The effect of increasing salinity and forest mortality on soil nitrogen and phosphorus mineralization in tidal freshwater forested wetlands

Tidal freshwater wetlands are sensitive to sea level rise and increased salinity, although little information is known about the impact of salinification on nutrient biogeochemistry in tidal freshwater forested wetlands. We quantified soil nitrogen (N) and phosphorus (P) mineralization using seasonal in situ incubations of modified resin cores along spatial gradients of chronic salinification (from continuously freshwater tidal forest to salt impacted tidal forest to oligohaline marsh) and in hummocks and hollows of the continuously freshwater tidal forest along the blackwater Waccamaw River and alluvial Savannah River. Salinification increased rates of net N and P mineralization fluxes and turnover in tidal freshwater forested wetland soils, most likely through tree stress and senescence (for N) and conversion to oligohaline marsh (for P). Stimulation of N and P mineralization by chronic salinification was apparently unrelated to inputs of sulfate (for N and P) or direct effects of increased soil conductivity (for N). In addition, the tidal wetland soils of the alluvial river mineralized more P relative to N than the blackwater river. Finally, hummocks had much greater nitrification fluxes than hollows at the continuously freshwater tidal forested wetland sites. These findings add to knowledge of the responses of tidal freshwater ecosystems to sea level rise and salinification that is necessary to predict the consequences of state changes in coastal ecosystem structure and function due to global change, including potential impacts on estuarine eutrophication.     

丘陵红壤茶园根际氮磷转化对不同强度酸雨胁迫的响应

南方丘陵红壤茶园长期受到酸沉降的胁迫,但茶树根际氮(N)、磷(P)转化过程对酸雨的响应及其机制尚不清楚.以江西典型丘陵红壤25年茶园为对象,开展pH 4.5、pH 3.5、pH 2.5及对照4种不同强度酸雨处理的原位模拟试验,于试验第3年测定根际和非根际土壤矿质N、速效P和相关酶的活性,并估测土壤N、P矿化速率,计算各变量的根际效应.结果表明: 与对照相比,pH 4.5、pH 3.5和pH 2.5处理根际土壤NO₃^--N含量分别降低了7.1%、42.1%和49.9%,矿质N分别降低了6.4%、35.9%和40.3%,速效P分别降低了10.5%、41.1%和46.9%;根际氨化速率分别降低了18.7%、30.1%和44.7%,N净矿化速率分别降低了3.6%、12.7%和38.8%,P矿化速率分别降低了31.5%、41.8%和63.0%,但不同处理之间根际硝化速率差异不显著;根际土壤脲酶和酸性磷酸酶活性均表现为随酸雨加重呈增强的趋势(P<0.05).非根际土壤除NH₄⁺-N外,其他有效N和P含量未随酸雨加重而改变;不同酸雨处理对非根际土壤氨化、硝化、N净矿化速率和P矿化速率的影响差异均不显著.根际NH₄⁺-N、NO₃^--N、矿质N、氨化和净矿化速率均随着酸雨强度加重由正效应转变为负效应,而脲酶和酸性磷酸酶活性由负效应转变为正效应,但速效P和P矿化速率始终表现为负效应,硝化速率始终为正效应.综上所述,连续酸雨加重总体上抑制了根际N、P转化,降低其有效性,且不同程度改变其根际效应,从而影响茶园养分循环.     

Soil nitrogen dynamics three years after a severe Araucaria–Nothofagus forest fire

