Introduction

Organic carbon, an essential element for life on Earth1, is an indispensable component of soil fertility, nutrient cycling2, and sustaining crop production. At the same time, it is challenge to enhance or even maintain the optimal extent of Soil Organic Carbon (SOC), especially in arid and semiarid regions3. In soil, carbon exists in organic (SOC) and inorganic carbon (SIC) forms; SOC is the dominant form. It influences the biogeochemical cycling of nutrients and soil physical properties, such as bulk density, moisture content, aggregation, and overall soil quality4. Total organic carbon can be categorized into the labile and non-labile fractions. Microbial biomass carbon (MBC), dissolved organic carbon (DOC), and potassium permanganate oxidizable organic carbon (PPOC) are considered in the labile fractions, and potassium permanganate oxidizable organic carbon II (PPOC II) is a non-labile fraction5. Labile fractions are highly sensitive to soil management, making them useful indicators for short- and medium-term effects6 of any cultural practices rather than long-term alterations7. The non-labile fractions of carbon are resistant8 and unreliable indicators of short-term changes in soil8,9. Soil aggregation and soil organic carbon (SOC) content are two closely linked components of soil health, functioning synergistically to enhance soil quality and carbon sequestration. Stable soil aggregates improve SOC retention by enhancing soil structure and physically protecting organic carbon from microbial decomposition10. In turn, increased SOC levels contribute to the formation and stabilization of soil aggregates, acting as a binding agent that promotes aggregate cohesion. This reciprocal relationship plays a vital role in carbon sequestration by slowing the decomposition of organic matter11 and immobilizing carbon within soil aggregates, serving as a gluing agent12.

Various factors influence the soil organic carbon content, of which nitrogen fertilization and cropping system are of prime importance. The cultivation of different crops involves variations in fertilization, biomass incorporation, irrigation, and nutrient cycling13. Leguminous crops fix atmospheric nitrogen and enhance soil organic carbon14, while cereals deplete the nutrient content in the soil. However, the effect of these cropping systems on aggregate-associated organic carbon fractions has not been studied much in the semiarid climatic conditions of India. Further, nitrogen is a key component in soil fertility and is directly associated with crop production15. Optimizing nitrogen levels is well known to increase crop production16, but it can also potentially increase the soil organic carbon content and its sequestration17,18, boost-up root biomass carbon19, and alter the nutrient availability. Similar to nitrogen, irrigation could be a potential management practice for optimum yield and carbon sequestration20.

During the last decades, numerous studies have shown that optimum nitrogen supply increases SOC and its fractions primarily due to enhanced root biomass and improved mineralization of soil organic matter17,18,19. However, the present study seeks to fill the gap by examining the impact of three different cropping systems (cereal-pulse-based) on not only SOC but also aggregate-associated SOC and its fractions in Bengaluru, semi-arid climatic conditions. In this study, the relationship between aggregate stability and different doses of nitrogen in various cropping systems has also been discussed. In literature, in most cases, the interactive effects of different cropping systems and nitrogen fertilization on aggregate-associated SOC and its fractions remain unclear. Furthermore, there is limited research on the impact of cropping systems on aggregate stability under two different moisture conditions, especially in the semiarid conditions of India. To explore this area, the present study was undertaken with the following objectives: (i) to study the influence of varying doses of nitrogen on aggregate-associated soil organic carbon and fractions; ii) to assess how different cropping systems influence aggregate-associated organic carbon and its fractions; iii) to determine how aggregate stability is affected by varying doses of nitrogen and adoption of different cropping system in rainfed and irrigated conditions.

Materials and methods

Experimental sites and weather conditions

The experiments were conducted in two different experimental plots, 500 m apart, one under rainfed conditions at a Dryland Agriculture farm, and another at an Agroforestry farm, GKVK, University of Agriculture Science (UAS), Bengaluru, under irrigated conditions21. The geographic location of both experiments is 130 05” N latitude and 770 34” longitude. The climatic condition at GKVK, Bengaluru, is semi-arid with an average annual rainfall of 926.4 mm. The potential evaporation during 2017 and 2018 was 1356 mm and 1291 mm per annum, respectively. Soil type was classified as Nitisols according to the World Reference Base (WRB) for Soil Resources21. The initial soil physicochemical properties of both sites are presented in Table 1.

