Potential Change of Soil Microbial Diversity in Anticipated Atmospheric-CO2 Elevation Triggers Rhizosphere Activation: A Meta-analysis.


 One of the key challenges in present time to meet out growing global food demand without damaging environment under constant threats of climate-extremes. Enhancement of nutrient use efficiency and build up intrinsic system tolerance through soil microbial manipulation has gained significant international support to address this challenge. Impact of elevated carbon dioxide (CO2) on soil microbial diversities both at present and future climatic scenario in spatial and temporal scale is highly debated with respect to its effects on soil functioning, nutrients dynamics and crop productivity and its practical consequences on resource conservation and food security. We conducted a meta-analysis on global database using 572 observations from 202 studies to investigate the effects of elevated CO2 on soil microbial biomass carbon (MBC), yield and structural (soil microbial populations) and functional (soil enzymatic activities) diversities across 22 countries and 108 crop species. Overall, our results revealed that MBC and functional diversity increases with elevated atmospheric-CO2 irrespective of temperature zone and crop type. However, data trends showed structural diversity has been gradually adapted under elevated CO2 across the region over decadal scale. Anticipated elevation of atmospheric CO2 increase rhizospheric activities and could make soil more input demanding and more so in temperate region. Therefore, to fetch the benefits of CO2 fertilization and to meet out the higher demand both plant and soil (microbes), real time judicious nutrient supply is necessary; otherwise, soil priming, loss of fixed soil carbon reserve and land degradation might threat the future food security.


Introduction
To address the issues of yield sustainability and food security under climate change risks, build-up of system resilience with higher resource use e ciency through microbial manipulation is a sustainable approach that harm the environment least. Elevation of atmospheric CO 2 level from 273 to 412 ppm (from 1750 to 2020) and mean annual surface temperature from 14 to 14.89°C (from 1750 to 2020) are evident due to climate change in last two centuries [1]. Further, atmospheric CO 2 concentration is going to be increased @ 1.5 ppm year −1 up to 2050 [2,3] which will affect the crop yield, soil nutrient and water balance and microbial diversities in near future. Soil microbes plays a crucial role in nutrient dynamics [4,5,6,7], soil aggregation [7,8] and yield [9,10]. More so, it has higher adaptability under stress [11,12,13] and ability to build up intrinsic system resilience (to drought, salinity, ood, nutrient scarcity) [14,15] and enhance nutrient use e ciency (e.g., N, P, K, Mn etc.) [16,17].
We know that elevation of atmospheric CO 2 in uences the above ground and belowground biomass production and carbon (C) allocations. The below ground C allocation primarily due to higher root exudation, higher availability of labile C as substrates for microbial enrichment and greater microbial activities [18]. The microbial biomass C and soil-enzymatic activities enhances due to elevated CO 2 which lead to higher nutrient mobilization, GHGs emissions and soil priming [19]. However, there are contrasting reporting about change in soil microbial diversities due to elevation of CO 2 and temperature, particularly population dynamics of bacteria and fungus and their functionality. Manny research group emphasized that there would be no change in microbial diversity as microbes are highly adaptable. Other groups provided evidence regarding shifting of structural as well as functional diversities both in negative and positive directions [19,20,21,22]. The debate is on, but the question is not on the change in the directions of diversities, it is the matter of functioning of crop rhizosphere in anticipated climate change scenarios. Whether the change in microbial functioning in rhizosphere would affect the crop yield, soil health and food security? Whether the nutrient demand would be more for getting the desired yield? Whether our soil would be going to loss for its reserve C due soil-priming in elevated CO 2 condition?
To answer these questions, we analysed scienti c evidence at global scale to assess the impacts of elevated CO 2 on soil microbial biomass carbon, microbial diversities (both structural and functional) and yield on spatial and temporal scale.

