Accelerated Soil Acidification Research Articles

The contribution of natural and anthropogenic causes to soil acidification rates under different fertilization practices and site conditions in southern China

Excessive application of mineral fertilizers has accelerated soil acidification in China, affecting crop production when the pH drops below a critical value. However, the contributions of natural soil acidification, induced by leaching of bicarbonate, and anthropogenic causes of soil acidification, induced by nitrogen (N) transformations and removal of base cations over acid anions, are not well quantified. In this study, we quantified soil acidification rates, in equivalents (eq) of acidity, by assessing the inputs and outputs of all major cations and anions, including calcium, magnesium, potassium, sodium, ammonium, nitrate, bicarbonate, sulphate, phosphate and chloride, for 13 long-term experimental sites in southern China. The acidification rates strongly varied among fertilizer treatments and with the addition of animal manure. Bicarbonate leaching was the dominant acid production process in calcareous soils (23 keq ha−1 yr−1) and in non-calcareous paddy soils (9.6 keq ha−1 yr−1), accounting for 80 % and 68 % of the total acid production rate, respectively. The calcareous soils were strongly buffered, and acidification led no or a limited decline in pH. In contrast, N transformations were the most important driver for soil acidification at one site with upland crops on a non-calcareous soil, accounting for 72 % of total acid production rate of 8.4 keq ha−1 yr−1. In this soil, the soil pH considerably decreased being accompanied by a substantial decline in exchangeable base cation. Reducing the N surplus decreased the acidification rate with 10 to 54 eq per kg N surplus with the lowest value occurring in paddy soils and the highest in the upland soil. The use of manure, containing base cations, partly mitigated the acidifying impact of N fertilizer inputs and crop removal, but enhanced phosphorus (P) accumulation. Combining mineral fertilizer, manure and lime in integrative management strategies can mitigate soil acidification and minimize N and P losses.

Straw return exacerbates soil acidification in major Chinese croplands

Straw return has been widely recommended as a practice of climate-smart agriculture. It is generally conceived that straw return also alleviates soil acidification, though this assumption has not been tested over large geographical areas. Here, we assess its contributions to soil acidification based on three datasets across China. Unexpectedly, straw return was found to predominately aggravate soil acidification in 145 investigated field experiments. The magnitude of straw-induced soil pH change was negatively correlated with initial soil pH. The critical initial soil pH in dry and paddy lands, above which straw return caused further soil acidification, were 6.7 and 5.4, respectively. We estimated that straw return would exacerbate soil acidification in 77% and 80% of dry and paddy lands, respectively. Our analysis disclosed the first evidence of accelerated soil acidification by straw return at national scale, providing new insights into both sustainable straw utilization and soil acidification remediation.

Nitrate improves aluminium resistance through SLAH-mediated citrate exudation from roots.

Aluminium (Al) toxicity is one of the major constraint for crop production in acidic soil, and the inappropriate utilization of nitrogen fertilizer can accelerate soil acidification. Despite previous studies investigating the regulation of nitrogen forms in Al toxicity of plants, the underlying mechanism, particularly at the molecular level, remains unclear. This study aims to uncover the potentially regulatory mechanism of nitrate (NO3 - ) in the Al resistance of maize and Arabidopsis. NO3 - conservatively improves Al resistance in maize and Arabidopsis, with nitrate-elevated citrate synthesis and exudation potentially playing critical roles in excluding Al from the root symplast. ZmSLAH2 in maize and AtSLAH1 in Arabidopsis are essential for the regulation of citrate exudation and NO3 - -promoted Al resistance, with ZmMYB81 directly targeting the ZmSLAH2 promoter to activate its activity. Additionally, NO3 - transport is necessary for NO3 - -promoted Al resistance, with ZmNRT1.1A and AtNRT1.1 potentially playing vital roles. The suppression of NO3 - transport in roots by ammonium (NH4 + ) may inhibit NO3 - -promoted Al resistance. This study provides novel insights into the understanding of the crucial role of NO3 - -mediated signalling in the Al resistance of plants and offers guidance for nitrogen fertilization on acid soils.

