Soil Soluble Organic Nitrogen Research Articles

Response of soil nitrogen components and its vertical distribution to rainfall redistribution during Robinia pseudoacacia forest restoration on the Loess Plateau

The effects of vegetation restoration on soil moisture dynamics and nutrient content have been widely studied; however, the knowledge on variation in rainfall redistribution and soil nitrogen component characteristics remain limited, particularly with respect to their relationship in restored ecosystems. Therefore, the characteristics of plant community structure, rainfall redistribution, and soil nitrogen components from farmland and Robinia pseudoacacia (RP) plantations of four restoration ages (12, 20, 30, and 45 a) were investigated and measured in the Wuliwan watershed in the Loess Plateau, China. The results showed that changes in plant community structure affected the rainfall redistribution processes. The throughfall and moisture entering the soil (MES) had the largest proportions (79.50 % and 60.25 %) in the initial and second distributions of atmospheric rainfall, respectively. With the increase in restoration age, the content of soil total nitrogen (STN), soil nitrate nitrogen (SNN), soil soluble total nitrogen (SSTN), soil soluble organic nitrogen (SSON), and soil soluble inorganic nitrogen (SSIN) all increased with restoration age in the upper soil (0–40 cm), fluctuated in the middle soil (40–100 cm), and increased and then decreased in the deeper soil (100–200 cm). In addition, the vegetation restoration process can indirectly affect soil nitrogen content and distribution patterns by affecting the rainfall redistribution process. In the upper soil, litter interception (LI) had a significant effect on STN by affecting SSIN and SSON. In the middle soil, both LI and MES had significant effects on STN through SSIN and SSON, respectively. In the deeper soil, LI had no significant effect on any nitrogen component, whereas MES had a significant effect on STN by affecting SSIN. Overall, this study indicates that RP plantation restoration has the potential to transform rainfall redistribution and soil nitrogen distribution, providing an important reference for the reciprocal feedback relationship between water and nitrogen.

Mutual feeding mechanism of carbon, nitrogen and enzyme activity in Northeast China black soil under snow cover change

Projected future changes in snow cover patterns associated with global warming in cold zone ecosystems could affect soil biochemical cycling. However, the effects of snow cover changes on soil available carbon, nitrogen and enzyme activities and their potential response mechanisms have not been clarified. Therefore, from November 2021 to April 2022, this study conducted a snow depth manipulation test of four treatments in the northeast black soil region, and divided the test period into five stages to measure soil temperature and humidity, microbial biomass, enzyme activity, and available carbon and nitrogen. The results showed that the decrease of snow cover increased the freeze-thaw cycle frequency and freezing temperature of soil, but decreased the soil water content. Soil total organic carbon and inorganic nitrogen contents were increased in early and deep snow periods, while snow treatment was on the contrary. Due to the release of soluble nutrients caused by frequent freeze-thaw processes, Soil soluble organic carbon and Soil soluble organic nitrogen contents increased with the decrease of snow depth in deep snow period, snowmelt period and subsequent early crop growth period. Snow treatment increased soil microbial carbon and nitrogen content in early winter and early spring because snow provided heat insulation. Soil enzyme activities increased with the increase of snow cover. Compared with the control, soil urease activities and sucrase activities increased by 18.5 % and 11.5 % under snow treatment, and decreased by 23.2 % and 10.8 % under snow reduction treatment. In addition, soil soluble organic matter was a controlling factor for soil microbial biomass and enzyme activity throughout winter. The direct effect of soil soluble organic carbon and nitrogen on soil enzymes will make soil enzymes participate in the cyclic transformation process of available carbon, thus forming a closed loop of mutual feedback between soil available carbon and nitrogen and enzymes. These results demonstrated that the changes of snow cover in the future will have certain effects on soil carbon and nitrogen cycles and enzyme activities and hence biogeochemical cycling in terrestrial system of earth.

