不同土地利用类型对丹江口库区土壤氮矿化的影响

氮(N)素是陆地生态系统净初级生产力的重要限制因子, 土地利用类型的变化对生态系统氮循环过程有着重要的影响。采用PVC顶盖埋管原位培养的方法, 对丹江口库区清塘河流域相邻的侧柏(Platycladus orientalis)人工林、人工种植灌木林地和农田3种土地利用类型的氮素矿化和硝化作用进行了研究。结果表明, 侧柏人工林、灌木林地和农田的NH₄⁺-N浓度(mg·kg^-1)依次为1.33 ± 0.20、1.67 ± 0.17和1.62 ± 0.13, 不同土地利用类型间的NH₄⁺-N浓度无显著性差异; 而3种土地利用类型下土壤NO₃^--N浓度(mg·kg^-1)差异显著, 农田NO₃^--N浓度(9.00 ± 0.73)显著高于侧柏人工林(1.27 ± 0.18)和灌木林地(3.51 ± 0.11)。NO₃^--N在灌木林地和农田中分别占土壤无机氮库的67.8%和84.8%, 是土壤无机氮库的主要存在形式; 而侧柏人工林中NO₃^--N和NH₄⁺-N浓度则基本相等。土壤硝化速率(mg·kg^-1·30 d^-1)从农田(7.13 ± 2.19)、灌木林地(2.56 ± 1.07)到侧柏人工林(0.85 ± 0.10)显著性降低。侧柏人工林、灌木林地和农田的矿化速率(mg·kg^-1·30 d^-1)依次为0.98 ± 0.12、2.52 ± 1.25和6.58 ± 2.29。矿化速率和硝化速率显著正相关, 但是矿化速率在不同的土地利用类型间差异不显著。培养过程中灌木林地和农田NH₄⁺-N的消耗大于积累, 氨化速率为负值, 导致灌木林地和农田矿化速率小于硝化速率。氮素的矿化和硝化作用受土壤含水量和土壤温度的影响, 并对土壤含水量更为敏感。土壤C:N与土壤矿化和硝化速率显著负相关。研究结果表明: 土地利用类型的变化会改变土壤微环境和土壤C:N, 进而会影响到土壤氮循环过程。     

草甸草原土壤与植物系统自然丰度氮稳定同位素的年际变化及其指示作用

氮是陆地生态系统生产力的首要限制性养分,利用自然丰度δ¹⁵N(¹⁵N/¹⁴N)可以有效指示生态系统氮循环过程。本试验研究了内蒙古草甸草原土壤与植物系统自然丰度δ¹⁵N、土壤净氮矿化潜势的年际变化。结果表明: 2017—2020年,土壤NO₃^--N含量(9.83～14.79 mg·kg^-1)均显著高于NH₄⁺-N含量(3.92～5.00 mg·kg^-1);土壤NH₄⁺的δ¹⁵N值(13.3‰～18.3‰)显著高于NO₃^-的δ¹⁵N值(3.76‰～6.14‰),土壤NO₃^-的δ¹⁵N值与土壤NO₃^-含量呈显著负相关;干旱年NH₄⁺的δ¹⁵N值相对较高,降水较高或较低年NO₃^-的δ¹⁵N值显著降低。干旱年土壤净氮矿化速率、净氨化速率显著高于湿润年,而土壤硝化速率与年降水量无显著相关性。植物δ¹⁵N值与土壤δ¹⁵N值无显著相关性,但与植物N含量呈显著负相关;豆科植物与非豆科植物δ¹⁵N值、N含量均呈显著正相关,在一定程度上表明豆科植物对非豆科植物的N吸收具有促进作用。研究结果可为草原土壤-植物系统氮循环过程及其对降水变化的响应提供数据支撑。     