Wildfires have shaped the biogeography of south Chilean Araucaria–Nothofagus rainforest vegetation patterns, but their impact on soil properties and associated nutrient cycling remains unclear. Nitrogen (N) availability shows a site‐specific response to wildfire events indicating the need for an increased understanding of underlying mechanisms that drive changes in soil N cycling. In this study, we selected unburned and burned sites in a large area of the National Park Tolhuaca that was affected by a stand‐replacing wildfire in February 2002. We conducted net N cycling flux measurements (net ammonification, net nitrification and net N mineralization assays) on soils sampled 3 years after fire. In addition, samples were physically fractionated and natural abundance of C and N, and ¹³C‐NMR analyses were performed. Results indicated that standing inorganic N pools were greater in the burned soil, but that no main differences in net N cycling fluxes were observed between unburned and burned sites. In both sites, net ammonification and net nitrification fluxes were low or negative, indicating N immobilization. Multiple linear regression analyses indicated that soil N cycling could largely be explained by two parameters: light fraction (LF) soil organic matter N content and aromatic Chemical Oxidation Resistant Carbon (COREC_arom), a relative measure for char. The LF fraction, a strong NH₄⁺ sink, decreased as a result of fire, while COREC_arom increased in the burned soil profile and stimulated NO₃^‐ production. The absence of increased total net nitrification might relate to a decrease in heterotrophic nitrification after wildfire. We conclude that (i) wildfire induced a shift in N transformation pathways, but not in total net N mineralization, and (ii) stable isotope measurements are a useful tool to assess post‐fire soil organic matter dynamics.     

Soil nitrogen availability and nitrification in Mediterranean shrublands of varying fire history and successional stage

The short-term effect of a single fire, and the long-term effect of recent fire history and successional stage on total and mineral N concentration, net nitrogen mineralization, and nitrification were evaluated in soils from a steep semi-arid shrubland chronosequence in southeast Spain. A single fire significantly increased soil mineral N availability and net nitrification. Increasing fire frequency in the last few decades was. associated with a sharp decrease in surface soil organic matter and total N concentrations and pools, and with changes in the long-term N dynamic patterns. The surface-soil extractable NH₄ ⁺:NO₃ ^– ratio increased throughout the chronosequence. All net mineralized N in laboratory incubations from all sites was converted to nitrate, suggesting that allelochemic inhibition of net nitrification is probably not important in this system. Net nitrification in samples during incubation increased through the sere. The maximum rate of net nitrification (k_max) increased through the first three stages of the sere. A linear relationship was found between total soil N and N mineralization, and both k_max and net nitrification for the first three stages of the sere, suggesting that total N and ammonification are likely to be the control mechanisms of nitrification within the sere. The oldest site exhibited the lowest specific k_max and the highest, potential soil respiration rate suggesting that a lower N quality and increasing competition for ammonium might also limit nitrification at least in the long-unburned garrigue site.     

毛竹林和阔叶林凋落物互置对土壤氮矿化的影响及微生物贡献

凋落物输入方式的改变导致凋落物数量和质量发生变化,进而对森林土壤氮矿化产生影响。选择未被入侵的次生阔叶林和毛竹入侵后形成的毛竹纯林为对象,对地表凋落物进行保留、去除与置换处理,采用室内培养法同时添加抗生素(链霉素和放线菌酮)分析真菌和细菌在土壤氮素矿化中的贡献。结果表明：(1)去除凋落物处理使阔叶林土壤净氨化速率增加27.0%,净硝化速率降低11.4%;毛竹林土壤净氨化速率增加23.4%。(2)置换凋落物处理使阔叶林土壤净氨化速率增加43.8%,净硝化速率降低33.5%;毛竹林土壤净硝化速率增加73.1%。(3)添加抗生素后,凋落物置换处理与对照相比,置换凋落物后阔叶林土壤真菌和细菌在净氨化中发挥主要作用;真菌在两种林分土壤净硝化中发挥主要作用,细菌在阔叶林土壤净硝化中发挥主要作用。(4)结合测定的凋落物化学性质可知,置换凋落物后引起真菌和细菌在土壤氮素矿化中贡献发生变化,是由于输入凋落物中木质素和纤维素含量的变化。综上,凋落物去除和置换改变了土壤氮素矿化速率,置换凋落物后改变了真菌和细菌对土壤氮素矿化的贡献。解析凋落物质量在土壤氮素矿化中的作用及微生物群落的相对贡献,有助于阐明毛竹入...     

