Abstract
As the largest portion of the terrestrial ecosystems, the arid and semi-arid grassland ecosystem is relatively sensitive and vulnerable to nitrogen (N) deposition. Mowing, the main management in Inner Mongolia grassland also has deep direct and indirect effect on N transformation by removing the nutrient from soils. However, the interaction effect of N addition and mowing on N transformation is still unclear, especially in semi-arid grassland. Here, we conducted a field-manipulated experiment to assess N addition (10 g N m−2 y−1) and mowing (in the middle of August) effects on soil net N mineralization rate across 4 growing seasons (2006–2009) in a semi-arid grassland in Inner Mongolia of northern China. We found that N addition with or without mowing led to significant effect on soil ammonification rate and net N mineralization rate, but had no significant effect on nitrification rates. Furthermore, mowing had no significant effect on soil net N mineralization, ammonification and nitrification rates. N addition and Mowing decreased microbial respiration and metabolic quotient, whereas the interaction of N addition and mowing had no significant effect on microbial respiration and metabolic quotient. Our results indicated that the effects of mowing and N addition did not interactively weaken soil net N mineralization rates in a semi-arid grassland of Northern China. Therefore, the anthropic management (i.e. mowing for hay once a year) with N addition may be a sustainable approach for restoration and reconstruction of vegetation in the abandoned grassland of Northern China.
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Introduction
Atmospheric nitrogen (N) deposition is one of the main nitric sources of the terrestrial ecosystem, and might aggravate under the influence of human disturbance and global climate change. Recent research has demonstrated that due to the continuous increasing of NO3− deposition and the significant decrease of wet NH4+ deposition, the total N deposition has changed from a rapid increase towards stability from 1980 to 2015 in China1. Meanwhile, N deposition has been shifted from being wet dominated to almost equal contributions of dry and wet deposition because of the increased atmospheric dry deposition and the decreased wet deposition1. Nitrogen addition has been recognized as an essential nutrient for plant production and microbial activities in terrestrial ecosystems, but it is mostly limited in terrestrial ecosystems2,3,4,5,6. There is mounting evidence for N fertilizer addition increased soil inorganic nitrogen content, but we know little about its effects on soil N transformation in semi-arid grassland. Although more and more studies have been conducted to report the impact of grassland management (i.e. mowing and N addition) on the structure and function of grassland ecosystems. Compared with other management, the research on the effect of mowing on grassland N transformation is a little, and the results are different, particularly the interactive effects of mowing and N addition on soil net N mineralization is also not clear.
Most of previous studies on the responses of soil N mineralization to N addition focused on soil physical and chemical characteristics, the frequency of N application and addition time, the amount of N addition and N fertilizer types7,8. Some researchers reported that N addition were promoting effects on the content of ammonium N, nitrate nitrogen and available nitrogen, and increased the microbial activity involved in soil N transformation, N transformation in typical and alpine grassland and wetland9,10. Normally grassland ecosystems are limited by N, appropriate rate of N addition increased the soil inorganic N content and organic matter content, alleviated the competition between plants and soil microbe, then promoted microbial N transformation. However, high N application rate inhibited microbial activity, and then reduced the N transformation rates. For example, there were also a few studies holding that lower N addition significantly increased soil net N mineralization rates, while higher N addition had opposite results in typical grassland of northern China. The inconsistent findings reported in previous studies might result from the impacts of experimental duration and other factors on the N addition responses of ecosystem N transformation. For example, N addition increased net N mineralization rate with the increasing N addition in a short time11,12. However, long-term N application decreased soil N mineralization rate, due to the decreased pH and decreased microbial activity and diversity13. The main reason is that long-term N addition lead to soil acidification, and increased the content of refractory organic matter and toxic substances in the soil, thus inhibiting soil microbial activity and structural diversity, thereby reducing soil N transformation. Furthermore, other environmental factors can further compromise N addition effects on ecosystem N transformation. Ecosystems may change toward P limitation as a consequence of ameliorated N limitation14, which will further restrain the N effect. All these factors contribute to the variation in ecosystem responses to N addition. Therefore, it is necessary to explore interactive effects of N addition and other factors on soil N transformation.
Mowing for harvesting hay is the most important ways in grassland management. The impacts of mowing on soil N transformation have been low documented and the results are inconsistent in different studies15,16. Mowing in grasslands may affect soil N transformation by changing soil microclimate (Soil temperature and water content). For instance, most of studies reported that long-term mowing increased soil temperature, decreases soil water content, which significantly affected soil microbial activity and diversity, then affect N transformation11,17,18,19,20. Some other studies showed that mowing increased plant root exudates and soil microbial biomass, reduced the C/N ratio of microbial biomass, then improved N transformation20,21. The loss of C substrate supply from photosynthesis and aboveground litter associated with mowing could reduce soil N transformation22. Successive and intensity mowing must lead the lost and unbalance of substance and energy, and lead up to degradation succession of the grassland2. The mechanism of mowing effects on soil N transformation is controlled by the complex interaction of changing soil microclimate and the availability of C substrate for root and microorganisms, and varies with vegetation type and soil texture. Therefore, more detailed studies in different ecosystems are needed.
