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.

Impact of manure and biochar additions on annual crop growth, nutrient uptake, and fate of 15N-labelled fertilizer in two contrasting temperate prairie soils after four years

Liquid hog manure (LHM) and solid cattle manure (SCM) co-applied with biochar could beneficially influence soil nitrogen (N) and phosphorus dynamics. A split-plot design was used at two sites (Brown and Black) in Saskatchewan to assess LHM and SCM (100 kg N ha-1) applied alone or in combination with biochars (8 Mg carbon ha-1) produced using slow or fast pyrolysis. Crop growth and nutrient uptake, along with fertilizer 15N recovery were followed over four years in a cereal-oilseed rotation. Crop growth on the Brown soil was more responsive to the treatments than the Black soil, reflecting lower fertility of the Brown soil. The manure and biochar, applied alone or in combination, had little impact on available soil phosphorus. Manure and biochar effects on crop growth and nutrient uptake were attributed to temporal effects on soil N immobilization-mineralization influencing plant available soil N. A negative impact of the fast pyrolysis biochar on growth and plant uptake was observed and attributable to its greater labile-carbon content, which likely promoted soil N immobilization. Synergism observed between SCM and the slow pyrolysis biochar may reflect enhanced net SCM-N mineralization and increased water-holding capacity. The majority (55-80%) of plant 15N recovery occurred during the first year, with 86% of fertilizer 15N conserved within the soil-plant system after four years. Greater (40%) plant 15N recovery without biochar addition, coupled with increased (38%) soil 15N recovery with added biochar, suggests biochar-related immobilization and/or sorption in the biochar-amended soils.

The mycorrhizal tragedy of the commons

<p>Ectomycorrhizal fungi (EMF) play a key role in the cycling of nitrogen (N) and carbon (C) in boreal forests. Trees receive growth-limiting N in exchange for allocating C to their mycorrhizal symbionts, but supplying the fungi with C can also cause N immobilization, which hampers tree growth. We present results from field and greenhouse experiments combined with mathematical modelling, showing that these are not conflicting outcomes.</p><p>Under N limitation, which is the general case in boreal forests, the plant host has been observed to continue supplying its ectomycorrhizal partner with C, and even increasing this C investment, while the fungus reduces mobilization of N to its host (Corrêa et al. 2008, 2010). N is thus withheld under conditions of limiting availability, and the host tree cannot unlock it by supplying the EMF with more C, because such an investment results in further diminishing N returns. Critical to this question is the observation that more than one fungus can form mycorrhiza on a given tree and that several trees can be connected to a given fungal individual (Southworth et al. 2005).</p><p>We hypothesize that plants sharing common ectomycorrhizal symbionts compete with each other for N by exporting C to the EMF network, and vice versa for a fungus. The fungi making up the EMF network export N to hosts if it is absorbed in excess of their own growth demand, which is limited by C; Exporting more than this would reduce their growth, exporting less would reduce their competitiveness for plant C (Näsholm 2013, Franklin 2014). This hypothesis has specific and predictable implications for relationship between plant C export to EMF and N uptake: At the community level, increasing plant C supply to EMF would increase both fungal N uptake and N use, but as soil N availability gradually becomes limiting, uptake should saturate while EMF N use continued to increase, leading to declining N export to plants.</p><p>We conducted two experiments, one in potted mesocosms and the other in a boreal forest setting. Belowground C flux was reduced by shading and/or stem strangling, which is a treatment whereby the flow of C to the root system is physically restricted by blocking transport through the phloem in the stem (Björkman 1944; Henriksson et al. 2015). Strangling a subset of seedlings growing in the same pot accomplishes two things: 1) the total belowground C flux is decreased, and 2) each seedling’s relative contribution to that flux is altered.</p><p>Based on measurements and mathematical modelling, we conclude that belowground C allocation by trees can indeed fuel N immobilization, reducing the amount of N to be distributed among the trees. But we also found that individual trees received nutritional benefits in proportion to their C contribution to the fungal network. We illustrate the evolutionary underpinnings of this situation by drawing on the analogous tragedy of the commons (Hardin 1968), where the shared mycorrhizal network is the commons, and explain how rising atmospheric CO<sub>2</sub> may lead to greater nitrogen immobilization in the future.</p>

Precision planting impacts on winter cereal rye growth, nutrient uptake, spring soil temperature and adoption cost