中亚热带丘陵红壤区森林演替典型阶段土壤氮磷有效性

在中亚热带典型丘陵红壤区选取裸露地、马尾松(Pinus massoniana)林地、针阔混交林地、常绿阔叶林地为研究对象,开展土壤氮(N)、磷(P)供应、有效性及其耦合过程的研究.结果表明,土壤有机C、全N、净矿化速率、中性磷酸酶活性表现为随森林演替进展呈现逐步提高的变化趋势;而土壤全P、C/N、C/P、氨化速率、硝化速率、树脂P、NaHCO3-P、NaOH-P、声波P、酸性P、总有效P、酸性磷酸酶活性未表现出此趋势;但反映N、P有效供应的指标,除氨化速率、树脂P和酸性磷酸酶外,在常绿阔叶林中均为最高.相关分析表明大部分N、P供应指标之间存在显著相关性(P<0.05).丘陵红壤区森林演替初级阶段P的限制性明显强于N,土壤N、P供应在森林演替进展过程中可以逐步得到优化而实现协调供应.以常绿阔叶林为中亚热带丘陵红壤区植被恢复的最终目标是可行和理想的.     

Soil nitrogen turnover in proximal and distal stem areas of European beech trees

In European beech (Fagus sylvatica L.) forests, a large proportion of the water and ion input to the soil results from stemflow which creates a soil microsite of high element fluxes proximal to the tree trunk. The soil proximal to the stem is considered to have different rates of nitrogen turnover which might influence the estimation of N-turnover rates at the stand scale. In a previous study we reported high nitrate fluxes with seepage proximal to the stems in a forest dominated by European beech in Steigerwald, Germany. Here, we investigated the soil nitrogen turnover in the top 15 cm soil in proximal (defined as 1 m² around beech stems) and distal stem areas. Laboratory incubations and in situ sequential coring incubations were used to determine the net rates of ammonification, nitrification, and root uptake of mineral nitrogen. In the laboratory incubations higher rates of net nitrogen mineralization and nitrification were found in the forest floor proximal to the stem as compared to distal stem areas. No stem related differences were observed in case of mineral soil samples. In contrast, the in situ incubations revealed higher rates of nitrification in the mineral soil in proximal stem areas, while net nitrogen mineralization was equal in proximal and distal areas. In the in situ incubations the average ratio of nitrification/ammonification was 0.85 in proximal and 0.34 in distal stem areas. The net nitrogen mineralization was 4.4 g N m^-2 90 day^-1 in both areas. Mineralized nitrogen was almost completely taken up by tree roots with ammonium as the dominant nitrogen species. The average ratio of nitrate/ammonium uptake was 0.69 in proximal and 0.20 in distal areas. The higher water content of the soil in proximal stem areas is considered to be the major reason for the increased rates of nitrification. Different nitrogen turnover rates in proximal stem areas had no influence on the nitrogen turnover rates in soil at the stand scale. Consequently, the observed high nitrate fluxes with seepage proximal to stems are attributed to the high nitrogen input by stemflow rather than to soil nitrogen turnover. This revised version was published online in August 2006 with corrections to the Cover Date.     

Net nitrogen mineralization and net nitrification along a tropical forest-to-pasture chronosequence

Soil inorganic nitrogen pools, net mineralization and net nitrification rates were compared during the dry season along a chronosequence of upland (terra firme) forest, 3-, 9- and 20-year-old pastures in the western Brazilian Amazon Basin state of Rondônia to investigate the influence of forest conversion to pasture on soil nitrogen cycles. Surface soil (0 to 10 cm) from forest had larger extractable inorganic nitrogen pools than pasture soils. In the forest, NO ₃ ^– pools equaled or exceeded NH ₄ ⁺ pools, while pasture inorganic N pools consisted almost exclusively of NH ₄ ⁺ . Rates of net N mineralization and net nitrification in seven -day laboratory incubations were higher in the seven - day forest than in the pastures. Net N mineralization rates did not differ significantly among different-aged pastures, but net nitrification rates were significantly lower in the 20-year-old pasture. Higher net N mineralization and net nitrification rates were measured in laboratory and in situ incubations of sieved soil, compared with in situ incubations of intact soil cores. Rates calculated in seven-day incubations were higher than determined by longer incubations. Sieving may increase N mineralization and/or decrease N immobilization compared with intact cores. We concluded that 7-day laboratory incubation of sieved soil was the most useful index for comparing N availability across the chronosequence of forest and pasture sites. High net nitrification rates in forest soils suggest a potential for NO ₃ ^– losses either through leaching or gaseous emissions.     