Impacts of Grazing Disturbance on Soil Nitrogen Component Contents and Storages in a Leymus chinensis Meadow Steppe

Long-term grazing leads to soil degradation in Inner Mongolia grassland. Based on the Hulunbeier meadow steppe, the variation characteristics of soil nitrogen content and storage in soil layers between 0–40 cm, under six different grazing intensities, and the response of vegetation and other physical and chemical properties of soil to grazing were studied. The main results were as follows: (1) Moderate grazing increased soil total nitrogen (TN), soluble total nitrogen (STN) and microbial biomass nitrogen (MBN) contents, while heavy grazing decreased MBN content. In the year with more rain, heavy grazing increased nitrate nitrogen (NO3−-N) content and storage, while less rain increased ammonium nitrogen (NH4+-N) content. (2) The proportion of 0–40 cm nitrogen components showed an upward trend in the year with more rain, and the opposite in the years with less rainfall with the increase of grazing intensity. Soil soluble organic nitrogen (SON) and NO3−-N storages decreased and MBN storage increased in rainy years. (3) Soil nitrogen component contents and storages were correlated with plant growth status, soil moisture (SM) and soil bulk density (SBD), and were significantly negatively correlated with soil temperature (ST) and pH (p < 0.05). The content and storage of soil nitrogen were affected by grazing, soil, vegetation, meteorological and other environmental factors. Moderate grazing was more conducive to the improvement of soil nitrogen storage capacity and the healthy development of grassland.

Identification of critical effect factors for prediction of spatial and intra-annual variability of shallow groundwater nitrate in agricultural areas

Shallow groundwater nitrate nitrogen (NO3−-N) concentrations in agricultural areas usually show high spatial and intra-annual variability. It is hard to predict such concentrations due to the complexity of influencing factors (e.g., different forms of N in soil, vadose zone characteristics, and groundwater physiochemical conditions). Here, a large number of groundwater and soil samples were collected monthly over two years at 14 sites to analyze the soil and groundwater physiochemical properties and the stable isotopes of δ15N and δ18O of groundwater NO3−-N in agricultural areas. Based on field observations, a random forest (RF) model was used to predict the groundwater NO3−-N concentrations and reveal the importance of effect factors. The results show that there are large spatiotemporal variations in NO3−-N, δ15N-NO3−, and δ18O-NO3− in groundwater. NO3−-N is the major dominant specie of inorganic N in groundwater, and the groundwater NO3−-N concentration in 24 % of the samples failed to meet the drinking water standard of the WHO (10 mg L−1). The RF model satisfactorily predicted groundwater NO3−-N concentrations with R2 of 0.90–0.94, RMSE of 4.54–5.07, and MAE of 2.17–3.38. Groundwater nitrite and ammonium are the most important factors related to NO3−-N consumption and production, respectively, in groundwater. Denitrification and nitrification were further identified by the relationships among δ15N-NO3−, δ18O-NO3−, and NO3−-N, and by the ranges of δ15N-NO3−, δ18O-NO3−, temperature, pH, DO, and ORP in groundwater. Soil-soluble organic nitrogen (S-SON) and the depth of groundwater table were identified as vital factors related to N sourcing and leaching. Overall, as a first approach to adopting a RF model for high spatiotemporal-resolution prediction of groundwater NO3−-N variations, the findings of this study enable a better understanding of groundwater N pollution in agricultural areas. Optimizing management of irrigation and N inputs is anticipated to reduce S-SON accumulation and mitigate the threat to groundwater quality in agricultural areas.