Responses of crop production and soil health to chemical nitrogen fertilization in a maize-wheat rotation system

Chemical nitrogen (N) fertilizer is generally applied at high rates to maximize crop yields and also many non-grain farmlands are being converted into grain production to ensure grain production safety in China now. To test the effects of chemical N on crop production and soil health, a 3-year trial with urea and controlled-release fertilizer was conducted in a maize (Zea mays L.)-wheat (Triticum aestivum L.) system shifted from vegetable farmland by evaluating the effects of N fertilization on crop yields, typical soil properties, and soil bacterial community using 16S rRNA gene sequencing. The maximum crop yield and aboveground N uptake efficiency was achieved at 150 kg N ha-1 for wheat and 200 kg N ha-1 for maize. After three cropping seasons, the soil N properties were notably altered. For each kg of N applied, soil pH decreased at a rate of 0.0004–0.0012 pH units. The soil total N (TN) and total carbon (TC) content reached a higher stability at certain cumulative N application rates. However, the C/N ratio linearly decreased as N rate increased. Consequently, N application significantly decreased soil bacterial diversity owing to the decrease of α-diversity indexes of Shannon and Pielou_e. At the phylum level, chemical N application significantly depleted members of Acidobacteria, Chloroflexi and Nitrospirae, but the growth of Actinobacteria, Gemmatimonadetes, Firmicutes and Patescibacteria were improved. Compared with urea, the controlled-release fertilizer did not significantly change crop yields, soil bacterial diversity, soil TN and TC content. Overall, our results indicated that chemical N promoted crop yields at optimum application rates, but excessive N application could result in a decline in soil health, specifically accelerated soil acidification, a decrease in soil bacterial diversity, and soil C and N imbalance.

Trade-off between short and long-term effects of mineral, organic or mixed mineral-organic fertilisation on grass yield of tropical permanent grassland

Since the discovery in the early 1950s of fixation of industrial atmospheric nitrogen through physico-chemical processes, mineral fertilisation has become the main means of fertilising cultivated land. However, several studies have shown that continued use of mineral fertilisers can accelerate soil acidification and reduce cation exchange capacity, water-holding capacity, and soil microbial activity, thereby reducing soil fertility. Other studies have underlined the importance of a nutrient balanced fertilisation protocol to sustain high grass yields. On the other hand, by recycling livestock manure, the use of organic fertiliser has been shown to lead to high soil productivity and to provide economic and environmental benefits to farmers. Our study focused on the short- and long-term effects of several types of fertilisers, (mineral, organic, and mixed organic-mineral), and several doses (ranging from 440 to 1,260kgN/ha/year) in a 15-year experiment on highly productive grassland. We showed that, in contrast to organic fertilisation, although the mineral treatments maintained soil nitrogen content, these treatments did not increase phosphorous and potassium concentrations, and in addition, led to soil acidification. Furthermore, although mineral fertilisation led to rapid increases in grass yield, its fertilisation effect weakened over time whereas some of the organic fertilisation treatments resulted in a gradual increase in grass yields. Significant differences in grassland productivity were observed between the effects of liquid manure and compost, liquid manure being more immediately effective while compost had a longer-term positive effect on grass yield. The mixed organic-mineral treatments were shown to be a good compromise, with a significant short-term increase in grass yields plus sustained grass production over time.

Effects of combined application of nitrogen, phosphorus, and potassium fertilizers on tea (Camellia sinensis) growth and fungal community

