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This review proposes a \"phenomenon-mechanism-regulation\" framework for understanding nitrogen immobilization during the conversion of green waste into growing media. Nitrogen immobilization acts as a double-edged sword: intense short-term immobilization, typically occurring within the first 1-2 weeks after substrate establishment, can rapidly deplete mineral nitrogen and induce plant nitrogen deficiency, whereas the immobilized nitrogen is subsequently incorporated into microbial biomass and lignin-associated organic pools, forming a slow-release reservoir that enhances nitrogen retention and reduces leaching losses. Owing to its extremely high C/N ratio (often >100) and the coexistence of labile carbon fractions and recalcitrant compounds (e.g., lignin and phenolics), green waste exhibits substantially stronger immobilization potential than conventional media. Empirical evidence indicates that nitrogen immobilization can reach 10-115 mg N·L within a few days in wood-derived substrates, and additional fertilization of up to 100 mg N·L may be required to maintain crop growth. Mechanistically, nitrogen immobilization is governed by the coupling of microbial assimilation-driven by stoichiometric C/N imbalance (typically triggered when C/N > 20-25)-and abiotic chemical fixation, including reactions between NH /NO and lignin-derived phenolics forming stable organic nitrogen compounds. The relative dominance of these pathways is jointly regulated by carbon quality, nitrogen form, and pH. Based on these mechanisms, regulatory strategies are summarized at multiple scales, including feedstock pretreatment to reduce labile carbon availability, substrate formulation to optimize C/N balance, and model-assisted intelligent fertigation to synchronize nitrogen supply with crop demand. Overall, this study provides a theoretical basis for improving green waste valorization and promoting sustainable horticultural production.

Nitrogen deposition is projected to increase rapidly in tropical ecosystems, but changes in soil-N-cycling processes in tropical ecosystems under elevated N input are less well understood. We used N-addition experiments to achieve N-enriched conditions in mixed-species, lowland and montane forests in Panama. Our objectives were to (1) assess changes in soil mineral N production (gross rates of N mineralization and nitrification) and retention (microbial immobilization and rapid reactions to organic N) during 1- and 9-yr N additions in the lowland forest and during 1-yr N addition in the montane forest and (2) relate these changes to N leaching and N-oxide emissions. In the old-growth lowland forest located on an Inceptisol, with high base saturation and net primary production not limited by N, there was no immediate effect of first-year N addition on gross rates of mineral-N production and N-oxide emissions. Changes in soil-N processes were only apparent in chronic (9 yr) N-addition plots: gross N mineralization and nitrification rates, NO 3 − leaching, and N-oxide emissions increased, while microbial biomass and NH 4 + immobilization rates decreased compared to the control. Increased mineral-N production under chronic N addition was paralleled by increased substrate quality (e.g., reduced C:N ratios of litterfall), while the decrease in microbial biomass was possibly due to an increase in soil acidity. An increase in N losses was reflected in the increase in 15 N signatures of litterfall under chronic N addition. In contrast, the old-growth montane forest located on an Andisol, with low base saturation and aboveground net primary production limited by N, reacted to first-year N addition with increases in gross rates of mineral-N production, microbial biomass, NO 3 − leaching, and N-oxide emissions compared to the control. The increased N-oxide emissions were attributed to increased nitrification activity in the organic layer, and the high NO 3 − availability combined with the high rainfall on this sandy loam soil facilitated the instantaneous increase in NO 3 − leaching. These results suggest that soil type, presence of an organic layer, changes in soil-N cycling, and hydrological properties are more important indicators than vegetation as an N sink on how tropical forests respond to elevated N input.

An ongoing roof experiment, where N and acid inputs were reduced to the recommended critical load levels, has been conducted since 1991 in an N-saturated spruce stand in Soiling, Germany. Our study was aimed at (1) quantifying the changes in gross rates of microbial N cycling under ambient and reduced N conditions, and (2) relating the soil N dynamics to the changes in N leaching and N status of trees. Two roofs were used, one to achieve \"ambient\" and the other reduced (\"clean rain\") inputs, with a roofless plot as a control for possible roof effects. In 2001, the ambient roof and ambient no-roof plots showed an apparent decrease in gross N mineralization rates and significantly lower microbial NH4 +immobilization rates and turnover rates of NH4 +and microbial N pools. The microbial NO3 -immobilization rates and NO3 -pool turnover rates were lower than the microbial NH4 +immobilization rates and NH4 +pool turnover rates, showing that less NO3 -cycled through microorganisms than NH4 +. There was also low abiotic NO3 -immobilization. High NO3 -input from throughfall and low microbial turnover rates of the NO3 -pool, combined with low abiotic NO3 -retention, may have contributed to the high NO3 -leaching losses in these ambient plots. The clean rain plot showed a slight increase in gross N mineralization rates and significantly higher microbial NH4 +immobilization rates and turnover rates of NH4 +and microbial N pools. Neither nitrification nor soil NO3 -was detectable. There was an increase in abiotic NO3 -immobilization. Foliar N concentration had decreased but was still adequate. An efficient cycling of NH4 +through microorganisms, combined with the high abiotic NO3 -immobilization, indicated efficient mineral N retention in the clean rain plot. These results indicated that long-term reduction of throughfall N and acid inputs had induced high but tightly coupled microbial NH4 +cycling and an increase in abiotic NO3 -retention, which contributed to the reversal of N saturation.

