Soil Net Nitrogen Mineralization Research Articles

Effect of nitrogen addition on soil net nitrogen mineralization in topsoil and subsoil regulated by soil microbial properties and mineral protection: Evidence from a long-term grassland experiment

Soil net nitrogen mineralization (Nmin), a microbial-mediated conversion of organic to inorganic N, is critical for grassland productivity and biogeochemical cycling. Enhanced atmospheric N deposition has been shown to substantially increase both plant and soil N content, leading to a major change in Nmin. However, the mechanisms underlying microbial properties, particularly microbial functional genes, which drive the response of Nmin to elevated N deposition are still being discussed. Besides, it is still uncertain whether the relative importance of plant carbon (C) input, microbial properties, and mineral protection in regulating Nmin under continuous N addition would vary with the soil depth. Here, based on a 13-year multi-level field N addition experiment conducted in a typical grassland on the Loess Plateau, we elucidated how N-induced changes in plant C input, soil physicochemical properties, mineral properties, soil microbial community, and the soil Nmin rate (Rmin)-related functional genes drove the responses of Rmin to N addition in the topsoil and subsoil. The results showed that Rmin increased significantly in both topsoil and subsoil with increasing rates of N addition. Such a response was mainly dominated by the rate of soil nitrification. Structural equation modeling (SEM) revealed that a combination of microbial properties (functional genes and diversity) and mineral properties regulated the response of Rmin to N addition at both soil depths, thus leading to changes in the soil N availability. More importantly, the regulatory impacts of microbial and mineral properties on Rmin were depth-dependent: the influences of microbial properties weakened with soil depth, whereas the effects of mineral protection enhanced with soil depth. Collectively, these results highlight the need to incorporate the effects of differential microbial and mineral properties on Rmin at different soil depths into the Earth system models to better predict soil N cycling under further scenarios of N deposition.

Effect of a Cushion Plant, Androsace tapete, on Soil Net Nitrogen Mineralization and Enzyme Activities during the Growing Season

Effect of a Cushion Plant, Androsace tapete, on Soil Net Nitrogen Mineralization and Enzyme Activities during the Growing Season

Dynamics of nitrogen mineralization and fine root decomposition in sub-tropical Shorea robusta Gaertner f. forests of Central Himalaya, India

This study aimed to examine the effects of spatial and temporal variability in edaphic, and climatic attributeson soil net nitrogen mineralization rate, and to understand the pattern of fine root decomposition of dominant and co-dominant tree species, and its influence on the nutrient cycling in forest ecosystems. Study was carried out at four different sites in sub-tropical forest ecosystems of Shorea robusta, in foothills of Central Himalayan region, India. Co-dominant tree species at four sites were Mallotus philippensis (site A), Glochidion velutinum (site B), Holarrhena pubescens (site C), and Tectona grandis (site D). Buried bag technique was used for nitrogen mineralization, while fine root decomposition was determined using fine root mesh bags. Seasonal variation, soil depth, soil characteristics, and site variability, all significantly (p < 0.05) affected nitrogen mineralization rates. Fine root decomposition was significantly affected by nutrient concentration of fine roots. Total mineral nitrogen was maximum at site D (16.24 ± 0.96 μg g−1 soil), while minimum at site C (10.10 ± 0.84 μg g−1 soil). Maximum nitrogen mineralization (13.18 ± 0.18 μg g−1 month−1) was recorded during summer season at site D, while the minimum nitrogen mineralization (3.20 ± 0.46 μg g−1 month−1) was recorded during rainy season at site C. Inorganic-N and net nitrogen mineralization was relatively higher in 0–20 cm soil layer than 20–40 cm and 40–60 cm soil layer. The fine roots showed 70.61–74.82 % weight loss on completion of 365 days of decomposition process. Maximum fine root decomposition was observed in the G. velutinum, and minimum in T. grandis. A significant positive correlation (p < 0.05) was observed between root nitrogen and carbon content, and decomposition rates per month. This study concluded that the spatial and temporal variability in soil nitrogen mineralization rates and fine root decomposition optimises nutrient cycling in forest ecosystems, which can contribute to the development of sustainable forest management practices.

