Induced N-limitation of bacterial growth in soil: Effect of carbon loading and N status in soil

Application of C-rich plant residues can change the soil system from C-limitation for microbial growth to limitation by other nutrients. However, the initial nutrient status of the soil may interact with the added amount of residues in determining limitation. We studied this interactive effect in soils from the Harvard Forest LTER, where annual addition of N since 1988 has resulted in soils with different N-status: No N (Unfertilized), 50 (Low N) and 150 (High N) kg N ha⁻¹. We hypothesized that adding C-rich substrate would change the soil from being C- to being N-limited for bacterial growth and that the extent of N-limitation would be higher with increasing substrate additions, while becoming less evident in soil with increasing N-status. We compared the effect of adding two C-rich substrates, starch (0, 10, 20, 40 mg g⁻¹ soil) and straw (0, 20, 40, 80 mg g⁻¹), incubating the soils for up to 3 and 4 weeks for starch and straw, respectively. Nutrient limitations were studied by measuring bacterial growth 3 days after adding C as glucose and N as NH₄NO₃ in a full factorial design. Initially bacterial growth in all soils was C-limited. As hypothesized, adding C-rich substrates removed the C-limitation, with lower amounts of starch and straw needed in the unfertilized and Low N soils than in the High N soil. Combinations of different N-status of the soil and amendment levels of starch and straw could be found, where bacterial growth appeared close to co-limited both by available C and N. However, at even higher amendment levels, presumable resulting in N-limitation, bacterial growth still responded less by adding N then C-limited soils by adding C. Thus, in a C-limited soil there appeared to be N available immediate for growth, while in an N-limited soil, easily available C was not immediately available.     

Threshold concentration of glucose for bacterial growth in soil

The activity of heterotrophic soil microorganisms is usually limited by the availability and quality of carbon (C). Adding organic substances will thus trigger a microbial response. We studied the response in bacterial growth and respiration after the addition of low amounts of glucose. First we determined if additions of glucose, at concentrations which did not result in an exponential increase in respiration after the lag phase, still stimulated bacterial growth. The second aim was to determine the threshold concentration of glucose needed to induce bacterial growth. Adding glucose-C at 1000 μg g⁻¹ soil resulted in an increased respiration rate, which was stable during 12 h, and then decreased without showing any exponential increase in respiration. Bacterial growth, determined as leucine incorporation, did not change compared to an unamended control during the first 12 h, but then increased to levels 5 times higher than in the control. Thus, after the lag phase, a period with increasing bacterial growth, but at the same time decreasing respiration rates, was found. Similar results, but with a more modest increase in bacterial growth, were found using 500 μg glucose-C g⁻¹ soil. Adding 50–700 μg glucose-C g⁻¹ resulted in increased respiration during 24 h correlating with the addition rate. In contrast, bacterial growth after 24 h was only stimulated by glucose additions >200 μg C g⁻¹ soil. Thus, there was a threshold concentration of added substrate for inducing bacterial growth. Below the threshold concentration growth and respiration appear to be uncoupled.     

华北灌溉区秸秆焚烧与直接还田生态效应研究

2年试验研究初步表明,华北灌溉地区夏季麦秸覆盖直接还田较焚烧处理可保持土壤水分,降低土壤温度,有利于玉米的生长发育,作物产量增加。秸秆焚烧初期的土壤速效养分含量(P、K)高于秸秆还田处理,尤其是速效磷含量远高于秸秆还田处理,但至收获时差异不大;秸秆焚烧比秸秆还田处理除草效果好,且可使部分病虫害减轻,但有的病虫害却因秸秆直接还田而加重。     

