ABSTRACT
To evaluate the effects of land-use types on the temporal dynamics of soil active carbon (C) and nitrogen (N), a field experiment was conducted from five adjacent sites, farmland (FL), abandoned land (AL), Platycladus orientalis artificial forest (PO), Robinia pseudoacacia artificial forest (RP) and Quercus variabilis artificial forest (QV) in the rocky mountainous of North China. Soil dissolved organic C and N (DOC, DON), soil microbial biomass C and N (MBC, MBN), ammonium-N (NH4+-N) and nitrate-N (NO3−-N) were determined during four seasons. The results showed that soil DOC, DON, MBC, MBN, NH4+-N, and NO3−-N contents in PO were higher than other land-use types, especially in QV. Soil DOC, DON, MBC, and MBN contents all showed a higher value in March. Both soil DOC/DON and MBC/MBN showed peak values in June. With the increase of soil depth, soil DOC, DON, MBC, and MBN contents all tended to decrease. Most of soil active C and N components had significant correlations with SOC, TN, AP, soil particle volume fraction and the activities of BG and PROT, while the main influencing factors for soil active C and N components were various. The key contributing factors to soil DOC and MBC was SOC, accounting for 62.6% and 67.9%, respectively. Soil TN was the main affecting factor of soil DON (98.4%) and MBN (75.7%). Soil NH4+-N and NO3−-N were, respectively, affected by volume fraction of silt particles (10.3%) and AP (15.0%). In conclusion, soil active C and N components are more likely to accumulate in the PO soil in spring, indicating tree species and temperature are the key factors driving soil active C and N turnover in this area.
1. Introduction
The biogeochemical cycles of carbon (C) and nitrogen (N) are fundamental to life on Earth (Finzi et al. Citation2011). The C-N coupled biogeochemical cycles are the basis of primary production, organic matter accumulation, and decomposition in terrestrial ecosystems. Soil active C and N are the most important and active components during soil C and N cycling, which are often used as evaluating indicators for the changes of soil quality and environment, and their temporal dynamics are of great significance to nutrient cycling, material flow and C-N balance (Kooch, Sanji, and Tabari Citation2019). Soil dissolved organic C and N (DOC, DON), soil microbial biomass C and N (MBC, MBN), ammonium-N (NH4+-N) and nitrate-N (NO3−-N) are important components of soil active C and N, all sensitive to environmental change (Parfitt et al. Citation2005). The formation and transformation process of soil active C and N are affected by biological, physicochemical factors and hydrological conditions, such as soil organic matter decomposition (Sutfin, Wohl, and Dwire Citation2016), temperature (Reinmann et al. Citation2019), soil nutrients (Wang et al. Citation2020), vegetation growth (Lars et al. Citation2013), precipitation process (Lv et al. Citation2014) and land-use change (Adrianto, Spracklen, and Arnold Citation2019). Therefore, soil active C and N components can be applied as important biological indicators to guide the management of soil ecosystem by reflecting the transformation of ecosystem functions (Van Leeuwen et al. Citation2017).
Land-use and land-cover change, that is, from natural ecosystem to semi-natural or artificial ecosystem, obviously influence climate at global and local scales (Wang et al. Citation2018). Land-use type, such as farmland, woodland, grassland, and so on, is mainly reflected in various land cover or different agricultural activities, significantly impact soil properties, including soil active C and N components (Kong et al. Citation2013). Many studies have demonstrated that soil nutrients differ significantly between different land-use types (Zhu et al. Citation2021). Land-use from natural to semi-natural or artificial ecosystem has different effects on plant productivity, environmental quality, and soil C sequestration (Zheng et al. Citation2021). Moreover, natural or semi-natural ecosystems usually have a higher C and N inputs and outputs than those from artificial ecosystem (Li, Niu, and Luo Citation2012). However, in a specific ecosystem, soil C and N biogeochemical cycles are restricted by environmental factors, nutrient concentration, and aboveground vegetation, thus affecting the soil active C and N components. Allmaras et al. (Citation2000) reported that proper land-use types could mitigate the increase of CO2 concentrations in the atmosphere by increasing the soil C storage. Obviously, it is very important to analyze soil active C and N components under specific environmental conditions among different land-use types for accurate evaluation of soil C and N turnover.