Fertilization is an effective strategy for enhancing tea yield and quality. In practice, fertilizer application in tea plantations is often characterized by N, P, or K deficiencies, which suppress tea growth. Fertilization also exerts indirect effects on tea yield and quality by affecting the microbial community in the soil which is critical for regulating nutrient cycling and soil multiplication. Due to their tolerance to adverse environments in tea plantations including low pH and high aluminum and fluorine content, fungi are strongly associated with tea growth and are involved in the decomposition of pruning residues. To elucidate the poorly understood effects of different fertilizer combinations on soil fungi and tea, soil and plant samples were collected from a five-year (2016–2020) field experiment involving combined application of N, P, and K fertilizers (NF: no fertilizer; PK, NK, and NP: fertilization under conditions of N, P, and K deficiency, respectively; and NPK: fertilization with N, P, and K). Yield and quality differences between NPK and PK were more significant than those between NPK and NK, and NPK and NP. Of these three nutrients, N was the primary limiting factor for tea growth and N deficiency had more significant effects on fungal diversity than P or K deficiencies. N addition accelerated soil acidification and promoted native soil organic carbon decomposition, thereby decreasing the fungal diversity and modifying fungal community composition and structure. Moreover, in the PK (N deficiency) treatment, the fungal network had the lowest modularity, indicating that N deficiency reduced the resistance of fungal communities to external disturbances. The relationship between tea and fungi at different soil depths revealed that yield and amino acid content of tea were more strongly correlated with fungi in the topsoil than in the subsoil. This indicates that soil samples collected from the top layer are more informative for investigating the relationship between tea and soil. Overall, these findings deepen our understanding of how combined fertilization affects tea growth and soil quality, thereby guiding optimal fertilization strategies toward maximization of tea yield and quality and minimization of environmental risks.

Microbial diversity and the abundance of keystone species drive the response of soil multifunctionality to organic substitution and biochar amendment in a tea plantation

AbstractHigh nitrogen (N) fertilizer inputs accelerate soil acidification and degradation in tea plantations, thus posing a threat to soil microbial diversity, species composition, and ecosystem service functions. The effects of organic fertilizer and biochar applications on improving soil fertility have been extensively studied on cropland; however, little is known about their effectiveness in promoting soil multifunctionality on rapidly expanding acidic soils in tea plantations. In this study, we conducted a two‐year field experiment in a subtropical tea plantation to investigate the effects of organic fertilizer substitution and biochar amendment on soil microbial communities and multifunctionality. The results showed that soil multifunctionality was enhanced in plots amended with organic fertilizer and biochar. Soil multifunctionality was significantly and positively correlated with alpha‐diversity of bacteria but not fungi. We also found that organic fertilizer substitution and biochar amendment improved soil multifunctionality by altering the abundance of keystone species. The abundance of keystone species classified as module hubs in the bacterial co‐occurrence network contributed significantly and positively to soil multifunctionality. In contrast, the keystone species categorized as module hubs in the fungal co‐occurrence network negatively affected soil multifunctionality. Soil pH was a key driver of soil microbial community composition, indicating that the increase in soil pH under organic fertilizer and biochar amendment had a crucial role in biological processes. These results suggest that organic substitution and biochar amendment are beneficial in preventing soil degradation and maintaining soil multifunctionality in subtropical tea plantations.

Soil acidification and factors controlling topsoil pH shift of cropland in central China from 2008 to 2018

Soil acidification can negatively impact the sustainability of agricultural systems. Spatially explicit estimations of soil acidification and the identification of local factors driving soil pH change are prerequisites for formulating and implementing site-specific measures to prevent and mitigate cropland soil acidification. Based on pairwise comparisons between 1443 pairs of topsoil pH observations at colocated sites in 2008 and 2018, this study revealed the spatial variability of cropland soil acidification and its influencing factors across Henan Province, central China. The results showed that the topsoil pH of cropland in Henan Province decreased by an average of 0.36 units, and more than 94% of the cropland in the study region suffered from varying degrees of topsoil pH decline from 2008 to 2018, demonstrating a large range in soil acidification. The most serious soil acidification of croplands occurred in the southern part of the province, while in the northern part of the province, the topsoil pH decreased to a lesser extent or even slightly increased. The topsoil pH shift of cropland was found to be closely associated with the N fertilizer application rate, base cation removal by crops, and factors influencing the soil response to acidification, i.e., soil calcareousness, initial acidity, clay content, moisture regimes and temperature regimes. Among the various factors, the N fertilizer application rate was identified as the most important factor related to soil acidification. Croplands face the risk of accelerating soil acidification due to continuous excessive nitrogen application, increasing the removal of base cations while promoting crop yield, and decreasing or even disappearing carbonate material in the soil.

