Forms of nitrogen inputs regulate the intensity of soil acidification

Soil acidification induced by reactive nitrogen (N) inputs can alter the structure and function of terrestrial ecosystems. Because different N-transformation processes contribute to the production and consumption of H⁺, the magnitude of acidification likely depends on the relative amounts of organic N (ON) and inorganic N (IN) inputs. However, few studies have explicitly measured the effects of N composition on soil acidification. In this study, we first conducted a meta-analysis to test the effects of ON or IN inputs on soil acidification across 53 studies in grasslands. We then compared soil acidification across five different ON:IN ratios and two input rates based on long-term field N addition experiments. The meta-analysis showed that ON had weaker effects on soil acidification than IN when the N addition rate was above 20 g N m⁻² year⁻¹. The field experiment confirmed the findings from meta-analysis: N addition with proportions of ON ≥ 20% caused less soil acidification, especially at a high input rate (30 g N m⁻² year⁻¹). Structural equation model analysis showed that this result was largely due to a relatively low rate of H⁺ production from ON as NH₃ volatilization and uptake of ON and NH₄⁺ by the dominant grass species Leymus chinensis (which are both lower net contributors to H⁺ production) result in less NH₄⁺ available for nitrification (which is a higher net contributor to H⁺ production). These results indicate that the evaluation of soil acidification induced by N inputs should consider N forms and manipulations of relative composition of N inputs may provide an effective approach to alleviate the N-induced soil acidification.     

Effect of cropping systems on soil chemical characteristics,with emphasis on soil acidification*

The soil under intensive cultivation and low addition of crop residues is exposed to erosion and reduction of organic matter. Increases in soil organic matter, cation exchange capacity (CEC) and nutrient availability may occur in no-till systems with legumes and with large additions of organic residues. Nevertheless, some legumes may increase soil acidification through the carbon and nitrogen cycles. An experiment was carried out over 10 years, with 10 cropping systems on a Dark Red Podzolic soil (Paleudult) to evaluate the effect of no-till cropping systems on soil chemical characteristics. Legume cropping systems resulted in the greatest soil organic C gain and the highest ECEC to a depth of 17.5 cm. The increase was greatest at 0 - 2.5 cm layer. Clover systems resulted in the highest soil acidification at 2.5 - 7.5 and 7.5 - 17.5 cm depths. The rate of soil pH decrease at 2.5 - 7.5 cm depth under clover+ t Spergula/maize system was 0.1 unit year^-1. Differences in soil acidification affected soil ECEC. Soil exchangeable cation data indicate that nitrate leaching increased soil acidification. Maize yields were greatest in legume systems due to increased N supply.     

Effects of deforestation and cultivation on soil CEC and contents of exchangeable bases: A case study in Simlipal National Park,India

Deforestation in the tropics seems to be a serious problem probably because of the reduction in soil CEC and the consequent losses of nutrients from the soils. Here, changes in these parameters as influenced by deforestation as well as vegetative cover were studied; statistical methods were applied to interpret the results. Cultivation causes a significant reduction in CEC, total content of the exchangeable bases and exchangeable Ca2+ and Mg2+ levels compared to the adjoining unmanaged forest land. Levels of exchangeable K⁺ and Na⁺, however, do not change significantly. Evergreen forest soils have the highest levels of CEC, total exchangeable bases, exchangeable Ca²⁺ and K⁺. Deciduous forest, grassland and cultivated soils have statistically similar contents of exchangeable Ca²⁺, Mg²⁺, K⁺ and Na⁺. Exchangeable Mg²⁺, however, is not affected by vegetative cover. Soil CEC shows fairly good correlation with the organic carbon content only in evergreen forest soils. In others, organic carbon apparently does not influence CEC significantly. All soils show excellent correlation between their CEC and total exchangeable bases. It is concluded that for regeneration of weathered tropical soils, an evergreen cover provides the most effective means; deciduous vegetation or grass cover do not seem promising.     

