Limiting global mean temperature changes to well below
2
To limit temperature increase due to climate change to well below
2
BECCS combines energy production from biomass and carbon capture at the
power plant with subsequent storage. Sources for biomass-based energy
production are crop and forestry residues (Smith, 2012; Smith et al.,
2012; Tokimatsu et al., 2017), dedicated bioenergy grass (BG) plantations
(Smith, 2012; Smith et al., 2012), or short-rotation woody biomass from
forestry (Cornelissen et al., 2012; Smeets and Faaij, 2007). Large-scale
AR, as well as bioenergy plantations, requires extensive landscape
modifications for growing forests or natural regrowth of trees in deforested
areas to increase terrestrial CDR (Kracher, 2017; Boysen et al.,
2017a; Popp et al., 2017; Humpenöder et al., 2014) and huge quantities of
irrigation water (Boysen et al., 2017b; Bonsch et al., 2016). The biomass
yields of AR and agricultural bioenergy crops directly correlate with
fertilizer application, which in turn could reduce CDR efficiency due to
related emissions of
Under intensive growth scenarios, nutrient supply is a critical factor. According to Liebig's law of the minimum, supplying high amounts of nitrogen (N) might shift growth limitation to other nutrients (von Liebig and Playfair, 1843). Some US forests are already showing changes in line with moving from an N-limited to a phosphorus-limited (P-limited) system caused by increases in N atmospheric deposition (Crowley et al., 2012) along with magnesium (Mg), potassium (K), and calcium (Ca) deficiencies (Garcia et al., 2018; Jonard et al., 2012). Poor nutrient supply, related to deficient mineral nutrition, may reduce tree growth (Augusto et al., 2017). Impacts on biomass production due to poor tree nutrition has been observed in European forests (Knust et al., 2016; Jonard et al., 2015), decreasing the carbon sequestration of forest ecosystems (Oren et al., 2001) – a factor rarely included in climate models, leading to overestimated CDR potential.
Specifically, global simulations with an N-enabled land surface model (Kracher, 2017) suggest that insufficient soil nitrogen availability for a representative concentration pathway 4.5 (RCP4.5) AR scenario (Thomson et al., 2011) could lead to a reduction in the cumulative forest carbon sequestration between the years 2006 and 2099 by 15 %. Goll et al. (2012) showed that carbon sequestration during the 21st century in the JSBACH land surface model was 25 % lower when N and P effects were considered.
Mineral weathering is a natural and primary source of geogenic nutrients
(e.g., Mg, Ca, K, and P; Hopkins and Hüner, 2008; Landeweert et al.,
2001; Waldbauer and Chamberlain, 2005; Singh and Schulze, 2015) and controls
atmospheric
P is a rather immobile soil nutrient, and only a small fraction of soil P is
readily available for plant uptake, limiting plant growth in a wide range of
ecosystems (Shen et al., 2011; Elser et al., 2007). P content in soils is a
result of a process controlled by the interactions of parent material
(primary rocks) with climate, tectonic uplift, and erosion history through
geological time (Porder and Hilley, 2011). The processes of P
transfer between biologically available and recalcitrant P pools influence
at most P availability (Porder and Hilley, 2011). Orthophosphate
(
The inclusion of soil hydraulic properties in the evaluation of EW effects
is important as the soil water content has a strong influence on average
crop yield. Practices that increase the plant-available water (PAW) are
thought to mitigate drought effects on crops (Rossato
et al., 2017). The water content of soils also seems to influence soil
erosion rates and surface runoff (Bissonnais and Singer, 1992). In
addition, soil water content influences soil
Deploying land-based NETs would imply large changes in a local landscape
nutrient and water cycle. At least 65 % of worldwide soils (
Since phosphorus (P) is a limiting nutrient in a wide range of ecosystems (Elser et al., 2007), we performed a P budget for an N-stock-based P demand from an AR scenario considering natural N supply (hereafter N-limited) and N fertilization (hereafter N-unlimited). We selected two N supply scenarios since the related P demand is proportional to biomass N stock, but in the main text we discuss only the N-limited AR scenario. We estimated the balanced supply of Mg, Ca, and K for each supplied P based on ideal Mg, Ca, and K demand of AR derived from databases of biomass nutrient content. Balanced nutrient supply is necessary to avoid shift of growth limitation to other nutrients, which can occur according to Liebig's law (von Liebig and Playfair, 1843). Shift of growth limitation to other nutrients is observed for some US forests that changed from an N-limited to a P-limited system after an increase in atmospheric N deposition (Crowley et al., 2012). Based on minimum and maximum harvest rates of bioenergy grass (BG), we estimated the related P and K export by harvest from the fields. We decided on these nutrients for BG since crops require large amounts of K and P, once N demand is covered. The amount of rock powder required for enhanced weathering (EW) to cover projected P gaps and to replenish exported nutrients was estimated. The projected impacts on soil hydrology due to EW deployment were carried out by pedotransfer functions since they are used to estimate soil hydraulic properties (Schaap et al., 2001; Whitfield and Reid, 2013; Wösten et al., 2001) and such approximations have proven to be a suitable approach (Vienken and Dietrich, 2011).
