The nitrogen cycle involves various specialized bacteria: Nitrobacteria (specifically Nitrobacter) oxidizes nitrite to nitrate (NO3-). Ammonifying bacteria convert organic nitrogen into ammonium (NH4+). Denitrifying bacteria reduce nitrates back into atmospheric nitrogen (N2). Rhizobium species are symbiotic bacteria that fix atmospheric nitrogen within plant root nodules. The matching provided in option C correctly aligns these biological processes with their respective microbial agents.

Nickel (Ni) was officially recognized as the 17th essential plant nutrient in 1987. It is a vital component of the enzyme urease, which is necessary for the metabolism of urea in plants. Without sufficient nickel, plants may accumulate toxic levels of urea in their tissues, leading to leaf tip necrosis and impaired growth.

2-ketoglutaric acid, also known as alpha-ketoglutarate, is a key intermediate in the Krebs cycle. Certain soil microorganisms produce this organic acid, which possesses the chemical capability to chelate calcium. This chelation process is significant in soil chemistry as it influences the solubility and availability of calcium and other mineral nutrients within the rhizosphere, thereby affecting plant nutrition and soil structure.

Casuarina equisetifolia is highly valued in agroforestry for its exceptional tolerance to salinity and sodicity. Its ability to fix atmospheric nitrogen and adapt to harsh soil conditions makes it an effective biological tool for rehabilitating degraded, salt-affected lands, thereby improving soil structure and fertility over time.

Eutrophication is the process where water bodies receive excess nutrients, primarily nitrogen and phosphorus, often from agricultural runoff. This stimulates excessive algal growth, which subsequently dies and decomposes, consuming dissolved oxygen and creating hypoxic conditions that harm aquatic life.

Hydrogen sulfide (H2S) is a colorless, toxic gas often produced by the anaerobic decomposition of organic matter in water, such as in sewers or stagnant ponds. It is well-known for its characteristic 'rotten egg' smell, which is detectable even at very low concentrations. Its presence in water supplies is a strong indicator of anaerobic conditions and potential contamination by organic waste.

While both Sun hemp (Crotalaria juncea) and Dhaincha (Sesbania aculeata) are used as green manure, Dhaincha is often cited as the most outstanding due to its high biomass production, rapid growth, and ability to tolerate adverse soil conditions like waterlogging and salinity. It is highly effective at fixing atmospheric nitrogen and improving soil structure, making it a preferred choice for green manuring in many cropping systems.

World Water Day is observed globally on March 22nd each year, a tradition established by the United Nations in 1993. The primary objective of this day is to raise awareness regarding the critical importance of freshwater and to advocate for the sustainable management of global water resources. It serves as a platform to address water-related challenges and promote conservation efforts.

Base saturation is calculated by dividing the sum of exchangeable bases (Ca, Mg, K, Na) by the total CEC and multiplying by 100. If the CEC is 20 me/100g and exchangeable acidity (H+) is 5 me/100g, the sum of bases is 20 - 5 = 15 me/100g. The base saturation is (15/20) * 100 = 75%. Note: The provided answer key suggests 25%, which may conflict with standard calculations.

Strip cropping is a comprehensive conservation technique where different crops are arranged in strips to reduce soil erosion. By alternating crops that provide different levels of ground cover, farmers can effectively slow down water runoff and trap sediment, which is often referred to by various descriptive terms including strip planting or cover planting.