Research progress on soil soluble organic nitrogen

Soluble organic nitrogen (SON) and inorganic N are two crucial nitrogen (N) forms in the cycling of N within terrestrial ecosystems, acting as either a "source" or a "sink" to the environmental N release. The mineralization, retention, leaching, and plant absorption of N in terrestrial ecosystems are closely related to SON. As a result, the role of SON in soil material circulation and nutrient flow has attracted much attention and has become one of the hotspots in various research fields, such as ecology, environmental science, soil science, and hydrology. We reviewed the research progress on soil SON, including the definition and quantification, the size and composition, the absorption and utilization by plants and microorganisms, the sources and influencing factors, and the transformation, migration, and leaching loss of SON. SON is a complex collection of multi-component soluble organic matter, mainly as recalcitrant components (difficult to degrade), with relatively low proportion as labile components (easily degradable). Due to the difference in the turnover time among recalcitrant and labile components, the roles of SON in N cycling and turnover cannot be fully represented by the SON quantity. Therefore, to accurately reflect the role of SON in N turnover, N uptake, and N leaching, it is necessary to establish new methods and distinguish between recalcitrant and labile SON components in future studies. When studying the role of SON in N conversion and N absorption, it is essential to focus on its labile components. When studying the contribution of soil SON to N leaching or runoff loss, it is necessary to focus on the recalcitrant components.

Dynamics and influencing factors of soluble organic nitrogen in paddy soil under different long-term fertilization treatments

Soil soluble organic nitrogen (SON) plays an important role in nitrogen cycling. However, the dynamic and influencing factors of SON in paddy soil are largely unknown. In this study, 36-year fertilization experiments with different fertilized (control (CK), chemical fertilizer (NPK), chemical fertilizer combined with cow manure (NPKM) and chemical fertilizer combined with straw (NPKS)) paddy soils were selected to investigate the dynamic changes and influencing factors of the SON content during the whole rice growth stage. We found that NPK, NPKM and NPKS increased the soil SON content by 20.61 %, 51.33 % and 52.50 %, respectively, compared with CK. Compared with the NPK treatment, the average SON contents in the NPKM and NPKS treatments were significantly increased by 25.46 % and 26.44 %, respectively, but there was no significant difference between the NPKM and NPKS treatments. Soil SON contents in different growth stages under the same fertilization treatments showed maturity stage > tillering stage > flowering stage > jointing stage > seedling stage. Structural equation model (SEM) analysis showed that bacterial biomass was an important factor affecting soil SON content, with a total path coefficient of 0.92, including direct path coefficients of 0.24 for soil SON and indirect path coefficients of 0.68 for soluble organic carbon, microbial biomass nitrogen and protease activity. Redundancy sequencing analysis showed that SON contents during different rice growth stages were significantly affected by the bacterial community, i.e., Chloroflexi in the early growth stage and Proteobacteria and Bacteroidetes in the later stage contributed 5.33 %, 37.40 %, and 2.17 % to the SON content variation, respectively. Conclusively, the dynamics of SON in paddy soil under long-term fertilization treatments are closely related to both fertilization treatments and the rice growth stage and are mainly affected by the bacterial community.

Driving factors of soluble organic nitrogen dynamics in paddy soils: Structure equation modeling analysis

Soil soluble organic nitrogen (SON) is one of the most active components in soil nitrogen pools; however, limited information is available with regard to its driving factors, as well as their pathways and degrees of influence. In this study, structural equation modeling was used to analyze the driving factors, their significance, and pathways that affected SON dynamics in a waterlogged experiment of two typical paddy soils incubated for 80 d after green manure application. Soil pH, Eh, microbial biomass, enzyme activity, and SON dynamics were used to construct the structural equation model. Results showed that soil microbial biomass carbon (MBC), protease, glutamine, and initial organic matter (OM) directly and significantly affected soil SON with path coefficients corresponding to 0.405, 0.547, 0.523, and–0.623 (P < 0.01), respectively. Soil microbial biomass carbon and initial OM affected the SON dynamics indirectly through protease and glutamine activity. In addition, pH indirectly affected SON dynamics by glutamine activity. It is implied that soil MBC, protease, glutamine, and initial OM are the key factors affecting SON dynamics in the waterlogged paddy soils after green manure application. Our research indicated that structural equation modeling could provide an effective method to clearly recognize the impact, significance, and pathways of multiple factors on SON dynamics in paddy soils.