Materials And Methods
Data inventories and data base A compressive meta-analysis was done on published data in peer reviewed journals and books, representing a signi cant assessment on this key issue. Because most of the studies on the effect of elevated CO 2 on plants were focused on yield, biomass production and photosynthetic e ciencies, not on soil microbial diversities and functionalities; both eld and laboratory studies containing tropical and temperate regions, on cereals, horticulture, grassland, and forest crops over last ve decades (1970-2020) were included in metadata base. Since, impact of climate change, speci cally the effect of elevated CO 2 on soil microbial diversities are distinctly depends on temperature, tropical and temperate zones were analysed separately irrespective of types of crops and CO 2elevation range. To assess, how the effect of elevated CO 2 varied across the two temperature zones on temporal scale on plants (agriculture/forest), yield, microbial biomass carbon (MBC) and microbial diversities (structural and functional) including bacterial and fungal population and soil enzymatic activities were grouped based on original data. For comparison, two temperature zones (tropical and temperate), three decadal time scales (1970-1990; 1990-2010; 2010-2020) and eight CO 2 -elevated ranges (0-10%; 10-20%; 20-30%; 30-40%; 40-50%; 50-60%; 60-70%; 70-80%) were considered. In total, there were 202 studies covering 22 countries and 108 plants /crops species in the data base.
We exhaustively search the literatures including research articles, books, book chapters that were peer-reviewed for studying the effects of elevated CO 2 on crop yields, microbial biomass carbon and structural (bacterial and fungal population) and functional (soil enzymatic activities) soil microbial diversities from 1970 to 2020. Search keywords included elevated CO 2 , open top chamber, free air CO 2 enrichment (FACE), climate change, crop yield, microbial biomass carbon, bacterial population, fungal population and soil enzymatic activities in the article, title, abstract and keywords. Not reviewed and non-English language publications were discarded. These searches produce around 2000 publications. Then the publications were screened based on following criteria; a) eld and laboratory experiments of elevated CO 2 on either yield or MBC or enzymatic activities or bacterial and fungal population or any of the two must be compared with ambient (reference/ control) condition; b) cereals (particularly grain) and forestry crops were both from temperate and tropical region were considered; c) in eld experiment yield was stated; d) location/ latitude-longitude of the experiment reported; e) management options and replications mentioned; f) different experiments even at same location at different times were also included. As this type of futuristic study are not quite common and there are limitations of crop phenology (height, duration), we tried to gather relevant published data for meta-analysis. We also rejected confusing and unclear results and methods according to our knowledge and more emphasis were goes to C 3 crops (cereals) as those are directly related to food security of resource poor countries. Speci c attention was given to reject and avoid the duplicate data (e.g., different experiment reported similar kind of data in different perspective). A portion of our researched publication on climate change/ elevated CO 2 experiments were avoided due to absence of control treatment that did not satisfy our criteria. Contain cases where speci c latitude-longitude were not provided, we extracted those from google earth-apps with the help of location name/ experimental station name. Vector map in the form of point shape le of 86 locations of the climate change/elevated CO 2 experiments are prepared and shown as bubble plot over the world map using ArcGIS ( Figure 1). Greater the size of bubble in the plot indicates higher number of experiments conducted on that location. Percent change of different parameter due to elevation of CO 2 (over ambient/ control) in different research nding were estimated/ analysed where directly not given in publication. In certain places where data were only given in gures or graphs, values were extracted through plot digitizer (http://plotdigitizer.source.net/).
Tropical and temperate zone were classi ed based on generalized climate classi cation scheme.
Nutrient and water management practices followed were recorded in each study and presented in basic data base (Supplementary Table 1

Metadata analysis
The classi ed data were subjected to one way analysis of variance (ANOVA) and correlation analysis using PROC GLM and PROC CORR of SAS (SAS institute, Cary, NC, USA), respectively to assess signi cance of change in yield, MBC, MDC, BP, FP, and enzymatic activities with respect to spatial changes, temporal changes and with changes in CO 2 levels. The summary statistic (mean, quartiles, minimum, maximum, and extreme observations/probable outliers) was presented using the box-and-whiskers plots of measurements along with the F-value and Prob >F.
Signi cance among the mean values were assessed at p<0.05.