The additional AR P demand, obtained for the 21st century for an
N-unlimited and N-limited AR scenario (Kracher, 2017), was
approximated by stoichiometric
Schematic steps and datasets used to derive geogenic nutrient
demand from simulated biomass changes; P gaps; reduced C sequestration; and
Ca, K, and Mg supply for balanced tree nutrition. Black colors: outputs from
land surface model JSBACH and agricultural production model MAgPIE. Yellow
colors: stoichiometric
Necessary Mg, Ca, and K supply for balanced tree nutrition based on P supply
was derived from N-stock-based Mg, Ca, and K additional demand normalized
to the N-stock-based additional P demand (Fig. 1).
The nutrient demand of bioenergy grass was estimated based on
stoichiometric
The idealized simulations for the AR system from Kracher (2017)
performed by the land surface model JSBACH (Reick et al.,
2013) for RCP4.5
were used (Thomson et al., 2011). The RCP4.5 scenario assumes
that the emissions peak is around 2040 and considers that forest lands
expand from their present-day extent (Thomson et al., 2011).
The coupled terrestrial nitrogen–carbon cycle model assumes N-unlimited and
N-limited conditions and considers harvest rates and transitions between
different anthropogenic and natural land cover types
(Hurtt et al., 2011) for a Gaussian grid
of approximately
The net primary productivity (NPP) calculation was based on atmospheric
We retrieved the annual changes in N and C content of different pools, i.e., wood (above and below ground, also including litter) and foliar (above and below ground, also including litter) for temperate, cold, tropical, and subtropical plant functional types climate growing forests and shrubs for the years 2006–2099 and annual model output.
Simulations of BG nutritional needs from the agricultural production model MAgPIE, a framework for modeling global land systems (Dietrich et al., 2018; Lotze-Campen et al., 2008; Popp et al., 2010), were used. The objective of MAgPIE is to minimize total costs of production for a given amount of regional food and bioenergy demand and a given climate target (here RCP4.5 to correspond to the AR simulations). In its biophysical core, the yields in the model are based on LPJmL (Bondeau et al., 2007; Beringer et al., 2011; Müller and Robertson, 2013), a dynamic global vegetation model, which is designed to simulate vegetation composition and distribution for both natural and agricultural ecosystems.
At the starting point of the simulation, the LPJmL bioenergy grass yields have been scaled using agricultural land use intensity levels (Dietrich et al., 2012) for different world regions accounting for the yield gap between potential and observed yields for the period 1995–2005. For the future yields (2005–2090), the development is then driven by investments into yield-increasing technologies (Dietrich et al., 2014) based on the socioeconomic boundary conditions of the system.
The MAgPIE output had a frequency of 10 a, and the global minimum and
maximum of each output year were taken to obtain the potential bioenergy
grass minimum (0.7 kg m
The P, Mg, Ca, and K additional demand is defined as the amount of P, Mg,
Ca, and K needed to realize the state of ecosystem N variables in each grid
cell and year according to JSBACH output (Fig. 1). It
was estimated from the spatially explicit information on average forest N
content of each stock and plant functional type for an N-unlimited and an
N-limited AR scenario from Kracher (2017). Since P limits forest
growth in a wide range of ecosystems
(Elser et al., 2007), we performed a P
budget for each AR scenario. The ideal P, Mg, Ca, and K biomass additional
demands were based on the difference in the simulated change in N pools at
that time with respect to the simulation year of 2006 multiplied by their
corresponding
Stoichiometric parameters for different pools and biomes used in this study.
The P, Mg, Ca, K, and N content of leaves obtained from a global leaf
chemistry database (Vergutz et al., 2012) was used to derive
the
Carbon sequestered in different afforestation–reforestation
scenarios for the 21st century period (2006–2099) for an RCP4.5
scenario, according to Kracher (2017).