The Response of Soil pH and Exchangeable Al to Alum and Lime Amendments

Intensive cultivation and unprecedented utilization of ammoniacal fertilizer has accelerated soil acidification in the southern Great Plains and many other parts of the world. During a two-year study that evaluated the impact of soil pH and aluminum (Al) toxicity on winter wheat yield potential, we observed a variance in the edaphic responses of the two study sites (Stillwater and Chickasha) to two soil amendments, Alum [Al2(SO4)3] and lime [Ca(OH)2]. We found that AlKCl values at Stillwater were 223% and 150% higher than Chickasha during Year 1 and Year 2, respectively, with similar soil pH. Additionally, Alsat values at Stillwater were 30.6% and 24.9% higher than Chickasha during Year 1 and Year 2, respectively. Surprisingly, when treated as a bivariate of Alsat, soil buffer indices differed in graphical structure. While Chickasha was identified with a cubic polynomial (p < 0.0001), Stillwater was characterized by linear regression (p < 0.0001). We have reason to believe that this divergence in edaphic response might be attributed to the organically bound Al, dissolved organic carbon (DOC), spatio-temporal variance, and adsorption reactions regulated by the solubility of Al(OH)+2 species in acidic soils.

Contribution of different proton sources to the acidification of red soil with maize cropping in subtropical China

Increasing acid deposition and intense nitrogen (N) fertilization have resulted in severe soil acidification in many regions of the world. However, the soil acidification rate, especially quantitative contributions of different proton (H+) sources remain unclear in various cropping systems. In this study, a two-year field lysimeter experiment was conducted with a set of different N fertilization treatments (0, 100, 200 and 400 kg N ha−1 yr−1) for maize crop in typical red soil in subtropical China. The pathways and budgets of different H+ sources were quantified by calculating the input–output balance of major elements. Results showed that fertilization significantly accelerated soil acidification by increasing the leaching of NO3– and base cations (K+, Ca2+, Na+, and Mg2+) as well as increasing the cation removals by plant uptake and harvest. Unbalanced plant uptake of anions and cations was the primary H+ source (64.4–80.5%) to soils. Plants have an important effect on soil acidification by redistributing cations in soils. Due to cation removals by plant uptake and harvest, there were significant decreases of soil exchangeable base cations under high urea-N treatments (200 and 400 kg N ha−1 yr−1) and significant increases of soil exchangeable H+ and Al3+ under three fertilization treatments. Of course, N transformation also plays an important role (12.1% to 38.8%). However, the effect of direct H+ deposition is minor in this area comparatively (5.78% to 7.34%). Quantification of acidification contribution of different sources is useful for the establishment of further soil remediation practices. The field management is considered very important to alleviate soil acidification, including increasing straw return and controlling fertilizer application.

Land use and management effects on volcanic soils in West Sumatra, Indonesia

In this study, we aimed to assess the influence of land use and management on the fertility characteristics and pedogenic indicators of volcanic soils. Our study area was a mountainous area located in West Sumatra, Indonesia, where local farmers intensively cultivate vegetables by applying high amounts of chemical fertilizers and agrochemicals. Some negative impacts of conventional management were observed in the vegetable fields: soil acidification and loss of organic matter, with descended soil pH and increased acidity as well as decreased contents of exchangeable bases and with decreased amount of organic C. These impacts were not effectively mitigated by positive factors such as increased P availability and decreased capacity for P fixation. Continued cultivation over the years compounded the effects of these changes. Organic farming was advantageous in terms of soil fertility maintenance as it alleviated negative impacts compared to conventional farming. Natural fallow was effective in replenishing soil fertility as indicated by a significant increase in organic C, total N, and exchangeable Ca and Mg contents compared to the vegetable fields. The degree of replenishment was enhanced with a longer period under fallow; however, the natural fallow decreased P availability and increased P fixation capacity. Calliandra calothyrsus fallow further accelerated soil acidification, regardless of the rapid increase in the organic C and exchangeable bases contents. The adverse effects of Calliandra fallow were associated with the formation of humus–aluminum complexes to an extent that it changed from Sil-andic to Alu-andic Andosols.