Nitrogen deposition contributes to soil acidification in tropical ecosystems

Elevated anthropogenic nitrogen (N) deposition has greatly altered terrestrial ecosystem functioning, threatening ecosystem health via acidification and eutrophication in temperate and boreal forests across the northern hemisphere. However, response of forest soil acidification to N deposition has been less studied in humid tropics compared to other forest types. This study was designed to explore impacts of long‐term N deposition on soil acidification processes in tropical forests. We have established a long‐term N‐deposition experiment in an N‐rich lowland tropical forest of Southern China since 2002 with N addition as NH₄NO₃ of 0, 50, 100 and 150 kg N ha^?1 yr^?1. We measured soil acidification status and element leaching in soil drainage solution after 6‐year N addition. Results showed that our study site has been experiencing serious soil acidification and was quite acid‐sensitive showing high acidification (pH_(H2O)<4.0), negative water‐extracted acid neutralizing capacity (ANC) and low base saturation (BS,< 8%) throughout soil profiles. Long‐term N addition significantly accelerated soil acidification, leading to depleted base cations and decreased BS, and further lowered ANC. However, N addition did not alter exchangeable Al³⁺, but increased cation exchange capacity (CEC). Nitrogen addition‐induced increase in SOC is suggested to contribute to both higher CEC and lower pH. We further found that increased N addition greatly decreased soil solution pH at 20 cm depth, but not at 40 cm. Furthermore, there was no evidence that Al³⁺ was leaching out from the deeper soils. These unique responses in tropical climate likely resulted from: exchangeable H⁺ dominating changes of soil cation pool, an exhausted base cation pool, N‐addition stimulating SOC production, and N saturation. Our results suggest that long‐term N addition can contribute measurably to soil acidification, and that shortage of Ca and Mg should receive more attention than soil exchangeable Al in tropical forests with elevated N deposition in the future.     

Prediction of long-term fertilizer nitrogen requirements of maize in the tropics using a nitrogen balance model

A simple N balance model was used to calculate fertilizer requirement for a target N uptake by maize. Nitrogen uptake from soil sources and target uptake of N with fertilizer N additions were obtained from fertilizer trials in Africa and Latin America. Most experiments had data for only one cropping period, although some from Latin America had data for four to six crops. The transfer coefficient of fertilizer N to the crop was adjusted to realize maximum recovery of fertilizer N under best methods of fertilizer application. The time constants of transfer of soil N to the crop were allowed to vary and were affected mainly by soil texture. Where 4 to 6 cropping periods were available good agreement between actual and predicted fertilizer N requirements was obtained. With this approach long-term fertilizer N requirements for 14 sites were predicted using first cropping period N uptake. This study showed that pools of organic N in more coarse-textured soils were usually smaller and declined more rapidly than in fine-textured soils. Labile organic N pools declined with time under all simulations, but approached equilibrium within 10 croppings seasons. Equilibrium N uptake from the soil organic N pool was predicted to be 31 kg ha^–1 for the more coarse-textured soils and 36 kg ha^–1 for the fine-textured soils. Long-term projections of fertilizer requirements using input data of the field experiments were reasonable, and effects of legume green manures and other amendments could be clearly evaluated.     

Uptake of fertilizer nitrogen and soil nitrogen by rice using15N-labelled nitrogen fertilizer

Data from five field experiments using labelled nitrogen fertilizer were used to determine the relative effects of soil nitrogen and fertilizer nitrogen on rice yield. Yield of grain was closely correlated with total aboveground nitrogen uptake (soil+fertilizer), less closely correlated with soil nitrogen uptake and not significantly correlated with fertilizer nitrogen uptake. When yield increase rather than yield was correlated with fertilizer nitrogen uptake, the correlation coefficient was statistically significant.Contribution from the Laboratory for Flooded Soils and Sediments, Agronomy Dept., Louisiana Agri. Exper. Sta., Louisiana State Univ., Baton Rouge, LA 70803, and Univ. of Florida, Agricultural Research and Education Center, Sanford, FL 32771.     

Modeling long-term crop response to fertilizer and soil nitrogen

A simple nitrogen balance model to calculate long-term changes in soil organic nitrogen, nitrogen uptake by the crop and recovery of applied nitrogen, is presented. It functions with time intervals of one year or one growing season. In the model a labile and a stable pool of soil organic nitrogen are distinguished. Transfer coefficients for the various inputs of nitrogen are established that specify the fractions taken up by the crop, lost from the system, and incorporated in soil organic nitrogen. It is shown how input data, model parameters and initial pool sizes can be derived and how the model can be used for calculating long-term changes in total soil organic nitrogen and uptake by the crop. For nitrogen applied annually as fertilizer or organic material the time course of nitrogen uptake and recovery of applied nitrogen is calculated. To test the sensitivity of the model, calculations have been performed for different environmental conditions with higher or lower risks for losses. The model has also been applied to establish fertilizer recommendations for a certain target nitrogen uptake by the crop. Finally, for agricultural systems where periods of cropping alternate with peroids of green fallow the time course of nitrogen uptake by the crop is calculated.     

