Drought effects on soil organic carbon under different agricultural systems

Drought is a natural hazard occurring with increasing frequency due to climate change. Drought events reduce soil water content and also soil organic carbon (SOC) content, with negative impacts on crop development and food security. This study investigates the impact of drought on SOC dynamics in agricultural systems and the influence of water availability and farm management practices in these impacts. The manuscript is a systematic review, based on Scopus database for scoping the literature on the topic. A total of 283 records were retrieved, but only 16 papers were relevant for the review. The main findings are: (1) water plays a key role in regulating SOC mineralization due to its impact on dynamics of soil microbial communities, necessitating further research on water management to mitigate carbon losses during drought; (2) different agricultural systems can have differing impacts on SOC under drought conditions depending on crop type (e.g. pastures are more resilient than arable systems) and farm management practices; and (3) SOC loss generally occurs after a drought event, regardless of farm management regime, but the contribution of drought to this loss requires further research. Best management practices, such as cover cropping and soil amendment, can minimize SOC losses, but further research is required to optimize these practices in counteracting the effect of drought. A better understanding of the effects of drought on SOC dynamics, and of short-term and long-term ways to mitigate these effects, is important to ensure soil health and crop productivity.


Introduction
Climate change is one of the main threats facing humanity, affecting several economic sectors, including agriculture (Planton et al 2008, Döll and Zhang 2010, Radić et al 2014, Arnell et al 2019, UNCCS 2019, Zemp et al 2019).Extreme events, such as droughts, are becoming increasingly common and are having severe impacts on agricultural productivity and food security (IPCC 2019).Agricultural droughts occur when soil moisture content in the root zone is at or below the permanent wilting point, i.e., below the optimal level expected for proper crop development in different growth stages, which results in plant stress and associated yield reduction or failure (Rickard 1960, Wilhite and Glantz 1985, Maracchi 2000, Hazaymeh and Hassan 2016).Many studies have quantified the negative effect of drought on crop yield, and subsequent economic losses.For example, a severe drought in 2012 led to 21% yield loss in maize production in the United States (Boyer et al 2013), droughts in Northern and Eastern Europe in 2018 affected winter wheat and barley production, leading to nearly 40% reductions in yield (Beillouin et al 2020), and water shortages in Australia in 2006-2007 reduced yield of wheat, barley, and cotton lint by 58%, 56%, and 50%, respectively (Roy et al 2021).These examples indicate a need to mitigate the impacts of drought, particularly since future projections indicate that the frequency and intensity of droughts are likely to increase in some regions of the globe that are already prone to droughts, such as the Mediterranean area, central Europe, southern Amazon, and southern Africa (IPCC 2019, Ionita et al 2022).
Besides affecting crop yields and food security, droughts can also have detrimental impacts on soil properties, such as decreasing soil organic carbon (SOC) content and affecting overall soil carbon dynamics (Naylor andColeman-Derr 2018, Schimel 2018).Drought events can weaken the capacity of terrestrial ecosystems to act as carbon sinks or even turn them into carbon sources (IPCC 2014, Piao et al 2019).Soil hosts the largest terrestrial carbon pool (Das et al 2022) and exploiting the potential for net carbon storage in soil is regarded as crucial for climate change mitigation (Amelung et al 2020, Evans et al 2022).Large-scale initiatives such as the European Green Deal, Common Agricultural Policy, and EU Mission: A Soil Deal for Europe recognize this potential and aim to conserve SOC, enhance soil health, and establish sustainable farming practices to achieve climate neutrality and biodiversity preservation (European Commission 2018, 2019, 2023).Other examples include the 4p1000 Initiative, which aims to increase global SOC stocks by 0.4% per year through supporting states and non-government stakeholders in efforts to achieve better management of SOC (Dignac et al 2017, Rumpel et al 2020, 4 Per 1000 Initiative 2021).That initiative is strongly focused on agricultural land and demands good farm management practices (Soussana et al 2019, Mikula et al 2020, Wiesmeier et al 2020).The RECSOIL ('Recarbonization of global soils') initiative, promoted by the Food and Agriculture Organization of the United Nations (FAO), aims to scale up sustainable soil management worldwide through good agronomic practices, in order to increase SOC content while also improving farmers' income and contributing to food security (FAO 2019).However, fulfilling the aims of these initiatives is likely to be quite challenging against a background of increasing soil degradation and desertification through SOC losses as a result of intensive farming and climate change (Bindraban et al 2012, Bhattacharyya et al 2015, Ferreira et al 2022).
Studies examining the effects of farm management practices on SOC stocks suggest that certain practices have the potential to increase SOC content (e.g., soil amendment, conservation tillage), while other practices contribute to decreasing soil carbon stocks (e.g., intensive tillage) (Haddaway et al 2017, Das et al 2022, Szostek et al 2022, Kiran et al 2023).However, little is known about the effects of these practices on SOC under drought conditions.With predicted increasing drought occurrence, it is crucial to understand soil carbon dynamics and identify farm management practices that can improve soil carbon content and crop yields under drought conditions.Some previous studies have reviewed the effects of drought on soil carbon dynamics in global natural ecosystems (Piao et al 2019, Deng et al 2021), but not specifically in agricultural systems such as crop production and pasture.A meta-analysis by Zhou et al (2016) examined responses in terms of soil carbon storage and associated carbon fluxes and pools to drought in terrestrial ecosystems, including agricultural systems, but did not consider the specificities of these systems, such as the farm management practices applied.
The aim of the present work was to perform a systematic review of the effects of drought on SOC dynamics in agricultural systems.Specific objectives were to: (1) assess the impact of drought on SOC stocks and how they are affected by water availability (i.e., irrigation), (2) examine the potential impact of agronomic factors (specifically type of farming system and farm management practices) in counterbalancing the effects of drought on SOC (sequestration and emissions), and (3) identify knowledge gaps and recommend future research lines.Understanding the effects of droughts on SOC in agricultural systems is important to support policy and decision making on strategies to enhance the resilience of agricultural systems to climate change.It can also support climate change mitigation through soil carbon storage and help meet several targets in the United Nations sustainable development goals (SDGs), such as: 'ensure sustainable food production systems and implement resilient agricultural practices that increase productivity and production' (target 2.4), 'strengthen resilience and adaptive capacity to climate-related hazards and natural disasters' (target 13.1), and 'combat desertification, restore degraded land and soil' (target 15.3).