氮添加对盐渍化草地根际土壤理化性质的影响

为探究氮输入和根际效应对盐渍化草地土壤理化性质的影响,对8个水平氮添加处理下(0、1、2、4、8、16、24和32 g N·m^-2·a^-1)晋北盐渍化草地根际和非根际土壤理化性质进行研究。结果表明: 氮添加显著降低根际土壤pH,显著增加根际和非根际土壤Ca²⁺、NO₃^--N和无机氮含量;随氮添加量的增加,根际和非根际土壤Ca²⁺、NO₃^--N、无机氮含量以及根际土壤全氮含量呈逐渐升高的趋势,而根际土壤Na⁺、K⁺、Mg²⁺、NH₄⁺-N和氨基酸含量以及非根际土壤全氮含量呈先升高后降低的趋势。主成分分析表明,根际土壤理化性质对低氮(≤8 g·m^-2·a^-1)和高氮添加(>8 g·m^-2·a^-1)的响应具有明显差异。根际土壤pH、有机酸和氨基酸含量分别比非根际土壤低0.71、44.3%和9.8%,而K⁺、Ca²⁺、Mg²⁺、NH₄⁺-N、无机氮、全碳和全氮含量分别比非根际土壤高51.0%、47.6%、20.8%、215.5%、139.3%、31.7%和65.3%,表明根际效应对盐渍化草地土壤理化性质的影响大于氮输入的影响。     

掺混氮肥配施抑制剂对土壤氮库的调控作用

采用冬小麦盆栽试验,探讨掺混氮肥(缓释肥N∶普通尿素N=1∶1)配施氮肥抑制剂NAM对冬小麦土壤铵态氮、硝态氮、微生物生物量氮和固定态铵含量及小麦产量、氮肥利用率的影响,分析不同处理土壤矿质氮库、微生物生物量氮库和固定态铵库的动态变化特征.试验共设6个处理,不施氮肥(CK)、普通尿素(U)、掺混氮肥(MU)、MU+2.5‰NAM(MUN₁)、MU+5‰NAM(MUN₂)和MU+7.5‰NAM(MUN₃).结果表明:与MU处理相比,MUN₂和MUN₃处理推迟了NH₄⁺-N峰值出现的时间;小麦整个生长季,添加NAM处理的土壤矿质氮平均含量比MU处理下降了5.3%～11.7%;分蘖期至抽穗期,MU处理的微生物生物量氮矿化量和矿化率分别为38.96 mg·kg^-1和91.5%,均高于U处理,而MUN₁、MUN₂和MUN₃处理分别为58.73 mg·kg^-1和83.3%、94.20 mg·kg^-1和94.6%、104.46 mg·kg^-1和96.3%,添加NAM处理固定态铵的释放量比MU处理提高了2.83~9.19 mg·kg^-1.通径分析结果显示,与MU处理相比,添加NAM减弱了土壤NH₄⁺-N库对NO₃^--N库的直接影响,增强了固定态铵库通过影响NH₄⁺-N库对NO₃^--N库的间接作用.同时,MUN₁、MUN₂和MUN₃处理的小麦籽粒产量较MU处理分别提高了31.6%、21.5%和22.9%,氮肥利用率分别提高了8.1%、13.5%和3.1%.综上,配施NAM通过对氮素释放及在土壤中转化的双重调控,延迟土壤NH₄⁺-N峰值出现的时间及后续向NO₃^--N的转化,提高微生物生物量氮和固定态铵的供氮作用,从而提高了作物产量和氮肥利用率.     

秸秆还田量对培肥农田黑土氮素初级转化速率的影响

秸秆还田是土壤培肥的重要措施之一,但培肥后土壤对氮素循环的调控功能是否具有可持续性以及与后续秸秆还田数量的关系尚不清楚。本研究对黑土旱地农田进行9年培肥处理后,设置了连续3年秸秆还田处理,秸秆还田量分别为年均秸秆产量(7500 kg·hm^-2)的100%、67%、33%和0,探讨不同秸秆还田量对培肥土壤(0～10 cm)氮素转化过程的影响。结果表明: 秸秆还田通过影响氮素初级转化速率,控制培肥土壤NH₄⁺-N和NO₃^--N的产生与消耗过程。当秸秆还田量低于67%时,NH₄⁺-N生成速率显著降低,而其消耗速率显著提高,土壤保留NH₄⁺-N的能力下降,而NO₃^--N生成速率增加,土壤NO₃^--N固持能力下降,增加NO₃^--N的积累及淋溶损失风险。对于东北旱地农田,利用秸秆归还进行黑土培肥,需要不低于67%收获量的秸秆持续还田才能维持土壤对氮素的保持功能。     

