Greenhouse gas emissions, carbon stocks and wheat productivity following biochar, compost and vermicompost amendments: comparison of non-saline and salt-affected soils

Understanding the impact of greenhouse gas (GHG) emissions and carbon stock is crucial for effective climate change assessment and agroecosystem management. However, little is known about the effects of organic amendments on GHG emissions and dynamic changes in carbon stocks in salt-affected soils. We conducted a pot experiment with four treatments including control (only fertilizers addition), biochar, vermicompost, and compost on non-saline and salt-affected soils, with the application on a carbon equivalent basis under wheat crop production. Our results revealed that the addition of vermicompost significantly increased soil organic carbon content by 18% in non-saline soil and 52% in salt-affected soil compared to the control leading to improvements in crop productivity i.e., plant dry biomass production by 57% in non-saline soil with vermicompost, while 56% with the same treatment in salt-affected soil. The grain yield was also noted 44 and 50% more with vermicompost treatment in non-saline and salt-affected soil, respectively. Chlorophyll contents were observed maximum with vermicompost in non-saline (24%), and salt-affected soils (22%) with same treatments. Photosynthetic rate (47% and 53%), stomatal conductance (60% and 12%), and relative water contents (38% and 27%) were also noted maximum with the same treatment in non-saline and salt-affected soils, respectively. However, the highest carbon dioxide emissions were observed in vermicompost- and compost-treated soils, leading to an increase in emissions of 46% in non-saline soil and 74% in salt-affected soil compared to the control. The compost treatment resulted in the highest nitrous oxide emissions, with an increase of 57% in non-saline soil and 62% in salt-affected soil compared to the control. In saline and non-saline soils treated with vermicompost, the global warming potential was recorded as 267% and 81% more than the control, respectively. All treatments, except biochar in non-saline soil, showed increased net GHG emissions due to organic amendment application. However, biochar reduced net emissions by 12% in non-saline soil. The application of organic amendments increased soil organic carbon content and crop yield in both non-saline and salt-affected soils. In conclusion, biochar is most effective among all tested organic amendments at increasing soil organic carbon content in both non-saline and salt-affected soils, which could have potential benefits for soil health and crop production.

soil.No. of spikelet were 33% more in non-saline soil, while 42% more in salt-affected soil when compared to their respective controls.Spike length followed the same trend as these were also found maximum in non-saline (40%) and salt-affected soil (29%) more in vermicompost amended soil compared to their controls.Maximum increase in root length was recorded in treatment with vermicompost (28%) in non-saline soil, while 25% increase in both vermicompost and compost amended soil in salt-affected soil.Plant dry biomass production was recorded maximum with 57% increase in non-saline soil with vermicompost, while 56% with the same treatment in salt-affected soil.The maximum increment in grain yield was noted with vermicompost treatment (44%) in non-saline, while 50% increase was obtained with vermicompost over the control in salt-affected soil (Table 2).

Plant physiological attributes
All the applied amendments significantly (p < 0.05) increased the chlorophyll contents in wheat in both nonsaline and salt-affected soils when compared with the control.Maximum chlorophyll increment was observed with vermicompost treatment (24%), while 22% increment was recorded in salt-affected soil after the application of the same treatments.Photosynthetic rate was recorded 47% and 53% increase over control with the same treatment application, i.e., vermicompost in both soils, respectively.Stomatal conductance also followed the same pattern with 60% and 12% increase over respective control in non-saline and salt-affected soils, respectively.The increase in relative water contents for non-saline and salt-affected soil treated with vermicompost were noted 38% and 27% over the control (Table 3).

Soil and plant nutrition acquisition
Same as for the plant growth, yield, and physiological parameters, all the applied amendments significantly (p < 0.05) increased N in wheat plant and soil in both non-saline and salt-affected soils compared with the control.However, the maximum N increase in plant was observed with the vermicompost treatment (62%) in non-saline, and 56% increase in salt-affected soil with the same treatment The total N in soil was found maximum in vermicompost amended soil up to 107% increase in non-saline, while 60% increase each in compost and vermicompost amended soils compared to respective controls (Table 4).