The AR C content (Fig. 2) from Kracher (2017) and the resulting N-stock-based Mg, Ca, and K demand were normalized
by the N-stock-based P demand to estimate the mean and range of the
The BG yield was obtained by the spatially explicit harvest rates within a
grid cell for an output frequency of 10 a and a period of 95 a
(1995–2090). The minimum 0.7 and maximum 3.6 kg m
The simulated forests from the AR scenario are perennial, unlike
bioenergy grasses which are completely harvested regularly due to their use
as biomass feedstock for BECCS. Thus, the natural system's nutrient supply
is insufficient for maintaining successive and constant yields, and the
nutrients exported by harvest need to be replenished (Cadoux et al.,
2012) to maintain high yields. The exported nutrients were calculated
following Eq. (2):
Geogenic P sources used for each geogenic P supply scenario.
The geogenic P source databases have different spatial resolutions
(Table 2); we resampled each of them to a coarser
The atmospheric dry and wet P deposition rates were taken from simulation
outputs for the 2006–2013 period and for the years 2030, 2050, and 2099
for an RCP4.5 scenario for a grid cell size of 1
Different sources of geogenic P.
The total soil P map from Yang et al. (2014a) was used to estimate the projected long-term available P in the soil system
(Fig. 3b). The total P supply by weathering for the
21st century (2006–2099) was based on Hartmann et al. (2014) maps
(Fig. 3c) that depict the chemical weathering as
a function of runoff and lithology, corrected for temperature and soil
thickness (Hartmann et al., 2014) and calibrated on 381 catchments in
Japan (Hartmann et al., 2009). A relationship between air temperature
and weathering rate was used, which was derived from reconstructed
weathering rates and different climate change scenarios for the recent past
(1860–2005) using the weathering model applied here. The relationship in
which P weathering increases by 9 % per 1
The potential P gap (
The Mg, Ca, and K necessary supply for balanced biomass nutrition
(
To cover the potential of different igneous rocks for EW strategies,
rhyolite and dacite (acidic rocks), andesite (intermediate rock), and basalt
(basic rock) were selected to project necessary amounts to cover P gaps from
the AR scenarios. Data on macronutrient concentrations (Mg, Ca, K, P) in
weight percent within these rocks were downloaded from the EarthChem web
portal (Fig. 4;
Statistical data of major element concentration in rocks with median
values (filled circles) and range (5th and 95th percentiles;
whiskers). Values from EarthChem web portal (
The nutrient supply was estimated assuming complete rock powder dissolution
in the system considering the median and range (5th or 95th
percentile) of chemical composition. The duration of complete rock powder
dissolution varies depending on the grain size (i.e., 1 a for grain
sizes between 0.6 and 90
However, the potential nutrient supply by EW for different amounts of rock
powder being deployed was also estimated following Eq. (9):
Large-scale deployment of rock powder on soils is expected to influence its texture. The deployed amount and texture of rock powder will somehow affect hydraulic conductivity, water retention capacity, and specific soil surface area. Pedotransfer functions (PTFs) are used to estimate soil hydraulic properties (Schaap et al., 2001; Whitfield and Reid, 2013; Wösten et al., 2001), and such approximations have proven to be a suitable approach (Vienken and Dietrich, 2011). PTFs make use of statistical analysis (Saxton and Rawls, 2006; Wösten et al., 2001), artificial neural networks, and other methods applied to large soil databases of measured data (Wösten et al., 2001). The equations from Saxton et al. (1986) performed the best estimations of soil hydraulic properties (Gijsman et al., 2002). Later on, Saxton and Rawls (2006) improved Saxton et al. (1986) PTFs, and they are used to estimate the effects on soil hydraulic properties due to deployment of basalt powder (Eqs. 10–18).
The potential changes in soil hydraulic properties, due to the application
of a fine basalt texture (15.6 % clay, 83.8 % silt, and 0.6 % fine
sand) or a coarse basalt texture (15.6 % clay, 53.8 % silt, and 30.6 %
fine sand), were estimated as a function of rock powder deployment for soils
corresponding to P gap areas from the N-unlimited AR scenario. According to
the international organization for standardization, the synthetic materials
can be classified according to their grain sizes; therefore, here the clay
comprises grain diameters
The initial hydrologic properties of topsoil were estimated for a depth of
0.3 m, as it is the average depth at which usual machinery can homogeneously mix
topsoil (Fageria and Baligar, 2008). Greater depths can be
reached but under higher energy and labor costs (Fageria and
Baligar, 2008). The global dataset of derived soil properties
(Batjes, 2005), which had textural information (sand, silt, and
clay content) for shallow soil depths (0.3 m), was used. The raster had a
resolution of 0.5
The necessary rock powder mass was estimated by Eq. (8) to close the
The impacts on soil texture by rock powder application considered the textures of applied basalt mass added to the initial soil mass by Eq. (23). A content of 15.6 % clay, 83.8 % silt, and 0.6 % fine sand for fine basalt powder and 15.6 % clay, 53.8 % silt, and 30.6 % fine sand for a coarse basalt powder was assumed.