北京东灵山大气氮沉降水平及变化特征

城市人为成因的气态活性氮排放影响空气质量，导致周边的陆地生态系统大气氮输入量持续增加。然而，陆地生态系统大气活性氮特别是溶解态无机氮(DIN)和溶解态有机氮(DON)的同步观测仍然较为缺乏，影响氮沉降生态效应的全面、准确评估。本研究观测了北京东灵山森林生态系统定位研究站2019年6月至2020年1月每周的混合沉降中铵态氮(NH₄⁺-N)、硝态氮(NO₃^--N)和总溶解态氮(TDN)浓度，计算了DON浓度和各形态氮的沉降通量，分析了它们的月际和干湿季差异及其变化机制。结果表明：该站点大气沉降中NH₄⁺-N、NO₃^--N、DON和TDN体积加权平均浓度分别为1.45±0.04、0.70±0.01、1.81±0.66和3.96±0.65 mg N·L^-1,TDN年沉降通量为25.00 kg N·hm^-2·a^-1,NH₄^+<...     

氮素添加和CO2浓度升高对白羊草根际和非根际土壤水溶性有机碳、氮的影响

采用盆栽控制试验对黄土丘陵区白羊草在不同CO₂浓度(400和800 μmol·mol^-1)和施氮水平(0、2.5、5.0 g N·m^-2·a^-1)条件下根际和非根际土壤水溶性有机碳(DOC)和水溶性有机氮(DON)的变化特征进行研究.结果表明: CO₂浓度升高对白羊草根际和非根际土壤DOC、水溶性总氮(DTN)、DON、水溶性铵态氮(NH₄⁺-N)、水溶性硝态氮(NO₃^--N)含量均无显著影响.施氮显著提高了根际和非根际土壤DTN、NO₃^--N含量和根际土壤DON含量,显著降低了根际土壤DOC/DON.在各处理条件下,根际土壤DTN、NO₃^--N和DON含量均显著低于非根际土壤,根际土壤DOC/DON显著高于非根际土壤.短期CO₂浓度升高对黄土丘陵区土壤水溶性有机碳、氮含量无显著影响,而氮沉降的增加在一定程度上改善了土壤中水溶性氮素缺乏的状况,但并不足以满足植被对水溶性氮素的需求.     

华北盐渍化草地土壤净氮矿化速率对不同水平氮添加的响应

对于养分贫瘠的盐渍化草地生态系统, 大气氮沉降如何影响土壤氮循环过程是一个目前尚未解决的问题。该研究在位于华北地区山西省右玉县境内的盐渍化草地建立了一个模拟氮沉降的试验平台, 设置8个氮添加水平, 分别为0、1、2、4、8、16、24、32 g·m^-2·a^-1 (N0、N1、N2、N4、N8、N16、N24、N32), 生长季5-9月, 每月月初以喷施的方式等量添加NH₄NO₃。从2017年5月到2019年10月, 运用顶盖PVC管法每月一次进行净氮矿化速率的测定同时计算了净氮矿化速率对不同水平氮添加的敏感性。主要结果表明: (1)高水平氮添加(N16、N24、N32)显著增加土壤无机氮库; (2)该盐渍化草地土壤氮矿化以硝化作用为主, 经过3年氮添加以后, 高氮添加(N24、N32)显著促进了土壤净硝化速率, 并且不同氮添加水平在不同的月份和年份中表现出差异性响应; (3)不同氮添加水平对土壤净氮矿化敏感性的影响在不同降水年份差异显著, 短期低水平氮添加提高了土壤净氮矿化的敏感性, 而高水平氮添加降低土壤净氮矿化敏感性; (4)盐渍化草地土壤净氮矿化速率与土壤温度和水分呈正相关关系, 与土壤pH呈负相关关系。因此, 在当前氮沉降增加的背景下, 北方盐渍化草地土壤氮矿化速率对低氮添加的敏感性较高, 结合氮沉降的特点, 未来模型预测应该同时考虑氮沉降对盐渍化草地的可能影响。     