Result
Impact CO 2 -elevation range on microbial diversities The yield increase was found maximum with 40-50% CO 2 -elevation; however, magnitudes of increase were higher at 60-70 and 70-80% CO 2 -elevation over ambient. The highest MBC was reported for the CO 2 -elevation range of 50-60% over ambient ( Figure 2). However, highest increase of MDC was noticed at 30-40% CO 2 elevation. Soil enzymatic activities were affected both positively and negatively. Negative effects were found maximum at 40-50% CO 2 elevation over ambient (Table 1, Figure 2). Correlation study revealed that percent increase of microbial biomass carbon and MDC were positively correlated with elevated CO 2 . Minimum correlation was found in case of increase of fungal population to CO 2 elevation (Table 1).   Table 2). The increase was higher during 1990-2010 compared to 2010-2020. The temporal change was not signi cant except for soil enzymatic activities with maximum in temperate region ( Figure 3). Table 2 Correlation study of percentage increase CO 2 with percentage increase of yield, and microbial diversities in respect to temporal scale (1970-1989; 1990-1999; 2010-2020 Figure 4).

Discussions
Overall, elevated CO 2 signi cantly affects the yield irrespective of region and decadal slabs. Those higher yields corresponded to higher photosynthetic e ciency [10,19,23] of C 3 crop led to higher biomass production and carbon (C) accumulation [19,24]. Further, the magnitude of percentage yield-increase and variation over decades reduces gradually, indicated greater adaptation of plant communities to elevated CO 2 [10,11] and or also due to technological advancement (i.e., introduction of climate resilient varieties and improved soil-water-plant management practices [15,25,26]. However, the crop speci c (cereals, forest, grass land, etc.) impacts on yield in our study were masked the temperature zone-effect under elevated CO 2 [27,28,29,30]. These nding are consistent with smaller data set (572 observations) in which considered both the crop types (both C 3 and C 4 ) together. In general, we found that cereals (mostly C 3 ) were positively affected (83 nos. crops; note presented separately), providing support to our conclusion [31,32]. However, detrimental consequence due to temperature rise associated with elevation of atmospheric CO 2 level is well established particularly in dry season crop in tropics [33,34].
To sustain global agricultural productivity and limit soil degradation in future climate change scenario (higher atmospheric CO 2 concentration), MBC and functional microbial diversities need to be regulated/managed judiciously [35,36,37]. Microbial biomass C found to be most sensitive indicator to rhizospheric functioning under elevated CO 2 irrespective of temperature zones and crop types [38,39,40,41]. In temperate region, a signi cant change in the MBC was reported with respect to percentage increase of atmospheric CO 2 [39,42]. However, in our study, the overall non-signi cant effects of elevated CO 2 on bacterial and fungal population indicated higher structural adaptability of microbial communities (bacterial and fungi) to climate change consequences (elevated atmospheric CO 2 ).
In our study, the impact of elevated CO 2 on microbial diversities was found more in temperate region than tropical due to relatively higher temperature build up in cooler region (due to CO 2 -elevation) which triggered microbial activities, soil enzymatic activities and respiration [14,43,44,45,46]. That means, we have been getting and going to get more functionally active rhizosphere in temperate zone compared to tropics in coming century [36,47,48].
It is often noticed that crop could not response to best management practices as expected due to unprecedented climate related stresses. So, their always exist risk for short to medium-term reduction in crop production possess a barrier for farmers [49,50]. Nutrient-water use e ciency causally related to microbial functionality at rhizosphere level. Our results con rm that there would be a change in functional diversity of microbes due to elevation of CO 2 (predicted in high certainty of climate change consequences) in root zone of cereals, grass, and forest crops both in tropical and temperate region. So, we are going to get a more active rhizosphere in temperate zone which could increase the availability of nitrogen (N), phosphorus (P), potassium (K), and micro-micronutrients. Obviously, the crops demand of those nutrients would be more to maintain proper C: N/P/S ratios in plant and to produce sustainable yield [16,17,51,52]. Therefore, real-time judicious nutrient management (both fertilizer and manure) is necessary in coming future; otherwise, we could loss the CO 2 -fertilization bene t in one hand and may emit more GHGs to atmosphere causing positive feedback to climate change [2,34,35]. As we found signi cant increase (6-59%) in MBC irrespective of crops and temperature zones, we can say that our soil would be hungry to meet the higher demand