Methodology
A systematic review was conducted of existing publications researching the effects of drought on SOC dynamics in agricultural soils.The review process followed the six steps in the framework developed by Koutsos et al (2019), i.e., (i) scoping, with development of a review protocol focusing on the research questions and study design, (ii) planning, including selection of main keywords and Boolean operators for use in the search string, (iii) identification, based on the search strategy defined in the previous step, (iv) screening, involving management and scrutiny of selected articles, (v) eligibility assessment of the articles retrieved, , and (vi) presentation/interpretation of the review findings and recommendations for future research.
The scoping step was performed in October 2022, on scientific literature from the Scopus database, considering its high reputation, praticability, and because it has a wider coverage when compared to other databases such as Web of Science (Martín-Martín et al 2018, Pranckutė 2021).The keywords chosen for the search were in accordance with the review goals and objectives, and were investigated with the Boolean operators 'OR' and 'AND' as follows: 'soil carbon' OR 'soil organic matter' AND 'drought' OR 'climate extreme' OR 'weather extreme' AND 'farm' OR 'cropland' OR 'agricult'.Since the manuscripts always introduce the abbreviations used, such as SOC and SOM (soil organic matter), our keywords are expected to cover manuscripts using these abbreviations.In the search, 283 records were identified and retrieved.These records were analyzed and relevant papers were selected according to the following four inclusion criteria: (1) documents written in the English language, (2) documents describing research (excluding literature reviews), (3) documents focusing on agricultural soils, and (4) documents evaluating SOC sequestration and/or emissions (soil respiration) before, during, and/or after a drought event.In order to meet criterion 2, 31 review papers, 15 conference papers, and 17 books and book chapters were automatically excluded, resulting in 220 research articles for analysis.
From the research articles retrieved, 167 records were excluded in a first document screening, for not fulfilling inclusion criteria 1 (n = 9), 2 (n = 25), 3 and 4 (n = 133).The 53 documents that passed the first screening were subjected to a second screening with full-text assessment, and 37 were excluded for not fulfilling exclusion criterion 4.After the second screening, 16 papers considered eligible for the systematic review were retained (figure 1).These papers were analyzed and information was collected in order to evaluate the effects of drought on SOC.Information regarding e.g., location of the research, climate system, intensity of drought, soil carbon parameters evaluated, and farm management practices investigated was also collected.