The main effects of N addition and mowing on soil N transformation on natural and artificial grassland were rarely reported, especially in abandoned grassland in northern China. Meanwhile, examination of the possible interactions between N addition and mowing on soil N transformation in semiarid grasslands has received little attention. The effect of mowing on soil N transformation might interact with N addition since mowing decreased soil carbon, while N addition increased N supply and enhanced plant nutrient uptake, amplifying the limitation of carbon availability23. Here, we hypothesized that N addition and mowing have a compensatory effect on nutrient cycling in N limited grassland of northern China. To investigate effects of mowing, N addition and their potential interactions on soil N mineralization, we conducted an N addition and mowing experiment in an abandoned grassland in Northern China through 2006 to 2009.
Materials and Methods
Site description
This study site was performed in the Restoration Ecological Research Station of Institute of Botany (42°02′N and 116°17′ E), Chinese Academy of Sciences (IBCAS), near Dunlun county, in Inner Mongolia, Northern China. The mean annual temperature is 2.1 °C, with mean monthly temperatures ranging from −17.5 °C in January to 18.9 °C in July. The mean annual soil temperature (0–5 cm depth) is 5.3 °C, ranged from minimum in January (−16.7 °C) to maximum in July (24.3 °C), with mean soil temperature from 14.9 to 24.3 °C during the growing seasons (May-September). Mean annual precipitation is 385.5 mm, almost >86% of which occurs in growing season (May – September), and the mean annual evaporation is 1748 mm. These data came from a long-term observation (1953–2007) at Restoration Ecological Research Station of Institute of Botany. The soil of the study site was classified as chestnut soil. The dominated vegetation is Artemisia frigida. Traditional land uses include livestock grazing and farming. Soil and plant characteristics were measured before start of the experiment in August of 2005 and reported by Wang et al.17. Duolun Country located in the ecotone between grassland and the dryland agriculture region in Northern China is an ecological fragile zone. In our study area, large parts of the natural steppe were converted to cropland in the 1960s. As a restoration measure, about 80% cropland was abandoned in 1995 and either remained without any further management or has been used for hay production or grazing17.
Experimental design
The experiment was conducted a randomized complete block design with twenty-four 4 m × 4 m plots, the distance between plots was 2 m. The four different treatments in the experiment were control (C), N addition (N), mowing (M) and N addition plus mowing (N + M) with six replicates. 10 g N m−2 yr−1 was added in the middle of growing season from 2006 to 2009. N fertilizer (as NH4NO3) was applied before the rains. Mowing was carried out in the middle of August in 2006–2009, leaving 3 cm of stubble, and moved away all plant material from the mowing plots. Details of the experiment design reported by Wang et al.17.
Sampling and measurements
Soil samples were collected in the middle of August in 2006–2009 when the most vigorous microbial activity occurred in this area. In order to avoid spatial heterogeneity, 5 cores (4.5 cm in diameter and 12 cm in depth) were collected from 0–10 cm depth in each plot at random locations, and then completely mixed into one composite fresh sample. Then plant roots, litter and large stones were removed by sieving (sieve mesh 2 mm), the soil samples were packed in bags in ice blocks and transported to the laboratory. Each sample was divided into two sub-samples and stored in the fridge at 4 °C. The first part for tests microbial biomass carbon (MBC) and microbial biomass nitrogen (MBN), the second part for tests potential microbial respiration (MR).
Microbial biomass was measured using the fumigation-extraction method24. Firstly, fumigated 20 g fresh soil samples for 24 h with ethanol-free CHCl3. Secondly, soil extracts from the fumigated and controlled samples were obtained by shaking soil samples with 50 ml of 0.5 M K2SO4 for 1 h, the supernatant was filtered using a 0.45 mm membrane filter. Finally, filtrate was measured using an automated TOC/TN analyzer (Analytik Jena AG, Jena, Germany) to determine extractable C and N. MBC and MBN were calculated from the difference between extractable C and N contents in the fumigated and non- fumigated samples using conversion factors (kEC and kEN) equal to 0.38 and 0.45, respectively25.
MR was measured by alkali absorption of CO2 evolved at 25 °C and optimal soil moisture for 1 week followed by titrating the residual OH− with a standardized acid26. Briefly, 25 g fresh soil was incubated in a 500 ml glass flask. The glass flask was connected with a glass tube in which 5 ml of 0.05 M NaOH solution was injected to capture the CO2 produced by the soil microbes. The metabolic quotient (qCO2) was calculated from the estimates of MR and MBC27.
