Silvipasture systems create land-use systems that are structurally and functionally more complex with greater efficiencies of resource capture and utilization (nutrients, light, and water) than traditional land management. The greater structural diversity includes nutrient cycling, soil conservation, carbon storage, biodiversity conservation, and enhancement of water quality. Soil quality under silvipasture systems is more similar to that under natural forest than under traditional or intensive agricultural systems.
The components such as livestock, pasture and timber trees are intimately interrelated in silvipastures. All these components have their impacts on soil. The individual and combination effect of tree, pasture and livestock on physical properties of soil depends upon the interaction between animal, pasture, tree component and management practices. Further impact of silvipasture is based on the type of tree species, pasture and types of animals. Effect of tree on soil in silvipasture depend up on the silviculture practices like pruning, thinning and pollarding and rate of leaf harvesting for fodder.
The impact of pasture intercropped in silvipasture, on soil depend upon the type of pasture i.e. grass or forage leguminous, duration of forage (seasonal or perennial forage and nitrogen fixing ability (leguminious or non leguminious). The soil properties and nutrient also altered by animal component of silvipasture.
The effect of animal component on soil depend upon types of animal (Bovines or ruminants) and type of management (free grazing or stall feeding).The properties of the soil such as soil porosity, aggregate stability, infiltration capacity, soil bulk density, soil texture, water holding capacity and soil compaction are altered by litter fall and root system of silvipasture system.
Silvipasture systems mimic characteristics of natural ecosystems such as multi-strata canopy, deep rooting trees and grass. So it provides more soil protection and maintenance of soil fertility similar to those under natural vegetation as compared to agriculture ecosystem.
The litter and root system are important component of tree that alters the soil physical properties. Litter fall is the main path for the return of dead organic matter and nutrients to the soil and humus formation. Litter with an associated unique micro flora and fauna impact on soil porosity, permeability, stable soil aggregates and water holding capacity. Trees through litter fall in turn, influences the physical properties of Soil.
Apart from litter, plant roots play essential role in altering the soil physical properties. Plant roots penetrating the soil leave macropores that improve water movement and gaseous diffusion. They contribute to the system of continuous pores in the soil and enhance the infiltration capacity of the soil.
The effect of living roots on soil- structural stability depends on the plant species. Monocotyledonous plants are superior to dicotyledonous plants and grasses are better than cereals in stabilizing aggregates, because the former contain a much larger root biomass with exudates. Silvipasture system can result in marked improvement in soil fertility.
There are several theories which explain the influence of silvipasture on soil conditions, notably that the incorporation of trees and pasture components may lead to:
1. Change in physiochemical properties of soil
2. Change in soil fertility status
3. Effect on organic matter
4. Effect on soil biology
5. Effect of soil mineralisation process
6. Nutrient recycle
Change in Physiochemical Properties of Soil:
a. Bulk Density, Porosity and Particle Density:
Tree and grass roots penetrating the soil leave macropores that improve water movement and gaseous diffusion. They contribute to the system of continuous pores in the soil and enhance the infiltration capacity of the soil. Li et al. (1992) observed that soil infiltration increases, because plant roots improve the noncapillary porosity of the soil and promote the formation of water-stable aggregates of 2-5 mm, and greater than 5 mm in diameter.
A higher soil infiltration capacity reduces the runoff volume and consequently soil erosion. Roots growing in the soil occupy space that was previously occupied by soil pore space and soil particles. Since root diameter is usually larger than soil pores, soil particles are pushed aside and the bulk density of the soil up to 8 mm near the root increases.
However, fine roots less than one millimetre in diametre can significantly decrease the bulk density of the soil and increase the soil porosity. This effect depends on the root diameter and the nature of the soil, and erosion resistance presumably derives from the large number of roots on top soil. According to Silva et al. (2011), silvipasture condition is intermediated land use system which gives benefit of forestry and reduces the negative impact obtained from agriculture.