Evaluating the mechanisms of the impacts of key factors on soil soluble organic nitrogen concentrations in subtropical mountain ecosystems

Soil soluble organic nitrogen (SON) concentrations in terrestrial ecosystems were influenced differently and substantially by both biotic and abiotic factors. This study aimed to ascertain the mechanisms of the impact of the key factors on the SON concentrations of subtropical mountain ecosystems in southeastern China using an integrative approach, which combined a field plot survey, gray relational analysis and structure equation modeling. The results showed that the soil organic matter, clay content, protease activity and bacterial biomass were the key factors controlling the dynamics of the SON concentrations in subtropical mountain ecosystems. Protease activity, by catalyzing the degradation of complex organic nitrogen to SON, had the highest direct influence on the SON concentrations among all of the impact factors with direct impact effect of 0.44. Organic matter, which serves as a primary source of SON and can increase soil protease activity and bacterial biomass, contributed the most significantly to the SON concentrations in both direct and indirect pathways with total impact effects of 0.87. Clay, by adsorbing SON and affecting organic matter accumulation and protease activity, also had important direct or indirect influences on the SON concentrations with total impact effects of 0.48. The impact of the bacterial biomass on the SON concentrations was likely to be concealed by accompanying nitrogen-degrading enzyme activity with total impact effects of 0.22. Thus, the organic matter, clay content and protease activity exerted greater total impact effects on the SON concentrations compared with the bacterial biomass. Protease activity and organic matter had a greater positive direct impact on the SON concentrations compared with the bacterial biomass and clay content, while organic matter also had greater positive indirect impacts on the SON concentrations than did the clay content. This study's results could help to elucidate the differential mechanism of SON dynamics among various terrestrial ecosystems.

Effects of Various Combinations of Fertilizer, Soil Moisture, and Temperature on Nitrogen Mineralization and Soluble Organic Nitrogen in Agricultural Soil

An 84-day laboratory incubation experiment was conducted to investigate the effects of different fertilizers (urea; manure), moisture conditions (60%, 75% and 90% water holding capacity) and temperatures (15, 25 and 35℃) on nitrogen mineralization. The experiment included 3 treatments:①CK, unfertilized control; ② Ur, adding urea at N 120 mg·kg-1; 3 UM, adding urea and manure (equal to adding N 120 mg·kg-1). Total inorganic nitrogen and soluble organic nitrogen (SON) were determined at different times throughout the experiment. The results showed that soil temperature and fertilization type had significant impacts on the net mineralization rates, cumulative mineralization, and the potentially mineralizable nitrogen (N0) (P<0.01). In addition, the soil net N mineralization rates and cumulative mineralization significantly (P<0.05) increased by 1.46-8.17 and 2.00-8.15 times, respectively, when fertilizers were added into soils. The soil net N mineralization rates and cumulative mineralization increased with the increase of temperature. Compared with CK treatment, Ur and UM treatments could significantly increase the content of soil soluble organic nitrogen(SON). There was a significant negative correlation between the content of SON and cumulative mineralization. It indicated that SON was involved in soil nitrogen mineralization as a non-negligible component. Increasing the temperature could significantly increase the mineralization rate and mineralization intensity of SON in soil, but the water content had no significant influence on the SON of the soils. Moreover, the authors found that fertilization treatment worked significantly in decreasing the Q10 value for soil N mineralization compared with CK treatment. Further, the Q10 value was significantly lowest in UM treatment(Q10=1.01). The results showed that the application of organic manure significantly reduced the sensitivity of the rate of nitrogen mineralization to temperature changes, which was beneficial in slowing down the release rate of mineral nitrogen under high temperatures and improving the nitrogen utilization efficiency of crops.