At August in 2006–2009 in each plot, two polyvinyl chloride tubes (4.5 cm in diameter and 12 cm in height) were hammered into the soil to a depth of 10 cm. The floor litter was removed before sampling. One of the two tubes from each subplot was retrieved and brought the soil in the tube back to the laboratory. The other tube with a breathable and impermeable sealing film on the top was incubated in situ for 30 days before being retrieved. Ammonification rate (Ramm), nitrification rate (Rnit) and net N mineralization rate (Rmin) were calculated from differences in soil NH4+-N and NO3−-N concentrations between the initial soil sample and the incubated sample. To analyze soil inorganic N concentrations, a 10 g fresh soil samples was taken from each sieved soil sample, and extracted with 50 ml of 0.5 M K2SO4 solution. The soil suspension was rigorously shaken for 1 h in a reciprocal shaker and then filtered through filter paper. Soil solutions were immediately analyzed for NH4+-N and NO3−-N on a FIAstar 5000 Analyzer (Foss Tecator, Denmark).
Soil temperature (ST) at a depth of 10 cm was measured between 9:00–10:00 am twice at August in each plot using a soil thermometer (Longstem Thermometer 6310, Spectrum Technologies, Plainfield, USA). Soil water content (SWC, 0–10 cm) was measured in each plot gravimetrically during the sampling period. The incubation tests were conducted in the laboratory of Institute of Botany, Chinese Academy of Sciences. All results were expressed on an oven-dried soil basis (105 °C, 24 h).
Statistical analysis
Statistical analyses focused on the data of August in 2006–2009. Three-way ANOVA was used to determine the effects of year, N, M and N + M on net N mineralization rate (Ramm, Rnit, Rmin), microbial biomass (MBC, MBN, MBC/MBN) and microbial respiration (MR, qCO2). The responses of SWC and ST, net N mineralization rate, microbial biomass and microbial respiration to N or M or N + M were performed with two-way ANOVA. Unless otherwise stated, the differences were considered statistically significant at P < 0.05. Simple and multiple linear and nonlinear regression analyses were used to examine the relationships between net N mineralization rate, microbial respiration and SWC, ST, microbial biomass. All statistical analyses were conducted using SPSS 23.0 (IBM SPSS, Inc., Chicago, Illinois, USA).
Results
Soil water content and temperature
Significant interannual variations of precipitation (387.2 mm, 185.6 mm, 318 mm and 172.1 mm from 2006 to 2009, respectively) in the growing season (August) were observed in this study site28. Meanwhile, there were significant interannual variations in both soil temperature (ST) and water content (SWC) from 2006 to 2009 (Table 1). ST and SWC in 2006 were significantly greater than that in 2007–2009 (Fig. 1; Table 1). N, M and N + M treatments had no significant effect on ST and SWC from 2006 to 2009 (Fig. 1). However, mean SWC in N + M treatment was significantly higher than that in N treatment only in 2006 (Fig. 1). In average, the main effect of N, M and N + M treatments had no significant effect on ST and SWC across 4 years (Fig. 1).
N addition and mowing effects on soil microbial biomass
Driven by the large variability amounts of moisture and temperature in the growing period across 4 year from 2006–2009, significant interannual variation in all of the microbial biomass carbon (MBC), microbial biomass nitrogen (MBN) and the ratio of microbial biomass carbon and nitrogen (MBC/MBN) were observed (Table 1). MBC was significantly lower in 2007 than that in 2006, 2008 and 2009 (Fig. 2a; Table 1). N, M and N + M treatments had no significant effect on soil MBC in 2006–2008 (Fig. 2a). However, MBC in N treatment was significantly greater than M and N + M treatments, which were significantly greater than C treatment in 2009 (Fig. 2a). In average, the main effect of N, M and N + M treatments had no significant effect on soil MBC across 4 years (Fig. 2a).
The effects of N, M and N + M treatments on soil MBN were inconsistent across 4 years (Fig. 2b). Soil MBN in N and N + M treatments were greater than that in M and C treatments in 2006 and 2007 (Fig. 2b). Soil MBN in C and M treatments were greater than N and N + M treatments in 2008 (Fig. 2b). In addition, soil MBN in N and N + M treatments were slightly greater than that in C and M treatments in 2009 (Fig. 2b). In average, soil MBN in N and N + M treatments were slightly greater than C and M treatments across 4 years (Fig. 2b).
N, M and N + M treatments significantly affected on soil MBC and MBN, then resulted in significant effect on the ratio of MBC/MBN (Fig. 2c). The ratio of MBC/MBN in C and M treatments were greater than N and N + M treatments in 2006 and 2007 (Fig. 2c). However, the ratio of MBC/MBN in N and N + M treatments were greater than C and M treatments in 2008 (Fig. 2c). In addition, N, M and N + M treatments had no significant effect in the ratio of MBC/MBN in 2009 (Fig. 2c). The results showed that the ratio of bacteria to fungi tend to be stable in temporal scale. In average, N, M and N + M treatments had no significant effect on the ratio of MBC/MBN across 4 years (Fig. 2c).