Plant composition change mediates climate drought, nitrogen addition, and grazing effects on soil net nitrogen mineralization in a semi-arid grassland in North China

Human activities induce alterations of the nitrogen (N) cycle, climate drought, and disturbance (e.g., livestock grazing) regimes at the global scale. Their individual, interactive, and combined effects on soil N cycling in grasslands are unclear. We investigated the N addition, drought, and grazing effects on the N mineralization, as well as their correlations with N-related variables, including the C4 species, shoot biomass (SB), root biomass (RB), plant total nitrogen (PTN), plant total carbon (PTC), soil total nitrogen (STN), soil total carbon (STC), and soil microbial N and C, during a three-year field experiment conducted in a semi-arid grassland in North China. The results showed that N addition increased the nitrate N (NO3−-N) and ammonium N (NH4+-N) concentrations, whereas drought decreased the NO3−-N concentration because of strengthened N limitation. Pronounced temporal variation in the N mineralization occurred under seasonal drought (maxima in August and September) and under its combination with N addition and grazing (minima in August). RB and the C4 species were positively correlated, whereas STC and the NO3−-N concentration were negatively correlated with the N mineralization under the combined influence of the three factors. The structural equation model showed that at the site affected by all three factors, drought indirectly increased the N mineralization by reducing the NO3−-N concentration, whereas N addition and grazing did not alter the N mineralization. N addition directly increased while indirectly reduced N mineralization by increasing the NO3−-N concentration. Additionally, N addition and grazing increased the C4 species and decreased the STC, consequently enhanced N mineralization. These results highlight the predominant role of drought, when combined with N addition and grazing, in controlling the N mineralization. The N supply balance in semi-arid grasslands could be stabilized in response to increased N addition, climate drought, and grazing.

Impacts of aboveground litter and belowground roots on soil greenhouse gas emissions: Evidence from a DIRT experiment in a pine plantation

Plant detritus alteration may influence soil greenhouse gas emissions, but the direction, extent and mechanisms remain uncertain. Here we conducted a DIRT (Detritus Input and Removal Treatment) experiment in a Pinus sylvestris var. mongolica plantation in Saihanba (Hebei Province, China), manipulating aboveground litter (L-: litter removal, L: normal litter, L+: litter doubling) and belowground roots (R-: root removal, R: with root) resulting in six treatments. We measured CO2, N2O, and CH4 fluxes during 2019 and 2020 growing seasons, along with soil physicochemical, enzymatic and microbial variables. Our findings revealed that soil CO2 and N2O emissions increased with litter input, while CH4 uptake decreased. The absence of roots decreased soil CO2 emission and CH4 uptake but increased N2O emission. Aboveground litter enhanced the carbon cycling variables, such as extractable organic carbon (EOC), the ratio of soil extractable organic carbon to extractable total nitrogen (EOC:ETN), soil carbon mineralization rate (Cmin), and peroxidase activity (PER), resulting in increased CO2 emission. In contrast, the absence of roots increased the nitrogen cycling variables, such as soil extractable total nitrogen (ETN), nitrate nitrogen content (NO3−-N), the ratio of soil extractable organic carbon to extractable total nitrogen (EOC:ETN), soil net nitrogen mineralization rate (Nmin), and the abundance of ammonia-oxidizing bacteria (AOB), leading to increased N2O emission. Moreover, soil CH4 uptake was mainly influenced by soil microclimate: aboveground litter input reduced CH4 uptake by lowering soil temperature, while belowground roots benefited CH4 uptake via reducing soil moisture by water uptake. Taken together, our study suggests that soil CO2, N2O, and CH4 emissions (or uptake) are influenced by both aboveground and belowground plant inputs through various pathways. While aboveground litter input promotes the emissions of greenhouse gasses, the mitigating effect of belowground roots on N2O and CH4 partially offsets the increment of CO2 emission in this plantation.