Background of the selected studies
Of the 16 selected research articles assessing the effects of drought on SOC in agricultural systems, the first paper addressing this topic was published in 2004, three papers were published between 2013 and 2016, and the remaining papers (n = 12) were published in the period 2018-2022 (table 1).The majority of the studies (n = 10) were conducted in crop production systems, three studies were performed on pastures, and other three studies covered both crop production systems and pastures.In crop production systems, field crops were the most commonly studied (n = 9), including wheat (Triticum aestivum L.), maize (Zea mays L.), oat (Avena sativa L.), soybean (Glycine max (L.) Merrill), and cotton (Gossypium spp.).Fruit and vegetable crops were less frequently investigated (n = 2 and n = 1, respectively).In pasture systems, a field mainly composed of switchgrass (Panicum virgatum L.) was examined in one study (Lai et al 2016), while in the other studies the fields generally contained a mixture of several herbaceous species, including perennial ryegrass (Lolium perenne L.), clovers (Trifolium spp.) (Moinet et al 2019, Ford et al 2021), orchard grass (Dactylis glomerata L.), and tall fescue (Festuca arundinacea   In terms of location, the studies were mainly performed in East Asia (China, n = 7) and North America (USA, n = 6) (figure 2).Few studies were conducted in Europe (n = 3) and Australia (n = 1).The studies were conducted in a semi-arid climate (n = 6), humid to semi-humid climate (n = 4), or maritime climate (n = 1) (five studies did not specify the climate setting).The studies were mainly conducted on soils with sandy (n = 4), loam (n = 4), and silty (n = 5) texture, while one study was conducted on clay soils and two studies did not specify the soil texture.Drought was reported in all studies, following the inclusion criteria in this review.However, only one study clearly reported the drought intensity using the Palmer Z Index (n = 1), while the remaining papers either generally reported extreme/severe drought (n = 9) or did not specify any drought intensity at all (n = 6).

Effect of drought under different climate and water management conditions
Four research articles analyzed soil carbon emissions under different precipitation regimes (Lai et al 2016, Sun et al 2018, 2021, Zhao et al 2021), one study evaluated both soil carbon sequestration and emissions under different soil moisture conditions (Singh et al 2021), one study assessed SOC under the effect of different irrigation inputs during a drought event in a semi-arid climate (Bhandari et al 2022), and one study evaluated the influence of soil type and soil water drainage capacity on soil carbon emissions under drought in a maritime climate (Ford et al 2021).Regarding the effects of precipitation, two studies evaluated the effects of drought on cropland and a jujube orchard, compared with abandoned grassland and shrubland, in a semi-arid continental climate (Sun et al 2018(Sun et al , 2021)).One study assessed the effects of four simulated precipitation patterns, considering different precipitation amounts (300 and 600 mm) distributed in different sub-events (10 or 100 mm), in a sub-humid continental monsoon climate (Zhao et al 2021).Another study examined the effects of precipitation and temperature on soil carbon emissions in a humid continental climate, modelling these impacts on soil carbon fluxes in different climate scenarios (Lai et al 2016).
Overall, the results indicated that precipitation regulates soil carbon flux by controlling soil moisture, since most of the studies found a strong positive linear correlation between soil respiration and soil water content in different land-use systems, indicating that higher soil moisture levels promote the biological processes responsible for carbon release (Sun et al 2018, Zhao et al 2021).However, type of land use influenced the intensity of soil carbon emissions, which appeared to be lower in agricultural systems than in abandoned grassland and shrubland (Sun et al 2021).The actual sequence of dry and wet periods and the duration/intensity of these periods also influenced soil carbon stocks.In one study, for example, in cropland under semi-arid conditions, a moderate rain event (6.2 mm h −1 ) following dry conditions (10 preceding days without rain) was found to induce around 12-fold higher soil carbon emissions than a moderate rain event (6.3 mm h −1 ) following wet conditions (one preceding day without rain) (Sun et al 2018).Zhao et al (2021) found that soil carbon emissions tended to be higher in a 30-day drought period than a 15-day drought period, which suggests that prolonged drought conditions may increase microbial growth recovery and enhance soil carbon mineralization.Similarly, Lai et al (2016) measured soil carbon emissions during four years (including one drought year) in a pasture system, and found higher carbon emissions during the drought year, supporting the hypothesis that drought leads to increased soil respiration.However, Lai et al (2016) did not find significant changes in carbon emissions in different modelled climate scenarios (temperature increase from 1 to 3 °C, precipitation change of between −30% and +30%).In contrast, in field experiments with variable soil moisture content, Singh et al (2021) found significantly lower soil respiration and significantly higher SOC content in a drought treatment (0 mm precipitation) than in rainfed and irrigated treatments (80-110 mm and 830-1030 mm, respectively), with no significant difference between the latter treatments.However, Bhandari et al (2022) found that SOC generally decreased during drought events and also reported significantly lower SOC content under low than high irrigation inputs.
Some studies indicated that soil water drainage capacity can have also impacts on soil carbon stocks depending on soil type.Ford et al (2021) assessed soil carbon emissions in two pastures with contrasting soil types: a well-drained Cambisol with silty-clay loam texture and a seasonally-wet Stagnosol with silty-clay texture.In the Cambisol, grazed soils were a net source of carbon emissions, exacerbated by drought (+6.8±1.2 t CO2 ha-1 year −1 without drought, +10.8±1.5 t CO2 ha −1 year −1 with drought), while in the seasonally-wet Stagnosol the pasture acted as a net sink of carbon, which could be mitigated by drought (−4.9±1.2 t CO2 ha −1 year −1 without drought, −0.12±1.3t CO2 ha −1 year −1 with drought).Drought had a negative impact on both soils, but soil type considerably influenced carbon emissions due to differences in water drainage capacity.

