F
DEP
B
HET
(97<$B
AOB
 
3$(9
*%(;8!-+;5:
$<$$
8(@$(;:
(8!;-O!;A:
8!;5O!;6:8-++F:
$$
$$8($(;:@
*%(-
-
7 8!;4:%!;4
$7
-
7$
7$
-
7$&
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Frequently asked questions

Q1. What are the contributions in "Hourly and daily rainfall intensification causes opposing effects on c and n emissions, storage, and leaching in dry and wet grasslands" ?

Here, the authors investigate the effects of expected twenty-first century changes in hourly and daily rainfall on soil carbon and nitrogen emissions, soil organic matter ( SOM ) stocks, and leaching using a coupled mechanistic carbon and nitrogen soil biogeochemical model ( BAMS2 ).