The silvipasture systems maintained the soil physical properties compared to soil under natural vegetation (Table 17.1). Continuous cropping adversely affects the physical properties evaluated, and the fallow time after cropping was not sufficient to allow recovery of physical conditions. It appears that silvipasture systems could be good alternative to recover soil physical conditions in arid region of India.
b. EC, pH and Cation Exchange Capacity:
Silvipasture system alters the EC and pH of soil. For instance, tree minimizes the salt deposition in the upper layers of the soil and prevents salt accumulation on the surface layer by improving water permeability; facilitating leaching of salts, decreasing the bicarbonate levels, and hence reduces soil pH and Electrical Conductivity (EC).
It also increases water holding capacity, infiltration rate and hydraulic conductivity. Similarly, enhancing Cation Exchange Capacity (CEC) reducing Exchangeable Sodium Percent (ESP) and improvement in desodification process all along the profile depth can be taken place through tree planting in salt affected soils (Table 17.2).
c. Available NPK and Organic Carbon:
Soils in arid region are prone to erosion hazards. Silvipasture play important role in soil conservation and erosion control. The importance of soil conservation in its broader sense, meaning conservation of fertility and prevention of erosion, has been studied and discussed by several authors.
In arid region, soil transects frequently show higher organic matter and better soil nutrient under trees. Trees improve soil fertility by process of maintaining soil organic matter, increasing nutrient inputs through nitrogen fixation, nutrient pumping from deep soil horizons and promote more closed nutrient cycling.
Sing et al. (2009) documented that the pH and EC were less in silvipasture system as compared to other landuse system like silviculture, agroforestry and agriculture system and SOC was also high in silvipasture system as compared to other land use system. Evidence for the effects of trees on soils comes from comparing soil properties under the canopy of individual trees with those in the surrounds without a tree cover. The changes in soil fertility under Acacia tortilis + Cenchrus ciliaris based silvipasture system and open pasture systems were studied by Mishra et al. (2010).
They recorded that under silvipasture system there were 28.81% , 8.93 %, 2.12% and 5.05% change in organic carbon per cent and available NPK respectively as compared to open pasture system that recorded 11.90%, 7.97%, 0.85% and 4.62% changes in organic carbon and Available NPK respectively over two years (Table 17.3).
The organic carbon content was found to be more in the vicinity of trees. It might be due to growth behaviour of trees, because fast growing trees added more organic matter through litter that enriched the fertility status of the soil. Highest content of organic matter and available nutrients under trees might be based on deciduous and leguminous nature of the trees. These results corroborate with the findings of Felkcr (1978) and Wiersum (1984).
Concentration and availability of nutrients under tree crown vary with distance from trees. The increase in organic matter and availability of nutrients under tree canopies was attributed to accumulation and mineralization of leaves, respectively. Soil organic matter, nutrients and physical and chemical properties improved significantly under trees as compared to open lands. Therefore silvipasture play important role in maintaining soil properties in arid region.
d. Silvipasture Impacts on Soil Microbial Biomass:
Microbial biomass plays a key role in the processes of soil organic matter dynamics and soil nutrient availability in the agricultural ecosystems. Soil microbial biomass comprises about 1-5% of total organic carbon in soil. It acts as a source and sinks for the plant nutrients, playing a crucial role in nutrient cycling and soil organic matter dynamics.
It is the prime agent involved in plant residue decomposition, nutrient conservation and cycling processes in the soil. The microbial biomass has therefore been used as an index of soil fertility, which depends on nutrient fluxes. Soil management practices strongly affect the size of microbial biomass, particularly the inputs of C substrates. Plant cover through its effect on quantity and quality of organic matter inputs influences microbial biomass.
The organic matter subjected to microbial decay in soils comes from several sources. The fast growing woody perennials in agroforestry provide an almost permanent litter cover; the decomposing organic matter in the form of litter being replenished by freshly falling material.
Significantly, soil microbial biomass shows greater variation due to land use management, type of tree and age of plantation. This is mainly attributed to addition of varying “quantity of organic matter inputs over the years through litter fall.
