A review of the importance of mineral nitrogen cycling in the plant-soil-microbe system of permafrost-affected soils – changing the paradigm

The paradigm that permafrost-affected soils show restricted mineral nitrogen (N) cycling in favor of organic N compounds is based on the observation that net N mineralization rates in these cold climates are negligible. However, we find here that this perception is wrong. By synthesizing published data on N cycling in the plant-soil-microbe system of permafrost ecosystems we show that gross ammonification and nitrification rates in active layers were of similar magnitude and showed a similar dependence on soil organic carbon (SOC) and total nitrogen (TN) concentrations as observed in temperate and tropical systems. Moreover, high protein depolymerization rates and only marginal effects of C:N stoichiometry on gross N turnover provided little evidence for N limitation. Instead, the rather short period when soils are not frozen is the single main factor limiting N turnover. High gross rates of mineral N cycling are thus facilitated by released protection of organic matter in active layers with nitrification gaining particular importance in N-rich soils, such as organic soils without vegetation. Our finding that permafrost-affected soils show vigorous N cycling activity is confirmed by the rich functional microbial community which can be found both in active and permafrost layers. The high rates of N cycling and soil N availability are supported by biological N fixation, while atmospheric N deposition in the Arctic still is marginal except for fire-affected areas. In line with high soil mineral N production, recent plant physiological research indicates a higher importance of mineral plant N nutrition than previously thought. Our synthesis shows that mineral N production and turnover rates in active layers of permafrost-affected soils do not generally differ from those observed in temperate or tropical soils. We therefore suggest to adjust the permafrost N cycle paradigm, assigning a generally important role to mineral N cycling. This new paradigm suggests larger permafrost N climate feedbacks than assumed previously.

Mid-Term Effects of Forest Thinning on N Mineralization in a Semi-Arid Aleppo Pine Forest

In order to assess the sustainability of silvicultural treatments in semiarid forests, it is necessary to know how they affect the nutrient dynamics in the forest. The objective of this paper is to study the effects of silvicultural treatments on the net N mineralization and the available mineral N content in the soil after 13 years following forest clearings. The treatments were carried out following a randomized block design, with four treatments and two blocks. The distance between the two blocks was less than 3 km; they were located in Chelva (CH) and Tuéjar (TU) in Valencia, Spain. Within each block, four experimental clearing treatments were carried out in 1998: T0 control; and T60, T75 and T100 where 60%, 75% and 100 of basal area was eliminated, respectively. Nitrogen dynamics were measured using the resin tube technique, with disturbed samples due to the high stoniness of the plots. Thirteen years after the experimental clearings, T100, T75 and T60 treatments showed a twofold increase in the net mineralization and nitrification rates with respect to T0 in both blocks (TU and CH). Within the plots, the highest mineralization was found in sites with no plant cover followed by those covered by undergrowth. These results can be explained in terms of the different litterfall qualities, which in turn are the result of the proportion of material originating from Pinus halepensis Mill. vs. more decomposable undergrowth residues.

Effect of crop rotation and cropping history on net N mineralization dynamics of a clay loam soil

Estimating soil N mineralization is critical to being able to balance fertilizer N requirements and their environmental impacts. In this study, net N mineralization was examined in soils under different crop rotations with each phase of the rotation present every year with biologically-based incubations in 2011 and 2015. Net N mineralization was significantly different among treatments when the current crop was soybean and the effect was dependent upon the previous crop and the cropping sequence. In particular, net increases in inorganic N were greater when the previous crop was winter wheat with/without red clover than if it were corn, and greater for the first year of soybean compared to the second year for rotations with two consecutive years of soybean in the 2011 incubation. However, cropping history did not influence net soil N mineralization when the current crop was either corn, winter wheat, or winter wheat + red clover. In 2015, the presence of red clover in the rotation increased net N mineralization in all phases of the rotation. These results suggest both current and previous crops should be considered when estimating the N supplying capacity (net mineralization) of the soil. Net mineralizable N was found to be significantly correlated with total amino sugars (P < 0.001), glucosamine (P < 0.001), and galactosamine (P = 0.003), which suggests that amino sugars could be used as an indicator of the N supplying capacity of soil.

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.

Assessing Nitrogen Availability in Biobased Fertilizers: Effect of Vegetation on Mineralization Patterns

Biobased nitrogen (N) fertilizers derived from animal manure can substitute synthetic mineral N fertilizer and contribute to more sustainable agriculture. Practitioners need to obtain a reliable estimation of the biobased fertilizers’ N value. This study compared the estimates for pig slurry (PS) and liquid fraction of digestate (LFD) using laboratory incubation and plant-growing experiments. A no-N treatment was used as control and calcium ammonium nitrate (CAN) as synthetic mineral fertilizer. After 100 days of incubation, the addition of PS and LFD resulted in a net N mineralization rate of 10.6 ± 0.3% and 20.6 ± 0.4% of the total applied N, respectively. The addition of CAN showed no significant net mineralization or immobilization (net N release 96 ± 6%). In the pot experiment under vegetation, all fertilized treatments caused N immobilization with a negative net N mineralization rate of −51 ± 11%, −9 ± 4%, and −27 ± 10% of the total applied N in CAN, PS, and LFD treatments, respectively. Compared to the pot experiment, the laboratory incubation without vegetation may have overestimated the N value of biobased fertilizers. Vegetation resulted in a lower estimation of available N from fertilizers, probably due to intensified competition with soil microbes or increased N loss via denitrification.