天山林区土壤总氮矿化过程对季节性冻融的响应

森林土壤总氮矿化对冻融过程的响应机制尚不明确,氮矿化速率和转化情况尚缺乏定量刻画。通过土壤原位法与室内培养分析相结合,利用¹⁵N同位素稀释技术,研究冻融期间天山林区乔木林地、灌丛、草地3种群落类型土壤总氮矿化及转化累积量的动态,分析土壤总氮矿化速率与土壤温度、含水率及微生物量氮(MBN)的相互关系。结果表明:(1)冻融过程及群落类型对总氮矿化速率和MBN含量有极显著的影响(P<0.01),秋、春季冻融期的总氮矿化速率相比冻结期更高;(2)季节性冻融期间,乔木林地土壤总氨化累积量在3种群落类型中最高(163.9 kg N hm^-2 a^-1),秋、春冻融期占整个时期的比值约为66%;而总硝化累积量在3种群落类型中相差较小,秋、春冻融期占比均约为77.4%;(3)土壤温度和含水率显著影响总氮矿化速率、净氮矿化速率和MBN速率,随土壤温度增加,总氨化速率(林地和灌丛)显著升高(P<0.05);随土壤含水率增加,净氨化速率(灌丛)和净硝化速率(灌丛)显著降低(P<0.05)。通过揭示天山林区土壤总氮矿化速率(总氨...     

亚热带不同林分土壤矿质氮库及氮矿化速率的季节动态

以亚热带地区天然林、格氏栲人工林和杉木人工林为对象,采取PVC管原位培养连续取样法,对不同林分土壤净氨化速率、净硝化速率及净氮矿化速率进行为期一年(2014年9月—2015年8月)的研究,分析林分类型和季节动态对土壤矿质氮库和净氮矿化速率的影响.结果表明: 硝态氮是该地区土壤矿质氮库的主要存在形式,天然林和杉木人工林土壤硝态氮含量分别占总土壤矿质氮库的55.1%～87.5%和56.1%～79.1%,林分间土壤铵态氮含量差异不显著,硝态氮含量差异显著,其中格氏栲人工林土壤硝态氮含量显著低于天然林和杉木人工林.土壤硝态氮库和矿质氮库在不同月份间差异显著,在植物非生长季节(10月至次年2月)较大,在植物生长季节(3—9月)较小.各林分全年土壤净硝化速率均较低,净氨化速率是净氮矿化速率的主要存在形式,林分类型对土壤净氨化速率有显著影响,其中杉木人工林显著低于天然林和格氏栲人工林.月份对土壤净氨化速率有显著影响,各林分土壤净氨化速率变化规律不一致,但均在11月和2月达到一年中的最低值.重复测量方差分析显示,林分类型和季节动态对土壤矿质氮库及氮矿化速率均有显著影响.温度和水分是影响土壤矿质库及氮矿化速率的重要因素,凋落物对土壤氮矿化速率的影响主要是通过质量控制而非数量控制.     

丹江口库区覆膜耕作土壤净氮矿化随夏玉米生长期的变化

农田土壤净氮矿化对土壤氮素流失和农业非点源污染有重要影响.以丹江口库区五龙池小流域夏玉米黄棕壤为例,进行原位矿化试验,通过与无覆膜耕作土壤相比较,研究覆膜耕作条件下土壤净氮矿化在夏玉米生长期内的变化.结果表明: 夏玉米整个生长期内,覆膜耕作土壤净氨化量、净硝化量和净氮矿化量均明显高于无覆膜土壤,分别高6.63、12.96和19.59 mg·kg^-1;覆膜耕作土壤净氨化速率表现为在苗期较高、抽穗期最低、成熟期增至最高的变化特征,而土壤净硝化和净氮矿化速率均呈现苗期较高、拔节期最低、成熟期升至最高的变化过程;覆膜耕作土壤净氮矿化速率均与土壤全氮和NO₃^--N含量、土壤含水量之间呈显著线性关系.覆膜可有效调节土壤水热条件,促进土壤净氮矿化.     

