Inter-microbial competition for N and plant NO3− uptake rather than BNI determines soil net nitrification under intensively managed Brachiaria humidicola

Brachiaria humidicola (syn. Urochloa humidicola) has been acknowledged to control soil nitrification through release of nitrification inhibitors (NI), a phenomenon conceptualized as biological nitrification inhibition (BNI). Liming and N fertilization as features of agricultural intensification may suppress BNI performance, due to a decrease in NI exudation, increased NH3 availability and promotion of ammonia oxidizing bacteria (AOB) over archaea (AOA). A 2-year three-factorial pot trial was conducted to investigate the influence of soil pH and soil microbial background (ratio of archaea to bacteria) on BNI performance of B. humidicola. The study verified the capacity of B. humidicola to reduce net nitrification rates by 50 to 85% compared to the non-planted control, irrespective of soil pH and microbial background. The reduction of net nitrification, however, was largely dependent on microbial N immobilization and efficient plant N uptake. A reduction of gross nitrification could not be confirmed for the AOA dominated soil, but possibly contributed to reduced net nitrification rates in the AOB-dominated soil. However, this putative reduction of gross nitrification was attributed to plant-facilitated inter-microbial competition between bacterial heterotrophs and nitrifiers rather than BNI. It was concluded that BNI may play a dominant role in extensive B. humidicola pasture systems, while N immobilization and efficient plant N uptake may display the dominant factors controlling net nitrification rates under intensively managed B. humidicola.

PENGARUH PEMBERIAN DOLOMIT DAN PUPUK KANDANG SAPI TERHADAP PERTUMBUHAN DAN PRODUKSI TANAMAN KEDELAI (Glycine max (L.) Merr) DI TANAH ULTISOL

This research was conducted at the ATC Experimental Garden, Faculty of Agriculture, Sriwijaya University, Indralaya, Ogan Ilir Regency, and began in September 2019 until February 2020. The analyzes of soil and plant have been carried out at the Laboratory of Chemistry, Biology, and Soil Fertility, Soil Department, Faculty of Agriculture, Sriwijaya University, Indralaya. This study aims at determining the effect of dolomite and cow manure on the growth and production of soybean (Glycine max (L.) Merr) in Ultisol Soil. This study used a factorial randomized block design with 2 treatment factors and 3 replications. The first factor is dolomite CaMg (CO3)2 consisting of two levels, 5 tons ha-1 and 10 tons ha-1. The second treatment factor is cow manure consisting of three levels, namely without manure, 10 tons ha-1, and 20 tons ha-1. The results indicated that interaction the giving of dolomite and cow manure had a significant effect in increasing soil pH, and plant N uptake. The giving of dolomite 10 tons ha-1 significantly affected the weight of 100 seeds and soybean production The giving of cow manure 20 tons ha-1 had a very significant effect on plant height, total number of pods, and the number of filled pods of soybean in Ultisols.

Nitrate and water uptake, rather than rhizodeposition, control denitrification in the presence of growing plants