The mechanisms of the different responses of soil gross nitrogen (N) transformation to increasing temperature or moisture in different types of soils are still unclear. Here, we conducted two 15N tracing experiments to investigate the effects of increasing temperature (15 °C and 25 °C) or moisture (30%, 45%, and 60% water-filled pore space (WFPS)) on soil gross N transformation rates for two soils (organic (O) and mineral (A) horizon soils) in a temperate forest. As the temperature increased from 15 to 25 °C or moisture increased from 30 to 60% WFPS, total mineralization rates increased by 4.5-fold and 2.5-fold respectively, total NH4+ immobilization rates increased by 173.2-fold and 7.6-fold respectively, and autotrophic nitrification rates increased by 0.7-fold and 0.6-fold respectively in the O horizon. Under the same treatment, the changes in autotrophic nitrification rates, NH4+ immobilization rates, and mineralization rates were much smaller in the A horizon than in the O horizon. We propose that the difference between the two horizons in the responses to changing temperature and moisture was due to the different resource status. The O horizon could provide more substrates than the A horizon, resulting in a stronger response of N processes to changing temperature and moisture. Resource status also affected the competition for substrates between NH4+ immobilization and autotrophic nitrification. The N transformation rates were higher in the O horizon than in the A horizon, consistent with higher microbial biomass N, microbial respiration rate, and amoA gene abundance in the O horizon. Our results suggest that the activity switch of microbes and the competition for resources are important biotic factors regulating potential responses of soil N cycling processes to changing abiotic factors.

Atmospheric nitrogen (N) deposition increasingly impacts remote ecosystems. At high altitudes, snow is a key carrier of water and nutrients from the atmosphere to the soil. Medium-sized subalpine grassland terraces are characteristic of agricultural landscapes in the French Alps and influence spatial and temporal snow pack variables. At the Lautaret Pass, we investigated snow and soil characteristics along mesotopographic gradients across the terraces before and during snowmelt. Total N concentrations in the snowpack did not vary spatially and were dominated by organic N forms either brought by dry deposition trapped by the snow, or due to snow-microbial immobilization and turnover. As expected, snowpack depth, total N deposited with snow and snowmelt followed the terrace toposequence; more snow-N accumulated towards the bank over longer periods. However, direct effects of snow-N on soil-N cycling seem unlikely since the amount of nitrogen released into the soil from the snowpack was very small relative to soil-N pools and N mineralization rates. Nevertheless, some snow-N reached the soil at thaw where it underwent biotic and abiotic processes. In situ soil-N mineralization rates did not vary along the terrace toposequence but soil-N cycling was indirectly affected by the snowpack. Indeed, N mineralization responded to the snowmelt dynamic via induced temporal changes in soil characteristics (i.e. moisture and T°) which cascaded down to affect N-related microbial activities and soil pH. Soil-NH4 and DON accumulated towards the bank during snowmelt while soil-NO3 followed a pulse-release pattern. At the end of the snowmelt season, organic substrate limitation might be accountable for the decrease in N mineralization in general, and in NH4 + production in particular. Possibly, during snowmelt, other biotic or abiotic processes (nitrification, denitrification, plant uptake, leaching) were involved in the transformation and transfer of snow and soil-N pools. Finally, subalpine soils at the Lautaret Pass during snowmelt experienced strong biotic and abiotic changes and switched between a source and a sink of N.

1 This work demonstrates that spatial distribution of understorey vegetation and gross N transformation rates in a mixed beach-oak forest is closely correlated within a distance of a few metres. The findings imply that plant diversity and productivity have a major influence on gross rates of N transformation and vice versa. 2 A geostatistical analysis was used to evaluate the spatial relationships between abundance and species composition of the understorey vegetation and in situ gross N mineralization, NH4 +immobilization and nitrification rates. 3 The gross N transformation rates and the plants spatial variation were correlated within the forest, but plant distribution was more dependent on the fraction of mineralized N that was nitrified than on individual N transformation rates. 4 The total cover of the understorey vegetation varied more in space than the species composition, and was higher in areas with high N transformation rates. 5 Plant species benefiting from high net nitrification rates were more common in areas with a low activity of mineralizing and nitrifying microorganisms, possibly because the net and gross rates were independent of each other. In fact, those species occurred most often in areas in which a large fraction of mineralized N was nitrified. 6 Beech and oak trees also had an effect on the spatial variation of the understorey vegetation. Beech trees provided conditions more suitable for plants benefiting from NO3 -, whereas the vascular plant cover was greater under oak trees, probably in response to a higher light interception than under beech trees. 7 Oak generally had a positive impact on gross N transformation rates compared with beech, perhaps reflecting differences in litter quality and climate caused by the two species. 8 The influence of trees alone could not explain the full magnitude of the variation of N transformation rates or the presence of overlapping areas with high mineralization and immobilization rates. These were probably caused by other factors, such as soil moisture content. 9 This work sheds new light on the small-scale spatial links between above-ground plant diversity and abundance, and below-ground microbial N transformations.