畜禽有机肥氮、磷在红壤中的矿化特征研究

选用腐熟的猪粪、鸡粪和第四纪红土发育的典型红壤为试验材料,通过室内培养试验,研究了土壤中矿质氮(NO-3-N和NH4+-N)及Olsen-p的动态变化.结果表明,有机肥中氮和磷的矿化具有不同特征.氮在红壤中的矿化阶段为:前4周缓慢释放,矿化速率为N 0.29～0.46mg/(kg·d);4～10周快速释放,矿化速率为N...     

不同施肥处理对红壤水稻土微生物生物量及呼吸强度的影响

通过布置室内盆栽试验,观测不同N、P肥施用量处理对不同母质和肥力水平红壤水稻土微生物生物量C、N和基础呼吸强度的影响.结果表明,不同红壤水稻土速效养分含量随施肥量的增加而增加,但微生物生物量、呼吸强度和呼吸熵并不一直呈上升趋势.第三纪和第四纪高肥水稻土微生物生物量C在超过2倍常规施肥量后呈下降趋势,第三纪低肥水稻土则随施肥量增加而呈上升趋势.不同土壤的微生物生物量N则在超过1.5倍常规施肥量后呈下降趋势,不同土壤问微生物生物量N变化表现为第四纪高肥水稻土>第三纪高肥水稻土>第三纪低肥水稻土.施肥条件下,红壤水稻土的基础呼吸强度在超过1.5倍常规施肥量后,即随施肥量增加而下降.但土壤呼吸熵随施肥量变化并没有一致的规律.     

无机氮与蔬菜废弃物耦合对土壤氮矿化的影响

为探明有机废弃物添加量与不同无机氮水平耦合对土壤氮矿化的影响,设计了3个甘蓝废弃叶添加量[B1:200 g.kg 1(土),B2:400 g.kg 1(土),B3:550 g.kg 1(土)]和4个无机氮水平[N0:0 mg.kg 1(土),N1:25mg.kg 1(土),N2:50 mg.kg 1(土),N3:100 mg.kg 1(土)]交互的控制培养试验(25℃,65%的田间持水量)。试验结果显示:各氮处理下土壤净累积氮矿化量是空白对照的4～5倍,N1水平下土壤净累积氮矿化量显著高于其他氮水平。各甘蓝废弃叶添加量处理下土壤净累积氮矿化量是空白对照的3～5倍,且B2添加量下土壤净累积氮矿化量显著高于B1和B3。统计分析表明,氮处理和甘蓝废弃叶添加量之间的交互效应不显著(P=0.275),甘蓝废弃叶的添加是影响氮矿化的主要因素(Eta2=0.16),而供氮水平为次要因素(Eta2=0.07)。B1添加量下,培养前期(0～20 d)土壤净累积矿化量逐渐升高,后期保持稳定水平;但B2和B3添加量下,培养前期(30 d)土壤呈现矿化、固持、再矿化现象,后期土壤净累积矿化量逐渐升高。氮矿化速率结果说明,甘蓝废弃叶添加后氮素矿化主要发生在培养前30 d。对培养期间土壤净累积氮矿化量随时间变化做一级动力方程模拟,拟合效果良好(R2=0.62～0.89)。     

模拟氮添加对亚热带杉木人工林土壤有机碳矿化的影响

为探讨氮添加对亚热带杉木人工林土壤有机碳矿化的影响，选择福建三明森林生态系统与全球变化国家野外科学观测研究站38年生杉木人工林土壤为研究对象，设置N0(0 mg/kg)、N10(100 mg/kg)、N25(250 mg/kg) 3个氮添加水平，并进行117 d的培养。结果表明：(1)氮添加后，土壤有机碳矿化速率在培养开始(0 d)即达到最大值，在培养前期(0～57 d)这一段时间内N0、N10和N25处理的有机碳矿化速率平均值显著下降了44%、45%、47%，而在整个培养期间3个处理有机碳平均矿化速率分别为9.97、9.27、8.89 mg/(kg·d)；(2)有机碳矿化累积量随培养时间延长显著增加，随氮添加增加显著降低，与N0处理相比，培养117 d后N10、N25处理有机碳矿化累积量平均值分别降低了3.4%、7.4%；(3)微生物生物量对氮添加响应并不显著，但真菌/细菌比随氮添加增加而增大。总体上，氮添加主要是通过改变土壤有机碳和氮抑制了土壤有机碳矿化。因此，氮添加后土壤中碳、氮养分含量的变化是有机碳矿化变化的主要原因，而微生物群落结构变化则不是主要因素。     