<p>The main prerequisites for denitrification are availability of nitrate (NO<sub>3</sub><sup>-</sup>) and easily decomposable organic substances, and oxygen deficiency. Growing plants modify all these parameters and may thus play an important role in regulating denitrification. Previous studies investigating plant root effects on denitrification have found contradictive results. Both increased and decreased denitrification in the presence of plants have been reported and were associated with higher C<sub>org</sub> or lower NO<sub>3</sub><sup>-</sup> availability, respectively. Accordingly, it is still unclear whether growing plants stimulate denitrification through root exudation or restrict it through NO<sub>3</sub><sup>-</sup> uptake. Furthermore, reliable measurements of N<sub>2</sub> fluxes and N<sub>2</sub>O/(N<sub>2</sub>O+N<sub>2</sub>) ratios in the presence of plants are scarce.</p><p>Therefore, we conducted a double labeling pot experiment with either maize (<em>Zea mays</em> L.) or cup plant (<em>Silphium perfoliatum</em> L.) of the same age but differing in size of their shoot and root systems. The <sup>15</sup>N gas flux method was applied to directly quantify N<sub>2</sub>O and N<sub>2</sub> fluxes in situ. To link denitrification with available C in the rhizosphere, <sup>13</sup>CO<sub>2</sub> pulse labeling was used to trace C translocation from shoots to roots and its release by roots into the soil.</p><p>Plant water uptake was a main factor controlling soil moisture and, thus, daily N<sub>2</sub>O+N<sub>2</sub> fluxes, cumulative N emissions, and N<sub>2</sub>O production pathways. However, N fluxes remained on a low level when NO<sub>3</sub><sup>-</sup> availability was low due to rapid plant N uptake. Only when both N and water uptake were low, high NO<sub>3</sub><sup>-</sup> availability and high soil moisture led to strongly increased denitrification-derived N losses.</p><p>Total CO<sub>2</sub> efflux was positively correlated with root dry matter, but there was no indication of any relationship between recovered <sup>13</sup>C from root exudation and cumulative N emissions. We anticipate that higher C<sub>org</sub> availability in pots with large root systems did not lead to higher denitrification rates, as NO<sub>3</sub><sup>-</sup> was limiting denitrification due to plant N uptake. Overall, we conclude that root-derived C stimulates denitrification only when soil NO<sub>3</sub><sup>-</sup> is not limited and low O<sub>2</sub> concentrations enable denitrification. Thus, root-derived C may stimulate denitrification under small plants, while N and water uptake become the controlling factors with increasing plant and root growth.</p>

Soil Mineral and Nitrate Nitrogen, Plant N Uptake, N Use Efficiencies and Yield as Influenced by Tillage and Nitrogen Management under Wheat Crop in Sub-Tropical Eastern India

Accumulation of mineral nitrogen (min-N) in soil profile (0-90 cm) was significantly (p<0.05) higher in conventional tillage (CT) than zero tillage (ZT) treatment plots both at 42 days after sowing (DAS) and 84 DAS of wheat, the increase over ZT being 58% and 44% respectively; but at the harvest stage an opposite trend was noted. Min-N accumulation in soil also varied significantly (p<0.05) amongst N application rates with its highest value at N150 followed by subsequent reductions with decreasing N levels at all stages of wheat growth, except at harvest where N0 level had the highest accumulation (341.06 kg ha-1) which is ascribed to poor growth and very low plant N uptake. Significant effect of tillage was found on grain yield, dry matter yield and higher plant N uptake recorded under ZT plots over CT. Of the N application levels, N150 produced the highest grain yield and it was significantly (p<0.05) higher than all other N treatments. ZT showed highest nitrogen use efficiencies [agronomic efficiency (AE), physiological efficiency (PE) and apparent recovery of nitrogen (ARN)] as compared to CT, whereas maximum values of AE (36.28) and PE (29.59) was observed at N120 and these were significantly higher than all other N treatments. As anticipated, highest ARN value was seen at N60 followed by its decrease with increasing levels of N application. Relative proportion of residual inorganic nitrogen (mineral vs. nitrate) in soil profile when compared at the harvest of wheat, a very high proportion of mineral (NH4+ + NO3-) as compared to nitrate nitrogen was evident under both the tillage treatments and more so in ZT than CT plots irrespective of N levels, indicating thereby the predominance NH4+ form of nitrogen in all the layers up to 90 cm soil profile which may be explained by lower nitrification rate due to high water storage in the soil profile during the entire wheat growing season. From the results, it is inferred that plots under ZT combined with N application @150 kg ha-1 proved superior to all other treatment combinations in respect of crop yield, nitrogen use efficiencies, plant N uptake and water storage in soil profile.