Besides texture and organic matter, intrinsic grain properties (e.g., the shape of grains and pores, tortuosity, specific surface area, and porosity) should be considered (Bear, 1972). The equations from Beyer (1964) are based on the nonuniformity of grain size distribution and density of the grain packing to estimate soil properties. Carrier (2003) uses information on the particle grain size distribution, the particle shape, and the void ratio in his equations to estimate soil properties. However, such detailed information on a global scale is missing, making Beyer (1964) and Carrier (2003) equations not applicable to our analysis.
The global C sequestration for the N-limited AR scenario is 190 Gt C, while for the N-unlimited AR scenario it is 34 Gt C higher. The AR model from Kracher (2017) shows an increase in biomass production in tropical and temperate zones (Fig. 2). The results only focus on the N-limited scenario since it considered natural N supply, but the results for the N-unlimited scenario are presented in the Supplement (Sect. Bii). The calculated P budgets according to Eq. (4) for the AR time of 2006–2099 (Fig. 5) considered different geogenic supply scenarios (scenario one – P from weathering and atmospheric P deposition; scenario two – the same as scenario one plus inorganic labile P and organic P) and the average and range of the N-stock-based P demand (calculated following Eq. 1) for the AR simulation from Kracher (2017).
Areas with potential P gap for the nutrient budget of the N-limited
AR scenario (after 94 a of simulation), assuming P concentrations within
foliar and wood material corresponding to mean values
(Table 1).
Global P gap, maximum estimated P gap, maximum C sequestration reduction, and global C reduction for the natural N supply (N-limited) AR scenario (projected C sequestration of 190 Gt C).
The ideal P biomass additional demand (calculated from Eq. 1) to
sequester 190 Gt C (N-limited AR scenario) amounts to 200 Mt P on a global
scale for a mean wood and leaves P content; for the 5th and 95th
percentile, the estimated P demand would be 71 and 345 Mt P, respectively.
The P budget (estimated from Eq. 4) for geogenic P supply scenario one
suggests that P deficiency areas are distributed around the world but with
more frequent occurrences in the Northern Hemisphere
(Fig. 5a) and the P gaps can potentially reach up to
Reduction of forest C sequestration due to geogenic P limitation.
C reduction estimated from stoichiometric
The P and N limitations cause an average C reduction of 47 % for the
geogenic P supply scenario one and 19 % for the geogenic P supply scenario
two (obtained by accounting for the C reduction from N limitation, which is
34 Gt C plus the C reduction from Table 3, and then
normalizing by the global sequestration for the N-unlimited scenario of
224 Gt C) or
Besides removing carbon from the atmosphere, EW can also amend soils by
supplying nutrients and increasing alkalinity fluxes (Leonardos et al.,
1987; Nkouathio et al., 2008; Beerling et al., 2018; Hartmann et al., 2013; Anda
et al., 2015). Since basalt has higher P content compared to acidic and
intermediate rocks (Porder and Ramachandran, 2013), it could be used as
raw material for EW to cover the estimated P gaps of
Fig. 5a and c. For a
median basalt P content of 500 ppm (cf., Sect. 2.5), it would be
necessary to apply
The total amount of basalt powder to close the estimated P gaps seen in
Fig. 5 would depend on the assumed geogenic P supply
scenario and chemical composition of wood and leaves, but for a mean P
chemical composition, at least
Mg, Ca, K, and P supply by basalt dissolution (logarithmic curve)
given as medians and ranges (5th and 95th percentiles; dark grey
areas). Horizontal filled boxes indicate the nutrient demand for the maximum
(17.1 g P m
Basalt deployment can also guarantee a balanced supply of Mg, Ca, and K for different deployment rates (Fig. 7), potentially preventing the shift of growth limitation to some of these nutrients within the P gapped areas (Fig. 5). Rhyolite, dacite, or andesite could be used as alternatives to basalt as a source of P, but these rocks generally have lower P content (Fig. 4). As a consequence, the necessary amount of rhyolite, dacite, or andesite would be higher than that of basalt. Even though, for a median rock nutrient content, if these rocks are used to close the projected P gaps, they can potentially supply the necessary amount of Ca, Mg, and K for balanced tree nutrition (Fig. 8).