The effect of long-term CO2 enrichment on carbon and nitrogen content of roots and soil of natural pastureland

Increasing levels of atmospheric CO2 may change C and N dynamics in pasture ecosystems. The present study was conducted to examine the impact of four years of CO2 enrichment on soil and root composition and soil N transformation in natural pastureland. Plots of open-top growth chambers were continuously injected with ambient CO2 (350 µL L–1) and elevated CO2 (625 µL L–1). Soil cores exposed to ambient and elevated CO2 treatment were incubated and collected each year. Net N-mineralization rates in soil (NH4 +-N plus NO3ˉ–-N), in addition to total C and N content (%) of soil and root tissues were measured. Results revealed that elevated CO2 caused a significant reduction in soil NO3 (P < 0.05), however, no significant CO2 effect was found on total soil C and N content (%). Roots of plants grown under elevated CO2 treatment had higher C/N ratios. Changes in root C/N ratios were driven by changes in root N concentrations as total root N content (%) was significantly reduced by 30% (P < 0.05). Overall, findings suggest that the effects of CO2 enrichment was more noticeable on N content (%) than C content (%) of soil and roots; elevated CO2 significantly affected soil N-mineralization and total N content (%) in roots, however, no substantial change was found in C inputs in CO2-enriched soil.

Carbon Availability and Nitrogen Mineralization Control Denitrification Rates and Product Stoichiometry during Initial Maize Litter Decomposition

Returning crop residues to agricultural fields can accelerate nutrient turnover and increase N2O and NO emissions. Increased microbial respiration may lead to formation of local hotspots with anoxic or microoxic conditions promoting denitrification. To investigate the effect of litter quality on CO2, NO, N2O, and N2 emissions, we conducted a laboratory incubation study in a controlled atmosphere (He/O2, or pure He) with different maize litter types (Zea mays L., young leaves and roots, straw). We applied the N2O isotopocule mapping approach to distinguish between N2O emitting processes and partitioned the CO2 efflux into litter- and soil organic matter (SOM)-derived CO2 based on the natural 13C isotope abundances. Maize litter increased total and SOM derived CO2 emissions leading to a positive priming effect. Although C turnover was high, NO and N2O fluxes were low under oxic conditions as high O2 diffusivity limited denitrification. In the first week, nitrification contributed to NO emissions, which increased with increasing net N mineralization. Isotopocule mapping indicated that bacterial processes dominated N2O formation in litter-amended soil in the beginning of the incubation experiment with a subsequent shift towards fungal denitrification. With onset of anoxic incubation conditions after 47 days, N fluxes strongly increased, and heterotrophic bacterial denitrification became the main source of N2O. The N2O/(N2O+N2) ratio decreased with increasing litter C:N ratio and Corg:NO3− ratio in soil, confirming that the ratio of available C:N is a major control of denitrification product stoichiometry.

Implications of the Thermodynamic Response of Soil Mineralization, Respiration, and Nitrification on Soil Organic Matter Retention

Considerable research has shown that modifications in global temperature regimes can lead to changes in the interactions between soil respiration and the sequestration of C and N into soil organic matter (SOM). We hypothesized that despite the interconnected nature of respiration, net N mineralization, and nitrification processes, there would be differences in their thermodynamic responses that would affect the composition of inorganic soil N and the potential for retention of N in SOM. To test this hypothesis, soil respiration, N mineralization and nitrification responses were evaluated during constant temperature incubations at seven temperatures (4–42°C) in tilled and no-till soils from two major agroecological zones in Oregon; Willamette Valley, and Pendleton located in the Columbia River Basin. We observed (1) significant thermodynamic differences between the three processes in all soils, (2) a distinctly different thermodynamic profile in Willamette vs. Pendleton, and (3) a dynamic response of Topt (optimal temperature for activity), and Tsmax (temperature of greatest rate response to temperature), and temperature sensitivity (ΔCp‡) over the incubation time course, resulting in shifts in the thermodynamic profiles that could not be adequately explained by changes in process rates. We found that differences in contributions of ammonia oxidizing archaea and bacteria to nitrification activity across temperature helped to explain the thermodynamic differences of this process between Willamette and Pendleton soils. A two-pool model of SOM utilization demonstrated that the dynamic thermodynamic response of respiration in the soils was due to shifts in utilization of labile and less-labile pools of C; and that the respiration response by Pendleton soils was more dependent upon contributions from the less-labile C pool resulting in higher Topt and Tsmax than Willamette soils. Interestingly, modeling of N mineralization using the two-pool model suggested that only the less-labile pool of SOM was contributing to N mineralization at most temperatures in all soils. The difference in labile and less-labile SOM pool utilization between respiration and N mineralization may suggest that these processes may not be as interconnected as previously thought.

Exploring Temperature-Related Effects in Catch Crop Net N Mineralization Outside of First-Order Kinetics

Catch crops are an effective method for reducing nitrogen (N) leaching in agriculture, but the mineralization of incorporated catch crop residue N is difficult to predict and model. We conducted a five-month incubation experiment using fresh residue from three catch crops (hairy vetch, fodder radish and ryegrass) with three temperature treatments (2 °C, 15 °C and 2–15 °C variable temperature) and two termination methods (glyphosate and untreated). Mineral N (ammonium and nitrate) in soil was quantified at 0, 1, 2, 4, 8 and 20 weeks of incubation. Ammonium accumulation from residue decomposition showed a lag at low and variable temperature, but subsequent nitrification of the ammonium did not. Mineral N accumulation over time changed from exponential to sigmoidal mode at low and variable temperature. Incubation temperature significantly affected mineralization rates in a first-order kinetics (FOK) model, while plant type and termination method did not. Plant type alone had a significant effect on the final mineralized fraction of added catch crop N. FOK models modified to accommodate an initial lag were fitted to the incubation results and produced better goodness-of-fit statistics than simple FOK. We suggest that initial lags in residue decomposition should be investigated for the benefit of mineralization predictions in cropping models.