添加生物质炭对红壤水稻土有机碳矿化和微生物生物量的影响

通过室内培育试验,研究了添加生物质炭对江西红壤水稻土有机碳矿化和微生物生物量碳、氮含量的影响。结果表明:红壤有机碳矿化速率在培育第2天达最大值后迅速降低,培养7天后下降缓慢并趋于平稳;添加生物质炭降低了土壤有机碳的矿化速率和累积矿化量,培养结束时,不加生物质炭的对照处理中有机碳的累积矿化量分别比添加0.5%和1.0%生物质炭的处理高10.0%和10.8%。此外,生物质炭的加入显著提高了土壤微生物生物量,添加0.5%生物质炭处理的土壤微生物生物量碳、氮含量分别比对照高111.5%～250.6%和11.6%～97.6%,添加1.0%生物质炭处理的土壤微生物生物量碳、氮含量分别比对照高58.9%～243.6%和55.9%～110.4%。相同处理中,干旱的水分条件下(40%田间持水量)微生物生物量要高于湿润的水分条件(70%田间持水量)。同时,添加0.5%和1.0%的生物质炭使土壤代谢熵分别降低2.4%和26.8%,微生物商减少了43.7%和31.7%。     

施肥和秸秆还田对红壤水稻土有机碳分布变异及其矿化特性的影响

利用始于1990年的湖南桃园红壤水稻土长期定位试验,研究了长期施肥和秸秆还田对红壤水稻土剖面有机碳含量和δ13C值分布,有机碳密度和变异幅度,以及有机碳矿化特性的影响.结果表明,长期施肥和秸秆还田使红壤稻田土壤表层有机碳含量显著升高,秸秆还田对有机质的累积作用明显优于仅施化肥处理.施肥或秸秆还田使0～25 cm和0～ 50 cm土层的有机碳密度均明显提高,且有机碳变异幅度也明显增大.0～10cm表层土壤δ13C值与对照的差异最大,而施化肥或秸秆还田处理之间的差异不大.各施肥处理土壤有机碳在培养初期的矿化速率较大,并在1～3周内迅速降低且逐渐达到稳定状态.秸秆还田使各处理有机碳矿化速率的提高较明显,各处理的平均矿化速率为CO247.75～31.16 ml/(kg.d),稳定矿化速率为CO240～60ml/(kg·d),12周培养期内累积矿化量大小为CK+C＞NP+C＞N+C＞NPK+C＞NP＞NPK＞N＞NK＞CK.长期施化肥和秸秆还田,均使土壤有机碳的累积矿化量提高,而秸秆还田作用更明显.因此,长期施肥和秸秆还田作为土壤有机碳累积的途径,有利于提高红壤水稻土的养分供应能力,并且能够维持稻田生态系统有机碳库的缓冲性和长期稳定性.     

华中地区两种典型菜地土壤中氮素的矿化特征研究

以华中地区两种典型的菜地土壤——黄棕壤和潮土为研究对象,利用室内连续培养试验研究了菜地土壤氮素的矿化规律和矿化特征.结果表明,黄棕壤和潮土氮素的矿化以硝态氮量较多,铵态氮较少.两种土壤的矿化速率随培养时间的延长而降低,培养13周以后黄棕壤的矿化速率为N 0.13 mg/(kg.d),潮土为N 0.32 mg/(kg.d).随着培养时间的延长,黄棕壤和潮土的累计氮矿化量缓慢增长.培养结束时黄棕壤矿化量为N 68.65 mg/kg,潮土矿化量为N109.37 mg/kg,分别占土壤全氮量的24.52％和21.45％.黄棕壤和潮土氮素的矿化势分别为N 74.63 mg/kg和123.45mg,/kg,分别占土壤全氮量的26.65％和24.21％.     