Potential macronutrient (Mg, Ca, and K) supply of different rocks
for closing projected P gaps of
For the simulation time span of 1995–2090 the minimum and maximum biomass
growth yields amount to 0.7 and 3.6 kg m
Projected K and P supply (logarithmic curve) by basalt dissolution
given as median ranges (5th and 95th percentiles) for bioenergy
grasses K and P demand (horizontal filled boxes) based on global minimum
0.7 and maximum 3.6 kg m
The baseline hydraulic properties for soils within the P gap areas from the
N-unlimited AR scenario, since this scenario represents the maximum effect,
were estimated by Eq. (10), and they show high variability. The projected
hydraulic conductivity (
Minimum and maximum soil hydraulic conductivity for areas coincident with the P gap areas of each geogenic P supply scenario, for the N-unlimited AR scenario (Fig. S7a).
The effects of rock powder deployment could be neglected, on average, for
upper limits of 50 and 205 kg basalt m
Relative impacts on soil saturated hydraulic conductivity (
Relative impacts on soil saturated hydraulic conductivity (
Closing the observed P gap areas in the N-unlimited AR scenario would
require a maximum deployment of 34 kg basalt m
Phosphorus (P) is a limiting nutrient in a wide range of ecosystems (Elser et al., 2007) and in temperate and tropical climate zones (Du et al., 2020). P deficiency might affect biomass growth of tropical (Herbert and Fownes, 1995; Tanner et al., 1998; Wright et al., 2011) and northern forests (Menge et al., 2012; Goswami et al., 2018) with mineral P already limiting biomass production in European forests (Jonard et al., 2015) and in forests in the USA (Garcia et al., 2018), as well as in agricultural areas (Ringeval et al., 2019; Kvakić et al., 2018). The uncertainty in which the P pool is available for long-term plant nutrition is high (Johnson et al., 2003; Sun et al., 2017), and we tackled this uncertainty assuming two potential geogenic P supply scenarios. Geogenic supply scenario two, assuming P from weathering and atmospheric deposition plus inorganic labile P and organic P, is a very optimistic assumption that might not correspond to reality based on the already-observed P limitation on different ecosystems (Elser et al., 2007). However, we cannot rule out that gradual shifts in soil organic P fractions occur, which make comparable amounts of P to that available in scenario two over time.
Impacts on soil hydrology estimated according to the equations of Saxton and
Rawls (2006) for basalt deployment mass coincident with areas with
potential P gaps for the nutrient budget of the N-unlimited AR scenario
assuming P concentrations within foliar and wood material corresponding to
mean values (Fig. S7a).
The numerical simulations of Kracher (2017) predicted biomass
growth for the 21st century (Fig. 2)
considering natural water supply,
More than 60 000 tree species are recorded worldwide (Beech et al., 2017), and a precise estimation regarding tree chemistry, which we attempted to represent by the considered ranges of wood and leaves chemistry from the databases, represents a challenge. However, different pathways and mechanisms control soil P availability to the plant (Vitousek et al., 2010), and they are not considered in our estimations, leading to conservative predictions. Adding soil P dynamics to models would allow for the reliable quantification of the C sequestration potential of AR (e.g., using P-enabled land surface models; Sun et al., 2017; Wang et al., 2010, 2017; Goll et al., 2012, 2017; Yang et al., 2014b).
Kracher (2017) has shown that N can limit biomass production and consequently C sequestration. To achieve the projected C sequestration of 190 Gt C for the N-limited scenario, the estimated P gaps must be closed. Potential P sources are industrial fertilizers, like diammonium phosphate (DAP) or rock powder (e.g., basalt). However, DAP potentially represents an extra input of ammonium to the groundwater, and it is expected, in the long term, that DAP deployment will acidify the soil (Fertilizer Technology Research Centre, 2016).
Most of the world's soils are acidic, with some being strongly acidic (IGBP-DIS, 1998), which generally favors the sorption of orthophosphate onto Fe- and Al-(hydro)oxide surfaces and clay minerals, essentially demobilizing P (Shen et al., 2011). Besides that, the long AR time span can undermine the effectiveness of DAP to supply P to forests due to the high soil acidification potential of DAP. Therefore, rock powder application can be an alternative as nutrients are slowly released and an increase in alkalinity fluxes is expected (Dietzen et al., 2018), which can raise and stabilize the pH of soils.