Temporal dynamics, especially temperature and precipitation, could influence the biological and geochemical cycles of soil elements in terrestrial ecosystems. Lee, Park, and Matzner (Citation2018) reported that temperature was the decisive factor of physicochemical dissolution of soil organic matter and macromolecular depolymerization. And Michakzik et al. (Citation2001) found that precipitation is an important source of dissolved organic matter, and provide more DON for soil. In addition, an increase in air temperature will lead to an increasing trend in soil temperature and a decreasing trend in soil moisture (Brzostek et al. Citation2012). Therefore, temperature, precipitation, and their interaction profoundly affect the formation and transformation process of soil active C and N components (Baldrian et al. Citation2013). However, the regulation mechanisms of land-use types, temporal dynamics and their interactions on soil active C and N turnover remains unclear.
As a vital ecological barrier, the rocky mountainous area of North China is one of the crucial regions of forest ecological engineering in China (Tong et al. Citation2014). In the past, ecosystem carrying capacity was not taken into account in the process of exploitation in this area, resulting in the degeneration of ecosystem, decreasing the soil fertility and accelerating the soil desertification. The good news is that since 1950s, the government has started an afforestation project in this area, and the degraded land has been gradually improved (Kong et al. Citation2020). And numerous studies have focused on the effect of the artificial afforestation on soil fertility, soil and water conservation and greenhouse gas emissions. However, as another prime kind of land-use types in this area, the effect of farmland on soil ecosystem restoration is still unclear. Therefore, these give motivation to investigate the temporal dynamics of soil active C and N components from three typical land-use types, i.e., farmland, abandoned land and artificial pure forests, in the rocky mountainous area of North China. The results could provide a comprehensive evaluation and screen out the suitable land-use types, which will give a new insight into the sustainable utilization of ecosystem.
2. Materials and methods
2.1. Study area and soil sampling
The study site, in the warm temperate continental climate zone, is located at the Yellow River Forest Ecosystem Research Station of Xiaolangdi Dam (35º01′N, 112º28′E; elevation 438 m), Chinese Forest Ecosystem Research Network (CFERN), Jiyuan city, Henan province, North of China (Kong et al. Citation2020). Annual mean temperature and precipitation are approximate 13.1°C and 641 mm, respectively. The seasonal distribution of precipitation is uneven, with rainfall accounting for 68% of whole year from June to September. The weather characteristics in the middle of March, June, September, and December of a year can represent the local four seasons. The soil is mainly alfisol.
Three typical land-use types were selected: a farmland (FL), an abandoned land (AL) and artificial forests (Platycladus orientalis (PO), Robinia pseudoacacia (RP), and Quercus variabilis (QV)). All land-use types are in close proximity (5 km apart), having similar slope position and aspect. Details of sampling lands are given in and Figure S1.
Table 1. Characteristics of sampling sites
Table 2. Soil basic data in the sampling plots
Sampling was done after the selection of representative plots under the five sampling sites. In each sample site, three 10 × 10 m plots were set based on forestry, pedagogical, and topographical properties. Five soil samples within each plot were collected by an S-shape from three soil layers, i.e., 0 ~ 10 cm, 10 ~ 20 cm and 20 ~ 30 cm. Fresh soils were sifted through 2 mm sifter and then refrigerated at 4°C until analyzed.
2.2. Analysis of soil active carbon and nitrogen
Soil MBC and MBN were determined by chloroform fumigation and potassium sulfate extraction method (Wu et al. Citation2020). Soil DOC, NH4+-N, and NO3−-N were measured by using potassium chloride extraction-SKALAR flow analyzer method (SKALAR San++, SKALAR Co., Netherlands). Soil DON was calculated as the difference between TN and mineral N (NH4+-N + NO3−-N).