模拟增温对青藏高原东部高寒灌丛土壤氮转化的影响

本文研究了青藏高原东部窄叶鲜卑花高寒灌丛生长季前期、生长季后期和非生长季3个生育期的土壤氮转化速率对模拟增温的响应,分析全球气候变暖对高寒灌丛土壤氮循环过程的影响.结果 表明:模拟增温使高寒灌丛土壤温度显著升高1.2℃,土壤水分显著降低2.5％.高寒灌丛生长季土壤净氮矿化(氨化和硝化)速率显著高于非生长季,但土壤净氮固...     

Seasonal patterns of ammonium and nitrate uptake in nine temperate forest ecosystems

Summary Seasonal patterns of net N mineralization and nitrification in the 0–10 cm mineral soil of 9 temperate forest sites were analyzed using approximately monthlyin situ soil incubations. Measured nitrification rates in incubated soils were found to be good estimates of nitrification in surrounding forest soils. Monthly net N mineralization rates and pools of ammonium-N in soil fluctuated during the growing season at all sites. Nitrate-N pools in soil were generally smaller than ammonium-N pools and monthly nitrification rates were less variable than net N mineralization rates. Nitrate supplied most of the N taken up annually by vegetation at 8 of the 9 sites. Furthermore, despite the large fluctuations in ammonium-N pools and monthly net N mineralization, nitrate was taken up at relatively uniform rates during the growing season at most sites.     

The influence of western hemlock and western redcedar on microbial numbers,nitrogen mineralization,and nitrification

Summary Microbial numbers in the forest floor and mineral soil (Al horizon) under large individual western hemlock (Tsuga heterophylla) and western redcedar (Thuja plicata) trees were compared. The lower pH and base saturation of hemlock samples was associated with higher fungal spore counts while cedar samples had higher total microbial counts and populations of ammonium oxidizing bacteria. Nitrogen mineralization rates were greater in laboratory incubations of hemlock soil but nitrification was only observed in incubations of cedar soil. These differences in nitrogen mineralization and nitrification are aspects of species-specific nutrient cycling regimes.     

间伐和林下植被剔除对毛竹林土壤氮矿化速率及其温度敏感性的影响

选择中亚热带毛竹人工林为研究对象,利用野外原位和室内培养相结合的方法,探讨不同间伐强度(25%间伐、50%间伐)和林下植被剔除对土壤氮矿化速率及其温度敏感性的影响。结果表明,25%间伐显著增加土壤氨化速率(P0.01),但降低硝化速率(P0.01);50%间伐显著增加土壤硝化速率(P0.01),而林下植被剔除显著降低土壤硝化速率(P0.01)。相关分析的结果表明,土壤氨化速率与有机碳(SOC)、全氮(TN)及全磷(TP)含量呈显著负相关关系;硝化速率与SOC、含水量(SWC)呈显著正相关关系,与铵态氮(NH~+_4-N)含量呈显著负相关关系。随着温度的升高,不同处理下的氨化速率均显著增加(P0.01),而硝化速率显著降低(P0.01)。25%间伐显著降低土壤净氮矿化和氨化过程的Q_(10)值,对硝化过程的Q_(10)值影响不显著;50%间伐对氨化和硝化过程的Q_(10)值影响均不显著;林下植被剔除对氨化过程的Q_(10)值影响不显著,但显著增加硝化过程的Q_(10)值。不同处理下的土壤氮矿化过程的Q_(10)值介于1.17—1.36之间。25%间伐和林下植被保留有利于毛竹林土壤氮素的供给。     

