**2.1 Natural ecosystem and biogeochemical cycles**

As farming activities are intertwined with natural processes of the ecosystem, farmers need to be conversant with the ecosystem and its natural processes that sustain life. The word "ecosystem" stands for a system of interconnected processes to achieve an objective in most efficient manner. The natural ecosystem is a lifesustaining environmental system operating in a geographical area that is composed of living (biotic) and non-living (abiotic) components interacting among themselves. Plants, animals, and other organisms are biotic components while land, air, water, sun, and weather are abiotic parts which interact among themselves as well as with adjacent ecosystems. The matter and energy are exchanged in all ecosystem interactions to sustain life. Life is sustained by the food that is initially produced from the inorganic matter (carbon dioxide) by photosynthesis. Subsequently, a food chain evolves where an organism is food of another organism. The organisms making own food from inorganic matter are called autotrophs. As the plants use sun's energy to make food by photosynthesis, they are also called photoautotrophs. Autotrophs are the primary producers of food in the food chain. The organisms who cannot make their own food and eat primary producers are called heterotrophs or consumers. Herbivores (plant eaters) and carnivores (animal eaters) are also known as "primary" and "secondary" consumers, respectively. Thus, life sustains on web of life called food web/chain. Microbes decompose dead bodies back to inorganic elements which are reused by autotrophs in making organic matter. This is the circular economy of nature which has no waste product. The photosynthesis is part of carbon cycle which, in turn, is part of biogeochemical cycles that control transformation and flow of elements among components of the earth system [4]. A cycle moving a particular element is known by the name of that element. Thus, we have cycles such as carbon cycle, nitrogen cycle, oxygen cycle, etc. Carbon and nitrogen being major constituents, these cycles are discussed here in detail.

Carbon cycle moves carbon from one reservoir/pool to another or from one ecosystem to another. Sediments, oceans, biosphere, and atmosphere are main reservoirs of carbon. The biosphere includes life above ground and soil life below ground. Photosynthesis, respiration, and decomposition are the main processes moving carbon from/to organisms. Photosynthesis fixes atmospheric carbon to plant biomass and then to soil life. Part of this carbon is turned into humus which stays in soil for thousands of years making soil as the biggest reservoir of terrestrial carbon. Carbon in sedimentary rocks of earth's crust is of the order of billions of billion tons while oceans store 38,000-billion-ton carbon at great depths. After earth's crust and oceans, soil is the biggest carbon reservoir containing 1500 billion tons organic carbon and 1000 billion tons inorganic carbon. Atmosphere contains about 750 billion tons of carbon mainly as CO2 while earth's biosphere store about 560 billion tons of carbon. Terrestrial carbon stock in gigaton (GT) or peta-gram (Pg) is summarized in **Table 1** below.

A pictorial view of the above figures in a pie chart is shown below (**Figure 1**) (Terrestrial Carbon Stock).

Earth's carbon cycle moves carbon between various pools but the store of carbon in these pools remain unchanged due to dynamic balance between inflows and

#### *Soil Science - Emerging Technologies, Global Perspectives and Applications*