of microbes. So, if we are not able to supply the demand of nutrients in available form there could be soil-priming [53,54,55]. Soil priming could exhaust the reserve of x-C and in long run could cause soil health deterioration and land degradation [53,56,57,58]. Non-signi cant structural changes of microorganisms due to CO 2 elevation indicated greater adoptability of them. So, there are ample possibility that those changes in microbial diversity could provide intrinsic stress (drought, ood, salinity) tolerance to crop. Therefore, intelligent water as well as nutrient management for harnessing the climate change-bene ts and saving the resources are in the card. There are positives of CO 2 -elevation (climate change consequences) both in plant, soil, and microbial context. But the mechanism and interrelationship need to understand properly. As per example, we must give more C, N, P, K and micronutrient input to soil considering both plant and microbes demands in future; otherwise, there would be soildegradation and crop-productivity deterioration [59,60].
It could not be determined from our study, whether typical biophysical conditions (e.g., soil texture/structure, soilsurface feature, residue-decomposition history etc.) or sub-optimal management practices are the reason for change in yield, MBC, and microbial diversity. Our hypothesis was that the dataset represents a holistic change in MBC, yield, microbial diversity and soil enzymatic activities across the regions and crops. Further, global scale study is needed to identify the combine effect of elevated CO 2 and temperature on microbial diversity in speci c water regimes.
However, climate change is a global phenomenon but the impacts on soil microbial diversity is typically site speci c [44,46]. Nevertheless, disproportionate number of world's poor farmers resides in Africa, South-Asia, Saharan-Africa, still struggling with food security are more vulnerable to climate change vagaries, land degradation and yield reduction. Microbial health of soil is the key indicator of soil-health and land degradation. Therefore, efforts need to be done for intelligent nutrient and water management through microbial mediated rhizosphere manipulation to make the soil-plant systems more climate smart.
Therefore, our result suggested the integrated microbe-nutrient-water management practices in stressed region to meet current and future production challenges and to fetch bene cial effect of higher rhizosphere activities (higher microbial functioning) in terms of better nutrient availability and C-build up ( Figure 5). This is a key nding in respect to the prediction (>90% certainty) that millions of hectares in temperate -Asia, America and Europe are going to fetch bene ts from climate change induced-CO 2 fertilization [9,49,61,62], still, if microbial mediated crop management is to be successful to cope up climate change vagaries and utilize the bene cial effect of CO 2 fertilization for sustaining crop productivity, it should be adjusted to local situations through an innovative coordinated approach that is sensitive to farmers preference, market-choice, and facilities available [30,46]. Therefore, we should give more focus to utilize and manipulate the functional aspect of microbes related to nutrient dynamics, water availability, C-sequestration for farming as short to mid-term strategies to climate change adaptation. Projected climate changes in respect to temperature and precipitation causing frequent drought/ ood, pest virulence, water scarcity is expected in critical agricultural hub of the world [14,15]. Depending on the type, intensity and severity of those climate change-vagaries, critical adaptation measures would be necessary to sustain global agricultural production at a desired level; the microbial mediated rhizosphere-manipulation could provide sustainable solution for improving water-nutrient use e ciencies, greenhouse gasses (GHGs) emissionmitigation and to impart intrinsic tolerance to the system (soil-plant-atmosphere continuum) to climate change threats [63,64,65].

Conclusion
Clearly, there are enhanced functional diversity and biomass carbon of microbes under elevated CO 2 which have both economic and environmental consequences. Future (coming 2-4 decades) crop-soil systems would require more nutrients, water, and carbon to produce sustainable yield; failed to provide that soil-priming and land degradation would be the risk. Climate change-land degradation-soil microbial health is in a vicious cycle. Unless we break the cycles, starting from microbial manipulation, positive feedback to climate change would continue. Hence, conscious efforts to improve real-time nutrient-water management in crops in response to change in microbial functional diversity in near future could "back re" and promote negative feedback to climate change.

Declarations
Con ict of Interest: The authors declare no competing interest.

Figure 1
Spatial distribution of experimental sites considered for meta-analysis in the study.