Mineral nitrogen use and impacts
The use of synthetic N undermines nutrients supplied through natural biogeochemical cycling. The life-cycle impacts of synthetic nitrogen fertiliser are significant and under-appreciated.
Analysis
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Methane Oxidizing Bacteria (MOB) namely Methylocapsa gorgona (also Methylocapsa acidiphila and Methylocapsa aurea) which is widely available in soil has the potential to utilize five additional atmospheric gases, carbon dioxide, carbon monoxide, hydrogen, nitrogen, and oxygen to supply its metabolism. It was once thought that this species only could utilize methane. This metabolic versatility might be the key to life on air and this discovery is essential for studying the biological methane sink both in soil and atmosphere.
Synthetic N addition undermines N supplied by arbuscular mycorrhizal fungi to native perennial grasses.
In 3 grassland ecosystems in China: N addition reduced soil pH and increased soil total N content at two meadow sites, P addition increased soil total P content at all three sites, but both N and P additions had minimal effects on soil organic C content. Bacteria and total microbial abundances did not change after N and P additions, while fungi and arbuscular mycorrhizal fungi (AMF) abundances were suppressed by N addition.
​Chen et al. (2019)​
Symbiotic N-fixers (Rhizobia and Actinobacteria) convert atmospheric N2 to plant-usable forms. Symbiotic N-fixers are responsible for a large fraction of biological soil-nitrogen inputs, which can increase nitrogen availability in forests in which N-fixers are locally abundant
Mineral nitrogen input decreases microbial biomass in soils under grasslands but not annual crops This is probably explained by the different carbon pathways in play in these different systems.
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In grasslands: long-term N enrichment of species-rich grasslands have shown major effects on AM functioning (Ames 2002). Under these conditions, P becomes the key nutrient that limits plant growth, and in grasslands plant P limitation can be exacerbated by the impairment of mycorrhizal functioning through excessive N enrichment (Ames 2002).
In forests: In short-term N addition experiments in the laboratory, EM fungi showed large increases in respiration in response to mineral N (Ek 1997; Bidartondo et al. 2001), and Suilloid species, which are particularly sensitive to forest N enrichment, gave up to five-fold greater increase in respiration compared with Paxillus involutus, which is considered relatively N tolerant. In the field, long-term (>10 years) N fertilization of forest plots reduced EM mycelial growth into sand ingrowth bags by approximately 50% (Nilsson and Wallander 2003). In highly N enriched soils the large C drain imposed on the hyphae by sustained uptake of high concentrations of mineral N is highly detrimental to the fungi, and the supply of C from the host plants to the fungi may be downregulated when the plants are no longer N-limited (Hobbie and Colpaert 2003).
Crop yield was 27% higher with organic amendment than mineral fertilizer. Organic amendment increased the amount of Soil Organic carbon (38%), total nitrogen (20%), microbial biomass carbon (51%) and microbial biomass nitrogen (24%) than mineral-only fertilization. Meta analysis of 690 experiments.
​Luo et al. (2018)​
Cereal production that now sustains a world population of more than 6.5 billion has tripled during the past 40 yr, concurrent with an increase from 12 to 104 Tg yrβˆ’1 of synthetic N applied largely in ammoniacal fertilizers. These fertilizers have been managed as a cost-effective form of insurance against low yields, without regard to the inherent effect of mineral N in promoting microbial C utilization. Such an effect is consistent with a net loss of soil organic C recently observed for the Morrow Plots, America's oldest experiment field, after 40 to 50 yr of synthetic N fertilization that substantially exceeded grain N removal. A similar decline in total soil N is reported herein for the same site and would be expected from the predominantly organic occurrence of soil N. This decline is in agreement with numerous long-term baseline data sets from chemical-based cropping systems involving a wide variety of soils, geographic regions, and tillage practices. The loss of organic N decreases soil productivity and the agronomic efficiency (kg grain kgβˆ’1 N) of fertilizer N and has been implicated in widespread reports of yield stagnation or even decline for grain production in Asia.
The climate change impact of cotton lint on a cradle-to-port basis was 1601 kg CO2e per tonne of cotton lint. The β€˜hot-spots’ within the emissions profile included the production and use of synthetic nitrogen (N) fertilisers (46% of emissions), the production and use of electricity and diesel used for irrigation (10%) and the production and use of diesel for farm machinery (9%). Example of how using LCA methodology highlights N use.
Chinese cropping systems are a net source of GHG emissions, and that total GHG emissions are about 12 times larger than carbon uptake by soil sequestration. The main sources of total GHG emissions are nitrogen fertilization (emissions during production and application), power use for irrigation, and soil N2O and CH4 emissions. Optimizing agronomic management practices, especially fertilization, irrigation, plastic mulching, and crop residues to reduce total GHG emissions from the whole chain is urgently required.
​Gao et al. (2018)​
The Total Energy Input required to produce 1kg durum wheat was higher in the No tillage-based system (11.05 MJ kg-1) than in the Intensive (6.80 MJ kg-1) and the Minimum one (6.78 MJ kg-1). The highest contribution to Energy Input derived from nitrogen fertiliser followed by diesel fuel.
Embodied energy in agricultural inputs. Incorporating a historical perspective
Two contrasted scenarios for the French agricultural system at the 2050 horizon have recently been designed and evaluated for their capacity to meet both the national population's food demand and environmental standards in terms of water pollution. The first scenario (O/S, for opening and specialization) assumes the continuation of the current trends of intensification, specialization, and opening to international markets. The second one (A/R/D, for autonomy, reconnection, and demitarian diet) assumes a radical change toward organic farming with diversification of crop rotations, reconnection of crop and livestock farming, and reduction of the proportion of animal proteins in the human diet. These simulations reveal that the overall CO2 emissions balance of the O/S scenario is far higher than those of the A/R/D because of the emissions associated with mineral fertilizer manufacture, and imported feed and mechanization of land management requiring a large amount of fossil fuel.
Based on the measured data, a source characterisation model was applied to estimate the methane emission rates from the upwind plants. Assuming that the estimates are representative of emissions during normal operations of a plant, we calculated the Nitrogen Gas (NG) loss rate (i.e. the ratio between NG emission rate and NG throughput). If the sampled plants are representative of the U.S. ammonia fertilizer industry, the industrial-averaged NG loss rate (Β± standard deviation) is estimated to be 0.34% (Β±0.20%), and the total methane emissions (Β± standard deviation) from this industry are estimated to be 29 (Β±18) Gigagram per year (Gg CH4/yr) in 2015–2016. This is significantly higher than the reported methane emissions of 0.2 Gg CH4/yr from the U.S. EPA’s Facility Level Information on Greenhouse Gas Tools (FLIGHT). This study begins to fill an important knowledge gap in quantifying methane emissions along the NG value chain, and demonstrates the capability of mobile sensing for characterizing airborne emissions.
Two prospective scenarios for French agriculture at the 2040 horizon are described.β€’ O/S pursues the current trends of agri-culture opening and specialization.β€’ A/R/D considers autonomy of farming systems, reconnection and a demitarian diet.β€’ The storylines are translated into a quantitative nutrient fluxes description.β€’ The A/R/D scenario can feed France with better environmental performances.
Three things that make the difference when modelling environmental impact of food systems:
Capping total calories produced per capita (3000).
Less animal and more plant foods
Reducing inputs through establishment of complex agro-ecosystems (Crop-crop and crop-animal synergies become the main drivers of land productivity instead of synthetic inputs)
LCA methodology counting the positive contribution of manure on pasture-based livestock systems as a replacement for mineral fertilizer.
​Leip et al. (2019)​
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