Moreover, we found that N, M and N + M treatments induced changes in soil microbial biomass were closely associated with ST and SWC (Fig. 3). Interestingly, MBC increased significantly with ST (R2 = 0.07, P < 0.05; Fig. 3a) and SWC (R2 = 0.07, P < 0.05; Fig. 3c), respectively. Meanwhile, MBN increased significantly with ST (R2 = 0.18, P < 0.001; Fig. 3b), but had no significant relationship with SWC (Fig. 3d).
N addition and mowing effect on soil microbial respiration
There were significant interannual variation in soil microbial respiration (MR) and metabolic quotient (qCO2) across 4 years (Table 1). N, M and N + M treatments had no significant effect on soil MR and qCO2 in 2006 and 2009 (Fig. 4). However, soil MR and qCO2 in C treatment were significantly greater than N + M treatment, M treatment, and N treatment in 2007, seperately (Fig. 4). In addition, soil MR and qCO2 in C treatment were significant higher than N and N + M treatments and M treatment in 2008 (Fig. 4). In average, soil MR in C treatment was greater than N and N + M treatments, which were significantly greater than mowing across 4 years (Fig. 4a). Meanwhile, soil qCO2 in C treatment was greater than N + M, which were significant greater than N and M across 4 years (Fig. 4b). Moreover, we observed a significant correlation between soil MR and ST (R2 = 0.22, P < 0.001; Fig. 5a) and SWC (R2 = 0.30, P < 0.001; Fig. 5b), respectively.
N addition and mowing effect on soil net ammonification, nitrification and N mineralization rates
Significant interannual variation in soil net ammonification, nitrification and N mineralization rates (Ramm, Rnit, Rmin) were detected (Table 1). N, M and N + M treatments had no significant effect on soil Ramm in 2006 and 2009 (Fig. 6a). However, soil Ramm in N + M was greater than M treatment, N treatment and control, respectively (Fig. 6a). N and N + M treatments in soil Ramm were significantly higher than that in M and C treatments in 2008 (Fig. 6a). In average, N and N + M treatments in soil Ramm were higher than control and M treatment across 4 years (Fig. 6a).
Soil Rnit in N and N + M treatments were significantly higher than C and M treatment in 2006 and 2009, seperately (Fig. 6b). Soil Rnit in M treatment was significantly higher than C, N and N + M treatments in 2007 (Fig. 6b). However, N, M and N + M treatments had no significant effects on soil Rnit in 2008 (Fig. 6b). Compared with control, soil Rnit in N, M and N + M treatments were slightly greater across 4 years (Fig. 6b).
N, M and N + M treatments had no significant effect on soil Rmin in 2006 (Fig. 6c). Soil Rmin in M treatment was significantly greater than that in N + M treatment, N treatment and control in 2007 (Fig. 6c). Soil Rmin in N and N + M treatments were significantly greater than C and M treatments in 2008 and 2009 (Fig. 6c). In average, N and N + M treatments in soil Rmin were greater than M treatment, which were significantly greater than C treatment across 4 years (Fig. 6c). Research suggested that ST, SWC and microbial biomass have been well documented to affect soil N mineralization rate in various ecosystems. It was supported by our observations that soil N mineralization rate was strongly correlated with ST, MBC and MBN at both temporal and spatial. For example, soil Ramm and Rmin had a significant non-linear correlation with ST (R2 = 0.22, P < 0.001; R2 = 0.22, P < 0.001, respectively; Fig. 7). Meanwhile, we observed a significant non-linear relationships between soil Rnit and MBC (R2 = 0.24, P < 0.001; Fig. 8a) and MBN (R2 = 0.12, P < 0.01; Fig. 8c), we also observed a significant non-linear relationships between soil Rmin and MBC (R2 = 0.10, P < 0.01; Fig. 8b) and MBN (R2 = 06, P < 0.05; Fig. 8d).
Discussion
Effects of N addition on microbial N transformation and microbial activity
The microbes and plant growth usually limited by available N in the N-limited temperate grassland. Thus, microbes and plant growth could be stimulated by increasing soil N content6,29,30. We found that N treatment was significantly increased soil ammonification rate (Ramm) and net N mineralization rate (Rnit), and slightly increased nitrification rate (Rnit) across 4 years in our field experiment, which were consisted with the results of the majority of previous research. For example, Dijkstra et al.31 found that N fertilizer stimulated net N mineralization in a grassland ecosystem in Central America. Even though soil temperature (ST) and soil water content (SWC) were important factors in controlling the process of N transformations. However, our results suggest that N treatment had no significant effects on ST and SWC in this area of grassland, which suggests that ST and SWC could not be well explained the changes of soil net N transformation rate in this area.