Drought effects under different agricultural systems
Two studies within our dataset compared the responses of different farming and land-use systems under drought conditions.However, each study evaluated different soil parameters.One study compared drought effects in crop production systems (cotton and sorghum) and conservation grassland in soil carbon sequestration, during a two-year drought stress period in a semi-arid climate (Li et al 2022).The drought event put pressure on both systems, but after the extreme weather the conservation grassland system showed significantly higher organic carbon content in the topsoil layer (at least two-fold) than the crop production system.The other study evaluated the impacts of different land-use systems (cropland, orchard, grassland, shrubland) on soil respiration response during an extreme drought period, with a precipitation pulse, and in the following three years (Sun et al 2021).During the drought year, mean growing soil respiration was identical in the different land-use systems, while in the following non-drought years soil respiration was significantly higher in the grassland (16%-53%) and shrubland systems (67%-126%) than in the cropland system.This was probably due to enhanced soil respiration response to high soil moisture as a result of higher SOC content (3.33 g kg −1 in grassland, 5.07 g kg −1 in shrubland versus 2.08 g kg −1 in cropland), greater fine root biomass (68 g m −2 in grassland, 102 g m −2 in shrubland versus 43 g m −2 in cropland), and lower dry bulk density (1.36 g cm −3 in both grassland and shrubland versus 1.42 g cm −3 in the cropland) (Sun et al 2021).The findings of Li et al (2022) and Sun et al (2021) underline the divergent responses of different land-use systems to drought, showing that, in relation to crop production systems, pastures may have a greater capacity to retain soil carbon but may also have greater soil respiration due to the presence of more organic matter that can be mineralized.
Although not comparing production systems, Li et al (2018) investigates the effects of soil properties on denitrification potential and capacity in three cropping sites over two yearsLi et al 2018).The authors found no significant differences in SOC content before and after a period of moderate drought (Palmer Z index −1.93 during the drought event versus −0.37 during average conditions in previous year) in any of the cropping sites.  2 presents a summary of the effects of drought on soil carbon under the types of farm management practices studied in the selected papers.According to the results, plastic mulch was insufficient to mitigate or reverse the effect of drought and in fact, when used over a longer period (30, 60, or 120 days), it led to greater carbon losses (0.59, 1.64, and 1.79 g kg −1 , respectively) than in a treatment without plastic mulching (0.45 g kg −1 ) (Li et al 2004, Zhang et al 2015).There was no difference in soil water content between the mulched and non-mulched treatments during the drought period, indicating that the difference between the treatments was most likely due to higher soil temperatures under the plastic mulch (+0.6 °C, average of 120 days from sowing to harvest) enhancing microbial metabolic processes contributing to SOC degradation (Li et al 2004).
Crop rotation gave differing results between studies.In one study, a long-term wheat-chickpea rotation lost less carbon (-0.13%) than a native pasture (−0.34%) (Singh and Whelan 2020), while in another study a vegetable rotation (broccoli-zucchini) with wheat led to higher losses (−0.90 t ha −1 ) than a vegetable rotation with a fallow period (−0.67 t ha −1 ) (Zhang et al 2015).Singh and Whelan (2020) found that conventional tillage and zero tillage gave slightly lower carbon losses than native pasture (−0.42%, −0.46%, and −0.53%, respectively).However, VeVerka et al (2019) found that watersheds without cover crop management and with vertical tillage showed significant decreases in SOC after a drought event, while watersheds managed with cover crops and no-till cropping displayed no significant variations.
In some studies, soil carbon retention was achieved with soil amendment treatments, such as biochar and polyacrylamide, actively increasing soil carbon rather than simply conserving carbon (Lashari et al 2013, Ma et al 2020).Annual application of polyacrylamide for five years (75 kg ha −1 and year) allowed soil carbon content during a drought event to remain higher (13 g kg −1 ) than in plots receiving no amendment (10 g kg −1 ) (Ma et al 2020).Application of biochar-poultry manure compost and a pyroligneous solution (a by-product of biochar production) in the first year of the study by Lashari et al (2013) and application of the same pyroligneous solution without biochar in the second year significantly increased soil carbon content during the drought period (+3.48 g kg −1 ) compared with an unamended control.
In pasture systems, one study compared the impact of changing soil volumetric water content during a drought event on soil respiration in 12-year-old permanent pasture under different management regimes (grazed by dairy cows or mowing without grazing) (Moinet et al 2019).The results showed that changing soil water content during a drought event had different impacts on soil respiration in grazed and mown plots.In grazed plots, soil respiration was higher for three days after water addition and decreased steadily after that, likely due to a reduction in plant root respiration, while soil respiration in mown plots did not show any clear pattern (Moinet et al 2019).Those authors attributed the difference between the regimes to differences in carbon reserves in the rooting system of grasses and unsynchronized cutting and grazing regimes.This suggests that farming practices and the timing of management decisions have the potential to modify the impact of soil water content on soil carbon dynamics within pasture systems.
Overall, the studies reviewed suggest that SOC is lost after a drought event, regardless of the type of management, i.e., some management practices may minimize carbon efflux relative to others, but in general the effect of drought outweighs the effect of management practices (table 2).However, improved understanding on the magnitude of the impact of drought on SOC loss is still required, since the studies included in this review do not investigate scenarios without drought.

