How to Enhance Carbon Sequestration in Drylands? | Soil Management

Given the vagaries of climatic and environmental constrains, increasing the net primary production is difficult in arid regions. This need to be address through different strategies including re-vegetation of degraded lands and restoring life of soil by increasing sequestration of organic and inorganic carbon. Strategies for enhancing soil organic carbon requires long term planning, which consists of selection of ideal land use and management for a piece of land depending up on the problems, potentiality and constraints because different type of land respond differently to the same management practice.

Agro-ecological zoning and segmentation of land based on the land quality class are the effective methods for compartmentalisation of land depending up on the land quality and constraints. A matching exercise between the land use requirement and available resources should be the next step for obtaining the best possible land use. Depending up on the farmer’s choice and available resources, land use policy should be optimised.

The associated management practices should also be the part of land use optimisation programme. A contingency crop planning should also be framed in the event of severe drought and complete failure of monsoon. The entire soil quality improvement plan should be monitored on a specific time frame.

Strategy # 1. Land Use and Contingency Planning:

Land suitability evaluation depending up on the rainfall, temperature, humidity, soil morphology and fertility helps to identify the right type of cultivars, grasses and tress for each land quality class/each agro-eco-sub zones. Optimum soil organic carbon density, which is a total reflection of soil quality, can also work as a tool for selecting right type of cultivars.

For optimizing SOC density, a soil suitability cumulative index for each land use can be derived by allocating the numerical value ranging from 20 to 80 to each soil characteristics, depending upon the land use requirement. Numerical value of each soil characteristics was normalized on the scale of 100 before their addition for cumulative soil suitability index.

Right type of land use for an area should have technological support, which includes development of agrisilvipasture system for erosion control, using combination of trees, shrubs and grasses as shelter belt. Crop improvement for drought mitigation, integrated nutrient and water management, residue management for enhancing soil fertility are other techniques that should come after the selection of ideal land utilization types. The combined effect of all factors may lead to the desired goal of soil organic carbon improvement in dry areas.

Strategy # 2. Contingency Planning for Drought Mitigation:

Keeping land without vegetative cover because of low rainfall punctuated with high drought frequency is the major reason for deteriorating soil quality in the dry regions. Therefore a sound contingency planning is needed to provide vegetative cover to the soils during drought and the planning should be job and income oriented, so that farmers can adopt and implement.

The land use planning need to include short duration varieties of pearl millet and legumes. Mix-cropping with pearl millet (Pennisetum glaucum), moth bean (Vigna aconitifolia), moong bean (Vigna radiata), clusterbean (Cyamopsis tetragonoloba) and sesame (Sesamum indicum) are considered as viable options. Cash crops that can survive limited water and soils of lower fertility such as isabgol, cumin, spices and condiments can be the part of contingency planning.

Shrubs like Lawsonia alba, Capparis decidua, Cassia angnstifolia, Commiphora wightii are drought hardy and may be panted in the field and on the field boundary with short duration varieties of crops. The inclusion of medicinal plants in the crop curriculum may be explored in drought prone areas.

Early drought declaration and monitoring using remote sensing and GIS technique is beneficial in selecting the right combination of trees, grasses, shrubs and cultivars for avoiding drought induced soil quality deterioration. High spatial and spectral resolution satellite images like Spot or Landsat combined with high frequency low-resolution satellite data in Geographic Information System (GIS) can be useful for drought monitoring and its early declaration. The results become more meaningful with use of navigation satellites such as Geographical Positioning Systems (GPS).

Strategy # 3. Crop and Land Management Practices for Enhancing Carbon Sequestration:

The quantity of carbon sequestration in soil is dependent on both environmental and management factors. The environmental factors include temperature, moisture status, soil types etc. and the management factors include return of crop residues into the soil, soil management including tillage, nutrient management, irrigation practices and crop husbandry followed etc. This article discusses various management practices that can improve sequestration of carbon in soil.

a. Nutrient management:

The dryland soils are poor in nutrients. However improvement of soil fertility with extensive use of fertiliser is not feasible due to lack of soil moisture. The application of organic manure and crop residues is essential which enhance the soil organic carbon status; however the availability of such materials for soil enrichment is limited due to competing demands from livestock sector.

The alternative strategy is to apply fertilisers in small quantities along with sowing, so that nutrients are efficiently utilised by crops and helping to add biomass. Adoption of recommended nutrient, based on soil test can not only improve soil fertility but also avoid wastage of fertilisers. Application of balanced fertilisers is helpful to realise increased biomass of crops with resultant impact on retention of belowground residues to the soil.

The impact on carbon addition and sequestration will be considerable if such practices are used in conjunction with reduced tillage and residue management. Hati et al. (2007) reported that under intensive cropping in semi-arid India, the application of balanced rate of fertilizers in combination with organic manure could sequester soil organic carbon in the surface layer, improve the soil physical environment and sustain higher crop productivity.