Experimental design and management

The experimental treatments were imposed and sustained in a similar pattern from 2016 till our sample collection (August 2017). Details of the experimental setup were as follows:

Two factorial (Split plot design) experimental field was divided into 36 plots which had three crop species per season as main treatments (finger millet, field bean, and maize crops) and three N-fertilizer level [no nitrogen (N1), medium (N2) and high (N3)] as subplot treatment at four replications (Fig. 1a and b). In the kharif season (August to December), cereal and pulses (maize—Zea mays, finger millet—Eleusine coracana, and lablab—Lablab purpureus) were cultivated in both sites, while in the rabi season (January to May), vegetable crops (tomato—Solanum lycopersicum L., eggplant—Solanum melongena L., and cabbage—Brassica oleracea L.) were followed in the irrigated field, while the rainfed field was left fallow. The levels of nitrogen fertilization imposed were high (N3), which received 200% of the recommended dose of nitrogen (RDN); medium (N2), which received 100% of the RDN; and low (N1), which received no nitrogen. The N3 level of nitrogen depicts the prevalent practice among the farmers, where they tend to apply double the amount of recommended dose of nitrogen. This was compared with the recommended N dose by the state agriculture university (N2) and control (N1). The recommended dose of nitrogen was determined by conducting multi-location and multi-temporal experiments in a given soil type and climatic condition for a particular crop by state agriculture universities, followed by approval from concerned state agriculture functionaries. The actual amount of N applied was determined as per the crop demand. The crops were managed for insect pest control and other aspects, specifically as per the guidelines of UAS. The fertilizers applied in the experiment are depicted in Table 2 along with other agronomical details of the study. Further explanation of the execution of the experiment can be referred in21.

Table 1 The physicochemical properties of the soil under irrigated and rainfed conditions.
Full size table
Fig. 1
figure 1

Field layout of experimental sites.

Full size image
Table 2 The details of agronomy and nutrient application in different cropping systems under irrigated and rainfed conditions.
Full size table

Soil sampling

Soil core samples (diameter 5 cm) were taken from the surface (0–15 cm) during summer 2017 to analyse aggregate stability and organic carbon and its fractions. The collected soil samples were divided into three portions; one portion was stored in the refrigerator at 4ºC for Microbial biomass carbon (MBC) analysis. Another portion was air-dried, ground, and sieved through a 0.5 mm sieve for SOC and other physicochemical properties analysis. The remaining third portion of the soil samples was air-dried and broken through its natural planes before being subjected to aggregate size separation.

Soil aggregate separation and aggregate stability

The soil samples were brought to the laboratory from the field and kept for air drying. These samples were then manually broken through their natural planes and then subjected to dry and wet sieving separation into different size groups based on the parameters to be analysed. Soil samples have been separated into macro- and microaggregates both by wet sieving, to determine aggregate-associated organic carbon fractions and mean weight diameter and by dry sieving methods, to determine tensile strength.

Wet Sieving

A standard sieve of 250 μm mesh size was used for the separation of aggregates into macroaggregates (> 250 μm) and microaggregates (< 250 μm). The method used for aggregate-size separation was adapted from22, who employed it over a range of soils. Briefly, a 100-g subsample (air-dried or rewetted) was submerged for 5 min on a 250 μm sieve. Aggregates were separated into two fractions by moving the sieve (by hand) up and down 3 cm, 50 times for 2 min, and both fractions were collected. All aggregate fractions were oven-dried at 45–50 °C and weighed23. These separated soil aggregates were used to determine total organic carbon and fractions of organic carbon. The soil’s aggregate stability (mean weight diameter) was determined by separating soil samples into six major groups using a customized eccentric tappet diving engine. The process of the separation is described below:

Wet sieving was conducted with a tower of sieves with mesh sizes of 0.25, 0.5, 1, 2, 4, and 8 mm using the customized sieving device. 20 g of air-dried soil aggregates were spread on the largest (8 mm) sieve, and after soaking for 30 min with water, the sieve tower was gently moved vertically up and down for 10 min in a barrel filled with tap water. The soil aggregates of each size fraction were oven-dried for approximately 24 h at 105 °C and then weighed afterwards. Parallel to each sieving, 10 g air-dried soil from the same sample was used to determine the gravimetric water content in air-dry conditions to correct the initial air-dry weight of the sample to oven-dry weight. The MWD was calculated using the formula given below:

$$\:\text{M}\text{W}\text{D}\:\left(\text{m}\text{m}\right)=\:\sum\:_{\text{i}=1}^{\text{n}}{{\upomega\:}}_{\text{i}}\stackrel{-}{\text{x}}\text{i}$$

Where Xi represents the mean diameter of each size fraction (mm) and wi is the proportion of the water-stable aggregates in the corresponding size group.

Tensile strength

Tensile strength was measured by crushing tests for different aggregate size classes and different levels of nitrogen. The samples were dry-sieved using a set of sieves as 0.25 mm, 0.5 mm, 1.00 mm, 2.00 mm, 4.00 mm, and 8.00 mm. The set of sieves was kept on a shaker for 5 min at a speed of 100 rpm. From each size class, individual soil aggregates were randomly handpicked and labelled. Five soil aggregates from each size group were crushed with a loading frame (Zwick Roell Allround Line, Ulm, Germany), and the stress required for disrupting the aggregate was determined. Tensile strength was calculated according to24 and the formula is provided below. Please consider25 for further details.

Tensile strength (kPa) = 0.576* F/d2 where, F = force (N); d = equivalent diameter (m)

Total organic carbon and its fractions

Total organic carbon was measured using a TOC analyzer (SHIMADZU TOC analyzer). DOC was calculated according to26. Microbial biomass carbon was determined by the fumigation-extraction method using 0.5 M potassium sulphate as extractant in a ratio of 1:4 (Soil: solution)27. Potassium permanganate oxidizable carbon was estimated using the methodology of5, while the other fraction of potassium permanganate oxidizable carbon (Non-labile organic carbon) was calculated by taking the difference between the TOC and PPOC. This fraction was more resistant to decomposition compared to PPOC.

Statistical analysis

Significant differences between pairs of treatment means were tested using the DMRT post hoc test at the 5% level of significance (p ≤ 0.05). A three-way analysis of variance (ANOVA) was carried out to assess the effect of nitrogen application, soil aggregation, and type of crop cultivation on different fractions of organic carbon and aggregate stability. The data were analysed in the SAS (Statistical Analysis System) version 9.3 statistical software (https://www.sas.com).

Results

Total organic carbon

Total organic carbon (TOC) content was significantly (p ≤ 0.05) higher in macroaggregates than microaggregates under both rainfed and irrigated conditions (Table 3). TOC content in macroaggregates (8.05 g kg−1) was about 6% higher than that of microaggregates (7.15 g kg−1) under irrigated conditions, while under rainfed conditions, this increase was 5.5% (7.48 g kg−1 TOC in macroaggregates and 6.70 g kg−1 TOC in microaggregates, respectively). Although the numerical values were different, the pattern of distribution of TOC was similar under both irrigated and rainfed conditions, where about 53% and 47% of TOC was found in macroaggregates and microaggregates, respectively. The TOC content in soil was significantly (p ≤ 0.05) affected by the type of cropping system practiced (Table 3). In irrigated conditions, the highest mean value of TOC (7.86 g kg−1) was observed under the maize cropping system, followed by field bean (7.68 g kg−1) and finger millet (7.26 g kg−1) cultivated soils. In rainfed conditions, the trend observed was maize (7.70 g kg−1) > finger millet (6.97 g kg−1) > field bean (6.62 g kg−1). TOC content significantly enhanced by raising the N levels from N1 to N3 with mean values of 6.41 to 8.77 g kg −1 in irrigated conditions. In rainfed conditions, it varied from 6.28 to 7.97 g kg−1 on increasing nitrogen doses from N1 to N3. The increase was 36.8% under irrigated conditions, whereas it was only 26.9% under rainfed conditions, indicating a lower response of TOC content to nitrogen fertilizer in the absence of irrigation (Table 3). The interaction effect of all three factors was found to be non-significant.