Nitrogen mineralization in upland Precambrian Shield catchments: Contrasting the role of lichen-covered bedrock and forested areas

The upland boreal forest at the Experimental Lakes Area (northwestern Ontario, Canada) is characterized by treed soil islands interspersed within lichen and moss-covered bedrock outcrops. N mineralization was 2.5-fold and net nitrification was 13-fold higher on an areal basis over bedrock surfaces because of high mineralization rates under lichen and moss patches. The higher average soil temperature in lichen and moss patches could not account for the difference in mineralization rates. Lichens did not provide a significant additional source of N because they did not fix atmospheric N. A refractory conifer litter with a high C:N probably favours the immobilization of N in forest islands. Buried bag and in situ core incubations yielded similar net N mineralization rates but core incubations underestimated net nitrification rates. Both methods did not adequately measure dissolved organic N (DON) production rates because soil disturbance caused high initial DON concentrations. The higher export of mineral N from bedrock surfaces is probably a combination of the lower retention of N in precipitation and leaching of mineralized N from lichen and moss patches.     

The pattern between nitrogen mineralization and grazing intensities in an Inner Mongolian typical steppe

Ungulate grazing is known to play a crucial role in regulating energy flow and nutrient cycling in grassland ecosystems. However, previous studies of the effect of grazing on soil N dynamics have showed controversial results. Some studies indicate that grazing stimulates N mineralization while others report that grazing suppresses N mineralization. In order to reconcile these contrasting results, we investigated the response pattern of nitrogen transformation to multiple grazing intensities in an Inner Mongolian steppe. In our study, we measured net nitrogen mineralization rates and nitrification rates during a whole growing season in a 17-year field experiment that had five grazing intensities (0.00, 1.33, 2.67, 4.00 and 5.33 sheep ha⁻¹). Primarily because of changes in temperature and moisture conditions, net N mineralization rates varied substantially during the growing season with higher values occurring in late July. No consistent differences in net N mineralization rates were observed between grazing intensity treatments at the monthly time scale. Compared to mineralization rates, net nitrification rates were generally low with slightly higher values occurring in late July and late August. Ungulate grazing stimulated the cumulative net N transformations (mineralization, nitrification and ammonification) at the annual time scale, and the most stimulation occurred at a moderate grazing intensity of 4.00 sheep ha⁻¹, whereas the highest grazing intensity of 5.33 sheep ha⁻¹ and the lighter grazing intensity of 1.33 sheep ha⁻¹ stimulated less. The general response of net N mineralization to grazing intensity gradient is roughly in the form of a normal distribution at the annual time scale. Our study demonstrated that grazing intensity in concert with soil moisture and temperature conditions imposed significant controls on soil N transformation and availability in this Inner Mongolian steppe.     

氮硅添加对青藏高原高寒草甸土壤氮矿化的影响

为了解全球气候变化背景下氮沉降对土壤氮矿化的影响及硅添加对土壤氮矿化的促进作用, 该试验设置不同浓度的氮肥单独添加(0、20、40、60 g·m ^-2, 分别为对照CK、N20、N40、N60)以及与硅肥配施(硅酸4 g·m ^-2, Si4), 测定不同处理下0-20、20-40、40-60 cm土层土壤硝态氮含量、铵态氮含量、净硝化速率、净氨化速率以及净矿化速率。结果显示: (1)单独添加氮肥, 各土层土壤硝态氮和铵态氮含量均随处理浓度的增加而增加, 0-20 cm土层N20、N40、N60处理下土壤硝态氮和铵态氮分别较CK增加63.48%、126.04%、247.03%和80.66%、152.52%、244.56%; 随着土层深度增加, 土壤硝态氮、铵态氮含量均有下降, 20-40、40-60 cm土层较0-20 cm土层硝态氮含量分别平均减少53.90%、76.05%, 铵态氮含量分别平均减少48.62%、68.23%。(2)土壤净硝化速率、净氨化速率及净矿化速率随着氮肥浓度增加均呈上升趋势。相同氮肥添加浓度下, 土壤净硝化速率、净氨化速率和净矿化速率随着土层深度增加逐渐下降(除CK外)。(3)与单独添加氮肥比较, 氮硅肥配施, 土壤氮含量有显著提高, 在0-20 cm土层硝态氮和铵态氮较CK分别增加98.78%、192.62%、330.16%和99.96%、195.82%、306.32%, 20-40、40-60 cm土层也有类似趋势。同时, 氮硅配施促进了土壤氮矿化行为, 在0-20 cm土层, N60Si4处理下的土壤净硝化速率、净氨化速率较单独施氮时分别增加35.88%、27.41%。以上结果表明, 与单独氮肥添加相比, 氮硅配施不但能提高土壤氮含量, 而且能促进土壤氮的矿化作用, 对大气氮沉降有一定的缓解作用。     