Effects of rainfall amount and frequencies on soil net nitrogen mineralization in Gahai wet meadow in the Qinghai-Tibetan Plateau

Global climate change has led to a significant increase in the frequency of extreme rainfall events in the Qinghai-Tibetan Plateau (QTP), thus potentially increasing the annual rainfall amounts and, consequently, affecting the net soil nitrogen (N) mineralization process. However, few studies on the responses of the soil net N mineralization rates to the increases in rainfall amounts and frequencies in alpine wet meadows have been carried out. Therefore, the present study aims to assess the effects of rainfall frequency and amount changes on the N fixation capacity of wet meadow soils by varying the rainfall frequency and amount in the Gahai wet meadow in the northeastern margin of the QTP during the plant-growing season in 2019. The treatment scenarios consisted of ambient rain (CK) and supplementary irrigation at a rate of 25 mm, with different irrigation frequencies, namely weekly (DF1), biweekly (DF2), every three weeks (DF3), and every four weeks (DF4). According to the obtained results, the increased rainfall frequency and amount decreased the soil mineral N stock and increased the aboveground vegetation biomass (AB) amounts and soil water contents in the wet meadows of the QTP. Ammonium (NH4+-N) and nitrate N (NO3–-N) contributed similarly to the mineral N contents. However, the ammonification process played a major role in the soil mineralization process. The effects of increasing rainfall amount and frequency on N mineralization showed seasonal variations. The N mineralization rate showed a single-peaked curve with increasing soil temperature during the rapid vegetation growth phase, reaching the highest value in August. In addition, the N mineralization rates showed significant positive correlations with soil temperatures and NH4+-N contents and a significant negative correlation with AB (P < 0.05). The results of this study demonstrated the key role of low extreme rainfall event frequencies in increasing the net soil N mineralization rates in the vegetation growing season, which is detrimental to soil N accumulation, thereby affecting the effectiveness of soil N contents.

Dynamics of soil net nitrogen mineralization and controlled effect of microbial functional genes in the restoration of cold temperate forests

Soil nitrogen mineralization (Nmin) generally controls the availability and circulation of soil nitrogen (N). To better understand the mechanisms of soil Nmin and environmental protection, this study explored the dynamics of net Nmin in soils from restored forests and the effect of soil physicochemical properties (SPPs) and soil microbial mineralization genes (SMGs) on Nmin. Field studies were conducted from July 2019 to April 2020 in five representative restored forests in the mountainous region of northwestern Hebei, China (i.e., Larix principis-rupprechtii forest (LF), Betula platyphylla forest (BF), mixed forest of Larix principis-rupprechtii and Betula platyphylla (MF), Picea asperata forest (SF) and Pinus sylvestris var. mongolica forest (MPF)), and soil net nitrogen mineralization rate (Rmin), SPPs and SMGs were determined after collecting soil samples in five plots. The influence of restored forests and seasonality on Rmin was investigated to determine the characteristics of the dynamics of soil net Nmin. Principal component analysis (PCA) and variation partitioning analysis (VPA) revealed that soil properties and functional genes could explain approximately 80 % of the variation in Nmin. SMGs significantly contributed to the variation in the soil net ammonification rate (Ramm) and nitrification rate (Rnit) compared to soil properties. SMGs were strongly correlated with Ramm\\Rnit (except in LF), implying that the shifts in functional genes may control Rmin. Given the debate on the role of soil properties and N cycling genes in driving soil N transformation, this study provides new insights into the controlled effect of microbial functional genes during the early stage of forest restoration in the cold temperate zone.