Table 3 Distribution of total organic carbon, microbial biomass carbon, potassium permanganate oxidizable carbon, Non labile organic carbon and dissolved organic carbon, under different nitrogen levels, cropping systems, and aggregate sizes.
Full size table

Microbial biomass carbon

Macroaggregates showed higher accumulation of microbial biomass carbon (MBC) compared to microaggregates in both the irrigated and rainfed conditions (Table 3). About 20.8% higher content of MBC was recorded in macroaggregates (0.499 g kg−1) compared to microaggregates (0.327 g kg−1) under the irrigated experiment, while this increase was 34% in rainfed conditions. Macro and microaggregates contained about 56% and 44% of the total amount of MBC in the rainfed condition, compared to 60% and 40% observed under the irrigated condition. The distribution of MBC significantly varied with the type of crop cultivated, and the highest content was observed in the fieldbean (0.450 g kg−1), followed by maize (0.441 g kg−1), and finger millet (0.349 g kg−1) cropping systems under irrigated conditions. In the case of the rainfed condition, maize had the highest (0.333 g kg−1) content, followed by field bean (0.323 g kg−1) and finger millet (0.312 g kg−1). Here, the interaction between the type of cropping system and soil aggregate size was found significant only under irrigated conditions and larger soil aggregates stored more MBC in all three cropping systems. The sequence of MBC content randomly varied in macroaggregates and microaggregates. Microbial biomass carbon (MBC) followed a similar pattern as TOC, and MBC content increased with nitrogen levels from N1 to N3. The overall increase in mean values from N1 (0.350 g kg−1) to N3 (0.486 g kg−1) was about 38.6%. The interaction of nitrogen with crop type was significant. The greatest increase in MBC with a rising nitrogen dose was found in finger millet (40%) cultivated soil in macroaggregates. In contrast, field bean cultivated soil showed the highest (108%) increase in microaggregates under irrigated conditions. In the rainfed experiment, with increasing N level from N1 to N3, the MBC content was enhanced from 0.260 to 0.394 g kg−1, respectively. The magnitude of increase due to N application was more pronounced with approximately 51% under rainfed conditions, compared to only about 38% under irrigated conditions.

Potassium permanganate oxidizable carbon

All the organic components that are easily oxidizable, including the labile humus material and polysaccharides, were estimated using potassium permanganate3. Potassium permanganate oxidizable carbon (PPOC) content was significantly influenced by nitrogen levels, soil aggregate sizes, and choices of cropping systems (Table 3). Aggregation leads to an increase in PPOC content, like other fractions of organic carbon and the mean value of PPOC in macroaggregates was 1.18 g kg−1 which was about 68% higher than microaggregates (0.70 g kg−1) in irrigated. In rainfed conditions, PPOC content in macroaggregates was 1.21 g kg−1 which was 7.1% higher than microaggregates (1.05 g kg−1). The interaction of aggregate size with N levels was found to be significant under irrigated conditions. The PPOC content increased by 52.1% in macroaggregates (from 0.96 g kg−1 in N1 to 1.46 g kg−1 in N3) and by 48.6% in microaggregates (from 0.70 g kg−1 in N1 to 1.05 g kg−1 in N3) on higher N dose application under irrigated conditions. The interaction effect of nitrogen level with soil aggregates was found non-significant under rainfed conditions, but an increasing trend was observed from N1 to N3. The effect of different cropping systems on soil PPOC distribution was significant under irrigated conditions, while no significant difference was observed in rainfed conditions. In irrigated conditions, the highest amount of PPOC was found in soil under finger millet cultivation (1.04 g kg−1), which is followed by the other two crops being on par with each other. PPOC content was significantly enhanced by raising the N levels in both irrigated and rainfed conditions. In the irrigated situation, the mean values varied from 0.85 to 1.01 g kg−1 from N1 to N3. There was a non-significant increase in PPOC content from the N1 to N2 level of nitrogen. In rainfed conditions, there was a significant increase of 19.4% in PPOC content with increasing nitrogen levels from N1 to N3.