Post-harvest soil analysis
All the applied amendments showed a positive effect on soil pH s , EC, and other ionic and nutritional characteristics in both non-saline and salt-affected soils after wheat growth.Applied organic amendments showed a minor but positive effect on soil pH.In non-saline soil, amendments have a minor decrease in soil PH , i.e., up to 3.30%, maximum decrease with the application of vermicompost over the control.While in salt-affected soil, a maximum decrease of 16% was observed with vermicompost compared to the control (Table 5).In non-saline soil, a minor decrease in EC was observed, i.e., 1% with both compost and vermicompost over the control, while 7% decrease was observed in salt-affected soil with same treatment.The SOC recorded maximum with vermicompost treatment up to 25 and 67% increase in non-saline and salt-affected soil, respectively with vermicompost.In soil MBC, 10 and 23% increase was recorded in non-saline and salt-affected soil, respectively with the application of vermicompost in both soils (see Table 5).The calculated SOC stocks at the end of the experiment are presented in Fig. 1.All the treatment except control enhanced the SOC stocks both type of soils, but the maximum increase was noted in vermicompost amended saline soil (17.96 t ha −1 ) compared to 11.80 t ha −1 in its control.The increase in SOC stocks in non-saline soil was also recorded in the same treatment which was 43.13 t ha −1 compared to 36.50 t ha −1 in its control (Fig. 1).

Greenhouse gas emissions
There were significant differences between the carbon and nitrous oxide emissions from non-saline and saltaffected soils (Fig. 2).Treatment with biochar application proved the most effective in restricting the carbon and nitrogen emissions and exhibited the least carbon emissions, i.e., 25% in non-saline soil followed by 60% and 54% in treatments with vermicompost and compost (Fig. 2).Similarly, biochar application also caused 11% of carbon emissions, followed by 16% and 38% in vermicompost and compost-amended salt-affected soil (Fig. 2a,b).Same as the case, biochar treatment also controlled maximum nitrogen (N 2 O) emissions compared to vermicompost and compost treatment.In non-saline soil, biochar treatment caused 0.87% less N emissions compared to 0.81% and 0.79% emissions in vermicompost-and compost-amended soil.In salt-affected soil, a similar trend was recorded where biochar caused 0.90% fewer N 2 O emissions compared to 0.85% and 0.87% in vermicompost and compost-amended salt-affected soil (Fig. 2c,d).

Global warming potential
Table 6 provides the estimated total global warming potential (GWP) in the non-saline and salt-affected soils applied with the organic amendments (i.e., biochar, vermicompost, and compost).In non-saline soil, the biochar treatment considerably reduced the GWP even compared to the control, whereas 77% and 81% higher GWP is estimated with treatment compost and vermicompost in comparison to the control.In the case of salt-affected soil, the application of biochar raised the GWP in comparison to the control, whereas the GWP of compost and vermicompost application was 2 times higher than the biochar amendment.

Correlation and principal component analysis
Correlation among the parameters Pearson's correlation coefficient was calculated to quantify relationships between various parameters of the study.Figure 3a shows Pearson's correlations and levels of significance for the relationship between the plant height, grain weight, yield, plant dry matter production, soil organic matter, no. of spikes, microbial biomass carbon, and relative water contents in non-saline soil.The nitrogen emissions were positively correlated with nitrogen in www.nature.com/scientificreports/plant and soil and microbial biomass carbon.The same is presented about the salt-affected soil in Fig. 3b, where plant height showed a positive relationship with grain yield, no. of spikes, dry matter, and microbial biomass carbon (Fig. 3b).

Principal component analysis (PCA)
The PCA analysis shows that apart from the germination percentage and greenhouse gas emissions, all the remaining components of the study (i.e., growth, yield and physiological attributes, and soil properties) accumulated 80.1% of the total variance (Fig. 4) in non-saline soil.The first principal component (PC1) explained 80.1% of the variance and reflected the positive coordination with the parameters.The second principal component (PC2) explained the 17.9% covariation of all increased factors.The points near the lines originating from the center depicted higher values as compared to distant points (Fig. 4a).In the salt-affected soil, PC1 showed 76.4% variance, while PC2 showed 15.6% variance.The same coordination pattern was also noted in the salt-affected soil as seen in non-saline soil (Fig. 4b).