黄土丘陵区植被恢复对土壤可溶性氮组分的影响

PDF HTML阅读 XML下载 导出引用 引用提醒 黄土丘陵区植被恢复对土壤可溶性氮组分的影响 DOI: 10.5846/stxb201603090416 作者: 作者单位: 西北农林科技大学林学院,西北农林科技大学农学院,西北农林科技大学林学院,西北农林科技大学农学院,西北农林科技大学农学院,西北农林科技大学资源环境学院,西北农林科技大学农学院 作者简介: 通讯作者: 中图分类号: s157.5 s714.8 基金项目: 国家自然科学青年基金项目（41301601）；陕西省科技统筹创新工程项目（2015KTCL02-07）；国家自然科学基金项目（41571501） Effects of vegetation restoration on soil soluble nitrogen in the Loess Hilly Region Author: Affiliation: College of Forestry,Northwest A&F University,,,,,,College of Agronomy,Northwest ADdDdF University Fund Project: 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:为探究黄土丘陵地区人工植被恢复对土壤氮素养分累积与有效性的影响，研究分析了植被恢复15年刺槐、柠条、刺槐侧柏混交、刺槐山桃混交以及荒草地土壤可溶性氮组分含量及其垂直分布特征。结果表明，与耕地相比，植被恢复显著提高了0-30 cm土壤可溶性氮组分含量，这也使0-30 cm土壤可溶性氮组分密度显著增加，可溶性有机氮密度增幅表现为柠条（262.2%）> 刺槐（232.8%）> 刺槐山桃混交、刺槐侧柏混交（34.5%） > 荒草地（-21.5%），硝态氮密度整体表现为柠条>刺槐>刺槐山桃混交>荒草地>刺槐侧柏混交，增幅为7.9%-182.8%，铵态氮密度以刺槐山桃混交增幅最大（110.3%），荒草地最小为2.6%。可溶性有机氮、硝态氮占全氮的比例以刺槐最高，分别提升了2.4倍和0.6倍，铵态氮占全氮的比例以刺槐山桃混交最高，提升了1.0倍。可溶性氮组分受微生物量碳氮的影响大于有机质和全氮，微生物量氮与可溶性氮组分的相关性优于微生物量碳，硝态氮对土壤有机质、全氮和微生物量碳氮的变化最为敏感。综上，植被恢复能够提高土壤可溶性氮组分含量、密度及其占全氮比例，增加土壤氮的有效性，以刺槐、柠条提升效果最好。 Abstract:Vegetation restoration has a significant impact on the physical and chemical properties of soil, and can improve soil organic carbon and total nitrogen. Soil soluble nitrogen pools have a major impact on the usually very small but rapidly cycling nitrogen pools such as NH4+-N. Therefore, they will possibly play an important role in the nitrogen transformation pathways and plant uptake. However, the response of soil soluble nitrogen to different vegetation restoration patterns in the Loess Hilly Region of China is still poorly understood. We studied the distribution of soil soluble nitrogen in soil layers at depths of 0-30 cm under five conversion lands: Robinia pseudoacacia (RP), Caragana korshinskii (CK), R. pseudoacacia and Platycladus orientalis mixed forest (RP+PO), R. pseudoacacia and Prunus davidiana mixed forest (RP + PD), and abandoned farmland (AF), which have been converted from slope farmland (SF) for 15 years in the Loess Hilly Region. Contents of soil soluble organic nitrogen (SON), ammonium nitrogen (NH4+-N), and nitrate nitrogen (NO3--N), and their densities and ratio to total nitrogen were determined to investigate the availability of soil nitrogen. Results show that the average contents of SON, NH4+-N, and NO3--N of vegetation recovery lands was significantly higher than that of the slope farmland in the 0-30 cm soil layer (P < 0.05). Compared with other vegetation recovery lands, the SON and NO3--N improvement trend was better for RP and CK in the 0-30 cm soil layer, whereas RP + PD vegetation recovery lands exhibited the most NH4+-N growth. After long-term vegetation recovery, the density of soil soluble nitrogen was also improved. Compared with SF, the SON rates increased in the 0-30 cm soil layer in the following order: CK (262.2%) > RP (232.8%) > RP + PD, RP + PO (average increase rate was 34.5%) > AF (-21.5%). The density of NO3--N in the 0-30 cm soil layer improved from 7.9 to 182.8% in the following order: CK > RP > RP + PD > AF > RP + PO. However, the maximum increase rate of NH4+-N density was seen in RP + PD, whereas the minimum increase rate was seen in AF. Moreover, the ratio of SON and NO3--N to total nitrogen in RP was increased by 2.4 and 0.6, respectively, the ratio of NH4+-N to total nitrogen in RP + PD has increased by 1, as compared to SF. The soil SON was not affected by soil depth, whereas the NO3--N and NH4+-N content decreased with soil depth in the profiles. Further, based on correlation and regression analyses, we found that the soil soluble nitrogen was affected by soil organic matter, total nitrogen, and microbial biomass carbon and nitrogen. However, among these factors, microbial biomass was more sensitive than the others, and microbial nitrogen had a higher influence than microbial carbon. From another perspective, the NO3--N content was sensitive to changes in soil organic matter, total nitrogen, and microbial biomass carbon and nitrogen in long-term vegetation recovery. These findings demonstrate that vegetation recovery can improve the content, density, and proportion of soil soluble nitrogen, and increase the availability of soil nitrogen. It was also observed that converting SF to RP and CK forests is an efficient method of nitrogen activation in the Loess Hilly Region of China. 参考文献 相似文献 引证文献