Re-establishing soil pH to (near-)neutral conditions, generally between 6.6
and 7, will provide new nutrient-holding sites at Fe- and Al-(hydro)oxide
surfaces and in soil organic matter, which makes the sorbed orthophosphate
plant available. An application of 8 kg m
To avoid shifts of nutrient limitation, the supply of macronutrients like Mg, Ca, and K might be proportional to P supply since Mg is required as an essential element in chlorophyll, Ca has a structural role, and K is responsible for water and ionic balance (Hopkins and Hüner, 2008). Rock powder can be used as a source of these nutrients, as suggested by different authors (Beerling et al., 2018; Hartmann et al., 2013; Straaten, 2007) and according to our results seen in Figs. 7 and 8. However, the potential of basalt powder to supply K, based on chemical composition, is lower than for other analyzed rocks. For median values, rhyolite has the highest content of K; however, if occurring in K-feldspars it will not be plant available. Blending these rocks in different proportions could result in a more balanced macronutrient supply (Leonardos et al., 1987).
The RCP8.5 scenario predicts that global agricultural areas (crop land and
pastures) are going to increase in the course of the 21st century due to a
decrease in forested area (Sonntag et al., 2016). Assuming a
future scenario of high atmospheric
Generally, natural soil P content is inadequate for the long-term cultivation of
agricultural plants. To overcome this issue, P is supplied by fertilizers to
reach or maintain optimum levels of crop productivity (Sharpley, 2000)
after several harvest rotations. In order to keep a positive
Overall, rock application has the potential to resupply the harvest-exported nutrients and partially or totally close the short- and long-term nutrient gaps in soil. Individual rock types, from basic (Mg, Ca) to acidic (K, Na), contain varying amounts of target nutrients, and mixing them might increase the overall nutrient supply capacity (Leonardos et al., 1987). Intrinsic mineralogical and/or petrographic structures can influence the release of nutrients (Ciceri et al., 2017), which makes them plant unavailable in some cases. K can also limit plant growth; it occurs in K-feldspars as a plant-unavailable form, in the case of acidic rocks, but becomes accessible after hydrothermal treatment (Liu et al., 2015; Ma et al., 2016a, b). However, research on release processes of other macro- and micronutrients and on nutrient release optimization (e.g., by hydrothermal decomposition) is necessary to be able to parameterize this effect in the soil environment.
Harvest rates control the nutrient export from bioenergy grass fields.
Therefore, an increase in harvest rate represents an increase in nutrient
export and vice versa. Thus, to keep a sustainable nutritional balance
of soils, the exported nutrients must be replenished; otherwise maintaining
the high harvest rates becomes unsustainable. Accounting for
other simulation setups or a numerical model different from MAgPIE might
change the harvest rates of this study. If we assume that the maximum
harvest rate of 3.6 kg m
AR and BECCS demand huge quantities of irrigation water (Boysen et al., 2017b; Bonsch et al., 2016), and it is projected that climate change will affect the water balance and consequently influence crop yields (Kang et al., 2009). Soils with higher water-holding capacity will tolerate the impacts of drought better (Kang et al., 2009). Therefore, practices that improve water availability to plants at the root system are used as strategies to mitigate drought effects (Rossato et al., 2017). We investigated whether deployment of rock powder can change the topsoil hydraulic conductivity and plant-available water (PAW) for different application ranges.
Concrete effects of EW on biomass productivity would depend on whether the changes
in the initial PAW values in topsoils reached PAW threshold values to
trigger biomass productivity (Sadras and Milroy, 1996). In
general, the average changes in topsoil PAW related to basalt powder
application would not be enough to trigger biomass growth. Therefore, areas
showing PAW changes from 14 % to 21 % would not trigger leaf and stem
expansion of maize, wheat, or soybean (Sadras and Milroy, 1996)
but could increase leaf and stem expansion of pearl millet
(Sadras and Milroy, 1996) after deploying 50 kg basalt m
The finest grain size able to be considered in the equations of Saxton and Rawls (2006) is
the clay fraction (grain diameter > 1 and < 3.9
During the weathering of rock powder, clay mineral genesis can occur and potentially increase the water-holding capacity of soils (Gaiser et al., 2000), which can subsequently change the estimated PAW. The added fresh silicate minerals to the soil by EW will have high reactivity releasing a significant number of nutrients, which increases soil nutrient pools. The increased nutrient availability will increase the potential of soils to stabilize carbon (Doetterl et al., 2018), and a positive effect on PAW is expected to occur based on Eqs. (15–17) and according to Olness and Archer (2005). The suitable amounts of rock powder applied depend on the target changes in the chosen soil and on the soil's intrinsic grain size distribution and organic matter content. Intrinsic grain properties like the shape of grains and pores, tortuosity, specific surface area, and porosity should be considered (Bear, 1972) for the evaluation of changes in soil hydraulic properties by pedotransfer functions and their consequences for dissolution kinetics. A large set of data from field and laboratory experiments covering different soil types, climatic regions, and plant species would enable a qualitatively and quantitatively reliable assessment of not only soil hydrology impacts but also dissolution rates and changes in the soils' mineralogy. The effects on soil microorganisms should be taken into account in order to correct the limits of rock powder deployment. The potential of rock powder to trigger plant suffocation, if gas exchange is prevented by water saturation of pores (Sairam, 2011), should also be considered before deployment.