Patterns and drivers of soil net nitrogen mineralization and its temperature sensitivity across eastern China

Patterns and drivers of soil net nitrogen mineralization and its temperature sensitivity across eastern China

Soil net N mineralization and hydraulic properties of carbonate-derived laterite under different vegetation types in Karst forests of China

Soil net nitrogen (N) mineralization (Nmin) is a key process in the forest N cycle regulating the N availability of plant growth. However, it is unclear how N transformation responds to soil hydraulic properties changes. The soil inorganic N pools and N transformation in the early growing season in karst forestlands were investigated by using an intact soil core in situ incubation method. Three different typical vegetation types were selected. The results showed that the mean values of NH4+-N, NO3−-N, and inorganic N were 1.05–1.36, 1.55–3.85, and 1.05–2.34 times greater for ferns than for shrubs. NO3−-N and NH4+-N mainly occur at soil depths of 0–5 cm and 5–15 cm, respectively. The soil Nmin was 2.21–232.03 times higher at 0–5 cm than at the 10–15 cm. Net N immobilization was found for the juvenile ferns and shrubs at 5–15 cm. The Nmin of juvenile and mature ferns was 1.90–11.78 times and 1.17–16.20 times higher than shrubs, respectively, and shrubs had the highest Ks (69.91 mm h−1) but the lowest water-holding capacity. Both ferns and shrubs were able to hold more water and available water was richest in mature fern soil, which provided an extra water source for fern growth. Principal component analysis (PCA) was used to test whether the measured variables affected Nmin, and the results showed that soil organic matter (SOM), pH, and saturated volumetric water content (θs) were the main soil factors affecting Nmin. In addition, the NH4+-N, NO3−-N, and inorganic N stocks were reduced by 3.98 %–59.04 %, 48.07 %–63.30 % and 8.18 %–57.37 % after rainwater input, respectively. Our findings suggest that soil inorganic N and Nmin in the karst forest were regulated by soil hydraulic properties. Changes in the soil hydraulic properties might therefore influence the functioning of soil N transformation.

Interannual variations in natural abundance of nitrogen stable isotopes in soil and plant and their implications in a meadow steppe

Nitrogen is the most limiting nutrient for ecosystems. The natural abundance of δ15N (15N/14N) can efficiently indicate ecosystem nitrogen cycling processes. We investigated the interannual variations in natural abundance of δ15N in soil-plant system and soil net nitrogen mineralization in a meadow steppe of Inner Mongolia. Results across the four sampling years (2017-2020) showed that the content of soil NO3--N (9.83-14.79 mg·kg-1) was significantly higher than that of NH4+-N (3.92-5.00 mg·kg-1) and that δ15N value of soil NH4+ (13.3‰-18.3‰) was significantly higher than that of NO3-(3.76‰-6.14‰). The δ15N value of soil NO3- was negatively correlated with soil NO3- content. The δ15N value of soil NH4+ was relatively higher in the dry years, while the δ15N value of soil NO3- significantly decreased in the wetter and drier years. Soil net mineralization and ammonification rates were significantly higher in the dry years than that of the wet years, while soil nitrification rates showed no correlation with annual precipitation. The δ15N values of plants were not related to that of soils, but nega-tively correlated with plant nitrogen content. Both δ15N values and nitrogen contents were significantly and positively correlated between the leguminous and non-leguminous plants, suggesting that legume could facilitate nitrogen uptake of non-leguminous plants. These results could provide supporting data for nitrogen cycling and their responses to changes in precipitation in grassland soil-plant systems.

Response of Soil Net Nitrogen Mineralization to a Litter in Three Subalpine Forests

