Search

Purpose: The mineralization rate of soil carbon (C) and nitrogen (N) is important for determining soil C storage as well as nutrient supply and retention. Soil C and N decomposition processes are microbially driven and are therefore expected to be influenced by the balance between soil resource availability and microbial resource demand. However, we lack understanding of how the microbial uptake and mineralization of soil C and N is affected by different levels of fertilization and crop rotation patterns in agricultural systems. Materials and methods: Soils from a field experiment including five levels of N fertilization (0 kg N ha−1 (control, 0), 84 kg N ha−1 (low N application), 95 kg N ha−1 (moderate N application), 105 kg N ha−1 (conventional N application) and 115.5 kg N ha−1 (high N application)) were used to determine soil C and N mineralization and retention, in either a tobacco plantation either under tobacco monoculture or tobacco-maize rotation. Results and discussion: Nitrogen fertilizer application increased net N mineralization (40–307%), net nitrification (150–400%), microbial NUE (131–373%) and CUE (16–57%) but decreased respiration (11–42%) in monoculture system, due to the significant higher DOC concentration and microbial C limitation. However, N fertilizer application increased net N mineralization (67–400%), net nitrification (50–544%), and microbial NUE (84–438%) but reduced microbial respiration (56–71%) and CUE (8–39%) in rotation system, due to the lower microbial activity caused by significant higher microbial C and N limitation and poorer C quality. Therefore, fertilization aggravated microbial resource limitation and lowered quality indicated by elemental stoichiometry in rotation system, leading to decoupling of microbial respiration and metabolism. Conclusions: Together, our results suggest that elemental stoichiometry and enzyme activities can be used to predict soil C and N cycling under different agricultural management practices. [ABSTRACT FROM AUTHOR]

An ex-situ 15 N–18O tracing experiment with soils collected from the valley and slope, respectively, of a subtropical secondary karst forest with three N addition levels, i.e., 0, 50, and 100 kg N ha−1 year−1 for each topographic position to investigate N2O production pathways. Autotrophic nitrification pathways (ammonia oxidation, nitrifier denitrification, and nitrification-coupled denitrification) accounted for > 70% of total N2O production, but denitrification pathways (heterotrophic denitrification and co-denitrification) were the minor source of N2O at both topographic positions. In the valley, chronic N addition stimulated ammonia oxidation-derived N2O, which was paralleled by increased ammonia-oxidizing archaea (AOA) amoA gene transcript abundance, but inhibited nitrifier denitrification- and nitrification-coupled denitrification–derived N2O along with suppressed ammonia-oxidizing bacteria (AOB) amoA gene transcript abundance and stimulated nosZII gene transcript abundance, respectively. On the slope, chronic N addition stimulated ammonia oxidation-derived N2O along with increased AOB amoA gene transcript abundance, and enhanced nitrifier denitrification-derived N2O congruent with increased AOB amoA and decreased nirK gene transcript abundances. In addition, chronic N addition reduced the relative contribution of heterotrophic denitrification to N2O production but had no significant influence on heterotrophic denitrification-derived N2O on the slope. Overall, our results provide a comprehensive view in terms of how topography-driven soil properties regulate N2O production and its pathways in a subtropical forest. [ABSTRACT FROM AUTHOR]

The influences of atmospheric nitrogen (N) deposition and topographical position on soil methane (CH 4) uptake have been intensively investigated; however, it remains unclear whether topographic position regulates the responses of soil CH 4 uptake to N deposition. Here, we examined the effects of N addition on soil CH 4 uptake in the valley and on the slope of a subtropical karst forest. Control (0 kg N ha−1 yr−1), moderate N addition (50 kg N ha−1 yr−1), and high N addition (100 kg N ha−1 yr−1) treatments were included at each topographic position. CH 4 fluxes were determined from 2017 to 2019 along with potential methane production rate (PMPR), potential methane oxidation rate (PMOR), mcrA and pmoA gene abundances, and plant and soil properties. The averaged annual CH 4 uptake under the control was 0.67 ± 0.07 kg CH 4 ha−1 yr−1 in the valley and 0.99 ± 0.32 kg CH 4 ha−1 yr−1 on the slope across the three years. High N addition consistently increased CH 4 uptake in the valley in 2017 and 2019 and on the slope in 2017 and 2018. The annual CH 4 uptake was stimulated by high N addition by 81.27 % in the valley owing to inhibited PMPR caused by increased NO 3 −, and to stimulated PMOR caused by increased NO 3 − and soil organic carbon. On the slope, annual CH 4 uptake was enhanced by high N addition by 75.83 % attributed to inhibited PMPR caused by increased fine root biomass and to stimulated PMOR induced by increased soil phosphorus availability. Our study highlights the importance of topography in regulating soil CH 4 uptake under elevated N addition. • N addition significantly increased soil CH 4 uptake in the valley and on the slope. • N addition stimulated CH 4 uptake due to increased NO 3 − and SOC in the valley. • N addition increased CH 4 uptake via raising available P and fine root on the slope. [ABSTRACT FROM AUTHOR]