Crop growth and yield parameters
The application of organic amendments significantly improves soil health attributes such as SOC, soil pH, moisture, and productivity in terms of crop yield 28 .In our study, vermicompost amendment produced the best results in terms of seed germination, plant growth, and yield attributes. Plant height, dry matter production, spikes number, length, etc., were found more in vermicompost-amended non-saline soil, compared to salt-affected soil with the same treatment.This could be due to the enrichment of both soils with readily available plant nutrients, which helps in better plant growth.Aslam et al. 29 reported that vermicompost application significantly improved plant growth and yield up to 5.37 t ha −1 .Due to large particulate surface areas, composts provide many micro sites for microbial activity and the strong retention of nutrients.Vermicompost contains most nutrients in plant-available forms that ultimately enhance the biochemical, yield, and quality attributes of the crops 30 .Some growth-improving products, such as hormones, humates, and amino acids, are also produced as a by-product of microbial and earthworm activity 31 .These properties of vermicompost might be the reason for the improvement Table 6.Global warming potential (kg CO 2 -equivalents ha −1 ) in non-saline and salt-affected soils.
Characterization factor for a 100-year time frame is 296 and 1 for N 2 O and CO 2 , respectively (IPCC 2001).
Values in parentheses representing the percent increase (+) or decrease (−).www.nature.com/scientificreports/ reported by Ibrahim et al. 34 .In salt-affected soils, compost increases crop production by improving soil biological and physical, and chemical properties and alleviates salt stress by neutralizing soil reaction 35 .Vermicompost also considerably increased plant height, fresh weight, and dry weight.While the effect of vermicompost compared with other treatments for root length, fresh root weight, and dry root weight remained non-significant.Befrozfar et al. 36 concluded that vermicompost application improves the growth parameters such as dry matter production, leaf area, and yield in non-saline and salt-affected soils.The growth of plants varied with the vermicompost treatments because growth-promoting substances are released at different rates in different treatments; this might be due to the amount of nutrient content and microbes promoting the plant growth 37 .

Plant physiological attributes
Organic amendments also showed positive impacts on the chlorophyll contents of plants in non-saline and salt-affected soils.Similar to the findings of this experiment, Abd El-Mageed et al. 38 also observed that biochar application improved stomatal conductance, relative chlorophyll contents (SPAD value), plant production, and photosynthetic efficiency.The properties of biochar, like high water-holding capacity and porous structure, maintain sufficient moisture in the soil that plant absorbs easily, and ultimately, plant shows an adequate level of relative water contents (RWC) 39 .Akhtar et al. 40 also observed improved plant water status with biochar application.The beneficial results with vermicompost and compost regarding gases exchange attributes in both non-saline and salt-affected soils were observed due to an increase in N contents with the application of these amendments that accelerated plant development and the leaf area index, which in turn increased light absorption.Therefore, when vermicompost and compost were applied, the chlorophyll contents of leaves increased 37 .
The improvement in RWC with the application of compost and vermicompost in non-saline and salt-affected soils could be justified by balancing between ion absorption and water loss.Compost and vermicompost play a significant role in an increase in RWC and N uptake, which is essential for photosynthesis and encourages plant growth and development 41 .