Forest litter accumulation can regulate the soil microclimate and alter nutrient distribution, but the effects of litter quality and seasonal differences on soil nitrogen (N) mineralization are still uncertain. The effects of litter change on the rates of net N mineralization, nitrification, and ammonification were studied through in situ incubation experiments in coniferous, mixed, and broad-leaved forests in the eastern Qinghai–Tibetan Plateau. Two litter treatments were established, one to allow the litter to enter the soil normally (remain litter) and the other to prevent the litter from entering the soil (remove litter). Soil samples were collected at the freezing (FS), thawing (TS), early growing (EGS), late growing (LGS), and early freezing (EFS) seasons during the 1.5-year incubation period. Compared to coniferous forests, the effects of litter removal on the net ammonification, nitrification, and N mineralization rates were more pronounced in broad-leaved forests, mainly during the growing and thawing seasons. Structural equation modeling indicated that microbial biomass N (MBN) was a common factor affecting the net ammonification, nitrification, and N mineralization rates in the three forest soils. The coniferous forest microbial biomass carbon (MBC), mixed forest soil moisture, broad-leaved forest soil N concentration, and C:N ratio were the unique influencing factors of the different forest types. The results showed that the effect of litter distribution on the soil net N mineralization mainly depended on forest type and season, suggesting that the litter composition and productivity in different seasons and forest types may alter the soil N cycling processes in subalpine forest ecosystems.

Influence of drought and dry-wet alternation on nitrogen transformation and low abundance microorganisms in tea garden soil

Aim: The soil water availability seriously limits the growth and development of tea plants, and soil microorganisms are an important medium to regulate soil nutrient cycling. In this study, the effects of water supply mode on soil nitrogen nutrition and soil microbes in tea gardens were investigated. Methodology: This experiment set up consisted two water supply modes (consecutive drought and dry-wet alternation) by using the soil microcosm incubation experiment, and four treatments were set: 20% water holding capacity for 21day (D21); 20% water holding capacity for 1-7 days and 60% water holding capacity for 8-21 days (D7W14); 20% water holding capacity for 1-14 day and 60% water holding capacity for 15-21 days (D14W7); 20% water holding capacity for 1-7 days, 60% water holding capacity for 8-14 days, 20% water holding capacity for 15-21 days (D7W7D7). Destructive sampling was carried out to determine soil NH4+-N, NO3--N, soil enzyme activities. 16S rRNA sequencing technique was used to determine the change in soil microbial diversity. Results: The results showed that the consecutive drought reduced the content of soil NH4+-N to 13.97 mg kg-1, and the net nitrogen mineralization was negative (-2.75 mg kg-1) after 21 days of incubation. Dry-wet alternation promoted the increase in of soil net nitrogen mineralization quantity and net nitrification quantity, which rose to 3.48-26.41 mg kg-1 and 8.07-23.11 mg kg-1, respectively. Different water supply modes had no significant impact on the structure of dominant soil microbial community, and the effect mainly focused on relative abundance, especially dry-wet alternation mode. Compared with the continuous drought treatment, the relative abundance of Nitrospirae, Actinobacteria, Chloroflexi, Patescibacteria, Latescibacteria, Rokubacteria, Acidobacteria were significantly different in different dry-wet alternation treatments, while the relative abundance of Nitrospirae, Acidobacteria, Latescibacteria, Gemmatimonadetes, Patescibacteria and Chloroflexi also increased or decreased significantly among different dry-wet alternation treatments. Among the physical and chemical factors of tea garden soil, NO3- had the most significant effect on the structure of microbial community. Interpretation: Different water supply can significantly affect the transformation of soil nitrogen and the change in soil bacterial community in tea garden, which provided a theoretical basis for tea garden to cope with adverse weather changes and maintain the stability of tea garden soil ecosystem.

Black locust coppice stands homogenize soil diazotrophic communities by reducing soil net nitrogen mineralization