Microbial biomass has been found to be a good indicator of N transformation processes at laboratory and field experiments32. Nitrogen treatment can increase or decrease microbial activity, yet it is not clear if these impacts are a direct function of the increase in N availability or a result of indirect effects of the fertilizer inputs on other soil chemical characteristics7,33. Results from our study showed that the microbial biomass and respiration had significantly changed after 4 years of N addition. In our study, N treatment had no significant effect on microbial biomass C (MBC) and the ratio of MBC/MBN. However, besides the data of 2008 still did not reveal a stimulating effect of N treatment on microbial biomass N (MBN), all other years showed such a stimulation effect. Meanwhile, N treatment had no effect on soil net N transformation rate in 2006, but increased soil net N transformation rate in other years. Accordingly, a stimulating effect of N treatment on microbial N transformation and MBN could be demonstrated. Nitrogen addition increased soil inorganic N pool and stimulated the decomposition of soil organic matter by the microbe-enzyme system. Thus, the stimulation of soil microbial N turnover following N addition is probably not only a response to increased soil N availability but also a feedback to N-induced increases in plant growth34. These results were consistent with previous studies, soil microorganisms immobilized a higher proportion of mineralized N after 8 years of N fertilization, and that was likely due to greater specific activity and turnover of microbial biomass in N treatment35. Our results also showed that N treatment slightly decreased microbial respiration (MR) and significantly reduced respiratory quotient (qCO2). Previous study indicated that N addition increased the available N content in grassland soil, and significantly increased the labile C sources in the topsoil. Meanwhile, N addition did not only increase the level of available N but also decreased soil pH, which may be the major reason for no significant effect on soil MR but significantly increased soil N transformation rate under N addition treatments.
Effects of mowing on microbial N transformation and microbial activity
Mowing (M) is an important land-use type in grassland ecosystem, it could reduce C inputs to soil and also remove a large amount of N out of grassland ecosystem, reduced the availability of decomposable substrates for soil microorganisms36. Many studies have shown that mowing promoted the aboveground and belowground parts of the plant growth and root secretion, affected the process of soil N transformation37. However, we found that mowing had no significant effects on soil microbial N transformation across 4 years in our field experiment. Consistent with our results, Maron and Jefferies38 examined the effects of mowing on N rich species richness, grassland yield and N retention, across 5 years spring mowing did not cause changes in net N mineralization rate. Two possible reasons could help to explain the little changes in soil N transformations under mowing observed in this study. Firstly, the possible reason why N transformation did not change was that soil temperature and moisture unaltered. Temporal variations in soil temperature and soil moisture can significantly drive microbial activities and N transformations in grassland ecosystems39. For example, Wang et al.40 found in a laboratory study that N transformation and microbial activity in typical temperate steppe near our field experiment site was driven by changes in soil temperature and moisture. The study about the effects of mowing on soil N mineralization by Snapp and Borden41 who found that the response of soil microbes to mowing might be affected by soil water content. However, mowing had no significant effect on soil temperature and water content in our study site. The mechanism of mowing effects on soil microbial N transformations were controlled by the complex interaction of changing soil microclimate. Another reason was that although the supply of C substrate to soil microorganisms decreased, but soil microbial activity might be unaltered. Our study showed that mowing had no significant effect on soil microbial biomass and slightly increased the ratio of MBC/MBN. Although the microbial community has changed, but the microbial biomass had no significant changes across 4 years. In addition, the supply of C substrate to soil microorganisms decreased during the same period in this study11. Previous studies suggested that the removal of plant aboveground biomass and litter with a large C:N ratio facilitated rapid N cycling by limiting carbon input to the soil, thus maintaining rapid N transformation in the microbial community24. Inconsistent with our research, some studies suggested that mowing reduced the ground organic carbon input to soil and reduced the energy for microbial activities, reduced the microbial biomass C and N ratio, thereby decreased the rate of N transformation42,43,44.
Mowing significantly decreases soil MR and qCO2. However, both increased45 and reduced46,47 soil temperature and moisture have been reported in response to mowing in the previous studies. Consistent with our results, Jia et al.48 and Han et al.11 found no response of soil respiration to mowing due to the unaltered plant growth and soil moisture. Given the strong regulation of soil water availability on root and microbial activity49,50,51, but the response of soil MR to mowing observed in this study might not be accounted for the unchanged ST and SWC. Mowing was also expected to affect soil MR through its influence on C substrate36. Previous studies suggested that a smaller metabolic quotient indicates a more efficient use of substrates by microorganisms, where a greater fraction of substrate C was incorporated into microbial biomass and less C per unit biomass was lost through respiration24. In addition, mowing reduced the supply of C and N substrate to soil microorganisms during the same period in this study11. These facts prove that mowing reduced the availability of decomposable substrates for soil microorganisms, and reduced C inputs to soil and lead to a large amount of N loss, caused substrate limitation to microorganisms37,47. Therefore, C and N substrate were the main factor affecting soil MR in this study.
