Nitrogen, Volume 2, Issue 1 (March 2021) – 7 articles

Nitrous oxide (N₂O) is one of the main greenhouse gases and its emissions from vegetable production systems have brought a sustainability challenge. The objective of this study was to evaluate the potential of reducing N₂O emissions from silt loam soil by mixing nitrogen (N)-rich broccoli (Brassica oleracea var. italica) residue with wheat straw or water-washed wheat straw. An experiment was conducted in randomized complete block design with five treatments; unamended or control (BS), wheat straw (+S), broccoli residue (+CR), broccoli residue and wheat straw (+CR+S) and broccoli residue and washed wheat straw (+CR+Sw) and was replicated four times. The +CR and +S were added at the rate of 3.5 kg and 2.0 kg fresh matter m⁻² and their mixtures, +CR+S and +CR+Sw, were incorporated in 3.5 kg of silt loam soil at 60% water-filled pore space (WFPS) and packed in soil microcosms. Nitrous oxide emissions were measured once a day during the 14-day of study period. Daily fluxes of N₂O were found to be reduced on +CR+W and +CR+Sw when compared to single-amended +CR treatment. Similarly, N₂O fluxes on +CR+Sw (2772 µg N m⁻² h⁻¹) were significantly lower than +CR+S (3606 µg N m⁻² h⁻¹) soon after the amendment but did not vary significantly thereafter. Moreover, the amendment mixture, +CR+S and +Cr+Sw, resulted in lower net N₂O emissions by 73.3% and 74.2%, respectively, relative to +CR treatment. While the results clearly suggest that the +CR+S or +CR+Sw reduced N₂O emissions, it necessitated further studies, possibly by increasing the frequency of sampling to clarify if washed wheat straw would further mitigate N₂O emissions from the vegetable production system. Full article

We investigated the nitrogen pools in monocultures of legume species widely used in reforestation in Brazil that have contrasting growth and nitrogen acquisition strategies. The plantations were established with the slow-growing and N₂-fixing tree Anadenanthera peregrina var. peregrina, and the fast-growing and non-fixing tree Schizolobium parahyba var. amazonicum. The measurements of N pools in the tree biomass and the soil followed standard methods and were carried out on 54 experimental plots. The N₂ fixation pools were evaluated by abundance natural of ¹⁵N and the N accretion methods. The soil N content was of similar magnitude between species and stand densities. The species showed similar amounts of N in the biomass, but divergent patterns of N accumulation, as well as the ¹⁵N signature on the leaves. S. parahyba accumulated most N in the stem, while A. peregrina accumulated N in the roots and leaves. However, the N accumulation in biomass of A. peregrina stand was less constrained by environment than in S. parahyba stands. The percentage of N derived from N₂ fixation in A. peregrina stands decreased with the increase of stand density. The biological N₂ fixation estimates depended on the method and the response of tree species to environment. Full article

Two contrasting estuarine copepods, Acartia tonsa and Eurytemora carolleeae, the former a broadcast spawner and the latter a brood spawner, were fed a constant carbon-based diatom diet, but which had a variable N:P content, and the elemental composition (C, N, P) of tissue and eggs, as well as changes in the rates of grazing, excretion, egg production and viability were measured. To achieve the varied diet, the diatom Thalassiosira pseudonana was grown in continuous culture at a constant growth rate with varying P supply. Both copepods altered their chemical composition in response to the varied prey, but to different degrees. Grazing (clearance) rates increased for A. tonsa but not for E. carolleeae as prey N:P increased. Variable NH₄⁺ excretion rates were observed between copepod species, while excretion of PO₄³⁻ declined as prey N:P increased. Egg production by E. carolleeae was highest when eating high N:P prey, while that of A. tonsa showed the opposite pattern. Egg viability by A. tonsa was always greater than that of E. carolleeae. These results suggest that anthropogenically changing nutrient loads may affect the nutritional quality of food for copepods, in turn affecting their elemental stoichiometry and their reproductive success, having implications for food webs. Full article