Overview of studies exploring the effect of drought on agricultural systems
The literature search conducted for this review showed an increasing number of published papers over the past 20 years reporting the effects of drought on SOC in crop production systems.The majority of the studies included in the review were performed in East Asia and North America and most were in a semi-arid climate.These studies include some of the most drought exporsure areas of the globe, but there is a lack of studies in areas with highest drought risk, such as India and North Africa (Carrão et al 2016).
Only a few studies in our dataset assessed both SOC content and soil carbon emissions, despite the fact that these parameters are related and can influence each other, so a wider perspective could be achieved by evaluating both parameters simultaneously.Low SOC content can induce low soil carbon emissions, due to lack of organic matter for microorganisms to process (Wang et al 2014).Consequently, during a drought period it is possible to detect lower soil carbon emissions from soils with low organic matter content than from soils with high organic matter levels (Sun et al 2021).Assessing only soil carbon emissions provides a partial view of the process and can lead to incomplete conclusions.Therefore, more investigations are needed on the effects of drought on both SOC levels and soil carbon emissions.This would provide valuable insights into the real-time reactions of soil carbon to drought and help understand the processes behind observed changes in SOC stocks.It is important, however, to stress that these findings are based on manuscripts retrieved by Scopus database, using the keywords and criteria specified in section 2. Other publications on this topic may be available and should be considered to support our findings.
Another research gap identified in this review was lack of standardization in reporting the intensity of drought.Although all studies selected for review mentioned the occurrence of drought, only one study clearly reported drought intensity using Palmer Z Index (Li et al 2018).Studies without clearly defined drought intensity are difficult to evaluate and compare, delaying progress in determining the effects of drought on SOC.Future studies should clearly report drought intensity, to give a better understanding of drought effects on SOC.The frequency of drought events and the impact on soil carbon dynamics also require further investigation.