In their study they observed that SOC content in control and 100% N plots remained around the initial SOC level (1.14 kg m ^-2), while the SOC content in 100% NPK and 100% NPK + FYM treatments attained a new equilibrium that was respectively 22.5 and 56.3% higher than the initial SOC level. Availability of other nutrients is crucial for sequestration of carbon since formation of humus requires the presence and availability of other essential nutrients in soil.

Such balanced application of manures and fertilisers improve soil fertility thereby enhancing water holding capacity leading to improved net primary productivity. Integrated nutrient management decrease the emission of N₂O, a greenhouse gas with considerable global warming potential, by reducing leaching and denitrification losses of fertilisers.

b. Farming System Approaches:

i. Mixed Cropping and Crop Rotations:

The vegetation in drylands ranges from grasslands, woodlands, shrubs and croplands. Natural vegetation in these ecosystems includes such species that are high in water use efficiency, capable of tolerating high solar irradiation and able to survive under irregular rainfall. Mixed cropping and crop rotations brings diversity in dryland agriculture.

It increases the above ground biomass and reduces the risk of crop failure in case of low rainfall or drought. More importantly, it diversifies the root pattern. If deep and medium rooted plants/crops are included along with or in rotation of shallow rooted crops, the carbon addition to various soil layers can be enhanced.

Apart from enhancing carbon allocation in the soil profile, such practices bring about increased soil biodiversity. Singh et al. (2007) reported higher organic carbon density in double cropped fields (pearl millet-legume) than mono-cropping. Monocropping reduces biological diversity in soil.

Legume based crop rotations aid in nutrient cycling in soil and inclusion of legumes in cropping system has been advocated for increasing soil carbon in drylands. Water being available only for limited periods in drylands, the crop selection should be such that they are efficient in water use with adaptations such as increased photosynthetic rates and improved root systems.

ii. Tillage Management:

Excessive tillage predisposes the soil and leads to loss of limited moisture and carbon. Conservation or reduced tillage can be very well practiced especially in coarse textured soils to increase water use efficiency and SOC. The beneficial effect of conservation tillage comes partly from its impact on water storage. Intensive tillage practices lead to disintegration of macro aggregates. A carbon sequestration rate of 0.2 to 1.5 Tg C ha ^-1 year^-1 was reported under no-till management in sub-Saharan Africa.

iii. Mulch and Crop Residues:

The major constraint in dryland farming is temperature and moisture. High temperature leads to evaporative loss of stored soil moisture, and increased transpiration rates adversely affect the plant growth. Arid areas witness significant diurnal variation in temperature. Day time temperatures vary between 35 to 45°C except for short cool dry season in winter. High surface temperature of soil layer leads to loss of soil moisture on account of increased evaporation and transpiration. Plants trying to compensate high temperatures with transpiration suffer from growth and development.

Uses of mulch and crop residues can help overcome such effects on plants growing in dry areas. Residues and mulch materials improve the physical, chemical and biological properties of soils. Apart from helping the plants with water shortage, the residues left on soil help to prevent degradation of soil structure and erosion. It also directly improves the soil quality by helping in nutrient cycling.

A long term study on crop residues by Mandal et al. (2007) showed that, for sustenance of SOC levels, a minimum of 2.9 Mg C per ha^-1 year^-1 to be added as C inputs. This data clearly indicate the importance of residue addition for maintaining the carbon levels. The main factor limiting the use of crop residues in dry areas is its competing uses for feed, fodder and fuel.

c. Management of Saline Wastelands:

Salt affected soils are one manifestation of land degradation in drylands. It results from the accumulation of excess salts in the root zone. They are developed from saline parent materials, intrusion of sea water, excess irrigation etc. These soils have low productivity and require adequate care and management. Unattended barren saline deserts are common feature in arid western India. Vast stretches of saline deserts are common occurrence in extreme arid areas.

These areas are under the risk of further degradation due to absence of plant cover, either due to grazing existing halophyte plants or due to combined effects of drought and salinity. Many palatable halophyte grasses have been identified by researchers.

These plants survive by various morphological and physiological adaptations. With little efforts these saline deserts can be re-vegetated to enhance the biomass and carbon input to these soils. Major halophytes adapted to such areas are Sporobolus marginatus, Aeluropus lagopoides, Urochondra setulosa. Certain non-grass halophytes such as Cressa cretica, Suaeda sp are also recommended. Based on some estimates these halophytes can assimilate 0.6 to 1.2 Gt of C per year and often 30-50% of assimilated carbon could enter into long term storage.

d. Silvipastoral Systems:

Planting trees in desertified and degraded lands help to increase biomass C and also adds significantly to soil C pool. The impact of trees in enriching the C pool also results from its effect on modifying micro-climate and improving soil structure that affect plant growth positively. Apart from the direct effects of increasing C storage in soil and vegetation, trees help to reduce C loss by reducing soil erosion and increasing soil aggregation. Trees in combination with suitable grass species in silvipastoral combination can increase organic matter content of soil.