Improvement of soil acidification and ammonium nitrogen content in tea plantations by long-term use of organic fertilizer

• Soil acidification is common in some Chinese tea plantations, which seriously affected growth of tea trees. Hence, it is essential to explore soil remediation in acidified tea plantations for sustainable development of the tea industry. We sought to determine how different fertilizers affect acidified soil and their N transformation in tea plantations.
• Different fertilizers were used on acidified tea plantation soils for 4 years (2017–2021), and changes in soil pH, indices related to soil N transformation and tea yield were analysed to construct interaction networks of these indices and find which had the largest influence on fertilization.
• Long-term use of sheep manure reduced soil acidification, increased soil pH, enhanced the number and intensity of N-fixing and ammonifying bacteria, urease, protease, asparaginase and N-acetamide glucose ribosidase activity and nifH gene expression. This treatment reduced the number and intensity of soil nitrifying and denitrifying bacteria, nitrate reductase and nitrite reductase activity, while the expression of amoA-AOA, nirK, nirS, narG and nosZ in turn increased ammonium N content of the soil, reduced nitrate N content, and enhanced tea yield. Topsis index weight analysis showed that ammonium N content in the soil had the largest impact among fertilization effects.
• Long-term use of sheep manure was beneficial in restoring the balance of the micro-ecosystem in acidified soil. This study provides an important practical basis for soil remediation and fertilizer management in acidified tea plantation soils.

     

Effects of litter incorporation and nitrogen fertilization on the contents of extractable aluminium in the rhizosphere soil of tea plant (Camallia sinensis (L.) O. Kuntze)

The effects of litter incorporation and nitrogen application on the properties of rhizosphere and bulk soils of tea plants (Camellia sinensis (L.) O. Kuntze) were examined in a pot experiment. Total of 8 treatments included four levels of tea litter additions at 0, 4.9, 9.8, and 24.5 g kg^–1 in combination with two N levels (154.6 mg kg^–1 and without). After 18 months of growth the rhizosphere soil was collected by removing the soil adhering to plant roots and other soil was referred to as bulk soil. The dry matter productions of tea plants were significantly increased by N fertilization and litter incorporation. The effect of litter was time-depending and significantly decreased the content of exchangeable Al (Al_ex, by 1 mol L^–1 KCl) and Al saturation at 9 months after litter incorporation whereas soil pH was not affected, although the litter contained high Al content. After 18 months, the contents of extractable Al by dilute CaCl₂, CuCl₂ + KCl, NH₄OAC, ammonium oxalate and sodium citrate (Al_CaCl2, Al_Cu/KCl, Al_NH4OAC, Al_Oxal, and Al_Cit respectively) and Al_ex, were not affected by litter application, except that of Al_CaCl2 in the rhizosphere soil which was decreased following litter additions. Nitrogen fertilization with NH₄ ⁺ (urea and (NH₄)₂SO₄) significantly reduced soil pH, the contents of exchangeable Ca, K, Mg and base saturation while raised extractable Al levels (Al_CaCl2, Al_Cu/KCl, Al_NH4OAC, and Al_ex). In the rhizosphere soils exchangeable K accumulated in all treatments while exchangeable Ca and Mg depleted in treatments without litter application. The depletions of Ca and Mg were no longer observed following litter incorporation. This change of distribution gradients in rhizosphere was possibly due to the increase of nutrient supplies from litter decomposition and/or preferable root growth in soil microsites rich in organic matter. Lower pH and higher extractable Al (Al_CaCl2, Al_ex, and Al_NH4OAC) in the rhizosphere soils, regardless of N and litter treatments, were distinct and consistent in all treatments. Such enrichments of extractable Al in the rhizosphere soil might be of importance for tea plants capable of taking up large amounts of Al.     