A greenhouse field experiment involving tomato (Solanum lycopersicum) was performed using different nitrogen (N) management regimes: sole application of differing rates of chemical N fertilizer (SC) (SC treatments: N0, N1, N2, and N3) and combined application of manure and chemical N fertilizer (MC) (MC treatments: MN0, MN1, MN2, and MN3). These were used to understand the relationship between comprehensive fruit composition, yield, and N fractions (soil mineral N; soil soluble organic N; soil microbial biomass N, and soil fixed ammonium) under greenhouse conditions. The results showed that the MC treatments significantly increased vitamin C and soluble sugar content compared with SC treatments. In addition, the MN2 treatment produced a high yield and had a positive effect on fruit composition. The N3 (563 kg N/ha) and MN3 (796 kg N/ha) treatments resulted in a high loss of N below the root zone (0-30 cm), consequently reducing N use efficiency. Soil mineral N, soil soluble organic N, and soil fixed ammonium tended to be higher during the first fruiting period, whereas soil microbial biomass N tended to be higher during the second fruiting period. MC treatments significantly increased the N fraction in the 0- to 30-cm soil layer; N fractions tended to be higher with the MN2 treatment. According to an optimum regression equation, soil fixed ammonium during the first fruiting period and soil microbial biomass N during the second fruiting period had a more significant influence on tomato yield and fruit composition. Overall, application MC at an appropriate rate (MN2: 608 kg N/ha) is a promising approach to achieving high yields and optimum taste, and it offers a more sustainable fertilizer management strategy compared with chemical N fertilization. [ABSTRACT FROM AUTHOR]

Greenhouse vegetable soils are characterized by high N 2 O emissions, but the functional importance of potential ammonia (NH 3 ) oxidation (PAO) and nitrite (NO 2 − ) consumption (PNC) by ammonia-oxidizing archaea (AOA) and bacteria (AOB) is poorly understood. Inhibitors of 1-octyne and 2-pheny l-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide (PTIO) in combination with potassium chlorate were used to examine the importance of AOA and AOB in NO 2 − production and consumption, to evaluate the soil-specific effects on N 2 O production under six long-term fertilization greenhouse vegetable soils across mainland China, and to predict their thermodynamic responses in a temperature gradient of 5–45 °C using the square root growth (SQRT) and macromolecular rate theory (MMRT) models. The ammonia oxidizers-driven PAO and PNC exhibited a strong response to temperature with maximum rates attained at 35 °C or 40 °C by AOA compared to 30 °C or 35 °C by AOB. Concomitantly, compared with ammonium amendment, NO 2 − addition stimulated N 2 O production by approximately 2–4-fold for AOA and 2–9-fold for AOB at 20–40 °C in four of six soils. AOA exhibited a broader range of temperatures (−33.2–56.8 °C vs −16.1–52.3 °C) and lower relative temperature sensitivity (Q 10 ) than AOB ( p  < 0.05). The heat capacity (ΔC P ‡ ) of AOA and AOB in PAO and PNC increased in relation to the optimum temperature (T opt ) and exhibited a greater negative value by AOB than AOA, indicating that specialized ammonia-oxidizer populations of PAO and PNC adapted to in situ soil environmental changes. Structural equation modeling revealed that potential AOA-driven N 2 O production was influenced by ΔC P ‡ and Q 10 and C/N, while AOB-driven N 2 O production by ΔC P ‡ and Q 10 and soil nitrate (NO 3 − ) content, all of which is influenced directly and indirectly by in situ climate parameters. Findings suggest the important roles of specific in situ climate and soil properties and provide new insights into the NO 2 − consumption in soil N 2 O production by ammonia-oxidizers. [ABSTRACT FROM AUTHOR]