Non labile organic carbon

This fraction of TOC can be considered as the passive fraction or the comparatively more resistant than other above fractions. The non-labile organic carbon (NLOC) content was significantly higher in macroaggregates under both irrigated and rainfed conditions (Table 3). In irrigated conditions, microaggregates had 6.44 g kg−1 NLOC while 6.86 g kg−1 in macroaggregates. Similarly, in the rainfed condition, macroaggregates showed a higher NLOC content of 6.27 g kg⁻¹ compared to 5.65 g kg⁻¹ in microaggregates. Although the absolute NLOC content was higher under irrigated conditions, the percentage increase in NLOC with larger aggregates was more pronounced under rainfed conditions (11%) compared to irrigated conditions (6.5%). The variation in non-labile organic carbon with cropping systems was also found significant, where the highest content was observed in soil under maize cultivation with mean values of 6.98 g kg−1 and non-significantly followed by the fieldbean cropping system (6.77 g kg−1) and a minimum amount of NLOC content in soil was observed under finger millet cropping system (6.23 g kg−1). In rainfed conditions, similarly, the highest content of non-labile carbon was reported in maize-cropped soil with mean values of 6.55 g kg−1, which was followed by finger millet (5.88 g kg−1) and field bean (5.46 g kg−1). NLOC content significantly increased with the application of nitrogen fertilizer. Under irrigated conditions, non-labile carbon increased by 37.9% (from 5.56 g kg−1 in N1 to 7.67 g kg−1 in N3) in the highest amount of N application compared to no nitrogen (Table 3). A similar trend was observed in rainfed condition with a 28.2% increase in the N3 level of nitrogen (from 5.25 g kg−1 in N1 to 6.73 g kg−1 in N3). The interaction among all the factors was found to be non-significant. Thus, in both water regimes (rainfed and irrigated), the overall content of non-labile carbon was higher in macroaggregates, and the effect of nitrogen fertilization was clearly evident, with an increase in non-labile carbon at higher N levels.

Dissolved organic carbon

Aggregation significantly affected the distribution of dissolved organic carbon (DOC) in soil where a higher DOC content was found in macroaggregates compared to microaggregates (Table 3). In irrigated condition, macroaggregates (1.14 g kg−1) had a 10.9% higher DOC content than microaggregates (0.92 g kg−1) while under rainfed condition the quantum was doubled and 20.9% higher DOC content was found in macroaggregates (0.38 g kg−1) compared to microaggregates (0.25 g kg−1). The amount of DOC varied significantly under varying cropping systems, with finger millet and maize yielding higher content of DOC, with mean values of 1.08 g kg−1 and 1.06 g kg−1, respectively, under irrigated conditions. However, under rainfed conditions, higher content was found in maize-cultivated soil with a mean value of 0.41 g kg−1, which was on par with the field bean cropping system (0.38 g kg−1). Significantly, the lowest content of DOC was found in the finger millet cropping system with a mean value of 0.17 g kg−1. DOC content in irrigated condition varied from 0.78 g kg−1 for N1 level to 1.31 g kg−1 for N3 level. In rainfed conditions, the quantum of DOC was lower in value and ranged from 0.17 to 0.50 g kg−1. Further, it was observed for both rainfed and irrigated conditions that an increasing N level leads to an increase in DOC. The interaction of nitrogen levels with cropping system and aggregate size was found to be significant, but only under irrigated condition. The content of DOC increased with both larger soil aggregate size and higher nitrogen doses. Notably, the effect of nitrogen fertilizers was most pronounced in soils cultivated with maize.