Growing winter cereal rye (Secale cereale) (WCR) has been identified as an effective in-field practice to reduce nitrate-N and phosphorus (P) losses to Upper Mississippi River Basin, USA. In the Midwestern USA, growers are reluctant to plant WCR especially prior to corn (Zea mays L.) due to N immobilization and establishment issues. Precision planting of WCR or ‘skipping the corn row’ (STCR) can minimize some issues associated with WCR ahead of corn while reducing cover crop seed costs. The objective of this study was to compare the effectiveness of ‘STCR’ vs normal planting of WCR at full seeding rate (NP) on WCR biomass, nutrient uptake and composition in three site-yrs (ARC2019, ARC2020, BRC2020). Our results indicated no differences in cover crop dry matter biomass production between the STCR (2.40 Mg ha−1) and NP (2.41 Mg ha−1) supported by similar normalized difference vegetative index and plant height for both treatments. Phosphorus, potassium (K), calcium (Ca) and magnesium (Mg) accumulation in aboveground biomass was only influenced by site-yr and both STCR and NP removed similar amount of P, K, Ca and Mg indicating STCR could be as effective as NP in accumulating nutrients. Aboveground carbon (C) content (1086.26 kg h−1 average over the two treatments) was similar between the two treatments and only influenced by site-yr differences. Lignin, lignin:N and C:N ratios were higher in STCR than NP in one out of three site-yrs (ARC2019) indicating greater chance of N immobilization when WCR was planted later than usual. Implementing STCR saved $8.4 ha−1 for growers and could incentivize growers to adopt this practice. Future research should evaluate corn response to STCR compared with NP and assess if soil quality declines by STCR practice over time.

Mineralization of pig slurry compost treated with retorted oil shale and dicyandiamide in two contrasting soils

The objective of this work was to evaluate carbon and nitrogen mineralization in the soil after the application of composts produced in an automated composting plant, using pig slurry (PS) with and without the addition of retorted oil shale (ROS) and dicyandiamide (DCD) during composting. Laboratory studies were carried out for 180 days on two soils with contrasting characteristics: sandy-loam Typic Paludalf and clay Rhodic Hapludox, which were managed for more than 10 years under a no-tillage system. The composts were thoroughly mixed with the soils. The mineralization of the C and N from the compost was evaluated by measuring continuously CO2 emissions and periodically mineral N (NH4+ + NO3-) content in the soils, respectively. The mineralization of the C from the compost without ROS and DCD was higher in the sandy-loam soil (20.5%) than in the clay soil (13.9%). Similarly, 19.4% of the total N from the compost was mineralized in the sandy-loam soil and 10.9% in the clay soil. The presence of ROS in the compost reduced C mineralization by 54%, compared with the treatment without additives, in the sandy-loam soil and caused net N immobilization in both soils during incubation. The addition of DCD during PS composting did not affect the mineralization of the C and N from the compost in both soils. The addition of ROS during the composting of PS favors the retention of the C from the compost in the soil, especially in the sandy-loam one, but results in a net N immobilization.

A Nitrification Inhibitor, Nitrapyrin, Reduces Potential Nitrate Leaching through Soil Columns Treated with Animal Slurries and Anaerobic Digestate

A leaching experiment was designed to study the effects of a commercial nitrification inhibitor containing nitrapyrin on nitrification, microbial nitrogen (N) immobilization, and nitrate leaching. Soil columns were treated with 100 mg N kg−1 from pig slurry, cattle slurry, and anaerobic digestate in a mixture with or without the nitrification inhibitor. Destructive sampling was carried out after 0, 7, and 28 days of incubation in the dark at 18 °C. At each sampling date, artificial rain (200 mm of 0.01 M calcium chloride over 4 h) was added to the soil columns. The leachate was collected, and the soil was removed from the columns and sectioned into 5 cm segments. Results indicated that after 28 days of incubation, nitrapyrin enhanced ammoniacal N accumulation in the top layers of the soil columns and reduced the nitrate concentration in the leachates with pig slurry and anaerobic digestate. Furthermore, in the soil columns treated with anaerobic digestate, nitrapyrin promoted microbial N immobilization. These findings suggest that the use of nitrapyrin in a mixture with animal slurry and anaerobic digestate has the potential to reduce nitrate leaching and increase N retention in the topsoil, affording both environmental and economic advantages.

The mycorrhizal tragedy of the commons

<p><strong>The mycorrhizal tragedy of the commons</strong></p><p>It is increasingly recognized that plant C allocation to mycorrhizal symbionts plays a critical role for plant nutrition and the future global CO<sub>2</sub> 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 (Näsholm et al., 2013), which makes it difficult to understand why the symbiosis has evolved and why it is so widespread.</p><p>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.</p><p>While some observations support this hypothesis, it had not yet been thoroughly tested experimentally – 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<sub>2</sub>.</p><p><strong>References</strong></p><p>Näsholm, T. et al., 2013. Are ectomycorrhizal fungi alleviating or aggravating nitrogen limitation of tree growth in boreal forests? New Phytologist, 198(1): 214-221.</p><p>Terrer, C. et al., 2019. Nitrogen and phosphorus constrain the CO2 fertilization of global plant biomass. Nature Climate Change, 9(9): 684-689.</p>