• N addition increased the rates of most N transformation processes in the valley. • N addition decreased the rates of most N transformation processes on the slope. • Topography modulates effects of N deposition on soil N transformations. Soil nitrogen (N) transformations play key roles in ecosystem productivity and other functions of terrestrial ecosystems via regulating soil N availability. Although the effects of elevated atmospheric N deposition on soil N transformations have been intensively investigated, it remains unclear whether the effects are mediated by topography, which impacts multiple soil abiotic and biotic properties. Here, we conducted an N addition experiment consisting of three treatments: control (0 kg N ha−1 yr−1), moderate N addition (50 kg N ha−1 yr−1), and high N addition (100 kg N ha−1 yr−1) in the valley and on the slope, respectively, of a subtropical karst forest. Under the control, protein depolymerization, amino acid uptake, nitrification and dissimilatory nitrate reduction to ammonium (DNRA) rates were significantly higher on the slope than in the valley attributed to the higher soil dissolved organic carbon, total dissolved N and pH on the slope. Nitrogen addition significantly increased the rates of protein depolymerization, amino acid uptake, mineralization, nitrification and DNRA through alleviating microbial carbon limitation in the valley, but decreased the rates of depolymerization, amino acid uptake, mineralization and DRNA by enhancing microbial carbon and phosphorus limitations on the slope. The increase in soil N transformations, microbial N use efficiency (NUE) and microbial biomass N but lowered microbial N turnover time resulted in a 94 % increase of total microbial necromass N in the valley under N addition. However, increase in NUE and microbial biomass N led to a 33 % decrease of necromass N due to enhanced microbial necromass destabilization on the slope under N addition. Our results suggest that the responses of soil N transformations to N addition may be mediated by topography, and hence highlight the importance of incorporating topography into Earth system models to better predict soil N dynamics in forest in the context of elevated atmospheric N deposition. [ABSTRACT FROM AUTHOR]

Field experiments were conducted to determine the effects of nitrogen (N) fertilization and manure addition on the soil-soluble nitrogen (SSN) (soil mineral N (SMN); soil-soluble organic N (SSON)) and soil-retained N (SRN) (soil fixed ammonium; soil microbial biomass N). The combined application of manure and inorganic N (different N fertilizer rates: M3N0, M3N1, M3N2, and M3N3; different manure rates: M0N2, M1N2, M2N2, and M3N2) was used in a greenhouse fertilization experiment. SSN and SRN increased with increasing N rate up to M3N2. SSON decreased with increasing manure rate and was the highest in the M1N2, whereas SMN and SRN were the highest in the M3N2, and increased with increasing manure rate on all sampling dates. Both SSN and SRN declined significantly with increasing soil depth in the different application rates of manure (p< .05). Moreover, the SSN and SRN significantly varied with plant growth and followed a different pattern during the growing season. SSN and FA peaked in the first ear fruit period, but SMBN was at its highest level in the second ear fruit period. There was a significant positive relationship (p< .05) between SSN and SRN throughout the plant growing season, and the annual apparent loss of N in the M3N3 was the highest. The combined application of inorganic N fertilizer and manure at an appropriate rate may be an effective strategy for maintaining the long-term health of greenhouses. [ABSTRACT FROM AUTHOR]

• N addition enhanced soil respiration by increasing microbial activity in the valley. • N addition increased soil respiration via increasing fine root biomass on the slope. • Greater response of soil respiration in the valley than that on the slope under N50. Soil respiration is the largest terrestrial source of atmospheric carbon dioxide, and hence plays a vital role in the carbon cycle-climate feedback. Although the effects of elevated atmospheric nitrogen (N) deposition on soil respiration have been intensively investigated, it remains unclear whether the effects are mediated by topography, which impacts multiple soil abiotic and biotic properties. Here, a field N addition experiment with three rates, i.e., 0, 50 and 100 kg N ha−1 yr−1 was performed in the valley and on the slope of a subtropical karst forest in southwest China to explore whether and how topography mediates soil respiration. Nitrogen addition increased soil respiration by 55–63 % in the valley due to increased microbial activity. However, high N addition stimulated soil respiration by 56 % on the slope due to enhanced fine root biomass. The response of soil respiration to moderate N addition was significantly greater in the valley than on the slope, likely owing to the lower N availability in the valley. Our results demonstrate that the responses of soil respiration to N addition were mediated by topography, and hence highlight the importance of incorporating topography into Earth system models to better predict forest carbon dynamics in the context of elevated atmospheric N deposition. [ABSTRACT FROM AUTHOR]