Effects of N addition on soil exchangeable cations in a young Keteleeria fortunei var. cyclolepis forest

ABSTRACTThe effects of simulated nitrogen (N) addition on soil exchangeable cations were assessed in a young Keteleeria fortunei var. cyclolepis forest in subtropical China. Three N addition schemes with three replications were designed for the forest control (without N addition, CK), low N (5 g N m−2 a−1, L-N) and high N (50 g N m−2 a−1, H-N). Results showed that continuous N addition resulted in increased, although not statistically significant, plant biomass of K. fortunei var. cyclolepis, while it significantly decreased cation exchange capacity (CEC), base saturation (BS) and exchangeable base cations (BCs) (especially Ca2+ and Mg2+ under high N addition). General soil properties (except total P) were significantly affected by high N addition compared with the control. High N addition significantly increased the concentration of foliar N, and decreased the concentrations of foliar P, Ca2+ and Mg2+. Soil pH was also significantly altered by increased N addition, indicating that N addition significantly accelerated soil acidification. This study found that N addition was negatively correlated with soil pH, CEC and Ca2+, and that loss of BCs might be related to increased NO3–.

The pH buffering capacity of Acrisols under cassava production in Southeastern Vietnam, Chau Thanh district

AbstractThis study examined the pH buffering capacity (pHBC) of haplic Acrisols under intensive cassava production in an upland area of Southeastern Vietnam where accelerated soil acidification has occurred. Soil samples (0–20, 20–40 and 40–60 cm) were taken at 12 sites under cassava and three sites under secondary dipterocarp forest as reference. The pH buffer curves were linear in the pHH2O range from 3.97 to 5.24, corresponding to a pHCaCl2 range from 3.74 to 5.20. Soil pHBC were low (1.16 ± 0.13 cmol/kg/pH) and correlated significantly with pH, Aluminium (Al) and Al‐related components. The results suggested that exchange acidity contributes significantly to soil's buffering capacity in acidic soils with low organic carbon. It also pointed to the possibility of using indicators of acidity to estimate soil pHBC. The low pHBC indicated a high risk for further acidification and also pointed to the possibility of using lime to remediate soils. Either acid buffering capacity or lime buffering capacity not pHBC in general should be considered, respectively, in acidification and liming studies. Factors and processes involved in soil acidification and liming need to be addressed as a background for soil remediation in the study area.

Spatio-temporal changes of cropland soil pH in a rapidly industrializing region in the Yangtze River Delta of China, 1980–2015

The impacts of rapid industrialization on spatio-temporal changes in soil pH have drawn major attention from the scientific community and decision-makers. A total of 1185 topsoil samples (0–20 cm) were collected from southern Jiangsu Province of China in 1980, 2000, and 2015, and the geostatistical sequential Gaussian simulation (SGS) was used to quantify the spatio-temporal patterns of changes in soil pH and identify the areas with significantly-declined pH. A significant topsoil acidification occurred in major croplands of the study area, with mean decrements of 0.56 and 0.52 units over the periods of 1980–2000 and 2000–2015, respectively. A significant decrease in pH occurred in the areas surrounding Taihu Lake during 1980–2000, and a further decrease was evident predominantly in the northeast during 2000–2015. Over the entire 35-year period, the soil pH for approximately 79% of the study area declined significantly. The decline over the first two decades (1980–2000) was largely attributed to increased chemical fertilizer inputs and the development of industries, while the further enhanced industrial emissions resulting from the significant expansion of factories were the primary reason for the accelerated soil acidification in the subsequent 15 years (2000–2015). Results from this study may serve as a basis for developing management strategies to achieve better soil quality in similar areas moving toward industrialization.

Plant acclimation to long-term high nitrogen deposition in an N-rich tropical forest

Anthropogenic nitrogen (N) deposition has accelerated terrestrial N cycling at regional and global scales, causing nutrient imbalance in many natural and seminatural ecosystems. How added N affects ecosystems where N is already abundant, and how plants acclimate to chronic N deposition in such circumstances, remains poorly understood. Here, we conducted an experiment employing a decade of N additions to examine ecosystem responses and plant acclimation to added N in an N-rich tropical forest. We found that N additions accelerated soil acidification and reduced biologically available cations (especially Ca and Mg) in soils, but plants maintained foliar nutrient supply at least in part by increasing transpiration while decreasing soil water leaching below the rooting zone. We suggest a hypothesis that cation-deficient plants can adjust to elevated N deposition by increasing transpiration and thereby maintaining nutrient balance. This result suggests that long-term elevated N deposition can alter hydrological cycling in N-rich forest ecosystems.