Black locust (BL, Robinia pseudoacacia) is considered a promising tree species for reforestation due to its great ability to fix nitrogen. However, after two or three coppice-harvesting rotations, the productivity of BL declines. Whether soil microbial communities are affected and how these groups correlate with the nitrogen mineralization process across multi-generation stands remains unclear. We investigated the composition and structure of free-living nitrogen-fixing microorganisms (diazotrophs) by sequencing the marker gene nifH and compared these results to levels of soil nitrogen mineralization in the bulk soil and rhizosphere in black locust plantations on Mount Tai, China. The results showed multi-generation BL coppice plantations decreased the total soil nitrogen (N), soil phosphorus (P), soil microbial biomass N (MBN), soil microbial biomass C (MBC), soil nitrification rate (Rn), soil ammonification rate (Ra), and net soil N mineralization rate (Rm), but significantly increased the concentration of soil NH4+-N to maintain sufficient NO3−-N. The dominant species in bulk soil and rhizosphere changed from Rhodopseudomonas (22.62% and 15.76%), unclassified_c_Alphaproteobacteria (22.37% and 29.28%), unclassified_o_Rhizobiales (15.40% and 13.31%), Bradyrhizobium (12.00% and 11.74%) in seedling plantations to Bradyrhizobium (45.95% and 47.86%) and Rhodopseudomonas (43.56% and 41.84%) in coppice plantations, respectively. Mantel test and Redundancy analysis (RDA) revealed that Rn, Ra, and Rm were the most important factors shaping the diazotrophic communities. Our results suggest that the multi-generation BL coppice plantation can homogenize soil diazotrophic communities, which is mainly regulated by the available N loss caused by nitrogen mineralization. Strengthening the management technology of coppice plantations will provide more beneficial external consumption.

Increases in substrate availability and decreases in soil pH drive the positive effects of nitrogen addition on soil net nitrogen mineralization in a temperate meadow steppe

Nitrogen (N) deposition is projected to increase over the coming decades, with consequences on soil N transformation. Soil N mineralization is an important process regulating soil N availability. However, it is unclear how the responses of soil N mineralization to increasing N deposition are controlled by biotic and abiotic factors. Here, we examined the effects of increasing N addition rates on soil net N mineralization, net nitrification, and N-cycling functional gene abundance in a temperate meadow steppe after six years treatments. We found that N addition significantly increased the rates of soil net N mineralization and nitrification. Nitrogen addition rates were positively correlated with the abundance of ammonia-oxidizing bacteria amoA gene, but negatively correlated with that of other N-related functional genes. The N-induced enhancement of net N transformations were predominantly driven by the enhanced quantity and quality of soil substrates, but not by N-related functional genes. Decreased soil pH also contributed to the enhancement of net N transformations. Our findings highlight the importance of soil substrate availability and soil pH in driving grassland soil N transformation processes under the scenario of increasing N deposition.

Effects of precipitation change and nitrogen addition on soil net N mineralization in a saline-alkaline grassland of Northern Shanxi Province, China.

To explore the responses of soil net nitrogen (N) mineralization rate to precipitation varia-tion and nitrogen deposition in salinized grassland, we set precipitation manipulation and nitrogen addition experiments in the typical agro-pastoral ecotone saline-alkaline grassland of Northern Shanxi Province, China. The in situ soil net N mineralization rate was determined by top-cover buried PVC cylinder from May to September in 2019. The results showed that there were seasonal dynamics in soil net N mineralization rate. Soil net N mineralization rate was not affected by increase/decrease precipitation (±50%), nitrogen addition (10 g·m-2·a-1) or the combination of nitrogen addition and increase 50% precipitation treatments. The combination of nitrogen addition and 50% reduction of precipitation significantly improved soil net nitrification rate and net N mine-ralization rate by 10.8 and 8.6 times, respectively. Soil net nitrogen mineralization rate was positively related to soil water content, and negatively related to soil pH. The effects of nitrogen addition on soil nitrogen mineralization rate were dependent on precipitation conditions. Soil water content and pH were important factors regulating soil net nitrogen mineralization rate in the saline-alkaline grassland of Northern Shanxi Province. Therefore, to roundly assess the response model of soil N mine-ralization process to global change, it is necessary to consider the interaction of precipitation changes and nitrogen addition, and the soil physical and chemical properties of salinized grassland.

