Control of NO3 − and N2O emissions in agroecosystems: A review

Abstract

Energized electron flows through biological systems sustain nature’s complexity. They drive bacterial, archaeal, and fungal oxidation–reduction processes and enable to introduce CO2 and N2 from the atmospheric pool. Electron flux-based food webs convert soil organic matter (SOM) in virgin forest and permafrost soils, over-fertilized agricultural land, grassland systems, compost/wastewater treatment plants, oceans, rain forests, savannahs, and forests of the temperate climate zones, and have their strategy adapted on the system in which they are active. Thus, the electron driving power is responsible in our industrializing world that carbon and nitrogen returns to the atmosphere presently with an annual N2O-N proportion of 0.5 to 4.2 terragrams (Tg) or an annual atmospheric N2O-N increase of 0.25 %. N2O is a 300-times more potent greenhouse gas than CO2. Nature’s water-soluble soil carbon (CH2O)/NO3 − ratio balancing is seen as a model of how N2O emissions could be kept in a tolerable range. Sub strategies beyond are (a) an annual 400–800 terragrams (Tg) photosynthate-C (90–95 % sugars) release into plant rhizospheres, (b) spot-wise N enriching animal excrement and wide C/N ratio litter fall distributions, (c) viral shunts or life shortcuts to supporting O2 consuming, N supplying, and denitrifying recycler communities, (d) subterranean organic–inorganic soil components mixing and O2 diffusion promoting NO3 − formation, and (e) the release of nitrification inhibiting compounds as neem, karanjin, or specific humic acids which help in controlling nitrate formation and denitrification. Soil microbial transport vehicles are fungal hyphae, plant roots, and subterranean animals. Through their activities, aerobic–anaerobic gradients in the soil crumb mosaics emerge. Plant root intertissue spaces, animal guts, and co-transported soil crumbs where under carbon-dominated CH2O/NO3 − ratios preferred microbes reside are mobile locations in well-aerated soils. In such reduction-equivalent surplus environments, denitrifying communities are forced to use during anaerobic respiration available nitrate-, nitrite ions, NO, and N2O economically. Though at carbon-dominated CH2O/NO3 − ratios more N2O is reduced to N2 than in nitrate surplus environments, a complete prevention of N2O emissions is not a reality and even not desirable from the climate point of view. After describing N2O formation and emissions from a compost pile, a municipal wastewater treatment plant, a constructed wetland, and mineral N-fertilizer, sewage sludge or nitrification inhibitor-stabilized N-fertilizer amended soils with their aerobic-anaerobic mosaics, this review tries to deduce exercisable CN (CH2O/NO3) ratio shaping and N2O emission lowering strategies for ecologists, agriculturists, and waste managers in our industrializing world.