2.3. Analysis of soil physicochemical and biological properties
Soil parameters including bulk density, soil water content (SWC), total porosity, soil particle composition volume fraction (classified into clay: <0.002 mm, silt: 0.002 ~ 0.02 mm, sand: 0.02 ~ 2 mm), soil pH, available phosphorus (AP) and available potassium (AK) were determined according to our previous study (Kong et al. Citation2020). Nitrophenol colorimetric method was used to detect the activity of soil β-1, 4-glucosidase (BG) (Chen et al. Citation2017). The activities of soil catalase (CAT) and urease (UREA) were measured by using potassium permanganate titration and sodium phenol-sodium hypochlorite colorimetric method, respectively (Guan, Zhang, and Zhang Citation1986). The activity of soil proteinase (PROT) was evaluated according to Folin-Ciocalteu colorimetric method (Ladd Citation1972). Soil total N (TN) was measured by soil elemental analyzer (EURO EA3000, EuroVector Italy).
2.4. Statistical analysis
The effects of land-use type, temporal dynamic and soil layer on the soil active C and N components were examined by using the multivariate analysis of variance (MANOVA). Stepwise multiple-regression analysis was conducted to determine the main parameters influencing the soil active C and N components. Both of them were performed using SPSS Statistics 20 (IBM, New York, USA). Relationship between soil active C and N components and soil properties were examined by redundancy analysis (RDA), performed by the software CANOCO 5.0 (Biometris-Plant research international, Wageningen, the Netherlands).
3. Results
3.1. Temporal dynamics of soil DOC and DON under three land-use types
Soil DOC and DON contents were significantly affected by the land-use types, temporal dynamics, soil layers, and their interaction (P < 0.01, ). The annual mean value of soil DOC content in PO at the layer of 0 ~ 30 cm (667 mg C kg−1) was significantly higher than FL (413 mg C kg−1), AL (245 mg C kg−1), RP (223 mg C kg−1) and QV (149 mg C kg−1) (). Soil DON and DOC/DON were not significantly different between FL and AL (, 1 c). Among the three kinds of artificial forests, soil DON content was higher in PO than that in RP and QV (). Soil DOC and DON contents under three land-use types had similar temporal dynamic trends, which showed peak values in March (, 1b). However, soil DOC/DON had a higher value in June than other months (). In addition, soil DOC and DON contents all decreased with the increase of soil depth (, 1b).
Table 3. General linear model analysis of the effects of Land-use types , temporal dynamics, and soil layer on soil active C and N components (n = 180)
Figure 1. Temporal dynamics of soil dissolved organic C (DOC) (a) and N (DON) contents (b) as well as their ratio (c) under three land-use types.
3.2. Temporal dynamics of soil MBC and MBN under three land-use types
Land-use type had a significant influence to soil MBC (P < 0.01), while no influence to soil MBN (). And soil MBC and MBN contents were significantly affected by the temporal dynamics and soil layers (P < 0.01, ). The annual mean value of soil MBC content in PO at the layer of 0 ~ 30 cm were 50%, 74%, 164%, and 130% higher than that in FL, AL, RP, and QV, respectively (). Except for March, the annual mean value of soil MBN content in PO at the layer of 0 ~ 30 cm were 105%, 312%, 244%, and 361% higher than that in FL, AL, RP, and QV, respectively (). Soil MBC and MBN contents under three land-use types showed peak values in March, then decreasing sharply in June, September, and December (, 2b). MBC and MBN contents at the 0 ~ 20 cm soil were significantly higher than those at 20 ~ 30 cm soil (, 2b).
Figure 2. Temporal dynamics of soil microbial biomass C (MBC) (a) and N (MBN) contents (b) as well as their ratio (c) under three land-use types.
3.3. Temporal dynamics of soil NH4+-N and NO3−-N under three land-use types
Land-use types significantly affected soil NH4+-N and NO3−-N contents (P < 0.01), while temporal dynamics and soil layers had no significant effect on them (, ). Soil NH4+-N content under three land-use types varied from 11 mg N kg−1 to 33 mg N kg−1. The annual mean value of soil NH4+-N content (0 ~ 30 cm) showed the trend of PO ≈ AL ≈ FL > QV > RP (). The soil NO3−-N content under three land-use types was from 7.4 mg N kg−1 to 27 mg N kg−1, annual mean value of soil NO3−-N content (0 ~ 30 cm) tended AL ≈ FL ≈ PO > RP > QV ().
