Maintaining higher grain production with less reactive nitrogen losses in China: A meta-analysis study.

Managing reactive nitrogen (Nr) in agricultural production is crucial for addressing the triple challenges of food security, climate change and environmental degradation. Intensive work has been conducted to investigate the effects of mitigation strategies on reducing Nr losses by ammonia emission (Nr-NH3), nitrous oxide emission (Nr-N2O) and nitrate leaching (Nr-NO3-) separately. This meta-analysis evaluated the efficiency of each strategy in mitigating Nr losses coupled with grain yield responses. The results indicate that producing one Megagram (Mg) of wheat grains caused higher Nr losses, twice that of rice and 17% that of maize. The Nr-NH3 and Nr-NO3- were the dominant sources of Nr losses of the three crops (96%), while Nr-NH3 only presented 86% of the total Nr losses for rice. Reducing the N rate strategy decreased the yield by 33% and the Nr losses by 62% compared with the conventional rate (150-250 kg N ha-1) as an average of the three crops. In contrast, increasing the N rate higher than 250 kg N ha-1 amplified the yield by 15% but also caused a 71% increase in Nr losses compared with the conventional rate. Although subsurface application decreased Nr losses by 5%, this study rejected this approach as an effective strategy due to a 4% yield decline on average of the grain crops. Slow-release fertilizers decreased Nr-NH3 and Nr-N2O losses by 41-58% and 54-89%, respectively, of the highest losses under urea in the three crops, but also led to yield reductions. Organic amendments achieved the highest drop in Nr-NO3- loss by 66% in maize coupled with yield declines. Biochar increased wheat and maize yields by 0.3 and 0.1 Mg, respectively, coupled with 1 kg reduction in Nr losses. On average, inhibitors augmented the grain yields by 0.2 Mg ha-1 for each 1 kg decline in Nr losses. In conclusion, for sustainable agricultural intensification, biochar (for wheat only) and inhibitors (for the three crops) are strongly recommended as mitigation strategies for Nr losses from grain crop production systems in China.

The food nitrogen footprint for African countries under fertilized and unfertilized farms.

Although nitrogen (N) is a limiting factor for food production (FP) in Africa, and African food security is seriously threatened by the phenomenon of soil N depletion, there is a dearth of information that shows the points to focus on throughout the chain of FP and food consumption (FC) in all African countries to minimize N loss while securing food N supply. Food N footprint (NF) is an indicator for tracing the losses of reactive N (Nr) with regard to the FP and FC chain. This is the first study to calculate the food NF for all African countries under fertilized and unfertilized farms, by calculating two sets of virtual N factors (VNFs; kg Nr released to the environment kg-1 N in consumed product): one for unfertilized farms (the unfertilized scenario) and one for fertilized farms (the fertilized scenario). The fertilized and unfertilized VNFs were utilized to calculate a weighted average set of VNFs (the combined scenario). From the percentage of farms that utilize N fertilizer, and the N percentage in production that comes from soil depletion, the proportion used for the combined scenario was determined. Soil N depletion factors (SNDFs; kg N taken from the unfertilized soil kg-1 N in food consumed) were also computed to identify the quantity of N extracted from the soil for food production. We have also provided the changes in N inputs, N outputs, and N use efficiency (NUE) for North Africa and Sub-Saharan Africa (SSA) during the last 57 years. The average total N input to croplands increased from 24 and 19 kg N ha-1 yr-1 in 1961-1965 to 100 and 42 kg N ha-1 yr-1 in 2010-2017 for North Africa and SSA, respectively. The NUE declined from 109% and 67% (1961-1965) to 47% and 63% (2010-2017) for North Africa and SSA, respectively. The total average per-capita food NF was 11 and 5.8 kg N cap-1 yr-1 in unfertilized farms; 21 and 14 kg N cap-1 yr-1 in fertilized farms; and 19 and 7.5 kg N cap-1 yr-1 under the combined scenario for North Africa and SSA, respectively. Vegetable-fruit and beef have the highest SDNFs in Africa. FP in Africa contributes approximately 70% of the total food NF. Therefore, if possible, the best way for Africans to reduce soil N depletion and N emissions is to encourage the production and consumption of livestock and crops products with less VNF and SNDF. However, African people do not have this luxury of choice because of poverty and ignorance. Therefore, African policy-makers must adopt integrated approaches that provide effective tools to control the production of animals and crops in conjunction with the improvement of NUE. Trying to completely change the African agricultural system is impossible, but strategies must be developed to reduce soil depletion in a gradual way, as well as a shift towards low-VNF foods.

How much nitrogen does Africa need to feed itself by 2050?

Nitrogen (N) fertilizers are very important for global food self-sufficiency (FSS), particularly for Africa, where the N input in agriculture is very low. This is the first work which studies and calculates the amount of N fertilizer that each country in Africa needs to feed itself by 2050. In this study, we used five different scenarios of inorganic fertilizer N (IFN) use and human diets to calculate the amount of N fertilizer needed to achieve FSS in Africa by 2050 and analyze the changes in N budget; N losses and N use efficiency (NUE). These scenarios include 1) business as usual (BAU), 2) equitable diet (EqD; self-sufficiency), 3) an IFN input 20% less than the EqD (S1), 4) an IFN input 40% less than the EqD (S2), and 5) a 20% increase in IFN input relative to the EqD (S3). Under the BAU scenario, production trends continue as they have over the past five decades, including an unhealthy human diet. In the EqD scenario, the priority is to meet the local demand for both animal and plant proteins with a healthy human diet. Under the EqD scenario, increasing the total N input from 35 kg N ha-1 yr-1 to 181 kg N ha-1 yr-1 during 2016-2050 is needed to achieve FSS in Africa. This increase in N fertilizer use represents unprecedented N inputs to African terrestrial ecosystems - at least 52 Tg N yr-1 - which would lead to inevitable increases in N losses. We also found that the NUE would decrease from 63% during 2010-2016 to 50% by 2050, whereas the total N surplus would increase from 13 kg N ha-1 yr-1 to 90 kg N ha-1 yr-1 by 2050. The estimated gaseous emissions would increase from 8 kg N ha-1 yr-1 to 61 kg N ha-1 yr-1 by 2050. Our findings conclude that, it is very important to consider the high N losses in Africa if the EqD scenario is applied. The S1 and S2 scenarios result in much less environmental N loss, and better NUE compared with the EqD scenario. Therefore, based on these findings we can recommend the implementation of the S2 scenario with an IFN dose of 77 kg N ha-1 yr-1, in parallel with the use of modern agricultural techniques and the increased use of organic inputs.