The goal of the European Nitrate Directive 91/676/CEE is to mitigate or prevent water pollution associated with the nitrogen (N) cascade. Vegetable crops have a high risk of nitrate leaching during autumn and winter. Information about the fate of N from artichoke (Cynara cardunculus L. var. scolymus (L.)) residues is reviewed and then supplemented with a three-year study with ¹⁵N-labelled residues in an artichoke-cauliflower (Brassica oleracea L. cv. botrytis) rotation in six lysimeters. After three years, 6% of N in artichoke residues was leached, 8% was exported by crops, while 86% remained in the lysimeter. Summed over the rotation, 16% of artichoke-residue N was absorbed by artichoke and 14% by cauliflower. Total aboveground N uptake by all crops during the entire rotation ranged from 370 to 534 kg N ha⁻¹, of which 207–311 kg N ha⁻¹ returned to the soil as residues. Increasing N-recycling efficiency and reducing the risk of N leaching while conserving crop productivity requires capturing N mineralized from soil organic N. Cauliflower performs this capture effectively during the drainage period. A break crop should be introduced after the first and second harvests of artichoke to further synchronize N mineralization and uptake and reduce leaching risk during the rotation. Full article

Soybean (Glycine max.) is one of the most important legumes cultivated worldwide. Its productivity can be altered by some biotic and abiotic stresses like global warming, soil metal pollution or over-application of herbicides like paraquat (1,1’-dimethyl-4,4’-bipyridinium dichloride). In this study, the effect of oxidative stress produced by paraquat addition (0, 20, 50 and 100 µM) during plant growth on symbiotic nitrogen fixation (SNF) and functionality of Bradyrhizobium diazoefficiens-elicited soybean nodules were evaluated. Results showed that the 50 µM was the threshold that B. diazoefficiens can tolerate under free-living conditions. In symbiosis with soybean, the paraquat addition statistically reduced the shoot and root dry weight of soybean plants, and number and development of the nodules. SNF was negatively affected by paraquat, which reduced total nitrogen content and fixed nitrogen close to 50% when 100 µM was added. These effects were due to the impairment of nodule functionality and the increased oxidative status of the nodules, as revealed by the lower leghaemoglobin content and the higher lipid peroxidation in soybean nodules from paraquat-treated plants. Full article

Seven winter and five summer vegetables produced under organic and conventional systems were collected from a supermarket seven times between January and April and between July and October for winter and summer vegetables, respectively, and their ascorbic acid and total phenolic content (compounds with proven antioxidant activity) as well as total antioxidant capacity, soluble solids and nitrates were determined. The results clearly indicated that, from the three factors studied (vegetable species, cropping system and sampling time), vegetable species made the highest contribution to ascorbic acid, phenolics, antioxidant capacity, soluble solids and nitrates. Results for each vegetable species showed that most organic vegetables appear to have lower nitrate content, some have higher phenolics, antioxidant capacity and soluble solids, and only few have higher ascorbic acid compared with conventional vegetables. The significance of the differences in nutritional and antioxidant value between organic and conventional vegetables is questionable, since vegetable species and sampling time can affect their nutritional value to a great or greater extent than the cropping system. Full article

Most nitrogen (N) in organic fertilizers must be mineralized to become available to plants, a process in which microorganisms play crucial roles. Droughts may impact microorganisms associated with the N cycle, negatively affecting N mineralization and plant N supply. The effects of drought on N-related processes may further be shaped by the farming system. We buried ¹⁵N-enriched plant material and reduced precipitation in conventionally and organically (biodynamically) managed wheat fields. On two sampling dates, we evaluated the soil water content, plant parameters and the plants’ ¹⁵N isotope signature. We intended to study the microbial communities associated with the N cycle to link potential treatment effects on plant N provisioning with characteristics of the underlying microbial community. However, floods impaired the experiment after the first sampling date, and the molecular work on the microbial communities was not performed. Focusing on the pre-flooding sampling date, our data suggested that processes associated with N transformation are sensitive to drought, but the role of the farming system needs further investigation. Since the underlying research question, the set-up and the lessons learned from this study may guide future experiments, we presented improvements to the set-up and provided ideas for additional analyses, hoping to promote research on this topic. Full article