Interaction effects of N addition and mowing on microbial N transformation and microbial activity
Generally, we hold the opinion that mowing may mask the N fertilizer effects on soil microbial communities by reducing plant litter input and root exudation and thus reducing soil microbial biomass and activities. However, soil microorganisms (ST and SWC) and the microbial variables (MBC, MBN and MBC/MBN) had no significant changes in our study. Meanwhile, N + M treatments had no significant effect on MR and qCO2. These results were in line with the studies by Johnson et al.52 and Stark and Kytöviita53, who found that fertilizer increased in situ soil respiration (plant plus microbial respiration) but did not affect the MR measured in the laboratory from sieved soil samples. Previous studies have shown that mowing probably resulted in a depletion of labile C sources in the topsoil and, together with increased N uptake by the plants as a response to mowing, soil microbes may have become C as well as N limited23. However, previous research found that N addition with or without mowing in grassland increased soil inorganic N, which alleviated the limitation for plant growth and enhanced its growth and photosynthetic capacity24. Our study found that N + M treatment was significantly increased the soil inorganic N pool and soil total organic C content, but had no significant effect on microbial biomass and activities. This could explain our finding that MR did not increase.
Mowing may significantly influence the direct impacts of N addition on the soil environment and on soil microbial N transformation responses to N addition54. We found that 4 years of continuous N application with mowing led to significant increase in Ramm and Rmin but had no significant effects on Rnit. Consistent with our study, Wang et al.24 found that mowing decreased SWC and increased ST at the same time, leading to low microbial activity, soil N mineralization rate decreased, but N + M treatment was significantly increased soil microbial N mineralization rate. Although N + M treatment had no significant effect on microbial biomass and activities, but significantly increased the soil inorganic N pool and soil total organic C content (Unpublished data). Results from our study indicate that 4 years of continuous N fertilizer application with mowing or without mowing led to significant changes in microbial N transformation in the abandoned grassland ecosystem where we studied, resulting in an increase in rates of net N mineralization. These conclusions confirmed the predictions of our theoretical hypothesis.
Conclusions
Nitrogen addition and mowing simultaneously increased net N mineralization rate in an abandoned grassland over a 4-year period experiment (2006–2009). Mowing slightly increased soil net N mineralization rates. However, N addition with or without mowing led to significant effects on microbial N transformation. In addition, N addition and Mowing treatments were decreased microbial biomass and microbial respiration according to 4 years, while N + M treatment had no significant effects on microbial biomass and microbial respiration. From our short time experiment, we made a conclusion that N + M treatment did not weaken soil net N mineralization rates in a semi-arid grassland of Northern China55,56.
References
Yu, G. R. et al. Stabilization of atmospheric nitrogen deposition in China over the past decade. Nature Geoscience., https://doi.org/10.1038/s41561-019-0352-4.
Zhang, Y., Cao, C. Y., Han, X. S. & Jiang, S. Y. Soil nutrient and microbiological property recoveries via native shrub and semi-shrub plantations on moving sand dunes in Northeast China. Ecological Engineering 53, 1–5 (2013).
Marklein, A. R. & Houlton, B. Z. Nitrogen inputs accelerate phosphorus cycling rates across a wide variety of terrestrial ecosystems. The New phytologist 193, 696–704 (2012).
Lue, X. T., Cui, Q., Wang, Q. B. & Han, X. G. Nutrient resorption response to fire and nitrogen addition in a semi-arid grassland. Ecological Engineering 37, 534–538 (2011).
Cao, C. Y., Jiang, S. Y., Ying, Z., Zhang, F. X. & Han, X. S. Spatial variability of soil nutrients and microbiological properties after the establishment of leguminous shrub Caragana microphylla Lam. plantation on sand dune in the Horqin Sandy Land of Northeast China. Ecological Engineering 37, 1467–1475 (2011).
Xia, J. Y. & Wan, S. Q. Global response patterns of terrestrial plant species to nitrogen addition. New Phytologist 179, 428–439 (2008).
LeBauer, D. S. & Treseder, K. K. Nitrogen limitation of net primary productivity in terrestrial ecosystems is globally distributed. Ecology 89, 371–379 (2008).
Bowden, R. D., Davidson, E., Savage, K., Arabia, C. & Steudler, P. Chronic nitrogen additions reduce total soil respiration and microbial respiration in temperate forest soils at the Harvard Forest. Forest Ecology and Management 196, 43–56 (2004).
Bai, J. B. et al. Effects of temperature and added nitrogen on carbon mineralization in alpine soils on the Tibetan Plateau. Ecology and Environmental Sciences 20, 855–859 (2011).
Yu, Z. Y., Zeng, D. H., Ai, G. Y. & Jiang, F. Q. Effects of nitrogen addition on soil nitrogen availability in sandy grassland. Chinese Journal of Ecology 26, 1894–1897 (2007).
Han, Y., Zhang, Z., Wang, C., Jiang, F. & Xia, J. Effects of mowing and nitrogen addition on soil respiration in three patches in an oldfield grassland in Inner Mongolia. Journal of Plant Ecology 5, 219–228 (2012).
Zhou, Y. et al. Soil microbial biomass,respiration,and metabolic quotient along an altitudinal gradient in Wuyi Mountain of southeastern China. Chinese. Journal of Ecology 28, 265–269 (2009).
Pietri, J. C. A. & Brookes, P. C. Relationships between soil pH and microbial properties in a UK arable soil. Soil Biology & Biochemistry 40, 1856–1861 (2008).