氮素添加和CO2浓度升高对白羊草根际和非根际土壤水溶性有机碳、氮的影响

采用盆栽控制试验对黄土丘陵区白羊草在不同CO₂浓度(400和800 μmol·mol^-1)和施氮水平(0、2.5、5.0 g N·m^-2·a^-1)条件下根际和非根际土壤水溶性有机碳(DOC)和水溶性有机氮(DON)的变化特征进行研究.结果表明: CO₂浓度升高对白羊草根际和非根际土壤DOC、水溶性总氮(DTN)、DON、水溶性铵态氮(NH₄⁺-N)、水溶性硝态氮(NO₃^--N)含量均无显著影响.施氮显著提高了根际和非根际土壤DTN、NO₃^--N含量和根际土壤DON含量,显著降低了根际土壤DOC/DON.在各处理条件下,根际土壤DTN、NO₃^--N和DON含量均显著低于非根际土壤,根际土壤DOC/DON显著高于非根际土壤.短期CO₂浓度升高对黄土丘陵区土壤水溶性有机碳、氮含量无显著影响,而氮沉降的增加在一定程度上改善了土壤中水溶性氮素缺乏的状况,但并不足以满足植被对水溶性氮素的需求.     

苦草根系对硝氮和氨氮的吸收

硝氮(NO₃^--N)和氨氮(NH₄⁺-N)是湖泊沉积物间隙水生物可利用氮源的主要形态。论文通过稳定性同位素¹⁵N示踪技术,通过模拟实验分别研究了苦草根系对NH₄⁺-N和NO₃^--N的吸收及其与氮浓度的关系。结果显示,苦草(Vallisnerianatan)根系对NH₄⁺-N的吸收显著高于NO₃^--N;根系吸收氮后向叶转移,而且NO₃^--N为氮源时其转移速率较高;NH₄⁺-N浓度的变化对苦草吸收NO₃^--N有影响,当NH₄⁺-N浓度小于0.072mmol/L时,根系对NO₃^--N的吸收随NH₄⁺-N浓度的增加而增加,随后降低并趋于平稳;同时,NO₃^--N浓度对苦草吸收NH₄⁺-N也有类似的影响。     

Plant carbon substrate supply regulated soil nitrogen dynamics in a tallgrass prairie in the Great Plains,USA: results of a clipping and shading experiment

Aims Land use management affects plant carbon (C) supply and soil environments and hence alters soil nitrogen (N) dynamics, with consequent feedbacks to terrestrial ecosystem productivity. The objective of this study was to better identify mechanisms by which land-use management (clipping and shading) regulates soil N in a tallgrass prairie, OK, USA.Methods We conducted 1-year clipping and shading experiment to investigate the effects of changes in land-use management (soil microclimates, plant C substrate supply and microbial activity) on soil inorganic N (NH 4 + ? N and NO 3 ? ? N), net N mineralization and nitrification in a tallgrass prairie.Important findings Land-use management through clipping and/or shading significantly increased annual mean inorganic N, possibly due to lowered plant N uptake and decreased microbial N immobilization into biomass growth. Shading significantly increased annual mean mineralization rates (P < 0.05). Clipping slightly decreased annual mean N nitrification rates whereas shading significantly increased annual mean N nitrification rates. Soil microclimate significantly explained 36% of the variation in NO 3 ? ? N concentrations (P = 0.004). However, soil respiration, a predictor of plant C substrate supply and microbial activity, was negatively correlated with NH 4 + ? N concentrations (P = 0.0009), net N mineralization (P = 0.0037) and nitrification rates (P = 0.0028) across treatments. Our results suggest that change in C substrate supply and microbial activity under clipping and/or shading is a critical control on NH 4 + ? N, net N mineralization and nitrification rates, whereas clipping and shading-induced soil microclimate change can be important for NO 3 ? ? N variation in the tallgrass prairie.     

