Introduction
There is a high demand for crops production as our population keeps increasing around the world. New methodologies have been developed to maintain a fast but effective way to produce high agricultural yields. On a daily basis, costumers have the option to purchase conventional and organic-grown vegetables, and might choose the latter due to the health benefits it claims to provide with its more natural-based harvesting conditions. Though there is a regimen that must be met for a product to be labeled as “organic”, it is possible for brands to falsely assert their crops to be grown under organic-mandated conditions. There are different factors that constitute the term of “organic”, including the prohibited use of synthetic fertilizers. In order to grow crops in a faster and low-cost way in conventional farming, synthetic fertilizers are added to get the necessary yield to sell in the market. These fertilizers contain
While organic farming does not rely on synthetic fertilizers, pesticides, or any other prohibited substance, it uses animal- and/or plant-based products for harvesting (USDA 2016). Though synthetic fertilizers allow the production of high yields a fast rate, the extensive use of these have had detrimental effects at a global scale. The N loss increments because of synthetic fertilizers have negatively affected the soil nitrogen use efficiency and productivity, increased N gas emissions, drinkable water quality, human and environmental health (Li et al. 2017). It is important to understand how fertilizers alter the N-cycling in the soil-plant systems in order to minimize the negative side effects. Also, it is critical to improve the monitoring of N to identify the N sources in soil, fertilizers, vegetation, and water.
Stable nitrogen isotopes
The stable nitrogen isotopes forms differ in atomic mass and abundance in the atmosphere, with 99.6337% of 14N and 0.3663% of 15N (Bedard-Haughn et al. 2007).
According to the standard 15N/14N isotope ratio, the more negative δ15N value is, the more depleted the sample is from the heavy isotope. Whereas a positive δ15N value indicates the sampling is more 15N-enriched. (Inácio et al. 2015)
About half of the N applied to the crops by synthetic fertilizers is lost due to processes in the soil such as nitrification, ammonia (NH3) volatilization, and denitrification (Li et al. 2017). When N availability is low in mycorrhizal plants, there is discrimination against 15N causing plants to be depleted in this heavy isotope. When there is higher N availability, plants do not rely as much in fungi and become embellished in 15N due to the increased mineralization resulting in gaseous N loss (Chapin et al. 2012).
Tracking methods
Stable N isotopes can be used for the detection of inaccurate organic labeling. The tracking of 15N allows the monitoring of synthetic fertilizer in agricultural products. There are different N isotope tracking mechanisms, such as 15N-enriched method and 15N-natural abundance method.
Case studies using δ15N tracking method
The synthetic fertilizers are known for being depleted in 15N, resulting in more negative or low positive values of δ15N ranging from -3.9 to +0.5 per mil (‰). In the other hand, organic fertilizers are more enriched in δ15N, having values from +5.3 to +7.2 ‰ in manures and +9.3 to +20.9‰ in composts. In order to understand more closely the limitations of the δ15N tracking method to differentiate fertilizers, a study in Brazil compared the c of soil, fertilizer, and plant samples from three harvesting sites. This experiment lasted a year and did the sampling at an organic farm with animal manure, an experimental organic farm with green manure (legumes) and fermented product without the use of compost, and a conventional farm with synthetic N fertilizer and animal manure. The results showed there was a substantial difference between the experimental farm and organic farm δ15N values in soil, rather not much significance between the organic farm and conventional farm soil δ15N values. This implies there is a higher δ15N signatures in soil when animal manure is applied. Additionally, the lettuce δ15N signatures in both organic farms were significantly different. The lettuce δ15N values in the organic farm were higher than in the experimental organic farm. This indicates the varying N sources in the lettuce samples at these two organic farms, being animal manure (higher δ15N) and green manure (lower δ15N). The lettuce at the experimental organic farm seemed to use the N from the fermented product and soil N with higher δ15N signatures, rather than utilizing the legume N with lower δ15N values. Conversely, there was an overlapping between the lettuce δ15N content in experimental organic farm and conventional farm. This overlay is suggested to be induced by the broader range of δ15N signatures in organic input there was at the conventional farm due to the use of both animal manure and synthetic fertilizer. Because the conventional farm had been supplied by synthetic N fertilizer, the δ15N values of the organic input and lettuce did not pair. This shows the effect of adding 15N-depleted fertilizer to the crops regardless of the manure, which resulted in lower δ15N values in the plant product. This experiment took into account the need to incorporate different sampling methods, such as organic and conventional with and without manure, to more accurately identify legitimately organic products. But overall, the study clearly demonstrated the implications fertilizers have on δ15N, with a pattern of lower values in crops grown with synthetic fertilizer and higher δ15N signatures in products harvested under organic farming conditions (Inácio et al. 2015).