Post-harvest changes in soil physico-chemical, biological characteristics and soil organic carbon turn over
It was observed that the compost and vermicompost amendments significantly improved the physio-chemical parameters of soil after harvesting of wheat crop.Previously, Antonangelo et al. 42 also reported maximum yields and availability of nutrients in soil after harvest in a treatment where compost and/or vermicompost was supplied.Similarly, Ros et al. 43 and Pratibha et al. 44 also found that the application of vermicompost in soils significantly improved the post-harvest physio-chemical attributes of the soil.Organic amendments significantly decreased EC and pH of salt-affected soils (Qadir et al. 2022).The application of compost and biochar improved the physical conditions of the soil, which increased the leaching ability of soil and thus resulted in a considerable reduction in salts in the root zone 45 .Biochar is claimed to achieve several sustainability goals, including C sequestration, soil health, and plant growth improvements 18,46 .In this study, addition of vermicompost and compost showed good results for increasing SOC in both non-saline and salt-affected soils.Nutrients richness and higher potential to supply nutrients to soils and plants increases SOC content, microbial activity such as nutrient cycling and nitrogen fixation, and soil fertility 47 .The application of compost and vermicompost increased more SOC in both soils compared to control and biochar, as biochar releases slow carbon and resists fast decomposition when added into soils.Soil organic carbon is a sink of atmospheric CO 2 , thereby counteracting the mechanism of global warming 48 .In this experiment, the higher rates of amendments application were responsible for the significant increase in SOC accumulation soils.In case of salt-affected soil, the soil was previously un-managed.The addition of biochar, vermicompost and compost increased soil aggregation by acceleration to form microaggregates and macroaggregates, which are then cemented or coagulated together 18 .In our investigation, the addition of organic amendments probably boosted the development of organic soil colloids, which encouraged the creation of organo-mineral complexes and caused aggregates to form and subsequent increase in SOC storage.The availability of nutrients was also boosted by the organic additions and decomposition.

Nitrogen in plant and soil
In vermicompost treatment, relatively higher N contents were recorded in both salt-affected and non-saline soil, which might be due to increased N as nitrogenous excretory products of earthworms in the vermicompost 49,50 .
Earthworms were reported to have a great impact on nitrogen transformation in vermicompost formation, and due to increased N mineralization, the major portion of the N retained in soil as the plant available form, i.e., nitrate and thus adequate N uptake by the plant when vermicompost was applied to soil 49,51 .The compost treatment also depicted an increase in N content as compared to the control, which may be due to the high mineral N content of compost and lower dry matter content.

Greenhouse gas emissions and global warming potential
The present study shows that biochar application is the most effective in restricting carbon and nitrogen emissions from soil and offsetting global warming potential (GWP) because of its higher ability to sequester C in the soil 18 .It is observed that the gas emissions in non-saline soil are considerably higher than the degraded soils.This may be due to higher microbial activity in non-saline soil compared to salt-affected soils.Application of organic amendments to soil can increase GHG emissions and can alleviate GWP 52 , even though they improve crop productivity and net carbon budget of the soils 53 .Vermicompost and compost increased the microbial and enzymatic activity in soil, which increased the mineralization rate of organic matter in the soil 31 and thus resulted in more CO 2 production 52 .This provides the reason for increased CO 2 emission and estimated total GWP with

Materials and methods
A pot trial was conducted during the winter season of 2021-2022 (15 Nov-6 April) in the glass house (31.2600N, 73.0419 E) at the Institute of Soil and Environmental Sciences (ISES), University of Agriculture Faisalabad (UAF).Bulk soil samples (depth 0-30 cm) were collected from the salt-affected area of Dijkot (31°11′25″ N 73°03′55″ E), District Faisalabad, Pakistan, while non-saline soil was collected from the farm area of ISES (31°26′20″ N 73°04′11″ E), UAF.These soils belong to aridisol class having clay loam texture and developed under arid condition through alluvium derived from Himalayas.Before analysis, the soil was air-dried, sieved using a 2-mm sieve, and processed further for the determination of physico-chemical and biological characteristics as detailed below.

Determination of soil physico-chemical and biological properties
Soil pH was determined using the pH meter (Hanna HI-83141), while electrical conductivity (EC) was measured using a pre-calibrated EC meter (Lovibond SensoDirect con200), while soil texture was determined using hydrometer through Bouyoucos method 54 .Soil organic matter and nutrient contents were quantified using standard analytical procedures i.e., soil organic matter was determined by the Walkley Black method 55 , while SOC was determined by multiplying SOM to 0.58.Total soil nitrogen was determined using the Kjeldahl apparatus by a two-step digestion and distillation process 56 .Available phosphorus was determined using the Olsen method 57 .Extractable potassium was determined using the method devised by Norman 58 .Microbial biomass carbon (MBC) was determined by fumigation-extraction method 59 .The total soluble salts (TSS), residual sodium carbonates (CO 3 2− and HCO 3 − ), and Cl − were measured using methods outlined by the US Salinity Laboratory Staff (1954) 60 .The sodium adsorption ration (SAR) was determined using the following equation; where r SAR is the SAR (mmol dm −3 ) 1/2 ), C Na is the concentration of sodium ions (mmol c L −1 ) and C Mg is con- centration of magnesium ions (mmol c L −1 ).