滇西北高原纳帕海湿地土壤氮矿化特征

采用树脂芯原位培育法,研究了纳帕海沼泽、沼泽化草甸和草甸土壤氮的矿化特征。结果表明,铵态氮（NH₄⁺-N）为沼泽、沼泽化草甸土壤中无机氮的主要存在形式,分别占无机氮含量的96.76%和75.24%,而硝态氮（NO₃^--N）为草甸土壤中无机氮的主要存在形式,占无机氮含量的58.77%。植物生长期内,纳帕海湿地土壤的净氮矿化速率表现为沼泽化草甸 > 草甸 > 沼泽,表明干湿交替的土壤环境更利于土壤氮矿化作用的进行,土壤中氮素有效性和维持植物可利用氮素的能力更强。整个生长季,沼泽和草甸土壤氮矿化为硝化作用,而沼泽化草甸土壤氮矿化为氨化作用。土壤硝态氮含量、有机质含量、碳氮比和含水量均对纳帕海沼泽、沼泽化草甸和草甸土壤的氮矿化产生显著影响。     

土壤动物与N素循环及对N沉降的响应

以主要的生态过程之一——N循环为对象,论述了土壤动物不仅对凋落物的分解有重要影响,而且在N素矿化和植物对N的吸收过程中也起着重要作用。同时,日益严重的全球变化问题之一——N沉降对土壤动物的多样性及其在生态系统中的功能构成了极大的威胁。另还对土壤动物与N循环研究的方法、土壤动物在N循环过程中的作用机制、热带地区的需求及N沉降下土壤动物的响应作了探讨,并提出,开展大尺度的专类研究及长期定位研究成为下一步研究的需要。     

Nitrogen Retention by Soil Biota; A Key Role in the Rehabilitation of Natural Grasslands?

Environmental stress is the main cause of the decline of species diversity in low‐productive fen meadows in the Netherlands. Attempts to restore species diverse fen meadows e.g. by sod cutting frequently fail. We supposed that unsuccessful efforts are due to ignoring the impact of environmental stress on the performance of soil biota, which play a key role in N‐immobilization and keeping available‐N for primary production low. We investigated both pristine and degraded natural sites and successfully and unsuccessfully restored sites of poor and rich fen meadows. We determined plant species composition, soil chemical properties, N‐pools in soil biota, N‐mineralization rates, and N‐fluxes. In pristine rich and poor fen meadows, mineral‐N was poorly available for primary production due to a strong N‐immobilization by soil biota. Annual N‐immobilization fluxes exceeded by far the annual N‐harvest by primary production. N‐immobilization in pristine fens was higher than in degraded fens. In successfully restored rich fens, net N‐mineralization was lower and N‐immobilization higher than in the unsuccessful category. From our results, we derived the hypothesis that in degraded or in unsuccessfully restored fens the soils internal N‐balance shifted from N‐immobilization to net N‐mineralization, favoring biomass production but disadvantaging plant species diversity. N‐retention driven by an active N‐immobilizing soil biological community, is likely a decisive process for successful recovery of plant species diversity in low productive fen meadows. We recommend that restoration techniques should stimulate a functionally diverse soil fauna, as this may enhance the storage of available nutrients in the soil food web.     

Simulation of nitrogen assimilation by heterotrophic soil microbial biomass

Assimilation of N by heterotrophic soil microbial biomass is associated with decomposition of organic matter in the soil. The form of N assimilated can be either low molecular weight organic N released from the breakdown of organic matter (direct assimilation), or NH⁺₄ and NO⁻₃ from the soil inorganic N pool, into which mineralized organic N is released (mineralization immobilization turnover). The kinetics of C and N turnover in soil is quantifiable by means of computer simulation models. NCSOIL was constructed to represent the two assimilation schemes. The rate of N assimilation depends on the rate of C assimilation and microbial C/N ratio, thereby rendering it independent of the assimilation scheme. However, if any of the N forms is labeled, a different amount of labeled N assimilation will be simulated by the different schemes. Experimental data on inorganic N and ¹⁵N and on organic ¹⁵N dynamics in soils incubated with ¹⁵N added as NH⁺₄ or organic N were compared with data simulated by different model schemes. Direct assimilation could not account for the amount of ¹⁵N assimilated in any of the experimental treatments. The best fit of the model to experimental data was obtained for the mineralization immobilization turnover scheme when both NH⁺₄ and NO⁻₃ were assimilated, in proportion to their concentration in the soil.     