草地土壤有机质不同组分氮库对长期氮添加的响应

土壤氮库对生态系统的养分循环至关重要。目前多数研究主要关注氮沉降对土壤总氮的影响, 而对土壤不同有机质组分的氮库对氮沉降响应的研究较为缺乏。该研究基于内蒙古典型草地的长期多水平施氮(0、8、32、64 g·m^-2·a^-1)实验平台, 利用土壤密度分级方法, 探究氮添加处理13年后典型草地中两种土壤有机质组分(颗粒态有机质(POM), 矿质结合态有机质(MAOM))氮含量的变化及调控机制。结果显示: 土壤总碳含量、POM和MAOM的碳含量在施氮处理间均没有显著差异。土壤总氮含量则随着施氮水平增加呈显著增加的趋势, 同时施氮处理下POM的氮含量显著上升, 而MAOM的氮含量没有变化。进一步分析发现, 施氮促进植物地上生物量积累, 增加了凋落物量及其氮含量, 从而导致POM的氮含量增加。由于MAOM主要通过黏土矿物等吸附土壤中小分子有机质形成, 其氮含量受土壤中黏粒与粉粒含量影响, 而与氮添加水平无显著相关关系。该研究结果表明长期氮添加促进土壤氮库积累, 但增加的氮主要分布在稳定性较低的POM中, 受干扰后容易从生态系统中流失。为了更准确地评估和预测氮沉降对陆地生态系统的氮循环过程的影响, 应考虑土壤中不同有机质组分的差异响应。     

氮肥调控剂对潮褐土中不同氮源氮素转化及油菜生长的影响

采用土壤培养和盆栽试验相结合的方法,研究了硝化抑制剂双氰胺(DCD)与纳米碳配合施用对尿素和碳酸氢铵在华北平原典型土壤潮褐土中转化的调控效果及其对油菜生长的影响.结果表明:尿素和碳酸氢铵在施入土壤后的2周内,土壤无机氮的供应强度差别较大,2周以后则基本相似.2种氮源对油菜生长及氮素利用的影响在生育前期(播种后34 d)差异显著,但最终达到商品生物量收获时,氮源之间差异不大.DCD对尿素和碳酸氢铵在潮褐土中的转化表现出显著的硝化抑制作用,其抑制强度和有效抑制时间随DCD用量的增加而增强,且以对碳酸氢铵施入土壤后的硝化抑制效果更好.在本研究条件下,DCD用量以占肥料纯氮量的1.0％ ～1.5％相对较佳,可显著提高油菜产量,改善叶色,降低植株硝酸盐含量,提高氮肥利用率.纳米碳与DCD配合施用对土壤铵氧化有明显的协同抑制效果,且可以显著刺激油菜前期的生长发育和氮素利用,降低油菜硝酸盐含量.     

长期施肥对潮棕壤有机氮组分的影响

依托长期定位试验(从1990年开始)的4个施肥处理(CK,不施肥;M,循环猪圈肥;NPK,单施化肥;NPK+M,化肥配施循环猪圈肥),探讨了长期施肥对潮棕壤有机氮各组分的影响并揭示了其年际变化特征.结果表明: 从试验开始至2015年,不同施肥模式下各有机氮组分呈现不同的变化趋势,其中酸解有机氮基本上表现为逐年增加的趋势;M和NPK+M处理氨基酸态氮也逐年上升,而CK和NPK处理氨基酸态氮持续下降,这可能与微生物对土壤中氨基酸的利用有关.在整个试验过程中(1990—2015年的平均值),NPK处理酸解铵态氮大幅度增加,比CK增加31.1%;在施加有机肥的2个处理中(M和NPK+M)有机氮各组分均有所提高.与M相比,NPK+M处理各有机氮组分增加的幅度更加明显,这表明有机无机肥配施能有效增加土壤有机氮,进而增强土壤供氮能力,提高土壤肥力水平.     

氮肥调控剂对潮褐土中不同氮源氮素转化及油菜生长的影响

采用土壤培养和盆栽试验相结合的方法,研究了硝化抑制剂双氰胺(DCD)与纳米碳配合施用对尿素和碳酸氢铵在华北平原典型土壤潮褐土中转化的调控效果及其对油菜生长的影响.结果表明： 尿素和碳酸氢铵在施入土壤后的2周内,土壤无机氮的供应强度差别较大,2周以后则基本相似.2种氮源对油菜生长及氮素利用的影响在生育前期(播种后34 d)差异显著,但最终达到商品生物量收获时,氮源之间差异不大.DCD对尿素和碳酸氢铵在潮褐土中的转化表现出显著的硝化抑制作用,其抑制强度和有效抑制时间随DCD用量的增加而增强,且以对碳酸氢铵施入土壤后的硝化抑制效果更好.在本研究条件下,DCD用量以占肥料纯氮量的1.0%～1.5%相对较佳,可显著提高油菜产量,改善叶色,降低植株硝酸盐含量,提高氮肥利用率.纳米碳与DCD配合施用对土壤铵氧化有明显的协同抑制效果,且可以显著刺激油菜前期的生长发育和氮素利用,降低油菜硝酸盐含量.     