The availability of carbonaceous materials and substrates such as sugars, amino acids and organic acids to the soil from the decomposing litter fall and decay of roots under the canopy of the tree are important for supplying energy for microbial populations. Soil microbial biomass and enzyme activities are increased under different tree based land use management.
Land management practices had a significant effect on the structure and composition of soil microbial community. Soil microbial communities were clearly clustered according to management systems, suggesting that different land management practices generate different and unique ecological niches to soil microbial communities. The management- induced changes on soil microbial community could be a consequence of modifications in their micro habitats and substrate availability that discourage or favour the growth of selected microbial groups.
Silvipasture system favoured the abundance of fungal biomarkers, which can represent an important change in soil quality because fungi can have beneficial properties. This includes hyphal growth and exudation of organic compounds that form and stabilize macroaggregates, which in turn improves aeration, root penetration and C protection. According to Vallejo et al. (2012) in silvipasture system and forest soil, microbial communities were more similar than in conventional pasture land communities (Table 17.4). Microbial biomass was improved in silvipasture system over conventional pasture.
In general, there were significant differences due to land management for all microbial properties most notably; conventional pasture caused a significant decrease in the density of fungi, arbuscular mycorrhizal fungi and actinomycetes, while gram-negative bacteria were significantly higher in conventional pasture in comparison with the other land management practices.
On the other hand, soil microbial communities were significantly different among the Prosopis juliflora based silvipasture system chronosequence. Similarly, total and gram-positive bacteria densities were significantly higher in the oldest silvipasture system of 12 years old (Table 17.4).
The higher inputs and diversity of ground cover residues under tree canopies provides substrates for microbial growth and stimulation of hydrolytic enzymes synthesis. Plant roots and the rhizosphere effect stimulate enzyme activity by creating favourable microhabitats for soil microbial community. In addition, the modification of organic carbon pool protects the free, catalytic soil enzymes stabilized in the soil matrix. P. juliflora also shifted the soil microbial community structure and composition, but overall had its biggest impact on soil nutrient availability, which can support the “islands of fertility”-theory.
Furthermore, it shows the importance of the inclusion of trees (P. juliflora) and shrubs for improving soil quality in silvipasture system. The soil microbial community structure and composition in silvipasture are more similar to those from native forest, which probably reflects a healthier soil than in conventional pasture. Hence, silvipasture based improvements are vital for long term productivity and sustainability of the soil in arid region, where level of soil biological activity is low due to lower soil organic matter.
e. Silvipasture and Nutrient Cycling:
Incorporating woody perennials on farmland can result in a marked improvement in soil fertility.
There are several theories which explain the influence of trees on soil conditions, notably that the incorporation of trees may lead to:
1. An increase in the organic matter content of soil through the addition of leaf litter, decaying roots and other plant parts;
2. More efficient nutrient cycling within systems, and thus better utilization of nutrients that are either inherently present in the soil or externally applied;
3. Biological nitrogen fixation, and improved solubility of relatively unavailable nutrients, such as phosphate, as a result of the activity of micro-organisms in the tree root zone;
4. An increase in the proportion of nutrients that are cycled through the plant layer, and therefore a decrease in nutrient loss through leaching;
5. A moderating effect of additional soil organic matter on extremes in soil acidity and alkalinity, and, consequently, improved release and availability of nutrients such as Phosphate and manganese that are sensitive to pH;
6. Increased activity of favourable micro-organisms in the root zone through improvement in the organic matter status and temperature of the soil;
7. Gradual improvement of the physical conditions of the soil-in permeability, water- holding capacity, aggregate stability, and soil temperature regimes.
The relative significance of these different effects will vary greatly depending on the component of silvipasture system like tree species, forage type and livestock type, soil and site conditions. Many of these effects also take a considerable time to develop the equilibrium; trees cannot be expected to have a dramatic effect on soil fertility overnight.