Q.The following diagram shows a simplified biogeochemical cycle: three ovals labelled 'Respiration', 'Burning of fossil fuels and wood', and 'Forest fire' each have an arrow pointing into a circle marked '?'; from that '?' circle an arrow points to a box labelled 'Plants', and from 'Plants' an arrow points upward to 'O2'. i) Name the compound whose cycle is shown in the above diagram. ii) What adverse effect does its excess amount cause on environment? iii) Suggest two ways of reducing its effect.

Biogeochemical cycles (nutrient cycling, or the cycling of materials) describe the continuous exchange of nutrients between organisms and their environment, essential for every organism's growth, development, maintenance and reproduction. There are two basic types: gaseous cycles, which move their element mainly through the atmosphere (oxygen, carbon and nitrogen cycles), and sedimentary cycles, which move their element mainly through solid-earth deposits (phosphorus, sulphur and calcium cycles). The carbon cycle circulates carbon between organisms and their environment, driven by the reciprocal processes of photosynthesis (pulling atmospheric CO2 into living tissue) and respiration (releasing it back); additional carbon returns to the atmosphere via burning of fossil fuels, deforestation, forest fires, volcanic eruptions and decomposition, while some is instead locked away long-term as peat, coal, oil, gas and buried sediment. The phosphorus cycle is a sedimentary cycle: phosphorus, essential to DNA, RNA, ATP, NADP and phospholipids, is scarce in the living biosphere but abundant in rock deposits, marine sediments and guano, from which weathering and mining release it into …