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.     

Nitrogen dynamics in alpine ecosystems of the northern Caucasus

Net N mineralization, nitrification, microbial biomass N and ¹⁵N natural abundance were studied in a toposequence of representative soils and plant communities in the alpine zone of the northern Caucasus. The toposequence was represented by (1) low-productive alpine lichen heath (ALH) of wind-exposed ridge and upper slope; (2) more productive Festuca varia grassland (FG) of middle slope; (3) most productive Geranium gymnocaulon/Hedusarum caucasicummeadow (GHM) of lower slope; (4) low-productive snowbed community (SBC) of the slope bottom. N availability, net N mineralization and nitrification were higher in soils of alpine grassland and meadow of the middle part of the toposequence compared with soils of lichen heath and snowbed community of extreme habitats in the alpine zone. There was no correlation between intensities of N transformation processes and favorable (low soil acidity, low C/N ratio, long vegetation period, relatively high temperature, absence of hydromorphic features) and unfavorable (opposite) factors, indicating that the intensity of N mineralization and nitrification in the alpine soils is controlled by a complex combination of these factors. Potential net N mineralization and nitrification in alpine soils determined in the short-term laboratory incubation were considerably higher than those determined in the long-term field incubation. The differences of potential nitrification between soils of various plant communities did not correspond to the field determined pattern indicating the importance of on-site climatic conditions for control of nitrification in high mountains. The result of comparison of N transformation potentials in incubated and native soils indicated that nitrification potential was significantly increased after long-term soil incubation. It means that net nitrification determined in the field was probably overestimated, especially in the meadow soils. A soil translocation experiment indicated that low temperature was an important factor limiting net N mineralization and nitrification in alpine soils: net N mineralization and especially nitrification increased when alpine soils were translocated into the subalpine zone and mean annual temperature increased by about 3°C. Additional N input increased N availability (NH₄ ⁺-N) and potential nitrification in soils of the lower part of the toposequense (GHM and SBC), and potential net N mineralization in two soils of extreme habitats (ALH and SBC). A positive correlation was found between soil ¹⁵N and net N mineralization and nitrification; the relative ¹⁵N enrichment was characteristic of grassland and meadow ecosystems. ¹⁵N of total soil N pool increased during the field mineralization experiment; there was a positive tendency between the change in ¹⁵N and net N mineralization and nitrification, however the relationship was not significant. Foliar ¹⁵N of dominant plant species varied widely within community, however, a tendency of higher foliar ¹⁵N for species growing on the soils with higher net N mineralization, nitrification and ¹⁵N was observed.     

Nitrogen mineralization in a green manure-amended soil as influenced by cropping history and subsequent crop

A greenhouse experiment was conducted to determine the influence of cropping variables on nitrogen dynamics in a soil amended with green manure. Surface soil from various long-term spring wheat rotations was amended with¹⁵N-labelled legume green manure (Lathyrus tingitanus) and subsequently cropped (canola [Brassica napus] and spring wheat [Triticum aestivum]) or incubated without a crop for 56 days in a greenhouse. Nitrogen mineralization from both the indigenous soil N and from green manure was suppressed in cropped soil. Net N mineralization in the uncropped and cropped treatments averaged 73 and 43 mg kg⁻¹, respectively. This difference was attributed, in part, to enhanced biological immobilization in the rhizosphere. Previous cropping practices also had significant effect on N mineralization, largely by their influence on indigenous organic matter quality. These observations suggest that short-term N mineralization is favored by fallowing soil after green manure application whereas N retention in organic matter is favored by immediate cropping. Contribution 3878873     