Figure 3. Temporal dynamics of soil ammonium-N (NH4+-N) (a) and nitrate-N (NO3−-N) contents (b) under three land-use types.
3.4. Relationships between soil active C and N components and other soil properties
RDA was performed on the correlations between environmental factors, other soil factors and soil active C and N components. The results showed that the first tow sequencing axes accounted for 71.61% (RDA1: 44.29%; RDA2: 27.32%) of the variation of soil active C and N components (). Soil active C and N components showed a significant negative correlation with soil sand volume fraction. Soil DOC, DON, MBC, MBN, and NO3−-N contents showed positive correlation with soil BG, CAT, PROT, and UREA activities, AN, SWC, and silt volume fraction, especially a strong positive correlation with soil SOC, TN, AP, and clay volume fraction. Interestingly, soil DOC/DON and MBC/MBN only showed a strong positive correlation with sand volume fraction ().
Figure 4. Redundancy analysis (RDA) for soil active C and N contents, soil physicochemical properties and environmental factors.
Stepwise multiple-regression analyses were conducted to study the main controlling factors influencing soil active C and N components (). Soil SOC and TN could explain 72.3% of the variation for soil DOC content, and soil TN could account for 98.4% of the variation for soil DON content. The key contributing factors to soil MBC content were soil SOC and TN, accounting for 75.1% of the variation. TN accounted for 75.7% variation for soil MBN content. Soil NH4+-N content was mainly affected by soil silt content (10.3%), and soil NO3−-N content was mainly affected by soil AP, clay, and SWC, and their contribution rate was 24.2%. Soil TN was the prime affecting factor of soil DOC/DON (7.7%) and MBC/MBN (2.6%).
Table 4. Multiple regression models for the relationship between soil active C and N components as well as soil properties (n = 180)
4. Discussion
4.1. Effects of land-use types on soil active C and N components
According to the variation of management measures, such as soil water management and tillage, different land-use types change the quantity and quality of plant residues into soil, and then alter the contents of soil active C and N components (Six et al. Citation2002). In the present study, the contents of most soil active C and N components were significantly higher in the PO soil than those of other land-use types, while they were the lowest in QV soil. It was probably due to the input of root exudate, decomposition rate of litter (Fanin, Moorhead, and Bertrand Citation2016) and soil physicochemical and biological conditions (). Furthermore, soil SOC, TN, and soil enzyme activities were the main controlling factors for soil active C and N components (, ). The return of a large number of root exudate and litter into the PO soil () provided a large number of effective C and substrates for soil microorganisms and enzymes, and then improved their activities, promoting the decomposition rate of litter, and thus causing the strong formation of available C and N () (Bai et al. Citation2021). In addition, PO soil also had better soil physicochemical and biological conditions (), which could provide a suitable habitat for soil microorganisms, indirectly accelerating the reproduction of soil microorganisms and improving the decomposition efficiency of litter. Interestingly, the soil MBC/MBN ratios of both PO and QV were higher than 7 (), indicating that the microbial communities of both soils were dominated by fungi (Campbell et al. Citation1991). However, QV showed a relatively low litter decomposition efficiency, which might be related to its poorer litter quality or faintly soil acid (pH < 6) (). We also found that soil active C and N components showed a low level in RP, might on account of the restriction of soil substrate concentration () (). Furthermore, after the farmland abandonment, soil MBC and MBN contents tended to decrease (, 2b). On the one hand, natural (like straw) or artificial fertilization left into FL soil increased soil nutrients, which indirectly leaded to the increase of microbial species and quantity. On the other hand, the large of bare areas in AL surface reduced soil water content and thus microbial activities (Sharma et al. Citation2004). Therefore, exogenous C and N inputs drove the turnover of soil active C and N in the rocky mountains of North China. In the process of land management, moderate fertilization or PO construction was more conducive to the sustainable development of soil ecosystem. However, this study only involved pure artificial forests, and a deep insight into the mixture plantation mode should be gained.