Soil aggregate stability

Mean weight diameter

Mean weight diameter (MWD) in rainfed condition ranged from 0.82 mm to 4.62 mm under different nitrogen levels and cropping systems (Table 4). There was no significant effect of nitrogen levels on MWD. However, a significant effect of the cropping system was observed but only in rainfed condition. The highest MWD was found in finger millet (4.09 mm) cropping system irrespective of nitrogen levels while the smallest MWD was observed maize cropping system (1.43 mm) while soil aggregates under field bean had intermediate MWD. The effect of the cropping systems and nitrogen levels as well as their interaction, in the irrigated condition were not statistically significant. A trend of increasing MWD with nitrogen levels was observed only for finger millet, while for field bean, there was no trend observed, and for maize, MWD decreased with increasing N-level. Overall, MWD was much higher for the irrigated compared to the rainfed site.

Table 4 Effect of different levels of nitrogen on mean weight diameter under three crops in rainfed and irrigated situations.
Full size table

Tensile strength

Tensile strength ranged from 262 to 1154 kPa under irrigated conditions and from 193 to 2,462 kPa under rainfed conditions (Table 5). Although nitrogen levels had no discernible impact on aggregate stability, a trend toward increased tensile strength was observed with rising nitrogen levels only for.

Table 5 Effect of different levels of nitrogen on tensile strength of different sizes of soil aggregates.
Full size table

larger soil aggregates with diameters greater than 8 mm. For smaller aggregates, the opposite trend, a decrease in tensile strength with increasing nitrogen level was observed. Tensile strength was seen to increase strongly as the size of the soil aggregates decreased; hence, in both irrigated and rainfed conditions, the aggregates with a diameter between 1 mm and 2 mm had the highest tensile strength. Significantly higher aggregate stability with mean values of 1,990 kPa and 1097 kPa was observed for aggregates of diameter 1–2 mm size fraction under rainfed and irrigated conditions, respectively. While comparing the irrigated and rainfed conditions, 1 mm to 2 mm diameter aggregates were more stable in rainfed condition while aggregates of larger size (> 2 mm diameter) were slightly more stable under irrigated condition accounting higher tensile strength.

Discussion

Effect of varying nitrogen doses and cropping systems on organic carbon and its fractions

In our study, the TOC content increased with increasing N levels due to enhanced root and shoot biomass input. Additional nitrogen application in the soil enhances soil organic carbon and its fractions because the carbon (C) bound in this additional root biomass can in turn contribute to potentially increasing SOC stocks28. In addition to root biomass C, rhizodeposition is another source of OC input from roots which was reported to be half the root biomass of crops29. Furthermore, higher root biomass pertaining to improved nitrogen doses, in turn, lowers soil temperature and moisture, slowing down the decomposition30,31. Some workers32 opine that higher N application can potentially increase the decomposition of native SOC, conversely low N availability is found to hinder soil C accumulation, whilst N fertilization has been shown to increase carbon storage33. This is due to higher net primary production (NPP) resulting from N addition and partly due to the improved stoichiometry of the residual C pool (C: N: P: S). Kirkby et al.33 demonstrated that storage of stable C fraction, irrespective of soil type and C input, is critically influenced by inorganic nutrient availability (especially N), which is due to the faster conversion of organic inputs into fine fraction soil C pools in the presence of the adequate amount of nutrients. Carbon accumulation in soil is a dynamic process that is affected by several factors, including the local temperature, moisture, and carbon input. In semiarid climatic conditions, reaching carbon equilibrium takes less time than in temperate conditions34. Emde et al.20 also observed that irrigated agriculture increased soil organic carbon. Excessive N application is associated with additional leaching losses of nitrogen35; however, various studies indicated that leaching losses of nitrogen were not only influenced by amount of inorganic N fertilization36 but soil type and management-related factors (soil texture, soil tillage, crop rotation, and others) also affect the amount of N leached37. The effect of higher nitrogen doses was more pronounced under irrigated conditions because water was not a limiting factor in plant growth and higher production of root and shoot biomass was observed than under water-limited (rainfed) conditions38.