The deep and extensive root systems of trees help to harvest nutrients from deeper layers and again sent back to soil through litters and root biomass. An increase of soil carbon stock from 24.3 Pg to 34.9 Pg was observed to be obtained by planting trees and grasses in degraded lands of Indian arid areas. Selection of appropriate plant species considering the regional climate and environmental characteristics is important. Growing of Prosopis and Acacia species in drylands is estimated to add carbon at the rate of 2 Mg ha^-1.

The usefulness of neem and acacia based silvipastoral systems in sequestering carbon in north west India was reported by Mangalassery et al. (2014) and Shamsudheen et al. (2009). The potentiality of tree based systems of Rajasthan in containing soil organic carbon loss correcting negative impact of high temperature and poor soil conditions were narrated by Singh et al. (2007). Due to high variation in rainfall amount, intensity and occurrence in dry areas, it is crucial to plan the planting activities to coincide with the onset of rainfall.

e. Water Management and Soil and Water Conservation Measures:

Water management is the key to carbon management in dryland ecosystems. This is because of the fact that drylands are characterised by high variability in rainfall and occurrence of prolonged droughts. Whole of the annual rainfall are received in two to three months in these regions. Moreover in majority of periods, major part of annual rainfall is received in a limited number of high intense rainfall events.

Water received during high intense rainfall events are not much useful as these are lost as runoff due to inherent soil properties which do not let water to infiltrate and store. The increased runoff carries fertile top soil through water erosion causing loss of carbon and nutrients. Low intensity rainfall is lost as evaporation. Capturing and conserving moisture is a key to dryland farming.

Two key aspects of water management in dryland farming are making more water available for cropping and using limited available water more efficiently. In water scarce dryland areas, the application of sewage sludge are reported to improve soil quality and increase carbon stock in soil.

However the application must be monitored to check the health hazards arising out of heavy metals. Drylands are prone to both water and wind erosion due to lack of soil cover. Both of these contribute to removal of top fertile soil. Sporadic high intensity rain events cause severe soil erosion, especially in areas not protected with adequate soil and water conservation measures. The absence of vegetation cover or sparse vegetation, a characteristic of arid region serves as conducive factor for wind erosion.

The estimated total annual emission of C due to erosion ranges from 0.23 to 0.29 Pg C per year. Adoption of adequate soil and water conservation measures can reduce soil and water erosion and help to capture and retain more rainfall. This apart from preventing loss of soil with carbon also enhances carbon build up by increasing biomass addition.

f. Management of Wastelands and Degraded Rangelands:

Suitable plants can be used for re-vegetation of wastelands and degraded rangelands. Spineless cactus (Opuntia sp.) can not only supplies the green fodder, but can also help prevent soil erosion thereby reducing loss of soil carbon and nutrients. If degradation in drylands are checked a net sequestration rate of 1 Pg C per year is achievable.

g. Scientific Livestock Management:

Integration of crop and livestock production is a common feature of dryland farming. Rangelands are an important ecosystem in drylands and grazing is a predominant land use for livelihood of local inhabitants. Since majority of rangelands are common property resources adequate care is not being exercised in using these communal lands. The grazing and rangelands in India is largely maintained as common property resources and faces heavy grazing pressure of herds of cattle, sheep and goats.

The overgrazing results in detrimental changes to the botanical composition of rangelands. Overgrazing triggers off succession and it may lead to dominance of less palatable perennials and annuals such as Oropetium sp, Aristida sp and Eragrostis sp etc. The over grazing damage to the rangelands can be attributed to destruction of shoot apex, decrease in root number and root biomass of different grass species, apart from their adverse effect on fertility and productivity of soil due to trampling by the hooves of livestock .

Heavy grazing and light grazing lead to 33 and 24% reduction in SOC and total nitrogen in the upper 0 to 30 cm depth. Uncontrolled grazing over and above carrying capacity of rangelands renders them barren. The effect of uncontrolled long term grazing is listed as increased variability in plant and soil resources after palatable plants are used up by livestock, foot pressure of livestock making soil susceptible to erosion, increase of unpalatable woody plants.

But the controlled and improved grazing provide the advantage of breaking the top crust of soil, by working through the hooves of livestock and thereby encourage better percolation of water for plant use and better yield of pasture. The adoption of scientific grazing management is helpful to improve carbon sequestration. Improved management practices such as fertiliser application, improved grazing management, sowing of legumes and grasses, introduction of earthworms and irrigation were found to enhance C sequestration from 0.11 to 3.04 Mg C.ha^-1 yr^-1, with a mean of 0.54 Mg C.ha^-1 .yr ^-1.

Strategy # 4. Monitoring Activities:

Monitoring of the soil organic carbon dynamics is essential to assess the sustainability of the soil resource in response to human induced pressures such as land use and soil contamination. Monitoring is defined as the repeated inventory of an item to determine trend and status. One method of monitoring soils is benchmark sampling.

The basic principle of benchmark sampling is to sample at the same location each year. Benchmark sites are representative of larger areas and are usually about a quarter acre (0.1 ha) in size. Sampling with this method is less expensive and time consuming than traditional grid sampling and is more consistent because it assumes the benchmark area is less variable than the larger area, which it represents.