Effect of residue and fertiliser management on soil fertility in a long-term sugarcane trial in South Africa

Understanding the impact of long-term continuous addition of sugarcane residues and fertiliser on soil fertility is important for development of best management practices. The experiment commenced in 1939. The fertility of the 0–10 cm soil layer was affected by the management options, but neither mulching nor fertiliser impacted significantly on soil organic carbon (SOC). However, the combination of mulching and fertiliser application increased the SOC content in the 0–2 cm soil layer significantly. It also increased soil acidity, as well as the concentration of iron, manganese, zinc and silicon, whereas reducing pH, the effective cation exchange capacity and the concentration of two of the basic cations, namely calcium and magnesium, in the 0–20 cm soil layer. Fertiliser affected yield significantly. In the absence of fertiliser, mulching had a significant effect on yield. Soil moisture and yield correlated linearly. In conclusion, increasing the addition of organic matter to continuously fertilised soils accelerated soil acidification leading to the loss of exchangeable basic cations. It is, therefore, important to take cognisance of the acidification potential of added residue under a greencane harvesting (mulching) system; this consideration emphasises the need for soil conditions to be rigorously managed on the basis of regular soil analysis.

Nitrogen additions inhibit nitrification in acidic soils in a subtropical pine plantation: effects of soil pH and compositional shifts in microbial groups

Plantation forests play a pivotal role in carbon sequestration in terrestrial ecosystems, but enhanced nitrogen (N) deposition in these forests may affect plantation productivity by altering soil N cycling. Hence, understanding how simulated N deposition affects the rate and direction of soil N transformation is critically important in predicting responses of plantation productivity in the context of N loading. This study reports the effects of N addition rate (0, 40, and 120 kg N ha−1 a−1) and form (NH4Cl vs. NaNO3) on net N mineralization and nitrification estimated by in situ soil core incubation and on-soil microbial biomass determined by the phospholipid fatty acid (PLFA) method in a subtropical pine plantation. N additions had no influences on net N mineralization throughout the year. Net nitrification rate was significantly reduced by additions of both NH4Cl (71.5) and NaNO3 (47.1%) during the active growing season, with the stronger inhibitory effect at high N rates. Soil pH was markedly decreased by 0.16 units by NH4Cl additions. N inputs significantly decreased the ratio of fungal-to-bacterial PLFAs on average by 0.28 (49.1%) in November. Under NH4Cl additions, nitrification was positively related with fungal biomass and soil pH. Under NaNO3 additions, nitrification was positively related with all microbial groups except for bacterial biomass. We conclude that simulated N deposition inhibited net nitrification in the acidic soils of a subtropical plantation forest in China, primarily due to accelerated soil acidification and compositional shifts in microbial functional groups. These findings may facilitate a better mechanistic understanding of soil N cycling in the context of N loading.

Effects of long-term nitrogen deposition on phosphorus leaching dynamics in a mature tropical forest

Elevated anthropogenic nitrogen (N) deposition is suggested to affect ecosystem phosphorus (P) cycling through altered biotic P demand and soil acidification. To date, however, there has been little information on how long-term N deposition regulates P fluxes in tropical forests, where P is often depleted. To address this question, we conducted a long-term N addition experiment in a mature tropical forest in southern China, using the following N treatments: 0, 50, 100, and 150 kg N ha−1 year−1. We hypothesized that (i) tropical forest ecosystems have conservative P cycling with low P output, and (ii) long-term N addition decreases total dissolved phosphorus (TDP) leaching losses due to reduced litter decomposition rates and stimulated P sorption deriving from accelerated soil acidification. As hypothesized, we demonstrated a closed P cycling with low leaching outputs in our forest. Under experimental N addition, TDP flux in throughfall was significantly reduced, suggesting that N addition may result in a less internal P recycling. Contrary to our hypothesis, N addition did not decrease TDP leaching, despite reduced litter decomposition and accelerated soil acidification. We find that N addition might have negative impacts on biological P uptake without affecting TDP leaching, and that the amount of TDP leaching from soil could be lower than a minimum concentration for TDP retention. Overall, we conclude that long-term N deposition does not necessarily decrease P effluxes from tropical forest ecosystems with conservative P cycling.