Effects of experimental warming on soil nitrogen transformation in alpine scrubland of eastern Qinghai-Tibet Plateau, China

We investigated the effects of warming on soil nitrogen cycling process in alpine scrub ecosystem, with an in-situ simulated warming experiment at Sibiraea angustata alpine scrubland on the eastern Qinghai-Tibet Plateau, China. We examined the responses of soil nitrogen transformation rate to warming in three critical periods (the early, late, and non-growing seasons). The results showed that warming increased soil temperature by 1.2 ℃, but decreased soil moisture by 2.5%. The soil net nitrogen mineralization rates (i.e., ammonification and nitrification) in the growing season were significantly higher than those in the non-growing season. The rates of soil net nitrogen fixation in the non-growing season were significantly higher than that in the growing season. Soil nitrification was the major process of soil nitrogen transformation in the early growing season, while soil ammonification was the major one in the late growing season and non-growing season. The effects of experimental warming on soil nitrogen transformation differed among those three periods. Experimental warming significantly increased soil net ammonification, nitrification, nitrogen mine-ralization and fixation in the early growing season, and enhanced soil net nitrification and nitrogen mineralization in the non-growing season. However, warming significantly decreased soil net nitrification, nitrogen mineralization and fixation in the late growing season and soil net ammonification in the non-growing season. Moreover, warming did not affect soil net nitrogen fixation rates in the non-growing season and soil net nitrification rates in the late growing season. Future climate warming would significantly change soil nitrogen transformation by accelerating soil nitrogen cycling in the alpine scrub ecosystem on the eastern Qinghai-Tibet Plateau.

Temporal variability in soil net nitrogen mineralization among forest regeneration patterns in eastern Tibetan Plateau

The dynamic effects of forest regeneration pattern after clear-cutting primary forest on soil N mineralization (Nmin) have not been well investigated and whether the effects vary with season is unclear in eastern Tibetan Plateau. In this study, we compared soil net Nmin, i.e. net ammonification rate (net Ra) and net nitrification rate (net Rn) among Picea asperata-broadleaf mixed forest (natural regeneration after planting P. asperata, MF), natural secondary forest (natural without assisted regeneration, NF) and P. asperata plantation (reforestation after clear-cutting primary forest, PF) from June to November, and measured soil microbial community and enzyme activities. Forest regeneration pattern significantly altered net Ra as well as net Nmin in June and November. Net Ra of MF, NF and PF in June was −0.29, −0.52 and 0.04 mg kg−1 d-1, respectively. NF was likely to have higher net NH4+ consumption in June presumably by the greater microbial utilization (biomass and enzymes) directly drove by the larger initial NH4+ concentration. NF had higher net NH4+ production (0.30 mg kg−1 d-1) in November than MF (0.15 mg kg−1 d-1) and PF (0.03 mg kg−1 d-1). Net Rn was insignificantly different among the regenerated forests and ranged between −0.13 and 0.39 mg kg−1 d-1 with a unimodal seasonal pattern. Nitrate dominated the inorganic N pool in PF while it was similar to ammonium in MF and NF in the middle of growing season, possibly suggesting an alteration of N preferences among the regenerated forests. Independent edaphic factors (including soil organic matter and available N) had greater explanations of net Ra variability (19.0%), while net Rn variability was more determined by enzyme activities (21.8%) and microbial community (13.1%). Overall, our results highlighted that the variations in soil net Nmin among three regenerated forests were dependent on season. Ammonium concentration before the start of growing season might play an important role in soil N dynamics. NF exhibited a larger N demand with regard to net Ra, microbial induces and N-acquisition enzymes. These provided essential information for understanding soil N dynamics and for developing management practices of subalpine forests in eastern Tibetan Plateau.

Global impacts of fertilization and herbivore removal on soil net nitrogen mineralization are modulated by local climate and soil properties.