Average tillage depth is 0.3 m, and greater depths can be reached with higher energy and labor costs (Fageria and Baligar, 2008). Since annual crops have an effective rooting depth typically in the range of 0.4–0.7 m (Madsen, 1985; Aslyng, 1976; Munkholm et al., 2003; Olsen, 1958), a deployment depth of 0.3 m seems to be reasonable.
Since tillage can trigger soil carbon loss (Reicosky, 1997; La Scala et
al., 2006), deploying rock powder at soil surface might be a solution. At
the soil surface, the long-term water percolation and/or bioturbation
(Fishkis et al., 2010; Taylor et al., 2015) can transport and mix
fine-grained material to deeper regions within the soil profile, which
potentially can change the
Detailed field studies to better comprehend downward transport of grained material through the soil profile, changes in soil water residence time, PAW, mineralogy, nutrient pools, CEC (Anda et al., 2015, 2013), and bioavailability of released trace metals (Renforth et al., 2015) are necessary. This would provide management recommendations for the diverse existing settings for EW application. In the present study, estimates for different basalt powder application upper limits are made for changes in soil hydraulic properties without accounting for downward transport of fine particles through the soil profile.
Besides avoiding clogging of pores of the topsoil by rock powder application to a certain extent, downward transport of rock powder can contribute freshly ground material that comes into contact with roots of trees or crops, which can enhance the weathering rates and create new sites to retain nutrients (Kantola et al., 2017; Anda et al., 2015).
Once the freshly ground material is in contact with the soil, different factors control the nutrient supply efficiency of rock powder. The nutrients from fresh material are initially inert, protected within the crystallographic structures of the minerals, and would become plant available only in solution or when associated with mineral surfaces (Appelo and Postma, 2005). The release of nutrients by weathering is controlled by film and intraparticle diffusion-limited mass transfer influenced by pH and ionic strength of the soil aqueous solution (Grathwohl, 2014), both being controlled by rooting exudates in the rhizosphere and the chemical composition of infiltrating waters.
Full dissolution is a simplification based on modeled scenarios (Taylor et al., 2015; Strefler et al., 2018). Under field conditions, soil water could rapidly reach near-equilibrium concentrations (Grathwohl, 2014), which would decrease weathering rates. The opposite would occur if near-equilibrium conditions could be disturbed by a sink of nutrients by nutrient root uptake (Stefánsson et al., 2001) or by percolation of water unequilibrated with soil porous water (Calabrese et al., 2017).
The nutrient (Mg, Ca, K, P, etc.) content of rocks can vary significantly.
Besides that, deploying rock powders with grain sizes > 90
Besides the potential to be used to rejuvenate soil nutrient pools (Leonardos et al., 1987), silicate rock powder can be used to reduce the risk of nitrate mobilization and is indicated for regions in which special care regarding water preservation is needed. However, extra input of sodium (Na) to the system, if the rock is rich in this element, could disturb this amelioration effect (Von Wilpert and Lukes, 2003). Besides decreasing nitrate mobilization, coapplication of rock powder with other fertilizers can increase the biomass production of crops (Anda et al., 2013; Leonardos et al., 1987; Theodoro et al., 2013).
An additional challenge of the application of rock products will be the assessment of the fate of weathering products, which might be transported eventually into river systems and alter geochemical baselines as evidenced by past land use changes in some large rivers (Hartmann et al., 2007; Raymond and Hamilton, 2018).
