*4.1.1.1 Aggregate formation and stabilisation*

Soil particles are bound together by temporary (i.e., fungal hyphae and roots) and transient binding agents (i.e., microbial- and plant-derived polysaccharides through organic matter decomposition) [27]. In presence of these agents, aggregation is promoted and with time the microbially restructured carbohydrate molecules get attached with finer soil particles like clay and silt which is a stable form as compared to particulate organic matter (POM). With elimination of soil disturbance (tillage), soil organic matter gets strongly bound to clay particles in the form of macroaggregates and microaggregates within the macroaggregates. Again, microaggregates within the macroaggregates constitute a secure habitat soil microorganism, soil disturbance destroys the microbial habitat, affects its activity. In non-disturbed soil, the particulate organic matter present in macroaggregates get to be predominantly stabilised within microaggregates owing to the slow turnover rate [28]. On the other hand, a higher turnover of POM is seen due to tillage because they get exposed to rapid microbial attack preventing its incorporation into microaggregates as fine POM. In short, tillage leads to carbon loss through breakdown of C-rich macroaggregates and a decrease in microaggregate formation. Research has shown that 90% of total difference in SOC in soils of varying type and climate

is due to the microaggregate-associated C fraction [29]. Thus, a slower turnover of this fraction in zero tillage allows greater protection and stabilisation of coarse POM over time through mineral-bound C decomposition product formation in the microaggregates-within-macroaggregates promoting long-term soil C sequestration in agricultural soils. The process of aggregate formation and protection under no tillage system is shown in the right flowchart whereas, disruption due to tillage is described in the left (**Figure 4**), The bold lines are implicative of higher amount.

**Figure 4.**

*Aggregate formation in a no-tillage as well as conventional tillage system. Adopted from [30].*

### *4.1.1.2 Microbial population and diversity*

Not only microbial habitat, but also macrofauna population is promoted under no tillage practices in absence of physical abrasion and habitat destruction as happens under conventional tillage practices.

Availability of protected habitat and higher C- input directly influence microbial population in a positive way. Generally, in tillage induced environment there is dominance of *r* strategists (with high reproduction rates and fast colonisation capacities) in soil biota shifting the ratio towards higher mesofauna vs. macrofauna or bacteria vs. fungi [31] and thus increased mineralisation versus humification [32] as well as low stability aggregate formation [27]. A fungal dominated system is considered to be a better carbon trapper because of higher metabolic growth efficiency of such class, which assimilate much of substate carbon in microbial biomass and by products but emit less CO2. Higher the metabolic growth efficiency, lesser the loss of mineral associated carbon as CO2 as the fungal products are more chemically resistant to decay [31]. The binding of microaggregates within macroaggregate by plant roots and microbial hyphae is described in **Figure 5**. The mechanism of higher microbial population (Fungi dominated) and aggregate stability are complementary to each other which is generally observed under high biomass input *Sustainable Carbon Management Practices (CMP) - A Way Forward in Reducing CO2 Flux DOI: http://dx.doi.org/10.5772/intechopen.97337*

conservation tillage system. A higher amount of microbially derived carbohydrate C, acid hydrolysable C, amino acids, amino sugars and glomalin content is observed under no tillage soil than a tilled one [33]. The complex interlinking of carbon substrate addition, improved soil physical structure and physical & biological activity enables higher carbon capture under a conservation agriculture management system. More number of binding agents in an undisturbed agricultural soil

#### **Figure 5.**

*Mechanism operated in soils under CA practice for enhancing C-pool size.*

#### **Figure 6.**

*Macro and micro aggregate formation in soil through binding agents. Adopted from [35].*


#### **Table 4.**

*Impact of conservation agriculture on SOC in different countries of Asia.*

promotes water stable aggregate formation and carbon sequestration within the structures. A higher enzymatic activity is also observed under CA.

The main social issue with farmers of IGP are, less time interval between harvesting of kharif crop and sowing of succeeding crop, fodder requirement of domestic animals, use of crop residue as a source of energy for domestic purpose. Mostly farmers adopt the simple way of residue management i.e., residue burning which is undoubtfully a huge source of CO2. In that case, may the carbon addition be very small due to residue return to the field that would otherwise have been emitted to the atmosphere, is a sure shot CO2 efflux mitigation principle (Powlson et al., 2016) [34] (**Figure 6**, **Table 4**).

#### **4.2 Cover crop**

The intercrops or catch crops can be grown in field instead of keeping the land fallow before sowing of the next fallow crop. A *cover crop* is a *crop* of a specific plant that is grown primarily for the benefit of the soil rather than the *crop* yield. Legumes such as vetch, clover, cowpea; green manure crops, a mixture of grasses like ryegrass, oats, winter rye etc. can be chosen as cover crops. In soils health prospect the benefits of cover crops are many starting from erosion control to nutrient trapping. In crops point of view they are excellent for reducing weed and pest infestation in the crop land resulting in a better crop stand. As a direct source of organic biomass to the land, growing cover crops is one of the most effective carbon management practices in Asia. The process of carbon management through cover crop is another interlinked phenomenon of soil erosion control by creating hindrance for the rain drops to splash on the ground directly, soil structural improvement and protection, microbial activity accelerator through supply of substate for their growth and carbon sequestration [37]. Legumes as cover crops enrich the soil with nitrogen whereas cereals and brassica are excellent nutrient scavengers (scavenge nitrogen from losses). A large part of the cover crop is added to the soil in the form of root biomass which was found to be a relatively stable carbon pool than the above ground residue [38]. No tillage legume can act as a potential sink of GHG with global warming potential of −971 to −2818 kg CO2 equivalent ha−1 year−1 as observed by Bayer et al. (2016) [39] in sub-tropical ultisols of Brazil. He also suggested that, these systems may act as a potential source of N2O emission but the net effect is fully offset by CO2 retention in soil organic matter which accounts for −2063 to −3940 kg CO2 ha−1 year−1. Along with below ground biomass, the cover crop is anyway an additional source of carbon enrichment to the soil as compared to a fallow period. A meta- analysis conducted by Poeplau and Don (2015) [40] concluded cover crop to be higher estimate management practice than sewage sludge application with an accumulation rate of 0.32 ± 0.08 Mg C ha \_1 yr. \_1 until saturation is reached in a soil depth of 22 cm (mean) in 30 sites worldwide (in Asia sites under study are from India and Japan). This cumulative carbon sequestration through cover cropping has the potential to compensate for 8% of the annual

#### *Sustainable Carbon Management Practices (CMP) - A Way Forward in Reducing CO2 Flux DOI: http://dx.doi.org/10.5772/intechopen.97337*

direct greenhouse gas emissions from agriculture [41]. Dynamics of nitrogen is very essential for carbon stabilisation in soil. C: N ratio, quality of nitrogen is a major factor controlling nitrogen dynamics in soil. a low C:N ratio plant like legume, early killing of cereal crop can release nitrogen faster into the following crop whereas high C:N ratio cereal grains slow down N release rate. Nitrogen is very much needed in balancing soil organic carbon. Thus, reduced tillage system and high C:N ratio residue can temporarily increase optimum N requirement in crop field that will add to long term carbon storage in soils. Cover crops can contribute this N either by scavenging residual N or by N2 fixation by legumes.