水稻自然生态应用技术研究

根据N通量的质量平衡原理,通过布置田间试验,测定作物的吸N量并据此估算红壤水稻土有机N、C的年矿化量。结果表明,红壤水稻土无N区作物的年吸N量为24~160kg/hm2,其中,高肥力土壤上约为110~160kg/hm2,中等肥力土壤上约为100~105kg/hm2,低肥力土壤上约为24~65kg/hm2;经校正计算得到红壤水稻土有机N的年矿化量为8~108kg/hm2,其中,高肥力土壤为70~110kg/hm2,中等肥力土壤为65kg/hm2,低肥力土壤为8~33kg/hm2;土壤有机C的年矿化量为130~1050kg/hm2,其中,高肥力土壤为1050kg/hm2,中等肥力土壤为750kg/hm2,低肥力土壤为440kg/hm2。土壤肥力水平、作物吸肥特性、土壤母质类型及土壤酸度是影响土壤有机N、C矿化的重要因素。     

不同培养温度下长期施肥红壤水稻土有机碳矿化特征研究

以长期不同施肥处理红壤水稻土为研究对象,布置不同温度下(15、25和35℃)的室内培养试验,研究有机碳矿化的温度敏感性及施肥对土壤有机碳矿化的影响,并分析土壤有机碳矿化与土壤理化性质和不同形态碳素之间的关系。结果表明,培养前期(0～7 d),土壤有机碳矿化速率快速下降,之后逐渐降低并最终趋于稳定。温度升高提高了土壤有机碳矿化速率、累积矿化量和矿化率。磷肥和有机肥的施用提高了土壤有机碳累积矿化量。各处理土壤有机碳矿化的温度敏感性系数Q10为1.31～1.75,施肥提高了土壤有机碳矿化的温度敏感性。Q10与有机碳、全量和速效氮磷、微生物生物量碳、易氧化有机碳和胡敏酸碳呈显著或极显著的正相关关系。3种培养温度下土壤有机碳累积矿化量均与pH呈显著负相关,与有机碳和全氮呈显著或极显著正相关。25℃和35℃培养时,土壤有机碳累积矿化量与微生物生物量碳、胡敏酸碳和富里酸碳显著或极显著正相关。     

15N标记秸秆在太湖地区水稻土上的氮素矿化特征研究

采用室内恒温培养试验研究了在太湖地区乌栅土和黄泥土上添加15N标记秸秆后,秸秆15N在矿质氮、微生物氮和不同粒径土壤组分中的分配情况,并应用氮同位素库稀释法测定了秸秆在两种土壤上的氮总矿化速率。结果表明:两种土壤添加秸秆后,土壤矿质氮量在7～28 d之间迅速下降,微生物氮在前7 d逐渐升高,随后维持稳定。随着秸秆的分解,秸秆15N进入矿质氮库和微生物氮库,矿质15N在第7天时最高,占添加秸秆15N的9.24%～12.3%,微生物15N在第14天时最高,占添加秸秆15N的21.3%～40.5%,随后矿质15N和微生物15N量均下降。在培养的第7～28天之间,矿质15N和微生物15N出现下降,可能存在秸秆氮的损失。培养56 d时,10.5%～13.3%的秸秆15N进入土壤53μm～2 mm组分,24.5%～26.5%进入2～53μm组分,30%进入<2μm组分,有5.7%～14.9%的秸秆氮损失掉,仍有15.4%～29.1%的秸秆未分解,秸秆在乌栅土上分解的更多,但损失也更多。添加秸秆后0.5 d时,秸秆在乌栅土和黄泥土上的氮总矿化速率分别为1.61 mg kg-1d-1和1.48 mg kg-1d-1;56 d时,秸秆在乌栅土和黄泥土上的氮总矿化速率分别为0.26 mg kg-1 d-1和0.36 mg kg-1 d-1。     

中国亚热带稻田土壤碳氮含量及矿化动态

Dynamics of soil organic matter in a cultivation chronosequence of paddy fields were studied in subtropical China. Mineralization of soil organic matter was determined by measuring CO2 evolution from soil during 20 days of laboratory incubation. In the first 30 years of cultivation, soil organic C and N contents increased rapidly. After 30 years, 0-10 cm soil contained 19.6 g kg^-1 organic C and 1.62 g kg^-1 total N, with the corresponding values of 18.1 g kg^-1 and 1.50 g kg^-1 for 10-20 cm, and then remained stable even after 80 years of rice cultivation. During 20 days incubation the mineralization rates of organic C and N in surface soil （0-10 cm） ranged from 2.2% to 3.3% and from 2.8% to 6.7%, respectively, of organic C and total N contents. Biologically active C size generally increased with increasing soil organic C and N contents. Soil dissolved organic C decreased after cultivation of wasteland to 10 years paddy field and then increased. Soil microbial biomass C increased with number of years under cultivation, while soil microbial biomass N increased during the first 30 years of cultivation and then stabilized. After 30 years of cultivation surface soil （0-10 cm） contained 332.8 mg kg^-1 of microbial biomass C and 23.85 mg kg^-1 of microbial biomass N, which were 111% and 47% higher than those in soil cultivated for 3 years. It was suggested that surface soil with 30 years of rice cultivation in subtropical China would have attained a steady state of organic C content, being about 19 g kg^-1.     