4.2. Effects of temporal dynamics on soil active C and N components
Soil active C and N components varied greatly across the four seasons (). The changes of temperature and precipitation drive main biological processes (photosynthesis, biological N fixation and microbial mineralization), all of which ultimately affected the active C and N components as well as their ratios (Tian et al. Citation2010; Zheng et al. Citation2021). In March, increasing temperature and thawing permafrost stimulated microbial activity, leading to a large number of available nutrients input (Devi and Yadava Citation2006) and lower C and N emission. Therefore, all land-use types in this study showed higher soil DOC, DON, MBC, and MBN contents in March (, 2). The suitable temperature and frequent rainfall in June promoted the vigorous growth of plants, which absorb N more intensively, thus inhibiting the growth of soil microorganisms (Liu, Liu, and Jin Citation2014). In the present study, we also found that soil MBC, MBN, DON, NH4+-N, and NO3−-N contents decreased in June. More rainfall in September and the lower temperature in December were the main factors affecting soil DOC, DON, MBC, and MBN contents (Figure S2). In addition, soil DOC/DON ratio showed a high value in June, indicating DOC was more likely to accumulate in the soil during this period.
Soil inorganic N is closely related to N mineralization, nitrification, and denitrification (Kong et al. Citation2013). However, neither NH4+-N nor NO3−-N showed significant temporal dynamics. The temperature in March, June, and September were in the optimum temperature range for nitrification (Yang et al. Citation2011), which made the nitrification bacteria involved in NH4+-N conversion more active. The temperature in December was significantly lower than 15°C, and the soil inorganic N content fluctuated smoothly due to the limitation of soil substrate concentration and microbial activity. In addition, soil freezing and melting in March and frequent precipitation in June and September aggravated soil denitrification, resulting in the loss of NO3−-N conversion into gas (Kong et al. Citation2020).
4.3. Effects of soil layer on soil active C and N components
Soil DOC, DON, MBC, and MBN contents under three land-use types were all higher in the surface soil layer (0 ~ 10 cm) than those in the deep soil layer (20 ~ 30 cm). This result could be explained by an abundant litter return, low soil density as well as good hydrothermal and aeration conditions in the surface soil layer (). In addition, poor ventilation and low nutrient content in the deep soil layer weakened microbial activities (Van Leeuwen et al. Citation2017) and affected their active C and N contents. Meanwhile, soil layer had no significant effect on the soil NH4+-N and NO3−-N contents under three land-use types (). For one thing, lateral roots of plants could absorb more inorganic N from the surface soil (Gao et al. Citation2013). On the other hand, because of the lower bulk density and the better ventilation condition in the surface soil (), soil NH4+-N is more easily converted to NO3−-N through nitrification process, and the soil has little adsorption of anions, thus soil NO3−-N is easily leached off (Wang et al. Citation2016).
5. Conclusion
Soil active C and N components are important indicators for soil biogeochemical cycles, which are extremely sensitive to land-use types and environmental factors. In this study, our results proved that land-use types, temporal dynamics and their interaction significantly affected most of soil active C and N components (i.e., soil DOC, DON, MBC, and MBN). Of these, soil active C and N contents, for instance, DOC, DON, MBC, and MBN in PO were significantly higher than that in other land-use types, especially in QV, indicating that soil in PO outfitted the advantage in term of C and N cycles. As the seasons advance, most of soil active C and N contents (i.e., DOC, MBC, and MBN) showed peak values in March, then tended to decrease smoothly. With the depth of the soil layer, soil DOC, DON, MBC, and MBN decreased significantly (P < 0.05), and accumulated in the surface layer, suggesting that the accumulation of soil active C and N mainly came from plant residues and was regulated by the water and heat variables. Further, soil SOC and TN were the main controlling factors for soil DOC, DON, MBC, and MBN contents. The activities of BG and PROT, AP content and soil particle volume fraction were found strongly correlated with soil active C and N components. Therefore, appropriate land management, such as PO construction or suitable fertilization, is beneficial to the sustainable development and utilization of the local soil ecosystem in the rocky mountainous area of North China.
Acknowledgments
The authors also thank Mr. Jia Chang-rong for providing information and helping select and set up experimental plots in the forestry farm.
Disclosure statement
No potential conflict of interest was reported by the author(s).
Correction Statement
This article has been republished with minor changes. These changes do not impact the academic content of the article.
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