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.

Coupled Effects of Reduced Chemical Fertilization and Biochar Supplementation on Availability and Transformations of Nitrogen and Phosphorus in Vegetable Farmland Soil: An In Situ Study in Southern China

Reduced fertilization technology is an eco-friendly strategy to minimize nitrogen (N) and phosphorus (P) surpluses and losses in vegetable production. However, little is known about the performance of chemical fertilizer reduction when supplemented with palm silk biochar (PSB) in subtropical acid soils. A short-term (60 d) field investigation under conditions of in situ incubation was conducted in vegetable farmland in southern China. The treatments included no fertilization (Control), 100% conventional fertilization (CF100), 90% conventional fertilization plus 10% PSB-based fertilization (CF90B10), 85% conventional fertilization plus 15% PSB-based fertilization (CF85B15), and 80% conventional fertilization plus 20% PSB-based fertilization (CF80B20). The CF90B10, CF85B15, and CF80B20 treatments had the same inputs of total N and P as the CF100 treatment. Reduced chemical fertilization generally decreased the soil NH4+-N regardless of the PSB substitution rate (10%, 15%, or 20%), incubation condition (top-covered or top-open: preventing or allowing the leaching process, respectively), and sampling time (1 day or 60 days). Conversely, compared with the CF100 treatment, both the CF85B15 and CF80B20 treatments did not lead to a significant decrease in the NO3−-N concentration in soil under top-open incubation conditions, but significantly (p < 0.05) increased soil NO3−-N under top-covered incubation conditions. The CF80B20 treatment significantly (p < 0.05) decreased soil Olsen-P in comparison with the CF100 treatment, regardless of the incubation condition and sampling time. After applying chemical fertilizer in combination with PSB, soil net ammonification and N mineralization tended to be reduced considerably, with substantial reductions of 39–76% and 24–45%, respectively; reversely, soil net nitrification was stimulated by an increased PSB substitution rate. As the rate of chemical fertilization decreased, the trends in NH4+-N and NO3−-N losses from the soil were similar to the trends observed in soil net ammonification and net nitrification, respectively. Additionally, there were no significant differences in the soil net P mineralization and Olsen-P loss between chemical fertilization alone and in combination with PSB application. Generally, the partial substitution of chemical fertilizer with PSB at a low application rate may not substantially reduce plant-available NO3−-N and Olsen-P. It can also contribute to the sustainable availability of N and P in vegetable farmland soil via a variety of transformation processes, such as mineralization, immobilization, and loss.

Dominant Tree Mycorrhizal Associations Affect Soil Nitrogen Transformation Rates Through Mediating Microbial Abundances in a Temperate Forest

Tree-fungal symbioses are increasingly recognized to affect soil nitrogen (N) transformations, yet the role of soil microbes in the process is largely unclear. Soil microbes directly interact with trees and are a primary driver of many N transformation processes. Here, we explored the linkage among tree mycorrhizal associations, soil microbes and N transformation rates in a temperate forest of Northeast China. Across a gradient of increasing ectomycorrhizal (ECM) tree dominance, we measured soil acid-base chemistry, bacterial and fungal abundances, N-hydrolyzing enzyme activities, abundances and community composition of ammonia-oxidizing archaea (AOA) and bacteria, and net N mineralization and net nitrification rates. Results showed that soil pH, exchangeable base cations, inorganic N concentrations and N transformation rates decreased with increasing ECM tree dominance. The ECM tree dominance was negatively related to soil bacterial and AOA amoA gene abundances, and positively to soil fungal abundances and β-N-acetylglucosaminidase activities. These shifts in soil microbial abundances and enzyme activities along the mycorrhizal gradient were linked with the increase in soil acidity with increasing ECM tree dominance. Structural equation models revealed that ECM tree dominance was not directly related to N transformation rates, but indirectly to net N mineralization rates via affecting bacterial and fungal abundances, and indirectly to net nitrification rates via influencing AOA amoA gene abundances. Collectively, our results indicate that soil microbes provide a mechanistic link between mycorrhizal associations and soil N transformations, and suggest that shifts in forest mycorrhizal associations under global change could have profound consequences for biogeochemical cycling of temperate forests.

Spatial and temporal variability of soil N2O and CH4 fluxes along a degradation gradient in a palm swamp peat forest in the Peruvian Amazon