Drought effects on soil organic carbon under distinct water availability
Water has a strong impact on microbial dynamics and thus on soil respiration, since it functions as a resource, solvent, and transport medium for soil microorganisms (Schimel 2018).Changes in soil water content significantly influence heterotrophic respiration (figure 3), which tends to decline with low water availability during drought events (Schimel 2018, Allisson andKathleen, 2008).In fact, water inputs usually play the main role in soil respiration, rather than soil temperature.For example, in a field experiment with rainout shelters, Arredondo et al (2018) found that soil water content explained around 70% of the variation in soil respiration, while soil temperature explained only around 25%, under a tropical semiarid grassland.In turn, Sun et al (2021) found that soil moisture and temperature explained 30% and 11%, respectively, of the total variation in soil respiration, in croplands under semarid continental climate.These results emphasize the importance of soil water content as a primary limiting factor for soil respiration and are consistent with findings in other studies on the impact of precipitation/irrigation of e.g., increases in soil respiration with increasing precipitation (Jensen et al 2003, Zhou et al 2009, Huang et al 2015, Mbonimpa et al 2015, Arredondo et al 2018), or with irrigation (Singh et al 2021).In contrast, Lai et al (2016) and Mbonimpa et al (2015) found higher carbon emissions during drought periods.This discrepancy in results may be because the sensitivity of soil respiration to drought stress is determined by multiple factors, such as type of agricultural system, frequency of droughts, and variations in organic matter availability, where lower availability of soil organic matter tend to reduces soil respiration (Wang et al 2014).Findings by Bhandari et al (2022) of significantly lower SOC content under low irrigation inputs also disagree with Singh et al (2021), who found significantly higher SOC content in a simulated drought treatment.Exposure to oxygen of SOC in soil aggregates during the drought event may have led to greater degradation of SOC in the study by Bhandari et al (2022), while the reduced microbial activity in the study by Singh et al (2021) may have reduced SOC uptake (Allison andTreseder 2008, Liu et al 2009).The variability of results between different studies makes general conclusions difficult to build and requires analysis of each case context.
Based on the papers reviewed, the intensity and duration of dry and wet periods can also impact soil carbon emissions (Sun et al 2018, Zhao et al 2021), with e.g., prolonged drought conditions before precipitation increasing microbial growth recovery and enhancing soil carbon mineralization (Zhao et al 2021).Despite low soil moisture content during drought periods increasing the stress on soil microbes, reducing soil respiration (Schimel 2018), increases in soil moisture through subsequent water inputs (as precipitation or irrigation) enhance microbial activity, which increases soil heterotrophic respiration, causing a carbon dioxide pulse-a phenomenon known as the Birch effect (Birch 1958, Schimel 2018).This pulse may explain why prolonged drought conditions before precipitation can increase microbial growth recovery and enhance soil carbon mineralization (Sun et al 2018, Zhao et al 2021).
The type of agricultural system can also impact soil carbon emissions (figure 3), with e.g., higher rates of soil respiration in grassland and shrubland than in cropland (Sun et al 2021).This difference is most likely explained by the higher organic matter content in both grassland and shrubland leading to increased soil carbon emissions due to higher availability of organic materials for processing by microorganisms (Wang et al 2014).
Another factor than can affect soil carbon emissions during drought is soil type.Soils with good drainage can be a net source of carbon emissions, while seasonally-wet soils may act as a net sink of carbon (Ford et al 2021).These observed effects of soil type on soil carbon emissions under drought are probably due to differences in soil water-holding capacity and soil aeration (Schimel 2018).Well-drained soils have high water infiltration rates and can drain water without depleting oxygen in the porous medium, which can stimulate microbial activity and carbon emissions (Pan et al 2016, Säurich et al 2019, Prananto et al 2020).In contrast, seasonally-wet soils with e.g., a silty-clay texture have high water-holding capacity and low infiltration rates, which can promote anaerobic conditions and limit microbial activity, leading to carbon sequestration (Davidson et al 2012, Keiluweit et al 2016).These results are consistent with previous findings suggesting that soil texture, porosity, and spatial distribution of water play a crucial role in soil carbon processes, since they influence microbial access to substrates through the pore network (Patel et al 2021).Moreover, soil particle size distribution strongly influences SOC sequestration, since SOC is usually stabilized by silt and clay particles.In some cases, up to 80% of SOC content is bound to these soil particles (Jolivet et al 2003, Christensen 2009, Angers et al 2011, Curtin et al 2015, Matus 2021).Soils with a high sand content tend to sequester less SOC (Christensen 2009).However, soil carbon sequestration can vary between soils within the same textural class (Li et al 2017(Li et al , 2022)), making the sequestration process highly variable between soil types.
Overall, our review indicated that water is a significant driver of soil carbon emissions, but that other relevant factors, such as microbial activity, organic material availability, and environmental conditions, can also play a key role in determining the intensity of the drought effect.Further research is needed to better understand the complex interactions between these factors, particularly under different farming systems and irrigation inputs.

Drought effects on soil organic carbon under different agricultural systems
The results collated in this review suggest that different types of farming and land-use systems can have differing impacts on SOC and respiration responses during drought periods (figure 3).Although drought may pressure both cropland and grassland systems, conservation grassland can recover more quickly after drought, with significantly higher SOC content in the topsoil layer than in crop production systems (Li et al 2022).These findings support the claim that farming practices in annual crop production systems, such as multiple tillage operations, reduce carbon sequestration (Kabiri et al 2016, Rittl et al 2017, Zhang et al 2018) and affect the resilience of crop systems in recovering soil carbon levels after a drought event (Li et al 2022).However, soil respiration rates can be higher in grassland and shrubland than in cropland, because of the higher organic matter content in such systems.Findings by Sun et al (2021) indicate that higher SOC content, soil moisture content and fine root biomass, and lower dry bulk density, most likely contribute to the higher soil respiration in grassland and shrubland than in cropland systems affected by drought.These results are consistent with findings that soils with higher levels of organic carbon tend to have higher rates of soil respiration, since there is more organic carbon available for microorganisms to process (Eberwein et al 2015, Pan et al 2016).In addition, higher organic carbon content will contribute to higher water-holding capacity and greater soil aeration, due to lower soil bulk density (Murphy 2015, Minasny andMcBratney 2018).These are factors that contribute positively to the metabolic processes of microorganisms and increased soil respiration (Yan et al 2015, Ben-Noah and Friedman 2018, Schimel 2018), as mentioned in section 3.3.