Climatic data
The weather data conditions prevailing in the experimental year 2021-2022 were obtained from the meteorological observatory, ISES, UAF located near at 31.2621 N and 73.0419 E (Fig. 5).As the Faisalabad lies in arid to semi-arid region based on the climate, the majority of the study span had continuous sunlight and less temperature variations from the averages of both winter and summer seasons.The annual maximum and minimum temperature were observed as 45.5 °C and 19.1 °C respectively, while the yearly precipitation ranged from 300 to 400 mm.

Experimental design and crop husbandry
The experiment was divided into two sets, each having four treatments.The first set was comprised of non-saline soil, while the second set was salt-affected soil.Both sets had the same treatments, i.e., control (un-amended), biochar, vermicompost and compost amendments.Treatments were applied at the rate of 1% of their organic carbon equivalent basis, i.e., 4.05 t ha −1 biochar, 11.80 t ha −1 vermicompost, and 9.15 t ha −1 compost.The pots had a capacity of 10 kg with diameter and depth of 25 cm were filled with (a) non-saline soil and (b) salt-affected soil.There were three treatments + control (per soil type).All these treatments were replicated three times and arranged in a completely randomized design in the glass house under natural conditions.Each pot was seeded with 8-10 seeds of wheat (Triticum aestivum L.) of the Akbar 2019 variety.After one week of seed germination, thinning was done by keeping 5 plants per pot, maximally spaced.Recommended doses of NPK were applied in all treatments using urea, (di-ammonium phosphate) (DAP), and sulfate of potash (SOP); applied N was 79 kg ha −1 , applied P was 57 kg ha −1 and applied K was 62 kg ha −1 .Canal water was used for irrigation and each irrigation was applied after 15 days of interval, while fertilizers were applied in 3 splits (at sowing, after 1st irrigation, and 2nd irrigation.The physico-chemical composition of the canal water and applied organic amendments are given in Tables 7 and 8, respectively.

Figure 1 .Figure 2 .
Figure 1.Effect of applied treatments on soil organic carbon stocks in saline and non saline soils after wheat harvesting.Error bars represent standard error.The data presented is average of three replications (n = 3), while error bars present standard errors.

Figure 3 .
Figure 3. Pearson correlation between the studied parameters in (a) non-saline, and (b) salt-affected soils.GP germination percentage, PH plant height, RL root length, GW grain weight, NSKLTS no. of spikelet, PDM plant dry matter, NSPKS no. of spikes, RWC relative water contents, SOM soil organic matter, Nplant nitrogen in plant, Nsoil nitrogen in soil, MBC microbial biomass carbon, Chl.chlorophyl contents, CO 2 carbon dioxide emissions, N 2 O nitrous oxide emissions, Stm.stomatal conductance, Pht.photosynthetic rate.

Figure 4 .
Figure 4. Principal component analysis explaining the effects of applied treatments on (a) non saline, and (b) salt-affected soil.

2 Figure 5 .
Figure 5. Meteorological parameters recorded during the study span.

Table 2 .
Impacts of applied treatments on different plant growth and yield attributes. Error bars represent the standard error (n = 3).Different letters indicate statistical difference (LSD, p < 0.05).

Table 3 .
Impacts of applied treatments on crop physiological attributes.The values after ± represent the standard error (n = 3).Different letters indicate statistical difference (LSD, p < 0.05).

Table 4 .
Impacts of applied treatments on soil and plant nitrogen uptake after wheat harvest.Error bars represent the standard error (n = 3).Different letters indicate statistical difference (LSD, p < 0.05).

Table 7 .
Properties of canal water used for irrigation during the study.Values after ± represent standard error (n = 3).EC electrical conductivity of the saturated soil paste extract, SAR sodium adsorption ratio, RSC residual sodium carbonates.

Table 8 .
Properties of organic amendments used in the study.Values given are average of 3 replicates (n = 3).Values after ± sign indicate standard error.EC electrical conductivity, OC organic carbon, CEC cation exchange capacity.