Lower content of MBC was observed under rainfed compared to irrigated condition which can be explained by a less favourable environment (higher water limitation) for microbe’s growth at the rainfed site39,40. For the irrigated site, higher air capacity and higher plant available water was reported compared to the rainfed site21 (Fuchs 2017 unpublished) leading to better aeration and moisture availability, which favours microbial growth and hence MBC content41. The lower content of MBC at the lowest N-level (N1) may be due to a low microbial population in poor nutrient conditions. As nitrogen content increases to optimum, the enhanced microbial population leads to more microbial biomass accumulation and, hence, higher MBC42. Higher PPOC & DOC content was observed under the N3 level of nitrogen due to increased root biomass and crop residue, which increased the input of organic matter and enhanced the microbial decomposition rate, thereby increasing these carbon fractions43,44. PPOC and DOC were considered as active fractions of carbon, and they mineralize faster when subjected to intensive cultivation, leading to lower amounts in soil45. NLOC representing a passive pool was higher in the site where more addition of organic matter was observed due to more fertilization46. These results are consistent globally and may apply to similar soil types and climatic conditions. Several workers observed higher organic carbon content in soil with increasing the dose of nitrogen30,31,47,48.

The cropping systems prevailing in any region is one of the key factors influencing the soil properties of that area, especially the nutrient distribution and organic carbon content49,50. Differential nutrient application as per the requirement of crop type as well as differential root rhizosphere deposition in the soil made varying amounts of organic matter in soil51,52. The maize cropping system showed the highest amount of TOC and its fractions in irrigated conditions except for PPOC. This might be due to more root biomass and a wider C: N ratio in the maize cropping system compared to other cropping systems. The contrasting results of PPOC may be due to the varying biochemical nature of root exudates, and the composition of root exudates exerts influence on various microbial community assemblages and functions53. PPOC being the labile fractions influenced in greater extent compared to TOC. Kushwah et al.54 also found that higher C sequestration potential of maize and pearl millet compared to finger millet and rice cropping system due to broader C: N ratio and greater root biomass in maize cropping system. However, in rainfed conditions, the field bean cropping system showed the highest amount of TOC and its fractions compared to other cropping systems. This was because of the prominent leguminous effect in rainfed condition, as crops were cultivated annually and one season used to be fallow; therefore, the effect of leguminous crops was clearly reflected in the soil22,55.

Aggregation and aggregate associated organic carbon

Organic carbon content and its various fractions (MBC, DOC, PPOC and NLOC) were higher in macroaggregates as compared to microaggregates which clearly support the finding of enhanced accumulation of organic carbon in macroaggregates56. This may be due to the fact that macroaggregates are formed from microaggregates where organic matter serves as a gluing agent15, hence contributing to higher organic carbon content57. It was also reported that organic matter acts as a binding agent58, and following the hierarchical theory of aggregation, macroaggregates are formed by the coalescing of microaggregates. DOC content was also higher in macroaggregates due to the accumulation of organic matter. Apart from this, macroaggregates have a higher porosity, allowing more amount of DOC to penetrate in macroaggregates as compared to microaggregates. Macroaggregates provide suitable conditions for microbial growth and, hence more labile form of carbon in this size group of aggregates rather than microaggregates59. MBC, PPOC, and NLOC also followed the trend, and higher content was observed in macroaggregates compared to microaggregates43,60. who also found a positive response of PPOC and MBC with increasing mean weight diameter of aggregates, suggesting that these fractions were binding agents for smaller microaggregates to form macroaggregates. In macroaggregates, carbon that was protected from mineralization can be considered one of the ways of carbon sequestration61. Some researchers57 also found that the content of MBC and mean weight diameter are positively correlated, and as the size of aggregates increases, the MBC amount increases. An increase in the content of the organic matter leads to an increase in the activity of microorganisms for which it acts as a source of food and energy, thus enhancing microbial biomass carbon in macroaggregates.

