N-Cycle

         It can be simply explained as applying the N-fertilizer so that there is adequate mixing with or movement into the soil. Once in the soil the N is either dissolved into soil solution and or adsorbed onto soil particles (clay surfaces or humus material). The ideal goal of adding N-containing fertilizer to a field is to place it where plant roots will contact and absorb it. Once in the plant roots it can be translocated within the plant and used to build plant components such as protein molecules. Each form of N fertilizer has features that can make it more or less subject to losses from the soil-plant system or positionally available to crops. To understand how losses occur it is useful to study the N-cycle.

The N- Cycle can be illustrated in different ways, but is often shown as a series of boxes or circles with arrows showing movement from and into the boxes. The boxes represent various components or pools of N that exist in the environment and each pool is made up of N in different molecular forms. 

         The majority of N present in the environment is N2 gas making up 78% of the atmosphere. The N2 gas is unavailable to plants unless it is combined with hydrogen and oxygen through a series of chemical or bio-chemical reactions called fixation. Fixation can occur naturally by soil microbes that take N2 gas and bio-chemically change it over to NH4+. Some of these microbes (i,e, Rhyzobia species bacteria) accomplish this by symbiotically fixing the N2 gas using energy from the legume plants on whose roots they live, while other free-living bacteria accomplish this on their own in the soil environment. Chemical N fixation can occur naturally by the strong oxidation reaction of lightning discharge in the atmosphere or industrially in fertilizer plants where hydrogen obtained from methane is combined with N2 gas and reacts in the presence of a catalyst under intense heat and pressure to form ammonia gas (NH3). It can be further industrially modified into ammonium, nitrate and urea. Plants primarily take up nitrogen into their root systems in either of the two ionic N forms ammonium (NH4+) or nitrate (NO3-). There can be some absorption of N through leaves as N2O, NH4+, NO3- or urea (CO(NH2)2) but this absorption by crops is a small amount compared to root uptake. The cation NH4+ is relatively immobile in soil and is adsorbed onto negatively charged soil colloids (clays and or organic matter). The anion NO3- is quite mobile in soil and moves by mass flow as water moves through the soil. The majority of N uptake by plants in most well-drained soils is in the NO3- form resulting from the normal oxidation of NH4+ to nitrite (NO2-) and further to NO3-. This NH4+ oxidation is called nitrification and is accomplished by two groups of soil bacteria.

         The amount of total N in the soil at any one time is mostly in the organic form (97-98%). It is contained in the organic molecules of soil humus, plant residues, soil fauna, soil microbes or animal wastes. These organic molecules are too large to be absorbed through plant root membranes and need to be decomposed and modified by soil fauna and microbes into the ionic forms NH4+ or NO3-. This whole decomposition and modification process is called mineralization, simply meaning changing organic N forms to mineral N forms. This is not a one-way trip for N as the ionic forms of NH4+ and NO3- can be returned to an organic form as soil microbes require and use N to decompose high carbon to nitrogen ratio containing plant residues.

         The whole N cycle is in a constant state of change and can be described as a type of dynamic equilibrium. Nitrogen fixation from the atmosphere to the soil-plant system is balanced off by a few possible pathways of losses. One is the process of denitrification of NO3- where NO3- is reduced to nitrous oxide (N2O) or N2 gases under conditions of low oxygen concentrations in the soil such as water-logged conditions. This is done by soil bacteria that need oxygen to carry out metabolic activities and usually use oxygen gas from the soil atmosphere but can use oxygen from NO3- when necessary. Another is volatilization losses of NH3 from the soil surface to the atmosphere by the reduction of NH4+ to NH3 or the hydrolysis of urea or other N-containing organic molecules resulting in free ammonia. Lastly N can also be lost from the soil-plant system through leaching of NO3- as water moves through and out of the soil into ground water or in some cases surface water.

         The goal of N fertilization is to add N-containing fertilizer in a form and place it in a location to minimize losses and maximize plant uptake. It is useful to look at the commonly available N-fertilizers with reference to the N-cycle to help understand how to minimize losses and maximize plant uptake. In summary the losses are volatilization of NH3 and denitrification and leaching of NO3-. In the short-term N-fertilizers that are in the NH4+ form, or react to form NH4+, and have good contact with the soil are not subject to losses. All N-fertilizers will end up primarily as NO3- however if given enough time and warmth (soil temperatures greater than 10oC but less than 30oC) and adequate but not excessive moisture.