秸秆还田下大气CO2浓度升高对水稻生长和CH4排放的影响

【目的】明确秸秆还田下大气CO₂浓度升高对水稻生长和稻田CH₄排放的影响,为气候变化下温室气体排放评估和丰产低碳的稻作技术创新提供理论参考和科学依据。【方法】利用开顶式气室(Open top chamber, OTC)进行田间试验,设置两个CO₂浓度处理,分别为正常大气CO₂浓度处理(简称aCO₂,CO₂浓度约为0.04%)和大气CO₂浓度升高处理(简称eCO₂,CO₂浓度约为0.055%),每个处理的田块混入等量的前茬小麦秸秆,探明秸秆还田下大气CO₂浓度升高对水稻产量等生长特性、稻田CH₄排放及微生物丰度的影响,揭示秸秆还田下大气CO₂浓度升高对CH₄排放的影响机制。【结果】大气CO₂浓度升高显著促进水稻的生长,使剑叶叶面积增加25.0%,地上生物量增加22.0%,产量提高29.0%。大气CO₂浓度升高显著增加了穗数、结实率和千粒重,但对穗粒数影响不显著。秸秆还田下,大气CO₂浓度升高有降低稻田CH₄排放的趋势,使单位产量CH₄排放量降低了39.4%。大气CO₂浓度升高使土壤甲烷氧化关键基因pmoA的拷贝数增加了20.0%,但对甲烷产生关键基因mcrA的拷贝数影响较小。【结论】秸秆还田条件下,未来大气CO₂浓度升高不仅提高了水稻产量,而且有利于减少稻田温室气体CH₄的排放。     

京郊畜禽粪肥资源现状及其替代化肥潜力分析

随着都市型规模化养殖业快速发展,畜禽粪尿废物大量排放因缺乏足够面积土地消纳所带来的环境压力很大,而粪尿中的氮磷钾养分与化肥一样对作物同等重要,如何合理利用这些废物资源决定了都市化农业的可持续发展。该文针对京郊养殖业和农用地面积现状,结合调研、收集畜禽养分排泄系数和农田养分需求等参数,估算京郊固液粪便养分资源现状及其替代化肥的潜力。结果表明:京郊畜禽固液粪便中N、P、K养分量分别为58.7×103、21.3×103、29.8×103 t,其中固体粪便N、P、K养分分别为43.1×103、20.3×103和19.7×103 t,京郊畜禽固液粪便可分别满足农田N、P、K养分需求量的99.3%、185.2%、62.7%。大部分区县粪肥中P养分产生量超过作物P需求量,粮田秸秆还田可带入的N、P、K养分分别为11.0×103、1.6×103和15.0×103 t,情景分析表明在秸秆还田条件下,按照磷素平衡原则估算本地区所能消纳的粪肥所带入N、P和K养分数量分别为18.3×103、9.9×103和10.3×103 t,同时需要补充N、K化肥分别为29.8×103和22.2×103 t,其余粪肥则需经过堆肥化处理并输往外地。经过堆肥处理,固体粪肥可提供的N、P、K养分分别下降了23%、11%和12%,外输固体粪肥堆肥可进一步减少农田氮磷负荷以及可能的环境风险。     

Growth rate of bacteria is affected by soil texture and extraction procedure

Effects of soil texture on the extraction efficiency of bacteria from soils and on biosynthetic activity of the extracted bacteria were studied. Bacterial extracts were prepared from three soils of different texture by homogenization (ultrasonication and mixing) or by homogenization-centrifugation at different speeds. Bacterial biosynthetic activity was estimated using thymidine and leucine incorporation techniques. In each step of the extraction procedure, a higher extractability of bacteria was obtained in finer soils than in coarse soil. Also cell-specific growth rates of bacteria were higher in the finer soils than in the coarse soil. However, in all soils, the extracted bacteria always had significantly lower cell-specific thymidine and leucine incorporation rates than the bacteria in soil slurries and thus did not represent so well the bacterial growth in the original soils. The total declines in cell-specific incorporation rates caused by the extraction were larger in fine soil (96-98%) than in coarse soil (90%), but bacteria in the coarse soil were more responsive to only minor intervention. The homogenization-centrifugation method eliminated the differences in bacterial biosynthesis found when working with soil slurries. Therefore, we recommend using of soil slurries or, optionally, soil suspensions to compare bacterial biosynthetic activity among soils of different textures.     

