A computational history and outlook of nitrogen use efficiency and precipitation-optimal fertilisation timings in agriculture

<p>Half of the nitrogen applied to arable-fields is lost through several processes linked to soil moisture. Low soil moisture limits nitrogen mobility reducing nitrogen-uptake while wetter conditions can increase nitrogen leaching. Rainfall ultimately governs soil moisture and the fate of nitrogen in soil. However, the interaction between rainfall and nitrogen use efficiency (NUE) remains poorly understood.</p> <p>We developed a field-scale modelling platform that describes coupled water and nitrogen transport, root growth and uptake, rainfall, the nitrogen-cycle and leaching to assess the NUE of split fertilisations with realistic rainfall patterns. The model was solved for every possible split fertilisation timing in 200+ growing seasons to determine optimal timings. Two previous field trials regarding rainfall and NUE had contrasting results: wetter years have enhanced fertiliser loss and drier years reduced plant nitrogen uptake. By choosing appropriate fertilisation timings in the model we could recreate the two contrasting trends and maintain variability in the data. However, we found by choosing other fertilisation timings we could mitigate the leaching in wetter years. Optimised timings could increase plant nitrogen uptake by up to 35% compared to the mean in dry years. Plant uptake was greatest under drier conditions due to mitigated leaching, but less likely to occur due to low nitrogen mobility. Optimal fertilisation timings varied dramatically depending on the rainfall patterns. Historic and projected rainfall patterns from 1950-2069 were used in the model. We found optimal NUE has a decrease from 2022-2040 due to increased heavy rainfall events and optimal fertilisation timings are later in the season but varied largely on a season-to-season basis.</p> <p>The results are a step towards achieving improved nitrogen efficiency in agriculture by using the ‘at the right time’ agronomic-strategy in the ‘4Rs’ of improved nitrogen fertilisation. Our results can help determine nitrogen fertilisation timings in changing climates.</p>

The Benefits of Mycorrhizae are Frequency-Dependent: A Case Study With a Non-Mycorrhizal Mutant of Pisum Sativum

ABSTRACTMutualisms are remarkably common in the plant kingdom. The mycorrhizal association which involves plant roots and soil fungi is particularly common, and found among members of the majority of plant families. This association is a resource-resource mutualism, where plants trade carbon-based compounds for nutrients, such as phosphorus and nitrogen, mined by the fungi.Evolutionary models usually assume that a mutation grants a small number of individual plants the ability to associate with mycorrhizal fungi, and that this subsequently spreads through the population resulting in the evolution of mutualism. This frequency-dependent hypothesis has been difficult to test, because it is rare to have members of the same species that are capable and incapable of forming the mutualism.Here we describe the results of an experiment that took advantage of a mutant pea (Pisum sativum L. R25) that is incapable of forming mycorrhizal (or rhizobial) associations, and differs from the wildtype (P. sativum cv. Sparkle) by a single recessive Mendelian allele (Pssym8). We grew each genotype either alone or in every combination of pairwise mixed- or same-genotype. We also present an evolutionary matrix game, which we parameterize from the experimental 15N results, that allows us to estimate the costs and benefits of the mutualism.We find that there was no difference between R25 and WT when grown with a competitor of the same genotype, but when R25 and WT compete, WT has a significant fitness advantage. From the model, we estimate that the benefit in units of fitness (g pod mass) obtained from direct plant nitrogen uptake is 22.2 g, and mycorrhizae increase this by only 0.6 g. The costs of plant nitrogen uptake are 9.4 g, while the cost of trade with mycorrhizae is 0.1g.From the model and experiment, we conclude that this relatively small cost-benefit ratio of the mycorrhizal association is enough to drive the evolution of mutualism in frequency-dependent selection. However, without the mutant R25 genotype we would not have been able to draw this conclusion. This validation of frequency-dependent evolutionary models is important for continued theoretical development.