&lt;p&gt;Mauritia flexuosa palm swamp, the prevailing Peruvian Amazon peatland ecosystem, is&lt;/p&gt;&lt;p&gt;extensively threatened by degradation. The unsustainable practice of cutting whole&lt;/p&gt;&lt;p&gt;palms for fruit extraction modifies forest's structure and composition and eventually&lt;/p&gt;&lt;p&gt;alters peat-derived greenhouse gas (GHG) emissions. We evaluated the spatio-temporal&lt;/p&gt;&lt;p&gt;variability of soil N&lt;sub&gt;2&lt;/sub&gt;O and CH&lt;sub&gt;4&lt;/sub&gt; fluxes and environmental controls along a palm swamp&lt;/p&gt;&lt;p&gt;degradation gradient formed by one undegraded site (Intact), one moderately degraded&lt;/p&gt;&lt;p&gt;site (mDeg) and one heavily degraded site (hDeg). Microscale variability differentiated&lt;/p&gt;&lt;p&gt;hummocks supporting live or cut palms from surrounding hollows. Macroscale analysis&lt;/p&gt;&lt;p&gt;considered structural changes in vegetation and soil microtopography as impacted&lt;/p&gt;&lt;p&gt;by degradation. Variables were monitored monthly over 3 years to evaluate intra- and&lt;/p&gt;&lt;p&gt;inter-annual variability. Degradation induced microscale changes in N&lt;sub&gt;2&lt;/sub&gt;O and CH&lt;sub&gt;4&lt;/sub&gt; emission&lt;/p&gt;&lt;p&gt;trends and controls. Site-scale average annual CH&lt;sub&gt;4&lt;/sub&gt; emissions were similar along the&lt;/p&gt;&lt;p&gt;degradation gradient (225.6 &amp;#177; 50.7, 160.5 &amp;#177; 65.9 and 169.4 &amp;#177; 20.7 kg C ha&lt;sup&gt;&amp;#8722;1&lt;/sup&gt; year&lt;sup&gt;&amp;#8722;1&lt;/sup&gt; at&lt;/p&gt;&lt;p&gt;the Intact, mDeg and hDeg sites, respectively). Site-scale average annual N&lt;sub&gt;2&lt;/sub&gt;O emissions&lt;/p&gt;&lt;p&gt;(kg N ha&lt;sup&gt;&amp;#8722;1&lt;/sup&gt; year&lt;sup&gt;&amp;#8722;1&lt;/sup&gt;) were lower at the mDeg site (0.5 &amp;#177; 0.1) than at the Intact (1.3 &amp;#177; 0.6) and&lt;/p&gt;&lt;p&gt;hDeg sites (1.1 &amp;#177; 0.4), but the difference seemed linked to heterogeneous fluctuations&lt;/p&gt;&lt;p&gt;in soil water-filled pore space (WFPS) along the forest complex rather than to degradation.&lt;/p&gt;&lt;p&gt;Monthly and annual emissions were mainly controlled by variations in WFPS, water&lt;/p&gt;&lt;p&gt;table level (WT) and net nitrification for N&lt;sub&gt;2&lt;/sub&gt;O; WT, air temperature and net nitrification&lt;/p&gt;&lt;p&gt;for CH&lt;sub&gt;4&lt;/sub&gt;. Site-scale N&lt;sub&gt;2&lt;/sub&gt;O emissions remained steady over years, whereas CH&lt;sub&gt;4&lt;/sub&gt; emissions&lt;/p&gt;&lt;p&gt;rose exponentially with increased precipitation. While the minor impact of degradation&lt;/p&gt;&lt;p&gt;on palm swamp peatland N&lt;sub&gt;2&lt;/sub&gt;O and CH&lt;sub&gt;4&lt;/sub&gt; fluxes should be tested elsewhere, the evidenced&lt;/p&gt;&lt;p&gt;large and variable CH&lt;sub&gt;4&lt;/sub&gt; emissions and significant N&lt;sub&gt;2&lt;/sub&gt;O emissions call for improved modeling&lt;/p&gt;&lt;p&gt;of GHG dynamics in tropical peatlands to test their response to climate changes.&lt;/p&gt;

Response of N2O emissions to logging residue piles of Norway spruce, Scots pine and silver birch

&lt;p&gt;Utilization of forest bioenergy is increasing; however, the overall environmental impacts of forest bioenergy utilization are not fully understood. Especially effects on N&lt;sub&gt;2&lt;/sub&gt;O emissions in mineral soils are less studied. With current harvesting practices, either whole-tree-harvest or stem-only-harvest, piles of logging residues are left on the forest floor. As a result, soil nitrogen (N) cycling processes can be accelerated on clear cut area under the piles, especially net nitrification. When N is transformed to more mobile form, the risk for N losses via nitrous oxide (N&lt;sub&gt;2&lt;/sub&gt;O) emissions from the forest floor may increase.&lt;/p&gt;&lt;p&gt;We studied how logging residue piles of three tree species, Norway spruce (Picea abies (L.) Karst.), Scots pine (Pinus sylvestris L.) and silver birch (Betula pendula Roth.), influence gaseous losses of N after clear-cut. A Norway spruce dominated mixed stand on a mineral soil site was clear-cut and N&lt;sub&gt;2&lt;/sub&gt;O emissions were monitored. There were four treatments; three tree species treatments consisting of 40 kg m&lt;sup&gt;-2&lt;/sup&gt; of fresh logging residues and control plot without residues as an additional treatment. Effects of logging residue piles on N&lt;sub&gt;2&lt;/sub&gt;O emissions were monitored over 4 growing season with closed chamber technic. Simultaneously soil temperatures were recorded over 2 growing season. Soil denitrification activity and the contribution of nitrification and denitrification to N&lt;sub&gt;2&lt;/sub&gt;O production were determined in laboratory experiments.&lt;/p&gt;&lt;p&gt;Logging residue piles lowered and balanced fluctuation of soil temperatures. N&lt;sub&gt;2&lt;/sub&gt;O fluxes peaked under the piles during the second and third growing season after the establishment of the piles; however inconsistent fluxes tended to be low. The production of N&lt;sub&gt;2&lt;/sub&gt;O was driven by both nitrification and denitrification processes, the proportion depending on the tree species. Our results indicate that logging residue piles accelerate N losses as gaseous form; however studies on the same field experiment shows that most of the N losses occur through soil percolation waters. Spruce residues tend to stimulate N&lt;sub&gt;2&lt;/sub&gt;O emissions longer compared to other tree species. There was a positive correlation with net nitrification and microbial biomass C (T&amp;#246;rm&amp;#228;nen et al. 2018, FORECO). These results have implications for sustainable and productive forest management practices and nutrition of re-growing vegetation.&lt;/p&gt;