Soil aggregate stability

Tensile strength is a very important parameter as it reflects the pressure needed for roots to penetrate soil aggregates but also determines soil strength and structural stability against external mechanical forces from tillage or traffic. An increase in tensile strength with decreasing aggregates sizes suggests stronger inter-particle bonding and more particle-to-particle contact points. Mineral particles are bound together with fungal, bacterial and plant debris into microaggregates and in turn form larger macroaggregates62,63. As a consequence, microaggregates at a lower hierarchical order exclude the pore spaces between the particles64,65. Thus, microaggregates are generally more stable than macroaggregates. Moreover, roots and hyphae are transient binding agents that are decomposed relatively quickly, resulting in a further decrease of macroaggregates62.

Comparing the irrigated versus the rainfed site revealed a higher tensile strength for 1–2 mm aggregates in the rainfed site while aggregates > 2 mm were slightly more stable under irrigated conditions. Latter could be explained by a higher microbial activity resulting in an enhances production of binding agents and the observed higher content of organic matter stabilising macroaggregates. The larger the aggregates the more binding agent would be involved and hence this effect was dominantly observed only under larger aggregates and not for aggregates smaller than 2 mm. The strong increase, however, in the stability of the smallest macroaggregates fraction (1–2 mm) at the rainfed site may be due to more pronounced cementing effects. Differences in texture – the irrigated site had higher clay contents in the (0–5 cm) soil layer (Table 1) - could also be a reason for higher tensile strength in the > 2 mm size fraction66. The rainfed experimental field is also quite strongly inhabited by termites (field observations), which prefer drier soil conditions. Termites are known to be involved in breaking down organic matter67. This may have disrupted aggregates due to a reduction of gluing organic matter, which could result in lower tensile strength of larger macroaggregates (> 2 mm) in the rainfed site.

Mean weight diameter (MWD) derived from wet sieving is a crucial parameter to describe aggregate stability against the impact of water. It can serve as an indicator of the tendency of soil slaking and crusting. The higher the MWD, the more stable the soil. MWD was not significantly affected by N-level, but an increasing trend was observed in both rainfed and irrigated conditions, inferring that soil aggregation was not influenced by the amount of nitrogen added to the soil68. Although higher N levels increase SOC content and its fractions, but did not improve the aggregate stability because higher root biomass sometimes hinders the stability of larger macroaggregates, disrupting the pre-existing aggregates. These results are in line with several other workers69,7071 who found that other root traits play an important role in soil aggregate stability rather than root biomass. Crop type influenced aggregate stability only under rainfed conditions, where higher MWD was found in finger millet cultivated soil, which might be due to finer root hairs with the intensive type of rooting, hence conferring lesser disturbances to the soil aggregates20. Also, finger millet is a hardy crop and thrives best in rainfed and limited resource availability.

Higher MWD were observed for the irrigated compared to the rainfed site. This may be attributed to a higher clay content (Table 1) at the irrigated site, resulting in more stable aggregates, as well as the effect of organic matter acting as a binding agent, which was also observed to be higher in the irrigated site.

Conclusion

In this study, aggregate associated organic carbon and its fraction were analysed in two different moisture condition (irrigated and rainfed) with different levels of nitrogen and their variation with cropping systems. Higher levels of N resulted in higher accumulation of organic carbon and its labile and non-labile fractions. Different cropping systems influenced soil organic carbon and its fraction and some fractions are higher in maize cultivated soil while some were higher under field bean or finger millet cultivated soils. In macroaggregates, TOC, MBC, PPOC, NLOC and DOC were accumulated more than microaggregates, due to involvement of organic matter as gluing agent. The stability of aggregates in terms of tensile strength had not shown any significant difference with the levels of nitrogen or under varying cropping systems. However, tensile strength varied with the size of the aggregates, and smaller aggregates had higher tensile strength. MWD was affected by different cropping systems but not by nitrogen levels in rainfed conditions, where higher MWD found in finger millet cultivated soil which may be related to a fine rooting pattern. Irrigated sites showed a higher amount of organic carbon and its fraction in comparison to rainfed conditions, and more stable aggregates were also found under irrigated conditions only. Thus, optimum N supply and inclusion of irrigation practices favour the accumulation of organic carbon along with its fractions, thereby forming more water-stable aggregates.