Our results illustrate the potential of enhanced weathering (EW) to act as a
nutrient source for nutrient-demanding AR and BG. This is an important, yet
often overlooked, aspect of EW besides
Besides the high chemical P content and relatively fast weathering rates, the equilibrated supply of Ca, K, and Mg puts the use of basalt powder one step ahead of other rocks as a potential alternative to industrial fertilizers. Regrowth of forests on abandoned agricultural land is a passive landscape restoration method (Bowen et al., 2007). In most of the cases soils become acidic on abandoned agricultural land in the long term (Hesterberg, 1993), which favors the leaching of nutrients (Haynes and Swift, 1986) and heavy metals (Hesterberg, 1993). As a consequence, the regrowth rate of forests might be limited in acidic soils. The use of basalt powder will keep a positive carbon budget; increase the soil pH (Anda et al., 2015, 2009), as basalt powder would act as a buffer maintaining soil pH under neutral to slight alkaline conditions; and close nutritional needs of AR and BG, and rock powder can be used to reduce the risk of nitrate mobilization (Von Wilpert and Lukes, 2003). However, to be able to assess the global potential of the combination of land-based biomass NETs with EW, it is necessary to explore related physicochemical changes in soil influenced by varying EW deployment rates, based on already-available data, and then develop improved EW models. They should be tested with field-based approaches. For example, tracking added elements through the ecosystem's soil and plant reservoirs probably needs test sites that use advanced methods of nutrient balance and isotope studies, as recently developed (Uhlig et al., 2017; Uhlig and von Blanckenburg, 2019).
In addition to the use for replenishing soil nutrient content, our research suggests that deployment of rock powder on the topsoil can enhance plant-available water (PAW) for different upper limits. Apart from controlling the nutrient release rates, the texture of deployed rock powder would influence the impacts on soil hydrology together with the initial soil texture. In general, EW appears to have considerable potential for water retention management of topsoils. This is an important characteristic that has not been explored before, since under a future scenario of climate change, EW can potentially mitigate or alleviate drought effects to a certain extent within areas used for AR and BG plantation. Field and laboratory experiments are needed to quantify soil hydraulic changes under a natural and controlled environment. Besides that, investigation of potential changes in coupling EW with other terrestrial NETs such as biochar is necessary, since biochar and EW can increase the amount of soil organic matter, a variable also responsible for increasing the water retention of soils.
We show that EW can be an important part of the solution to the problem of nutrient limitation that AR and BG might suffer from. Specifically, its potential for hydrological management of soils was shown, and it could be used in areas where seasonality and droughts might affect the biomass growth. The use of enhanced weathering for hydrological management coupled to land-based NETs is worth investigating. Global management of carbon pools will need a full-ecosystem understanding, addressing nutrient fluxes and related soil mineralogy changes, soil hydrology, impacts on soil microorganisms, and responses of plants to the diverse array of soil types and climates. Applied ecosystem engineering is likely to be a future nexus discipline which needs to link local ecosystem processes with a global perspective on carbon pools within a universal effort to manage the carbon cycle.
Data used for estimating initial soil hydraulic properties can be downloaded at
Data and scripts used from the Kracher (2017) AR model are archived by the Max Planck Institute for Meteorology and can be obtained by contacting publications@mpimet.mpg.de.
Data used for the estimation of nutrient export for bioenergy grass are available upon request to wagner.o.garcia@gmail.com.
The hardwood and softwood tree chemistry database can be downloaded at
The global leaf database can be downloaded at
The Global Gridded Soil Phosphorus Distribution Maps at a 0.5
The used observation-based estimates of P release are available upon request to the original author (Hartmann et al., 2014).
The used global atmospheric P deposition data are available upon request to the original author (Wang et al., 2017).
The supplement related to this article is available online at:
This article was conceived by the joint work of all the authors, who participated in the discussions and writing, led by WdOG. The study was designed by WdOG, JH, and TA. WdOG compiled all the data used and conducted the calculations. KK and AP supplied the MAgPIE model simulations and the stoichiometric ratios used for bioenergy grass. LRB contributed to handling the JSBACH model outputs, and DG contributed to the methodology used to obtain the P-stock-based demand for AR.
The authors declare that they have no conflict of interest.
This study was funded by the German Research Foundation (Deutsche Forschungsgemeinschaft – DFG) priority program DFG SPP 1689 – Climate Engineering: Risks, Challenges, Opportunities? – and specifically the CEMICS2 project - DFG-project HA4472/10-2. In addition this work was supported by the DFG under Germany’s Excellence Strategy – EXC 2037 Climate, Climatic Change, and Society – project no. 390683824, and contributes to the Center for Earth System Research and Sustainability (CEN), University of Hamburg. Daniel Goll is funded by the IMBALANCE-P project of the European Research Council (ERC-2013-SyG-610028). We are grateful for the constructive comments and suggestions from the reviewers and editor.
This study was funded by the German Research Foundation priority program DFG SPP 1689 – Climate Engineering: Risks, Challenges, Opportunities? – and specifically the CEMICS2 project – DFG-project HA4472/10-2.
This paper was edited by Alexey V. Eliseev and reviewed by Daniel Ibarra and one anonymous referee.