小麦玉米秸秆连续还田对土壤有机质红外光谱特征及氮素形态的影响

通过6年的田间定位试验,探讨了CK(不施肥,秸秆不还田)、SF(施肥,秸秆不还田)、T1(施肥,玉米秸秆还田)、T2(施肥,小麦秸秆还田)、T3(施肥,玉米小麦秸秆还田)5种处理对土壤氮素形态和有机质红外光谱特征的影响。结果显示:与SF相比较,T1、T2、T3处理使土壤有机氮含量分别增加3.7%、15.9%和18.5%,土壤无机氮含量分别减少15.5%、15.9%和24.0%,其中铵态氮分别降低11.3%、6.0%和12.0%,土壤硝态氮含量分别降低19.3%、22.9%和32.1%;与SF相比较,T1、T3处理土壤有机质(SOM)的C/N分别降低2.8%和1.4%,T2处理SOM的C/N提高1.4%;C/O分别提高9.2%、12.8%和12.1%;而H/C分别降低4.6%、5.5%和4.6%。红外图谱分析显示,T1、T2、T3处理引起3 500~3 200 cm?1处的吸收峰增加,2 924 cm?1处出现了新的弱峰,表明SOM的脂肪族特征增加,且以1 630 cm?1处为中心的宽带吸收峰强度明显增加,SOM芳构化程度增强。研究表明,施肥显著提高了土壤有机氮、无机氮含量,以及土壤有机质的C/N和C/O。而秸秆还田降低了土壤无机氮,提高了土壤有机氮,使SOM的C/N、H/C下降,C/O上升,同时提高了SOM中酚基、羟基、羧基、芳香碳和酰胺含量,其中以小麦、玉米秸秆双季还田的效果最为显著。     

稻草还田对烟田土壤性状和烟草产量及品质的影响

在永州和浏阳进行的稻草还田田间试验结果表明，稻草还田无论是以覆盖还是翻埋方式还田，对烟田土壤性状和烟草产量、品质均有明显的影响。稻草覆盖能在烟草生长后期高温季节对土壤有一定的保湿作用，同时还能增加0～5cm土层土壤的微生物数量，提高烤烟的产量、产值、纯收入和内、外品质；稻草翻埋能增加15～20cm土层土壤的微生物数量，同样能提高烤烟的产量、产值、纯收入和内、外品质。为了避免稻草在腐解过程中产生不利影响，建议稻草还田时应定量（4500kg／hm^2），稻草翻埋还田的应尽量早翻埋，在晚稻收割以后就可以将稻草翻埋下去。     

Role of earthworms in nitrogen cycling during the cropping phase of shifting agriculture (Jhum) in north-east India

We investigated the role of earthworms in the N cycle in a shifting agriculture system under a 5- and a 15-year Jhum system fallow period intervening between two croppings on the same site. Earthworms participated in the N cycle through worm cast egestion, mucus production, and dead tissue decomposition. Soil N was initially depleted by volatilization during slash and burn operations, and subsequently during cultivation processes. These losses were more pronounced under the 15-year Jhum system. We also studied the addition of N to the system in crop residues, through weed recycling, or in compost applied as organic manure under both the 5- and the 15-year Jhum systems. The total soil N made available for uptake by the plant through the activity of earthworms in this agro-ecosystem was higher than the total input of N to the soil through the addition of slashed vegetation, inorganic and organic manure, and recycled crop residue and weeds. Therefore, in highly leached soils of the humid tropics, worm activity is particularly, important because of rapid incorporation of litter into the mineral soils and because of local concentrations of nutrients in the surface soil layers.     

Comparison of factors limiting bacterial growth in different soils

Lack of carbon has been assumed to be the most common limiting factor for bacterial growth in soil, although there are reports of limitation by other nutrients, e.g. nitrogen and phosphorus. We have studied which nutrient(s) limited instantaneous growth rates of bacteria in 28 Swedish soils using the thymidine or leucine incorporation technique to measure increased growth rate after adding different combinations of organic carbon (glucose), nitrogen and phosphorus. The soils ranged in pH between 3.1 and 8.9, in organic matter content between 1% and 91% and in soil C/N ratio between 10 and 28. We also tested the effect of adding different amounts of carbon on the bacterial change in growth rate for two soils with different organic matter content. We found that bacterial growth in most of the 28 soils was limited by a lack of carbon, indicated by an increased bacterial growth rate 48 h after adding glucose. In some soils, adding carbon together with nitrogen increased the bacterial growth rates even further. In three soils no effects were seen upon adding nutrients separately, but adding carbon and nitrogen together increased bacterial growth rates. Nitrogen addition tended to decrease bacterial growth rates, while phosphorus addition had little effect in most soils. No correlations were found between the soil C/N ratio, ammonium or nitrate content in soil and bacterial growth limitation, indicating that even soils with a C/N ratio of 28 could be carbon limited. Although the interpretation of the effects of a single limiting nutrient was in most cases straightforward, an interaction between the amount of carbon added and the organic matter content of the soil confounded the interpretation of the extent of a second limiting nutrient.