Nitrogen Uptake and Use Efficiency in Sweet Basil Production under Low Tunnels

Low tunnels (LTs) enhance vegetative growth and production in comparison with open field, but it is not known whether nitrogen (N) requirements and use efficiency increase or decrease for optimal crop performance. Therefore, the purpose of this study was to determine differences in N requirement, uptake, and use efficiency in basil grown under LTs compared with open field. The experimental design each year was a split plot with four replications. The main effect (plots) was N fertilizer application rate (0, 37, 74, 111, 148, and 185 kg·ha−1) and the secondary effect (subplots) was production system (LTs covered with spun-bonded rowcover vs. open field). Plant height and stem diameter were greater under LT than open field; however, they were unaffected by N fertilizer rate. Total fresh and dry weight increased with LT by 61% and 58% and by 50% and 48% in 2017 and 2018, respectively. Optimum N rates for fresh weight (98% of peak yield) were 124 and 104 kg·ha−1 N under LT and open field, respectively. Leaf N concentration decreased under LT, but total plant N uptake increased because of increased dry weight. Without fertilization, soil available N use efficiency (SNUE) for dry weight increased by 45% and 66% in 2017 and 2018, respectively. Mixed results were obtained for N fertilizer use efficiency (NFUE) in response to N rate. In conclusion, LT increased summer production of sweet basil, total plant N uptake, and SNUE.

NO3- uptake and C exudation – do plant roots stimulate or inhibit denitrification?

&lt;p&gt;Growing plants affect soil moisture, mineral N and organic C (C&lt;sub&gt;org&lt;/sub&gt;) availability in soil and may thus play an important role in regulating denitrification. The availability of the main substrates for denitrification (C&lt;sub&gt;org&lt;/sub&gt; and NO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt;) is controlled by root activity and higher denitrification activity in rhizosphere soils has been reported. We hypothesized that (I) plant N uptake governs NO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt; availability for denitrification leading to increased N&lt;sub&gt;2&lt;/sub&gt;O and N&lt;sub&gt;2&lt;/sub&gt; emissions, when plant N uptake is low due to smaller root system or root senescence. (II) Denitrification is stimulated by higher C&lt;sub&gt;org&lt;/sub&gt; availability from root exudation or decaying roots increasing total gaseous N emissions while decreasing their N&lt;sub&gt;2&lt;/sub&gt;O/(N&lt;sub&gt;2&lt;/sub&gt;O+N&lt;sub&gt;2&lt;/sub&gt;) ratios.&lt;/p&gt;&lt;p&gt;We tested these assumptions in a double labeling pot experiment with maize (Zea mays L.) grown under three N fertilization levels S / M / L (no / moderate / high N fertilization) and with cup plant (Silphium perfoliatum L., moderate N fertilization). After 6 weeks, all plants were labeled with 0.1 g N kg&lt;sup&gt;-1&lt;/sup&gt; (Ca(&lt;sup&gt;15&lt;/sup&gt;NO&lt;sub&gt;3&lt;/sub&gt;)&lt;sub&gt;2&lt;/sub&gt;, 60 at%), and the &lt;sup&gt;15&lt;/sup&gt;N tracer method was applied to estimate plant N uptake, N&lt;sub&gt;2&lt;/sub&gt;O and N&lt;sub&gt;2&lt;/sub&gt; emissions. To link denitrification with available C in the rhizosphere, &lt;sup&gt;13&lt;/sup&gt;CO&lt;sub&gt;2&lt;/sub&gt; pulse labeling (5 g Na&lt;sub&gt;2&lt;/sub&gt;&lt;sup&gt;13&lt;/sup&gt;CO&lt;sub&gt;3&lt;/sub&gt;, 99 at%) was used to trace C translocation from shoots to roots and its release by roots into the soil. CO&lt;sub&gt;2&lt;/sub&gt; evolving from soil was trapped in NaOH for &amp;#948;&lt;sup&gt;13&lt;/sup&gt;C analyses, and gas samples were taken for analysis of N&lt;sub&gt;2&lt;/sub&gt;O and N&lt;sub&gt;2&lt;/sub&gt; from the headspace above the soil surface every 12 h.&lt;/p&gt;&lt;p&gt;Although pots were irrigated, changing soil moisture through differences in plant water uptake was the main factor controlling daily N&lt;sub&gt;2&lt;/sub&gt;O+N&lt;sub&gt;2&lt;/sub&gt; fluxes, cumulative N emissions, and N&lt;sub&gt;2&lt;/sub&gt;O production pathways. In addition, total N&lt;sub&gt;2&lt;/sub&gt;O+N&lt;sub&gt;2&lt;/sub&gt; emissions were negatively correlated with plant N uptake and positively with soil N concentrations. Recently assimilated C released by roots (&lt;sup&gt;13&lt;/sup&gt;C) was positively correlated with root dry matter, but we could not detect any relationship with cumulative N emissions. We anticipate that higher C&lt;sub&gt;org&lt;/sub&gt; availability in pots with large root systems did not lead to higher denitrification rates as NO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt; was limited due to plant uptake. In conclusion, plant growth controlled water and NO&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt; uptake and, subsequently, formation of anaerobic hotspots for denitrification.&lt;/p&gt;