Recent carbon and nitrogen accumulation and acidification in soils of two Scots pine ecosystems in Southern Germany

Changes of the soil chemical status during the recent 22–30 years at two historically degraded forest sites in southern Germany (Pfaffenwinkel, Pustert) stocked with mature Scots pine (Pinus sylvestris L.) stands were studied by repeated soil inventories conducted in 1974, 1982–1984, 1994, and 2004 on replicated control plots of fertilization experiments, allowing a statistical analysis. Additionally, the nutritional status of the stands at all plots was monitored from 1964 until 2004 by annual or bi-annual analysis of current-year foliage, and stand growth was assessed by repeated stand inventories carried out in 3- to 9-year intervals. For both sites, a statistically significant systematic decrease of the forest floor C/N ratio between 1974 and 2004 from 35.4 to 29.2 (Pfaffenwinkel) and from 36.5 to 23.0 (Pustert) was observed. The soils at both sites also showed a considerable accumulation of organic carbon (210 and 400 kg C ha⁻¹ year⁻¹ for Pfaffenwinkel and Pustert, respectively) and nitrogen (13 and 18 kg N ha⁻¹ year⁻¹). In addition, the mineral topsoil at both sites has acidified considerably, indicated by significantly decreased pH values (Pustert only; mean decrease 0.1 units per decade), base saturation, and base cation stocks. The trend of N enrichment and base cation loss in the soils is mirrored by the trends of stand nutrition at both sites, which are characterized by improved N nutrition and reduced supply with K, Mg (Pustert only), and Ca. The results confirm findings of other studies indicating a recent N eutrophication and acidification of forest soils in Central Europe and southern Scandinavia. Since soils with historic degradation due to earlier non-sustainable forest utilization are widespread in Central Europe, the results obtained on our study sites probably apply for large forested areas, suggesting a significant potential of Central European forests to sequester atmospheric carbon and nitrogen not only in stand biomass, but also in the soil.     

川西北泡沙参种群根系生长动态及其与环境因素关系研究

对川西北地区泡沙参种群根系生物量累积和形态发育过程及其与环境因素的关系进行了研究.结果表明,泡沙参种群根系生物量累积一般规律符合Logistic增长过程.较高的泡沙参根系生物量累积和形态发育时期可以持续到15～17年生以后,根系采收直径应在1.7cm以上.动物啃食、人为采挖、土壤、气候等环境条件与不同海拔各种群的根系生物量累积等有着密切联系,中海拔地区(2800～3300m)人为干扰少,土壤和水热条件适宜,根系生物量和形态发育达到较高水平,适合高产栽培.而低海拔和高海拔地区的不利环境条件限制了泡沙参根系生长.在进行野生资源保护利用以及人工栽培泡沙参时应充分考虑环境因素,努力减少放牧、采挖等人为破坏,有条件的地区应实行分区禁牧、禁采挖,为泡沙参种群恢复和药材品质提高创造条件.     

减磷配施有机肥对水旱轮作紫色水稻土磷素淋失的消减效应

为探索长江流域稻油轮作系统水稻季减少农田磷素流失的最佳施肥模式和有效耕作措施,降低其对长江水质的威胁。采用渗漏池长期田间原位定点试验并结合室内实验分析,研究了化肥配施猪粪有机肥和水稻秸秆还田对土壤磷素淋溶迁移的影响。结果表明在水稻生长期内土壤淋溶水中磷素浓度随时间延长呈逐渐下降的趋势,前期波动幅度大且下降迅速,到55d之后逐步稳定达到平衡。总可溶性磷(TDP)是渗漏水磷素的主要形态。土壤淋溶水中总磷(TP)和总可溶性磷(TDP)含量均表现为优化施肥+猪粪有机肥(MP)优化施肥+秸秆还田(SP)优化施肥(P)优化施肥量磷减20%+猪粪有机肥(MDP)优化施肥量磷减20%+秸秆还田(SDP)不施磷肥(P0)。土壤总磷(TP)淋失负荷在0.295—0.493 kg/hm2之间。施用有机肥提高了淋溶水中的磷素含量,促进了土壤中磷素的淋失,同时显著提高了土壤中有效磷的含量,猪粪有机肥的促进作用比水稻秸秆大。减少化肥施用量有利于降低土壤磷素淋失。在综合考虑农业生产省本增效和控制农田面源污染的情况下,可以采取减量化肥配施有机肥的施肥模式。     