Menge, D. N. L. & Field, C. B. Simulated global changes alter phosphorus demand in annual grassland. Global Change Biology 13, 2582–2591 (2007).
Gusewell, S., Jewell, P. L. & Edwards, P. J. Effects of heterogeneous habitat use by cattle on nutrient availability and litter decomposition in soils of an Alpine pasture. Plant and Soil 268, 135–149 (2005).
Leriche, H. et al. Which functional processes control the short-term effect of grazing on net primary production in grasslands? Oecologia 129, 114–124 (2001).
Wang, C. H. et al. The effects of biomass removal and N additions on microbial N transformations and biomass at different vegetation types in an old-field ecosystem in northern China. Plant and Soil 340, 397–411 (2011).
Aleklett, K., Leff, J., Fierer, N. & Hart, M. Wild plant species growing closely connected in a subalpine meadow host distinct root-associated bacterial communities. Peerj 3, e804 (2015).
DuPont, S. T., Culman, S. W., Ferris, H., Buckley, D. H. & Glover, J. D. No-tillage conversion of harvested perennial grassland to annual cropland reduces root biomass, decreases active carbon stocks, and impacts soil biota. Agriculture Ecosystems and Environment 137, 25–32 (2010).
Shi, Y. et al. Organic and inorganic carbon in the topsoil of the Mongolian and Tibetan grasslands: pattern, control and implications. Biogeosciences 9, 2287–2299 (2012).
Potthast, K., Hamer, U. & Makeschin, F. Land-use change in a tropical mountain rainforest region of southern Ecuador affects soil microorganisms and nutrient cycling. Biogeochemistry 111, 151–167 (2012).
Cao, G. M. et al. Grazing intensity alters soil respiration in an alpine meadow on the Tibetan plateau. Soil Biology & Biochemistry 36, 237–243 (2004).
Reich, P. B., Hungate, B. A. & Luo, Y. Carbon-Nitrogen Interactions in Terrestrial Ecosystems in Response to Rising Atmospheric Carbon Dioxide. Annual Review of Ecology, Evolution, and Systematics 37, 611–636 (2006).
Vance, E. D., Brookes, P. C. & Jenkinson, D. S. An extraction method for measuring soil microbial biomass C. Soil Biology & Biochemistry 19, 703–707 (1987).
Lovell, R. D., Jarvis, S. C. & Bardgett, R. D. Soil microbial biomass and activity in long-term grassland: effects of management changes. Soil Biology & Biochemistry 27, 969–975 (1995).
Hu, S. & Bruggen, V. Microbial dynamics associated with multiphasic decomposition of 14 C- labeled celllose in soil. Microbial Ecology 33, 134–143 (1997).
Wardle, D. A. & Ghani, A. A critique of the microbial metabolic quotient (qCO2) as a bioindicator of disturbance and ecosystem development. Soil Biology & Biochemistry 27, 1601–1610 (1995).
Wang, C. H. et al. Nitrogen addition and mowing affect microbial nitrogen transformations in a C4 grassland in northern China. European Journal of Soil Science 66, 485–495 (2015).
Niu, S. L. et al. Nitrogen effects on net ecosystem carbon exchange in a temperate steppe. Global Change Biology 16, 144–155 (2010).
Galloway, J. N. et al. Transformation of the nitrogen cycle: Recent trends, questions, and potential solutions. Science 320, 889–892 (2008).
Dijkstra, F. A. et al. Climate change alters stoichiometry of phosphorus and nitrogen in a semiarid grassland. New Phytologist 196, 807–815 (2012).
Cookson, W. R. et al. The contribution of soil organic matter fractions to carbon and nitrogen mineralization and microbial community size and structure. Soil Biology & Biochemistry 37, 1726–1737 (2005).
Craine, J. M., Morrow, C. & Fierer, N. Microbial nitrogen limitation increases decomposition. Ecology 88, 2105–2113 (2007).
Williams, M. A., Rice, C. W. & Owensby, C. E. Nitrogen competition in a tallgrass prairie ecosystem exposed to elevated carbon dioxide. Soil Science Society of America Journal 65, 340–346 (2001).
Fisk, M. C. & Fahey, T. J. Microbial biomass and nitrogen cycling responses to fertilization and litter removal in young northern hardwood forests. Biogeochemistry 53, 201–223 (2001).
Wan, S. Q. & Luo, Y. Q. Substrate regulation of soil respiration in a tallgrass prairie: Results of a clipping and shading experiment. Global Biogeochemical Cycles 17 (2003).
Holst, J. et al. Microbial N turnover and N-oxide (N2O/NO/NO2) fluxes in semi-arid grassland of Inner Mongolia. Ecosystems 10, 623–634 (2007).
Maron, J. L. & Jeffries, R. L. Restoring enriched grasslands: Effects of mowing on species richness, productivity, and nitrogen retention. Ecological Applications 11, 1088–1100 (2001).