Drought effects on soil organic carbon under different farm management practices
Our review suggests that drought can have a negative impact on SOC irrespective of the farm management practices used.However, it should be noted that over time there are slight fluctuations in SOC levels even in scenarios without drought, determined by the type of soil and its mineralogy, type of climate, farming system, and farm management (Trumbore 1997, Fließbach et al 2007).In general, SOC-rich soils exhibit lower gains or higher losses than SOC-poor soils because the capacity of soils to store SOC saturates due to biophysical factors, particularly the amount of silt and clay-sized minerals that protect SOC from microbial decomposers (Slessarev et al 2023).As previously mentioned, the results found in our review do not present scenarios without drought to perform as a control, Ford et al (2021), and allow a better understanding of the impact of drought intensity on SOC losses.
In any case, our review shows that after drought events the SOC content decreased in both bare and plasticmulched soils (Li et al 2004), tilled and zero-tilled soils (VeVerka et al 2019, Singh and Whelan 2020), and fallow and non-fallow crop rotation systems (Zhang et al 2015).However, some practices seemed to minimize carbon losses more than others.For example, soil without mulch lost much less carbon than soil under plastic mulch, probably due to elevated temperatures under the plastic mulch during the drought period accelerating microbial metabolic processes contributing to organic carbon degradation (Li et al 2004).Plastic mulch can affect SOC differently depending on the type of farming and crop production system, e.g., it can significantly increase SOC in rainfed uplands and decrease SOC content in irrigated uplands and paddy fields (Yu et al 2021).Thus, it is essential to carefully evaluate the use of plastic mulching in drought-prone areas and consider alternative soil and water conservation practices.No study in our dataset evaluated SOC during drought under organic mulch, but this is a practice which should be considered in future research since organic mulches are known to increase SOC content due to accumulation of organic material, and are used worldwide (Blanco-Canqui and Lal 2007, Huang et al 2008, Bajorienė et al 2013).Tillage practices can impact SOC content differently (Ferreira et al 2020).Conventional tillage is known to decrease SOC stocks, due to increased soil aeration stimulating aerobic processes, leading to increased soil carbon mineralization (Kabiri et al 2016, Haddaway et al 2017, Zhang et al 2018, Kan et al 2020, Szostek et al 2022).In contrast, zero tillage or reduced tillage can contribute to soil carbon storage (Barão et al 2019).Alvarez (2006) compiled data from 161 agricultural study sites with contrasting tillage practices and found that both zero tillage and reduced tillage using a chisel, disc or sweep implement gave significant increases in SOC (on average more 2.2 t ha −1 ) compared with conventional tillage by moldboard/ disc plough.Haddaway et al (2017) systematically reviewed 351 studies on tillage impacts on SOC, with the results suggesting that SOC concentration in the topsoil layer (0-15 cm) was significantly higher under zero tillage than under intermediate-intensity tillage with non-inversion machinery (+1.18±0.34g kg −1 ) or highintensity tillage with full inversion machinery (+2.09±0.34g kg −1 ).In fact, tillage practices might explain to some extent the results obtained by VeVerka et al (2019) and Zhang et al (2015) as regards the effects of cover cropping and crop rotation, respectively, on SOC content.After a drought period, VeVerka et al (2019) found no changes in SOC content in fields managed with cover cropping and zero tillage, but a decrease in SOC in fields with vertical tillage.Several studies have shown the effect of cover crops in increasing SOC levels (Blanco-Canqui et al 2015, Bai et al 2018).In addition to their role in enhancing SOC, cover crops offer multifaceted benefits in agricultural ecosystems, including enhance soil moisture retention by reducing transpiration and lowering soil temperatures due to the protective cover provided (Joyce et al 2002, Chen and Weil 2010, Blanco-Canqui et al 2015, Tribouillois et al 2016, Nasir Ahmad et al 2020).These impacts can be relevant for potential mitigation of drought stress.For instance, Zhang et al (2015) found that a crop rotation with a fallow period led to lower soil carbon losses than the same rotation with a wheat crop instead of fallow.Again, the additional tillage for wheat production may have contributed to the higher soil carbon losses during the drought period.Therefore, more research should be conducted to identify alternative reduced tillage management practices for drought-prone areas without compromising crop productivity, especially for arid and semi-arid areas where soil crusting is a major constraint in cropland (Ferreira et al 2018).The results from our review also indicated that soil amendments, such as biochar and polyacrylamide, can actively increase SOC content.Biochar is a highly carbon-rich product that has been suggested as a promising tool against climate change (Woolf et  compared the effects of cover crops, conservation tillage, and biochar application on SOC sequestration in a meta-analysis of 417 peer-reviewed articles and found that, on average, biochar application was most effective in increasing SOC content (39%), followed by cover crops (6%) and conservation tillage (5%).Comparisons of different farm management practices in a meta-analyses of 84 studies under tropical and subtropical conditions (Das et al 2022) and 295 studies in India (Kiran et al (2023) showed that in both cases, biochar was the most effective practice in increasing SOC sequestration.Lashari et al (2013) also reported significant SOC enhancement during a drought period through the application of biochar, poultry manure compost and a pyroligneous solution, while Ma et al (2020) showed that continuous application of polyacrylamide can prevent SOC loss during drought events.These findings collectively highlight the potential of soil amendments in balancing carbon losses due to extreme droughts, but more research is needed to quantify the mitigation potential and to identify the best amendment(s) to use, the optimum amount and frequency of application, and the impact on soil carbon balance under different drought conditions.Further research should also focus on biochar to better understand (i) its impact on temporal variability of SOC, given the general decreasing annual carbon sequestration rates (Kan et al 2020), (ii) the impact of different types of biochar (from distinct pyrolyzed biomass) on SOC variability, (iii) its impact on the bioavailability of pottentially toxic elements under different soil water regimes, given the influence on stable chemical fractions (Boostani et al 2021), and (iv) its impact on the emission of greenhouse gases (e.g.nitrous oxide), since it has been shown to decrease the efficacy of some measures under different soil moisture levels (Pokharel and Chang 2021).