土壤微生物生物量氮对不同腐熟度有机肥的响应及对土壤矿质氮的调控作用

为探究化肥配施不同腐熟度有机肥对土壤微生物生物量氮(MBN)的影响及土壤MBN调控土壤矿质氮的作用,将堆肥过程与土壤培养试验相结合,设置常规化肥对照(CK)、化肥+腐熟度为50%(种子发芽指数为50%,下同)的有机肥(CO1)、化肥+腐熟度为80%的有机肥(CO2)、化肥+腐熟度为100%的有机肥(CO3)共4个处理,测定土壤MBN、矿质氮(NH₄⁺-N、NO₃^--N)、净硝化速率、微生物生物量碳(MBC)、可溶性有机碳(DOC)、脲酶和蛋白酶,并揭示土壤MBN对矿质氮的调控作用。结果表明: 到培养结束时,与CK处理相比,有机肥处理(CO1、CO2、CO3)的土壤MBN、NH₄⁺-N含量显著提高50.1%～62.4%、109.9%～147.1%,土壤NO₃^--N、净硝化速率显著降低23.3%～46.8%、26.2%～51.5%,土壤MBC、DOC含量、脲酶和蛋白酶活性分别显著提高33.8%～69.6%、7.4%～20.8%、11.2%～69.0%、9.4%～25.1%,且CO2、CO3的变化幅度均显著高于CO1。冗余分析和结构方程模型显示,较高腐熟度有机肥(腐熟度≥80%)对MBC、MBN、NH₄⁺-N含量、脲酶和蛋白酶活性具有正向调控作用,对土壤净硝化速率具有负向调控作用。化肥配施较高腐熟度有机肥可以明显增加土壤MBN,提升脲酶、蛋白酶活性,增加NH₄⁺-N含量,降低土壤净硝化速率。因此,在实际应用中,建议采用腐熟度为80%的有机肥与化肥配施,减少有机肥生产成本及时间,实现有机固体废弃物的资源化利用。     

Yield and crop parameters of wetland rice as influenced by soil and fertilizer nitrogen

Summary The behavior of soil N, fertilizer N and plant N was studied in a greenhouse experiment with 2 plant densities of rice (IR 36) under flooded conditions. Increasing plant density from 25 hills m² to 50 hills m² increased tiller number and panicle number but had no influence on grain yield. The yield of grain was linearly related to N content of the above ground dry matter at harvest (r²=.96) and thus the effect of manipulating the N supply on yield was directly related to N uptake.Mixing of (NH₄)₂SO₄ with the soil volume before transplanting resulted in increases in N in the aboveground dry matter equal to 87% of the applied N. When (NH₄)₂SO₄ was broadcast into the flood water at 4 stages of growth beginning 25 DAT, the corresponding increase was 77% of the applied N. When (NH₄)₂SO₄ was split between shallow mixing before transplanting and a broadcast application of 32 DAT, the corresponding increase was 42%. Thus several applications of fertilizer N increased grain production per unit of applied N.Inorganic N extractable by KCl was a useful but not an infailible guide to the behavior of the soil and fertilizer inorganic N.     

Significant soil acidification across northern China's grasslands during 1980s–2000s

Anthropogenic acid deposition may lead to soil acidification, with soil buffering capacity regulating the magnitude of any soil pH change. However, little evidence is available from large‐scale observations. Here, we evaluated changes in soil pH across northern China's grasslands over the last two decades using soil profiles obtained from China's Second National Soil Inventory during the 1980s and a more recent regional soil survey during 2001–2005. A transect from the central‐southern Tibetan Plateau to the eastern Inner Mongolian Plateau, where Kriging interpolation provided robust predictions of the spatial distribution of soil pH, was then selected to examine pH changes during the survey period. Our results showed that soil pH in the surface layer had declined significantly over the last two decades, with an overall decrease of 0.63 units (95% confidence interval = 0.54–0.73 units). The decline of soil pH was observed in both alpine grasslands on the Tibetan Plateau and temperate grasslands on the Inner Mongolian Plateau. Soil pH decreased more intensively in low soil carbonate regions, while changes of soil pH showed no significant associations with soil cation exchange capacity. These results suggest that grassland soils across northern China have experienced significant acidification from the 1980s to 2000s, with soil carbonates buffering the increase in soil acidity. The buffering process may induce a large loss of carbon from soil carbonates and thus alter the carbon balance in these globally important ecosystems.