Influence of Soil Properties on Maize and Wheat Nitrogen Status Assessment from Sentinel-2 Data

Soil properties variability is a factor that greatly influences cereals crops production and interacts with a proper assessment of crop nutritional status, which is fundamental to support site-specific management able to guarantee a sustainable crop production. Several management strategies of precision agriculture are now available to adjust the nitrogen (N) input to the actual crop needs. Many of the methods have been developed for proximal sensors, but increasing attention is being given to satellite-based N management systems, many of which rely on the assessment of the N status of crops. In this study, the reliability of the crop nutritional status assessment through the estimation of the nitrogen nutrition index (NNI) from Sentinel-2 (S2) satellite images was examined, focusing of the impact of soil properties variability for crop nitrogen deficiency monitoring. Vegetation indices (VIs) and biophysical variables (BVs), such as the green area index (GAI_S2), leaf chlorophyll content (Cab_S2), and canopy chlorophyll content (CCC_S2), derived from S2 imagery, were used to investigate plant N status and NNI retrieval, in the perspective of its use for guiding site-specific N fertilization. Field experiments were conducted on maize and on durum wheat, manipulating 4 groups of plots, according to soil characteristics identified by a soil map and quantified by soil samples analysis, with different N treatments. Field data collection highlighted different responses of the crops to N rate and soil type in terms of NNI, biomass (W), and nitrogen concentration (Na%). For both crops, plots in one soil class (FOR1) evidenced considerably lower values of BVs and stress conditions with respect to others soil classes even for high N rates. Soil samples analyses showed for FOR1 soil class statistically significant differences for pH, compared to the other soil classes, indicating that this property could be a limiting factor for nutrient absorption, hence crop growth, regardless of the amount of N distributed to the crop. The correlation analysis between measured crop related BVs and satellite-based products (VIs and S2_BVs) shows that it is possible to: (i) directly derive NNI from CCC_S2 (R2 = 0.76) and either normalized difference red edge index (NDRE) for maize (R2 = 0.79) or transformed chlorophyll absorption ratio index (TCARI) for durum wheat (R2 = 0.61); (ii) indirectly estimate NNI as the ratio of plant nitrogen uptake (PNUa) and critical plant nitrogen uptake (PNUc) derived using CCC_S2 (R2 = 0.77) and GAI_S2 (R2 = 0.68), respectively. Results of this study confirm that NNI is a good indicator to monitor plants N status, but also highlights the importance of linking this information to soil properties to support N site-specific fertilization in the precision agriculture framework. These findings contribute to rational agro-practices devoted to avoid N fertilization excesses and consequent environmental losses, bringing out the real limiting factors for optimal crop growth.

Temporal Dynamics of Nitrous Oxide Emission and Nitrate Leaching in Renovated Grassland with Repeated Application of Manure and/or Chemical Fertilizer

Managed grassland is occasionally renovated to maintain plant productivity by killing old vegetation, ploughing, and reseeding. This study aimed to investigate the combined effect of grassland renovation and long-term manure application on the temporal dynamics of nitrous oxide (N2O) emission and nitrate nitrogen (NO3−–N) leaching. The study was conducted from September 2013 to September 2016 in a managed grassland renovated in September 2013. In this grassland, two treatments were managed—chemical fertilizer application (F treatment) and the combined application of chemical fertilizer and beef cattle manure (MF treatment)—for eight years before the renovation. The control treatment without fertilization (CT) was newly established in the F treatment. The soil N2O flux was measured using a closed chamber method. A leachate sample was collected using a tension-free lysimeter that was installed at the bottom of the Ap horizon (25 cm deep), and total NO3−–N leaching was calculated from leachate NO3−–N concentration and drainage volume was estimated by the water balance method. In the first year after renovation, the absence of plant nitrogen uptake triggered NO3−–N leaching following rainfall during renovation and increased drainage water after thawing. NO3−–N movement from topsoil to deeper soil enhanced N2O production and emission from the soil. N2O emission in MF treatment was 1.6–2.0 times larger than those of CT and F treatments, and NO3−–N leaching in MF treatment was 2.3–2.6 times larger than those of CT and F treatments in the first year. Mineral nitrogen release derived from long-term manure application increased NO3−–N leaching and N2O emission. In the second year, N2O emission and NO3−–N leaching significantly decreased from the first year because of increased plant N uptake and decreased mineral nitrogen surplus, and no significant differences in N2O emission and NO3−–N leaching were observed among the treatments. In the second and third years, NO3−–N leaching was regulated by plant nitrogen uptake. There were no significant differences in NO3−–N leaching among the treatments, but N2O emission in MF treatment was significantly smaller than in the F treatment. Long-term manure application could be a possible option to mitigate N2O emission in permanent grassland; however, the risk of increased NO3−–N leaching and N2O emission in the renovation year induced by manure nitrogen release should be noted.