Production,nutrient availability,and elemental balances of two meadows affected by different fertilization and water table regimes in The Netherlands

The restoration of degraded peat-grasslands is an important nature conservation goal in The Netherlands. We investigated the effects of ceased fertilization (15 years) combined with a groundwater-raised water table (6 years) on the production of the peat-grassland vegetation and soil nutrient availability in a meadow. Furthermore, we evaluated whether and how this difference between meadows affected the balances between nutrient inputs and outputs in the ecosystem. We used an adjacent fertilized meadow in which the water table followed agricultural practice as a control. Yield of the grassland vegetation was significantly lower in the wet than in the control meadow. The tissue concentrations of N, P, and K in the harvested vegetation were significantly lower, but those of Ca higher in the wet than in the control meadow. The difference between both meadows significantly affected the annual nitrification rate, but not the annual C and N mineralization rates and the annual net P and K release rates. The difference between both meadows also significantly affected the seasonal nitrification and K release rates. Season exerted a significant effect on the seasonal C and N mineralization and nitrification rates. The elemental balances and relative contributions of the balance terms to elemental inputs and outputs varied considerably with element. Annually, the wet meadow lost N, P and K, while the control meadow gained these elements. The elemental demand of the grassland vegetation in the wet meadow was met for N for a large part by mineralization and for the remainder by atmospheric deposition, for P it was in the same order of magnitude as the net soil-P release, as it was for K. It is to be expected that the soil resources of N, P and K will continue to decrease under a continued regime of ceased fertilization and a raised water table, with those of N decreasing with the same rate, of P morerapidly and of K more slowly than estimated from regressions.     

降水变化对科尔沁沙地樟子松人工林土壤氮矿化和淋溶的影响

为研究降水量减少对沙地森林土壤氮循环过程的影响,以科尔沁沙地15年生樟子松人工林为研究对象,野外模拟不同降水量（自然降水、减少30%和50%）对沙地樟子松人工林土壤无机氮（SIN）含量、氮矿化速率和淋溶动态的影响。研究结果发现,沙地樟子松人工林SIN主要以硝态氮形态存在,模拟降水减少降低土壤硝态氮含量（P<0.05）和硝态氮/SIN值（P<0.001）,而增加土壤铵态氮含量（P<0.05）。与自然降水相比,降水减少降低土壤净硝化速率和净矿化速率（P=0.002）,但不同降雨处理的土壤净氨化速率差异不显著（P=0.86）。科尔沁沙地樟子松人工林土壤以硝态氮淋溶为主,不同降雨处理土壤硝态氮淋溶量差异不显著（P=0.09）,但模拟降水减少降低土壤铵态氮淋溶（P=0.04）。此外,沙地樟子松人工林SIN含量、净氮矿化速率和淋溶量具有明显月动态特征,与降雨月动态规律基本一致。降水处理和采样时间对SIN含量和净氮矿化速率具有显著交互作用,但土壤氮淋溶量的交互作用不显著。可见,降水变化能够显著影响科尔沁沙地樟子松人工林土壤氮有效性、氮矿化速率和淋溶等过程,未来干旱加剧可能降低科尔沁沙地樟子松人工林土壤氮的可利用性。