Responses of soil nitrous oxide flux to soil environmental factors in a subtropical coniferous plantation: A boundary line analysis

Soil N2O responds differently to various soil variables; moreover these driving variables usually have low explanatory capacity of soil N2O flux variability. Anthropogenic nitrogen (N) deposition inputs to the subtropical plantation forests markedly increase N2O emission from soils, but the relative contribution of deposited NH4+ and NO3− is not well understood. Based on a two-type (NH4Cl and NaNO3) and three-level (0, 40, 120 kg N ha−1 yr−1) N addition experiment, we simultaneously determined soil N2O flux and related soil variables once a week in 2015. Boundary line analysis was used to describe the maximum responses of soil N2O fluxes to different soil variables. Four years of N addition led to significant accumulations of soil NH4+-N and NO3−-N, thereby accelerating soil acidification. Nitrogen addition significantly increased soil N2O fluxes by 4.7–19.3 folds relative to control; moreover, the promotions to soil N2O emission and soil acidification were greater with ammonium-N addition than with nitrate-N addition. The relationships between the maximum soil N2O fluxes and individual soil variables were well fitted to the Gaussian equations, and the optimum values of soil temperature, moisture, pH, and NO3−-N content indicate that nitrification dominated soil N2O production. Overall, our results suggest that exogenous NH4+ input to the subtropical plantation appears to have greater negative effects on soil acidification and N2O emission compared with NO3− input. Boundary line analysis well depicts the shape and magnitude of response functions of soil N2O fluxes against soil variables, which should be incorporated into terrestrial ecosystem process models.

Soil pH rather than nutrients drive changes in microbial community following long-term fertilization in acidic Ultisols of southern China

Long-term intensive cultivation and heavy fertilization improve the nutritional conditions in acidic Ultisols, but also sharply accelerate soil acidification. However, the impact of such dramatic environmental changes on soil microorganisms is unclear. The aims of this work were to investigate the responses of microbial community composition and metabolic function to long-term fertilization, and to determine the key factors that primarily drive microbial changes in acidic Ultisols. A long-term fertilization experiment under a winter wheat–summer maize rotation was established in 1990 in acidic Ultisols of southern China. Soils were collected from four treatments in June 2014: (1) non-fertilization control (CK); (2) only N fertilization (N); (3) N, P, and K fertilization (NPK); and (4) NPK plus manure (NPKM; 70% of total N obtained from manure). The amount of N used in all N treatments was 300 kg N ha−1 year−1. The soil pH, cation exchange capacity (CEC), soil organic carbon (SOC), total nitrogen (TN), phosphorus (TP) and potassium (TK), available nitrogen (AN), phosphorus (AP), and potassium (AK) were measured. Soil microbial community composition and metabolic function were determined by phospholipid fatty acids analysis (PLFA) and community-level physiological profile (CLPP) method, respectively. Compared with CK, NPKM significantly increased total PLFA biomass and average well color development (AWCD); NPK increased total PLFA biomass by 2.2 times, but its AWCD was not significantly different from CK, indicating that microbial metabolic efficiency in NPK decreased. N decreased total PLFA biomass by 27.9%, while almost completely inhibiting metabolic activity. NPKM maintained microbial functional diversity indexes at similar levels as CK, while NPK and N significantly decreased microbial functional diversity indexes. Redundancy analysis (RDA) revealed that soil microbial community composition and metabolic pattern were more stably maintained by application of manure compared to chemical fertilizers. Soil pH showed the primary effect on microbial community composition, metabolic activity, and functional diversity indexes. This research demonstrated that the negative effects of Ultisol acidification induced by long-term application of chemical N fertilizer on microorganisms overwhelmed the positive effects of soil nutrition improvement. The inhibiting effect of serious acidification on microbial metabolic function was stronger than that on community composition. Microorganisms live in a low active metabolic state to resist serious Ultisols acidification. Therefore, fertilization in acidic Ultisols should be based on the premise of preventing soil further acidification.