The mycorrhizal tragedy of the commons

&lt;p&gt;&lt;strong&gt;The mycorrhizal tragedy of the commons&lt;/strong&gt;&lt;/p&gt;&lt;p&gt;It is increasingly recognized that plant C allocation to mycorrhizal symbionts plays a critical role for plant nutrition and the future global CO&lt;sub&gt;2&lt;/sub&gt; fertilization effect on plants (Terrer et al., 2019). At the same time its future impacts are hard to predict because we do not fully understand the mechanisms underlying the symbiosis. The traditional view of mycorrhizal symbiosis always helping plants has been challenged by observations of negative effects, e.g. on tree N uptake (N&amp;#228;sholm et al., 2013), which makes it difficult to understand why the symbiosis has evolved and why it is so widespread.&lt;/p&gt;&lt;p&gt;We propose, and tested, a theory explaining the contrasting findings by showing that mycorrhizal symbiosis can be both mutualistic and parasitic at the same time. Plants and fungi are connected in a mycorrhizal network where each fungus has multiple plant partners and vice versa. Each plant can gain additional N at the expense of the other plants by supplying more C to the fungi, i.e. paying a higher C price for N. At the same time the additional C supply increases N immobilization in fungal biomass, which reduces the total N export to all plants. Thus, an individual plant can gain N at the expense of its neighbors while the negative side effects are shared among all, resulting in a tragedy of the commons effect that reduces plant N uptake and drives N immobilization in the soil.&lt;/p&gt;&lt;p&gt;While some observations support this hypothesis, it had not yet been thoroughly tested experimentally &amp;#8211; until now. Based on laboratory and field experiments in boreal pine forest we tested both key components of this hypothesis - individual level mutualism and the community parasitism (decline in plant N uptake). We also estimated the strength of the fungal discrimination among its plant partners, which drives the competitive C for N trading. Finally, we highlight potential consequences of these mechanisms for boreal forest C allocation and responses to rising CO&lt;sub&gt;2&lt;/sub&gt;.&lt;/p&gt;&lt;p&gt;&lt;strong&gt;References&lt;/strong&gt;&lt;/p&gt;&lt;p&gt;N&amp;#228;sholm, T. et al., 2013. Are ectomycorrhizal fungi alleviating or aggravating nitrogen limitation of tree growth in boreal forests? New Phytologist, 198(1): 214-221.&lt;/p&gt;&lt;p&gt;Terrer, C. et al., 2019. Nitrogen and phosphorus constrain the CO2 fertilization of global plant biomass. Nature Climate Change, 9(9): 684-689.&lt;/p&gt;