Soil nitrogen (N) availability is critical for grassland functioning. However, human activities have increased the supply of biologically limiting nutrients, and changed the density and identity of mammalian herbivores. These anthropogenic changes may alter net soil N mineralization (soil net Nmin ), that is, the net balance between N mineralization and immobilization, which could severely impact grassland structure and functioning. Yet, to date, little is known about how fertilization and herbivore removal individually, or jointly, affect soil net Nmin across a wide range of grasslands that vary in soil and climatic properties. Here we collected data from 22 grasslands on five continents, all part of a globally replicated experiment, to assess how fertilization and herbivore removal affected potential (laboratory-based) and realized (field-based) soil net Nmin . Herbivore removal in the absence of fertilization did not alter potential and realized soil net Nmin . However, fertilization alone and in combination with herbivore removal consistently increased potential soil net Nmin. Realized soil net Nmin , in contrast, significantly decreased in fertilized plots where herbivores were removed. Treatment effects on potential and realized soil net Nmin were contingent on site-specific soil and climatic properties. Fertilization effects on potential soil net Nmin were larger at sites with higher mean annual precipitation (MAP) and temperature of the wettest quarter (T.q.wet). Reciprocally, realized soil net Nmin declined most strongly with fertilization and herbivore removal at sites with lower MAP and higher T.q.wet. In summary, our findings show that anthropogenic nutrient enrichment, herbivore exclusion and alterations in future climatic conditions can negatively impact soil net Nmin across global grasslands under realistic field conditions. This is an important context-dependent knowledge for grassland management worldwide.

Soil microbial biomass and extracellular enzymes regulate nitrogen mineralization in a wheat-maize cropping system after three decades of fertilization in a Chinese Ferrosol

Soil net nitrogen (N) mineralization is a vital process that impacts the global N cycling and regulates the N availability for plant development. The objectives of this study were to evaluate the response of N mineralization to long-term organic versus inorganic fertilization and to quantify the relationships between N mineralization and soil microbial characteristics in the ferrosol (red soil) of South China after 30 years of mineral and manure application in a wheat-maize cropping system. Soil was sampled from a wheat-maize rotation system, consisting of five treatments. The treatments included (1) CK (no fertilizer), (2) PK (synthetic phosphorus and potassium fertilizer), (3) NK (synthetic nitrogen and K fertilizer), (4) N (synthetic N fertilizer), and (5) NPKM (synthetic NPK fertilizer and manure). The sampled soil was analyzed for physicochemical parameters and incubated for the determination of N mineralization, soil microbial biomass carbon (SMBC), soil microbial biomass nitrogen (SMBN), and associated soil enzymes related to C and N cycling. Results showed that NPKM increased soil organic carbon (SOC), available P (AP), total P (TP), and total nitrogen (TN) by 132%, 880%, 293%, and 76% respectively, over the control (CK). Among different treatments, soil microbial biomass nitrogen (SMBN) and carbon (SMBC) were highest under the NPKM treatment. N-cycling and P-cycling enzyme activities also showed significant differences among treatments. N-Acetyl-β-d-glucosaminidase (NAG), leucine-aminopeptidase (LAP), and acid phosphatase (AcP) activities were also highest under the NPKM treatment, at 650.36, 32.36, and 23.41 (mol g−1 h−1), respectively. A linear increase was observed in the NO3−-N and NH4+-N concentrations throughout the 90-day incubation period. NPKM showed a maximum N mineralization potential (No) and mineralization rate constant, k (NMR), at the end of the incubation period. A principal component analysis (PCA) interpreted the differences among fertilization and their effects on net N mineralization. A significant (p ≤ 0.05) positive correlation was observed between SMBC (R2 = 0.87), SMBN (R2 = 0.92), enzyme activities, and the No. Structural equation modeling (SEM) revealed that SOC, TN, and TP directly affected mineralization, while SMBC and SMBN indirectly affected the net mineralization. Manure input increased the extracellular enzymes in soil, which accelerated the net N mineralization due to enhanced soil microbial activities. Consequently, long-term manure addition appears to be an optimal approach to meet the nutrient demands and to enhance the N availability in a wheat-maize cropping system.

Response of soil net nitrogen mineralization to different levels of nitrogen addition in a saline-alkaline grassland of northern China

Response of soil net nitrogen mineralization to different levels of nitrogen addition in a saline-alkaline grassland of northern China