Soil enzyme activities as potential indicators of soluble organic nitrogen pools in forest ecosystems of Northeast China

Soluble organic nitrogen is considered to reflect the effect of forest types on soil nitrogen status. As a major process affecting the soil-soluble organic nitrogen pool, degradation of insoluble organic nitrogen in the production of soluble organic nitrogen is mediated by a suite of soil enzymes. This study aims to examine soil-soluble organic nitrogen pools and their relationships with the activities of soil enzymes in natural secondary forest stands and adjacent larch plantation stands. Four pairs of larch plantation stands and secondary forest stands were randomly selected from a mountainous area, and the top 15 cm of the mineral soils were sampled from each field. The soil-soluble organic nitrogen concentrations were up to 2-fold greater in the secondary forest stands than in the larch plantation stands, whereas the ratio of soluble organic nitrogen/total nitrogen was comparable between the two forest types. The concentrations of soluble organic nitrogen were positively correlated with approximately 2-fold differences in urease and protease activities, a 1.2-fold difference in N-acetyl-β-glucosaminidase and a 1.7-fold difference in l-asparaginase between the two forest types. Our results suggest that relationships between soil-soluble organic nitrogen and enzyme activities are independent on sampling time, and that the soil enzyme activities can be used as potential indicators of soil soluble organic nitrogen pools in the temperate forest ecosystem.

Vertical distribution of soluble organic nitrogen, nitrogen mineralization, nitrification, and amidohydrolase activities in a manure‐treated soil

AbstractRecent studies indicate that soil soluble organic nitrogen (SON) plays an important regulatory role in the soil–plant N cycle. The aims of this study were to identify the vertical distribution of SON and its correlation with N mineralization, nitrification, and amidohydrolase activities, in a soil repeatedly amended with cow manure or chemical fertilizer. For this purpose, soil samples were collected from 0–20, 20–40, 40–60, 60–80, and 80–100 cm depths of a calcareous soil, which has been annually amended for 5 y with cow manure (CM) at two rates of 50 (CM50) and 100 (CM100) Mg CM ha–1 y–1. Treatments with chemical fertilizer (CF) and a control (CT) were also included. Soluble organic N, N mineralization, nitrification rates, as well as L‐glutaminase and L‐asparaginase activities were determined. Both CM50 and CM100 enhanced SON content throughout the soil profile. Nitrogen‐mineralization rate (Nm) was increased at the 0–20 cm depth of the CM100 treatment and remained unaffected at the deeper depths. Nitrification rate (Nn) was significantly higher at the 0–60 cm depth of CM100 compared to CF and CT. L‐glutaminase and L‐asparaginase activities were significantly increased at the 0–40 cm depth in both CM50 and CM100 compared to CF and CT. The amidohydrolase activities could not be detected below 40 cm, regardless of the fertilizer treatments. Our results suggest that SON makes a minor contribution to N mineralization in deep soil layers. It was also concluded that changes in the SON throughout the soil profile were not associated with changes in the N‐transformation rates (Nm and Nn) and amidohydrolase activities. While we conclude that SON is a major N pool in the whole profile of the manure applied soil further investigation is required to characterize SON and to investigate the bioavailability of SON for microbial activity in different soil depths.