Tree Patches Show Greater N Losses but Maintain Higher Soil N Availability than Grassland Patches in a Frequently Burned Oak Savanna

Long-term prescribed fires have increased woody canopy openness and reduced nitrogen (N) cycling (that is, net N mineralization) in an oak savanna in Minnesota, USA. It is unclear how fire-induced shifts from oak-dominated to C4 grass-dominated vegetation contribute to this decline in N cycling compared to direct effects of increasing fire frequency promoting greater N losses. We determined (1) the magnitude of decline in net N mineralization in oak versus grass-dominated patches with increasing fire frequency and (2) if differences in net N mineralization between oak and grass patches in frequently burned oak savanna (burned 8 out of 10 years on average during the last 40 years) could be attributed to differences in N losses through volatilization and leaching or to plant traits affecting decomposition and mineralization. In situ net N mineralization declined with increasing fire frequency overall, but this decline was less in oak- than in grass-dominated patches, with oak-dominated patches having more than two times higher net N mineralization than grass-dominated patches. Greater net N mineralization in oak-dominated patches occurred despite greater N losses through volatilization and leaching (on average 1.8 and 1.4 g m⁻² y⁻¹ for oak- and grass-dominated patches, respectively), likely because of higher plant litter N concentration in the oak-dominated patches. As total soil N pools in the first 15 cm did not differ between oak- and grass-dominated patches (on average 83 g N m⁻²), N inputs from atmospheric deposition and uptake from deep soil layers may offset higher N losses. Our results further show that net N mineralization rates decline within 5 years after tree death and subsequent colonization by C4 grasses to levels observed in grass-dominated patches. Although long-term prescribed fires often directly reduce N stocks and cycling because of increased N losses, this study has shown that fire-induced shifts in vegetation composition can strongly contribute to the declines in N cycling in systems that are frequently disturbed by fires with potential feedbacks to plant productivity.     

Effects of precipitation intensity and temporal pattern on soil nitrogen mineralization in a typical steppe of Nei Mongol grassland

Aims Our objective is to: 1) explore the dynamics of soil nitrogen (N) mineralization in a grassland ecosystem in response to the changes in precipitation intensity and temporal distribution, and 2) identify the controlling factors.Methods The two study sites located in a typical steppe of the Nei Mongol grassland were fenced in 2013 and 1999, respectively. Our field experiment includes manipulations of three levels of precipitation intensity (increased 50%, decreased 50%, control) in three temporal patterns (increased or decreased precipitation for three years; increased or decreased precipitation for two years and no manipulation for one year; increased or decreased precipitation for one year and no manipulation for one year).Important findings 1) The soil net N mineralization and net nitrification rates decreased with changes in the temporal distributions of precipitation from one year to three years, with the maximum values of soil net N mineralization and nitrification rates observed in the treatments of increased or decreased precipitation for one year and no manipulation for one year (+PY1 or -PY1). This indicates that the high precipitation intensity and longer precipitation may have negative effects on soil net N mineralization and nitrification rates, while the moderate soilmoisture and temperature may stimulate soil mineralization. 2) The soil net N mineralization and nitrification rates, soil cumulative N mineralization, and nitrification in the fenced site in 1999 were higher than those in the site fenced in 2013, implying that a long-term enclosure may have promoted nutrient storage and soil quality restoration. 3) The long-term treatments of increased or decreased precipitation had significant effects on soil water content and temperature, whereas the short-term, discontinuous precipitation produced minor effects on soil moisture and temperature. Moreover, the controlling factors for soil N mineralization were different between the two fields. Soil moisture had a major effect on soil inorganic N content and net N mineralization rate in the site fenced in 2013, while soil temperature played a dominant role in the site fenced in 1999, with the net N mineralization rate depressed by higher soil moisture. Our findings suggest that the precipitation intensity and temporal distribution had important impacts on soil N mineralization in the Inner Mongolia grassland; these effects was site-dependent and particularly related to soil texture, community composition, and disturbance, and other factors.