Nitrification activities and N mineralization in paddy soils are insensitive to oxygen concentration

Oxygen concentration is considered to be the most important factor influencing nitrification and mineralization rates in agricultural soils. However, the sensitivities of nitrification and N mineralization in paddy soils to oxygen concentrations are not well known. We examined nitrification activities and N mineralization rates of six paddy soils with pH ranging from 5.23 to 7.83 and incubated at 25°C and 60% water-holding capacity in laboratory after ammonium was added at concentrations of 10, 30 and 50?mg?N?kg^?1 of soil and the headspace gases were replaced with stock gases whose oxygen concentrations were 20%, 10% and 2%, respectively. The tested paddy soils had a very wide range of nitrification activities so that the nitrate ratio in inorganic N varied from >?95% after 1 day incubation to?相似文献   

围海造田长期耕种稻田和旱地土壤氮矿化速率及供氮潜力比较

围海造田是沿海地区拓展土地面积的主要途径。土壤氮矿化参数是揭示围海造田土壤肥力演变和土壤氮供应的重要指标,但是我国沿海造田土壤的相关研究少有报道。本研究以杭州湾南岸海积平原上慈溪市1000年和520年筑塘造田区为对象,选择4个代表性采样点,每个点从低洼稻田采集1个表层混合水稻土,在其相邻高地采集1个表层混合旱地土壤,共8个样品。采用间隙淋洗法研究了土壤样品氮矿化动力学特征。结果如下: 119 d培养试验证实水稻土和旱地土壤有机氮矿化动力学符合一级反应动力学方程Nt=N0(1-e-kt); 水稻土有机氮矿化势（N0）为82.7～161.9 mg/kg（平均114 mg/kg）,占有机氮的7.3%,旱地土壤N0为63.9～104.4 mg/kg（平均83.4 mg/kg）,占有机氮的7.3%; 水稻土有机氮矿化速率（k）为0.033～0.114/d（平均0.064/d）,旱地土壤k为0.007～0.023/d（平均0.020/d）。土壤综合供氮指标（N0k）,水稻土为3.8418.46 mg/(kgd)［平均8.0 mg/(kgd)］,旱地土壤为0.54～2.66 mg/(kgd)［平均1.6 mg/(kgd)］。水稻土总氮含量为1.4～2.0 g/kg （平均1.6 g/kg）,旱地为0.87～2.0 g/kg（平均1.3 g/kg）。可见,水稻土氮库、供氮潜力和速率均大于相邻旱地土壤。因此,从土壤氮肥力来讲,相对于旱地,围海形成的水稻田更具有可持续利用性。     

Soil biochemical and chemical changes in relation to mature spruce (Picea abies) forest conversion and regeneration

To investigate soil changes from forest conversion and regeneration, soil net N mineralization, potential nitrification, microbial biomass N, L‐asparaginase, L‐glutaminase, and other chemical and biological properties were examined in three adjacent stands: mature pure and dense Norway spruce (Picea abies (L.) Karst) (110 yr) (stand I), mature Norway spruce mixed with young beech (Fagus sylvatica) (5 yr) (stand II), and young Norway spruce (16 yr) (stand III). The latter two stands were converted or regenerated from the mature Norway spruce stand as former. The studied soils were characterized as having a very low pH value (2.9 – 3.5 in 0.01 M CaCl₂), a high total N content (1.06 – 1.94 %), a high metabolic quotient (qCO₂) (6.7 – 16.9 g CO₂ kg^–1 h^–1), a low microbial biomass N (1.1 – 3.3 % of total N, except LOf₁ at stand III), and a relatively high net N mineralization (175 – 1213 mg N kg^–1 in LOf₁ and Of₂, 4 weeks incubation). In the converted forest (stand II), C : N ratio and qCO₂ values in the LOf₁ layer decreased significantly, and base saturation and exchangeable Ca showed a somewhat increment in mineral soil. In the regenerated forest (stand III), the total N storage in the surface layers decreased by 30 %. The surface organic layers (LOf₁, Of₂) possessed a very high net N mineralization (1.5 – 3 times higher than those in other two stands), high microbial biomass (C, N), and high basal respiration and qCO₂ values. Meanwhile, in the Oh layer, the base saturation and the exchangeable Ca decreased. All studied substrates showed little net nitrification after the first period of incubation (2 weeks). In the later period of incubation (7 – 11 weeks), a considerable amount of NO₃‐N accumulated (20 – 100 % of total cumulative mineral N) in the soils from the two pure spruce stands (I, III). In contrast, there was almost no net NO₃‐N accumulation in the soils from the converted mixed stand (II) indicating that there was a difference in microorganisms in the two types of forest ecosystems. Soil microbial biomass N, mineral N, net N mineralization, L‐asparaginase, and L‐glutaminase were correlated and associated with forest management.     