Assessing soluble organic nitrogen pools in horticultural soils: a case study in the suburbs of Shanghai (China)

Soil soluble organic nitrogen plays a significantly important role in nitrogen biogeochemical cycling. The aim of this study was to investigate the amount of soluble organic and inorganic nitrogen pools extracted by water, salt solutions, and centrifugal-drainage technique from different horticultural management system soils. Approximately 5.4–16.6, 4.4–46.5, 7.1–18.2, 8.8–27.8, 1.8–5.6, and 1.7–4.4 mg kg − 1 soluble organic nitrogen were obtained by 1M KCl, 0.5M K2SO4, 10 mM CaCl2, 1/15 mM phosphate buffer (pH 7.0), water, and centrifugal-drainage technique, respectively, in the 0–20 cm horticultural soils. Large variation in soluble organic nitrogen pools was observed across the sites in the present study and the volumes of soluble organic nitrogen pools generally followed the order: organic production system soil > conventional production system soil > transitional production system soil. In relative contrast to soluble organic nitrogen, organic management, known primarily for the absence of artificial fertilizers and pesticides, resulted in significantly lower levels of both soluble inorganic nitrogen and total soluble nitrogen in the soil. The degree of soluble inorganic nitrogen recovery appears highly dependent on the different chemical extractants and generally followed the series: phosphate buffer (pH 7.0) > KCl≈K2SO4 > CaCl2 > water > centrifugal-drainage technique. The soluble organic carbon and nitrogen in all soil extracts were all positively related to soil total carbon, total nitrogen, and electric conductivity value. This suggested that soil total carbon and nitrogen played a central role in the retention of soluble organic nitrogen in soils. Further studies on investigating the soluble organic nitrogen degradation pathway and its bottleneck are warranted.

Genotype and slope position control on the availability of soil soluble organic nitrogen in tea plantations

Soluble organic nitrogen (SON) plays a vital role in ecosystem N cycling processes and is controlled by a number of biotic and abiotic factors. We compared soil SON availability, microbial biomass, protease and asparaginase activities and phospholipids fatty acid (PLFA) profiles at the 0–15 and 15–30 cm layers in 10 year old tea plantations of two genotypes—Oolong tea (Camellia sinensis (L.) O. Kuntze cv. Huangjingui) (designated as ‘OT’) and Green tea (C. sinensis (L.) O. Kuntze cv. Fuyun 6) (designated as ‘GT’)—established at different slope positions. Concentrations of soil SON measured by the 2 M KCl extraction under the OT plantation were greater than under the GT plantation, while concentrations of soil SON were greater in the middle slope (MS) and lower slope (LS) positions than in the upper slope (US) position. Trends in soil microbial biomass C and N and protease and asparaginase activities between the two genotypes and across the slope positions were similar to the SON pools. The fungal-to-bacterial ratios were higher in the US position than in the MS and LS positions and higher under the GT plantation than under the OT plantation. Results from this study support that the genotype and the slope position are key factors controlling the availability of soil SON in tea plantations and also imply the importance of plant traits (e.g. litter quantity and chemistry) and soil texture in determining overall soil N availability and transformation processes and microbial community composition at the landscape level.