Makarov, M. I. et al. Nitrogen dynamics in alpine ecosystems of the northern Caucasus. Plant and Soil 256, 389–402 (2003).
Wang, C. H., Wan, S. Q., Xing, X. R., Zhang, L. & Han, X. G. Temperature and soil moisture interactively affected soil net N mineralization in temperate grassland in Northern China. Soil Biology & Biochemistry 38, 1101–1110 (2006).
Snapp, S. S. & Borden, H. Enhanced nitrogen mineralization in mowed or glyphosate treated cover crops compared to direct incorporation. Plant and Soil 270, 101–112 (2005).
Matthew Robson, T., Baptist, F., Clement, J.-C. & Lavorel, S. Land use in subalpine grasslands affects nitrogen cycling via changes in plant community and soil microbial uptake dynamics. Journal of Ecology 98, 62–73 (2010).
Uhlirova, E., Simek, M. & Santruckova, H. Microbial transformation of organic matter in soils of montane grasslands under different management. Applied Soil Ecology 28, 225–235 (2005).
Antonsen, H. & Olsson, P. A. Relative importance of burning, mowing and species translocation in the restoration of a former boreal hayfield: responses of plant diversity and the microbial community. Journal of Applied Ecology 42, 337–347 (2005).
Owensby, C. E., Hyde, R. M. & Anderson, K. L. Effects of Clipping and Supplemental Nitrogen and Water on Loamy Upland Bluestem Range. Journal of Range Management 23, 341–& (1970).
Bremer, D. J., Ham, J. M., Owensby, C. E. & Knapp, A. K. Responses of soil respiration to clipping and grazing in a tallgrass prairie. Journal of Environmental Quality 27, 1539–1548 (1998).
Wan, S., Luo, Y. & Wallace, L. L. Changes in microclimate induced by experimental warming and clipping in tallgrass prairie. Global Change Biology 8, 754–768 (2002).
Jia, X. X., Shao, M. A. & Wei, X. R. Responses of soil respiration to N addition, burning and clipping in temperate semiarid grassland in northern China. Agricultural and Forest Meteorology 166, 32–40 (2012).
Liu, W. X., Zhang, Z. & Wan, S. Q. Predominant role of water in regulating soil and microbial respiration and their responses to climate change in a semiarid grassland. Global Change Biology 15, 184–195 (2009).
Zhang, N. L. et al. Impacts of urea N addition on soil microbial community in a semi-arid temperate steppe in northern China. Plant and Soil 311, 19–28 (2008).
Xu, W. & Wan, S. Water- and plant-mediated responses of soil respiration to topography, fire, and nitrogen fertilization in a semiarid grassland in northern China. Soil Biology & Biochemistry 40, 679–687 (2008).
Johnson, L. C. et al. Plant carbon-nutrient interactions control CO2 exchange in Alaskan wet sedge tundra ecosystems. Ecology 81, 453–469 (2000).
Stark, S. & Kytoviita, M. M. Simulated grazer effects on microbial respiration in a subarctic meadow: Implications for nutrient competition between plants and soil microorganisms. Applied Soil Ecology 31, 20–31 (2006).
Shaver, G. R. et al. Global warming and terrestrial ecosystems: A conceptual framework for analysis. Bioscience 50, 871–882 (2000).
Zhang, L., Huang, J. H., Bai, Y. F. & Han, X. G. Effects of Nitrogen Addition on Net Nitrogen Mineralization in Leymus Chinensis Grassland, Inner Mongolia, China. Acta Phytoecologica Sinica 33, 563–569 (2009).
Aber, J. D. & Magill, A. H. Chronic nitrogen additions at the Harvard Forest (USA): the first 15 years of a nitrogen saturation experiment. Forest Ecology and Management 196, 1–5 (2004).
Acknowledgements
We gratefully thank the staff in the Dunlun stations for access and permission. This work was partially supported by the Chinese National Key Development Program for Basic Research (2017YFA0604802, 2016YFC0500703) and National Natural Science Foundation of China (31572452,41573063) and funding from Key Laboratory of Vegetation Ecology, Ministry of Education. Data share or any questions should contact with Dr. Changhui Wang (wangch@ibcas.ac.cn) & Prof. Kuanhu Dong (E-mail: dongkuanhu@126.com).
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Yükun Luo, Changhui Wang and Yan Shen wrote this manuscript and analyzed the data and made all the figures; Changhui Wang & Kuanhu Dong designed experiment and revised the manuscript; Wei Sun analyzed ammonia and nitrate concentration and give suggestion for the scientific questions; Luo Y., Wang C., Sun W., and Dong K. wrote the main manuscript text and Luo Y. and Shen Y. prepared Figures 1–8.
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Luo, Y., Wang, C., Shen, Y. et al. The interactive effects of mowing and N addition did not weaken soil net N mineralization rates in semi-arid grassland of Northern China. Sci Rep 9, 13457 (2019). https://doi.org/10.1038/s41598-019-49787-6
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DOI: https://doi.org/10.1038/s41598-019-49787-6
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