旱地土壤氮素矿化参数与氮素形态的关系

应用间歇淋洗培养方法 ,以长期不同培肥定位试验土壤为研究对象 ,求得土壤氮素矿化参数 ,并探讨氮素矿化潜势 (N0)、碱解氮、微生物氮、可浸提易矿化氮、全氮之间的关系。结果表明 ,在 35℃和 20℃条件下培养 ,一级动力学模型能够很好的拟合试验数据 ,模拟方程和模拟参数均达到极显著水平。经过 15年的培肥和轮作 ,无论是单施氮肥区 ,还是氮肥与有机肥配合施用区 ,N0均有不同程度的增加 ,这标志着土壤活性有机氮库增加。k值变化范围在0.004628～0.013148d-1之间 ,说明可矿化氮以每天 0.46 %～1.31%的平均速率矿化释放。而且 ,在本试验条件下 ,淋洗液中均含有一定数量的可溶性有机态N ,因此进行氮素矿化研究时 ,同时测定NH4-N、NO3-N和Norg的含量是必要的。 35℃下 ,N0 占全氮的比例为 7.23%～17.36% ,变异系数30.4% ;易矿化有机态氮占全氮的比例为0.27%～0.48% ,变异系数 200% ;碱解氮占全氮的比例为 5.55%～6.54% ,变异系数仅 5.8% ;微生物氮占全氮的比例在 2.16%～5.18%之间 ,变异系数28.8%。从几种指标测得的平均值看 ,N0碱解氮 微生物氮 易矿化氮 ,而变异系数是N0微生物氮 易矿化氮 碱解氮。虽然N0的绝对值远高于田间实际矿化量 ,     

Carbon and nitrogen mineralization of added particulate and macroorganic matter

Particulate organic matter (POM) is more sensitive than total SOM to changes in management practices and, accordingly, may indicate changes in soil quality. A soil incubation study was conducted to determine the effects of added POM (75 to 250 μm size fraction), or macroorganic matter (MOM, 250 to 2000 μm size fraction) on C and N mineralization and microbial C and N content. A 1 kg composite made from 16 predominantly silt loam soils was amended with 10 g of POM, MOM or MOM ground to a reduced size of 75 to 250 μm (GMOM). The MOM amendment equaled 4.55-fold and POM equaled 1.60-fold of total MOM and POM found in the composite soil. Carbon mineralization of MOM and POM after 8 weeks was approximately 9 and 4%, respectively of the total MOM and POM-C added. Reducing the size of MOM to 75 to 250 μm did not affect mineralization. Nitrogen mineralization was slightly greater in the amended soils after 8 weeks and equaled 5 to 6% of the MOM or POM-total N added. Contribution of POM to total mineralized N from soil organic matter (SOM) in the composite soil was proportional to the POM content in SOM or approximately 12%. Amended soils had 25 to 42% more biomass-C than the control soil 2 weeks after amendment application. After 8 weeks, the amended soils contained about 32% more biomass-C. This increase in biomass-C at 8 weeks accounted for approximately 2% of the added C. At 8 weeks, microbial biomass-N in GMOM-, MOM- and POM-amended soils was about 56, 46 and 14% higher, respectively, than in the control soil. These increases were approximately 8% of the MOM-N added and 2% of the POM-N added. Increases in POM resulted in increases in soil respiration and microbial biomass-C and N, which also are suggested indicators of soil quality. Therefore, POM may be a suitable soil quality indicator that provides similar information as soil respiration or microbial biomass determinations.     