Soil soluble organic nitrogen and active microbial characteristics under adjacent coniferous and broadleaf plantation forests

Soil soluble organic nitrogen (SON) is considered as a sensitive indicator of soil nitrogen (N) status and plays an important role in N cycling in forest ecosystems. Most work on forest soil SON to date has been conducted in temperate areas. The information about soil SON pools and dynamics in tropical and subtropical areas is limited. The aim of this study was to investigate the effects of different forest types on soil SON availability and associated microbial properties. Soil samples were collected at two depths (0–20 and 20–40 cm) from adjacent 12-year-old plantation forests of coniferous Chinese fir (Cunninghamia lanceolata (Lamb) Hook; Coniferous Fir), coniferous–broadleaf mixed species of broadleaf Nagai Podocarpus and coniferous Pond cypress (mixed species), and broadleaf Phoebe nanmu (broadleaf Phoebe) in subtropical China. Three extraction methods (water, hot water, and 2 M KCl) were used for measurements of SON. Soil microbial biomass carbon (C) and N and activities of urease, protease, and l-asparaginase were also measured. Hot water and KCl extractable SON pools were greater in the surface soils (0–20 cm) under the broadleaf Phoebe than under the Coniferous Fir, while those under the mixed species were intermediate. The microbial biomass C and N as well as activities of protease and urease were higher in the soils under the broadleaf Phoebe and mixed species than under the Coniferous Fir. Soluble organic C and SON pools were positively related to soil total C and N and microbial biomass C and microbial biomass N. Activities of urease, protease, and asparaginase were highly correlated with SONw, SONhw, and SONKCl. This work has demonstrated that forest type had significant impacts on the pool size of soil SON. This may be attributed to the different quantity and quality of organic inputs from different forest systems. Positive relationships between microbial biomass and enzyme activities with the SON pools clearly demonstrated that microbial communities play a vital role in the dynamics of soil SON in forest ecosystems. It is recommended that soil SON measured by hot water extraction be incorporated into the routine measurement of soil available N in forest ecosystems. Further study on the role of specific microbial functional groups in the SON transformation and the chemical nature and seasonal dynamics of SON pools in forest ecosystems is warranted.

Soluble organic nitrogen pools in adjacent native and plantation forests of subtropical Australia

Soil soluble organic nitrogen (SON) can play an important role in soil nitrogen (N) cycling in forest ecosystems. This study examined the effect of land-use change from a native forest (NF) to a first rotation (1R) and subsequent second rotation (2R) hoop pine ( Araucaria cunninghamii) plantation on soil SON pools. The impact of residue management on SON pools was also investigated in the 2R forest, where SON was measured in tree rows (2R-T) and windrows (2R-W). Various extraction techniques were used to measure SON pool size in the 0–10, 10–20 and 20–30 cm layers of soil. The results showed that land-use change had a significant impact on soil SON pools. In the 0–10 cm layer, 3.2–8.7, 14–23, 20–28, 60–160 and 127–340 mg SON kg −1 were extracted by water, 0.5 M K 2SO 4, 2 M KCl, hot water and hot 2 M KCl, respectively. The size of the SON pools and the potential production of SON (PPSON) were generally highest in the NF soil and lowest in the 2R-T soil, and in all forest types decreased with soil depth. The larger SON pools in the NF soil coincided with lower soil, litter and root C:N ratios, suggesting that the difference in the size of SON pools between the NF and 1R soil may be related to differences in the quality of organic matter input under the different forest ecosystems. Differences in the size of SON pools between the 1R soil and the 2R soils and between the 2R-T soil and the 2R-W soil may be related to the quantity of organic matter input and time since disturbance. Significant relationships were found between the SON extracted by 0.5 M K 2SO 4 (SON ps) and 2 M KCl (SON KCl), and also among the SON extracted by hot 2 M KCl (SON hKCl), hot water (SON hw) and water (SON w), suggesting that the organic N released by these groups of extracts may be at least partly from similar pools.

On the Nature and Ecological Functions of Soil Soluble Organic Nitrogen (SON) in Forest Ecosystems

On the Nature and Ecological Functions of Soil Soluble Organic Nitrogen (SON) in Forest Ecosystems