Knowledge gaps
This review identified the following knowledge gaps about the impacts of droughts on SOC in agricultural systems: • Most studies were conducted in East Asia and North America, under semi-arid climates.Additional research is needed in other regions/climate systems, especially in drought-prone areas where water scarcity is an increasing problem.
• Few studies have investigated both SOC and soil carbon emissions simultaneously.More research is needed to improve understanding of drought effects and implications for agricultural systems.
• There is a need for research on the effects of drought combined with soil moisture dynamics, in order to identify possible thresholds and explain conflicting findings reported in the literature, while providing important insights into optimizing irrigation management to support soil health.
• Further research is needed to identify the role of soil type, soil properties (e.g., texture, aggregates) and climate settings on soil carbon dynamics under drought conditions.
• Additional research is required to clarify the potential of each farm management practice in improving carbon sequestration during a drought period.It would be relevant to identify best practices for distinct environmental settings and crops, including e.g., instensity and frequency of application.
• Long-term effects of drought on SOC in agricultural soils are not well studied.Model-based simulations can be useful in revealing long-term effects and supporting policymakers and stakeholders in the task of developing and implementing management strategies to enhance drought resilience to soil degradation.

Figure 1 .
Figure 1.Methodological framework used in selection of relevant articles for the systematic review.
Schreb)(Moinet et al 2019).According to the available data, in one case the field studied was grazed by dairy cows(Moinet et al 2019) and in another case by mixed livestock, mainly mountain sheep throughout the year and beef cattle between spring and early autumn(Ford et al 2021).Most of the studies involved field experiments (n = 13), while a minority included both field experiments and modelling to simulate carbon dynamics (n = 2), or simulations through pot experiments (n = 1).Studies involving modeling used the DAYCENT model(Lai et al 2016) and linear mixed-effects models(Ford et al 2021).Half of the studies evaluated soil carbon analytically in the laboratory, identifying the organic carbon quantity stored in the soil (n = 8), some studies evaluated soil carbon emissions (soil respiration) in situ, using dynamic chamber systems placed at the soil surface (n = 6), and only a few studies evaluated both soil carbon content and soil carbon emissions (n = 2).

Figure 2 .
Figure 2. Geographical distribution of the selected studies.Year next to location refers to the drought period.Different colours correspond to different soil textures.

3. 4 .
Drought effects under farm management practices Regarding the effects of specific farm management practices, six of the research articles evaluated the effects of drought on SOC under: mulching (Li et al 2004, Zhang et al 2015), crop rotation (Zhang et al 2015, Singh and Whelan 2020), tillage (Singh and Whelan, 2020), cover crops (VeVerka et al 2019), and soil amendment application (Lashari et al 2013, Ma et al 2020).All those studies were conducted under real production conditions (field experiments) and all lasted at least two years and up to a maximum of five years.Table

Figure 3 .
Figure 3. General dynamics of soil respiration (S R ) depending on: (a) soil water availability, and on (b) type of farming system.SOC: soil organic carbon.The diagram depicts only general trends.

Table 1 .
Resume of the eligible papers for the systematic review.
Revegetation modifies patterns of temporal soil respiration responses to extreme-drying-and-rewetting in a semiarid ecosystem Croplands and Pastures 14 VeVerka et al Soil health indicator responses on Missouri claypan soils affected by landscape position, depth, and management practices Croplands 15 Zhang et al Effects of plastic mulch and crop rotation on soil physical properties in rain-fed vegetable production in the mid-Yunnan plateau, China Croplands 16 Zhao et al Diverse soil respiration responses to extreme precipitation patterns in arid and semiarid ecosystems Croplands

Table 2 .
Summary of reported effects of drought on soil carbon under different farm management practices.Reported variation refers to differences in values obtained in topsoil sampling before and after a drought event.SOC: soil organic carbon, SOM: soil organic matter.