设施和露天栽培下有机菜地土壤氮素矿化和硝化作用的比较研究

矿化作用和硝化作用是土壤氮素转化的主要途径,通过室内培养试验,对设施和露天栽培方式下有机菜地土壤氮素的矿化与硝化作用进行了比较研究。结果表明,除培养第1d外,设施有机菜地土壤氮素矿化量、矿化率在整个培养期间都显著高于露天有机菜地土壤;设施有机菜地土壤硝化量、硝化率在培养前两周内高于露天有机菜地土壤;设施有机菜地土壤矿化与硝化作用总体比露天有机菜地土壤强烈。矿化作用可能与全氮、C/N、微生物活性关系密切,而硝化作用强弱可能与微生物活性有关。无论施肥与否,设施有机菜地土壤N2O排放速率在培养期间总体高于露天有机菜地土壤,前者N2O累积排放量显著高于后者,这可能与土壤C/N有关。     

干湿交替对水稻土碳氮矿化的影响

通过室内培育试验,研究干湿交替条件下长期不同施肥处理水稻土微生物生物量和理化性状变化对土壤C、N矿化的影响机制.结果表明,与连续淹水(Cw)处理相比,干燥处理不仅显著地提高了所有施肥处理土壤有机C的矿化速率,其幅度为78%～204%,而且也提高了各处理土壤微生物生物量C和N,其幅度分别为55%～77%和57%～72%;干燥后淹水处理土壤有机C矿化速率的提高幅度为74%～95%,呈先降低再升高的趋势.土壤N的矿化在干湿交替过程的干燥处理中降低34%～78%:干燥后淹水过程仅使NPK处理的升高21%,而CK和NPKOM处理分别降低5%和13%.在培养过程中土壤Eh值仅在-60～60 mV范围时,与土壤微生物生物量C之间有显著的负相关关系.在干湿交替的干燥过程,随土壤pH值的升高土壤微生物生物量C有增加的趋势,在淹水条件下土壤pH值则仅与NPKOM处理土壤微生物生物量C之间有明显的负相关关系.干湿交替条件下土壤 pH和 Eh 值、微生物群落组成和数量与有机质的矿化之间的相互作用关系复杂,三者间的作用机理需进一步研究.     

Estimation of sulfur mineralization and relationships with nitrogen and carbon in soils

A 25-week laboratory study was carried out to determine sulfur, carbon, and nitrogen mineralization rates in soil samples obtained from representative soils in France. Their relationship with some of the soil properties was investigated to find a predictor of mineralized S in soils. At 20°C and 80% water-holding capacity, the S mineralization rate ranged from 0.02 to 0.16 mg kg⁻¹ day⁻¹. It was significantly positively related to soil organic C and N and to C and N mineralization rates. It was weakly related to total soil S. The results suggest that the S mineralization is predominantly driven by heterotrophic microbial activity. A predictive equation for S mineralization based on soil C content, soil pH, and clay content is proposed.     

Effect of freeze-thaw events on mineralization of soil nitrogen

Summary In humid regions of the United States there is considerable interest in the use of late spring (April–June) soil NO ₃ ^– concentrations to estimate fertilizer N requirements. However, little information is available on the environmental factors that influence soil NO ₃ ^– concentrations in late winter/early spring. The influence of freeze-thaw treatments on N mineralization was studied on several central Iowa soils. The soils were subjected to temperatures of-20°C or 5°C for 1 week followed by 0–20 days of incubation at various temperatures. The release of soluble ninhydrin-reactive N, the N mineralization rate, and net N mineralization (mineral N flush) were observed. The freeze-thaw treatment resulted in a significant increase in the N mineralization rate and mineral N flush. The N mineralization rate in the freeze-thaw treated soils remained higher than in non-frozen soils for 3–6 days when thawed soils were incubated at 25°C and for up to 20 days in thawed soils incubated at 5°C. The freeze-thaw treatments resulted in a significant release of ninhydrin-reactive N. These values were closely correlated with the mineral N flush (r ²=0.84). The release of ninhydrin-reactive N was more closely correlated with biomass N (r ²=0.80) than total N (r ²=0.65). Our results suggest that freeze-thaw events in soil disrupt microbial tissues in a similar way to drying and re-wetting or chloroform fumigation. Thus the level of mineral N released was directly related to the soil microbial biomass. We conclude that net N mineralization following a spring thaw may provide a significant portion of the total NO ₃ ^– present in the soil profile.